南
台
科
技
大
光電工程研究所
透明電極應用之發光二極體
專利資料庫之尋找
碩研光電一甲 第二組
陳興豪 M94L0215
林鴻濱 M94L0214
指導老師：林正峰 老師
-1-
學
中文專利資料庫
專利數目：共 12 篇
資料庫單位：中華民國專利公報檢索系統
(簡易模式,不限欄位)
網址：http://patentog.tipo.gov.tw/tipo/miscmain.htm
關鍵字：透明電極 or 透明導電薄膜 and 發光二極體 and
ITO and ZnO
-2-
專利類型
發明
公告/公開號 00583703
專利名稱
光電單元及其透明導電基板
專利影像
公告/公開日 2004/04/11
期
公報卷期
3111
申請日期
2002/10/11
申請案號
091123523
國際分類號
H01J-001/62
發明人/地址
林明德 新竹市新竹科學工業園區竹村二路十二之三號 中華民國;
/國家
王冠儒 嘉義縣義竹鄉義竹村九十五號 中華民國
申請人/地址
連勇科技股份有限公司 HIGHLINK TECHNOLOGY
/國家
CORPORATION 新竹縣竹北市四維街四十號 中華民國
專利代理人
蔡坤財 臺北市中山區松江路一四八號十二樓
摘要
本發明提供一種光電單元及其透明導電基板。本發明
之透明導電基板包括透明板、 透明電極 薄膜、絕緣部、及
焊接隆起部等,其中 透明電極 薄膜與絕緣部形成於透明板
上,且絕緣部將 透明電極 薄膜分隔成兩互相不導通之第一
透明電極 薄膜區及第二 透明電極 薄膜區,而焊接隆起部形
成於第二 透明電極 薄膜區上。本發明之光電單元則包括上
述透明導電基板、光電元件、及連接線等,其中光電元件
之一電極電連接至上述第一 透明電極 薄膜區,而光電元件
之另一電極則經由連接線與上述焊接隆起部電連接。
專利類型
發明
公告/公開號 00573330
專利名稱
具有覆晶封裝結構之發光半導體裝置
專利影像
公告/公開日 2004/01/21
-3-
期
公報卷期
3103
申請日期
2002/03/21
申請案號
091105514
國際分類號
H01L-021/56
發明人/地址
林明德 新竹市新竹科學工業園區竹村二路十二之三號 中華民國;
/國家
王冠儒 嘉義縣義竹鄉義竹村九十五號 中華民國;
莊啟棠 新竹市北大路四六八號八樓之三 中華民國
申請人/地址
連勇科技股份有限公司 HIGHLINK TECHNOLOGY
/國家
CORPORATION 新竹市新竹科學工業園區力行五路一號二樓 中華
民國
專利代理人
許峻榮 新竹市民族路三十七號十樓
摘要
一種具有覆晶(flip chip)封裝結構的白光發光半導
體裝置,該裝置包含一封裝基板以及一 發光二極體 。此 發
光二極體 則包含一基板、一第一型態半導體層、一主動層
及一第二型態半導體層。第一型態半導體層表面具有一第
一電極,而第二型態半導體層表面具有一第二電極。此 發
光二極體 之基板底部表面則塗佈一層螢光層。封裝基板表
面具有一第一絕緣層。此第一絕緣層之上具有一第一焊接
隆起部與一第二焊接隆起部。此 發光二極體 係利用第一電
極與第一焊接隆起部之連接、第二電極與第二焊接隆起部
之連接而接合至封裝基板之上,進而構成此白光發光半導
體裝置。
專利類型
發明
公告/公開號 00563250
專利名稱
全彩顯示裝置
專利影像
公告/公開日 2003/11/21
期
公報卷期
3033
-4-
申請日期
2002/10/11
申請案號
091123521
國際分類號
H01L-029/76
發明人/地址
林明德 新竹市新竹科學工業園區竹村二路十二之三號 中華民國;
/國家
王冠儒 嘉義縣義竹鄉義竹村九十五號 中華民國;
黃世晟 臺北市北投區吉利街二五七巷三弄八號 中華民國;
葉明發 基隆市中正區和二路三十號五樓 中華民國;
鍾伊泰 臺中市西區忠信街六號 中華民國
申請人/地址
連勇科技股份有限公司 新竹縣竹北市四維街四十號 中華民國
/國家
專利代理人
蔡坤財 臺北市中山區松江路一四八號十二樓
摘要
本發明提供一種全彩顯示裝置,其中本發明之全彩顯
示裝置至少包括複數個像素單元,而每個像素單元包括基
座、複數個透明導電基板、複數個發光元件、以及複數個
電極部等。基座上形成有至少三個開口,且每一開口之底
面為反射面,而每個透明導電基板則分別覆蓋每個開口。
各發光元件分別設置於各透明導電基板之一側且分別容
置於各相對應之開口中。各電極部形成於基座上,且各透
明導電基板之電極分別與各發光元件之電極及各電極部
電連接。
專利類型
發明
公告/公開號 00561636
專利名稱
光電裝置
專利影像
公告/公開日 2003/11/11
期
公報卷期
3032
申請日期
2002/10/11
申請案號
091123522
國際分類號
H01L-033/00
-5-
發明人/地址
林明德 新竹市新竹科學工業園區竹村二路十二之三號 中華民國;
/國家
王冠儒 嘉義縣義竹鄉義竹村九十五號 中華民國
申請人/地址
連勇科技股份有限公司 新竹縣竹北市四維街四十號 中華民國
/國家
專利代理人
蔡坤財 臺北市中山區松江路一四八號十二樓
摘要
本發明提供一種光電裝置,至少包括透明導電基板、
光電元件、及基座。透明導電基板至少包括透明板、形成
於透明板上之 透明電極 薄膜、形成於透明板上之絕緣部,
其中絕緣部係將 透明電極 薄膜分隔成兩互相不導通之第
一 透明電極 薄膜區以及第二 透明電極 薄膜區。光電元件係
設置於透明導電基板上,且光電元件之正電極及負電極分
別與第一 透明電極 薄膜區及第二 透明電極 薄膜區電連
接。基座形成有開口,且開口之底面係為反射面,其中光
電元件係容置於開口中,且光電元件懸離或接觸於開口之
底面。
專利類型
發明
公告/公開號 00543128
專利名稱
可表面黏著並具有覆晶封裝結構之發光半導體裝置
專利影像
公告/公開日 2003/07/21
期
公報卷期
3021
申請日期
2001/07/12
申請案號
090117161
國際分類號
H01L-021/60 ; H01L-033/00
發明人/地址
林明德 新竹市新竹科學工業園區竹村二路十二之三號 中華民國;
/國家
王冠儒 嘉義縣義竹鄉義竹村九十五號 中華民國
申請人/地址
連勇科技股份有限公司 新竹市新竹科學工業園區力行五路一號二樓
/國家
中華民國
專利代理人
許峻榮 新竹市民族路三十七號十樓
-6-
摘要
本發明係一種發光半導體裝置之封裝結構,其係結合
覆晶封裝結構與表面黏著裝置二者之特色。此發光半導體
裝置之封裝結構包含一絕緣基板以及一 發光二極體 , 發光
二極體 則包含一基板、一形成於基板上並具有一第一電極
區域之第一型態半導體層、一形成於第一型態半導體層上
並具有一第二電極區域之第二型態半導體層、一形成在第
一電極區域之第一電極、以及一形成在第二電極區域之第
二電極。本發明之特點係利用覆晶之方式將 發光二極體 固
接於絕緣基板之上,絕緣基板之二側端面上並設有二電極
層,以使本發明之發光半導體裝置可以表面黏著之方式安
裝。
專利類型
發明
公告/公開號 00531907
專利名稱
Ⅲ族氮化物 發光二極體
專利影像
公告/公開日 2003/05/11
期
公報卷期
3014
申請日期
2002/02/18
申請案號
091102707
國際分類號
H01L-033/00
發明人/地址
汪信全 臺北縣三重市正義北路三六四巷八號 中華民國;
/國家
陳錫銘 臺南市東區東平路二十三號九樓 中華民國
申請人/地址
聯銓科技股份有限公司 臺南縣學工業園區台南縣南科三路十五號四
/國家
樓 J 室 中華民國
專利代理人
蔡坤財 臺北市中山區松江路一四八號十二樓
摘要
一種Ⅲ族氮化物 發光二極體 ,係在活性層與基板之間
加入一層退化接面,此退化接面係由一層 n+型層與其上的
一層 p+型層所構成。藉由此退化接面,可使Ⅲ族氮化物 發
光二極體 形成上 n 下 P 的形式,而 發光二極體 上的導電電
-7-
極也皆為 n 型。應用本發明Ⅲ族氮化物 發光二極體 ,由於 n
型氮化物的可摻雜濃度較 p 型氮化物材料高,所以有較好
的電流分散能力。另外,更由於 p 型氮化物的暴露部分減
少,因此較不易受到氫鈍化的影響。
專利類型
發明
公告/公開號 00515116
專利名稱
發光二極體 結構
專利影像
公告/公開日 2002/12/21
期
公報卷期
2936
申請日期
2001/12/27
申請案號
090132481
國際分類號
H01L-033/00
發明人/地址
許進恭 台南縣將軍鄉將貴村七十號 中華民國
/國家
申請人/地址
元砷光電科技股份有限公司 台南科學工業園區台南縣新市鄉大順
/國家
九路十六號 中華民國
專利代理人
詹銘文 台北巿羅斯福路二段一○○號七樓之一;
蕭錫清 台北市羅斯福路二段一○○號七樓之一
摘要
一種 發光二極體 結構,其架構於一基底上。首先有一
低溫成長之成核層形成於基底上,接著導電緩衝層係位在
成核層之上,用以使後續的長晶製程可以順利。主動層結
構則位於上束縛層與下束縛層之間,主動層結構係包括摻
雜之三-五族元素為主所構成的半導體材料。接觸層則位
在上束縛層結構上。接著在接觸層上形成一反轉穿隧層,
其導電型與接觸層的導電型不同。再者有一 透明電極 位在
反轉穿隧層上;以及陰極電極,其與導電緩衝層接觸,並
且與主動層結構與該 透明電極 隔離。
-8-
專利類型
發明
公告/公開號 00492202
專利名稱
具有覆晶組態之防止靜電放電結構之 III-V 族 發光二極體 結構
專利影像
公告/公開日 2002/06/21
期
公報卷期
2918
申請日期
2001/06/05
申請案號
090113545
國際分類號
H01L-033/00
發明人/地址
許進恭 台南縣將軍鄉將貴村七十號 中華民國
/國家
申請人/地址
元砷光電科技股份有限公司 台南科學工業園區台南縣新市鄉大順
/國家
九路十六號 中華民國
專利代理人
詹銘文 台北巿羅斯福路二段一○○號七樓之一
摘要
一種具有覆晶組態之防止靜電放電破壞之Ⅲ-Ⅴ族 發
光二極體 結構。在一透明基底上所形成的導電緩衝層定義
成第一導電緩衝層與第二導電緩衝層。之後,在第一導電
緩衝層上依序形成主動結構層、接觸層、電極等 發光二極
體 結構。另外,則在第二導電緩衝層上形成一金屬電極以
構成蕭特基二極體。或者,在第二導電緩衝層中再形成摻
雜區域,以構成同質接合面(homo-junction)二極體結構。
第二導電緩衝層上之二極體的陰極與陽極分別電性耦接到
第一導電緩衝層上之 發光二極體 的陽極與陰極。
專利類型
發明
公告/公開號 00488088
專利名稱
發光二極體 結構
專利影像
公告/公開日 2002/05/21
-9-
期
公報卷期
2915
申請日期
2001/01/19
申請案號
090101241
國際分類號
H01L-033/00
發明人/地址
郭啟文 台南市青年路一二三號五樓之一 中華民國;
/國家
許世昌 台南市青年路一二三號六樓之二 中華民國
申請人/地址
元砷光電科技股份有限公司 台南科學工業園區台南縣南科三路十
/國家
五號四樓 S 室 中華民國;
郭啟文 台南市青年路一二三號五樓之一 中華民國
專利代理人
詹銘文 台北巿羅斯福路二段一○○號七樓之一
摘要
一種具有超晶格接觸層之 發光二極體 結構,其架構
於一基底上。首先有一低溫成長之晶核層形成於基底
上,接著導電緩衝層係位在晶核層之上,用以使後續的
長晶製程可以順利。主動層結構則位於上束縛層與下束
縛層之間,主動層結構係包括摻雜之三-五族元素為主所
構成的半導體材料。超晶格接觸層則位在上束縛層結構
上。再者有一 透明電極 位在超晶格接觸層上;以及電極,
其與導電緩衝層接觸,並且與主動層結構與該 透明電極
隔離。
專利類型
發明
公告/公開號 00444358
專利名稱
發光元件及其製作
專利影像
公告/公開日 2001/07/01
期
公報卷期
2819
申請日期
2000/01/19
申請案號
089100791
國際分類號
H01L-021/8252
- 10 -
發明人/地址
何晉國 台北巿光復南路六巷四十五號二樓 中華民國;
/國家
陳澤潤 台北巿民族西路一六二巷四號五樓 中華民國;
鄭振雄 新竹巿光復路一段三五四巷十六弄八號七樓 中華民國;
游原振 新竹巿光復路一段一○八巷一三二號七樓之一 中華民國;
鍾長祥 新竹市埔頂路一三三號六樓之二 中華民國;
陳權威 宜蘭縣羅東鎮北成街二三七巷一號 中華民國;
邱建嘉 台北市民權東路三段一八四巷一號六樓 中華民國
申請人/地址
財團法人工業技術研究院 新竹縣竹東鎮中興路四段一九五號 中華
/國家
民國
摘要
一種高亮度的發光元件及其製作方法,其利用一複合
透明導電薄膜 做為 透明電極 ,可增加發光元件的發光效
率。上述複合 透明導電薄膜 不需使用 p 型 GaAs,即可與 p 型
(AlvxGa1-x)0.5In0.5P(0.5≦x≦l)、(AlvxGa1-x)As(0.4≦x<L)
或 GaP 等半導體形成低的接觸電阻。其架構主要包括:一 n
型 GaAs 基板;異質結構,形成於上述 n 型 GaAs 基板上,依
序包括 n 型 A10.5In0.5P 被覆層、AlGaInP/GaInP 量子井發光
活性層及 p 型 Al0.5In0.5P 被覆層;一半導體氧化物薄膜,形
成於上述異質結構上;一 透明導電薄膜 ,形成於上述半導
體氧化物薄膜上;一正面金屬電極,形成於上述 透明導電
薄膜 之上;一背面金屬電極,形成於上述 n 型 GaAs 基板的
另一面,且與上述 n 型 GaAs 基板形成歐姆接觸。上述半導
體氧化物薄膜與 透明導電薄膜 結合形成複合 透明電極 , 透
明導電薄膜 可為導電性氧化物或氮化物,而半導體氧化物
為過渡元素之氧化物。
專利類型
發明
公告/公開號 00439304
專利名稱
氮化鎵系列Ⅲ-Ⅴ族化合物半導體元件
專利影像
公告/公開日 2001/06/07
期
公報卷期
2816
- 11 -
申請日期
2000/01/05
申請案號
089100082
國際分類號
H01L-033/00
發明人/地址
何晉國 台北巿光復南路六巷四十五號二樓 中華民國;
/國家
鍾長祥 新竹巿埔頂路一三三號六樓之二 中華民國;
邱建嘉 台北巿民權東路三段一八四巷一號六樓 中華民國;
陳澤潤 台北巿民族西路一六二巷四號五樓 中華民國
申請人/地址
財團法人工業技術研究院 新竹縣竹東鎮中興路四段一九五號 中華
/國家
民國
摘要
本發明揭露一種氮化鎵系列Ⅲ-V 族化合物半導體元
件,係以一半導體氧化物膜與一透明導電膜構成其 p 型電
極,利用前者與 p 型半導體層形成良好的歐姆接觸,後者
使電流能均勻地擴散至整個 p 型半導體層表面;由於半導
體氧化物膜與透明導電膜均具有良好的光穿透性,使得 p
型電極易成為透明結構,其穿透率在可見光波長範圍內達
75%以上,因此可大幅提升發光元件之發光效率,而且此
種製程簡單可靠,可大幅提昇生產良率及元件可靠度,同
時亦能簡化製程步驟,降低生產成本。
- 12 -
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關鍵字: transparent and conductive and LED
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United States Patent
Yamamoto ,
6,965,467
et al.
November 15, 2005
Title: Particles for display device, image display
medium using the same, and image forming device
Abstract
Provided are particles for use in a display device, in which
particles cohesive force between the particles and a specific
gravity are reduced, and an image display medium which can
ensure a stable display image over a long period of time, and
an image forming device. The particles for a display device are
such that the cohesive force between the particles and the
specific gravity are reduced. Further, the present invention
can provide the image display medium, in which a driving voltage
can be set to be low, and which can ensure a stable display image
over a long period of time even if there are shocks from an
exterior or static states over long periods and the image
forming device utilizing this image display medium.
Inventors: Yamamoto; Yasuo (Minamiashigara, JP); Hiraoka; Satoshi (Minamiashigara,
JP)
Assignee: Fuji Xerox Co., Ltd. (Tokyo, JP)
Appl. No.: 294873
Filed:
November 15, 2002
Foreign Application Priority Data
- 13 -
Dec 12, 2001[JP]
2001-378844
Aug 22, 2002[JP]
2002-241787
Current U.S. Class:
359/290; 359/296; 347/112; 347/153; 345/107
Intern'l Class:
G02B 026/00
Field of Search:
359/296,290 347/112,153,111,122,151 345/107,85
250/378 348/383 430/19,32,41 399/158,131
References Cited [Referenced By]
U.S. Patent Documents
4126528
Nov., 1978
Chiang.
6373461
Apr., 2002
Hasegawa et al.
6400462
Jun., 2002
Hille.
6400492
Jun., 2002
Morita et al.
6407763
Jun., 2002
Yamaguchi et al.
6411316
Jun., 2002
Shigehiro et al.
6524153
Feb., 2003
Ikeda et al.
2001/0024577
Sep., 2001
Matsuura et al.
2003/0227665
Dec., 2003
Kawai.
Foreign Patent Documents
A 2001-312225
Nov., 2001
JP.
Other References
Gugrae-Jo et al., "New Toner Display Device (I)", Japan Hardcopy, Ronbunshu,
pp. 249-252, 1999.
Primary Examiner: Dang; Hung Xuan
Assistant Examiner: Martinez; Joseph
Attorney, Agent or Firm: Oliff & Berridge, PLC
Claims
- 14 -
1. Particles for use in a display device, wherein the particles are filled in, and movable,
between a pair of substrates disposed so as to face one another in a display device, the
particles comprising polymer particulate, a color material and a resin, and having a
property of being able to be charged one of positive and negative, wherein the charged
particles are moved by an electric field with air serving as a medium.
2. An image display medium, comprising:
a pair of substrates disposed so as to oppose each other and form a void, and particle
groups formed from at least two types of particles filled into, and movable within, the void
between the pair of substrates, and at least one type of said at least two types of particles
has a property of being able to be charged positive and at least one other type of said at
least two types of particles has a property of being able to be charged negative, and the
particles which are able to be charged positive and the particles which are able to be
charged negative have respectively different colors,
wherein at least one of the particles which are able to be charged positive and at least one
of the particles which are able to be charged negative are particles for a display device
comprising polymer particulate, a color material and a resin, and the charged particles are
moved by an electric field with air serving as a medium.
3. An image forming device forming an image on an image display medium, comprising:
a pair of substrates disposed so as to face each other, and particle groups formed from at
least two types of particles filled into, and movable within, a void between the pair of
substrates, and at least one type of said at least two types of particles has a property of
being able to be charged positive and at least one other type of said at least two types of
particles has a property of being able to be charged negative, and the particles which are
able to be charged positive and the particles which are able to be charged negative have
respectively different colors, and at least one of the particles which are able to be charged
positive and the particles which are able to be charged negative are particles for a display
device comprising polymer particulate, a color material and a resin; and
electric field generating means for generating an electric field corresponding to an image
between the pair of substrates,
- 15 -
wherein the charged particles are moved by the electric field with air serving as a medium.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image display medium at which repeated rewriting is
possible, to particles for a display device used in the image display medium, and to an
image forming device.
2. Description of the Related Art
Conventionally, display techniques such as twisting ball display (display by the rotation of
particles which are painted two colors), electrophoresis, magnetic migration, thermally
rewritable media, liquid crystal having a good memory property, and the like, have been
proposed as image display media at which repeated rewriting is possible. These display
techniques have an excellent memory property with respect to images, but have the
problems that the display surface cannot display white color such as in the case of paper,
and the contrast is low.
The following display technique has been proposed ("Japan Hardcopy" '99, Ronbunshu, pp.
249-252) as a display technique using a toner and overcoming the above-described
problems: a conductive colored toner and white particles are filled between opposing
electrode substrates, and charges are injected into the conductive colored toner via a charge
transporting layer provided at the inner surface of the electrode of the non-display substrate.
The conductive colored toner, in which the charges have been injected, is moved by the
electric field between the electrode substrates, toward the display substrate which is
positioned so as to oppose the non-display substrate. The conductive colored toner adheres
to the inner side of the substrate at the display side, and an image is displayed by the
contrast between the conductive colored toner and the white particles. In this display
technique, the entire image display medium is structured by solid bodies, and the display
technique is superior in that the display of white and black (color) can be switched by
100% in theory. However, in this technique, there exists conductive colored toner which
- 16 -
does not contact the charge transporting layer provided at the inner surface of the electrode
of the non-display substrate, and conductive colored toner which is isolated from the other
conductive colored toner. Because charges are not injected into these conductive colored
toner, these conductive colored toner randomly exist within the substrate without being
moved by the electric field. Thus, there is the problem that the density contrast deteriorates.
In order to overcome such a problem, Japanese Patent Application Laid-Open (JP-A) No.
2001-312225 discloses an image display medium comprising a pair of substrates, and
plural types of particle groups which have respectively different colors and different
charging characteristics and which are filled between the substrates so as to be movable
between the substrates due to an applied electric field. In accordance with this proposed
technique, a high degree of whiteness and density contrast can be obtained. The particles
proposed therein are structured such that the applied voltage needed to display a
black-and-white image is several hundred volts, and by reducing the voltage, an increase in
the degrees of freedom in designing the driving circuit is made possible.
However, decreasing the applied voltage used in driving results in the problem that the
attraction between the substrates and the particles is decreased, and due to shocks from the
exterior and static states over long periods of time, the particles fall off from the substrate.
In particular, when particles of a large specific gravity containing a color material whose
mass is large are used, the falling off from the substrate becomes even worse due to the
specific gravity and the cohesion between particles, and it is difficult to hold a stable
display image.
SUMMARY OF THE INVENTION
The present invention overcomes the above-described drawbacks of the conventional art,
and achieves the following objects. Namely, an object of the present invention is to provide
particles for a display device in which the cohesive strength between particles and the
specific gravity of the particles are reduced. Another object of the present invention is to
provide an image display medium in which the driving voltage can be set to be low, and
which can ensure a stable display image over a long period of time even if there are shocks
from the exterior or static states over long periods of time, and to provide an image
forming device using the image display medium.
The present inventors focused their attention on attenuating the cohesive strength between
particles and the adhesion of fine particles having a large specific gravity to the surface of
a substrate, and, as a result of their diligent research, found that, by improving these
- 17 -
properties to appropriate levels, the above-described drawbacks could be overcome. The
present inventors thereby arrived at the present invention.
The particles for a display device of the present invention have the property of being able
to be charged positive or negative, and have color, and contain polymer particulates therein.
Further, it is preferable that a portion of or all of the polymer particulates are hollow
particles.
Polymer particulates, which have a relatively low specific gravity, are contained in the
particles for a display device of the present invention, and the added amount of the color
material which forms the color is reduced. It is thereby possible to reduce the specific
gravity of the particles. In particular, by making the contained polymer particulates be
hollow particles, the specific gravity can be set to be even lower.
It is preferable that the color material which makes the color appear is formed from a
pigment having a color difference (ΔE*ab) of light resistance of no more than 2.0 and
having a color difference (ΔE*ab) of heat resistance of no more than 2.0 at 130° C. or
higher, as obtained on the basis of pigment testing method JIS K 5101. By using such a
pigment, the range of applications of the particles for a display device can be broadened.
The image display medium of the present invention has: a pair of substrates disposed so as
to face each other, and particle groups formed from at least two types of particles filled
into a void between the pair of substrates, and at least one type of the at least two types of
particles has a property of being able to be charged positive and at least one other type of
the at least two types of particles has a property of being able to be charged negative, and
the particles which are able to be charged positive and the particles which are able to be
charged negative have respectively different colors, wherein at least one of the particles
which are able to be charged positive and at least one of the particles which are able to be
charged negative are particles for a display device comprising polymer particulates therein.
It is preferable that a portion of or all of the polymer particulates are hollow particles.
Moreover, it is preferable that the color material which makes the color appear is formed
from a pigment having a color difference (ΔE*ab) of light resistance of no more than 2.0
and having a color difference (ΔE*ab) of heat resistance of no more than 2.0 at 130° C. or
higher, as obtained on the basis of pigment testing method JIS K 5101.
In the present invention, it is important that the particles which can be charged positive and
the particles which can be charged negative have respectively different colors, and that the
specific gravity of at least one of the types of particles is low. Because the colors are
- 18 -
different, it is possible to obtain a high contrast between the image regions, which are
formed from the group of particles which can be charged positive, and the image regions,
which are formed from the group of particles which can be charged negative. Moreover, by
reducing the specific gravity of the particles, the adsorbency between the particles and the
substrates can be increased. Thus, it is also possible to lower the driving voltage needed for
image display. Further, it is possible to ensure a stable displayed image over a long period
of time even if there are shocks from the exterior or if there are static states for long
periods. In addition, by utilizing particles for a display device which contain a pigment
having excellent light-resistance and heat-resistance, the image display medium of the
present invention can be suitably used as, for example, a display using a backlighting
system.
In the image display medium of the present invention, it is preferable that one of the
particles which can be charged positive and the particles which can be charged negative are
white. By making at least one of these types of particles white, the coloring strength of the
other particles and the density contrast can be improved. Moreover, the white particles
contain a color material, and it is preferable that the color material is titanium oxide. By
using titanium oxide, in the range of wavelengths of visible light, a high concealability can
be obtained, and the contrast can be improved even more. In addition, in light of the
relationship between dispersability and concealability, it is preferable that the titanium
oxide is formed from at lest two types of titanium oxide having respectively different
particle diameters.
The image forming device of the present invention forms an image on an image display
medium having a pair of substrates disposed so as to face each other, and particle groups
formed from at least two types of particles filled into a void between the pair of substrates,
and at least one type of the at least two types of particles has a property of being able to be
charged positive and at least one other type of the at least two types of particles has a
property of being able to be charged negative, and the particles which are able to be
charged positive and the particles which are able to be charged negative have respectively
different colors, and at least one of the particles which are able to be charged positive and
the particles which are able to be charged negative are particles for a display device
including polymer particulates therein, and the image forming device comprises: electric
field generating means for generating an electric field corresponding to an image, between
the pair of substrates.
BRIEF DESCRIPTION OF THE DRAWINGS
- 19 -
FIG. 1 is a schematic structural diagram of an image forming device of a first embodiment.
FIG. 2 is a schematic structural diagram of an image forming device of a second
embodiment.
FIG. 3 is a cross-sectional view of an image forming portion in an arbitrary plane of FIG. 2.
FIG. 4 is a cross-sectional view of an image forming portion in an arbitrary plane of FIG. 2.
FIG. 5 is a cross-sectional view of an image forming portion in an arbitrary plane of FIG. 2.
FIG. 6 is a schematic structural diagram of an image forming device of a third embodiment.
FIGS. 7A through 7C are diagrams showing patterns of electrodes of a printing electrode.
FIG. 8 is a schematic structural diagram of the printing electrode.
FIG. 9 is a schematic structural diagram of an image forming device of a fourth
embodiment.
FIG. 10 is a graph showing electric potentials at an electrostatic latent image carrier and an
opposing electrode.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Hereinafter, detailed explanation will be given of the particles for a display device, the
image display medium using the particles for a display device, and the image forming
device, of the present invention.
[Structure of Particles for a Display Device of the Present Invention]
The particles for a display device of the present invention have the property of being able
to be charged positive or negative, and have color, and polymer particulates must be
contained therein.
The particles for a display device of the present invention are formed from at least polymer
particulates, a color material, and a resin. As needed, a charge controlling agent may be
included therein, or the color material may also function as a charge controlling agent.
- 20 -
Polymer particulates having a relatively low specific gravity are contained in the particles
for a display device of the present invention, and the contained amount, in the particles for
a display device of the present invention, of the color material which has a high specific
gravity is reduced. The overall specific gravity of the particles for a display device can
thereby be reduced. Namely, in the particles for a display device, by substituting a portion
of the color material, which affects the specific gravity the most, with polymer particulates
which have a low specific gravity, the overall specific gravity of the particles for a display
device can be reduced.
In this way, the particles for a display device of the present invention have a reduced
specific gravity, and the cohesiveness between the particles also can be reduced. Moreover,
due to the scattering of light by the polymer particulates which are contained in the
particles for a display device, the optical reflection density can be made to be high even if
the contained amount of color material is reduced.
(Polymer Particulates)
Conventionally known polymers can be used as the polymer particulates. However, it is
preferable to use polymer particulates whose specific gravity is lower than that of the color
material with which they are used. Moreover, when the polymer particulates themselves
have color, it is preferable to appropriately select and use the polymer particulates in
consideration of the color of the color material with which they are used. In addition,
although the resins which will be listed hereinafter can be used as the resin which is used
together with the polymer particulates, methacrylate resins and acrylate resins are
preferably used.
As the polymer particulates, specifically for example, polystyrene resin, polymethyl
methacrylate resin, urea-formalin resin, styrene-acrylate resin, polyethylene resin,
polyvinylidene fluoride resin and the like can be used singly or plural types thereof can be
used in combination. However, the polymer particulates are not limited to these resins.
These resins preferably have a cross-linked structure, and more preferably have a refractive
index which is higher than that of the resin phase with which they are used.
Polymer particulates of any configuration, such as spherical, amorphous, flat, or the like,
may be used. However, it is more preferable that the polymer particulates are spherical.
Polymer particulates may be used provided that their volume average particle diameter is
- 21 -
less than that of the particles for a display device. However, the volume average particle
diameter of the polymer particulates is preferably 10 μm or less, and more preferably 5
μm or less. It suffices for the particle size distribution to be sharp, and a monodisperse
particle size distribution is particularly preferable.
From the standpoint of preparing the particles for a display device which have a lower
specific gravity, a portion of or all of the polymer particulates are preferably formed from
hollow particles. Although hollow particles can be used provided that their volume average
particle diameter is less than that of the particles for a display device, the volume average
particle diameter of the hollow particles is preferably 10 μm or less, and more preferably
5 μm or less. In particular, from the standpoint of scattering of light, the volume average
particle diameter of the hollow particles is still more preferably 0.1 to 1 μm, and is
particularly preferably 0.2 to 0.5 μm.
Here, "hollow particle" means a particle which has a void at the interior of the particle. The
void is preferably 10 to 90%. Moreover, "hollow particles" may be particles in the form of
a hollow capsule, or particles in which the outer wall of the particle is porous.
It is particularly preferable to include hollow particles in the white particles for a display
device because they can improve the concealability and can increase the degree of
whiteness by utilizing the scattering of light. In hollow particles which are in a hollow
capsule form, this scattering of light is caused by the difference in the refractive indices at
the interface between the resin layer at the outer shell portion and the air layer at the
particle interior. In hollow particles whose outer wall is porous, the scattering of light is
caused by the difference in refractive indices between the outer wall and the cavities.
In the particles for a display device of the present invention, the added amount of the
polymer particulates is, with respect to the entire amount of the particles for a display
device, preferably 1 to 40% by mass, and more preferably 1 to 20% by mass. If the added
amount of the polymer particulates is less than 1% by mass, there are cases in which it is
difficult for the effect of reducing the specific gravity by the addition of the polymer
particulates to appear. Further, if the added amount of the polymer particulates is greater
than 40% by mass, there are cases in which the manufacturability, such as the
dispersability and the like, at the time of preparing the particles for a display device of a
preferred form deteriorates.
(Color Material)
- 22 -
Examples of the color material are as follows.
Examples of black color materials are black color materials which are either organic or
inorganic and are either a dye or a pigment, such as carbon black, titanium black, magnetic
powder, oil black, and the like.
Examples of white color materials are white pigments such as rutile-type titanium oxide,
anatase-type titanium oxide, zinc white, white lead, zinc sulfide, aluminum oxide, silicon
oxide, zirconium oxide, and the like.
In addition, as color materials having chromatic colors, phthalocyanine-based,
quinacridone-based, azo-based, and condensed-type insoluble lake pigments and dyes and
pigments of inorganic oxides can be used. Specifically, aniline blue, chalcoil blue, chrome
yellow, ultramarine blue, Dupont oil red, quinoline yellow, methylene blue chloride,
phthalocyanine blue, malachite green oxalate, lamp black, rose bengal, C.I. Pigment
Red48:1, C.I. Pigment Red 122, C.I. Pigment Red 57:1, C.I. Pigment Yellow 97, C.I.
Pigment Blue 15:1, C.I. Pigment Blue 15:3 and the like can be suitably listed as
representative examples.
These dyes and pigments may, as needed, be subjected to a surface treatment or the like for
improving the dispersability thereof.
As a color material having a chromatic color, it is particularly preferable to use a pigment
(which, for convenience, will hereinafter be called "specific pigment") whose color
difference (ΔE*ab) of light resistance is 2.0 or less and whose color difference (ΔE*ab)
of heat resistance is 2.0 or less at 130° C. or more, as obtained on the basis of pigment
testing method JIS K 5101. As mentioned above, this specific pigment has high light
resistance and heat resistance, which means that it does not discolor due to light or heat.
This specific pigment has the excellent advantage that, by dispersing the specific pigment
extremely finely in order to obtain a high-level color rendering property, in applications
such as, for example, displays using a backlighting method or the like, the desired
transparency can be ensured and even more vivid color can be obtained as compared with
generally used organic pigments such as paints or inks or the like.
Examples of the specific pigment which has a chromatic color are pigments used in color
filters, and the like, such as blue pigments having a maximum absorption wavelength in the
range of 400 nm to 500 nm, green pigments having a maximum absorption wavelength in
- 23 -
the range of 500 nm to 600 nm, red pigments having a maximum absorption wavelength in
the range of 600 nm to 700 nm, and the like. More specifically, examples of blue pigments
are C.I. Pigment Blue 15 (15:3, 15:4, 15:6 and the like), 21, 22, 60, 64 and the like;
examples of green pigments are C.I. Pigment Green 7, 10, 36, 47, and the like; and
examples of red pigments are C.I. Pigment Red 9, 97, 122, 123, 144, 149, 166, 168, 177,
180, 192, 215, 216, 224 and the like.
The specific pigment is preferably used as a master batch pigment. Here, "master batch"
means a preliminary mixture for a final molded product (in the present invention, the
particles for a display device), which mixture has been conceived of in order to improve
the economy of compounding of the color material, the dispersion of the color material,
and the uniformity of the color material, as well as improve the ease of injection molding,
extrusion molding, measurement, and the like. The master batch is formed by mixing, at a
high concentration (usually 5 to 50% by mass) a pigment having a desired color into a raw
material resin, and kneading the mixture, and working the mixture into the form of pellets
(or into the form of flakes or into a plate-like form).
Examples of the raw material resin used in the master batch pigment are homopolymers
and copolymers of radically polymerizable monomers such as styrene, methylstyrene,
chlorostyrene, vinyl acetate, vinyl propionate, methyl acrylate, ethyl acrylate, propyl
acrylate, n-butyl acrylate, isobutyl acrylate, n-octyl acrylate, dodecyl acrylate, 2-ethylhexyl
acrylate, stearyl acrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate,
n-butyl methacrylate, isobutyl methacrylate, n-octyl methacrylate, dodecyl methacrylate,
2-ethylhexyl methacrylate, stearyl methacrylate, acrylonitrile, methacrylonitrile,
acrylamide, methacrylamide, glycidyl acrylate, glycidyl methacrylate, acrylic acid,
methacrylic acid, 2-vinylpyridine, and the like, as well as polyester resin, polyamide resin,
epoxy resin, and the like.
The method of manufacturing the master batch pigment is as follows. First, the special
pigment and the raw material resin are ground and dispersed in an organic solvent so as to
prepare a pigment dispersed liquid. Here, a medium stirring mill such as a sand mill, a ball
mill, an attritor or the like can be used in the grinding/dispersing processing. The
grinding/dispersing processing may be carried out either in batches or continuously.
Thereafter, the organic solvent is removed from the pigment dispersed liquid. Then,
grinding is carried out so as to manufacture a master batch pigment in which the specific
pigment is uniformly dispersed in the raw material resin.
When the particles for a display device of the present invention are manufactured by using
- 24 -
the master batch pigment obtained in this way, the master batch pigment is used in the
form of being added to and dispersed in a monomer.
Examples of the color material which is also used as the charge controlling agent are
substances which have a charge attracting group or a charge donating group, metal
complexes, and the like. Specific examples include C.I. Pigment Violet 1, C.I. Pigment
Violet 3, C.I. Pigment Violet 23, C.I. Pigment Black 1, and the like.
If the specific gravity of the color material is 1, the added amount of the color material is
preferably in a range of 1 to 60% by mass, and more preferably in a range of 5 to 50% by
mass, with respect to all of the particles.
Further, when the color material is the specific pigment, if the specific gravity of the color
material is 1, the added amount of the color material is preferably in a range of 1 to 60%
by mass, and more preferably in a range of 5 to 30% by mass, with respect to all of the
particles.
(Resin)
Examples of the resin are polyvinyl resins such as polyolefin, polystyrene, acrylic resin,
polyacrylonitrile, polyvinyl acetate, polyvinyl alcohol, vinyl chloride, polyvinyl butyral,
and the like; vinyl chloride—vinyl acetate copolymer; styrene-acrylate copolymer; styrene
silicon resins formed by organosiloxane bonds, and modified resins thereof; fluorine resins
such as polytetrafluoroethylene, polyvinyl fluoride, and polyvinylidene fluoride; polyester,
polyurethane, polycarbonate; amino resins; epoxy resins; and the like. A single one of
these resins may be used or plural resins may be used by being mixed together. These
resins maybe cross-linked. Moreover, known binder resins, which are known as main
components in toners used in conventional electrophotographic methods, may be used
without problem as the resin. In particular, its is preferable to use a resin containing a
cross-linked component.
(Other Additives)
As needed, a charge controlling agent may be added to the particles for a display device of
the present invention, in order to control the charge ability. Known charge controlling
agents which are used in toner materials for electrophotography can be used as the charge
controlling agent. Examples include cetyl pyridyl chloride, quaternary ammonium salts
such as P-51 and P-52 (manufactured by Orient Chemical Industries, Ltd.) and the like,
- 25 -
salicylic acid based metal complexes, phenol condensation products, tetraphenyl
compounds, calixarene compounds, as well as metal oxide particulates and metal oxide
particulates which are surface treated by various types of coupling agents.
The charge controlling agent is preferably colorless, or has low coloring strength, or is a
color similar to the color of the overall particles in which the charge controlling agent is
contained. By using a charge controlling agent which is colorless, or has low coloring
strength, or is a color similar to the color of the overall particles in which the charge
controlling agent is contained (i.e., is a color similar to the color of the color material
contained in the particles), the impact on the hue of the selected particles can be lessened.
Here, "colorless" means not having color, and "low coloring strength" means that there is
little effect on the color of the overall particles in which the charge controlling agent is
contained. Further, "is a color similar to the color of the overall particles in which the
charge controlling agent is contained", means that, although the charge controlling agent
itself has a hue, it is the same color as or a similar color to the color of the overall particles
in which the charge controlling agent is contained, and as a result, there is little effect on
the color of the overall particles in which the charge controlling agent is contained. For
example, in particles containing a white pigment as the color material, a white color charge
controlling agent would fall under the scope of "is a color similar to the color of the overall
particles in which the charge controlling agent is contained". In any case, regardless of
whether the color of the charge controlling agent is "colorless", "low coloring strength", or
"a color similar to the color of the overall particles in which the charge controlling agent is
contained", it suffices that the color of the charge controlling agent is such that the color of
the particles containing the charge controlling agent becomes the desired color.
The added amount of the charge controlling agent is preferably 0.1 to 10% by mass, and
more preferably 0.5 to 5% by mass. Further, with regard to the size of the dispersed unit of
the charge controlling agent within the particles, a volume average particle diameter of 5
μm or less is suitably used, and a volume average particle diameter of 1 μm or less is
preferable. Moreover, the charge controlling agent may exist in a compatible state in the
particles.
It is preferable to add a resistance adjusting agent to the particles for a display device of
the present invention. By adding a resistance adjusting agent, the exchange of charges
between particles can be made faster, and early stabilization of the display image can be
achieved. Here, "resistance adjusting agent" means a conductive fine powder, and
particularly preferably, a conductive fine powder which generates an appropriate level of
- 26 -
exchange of charges or leakage of charges. By also including a resistance adjusting agent
in the particles for a display device of the present invention, it is possible to avoid an
increase in the amount of charge of the particles, i.e., so-called "charging-up", due to
friction between the particles and friction between the particles and the surface of the
substrate over a long period of time.
An appropriate example of the resistance adjusting agent is an inorganic fine powder
whose volume resistivity is 10×106 Ωcm or less, and preferably 10×104 Ωcm or less.
Specific examples include particulates coated with any of various types of conductive
oxides such as tin oxide, titanium oxide, zinc oxide and iron oxide, such as, for example,
titanium oxide coated with tin oxide and the like. It is preferable that the resistance
adjusting agent is colorless, has low coloring strength, or is a color similar to the color of
the overall particles in which the resistance adjusting agent is contained. The meanings of
these terms are similar to those described above in the discussion of the charge controlling
agent. The added amount of the resistance adjusting agent does not present problems
provided that it is in the range of not interfering with the color of the colored particles, and
an added amount of 0.1 to 10% by mass is preferable.
The particle diameter of the particles for a display device of the present invention cannot
be stipulated unconditionally. However, in order to obtain a good image, the volume
average particle diameter thereof is preferably about 1 to 100 μm and more preferably
about 3 to 30 μm. It suffices for the particle size distribution thereof to be sharp, and a
monodisperse particle size distribution is more preferable.
(Method of Manufacturing Particles for Display Device)
Examples of methods of manufacturing the particles for a display device of the present
invention are wet manufacturing methods which manufacture spherical particles, such as
suspension polymerization, emulsion polymerization, dispersion polymerization and the
like, and conventional grinding/classifying methods which manufacture
non-uniformly-shaped particles. Further, in order to obtain a uniform shape of the particles,
a heat treatment can also suitably be carried out.
As a method of making the particle size distribution uniform, the particle size distribution
can be adjusted by classifying. This can be carried out by, for example, a vibrating sieve,
an ultrasonic sieve, a pneumatic sieve, a wet sieve, rotor rotary-type classifying devices
using the principles of centrifugal force, wind power based classifying devices, and the like.
However, the present invention is not limited to the same. The particle size distribution can
- 27 -
be adjusted to the desired particle size distribution with a single device, or by combining
plural devices. In a case in which particularly precise adjustment is to be carried out, it is
preferable to use a wet sieve.
The following is a suitable example of a method of controlling the shape of the particles (a
method of controlling the shape factor). A so-called suspension polymerization method is a
method in which a polymer is dissolved in a solvent, and a colorant is mixed in, and in the
presence of an inorganic dispersing agent, the mixture is dispersed in a water-based solvent
such that particles are formed. In this suspension polymerization method, an organic
solvent, which is compatible with a monomer (i.e., has no or little compatibility with a
solvent) and is not polymerizable, is added and suspension polymerization is carried out.
Examples of a method can be suitably given, which method suitably selects a drying
method for removing the organic solvent by steps of forming, removing and drying the
particles. A freeze-drying method disclosed in JP-A No. 10-10775 is a suitable example of
the method of appropriately selecting a drying method for removing the organic solvent.
The freeze-drying method is preferably carried out at -10° C. to -200° C. (preferably, -30° C.
to -180° C.). Further, the freeze-drying method is carried out at a pressure of around 40 Pa
or less, and is particularly preferably carried out at 13 Pa or less. Here, examples of the
organic solvent are ester solvents such as methyl acetate, propyl acetate and the like; ether
solvents such as diethylether; ketone solvents such as methylethyl ketone, methylisopropyl
ketone, methylisobutyl ketone, and the like; hydrocarbon solvents such as toluene,
cyclohexane and the like; halogenated hydrocarbon solvents such as dichloromethane,
chloroform, trichloroethylene, and the like; and the like. It is preferable that these solvents
can dissolve polymers, or that the proportion which dissolves in water is about 0 to 30% by
mass. Further, in carrying out the method on an industrial scale, cyclohexane is particularly
preferable in consideration of the stability, cost, and produceability.
Further, the particle shape can also be controlled by the method disclosed in JP-A No.
2000-292971 of making small particles cohere and unite so as to enlarge the small particles
to a desired particle diameter, or a method of heating or a method of applying mechanical
impact force (e.g., by a hybridizer (manufactured by Nara Machinery Co., Ltd.), an ang
mill (manufactured by Hosokawa Micron Corporation), a θ composer (manufactured by
Tokuju Corporation), or the like) to particles obtained by conventionally known
fusing/kneading, grinding, classifying and other methods, or the like.
[Structure of Image Display Medium of the Present Invention]
The image display medium of the present invention comprises a pair of substrates which
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are disposed so as to oppose one another, and particle groups formed from at least two or
more types of particles filled into the void between the pair of substrates. Of the two or
more types of particles, at least one type has the property of being able to be charged
positive, and at least one another type has the property of being able to be charged negative.
The particles which can be charged positive and the particles which can be charged
negative have respectively different colors. At least one of the particles which can be
charged positive and the particles which can be charged negative are the above-described
particles for a display device of the present invention.
(Particle Groups Formed From Two or More Types of Particles)
The particle groups of the present invention which are formed from two or more types of
particles have the feature that, in the particle groups, at least one type of particles (first
particles) has the property of being able to be charged positive, and at least one another
type of particles (second particles) has the property of being able to be charged negative,
and the particles able to be charged positive and the particles able to be charged negative
have respectively different colors.
In the image display medium of the present invention, the previously-mentioned problems
can be overcome by reducing the specific gravity of the particles of at least one of the first
particles and the second particles. Namely, in the image display medium of the present
invention, by using, as the particles of at least one of the first particles and the second
particles, the particles for a display device of the present invention whose specific gravity
has been reduced, the cohesiveness between particles and the peeling away from the
substrate can be reduced, and a stable display image can be maintained. Accordingly, at the
image display medium of the present invention, the driving voltage can be set to be low,
and even if there are shocks from the exterior or static states for long periods of time, a
stable display image can be ensured over a long period of time.
Note that, in the above description, expression were used which were based on the
assumption that there was one type of particles charged positive (the first particles) and one
type of particles charged negative (the second particles). However, there are no problems if
there is only one type of each of positively charged particles and negatively charged
particles, or if there are two or more types of each. Even when there are two or more types
of each, if one type there among is formed by the particles for a display device of the
present invention, the effects of the present invention due to operational mechanisms which
are the same as those described above can be achieved.
- 29 -
Hereinafter, in the image display medium of the present invention, the first particles and
the second particles together, i.e., both of the particles which can be charged positive and
the particles which can be charged negative, will collectively be called the "display
particles". Both of these display particles are preferably structured by the above-described
particles for a display device of the present invention. However, as will be described
hereinafter, conventionally known particles, which do not contain polymer particulates
therein, can also be used together.
Particles, which are formed from at least a color material and a resin and whose color
material and resin are the same as those of the above-described particles for a display
device of the present invention, can be used as the conventionally known particles which
can be used together. Further, in the same way as described above, these particles may, as
needed, contain a charge controlling agent, and the color material may also serve as the
charge controlling agent.
In the image display medium of the present invention, it is preferable that one type of the
display particles is white, i.e., it is preferable that one type of the display particles contains
a white color material. By making one type of the particles white, the coloring strength of
the other type of particles and the density contrast can be improved. Here, titanium oxide is
preferable as the white color material for making one type of particles white. By using
titanium oxide as the color material, in the range of wavelengths of visible light, the
concealing force increases, and the density contrast can be improved even more.
Rutile-type titanium oxide is particularly preferable as the white color material.
It is preferable that the titanium oxide used in the present invention is two or more types of
titanium oxides having respectively different particle diameters. Generally, the
dispersability of titanium oxide is poor. Even if the dispersability is improved, with those
titanium oxide particles whose diameters are large, the occurrence of secondary and
tertiary cohesion is more rapid, the dispersion stability deteriorates, and there are cases in
which the concealing force cannot be sufficiently exhibited, all in accordance with the
larger specific gravity of such larger-diameter titanium oxide particles. On the other hand,
those titanium oxide particles whose particle diameters are small cannot sufficiently cause
scattering of light, and there are cases in which the concealing force is poor. Accordingly,
by using in combination two or more types of titanium oxides having different average
particle diameters, both dispersion stability and concealability can be improved.
The primary particle diameter of at least one type of titanium oxide which can be used is
preferably 0.1 μm to 1.0 μm which is a particle diameter which results in high optical
- 30 -
concealability. The primary particle diameter of the other titanium oxide is preferably less
than 0.1 μm.
The titanium oxide which has the small particle diameter may be subjected to a surface
treatment. Substances in which any of various types of coupling agents or organic
substances is dissolved in a solvent can be used as the surface treating agent, provided that
it is in the range of not affecting the degree of whiteness.
Here, because the specific gravity of white color display particles containing titanium
oxide is particularly large as compared with that of the display particles having other color
materials, it is particularly preferable that the above-described particles for a display device
of the present invention are used as the display particles. Further, by making the polymer
particulates included in the particles for a display device be hollow particles, the degree of
whiteness can be increased and better contrast can be expected.
Note that, in the present invention, one type of display particles is not limited to being
white. For example, one type of display particles may be black. In this case, for example, it
is particularly effective to carry out display by switching between a character or symbol
which is black and a character or a symbol which is another color.
It is necessary to prepare the display particles such that one type thereof has the property of
being able to be charged positive and another type thereof has the property of being able to
be charged negative. When different types of particles are charged by colliding and being
rubbed, one type is charged positive and another type is charged negative due to the
positional relationship of both tribo series. Thus, for example, by appropriately selecting
the charge controlling agent, the positions of the tribo series can be appropriately adjusted.
With regard to the particle size of the display particles, by making the particle diameters
and the distributions of the white particles and the black particles substantially the same for
example, it is possible to avoid a so-called adhered state in which the larger particle
diameter particles are surrounded by the smaller particle diameter particles as in the case of
a two-component developer. Thus, a high white density and a high black density can be
obtained. The coefficient of variation is preferably about 15% or less, and monodisperse is
particularly preferable. There are cases in which small particle diameter particles adhere to
the peripheries of large particle diameter particles, and the inherent color density of the
large particles decreases. Further, there are cases in which the contrast varies also due to
the mixing ratio of the white and black particles. The mixing proportion is preferably of an
extent such that the display surface areas of the display particles are equivalent. If the
- 31 -
mixing proportion deviates greatly from such a mixing proportion, the color of the particles
whose proportion is greater may be stronger. However, this does not apply in a case in
which it is desired to increase the contrast in a display of a darker color tone and the
display of a lighter color tone of the same color, or in a case in which it is desired to carry
out display in a color formed by mixing together two types of colored particles.
(Substrate)
A pair of the substrates are disposed so as to oppose one another, and the display particles
are filled into the void between the pair of substrates. In the present invention, the substrate
is a conductive plate-shaped body (a conductive substrate). In order to achieve the
functions of the image display medium, at least one of the pair of substrates must be a
transparent conductive substrate. Here, the transparent conductive substrate is the display
substrate.
The conductive substrate may be a structure in which the substrate itself is conductive, or
may be a structure in which the surface of an insulating support is subjected to a
processing for making it conductive. The conductive substrate may be either crystal or
non-crystal. Examples of conductive substrates in which the substrates themselves are
conductive are metals such as aluminum, stainless steel, nickel, chromium or the like and
alloys thereof, and semiconductors such as Si, GaAs, GaP, GaN, SiC, ZnO and the like,
and the like.
Examples of the material of the insulating support are polymer films, glass, quartz,
ceramics and the like. The process for making the insulating support conductive can be
carried out by forming a film by vapor depositing, spattering, ion plating or the like any of
the metals which are listed above as specific examples of the material for the conductive
substrate when the substrate itself is conductive, or gold, silver, copper, or the like.
A conductive substrate, at which a transparent electrode is formed on one surface of an
insulating transparent support, or a transparent support, which itself is conductive, is used
as the transparent conductive substrate. Examples of materials of transparent supports
which are themselves conductive are transparent conductive materials such as ITO, zinc
oxide, tin oxide, lead oxide, indium oxide, copper iodide, and the like.
Films or plate-shaped bodies of transparent inorganic materials such as glass, quartz,
sapphire, MgO, LiF, CaF2, and the like, or of transparent organic resins such as fluorine
resins, polyester, polycarbonate, polyethylene, polyethylene terephthalate, epoxy and the
- 32 -
like, can be used as the insulating transparent support. Or, an optical fiber, a Selfoc optical
plate, or the like can be used as the insulating transparent support.
Structures which are formed by methods such as vapor deposition, ion plating, spattering
and the like by using a transparent conductive material such as ITO, zinc oxide, tin oxide,
lead oxide, indium oxide, copper iodide or the like, or structures in which a metal such as
Al, Ni, Au or the like is formed thin enough to be semi-transparent by vapor deposition or
spattering or the like, can be used as the transparent electrode provided on one surface of
the transparent support.
The surfaces of these substrates at the sides which oppose one another affect the charged
polarity of the particles. Thus, providing thereat a protective layer in the form of an
appropriate surface is preferable. The protective layer can be appropriately selected mainly
from the standpoints of adhesion to the substrate, transparency, and the electrode rows, as
well as from the standpoint of the ability thereof to not dirty the surface. Specific examples
of materials of the protective layer are polycarbonate resin, vinyl silicone resin,
fluorine-group-containing resins, and the like. A resin, that is compatible with structure of
the main monomer of the particles used and whose friction charging difference with the
particles are small, is selected.
[Embodiments of the Image Forming Device of the Present Invention]
Hereinafter, embodiments of the image forming device of the present invention, which
uses the image display medium of the present invention, will be described in detail with
reference to the drawings. Note that members having similar functions are denoted by the
same reference numerals throughout all of the figures, and there are cases in which
description thereof will be omitted.
First Embodiment
FIG. 1 illustrates an image display medium relating to the present embodiment, and an
image forming device, relating to the present embodiment, for forming an image on the
image display medium.
As shown in FIG. 1, an image forming device 12 relating to the present first embodiment
has a voltage applying means 201. An image display medium 10 is structured by spacers
204, black particles 18, and white particles 20 being filled in between a display substrate
14, which is at the side that the image is displayed, and a non-display substrate 16 which
- 33 -
opposes the display substrate 14. As will be described layer, a transparent electrode 205 is
attached to each of the display substrate 14 and the non-display substrate 16. The
transparent electrode 205 of the non-display substrate 16 is grounded, whereas the
transparent electrode 205 of the display substrate 14 is connected to the voltage applying
means 201.
Next, details of the image display medium 10 will be described.
For example, 7059 glass substrates, to which 50×50×1.1 mm transparent electrode ITOs
are attached, are used as the display substrate 14 and the non-display substrate 16 which
form the outer sides of the image display medium 10. An inner side surface 206 of the
glass substrate, which inner side surface 206 contacts the particles, is coated by a 5 μm
thick polycarbonate resin (PC-Z). The center portions of the silicon rubber plates 204
which are 40×40×0.3 mm are each cut-out in a square of 15×15 mm so as to form a space,
and these silicon rubber plates are set on the non-display substrate 16. For example, the
white particulates 20, which are spherical and contain titanium oxide and have a volume
average particle diameter of 20 μm, and the black particulates 18, which are spherical and
contain carbon and have a volume average particle diameter of 20 μm, are mixed together
in a mass ratio of 2-to-1. About 15 mg of these mixed particles are shaken through a screen
into the spaces cut-out in squares in the silicon rubber plates. Thereafter, the silicon rubber
plates are fit tightly to the display substrate 14. The region between the substrates is
pressurized and held by double clips, such that the silicon rubber plates and the both
substrates are tightly fit together and the image display medium 10 is formed.
Second Embodiment
Hereinafter, a second embodiment of the present invention will be described in detail with
reference to the drawings.
FIG. 2 illustrates an image forming device 12 which relates to the present embodiment and
which is for forming an image on the image display medium 10 using a simple matrix.
Electrodes 403An and 404Bn (where n is a positive number) are arranged in a simple
matrix structure. A plurality of particle groups having different chargeabilities are filled
into the spaces between the electrodes 403An, 404Bn. Electric potential is generated at the
respective electrodes 403An, 404Bn by an electric field generating device 402 formed by a
waveform generating device 402B and a power source 402A, or by an electric field
generating device 405 formed by a waveform generating device 405B and a power source
405A. By a sequencer 406, the electric potential driving timing of the electrodes is
- 34 -
controlled and the driving of the voltages of the respective electrodes is controlled. An
electric field, by which the particles can be driven in units of one row, can be applied to the
electrodes 403A1 through An of one surface, and an electric field corresponding to image
information can simultaneously be applied within the surface to the electrodes B1 through
Bn of the other surface.
FIGS. 3, 4, and 5 show cross-sections of the image forming portion in an arbitrary plane of
FIG. 2. The particles are contacting the electrode surface or the substrate surface, and at
least one surface of the substrate is transparent and the color of the particles passes through
and can be seen from the exterior. The electrodes 403A, 404B may be embedded in and
made integral with the substrate as shown in FIGS. 3 and 4, or may be formed to as to be
set apart from the substrates as shown in FIG. 5.
By setting the electrical field appropriately in the above-described device, display in
accordance with simple matrix driving is possible. Note that, provided that the particles
have a threshold value of movement with respect to the electric field, driving is possible,
and the colors, charged polarities, amounts of charge, and the like of the particles are not
restricted.
Third Embodiment
Hereinafter, a third embodiment of the present invention will be described with reference
to the drawings. The third embodiment is an image forming device using a printing
electrode.
As shown in FIG. 6 and FIG. 7A, the printing electrode 11 is formed from a substrate 13
and a plurality of electrodes 15 whose diameter is, for example, 100 μm. The image
forming device 12 is equipped with the printing electrode 11, an opposing electrode 26, a
power source 28, and the like.
As shown in FIG. 7A, the plurality of electrodes 15 are aligned at one side surface of the
display substrate 14 in one row at predetermined intervals in accordance with the
resolution of the image and along a direction (the main scanning direction) substantially
orthogonal to the conveying direction (the direction of arrow B in FIG. 6) of the image
display medium 10. The electrodes 15 may be square as shown in FIG. 7B, or may be
disposed in a matrix form as shown in FIG. 7C.
As shown in FIG. 8, an AC power surface 17A and a DC power source 17B are connected
- 35 -
to the respective electrodes 15 via a connecting control section 19. The connecting control
section 19 is formed by a plurality of switches which are switches 21A, ones of ends of
which are connected to the electrodes 15 and the others of ends of which are connected to
the AC power source 17A, and switches 21B, ones of ends of which are connected to the
electrodes 15 and the others of ends of which are connected to the DC power source 17B.
The switches are controlled on and off by a control section 60, so as to electrically connect
the electrodes 15, and the AC power source 17A and the DC power source 17B. In this
way, AC voltage or DC voltage, or voltage on which AC voltage and DC voltage are
superimposed, can be applied.
Next, operation of the present third embodiment will be described.
First, the image display medium 10 is conveyed by an unillustrated conveying means in the
direction of arrow B in FIG. 6. When the image display medium 10 is conveyed to
between the printing electrode 11 and the opposing electrode 26, the control section 60
instructs the connecting control section 19 to turn all of the switches 21A on. AC voltage is
thereby applied from the AC power source 17A to all of the electrodes 15.
Here, the image display medium is a medium in which two or more types of particles
groups are filled in the space between a pair of substrates which do not have electrodes.
When AC voltage is applied to the electrodes 15, the black particles 18 and the white
particles 20 within the image display medium 10 move reciprocally between the display
substrate 14 and the non-display substrate 16. In this way, the black particles 18 and the
white particles 20 are frictionally charged due to the friction between the particles and the
friction between the substrates and the particles. For example, the black particles 18 are
charged positive, and the white particles 20 are not charged or are charged negative.
Hereinafter, description will be given assuming that the white particles 20 are charged
negative.
Then, the control section 60 instructs the connecting control section 19 to turn on only the
switches 17B which correspond to the electrodes 15 at positions corresponding to the
image data, such that DC voltage is applied to the electrodes 15 at the positions
corresponding to the image data. For example, DC voltage is applied to the non-image
portions, and DC voltage is not applied to the image portions.
In this way, when DC voltage is applied to the electrodes 15, as shown in FIG. 6, the black
- 36 -
particles 18, which were charged positive and were at the portion where the printing
electrode 11 opposed the display substrate 14, move toward the non-display substrate 16
due to the working of the electric field. Moreover, the white particles 20, which were
charged negative and were at the non-display substrate 16 side, move toward the display
substrate 14 due to the working of the electric field. Accordingly, because only the white
particles 20 appear at the display substrate 14 side, no image is displayed at the portions
corresponding to the non-image portions.
On the other hand, when DC voltage is not applied to the electrodes 15, the black particles
18, which were charged positive and were at the portion were the printing electrode 11
opposed the display substrate 14, are maintained as is at the display substrate 14 side due
to the working of the electrode field. Moreover, the black particles 18, which were charged
positive and were at the non-display substrate 16 side, move toward the display substrate
14 side due to the working of the electric field. Accordingly, because only the black
particles 18 appear at the display substrate 14 side, an image is displayed at the portions
corresponding to the image portions.
In this way, because only the black particles 18 appear at the display substrate 14 side, an
image is displayed at the portions corresponding to the image portions.
In this way, the black particles 18 and the white particles 20 move in accordance with the
image, and the image is displayed at the display substrate 14 side. Note that, if the white
particles 20 are not charged, only the black particles 18 move due to the effect of the
electric field. Because the black particles 18 at the regions where no image is displayed
move toward the non-display substrate 16 and are concealed from the display substrate 14
side by the white particles 20, the image can be displayed. Further, even after the electric
field which was generated between the substrates of the image display medium 10
disappears, the displayed image is maintained due to the inherent adhesion of the particles.
Further, if an electric field is generated between the substrates, these particles can move
again. Therefore, images can repeatedly be displayed by the image forming device 12.
In this way, because the charged particles are moved by an electric field with air serving as
the medium, the stability is good. Moreover, because air has low viscous resistance, the
high-speed response property is satisfactory.
Fourth Embodiment
Hereinafter, a fourth embodiment of the present invention will be described with reference
- 37 -
to the drawings. The fourth embodiment is an image forming device using an electrostatic
latent image carrier.
The image forming device 12 of the present fourth embodiment is illustrated in FIG. 9. The
image forming device 12 is equipped with an electrostatic latent image forming section 22,
a drum-shaped electrostatic latent image carrier 24, the opposing electrode 26, the DC
voltage power source 28, and the like.
The electrostatic latent image forming section 22 has a charging device 80 and a light
beam scanning device 82. The photosensitive drum 24 can be used as the electrostatic
latent image carrier 24. The photosensitive drum 24 is a structure in which a
photoconductive layer 24B is formed on a conductive substrate 24A which is drum-shaped
and is formed of aluminum, SUS, or the like. Any of known materials an be used as the
material of the photoconductive layer. For example, inorganic photoconductive materials
such as α-Si, α-Se, As2Se3 and the like, and organic photoconductive materials such as
PVK/TNF and the like can be used. These materials can be used to form the
photoconductive layer 24B by plasma CVD, vapor deposition, dipping, or the like. As
needed, a charge transporting layer or an overcoat layer or the like may be formed.
The charging device 80 uniformly charges the surface of the electrostatic latent image
carrier 24 to a desired electric potential. It suffices for the charging device 80 to charge the
surface of the photosensitive drum 24 to an arbitrary electric potential. In the present
embodiment, as the charging device 80, a corotron is used which applies high voltage to an
electrode wire, generates corona discharge between the electrode wire and the electrostatic
latent image carrier 24, and uniformly charges the surface of the photosensitive drum 24.
In addition, any of various types of known charging devices may be used such as devices
which make conductive roller members, brushes, film members or the like contact the
photosensitive drum 24, apply voltage thereto, and charge the surface of the photosensitive
drum, or the like.
The light beam scanning device 82 irradiates light in the form of an extremely small spot
on the basis of an image signal onto the surface of the charged electrostatic latent image
carrier 24, so as to form an electrostatic latent image on the electrostatic latent image
carrier 24. It suffices that the light beam scanning device 82 is a structure which, in
accordance with the image information, irradiates a light beam onto the surface of the
photosensitive drum 24 and forms an electrostatic latent image on the photosensitive drum
24 which has been uniformly charged. In the present embodiment, the light beam scanning
device 82 is an ROS (Raster Output Scanner) device which, by a focussing system having
- 38 -
a polygon mirror 84, a bend-back mirror 86, an unillustrated light source, lens, and the like,
scans light onto the surface of the photosensitive drum 24 by the polygon mirror 84 while
turning the laser beam, which has been adjusted to a predetermined spot diameter, on and
off in accordance with the image signal. Other than the ROS device, an LED head, in
which LEDs are aligned in accordance with the desired resolution, or the like may be used
as the light beam scanning device 82.
Note that the conductive substrate 24A of the electrostatic latent image carrier 24 is
grounded. Moreover, the electrostatic latent image carrier 24 rotates in the direction of
arrow A in FIG. 9.
The opposing electrode 26 is formed by, for example, a conductive roller member which is
elastic. In this way, the opposing electrode 26 can be set in even closer contact with the
image display medium 10. The opposing electrode 26 is disposed at a position which is at
the side of the image display medium 10, which is being conveyed by an unillustrated
conveying means in the direction of arrow B in FIG. 9, opposite the side at which the
electrostatic latent image carrier 24 is disposed. The DC voltage power source 28 is
connected to the opposing electrode 26. Bias voltage VB is applied to the opposing
electrode 26 by the DC voltage power source 28. This bias voltage VB which is applied is,
for example, as shown in FIG. 10, an intermediate electric potential between VH, which is
the electric potential of the portions on the electrostatic latent image carrier 24 which are
charged with positive charges, and VL, which is the electric potential of the portions which
are not charged. Further, the opposing electrode 26 rotates in the direction of arrow C.
Next, operation of the present fourth embodiment will be described.
When the electrostatic latent image carrier 24 begins to rotate in the direction of arrow A
in FIG. 9, an electrostatic latent image is formed on the electrostatic latent image carrier 24
by the electrostatic latent image forming section 22. On the other hand, the image display
medium 10 is conveyed in the direction of arrow B in FIG. 9 by the unillustrated
conveying means, so as to be conveyed between the electrostatic latent image carrier 24
and the opposing electrode 26.
Here, the bias voltage VB shown in FIG. 10 is applied to the opposing electrode 26. The
electric potential of the electrostatic latent image carrier 24 at the position opposing the
opposing electrode 26 is VH. Thus, when the portion of the electrostatic latent image
carrier 24 opposing the display substrate 14 is charged with positive charges (a non-image
portion), and when the black particles 18 adhere to the portion of the display substrate 14
- 39 -
opposing the electrostatic latent image carrier 24, the black particles 18 which are charged
positive move from the display substrate 14 side toward the non-display substrate 16 side
and adhere to the non-display substrate 16. In this way, because only the white particles 20
appear at the display substrate 14 side, an image is not displayed at the portion
corresponding to the non-image portion.
On the other hand, when the portion of the electrostatic latent image carrier 24 which
opposes the display substrate 14 is not charged with positive charges (an image portion),
and when the black particles 18 adhere to the portion of the non-display substrate 16
opposing the opposing electrode 26, the electric potential of the electrostatic latent image
carrier 24 at the position opposing the opposing electrode 26 is VL. Thus, the charged
black particles 18 move from the non-display substrate 16 side toward the display substrate
14 side, and adhere to the display substrate 14. In this way, because only the black particles
18 appear at the display substrate 14 side, an image is displayed at the portion
corresponding to the image portion.
In this way, the black particles 18 move in accordance with the image, and the image is
displayed on the display substrate 14 side. Note that even after the electric field, which was
generated between the substrates of the image display medium 10, disappears, the
displayed image is maintained due to the inherent adhesion of the particles and the image
force between the particles and the substrates. Further, if an electric field is generated
between the substrates, the black particles 18 and the white particles 20 can again move,
and therefore, images can repeatedly be displayed by the image forming device 12.
In this way, because bias voltage is applied to the opposing electrode 26, even if the black
particles 18 are adhering to either of the display substrate 14 or the non-display substrate
16, the black particles 18 can be moved. Thus, there is no need to make the black particles
18 adhere in advance to one of the substrates. Further, an image which has high contrast
and sharpness can be formed. In addition, because the charged particles are moved by the
electric field with air being the medium, the stability is good. Moreover, because the
viscous resistance of air is low, a satisfactory high-speed response property can be obtained.
Embodiments of the image forming device of the present invention utilizing the image
display medium of the present invention have been described above with reference to the
figures. However, the present invention is not limited to these embodiments, other than the
fact that the above-described display particles are utilized, and can be structured as desired.
Further, in the above description, black and white were used as the combination of the
colors of the particles. However, the present invention is not limited to this combination,
- 40 -
and the particles which have color can be appropriately selected as needed.
EXAMPLES
Hereinafter, the present invention will be described more concretely with reference to
Examples. However, it is to be noted that these Examples are not intended to limit the
present invention. Note that, in the following Examples and Comparative Examples, the
effects of the present invention are confirmed by utilizing the image display medium and
the image forming device relating to the first embodiment in the above section
"Embodiments of the Image Forming Device of the Present Invention" (i.e., the image
display medium and the image forming device of the structure of FIG. 1), and by changing
the structures of the white particles 20 and the black particles (or blue particles) 18. At this
time, the size, materials, and the like of the respective members are the same as described
in the above section "Embodiments of the Image Forming Device of the Present Invention".
(Preparation of White Particles—1)
Preparation of Dispersion Liquid A
The following composition was mixed together and subjected to ball mill grinding for 20
hours by 10 mmΦ zirconia balls so as to prepare dispersion liquid A.
<Composition>
cyclohexyl methacrylate
64 parts by mass
titanium oxide 1 (white pigment)
25 parts by mass
(primary particle diameter 0.3 μm, TIPAQUE CR63
manufactured by Ishihara Sangyo Kaisha, Ltd.)
polymer particles
10 parts by mass
(primary particle diameter 0.3 μm, SX866 (A)
manufactured by JSR)
charge controlling agent
1 part by mass
(COPY CHARGE PSY VP2038 manufactured by
Clariant Japan)
- 41 -
Preparation of Dispersion Liquid B
The following composition was mixed together and finely ground in a ball mill in the same
way as dispersion liquid A, so as to prepare dispersion liquid B.
<Composition>
calcium carbonate
40 parts by mass
water
60 parts by mass
Preparation of Mixed Liquid C
The following composition was mixed together, deaerated for 10 minutes by an ultrasonic
device, and then stirred by an emulsifier so as to prepare mixed liquid C.
<Composition>
2% cellogen aqueous solution
4.3 g
dispersion liquid B
8.5 g
20% saline solution
50 g
35 g of dispersion liquid A, 1 g of divinylbenzene, and 0.35 g of polymerization initiator
AIBN were measured out and sufficiently mixed together, and the mixture was deaerated
for 10 minutes in an ultrasonic device. This mixed liquid was added into above mixed
liquid C, and the mixture was emulsified in an emulsifier. Next, the emulsified liquid was
placed in a bottle which was then plugged with a silicone stopper. Using an injection
needle, the interior of the bottle was sufficiently deaerated and the pressure thereof reduced,
- 42 -
and nitrogen gas was filled in. Then, a reaction was carried out for 10 hours at 70° C. such
that particles were obtained. The obtained particulate powder was dispersed in
ion-exchanged water, and the calcium carbonate was dissolved with hydrochloric acid
water, and the mixture was filtered. Thereafter, sufficient washing with distilled water was
carried out, and the particle size was made uniform by sieving through nylon sieves having
apertures of 20 μm and 25 μm. The particles were dried, and white particles—1
(particles for a display device of the present invention) having an average particle diameter
of 22 μm were obtained.
(Preparation of White Particles—2)
White particles—2 (particles for a display device of the present invention) were prepared
in the same way as the preparation of the white particles—1, except that following
dispersion liquid A′ was used in place of dispersion liquid A.
The average particle diameter of the obtained white particles—22 was 22 μm.
Preparation of Dispersion Liquid A′
The following composition was mixed together and subjected to ball mill grinding for 20
hours by 10 mmΦ zirconia balls, so as to prepare dispersion liquid A′.
<Composition>
cyclohexyl methacrylate
64 parts by mass
titanium oxide 1 (white pigment)
25 parts by mass
(primary particle diameter 0.3 μm, TIPAQUE CR63
manufactured by Ishihara Sangyo Kaisha, Ltd.)
titanium oxide 2 (white pigment)
5 parts by mass
(primary particle diameter 0.8 μm, STT-30 EHJ
manufactured by Titan Kogyo)
polymer particle (hollow particles)
5 parts by mass
(primary particle diameter 0.3 μm, SX866 (A)
manufactured by JSR)
- 43 -
charge controlling agent
1 part by mass
(BONTRON E89 manufactured by Orient Chemical
Industries, Ltd.)
(Preparation of White Particles—3)
White particles—3 were prepared in the same way as the preparation of the white
particles—2, except that following dispersion liquid A" was used in place of dispersion
liquid A′. The average particle diameter of the obtained white particles—3 was 21 μm.
Further, the specific gravity of the obtained white particles—3 was about 1.3 times that of
white particles—1, and about 1.2 times that of white particles—2.
Preparation of Dispersion Liquid A"
The following composition was mixed together and subjected to ball mill grinding for 20
hours by 10 mmΦ zirconia balls, so as to prepare dispersion liquid A".
<Composition>
cyclohexyl methacrylate
55 parts by mass
titanium oxide 1 (white pigment)
44 parts by mass
(primary particle diameter 0.3 μm, TIPAQUE CR63
manufactured by Ishihara Sangyo Kaisha, Ltd.)
charge controlling agent
1 part by mass
(COPY CHARGE PSY VP2038 manufactured by
Clariant Japan)
(Preparation of Black Particles—1)
Black particles—1 were prepared in the same way as the preparation of the white
particles—1, except that following dispersion liquid K was used in place of dispersion
liquid A. The average particle diameter of the obtained black particles—1 was 23.2 μm.
- 44 -
Preparation of Dispersion Liquid K
The following composition was mixed together and subjected to ball mill grinding for 20
hours by 10 mmΦ zirconia balls, so as to prepare dispersion liquid K.
<Composition>
methyl methacrylate
81 parts by mass
diethylamino ethylmethacrylate
4 parts by mass
carbon black graft polymer
15 parts by mass
(CX-GLF-0215S manufactured by Nippon Shokubai
Co., Ltd.)
(Preparation of Black Particles—2)
Black particles—2 (particles for a display device of the present invention) were prepared
in the same way as the preparation of the white particles—1, except that following
dispersion liquid K′ was used in place of dispersion liquid A. The average particle
diameter of the obtained black particles—2 was 22.5 μm.
Preparation of Dispersion Liquid K′
The following composition was mixed together and subjected to ball mill grinding for 20
hours by 10 mmΦ zirconia balls, so as to prepare dispersion liquid K′.
<Composition>
methyl methacrylate
71 parts by mass
diethylamino ethylmethacrylate
4 parts by mass
titanium black (black pigment)
15 parts by mass
- 45 -
polymer particulates
10 parts by mass
(primary particle diameter 3.0 μm, SX8703 (A)-02
manufactured by JSR)
(Preparation of Blue Particles—1)
Blue particles—1 (particles for a display device of the present invention) were prepared in
the same way as the preparation of the white particles—1, except that following dispersion
liquid L was used in place of dispersion liquid A. The average particle diameter of the
obtained blue particles—1 was 23 μm.
Preparation of Dispersion Liquid L
The following composition was mixed together and subjected to ball mill grinding for 40
hours by 10 mm∠ zirconia balls, so as to prepare dispersion liquid L.
<Composition>
methyl methacrylate monomer
85 parts by mass
diethylamino ethylmethacrylate
1 part by mass
Pigment Blue 15:3 (blue pigment)
4 parts by mass
(Fastgen Blue 5375 manufactured by Dainippon Ink &
Chemicals Inc.)
polymer particles
10 parts by mass
(primary particle diameter 3.0 μm, SX8703 (A)-02
manufactured by JSR)
(Preparation of Blue Particles—2)
Blue particles—2 (particles for a display device of the present invention) were prepared in
the same way as the preparation of the blue particles—1, except that the blue pigment
- 46 -
(Pigment Blue 15:3) in dispersion liquid L was replaced with a specific pigment of the
present invention (Pigment Blue 15:6 (Cyanine Blue 5203 manufactured by
Dainichiseika)). The average particle diameter of the obtained blue particles—2 was 14.91
μm. Further, it was confirmed, by measurement based on JIS K 5101, that the blue
particles—2 had excellent light-resistance.
(Preparation of Blue Particles—3)
Blue particles—3 (particles for a display device of the present invention) were prepared in
the same way as the preparation of the blue particles—1, except that the blue pigment
(Pigment Blue 15:3) in dispersion liquid L was replaced with master batch pigment M1
obtained by the following manufacturing method. The average particle diameter of the
obtained blue particles—3 was 13.60 μm. Further, it was confirmed, by measurement
based on JIS K 5101, that the blue particles—3 had excellent light-resistance. Moreover,
when a small amount of the blue particles—3 was observed under an optical microscope, it
was observed that the master batch pigment M1 was dispersed uniformly.
Manufacture of Master Batch Pigment M1
30 parts of a specific pigment (Pigment Blue 15:6 (Cyanine Blue 5203 manufactured by
Dainichiseika)) as a blue pigment, and 40 parts of a styrene/methyl methacrylate resin were
mixed together with 30 parts of toluene, and the mixture was ground and dispersed by a
circulating batch grinding system. An Apex Mill having a content volume of 1 liter (AM-1
manufactured by Kotobuki Engineering and Manufacturing Co., Ltd.) was used as the
grinding/dispersing device. After the grinding processing was carried out for two hours, a
pigment dispersed liquid was obtained. The conditions for the grinding/dispersing were as
follows: the grinding medium was zirconia of a diameter of 2.0 mm, the rotational speed of
the rotor was 1700 rpm, and the supply pressure was 1.0 to 1.3 kg/cm2.
The solvent was evaporated from this pigment dispersed liquid, such that a pigment resin
containing about 40% by mass of pigment solids was obtained. This pigment resin was
then coarsely ground to obtain the master batch pigment M1.
(Preparation of Blue Particles—4)
Blue particles—4 (particles for a display device of the present invention) were prepared in
the same way as the preparation of the blue particles—1, except that a master batch
pigment M2 was prepared by replacing the blue pigment (Pigment Blue 15:6 (Cyanine
- 47 -
Blue 5203 manufactured by Dainichiseika)) used in the preparation of blue particles—3
with Fastgen Blue EP-CF (manufactured by Dainippon Ink & Chemicals Inc.), and this
master batch pigment M2 was used as the blue pigment in dispersion liquid L. The average
particle diameter of the obtained blue particles—4 was 13.27 μm. Further, it was
confirmed, by measurement based on JIS K 5101, that the blue particles—4 had excellent
light-resistance. Moreover, when a small amount of the blue particles—4 was observed
under an optical microscope, it was observed that the master batch pigment M2 was
dispersed uniformly.
Examples 1 through 8, Comparative Example 1
White particles, black particles and blue particles were respectively mixed together in
accordance with Table 1 to as to prepare display particles 1 through 9. These display
particles 1 through 9 were filled into the void between substrates disposed to oppose one
another (the display substrate 14 and the non-display substrate 16) in the image display
medium relating to the previously-described first embodiment and the image forming
device for forming an image on the image display medium. Image display media of
Examples 1 through 8 and Comparative Example 1 were thereby prepared. At this time, the
compounding ratio (based on the number of particles) of the white particles and the black
particles or the blue particles was white particles:black particles or blue particles =2:1.
(Evaluation)
The following evaluations were carried out on the obtained image display media and image
forming devices.
Driving Voltage
When DC voltage of 100 V is applied to the transparent electrode of the display substrate
14 of the above-described image display medium 10 in which is filled a predetermined
amount of two types of particles which are the white particles 20 and the black particles (or
blue particles) 18 mixed together in a mass ratio of 2:1, a portion of the white particles 20,
which are at the non-display substrate 16 side and which are charged with a negative
polarity, begin to move toward the display substrate 14 side due to the working of the
electric field. When DC voltage (driving voltage) is applied, a large number of the white
particles 20 move toward the display substrate 14 side such that the display density is
substantially saturated. At this time, the black particles (or blue particles) 18 which are
charged with a positive polarity move toward the non-display substrate 16 side. Thereafter,
- 48 -
even when the voltage is made to be 0 V, the particles do not move on the display substrate,
and there is no change in the display density. The DC voltage applied at this time is the
driving voltage, and this driving voltage is shown in Table 1.
Long Term Stability of Image
As described above, by applying voltage between the display substrate 14 and the
non-display substrate 16 and making a desired electric field work on the particle groups,
the particles 18, 20 move between the display substrate 14 and the non-display substrate 16.
By switching the polarity of the applied voltage, the particles 18, 20 move in different
directions between the display substrate 14 and the non-display substrate 16. By repeatedly
switching the polarity of the voltage, the particles 18, 20 move back and forth between the
display substrate 14 and the non-display substrate 16. In this process, the particles 18 and
the particles 20 are charged to respectively different polarities due to the collisions between
the particles 18, 20, and the colliding of the particles 18, 20 and the display substrate 14 or
the non-display substrate 16. The black particles (or the blue particles) 18 are charged to a
positive polarity, and the white particles 20 are charged to a negative polarity. The particles
18, 20 move in respectively different directions in accordance with the electric field
between the display substrate 14 and the non-display substrate 16. When the electric field
is fixed to one direction, the respective particles 18, 20 adhere to the display substrate 14
or the non-display substrate 16, such that an image, which has high contrast, uniform high
density, and no non-uniformity of the image, is displayed. The reflection densities of the
respective images before and after switching of the polarity of the voltage in a case in
which the switching of the polarity of the voltage was repeated for 5000 cycles in a
one-second interval, and then repeated 3000 cycles in a 0.1 second interval to a total
number of 8000 cycles, were measured, and were used to functionally evaluate the
long-term stability of the image.
Here, the method of functionally evaluating the long-term stability of the image was
carried out as follows. Five places within a 20 mm×20 mm patch of each of the respective
images before and after the switching of the polarity of the voltage were measured by the
density measuring device X-Rite 404. The average value of the reflection densities of the
five places was computed for each image, and evaluations were made by comparing these
average values. In the evaluation, if the difference between the average reflection density
of the image before the polarity of the voltage was switched, and the average reflection
density of the image after the polarity of the voltage was switched (i.e., the value of the
fluctuation in the average reflection density) was ±0.05 or less, the long-term stability of
the image was judged to be good.
- 49 -
TABLE 1
value of
functuablack or
tion of
white
blue
average
particles
particles
driving
reflection
20
18
voltage
density
Ex. 1
Ex. 2
Ex. 3
Ex. 4
Ex. 5
Ex. 6
Ex. 7
Ex. 8
display
white
black
particles
particles
particles
1
-1
-1
display
white
black
particles
particles
particles
2
-2
-1
display
white
black
particles
particles
particles
3
-1
-2
display
white
black
particles
particles
particles
4
-2
-2
display
white
blue
particles
particles
particles
5
-1
-1
display
white
blue
particles
particles
particles
6
-1
-2
display
white
blue
particles
particles
particles
7
-1
-3
display
white
blue
particles
particles
particles
- 50 -
200 V
-0.03
200 V
+0.03
210 V
-0.02
205 V
+0.04
200 V
+0.03
200 V
+0.03
200 V
+0.02
200 V
-0.04
Comp. Ex. 1
8
-1
-4
display
white
black
particles
particles
particles
9
-3
-1
400 V
-0.10
From these results, it can be understood that, in Examples 1 and 2 which used, as the white
particles 20, the white particles—1 and the white particles—2 which are particles for a
display device of the present invention, the driving voltage was 200 V which was low. This
driving voltage is a value which is about half of that of Comparative Example 1. Further,
because the value of fluctuation of the average reflection density was smaller than the
value used for judging the long-term stability to be good, it was clear that the long-term
stability of the displayed image was good.
Further, in the same way as in Examples 1 and 2, good effects were also obtained in
Examples 3 through 8 which utilized, as the white particles 20, the white particles—1 and
the white particles —2 which are particles for a display device of the present invention,
and which utilized, as the black particles or the blue particles 18, the black particles—2 or
the blue particles—1 through 4 which are particles for a display device of the present
invention.
On the other hand, in Comparative Example 1 which did not use the particles for a display
device of the present invention as the display particles, the driving voltage was 400 V
which was high, and it was clear that that a high driving voltage was needed to form the
image. Further, the value of fluctuation of the average reflection density exceeded the
value used for judging the long-term stability to be good, and it was thus clear that the
long-term stability of the displayed image was poor.
The same effects were also obtained when the above-described Examples and Comparative
Example were applied to the image display media and the image forming devices relating
to the second through fourth embodiments as well.
As described above, in accordance with the present invention, there are provided particles
for a display device in which the cohesive force between the particles is reduced and
whose specific gravity is reduced. Moreover, in accordance with the present invention,
there are provided an image display medium whose driving voltage can be set low, and
- 51 -
which can ensure a stable displayed image over a long period of time even if there are
shocks from the exterior or static states for long periods of time, and an image forming
device using this image display medium.
*****
第二筆
United States Patent
Shei ,
6,914,268
et al.
July 5, 2005
Title: LED
device, flip-chip LED package
and light reflecting structure
Abstract:
A light emitting diode (LED) device is provided. The LED device includes a device
substrate, a first doped layer of a first conductivity type, a light emitting layer, a second
doped layer of a second conductivity type, a transparent conductive oxide layer, a
reflecting layer and two electrodes. The first doped layer is deposited on the device
substrate, the light emitting layer is deposited on a portion of the first doped layer, and the
second doped layer is deposited on the light emitting layer. The first and the second doped
layers are comprised of III-V semiconductor material respectively. The transparent
conductive oxide layer is deposited on the second doped layer, and the reflecting layer is
deposited on the transparent conductive oxide layer. The two electrodes are deposited on
the reflecting layer and the first doped layer respectively.
Inventors: Shei; Shih-Chang (Tainan County, TW); Sheu; Jinn-Kong (Tainan, TW)
Assignee: South Epitaxy Corporation (Tainan, TW)
Appl. No.: 708203
Filed:
February 16, 2004
Current U.S. Class:
257/99
Intern'l Class:
Field of Search:
H01L 033/00
257/12,13,14,15,16,17,18,19,79,94,95,96,97,98,99,100,101,102,103
References Cited [Referenced By]
- 52 -
U.S. Patent Documents
6497944
Dec., 2002
Oku et al.
6791119
Sep., 2004
Slater et al.
2002/0163302
Nov., 2002
Nitta et al.
2004/0113156
Jun., 2004
Tamura et al.
Primary Examiner: Flynn; Nathan J.
Assistant Examiner: Quinto; Kevin
Attorney, Agent or Firm: Jiang Chyun IP office
Claims
1. A light emitting diode (LED) device, comprising:
a device substrate;
a first doped layer, formed on the device substrate;
a light emitting layer, formed on the first doped layer;
a second doped layer, formed on the light emitting layer, wherein the second doped layer
and the first doped layer are comprised of a semiconductor material of a III-V group
compound with different conductivity type;
a strained-layer superlattice contact layer
a transparent conductive oxide layer as an ohmic contact layer, wherein the transparent
conductive oxide layer is deposited on the strained-layer superlattice contact layer, wherein
a thickness of the transparent conductive oxide layer is (2 m+1)λ/2 n (m is 0 or a positive
integer), wherein λ is a wavelength of a light emitted from the light emitting layer and n
is a refractive index of the transparent conductive oxide layer;
a reflecting layer, deposited on the transparent conductive oxide layer; and
two electrodes, formed on the reflecting layer and a portion of the first doped layer,
respectively.
2. The LED device of claim 1, wherein the strained-layer superlattice contact layer
comprise n-type or p-type III-V semiconductor multi-layer structures.
3. The LED device of claim 1, wherein the semiconductor material of the III-V group
compound is gallium nitride (GaN), gallium phosphide (GaP) or gallium phosphide
arsenide (GaAsP).
- 53 -
4. The LED device of claim 1, wherein the light emitting layer comprise a quantum-well
light emitting layer.
5. The LED device of claim 1, wherein a material of the transparent conductive oxide layer
is indium tin oxide (ITO), cerium tin oxide (CTO), antimony tin oxide (ATO), aluminum
zinc oxide (AZO) indium zinc oxide (IZO), zinc oxide (ZnO), cadmium tin oxide, ZnGa2O4,
SnO2:Sb, Ga2O3:Sn, AgInO2:Sn, In2O3:Zn, CuAlO2, LaCuOS, NiO, or CuGaO2, SrCu2,O2.
6. The LED device of claim 1, wherein the first doped layer is comprised of a N-type
doped layer, and the second doped layer is comprised of a P-type doped layer.
7. The LED device of claim 1, wherein the first doped layer is comprised of a P-type
doped layer, and the second doped layer is comprised of a N-type doped layer.
8. A light emitting diode (LED) device, comprising:
a device substrate;
a first doped layer, formed on the device substrate;
a light emitting layer, formed on the first doped layer;
a second doped layer, formed on the light emitting layer, wherein the second doped layer
and the first doped layer are comprised of a semiconductor material of a III-V group
compound with different conductivity type;
a strained-layer superlattice contact layer;
a transparent conductive oxide layer as an ohmic contact layer, wherein the transparent
conductive oxide layer is deposited on the strained-layer superlattice contact layer;
a transparent insulating layer as a passivation layer, wherein the transparent insulating
layer is deposited on transparent conductive oxide layer;
a reflecting layer, deposited on the transparent insulating layer and a portion of the
transparent conductive oxide layer; and
two electrodes, formed on the reflecting layer and a portion of the first doped layer,
respectively.
9. The LED device of claim 8, wherein a thickness of the transparent conductive oxide
layer is (2 m+1)λ/2 m (in is 0 or a positive integer), wherein λ is a wavelength of a light
emitted from the light emitting layer and n is a refractive index of the transparent
conductive oxide layer.
10. The LED device of claim 8, wherein a thickness of the transparent insulating layer is (2
m+1)λ/2 k (m is 0 or a positive integer), wherein λ is a wavelength of a light emitted
from the light emitting layer and k is a refractive index of the transparent insulating layer.
- 54 -
11. The LED device of claim 8, wherein the strained-layer superlattice contact layer
comprise n-type or p-cype III-V semiconductor multi-layer structures.
12. The LED device of claim 8, wherein the semiconductor material of the III-V group
compound is gallium nitride (GaN), gallium phosphide (GaP) or gallum phosphide
arsenide (GaAsP).
13. The LED device of claim 8, wherein the light emitting layer comprise a quantum-well
light emitting layer.
14. The LED device of claim 8, wherein a material of the transparent conductive oxide
layer is indium tin oxide (ITO), cerium tin oxide (CTO), antimony tin oxide (ATO),
aluminum zinc oxide (AZO) indium zinc oxide (IZO), zinc oxide (ZnO), cadmium tin
oxide, ZnGa2O4, SnO2:Sb, Ga2O3:Sn, AgInO2:Sn, In2O3:Zn, CuAlO2, LaCuOS, NiO, or
CuGaO2, SrCu2O2.
15. The LED device of claim 8, wherein a material of the transparent conductive oxide
layer is SiO2, SiNx, Al2O3, AlN, BeO, ZnO.
16. The LED device of claim 8, wherein the first doped layer is comprised of a N-type
doped layer, and the second doped layer is comprised of a P-type doped layer.
17. The LED device of claim 8, wherein the first doped layer is comprised of a P-type
doped layer, and the second doped layer is comprised of a N-type doped layer.
18. A flip-chip light emitting diode (LED) package structute comprising:
a package substrate; and
a LED device, faced-down and flipped on the package substrate and electrically connected
to the package substrate, wherein the LED device comprises:
a device substrate;
a first doped layer, formed on the device substrate;
a light emitting layer, formed on the first doped layer;
a second doped layer, formed on the light emitting layer, wherein the second doped layer
and the first doped layer are comprised of a semiconductor material of a III-V group
compound with different conductivity type;
a strained-layer superlattice contact layer
a transparent conductive oxide layer as an ohmic contact layer, wherein the transparent
conductive oxide layer is deposited on the strained-layer superlattice contact layers,
wherein a thickness of the transparent conductive oxide layer is (2 m+1)λ/2 n (m is 0 or a
positive integer), wherein λ is a wavelength of a light emitted from the light emitting
- 55 -
layer and n is a refractive index of the transparent conductive oxide layer;
a reflecting layer, deposited on the transparent conductive oxide layer; and
two electrodes, formed on the reflecting layer and a portion of the first doped layer,
respectively.
19. A flip-chip light emitting diode (LED) package structure, comprising:
a package substrate; and
a LED device, faced-down and flipped on the package substrate and electrically connected
to the package substrate, wherein the LED device comprises:
a device substrate;
a first doped layer, formed on the device substrate;
a light emitting layer, formed on the first doped layer;
a second doped layer, formed on the light emitting layer, wherein the second doped layer
and the first doped layer are comprised of a semiconductor material of a III-V group
compound with different conductivity type;
a strained-layer superlattice contact layer;
a transparent conductive oxide layer as an ohmic contact layer, wherein the transparent
conductive oxide layer is deposited on the strained-layer superlattice contact layer;
a transparent insulating layer as a passivation layer, wherein the transparent insulating
layer is deposited on transparent conductive oxide layer;
a reflecting layer, deposited on the transparent insulating layer and a portion of the
transparent conductive oxide layer; and
two electrodes, formed on the reflecting layer and a portion of the first doped layer,
respectively.
20. The flip-chip LED package structure of claim 19, wherein a thickness of the
transparent conductive oxide layer is (2 m+1)λ/2 n (m is 0 or a positive integer), wherein
λ is a wavelength of a light emitted from the light emitting layer and n is a refractive
index of the transparent conductive oxide layer.
21. The flip-chip LED package structure of claim 19, wherein a thickness of the
transparent insulating layer is (2 m+1)λ/2 k (m is 0 or a positive integer), wherein is a
wavelength of a light emitted from the light emitting layer and k is a refractive index of the
transparent insulating layer.
22. A light reflective structure for a light emitting diode (LED), comprising:
- 56 -
a transparent conductive oxide layer deposited on a semiconductor layer;
a transparent insulating layer deposited on the transparent conductive oxide layer; and
a reflecting layer deposited on the transparent insulating layer.
23. The light reflective structure of claim 22, wherein a thickness of the transparent
conductive oxide layer is (2 m+1)λ/2 n (m is 0 or a positive integer), wherein λ is a
wavelength of a light emitted from the light emitting layer and n is a refractive index of the
transparent conductive oxide layer.
24. The LED device of claim 22, wherein a thickness of the transparent insulating layer is
(2 m+1)λ/2 k (m is 0 or a positive integer), wherein λ is a wavelength of a light emitted
from the light emitting layer and k is a refractive index of the transparent insulating layer.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the priority benefit of Taiwan application serial no.92120195, filed
on Jul. 24, 2003.
BACKGROUND OF INVENTION
1. Field of the Invention
The invention relates in general to a structure of a semiconductor light emitting device, and
more particularly, to a structure of a light emitting diode (LED) device, to a package
structure of a flip-chip LED device, and to a light reflective structure being applicable for a
LED.
2. Related Art of the Invention
In general, a light emitting diode (LED) constructed by an III-V semiconductor material
can be provided as a wide bandgap light emitting device. The wavelength of the light
emitted from the wide bandgap light emitting device ranges from infrared (IR) to
ultraviolet (UV); therefore the entire spectrum of visible light is also covered. In recent
years, due to the rapid development of the high illumination of the gallium nitride (GaN)
LEDs having a blue/green light, the full-color LED display, white light LED and the LED
for traffic signals are put into practice. Therefore, the application of a variety of LED also
becomes more popular.
In principle, a fundamental structure of a LED device includes an epitaxy layer of a P-type
and a N-type III-V group compound and a light emitting layer in-between. The light
emitting efficiency of the LED device is dependent on the internal quantum efficiency of
- 57 -
the light emitting layer and the light extraction efficiency of the device. A method of
increasing the internal quantum efficiency includes, for the most part, improving the
quality of the light emitting layer and the design of the structure. The method of increasing
the light extraction efficiency includes, for the most part, decreasing the light loss caused
by the absorption of the light emitted from the light emitting layer due to the reflection of
the light inside the LED device.
In a conventional gallium nitride (GaN) LED device grown on the first substrate, such as
sapphire, having an insulating property, since the positive and the negative electrodes of a
gallium nitride (GaN) LED device are deposited on, in general, the same side of a first
surface, and the positive electrode will screen out the emitted light from light emitting
layer. Therefore, the packaging for a gallium nitride (GaN) LED normally uses the flip
chip method. Thus, the emitted light will pass through the second surface. Moreover, a
reflecting layer is formed on the topmost surface of GaN LED that faces the second
substrate, in order to emit most of the emitted light towards the second surface of a GaN
LED. Another advantage of using the flip-chip package process is, if a proper surface
mount (so called surmount) substrate, for example, a silicon substrate is provided, the heat
dissipation of the LED device is enhanced, especially under a high current operation.
Accordingly, not only the light extraction efficiency is increased, the internal quantum
efficiency of the light emitting layer will also be maintained.
Moreover, in order to improve the electrical property of the LED device, a
semi-transparent nickel (Ni)/gold (Au) ohmic contact layer is first formed on the epitaxy
layer surface, and a thermal process is performed to form adesirable ohmic contact,
followed by forming a reflecting layer thereon. However, since the absorption of light of
the Ni/Au layer is high (the transparency of that is about 60% to about 70%), and due to
the thermal process, the interface between the epitaxy layer and the Ni/Au layer becomes
too rough to reflect light. Accordingly, the light reflective efficiency of the bottom of the
flip-chip LEDs device will be reduced.
SUMMARY OF INVENTION
Accordingly, the present invention is to provide a light reflective structure, which is
applicable for a LED device to enhance the extraction efficiency of light.
Another object of the present invention is to provide a LED device having a light reflective
structure of the present invention, wherein the extraction efficiency of light is enhanced.
It is yet another object of the present invention to provide a flip-chip LED package
structure having a light reflective structure of the present invention, wherein the extraction
efficiency of light is enhanced.
In order to achieve the above objects and other advantages of the present invention, a light
reflective structure for a LED device is provided. The light reflective structure includes, for
example but not limited to, a transparent conductive oxide layer deposited on a
semiconductor layer, a transparent insulating layer deposited on the transparent conductive
oxide layer, and a reflecting layer deposited on the transparent insulating layer. The
transparent conductive oxide layer is provided as an ohmic contact layer for the
semiconductor layer. The transparent insulating layer is provided as a passivation layer for
- 58 -
the transparent conductive oxide layer. When the wavelength of the light emitted from the
LED device is λ, and the refractive index of the transparent conductive oxide layer is n,
the thickness of the transparent conductive oxide layer is preferably to be (2 m+1)λ/2 n
(m is 0 or an positive integer). When the refractive index of the transparent insulating layer
is k, the thickness of the transparent insulating layer is preferably to be (2 m+1)λ/2 k (m
is 0 or an positive integer). Therefore, a constructive interference of the lights is achieved.
In order to achieve the above objects and other advantages of the present invention, a light
reflective structure applicable for a LED device is provided. The light reflective structure
includes a transparent conductive oxide layer deposited on a semiconductor layer, and a
reflecting layer deposited on the transparent conductive oxide layer. The transparent
conductive oxide layer is provided as an ohmic contact layer for the semiconductor layer.
When the wavelength of the light emitted from the LED device is λ, and the refractive
index of the transparent conductive oxide layer is n, the thickness of the transparent
conductive oxide layer is preferably to be (2 m+1)λ/2 n (m is 0 or a positive integer).
Therefore, a constructive interference of the lights is achieved.
The LED device of the present invention includes a first substrate called device substrate, a
first doped layer, a light emitting layer, a second doped layer, a transparent conductive
oxide layer, a reflecting layer, and two electrodes. The first doped layer is deposited on the
device substrate, the light emitting layer is deposited on the first doped layer, and the
second doped layer is deposited on the light emitting layer. The second doped layer and the
first doped layer are constructed from an III-V group compound of semiconductor material
with different conductivity type. The transparent conductive oxide layer is deposited on the
second doped layer, and is provided as an ohmic contact layer. The transparent insulating
layer is deposited on the ohmic contact layer to serves as a passivation layer. The reflecting
layer is deposited on the transparent insulating layer. The two electrodes are formed on the
reflecting layer and the first doped layer, respectively.
The LED device of the present invention includes a first substrate called device substrate, a
first doped layer, a light emitting layer, a second doped layer, a transparent conductive
oxide layer, a reflecting layer, and two electrodes. The first doped layer is deposited on the
device substrate, the light emitting layer is deposited on the first doped layer, and the
second doped layer is deposited on the light emitting layer. The second doped layer and the
first doped layer are constructed from an III-V group compound of semiconductor material
with different conductivity type. The transparent conductive oxide layer is deposited on the
second doped layer, and is provided as an ohmic contact layer. The reflecting layer is
deposited on the transparent conductive oxide layer. The two electrodes are formed on the
reflecting layer and the first doped layer, respectively.
The flip-chip LED package structure of the present invention includes a package substrate
called second substrate or submount substrate and a LED structure on the first substrate, in
which the LED is faced-down over the package substrate and is electrically connected to
the package substrate. The LED includes a first substrate (device substrate), a first doped
layer, a light emitting layer, a second doped layer, a transparent conductive oxide layer, a
transparent insulating passivation layer, a reflecting layer, and two electrodes. The first
doped layer is deposited on the first substrate, the light emitting layer is deposited on the
first doped layer, and the second doped layer is deposited on the light emitting layer. The
second doped layer and the first doped layer are constructed from an III-V group
- 59 -
compound of semiconductor material with different conductivity type. The transparent
conductive oxide layer is deposited on the second doped layer, and is provided as an ohmic
contact layer. The transparent insulating layer is deposited on the ohmic contact layer to
serves as a passivation layer. The reflecting layer is deposited on the transparent insulating
layer. The two electrodes are deposited on the reflecting layer and the first doped layer,
respectively.
The flip-chip LED package structure of the present invention includes a package substrate
called second substrate or submount substrate and a LED structure on the first substrate, in
which the LED is faced-down over the package substrate and is electrically connected to
the package substrate. The LED includes a first substrate (device substrate), a first doped
layer, a light emitting layer, a second doped layer, a transparent conductive oxide layer, a
reflecting layer, and two electrodes. The first doped layer is deposited on the first substrate,
the light emitting layer is deposited on the first doped layer, and the second doped layer is
deposited on the light emitting layer. The second doped layer and the first doped layer are
constructed from an III-V group compound of semiconductor material with different
conductivity type. The transparent conductive oxide layer is deposited on the second doped
layer, and is provided as an ohmic contact layer. The reflecting layer is deposited on the
transparent insulating layer. The two electrodes are deposited on the reflecting layer and
the first doped layer, respectively.
Accordingly, in the present invention, the material of the ohmic contact layer includes a
transparent conductive metal oxide, and a thermal process for achieving a good ohmic
contact is not required for the transparent conductive metal oxide. Therefore, the interface
between the ohmic contact layer and the second doped layer is smooth, and thus the
interface can be provided as a reflecting surface. Moreover, in the present invention, the
absorption to visible light of the transparent conductive metal oxide can be reduced to less
than 10% (for example, when the oxide is an indium tin oxide (ITO; therefore, the
absorption of the ohmic contact layer to the LED device is reduced drastically.
It is to be understood that both the foregoing general description and the following detailed
description are exemplary, and are intended to provide further explanation of the invention
as claimed.
BRIEF DESCRIPTION OF DRAWINGS
The accompanying drawings are included to provide a further understanding of the
invention, and are incorporated in and constitute a part of this specification. The drawings
illustrate embodiments of the invention and, together with the description, serve to explain
the principles of the invention.
FIG. 1 is a cross-sectional view illustrating a structure of a LED device and a enlarged
view of a portion adjacent to a interface of the transparent conductive oxide layer of the
LED device according to a preferred embodiment of the present invention.
FIG. 2 is a cross-sectional view illustrating another structure of a LED device.
FIG. 3 is a cross-sectional view illustrating a flip-chip LED package structure achieved
after a flip-chip package process of the LED device of FIG. 1 and FIG. 2.
- 60 -
DETAILED DESCRIPTION
The present invention now will be described more fully hereinafter with reference to the
accompanying drawings, in which preferred embodiments of the invention are shown. This
invention may, however, be embodied in many different forms and should not be construed
as limited to the embodiments set forth herein; rather, these embodiments are provided so
that this disclosure will be thorough and complete, and will fully convey the scope of the
invention to those skilled in the art. Like numbers refer to like elements throughout.
FIG. 1 is a cross-sectional view illustrating a structure of a LED device and a enlarged
view of a portion adjacent to a interface of the transparent conductive oxide layer of the
LED device according to a preferred embodiment of the present invention. Referring to
FIG. 1, the LED device includes a device substrate 100, a N-type doped layer 110, a light
emitting layer 120, a P-type doped layer 130, a strained-layer superlattice (SLS) contact
layer 135, a transparent conductive oxide layer 140, a reflecting layer 150, and an anode
160 and a cathode 170. In FIG. 1, an active layer constructed by a N-type doped layer 110,
a light emitting layer 120 and a P-type doped layer 130 is formed, for example but not
limited to, by performing a series of epitaxy processes sequentially on the device substrate
100. Moreover, in the succeeding process, a portion of the N-type doped layer 110, a
portion of the light emitting layer 120 and a portion of the P-type doped layer 130 are
removed, for example but not limited to, by etching or by another method. Therefore, each
of the layers 110, 120, 130 and 135 are patterned to form a plurality of isolated island
structure (MESA). It is noticed that, in the isolated island structure above, a portion of the
P-type doped layer 130 and SLS contact layer 135 over the cathode 170, the light emitting
layer 120 and a portion of the N-type doped layer 110 are removed. The cathode 170 thus
can be electrically connected with the N-type doped layer 110.
Referring to FIG. 1, in the present embodiment, the transparent conductive oxide layer 140
is deposited on the SLS contact layer 135, while the reflecting layer 150 is deposited on the
transparent conductive oxide layer 140 and the anode 160 is deposited on the reflecting
layer 150.
The device substrate 100 includes, for example but not limited to, a sapphire substrate. The
materials of the N-type doped layer 110, light emitting layer 120, the P-type doped layer
130, and SLS contact layer 135 are comprised of a III-V group compound of
semiconductor material, including but not limited to, a gallium nitride (GaN), a gallium
phosphide (GaP) or a gallium phosphide arsenide (GaAsP). The light emitting layer 120
includes, for example but not limited to, a single or a multi quantum well structure, to
enhance the light emitting efficiency. A material of the transparent conductive oxide layer
140 preferably includes an indium tin oxide (ITO), but also may include, for example but
not limited to, such as ITO, CTO, IZO, ZnO:Al, ZnGa2O4, SnO2:Sb, Ga2O3:Sn, AgInO2:Sn,
In2O3:Zn, CuAlO2, LaCuOS, NiO, CuGaO2, SrCu2O2, and so on or other transparent
conductive material having similar properties. A material of the reflecting layer 150
includes, for example but not limited to, an aluminum (Al), a silver (Ag), Ni/Ag, Ni/Al,
Mo/Ag, Mo/Al, Ti/Ag, Ti/Al, Nd/Al, Nd/Ag, Pd/Al, Pd/Ag, Cr/Al, Cr/Ag and materials of
the anode 160 and the cathode 170 include, for example but not limited to, a bi-layer or
tri-layer metal system, such as Cr/Au, Ti/Au, Cr/Pt/Au and Ti/Pt/Au.
- 61 -
As shown in the enlarged view of FIG. 1, since the transparent conductive oxide layer 140
does not require a thermal process for increasing the ohm contact efficiency, the interface
between the transparent conductive oxide layer 140 and the SLS contact layer 135 is
smooth. A desirable reflecting effect is thereby achieved. Moreover, according to the
theory of light interference, when the light emitting wavelength of the LED device is λ,
and the refractive index of the transparent conductive oxide layer 140 is n, the thickness of
the transparent conductive oxide layer 140 is preferably to be (2 m+1)λ/2 n (m is 0 or an
positive integer such as 1, 2, 3, etc.). Thus, the reflecting light from the interface between
the transparent conductive oxide layer 140 and the reflecting layer 150, and the reflecting
light from the interface of the SLS contact layer 135 and the transparent conductive oxide
layer 140 can generate a constructive interference effect.
FIG. 2 is a cross-sectional view illustrating another structure of a LED device. Referring to
FIG. 2, the LED device includes a device substrate 200, a N-type doped layer 210, a light
emitting layer 220, a P-type doped layer 230, a strained-layer superlattice (SLS) contact
layer 235, a transparent conductive oxide layer 240, a transparent insulating passivation
layer 245, a reflecting layer 250, and an anode 260 and a cathode 270. In FIG. 2, an active
layer constructed by a N-type doped layer 210, a light emitting layer 220 and a P-type
doped layer 230 is formed, for example but not limited to, by performing a series of
epitaxy processes sequentially on the device substrate 200. Moreover, in the succeeding
process, a portion of the N-type doped layer 210, a portion of the light emitting layer 220,
a portion of the P-type doped layer 230 and a SLS contact layer 235 are removed, for
example but not limited to, by etching or by another method. Therefore, each of the layers
210, 220, 230 and 235 are patterned to form a plurality of isolated island structure (MESA).
It is noticed that, in the isolated island structure above, a portion of the P-type doped layer
230 and SLS contact layer 235 over the cathode 270, the light emitting layer 220 and a
portion of the N-type doped layer 210 are removed. The cathode 270 thus can be
electrically connected with the N-type doped layer 210.
Referring to FIG. 2, in the present embodiment, the transparent conductive oxide layer 240
is deposited on the SLS contact layer 235, and the transparent insulating passivation layer
245 is deposited on the transparent conductive oxide layer 240 while the reflecting layer
250 is deposited on the transparent insulating passivation layer 245 and the anode 260 is
deposited on the reflecting layer 250.
The device substrate 200 includes, for example but not limited to, a sapphire substrate. The
materials of the N-type doped layer 210, light emitting layer 220, the P-type doped layer
230, and SLS contact layer 235 are comprised of a III-V group compound of
semiconductor material, including but not limited to, a gallium nitride (GaN), a gallium
phosphide (GaP) or a gallium phosphide arsenide (GaAsP). The light emitting layer 220
includes, for example but not limited to, a single or a multi quantum well structure, to
enhance the light emitting efficiency. A material of the transparent conductive oxide layer
140 preferably includes an indium tin oxide (ITO), but also may include, for example but
not limited to, such as ITO, CTO, IZO, ZnO:Al, ZnGa2O4, SnO2:Sb, Ga2O3:Sn, AgInO2:Sn,
In2O3:Zn, CuAlO2, LaCuOS, NiO, CuGaO2, SrCu2O2, and so on. or other transparent
conductive material having similar properties. A material of the transparent insulating
passivation layer 245 includes, for example but not limited to, a SiO2, SiNx, Al2O3, AlN,
BeO, ZnO, and so on. A material of the reflecting layer 250 includes, for example but not
limited to, an aluminum (Al), a silver (Ag), Ni/Ag, Ni/Al, Mo/Ag, Mo/Al, Ti/Ag, Ti/Al,
- 62 -
Nd/Al, Nd/Ag, Pd/Al, Pd/Ag, Cr/Al, Cr/Ag and materials of the anode 260 and the cathode
270 include, for example but not limited to, a bi-layer or tri-layer metal system, such as
Cr/Au, Ti/Au, Cr/Pt/Au and Ti/Pt/Au.
As shown in the enlarged view of FIG. 2, since the transparent conductive oxide layer 240
does not require a thermal process for increasing the ohm contact efficiency, the interface
between the transparent conductive oxide layer 240 and the SLS contact layer 235 is
smooth. A desirable reflecting effect is thereby achieved. Moreover, according to the
theory of light interference, when the light emitting wavelength of the LED device is λ,
and the refractive index of the transparent conductive oxide layer 140 is n, the thickness of
the transparent conductive oxide layer 240 is preferably to be (2 m+1)λ/2 n (m is 0 or an
positive integer such as 1, 2, 3, etc.). Moreover, according to the theory of light
interference, when the light emitting wavelength of the LED device is λ, and the
refractive index of the transparent insulating passivation layer 245 is k, the thickness of the
transparent insulating passivation layer 245 is preferably to be (2 m+1)λ/2 k (m is 0 or an
positive integer such as 1, 2, 3, etc.). Thus, the reflecting light from the interface between
the transparent insulating passivation layer 245 and the reflecting layer 250, and the
reflecting light from the interface of the SLS contact layer 235 and the transparent
conductive oxide layer 240 can generate a constructive interference effect.
FIG. 3 is a cross-sectional view illustrating a flip-chip LED package structure obtained
after the flip-chip packaging of the LED device of FIG. 1 and FIG. 2. Referring to FIG. 3,
the LED device of FIG. 1 or FIG. 2 is flipped over a package substrate 300, the package
substrate 300 includes, for example but not limited to, a silicon substrate. The LED device
of FIG. 1 and the package substrate 300 are electrically connected via a bump 380 and a
bump 390. The bump 380 is electrically connected with the anode 160 and the package
substrate 300, and the bump 390 is electrically connected with the cathode 170 and the
package substrate 300. Since the reflecting layer 150 is between the top layer of the FIG. 1
and the package substrate 300, and faces to the package substrate 200. Thus, the light
emitted from the light emitting layer 120 is reflected by the multi-layer structures including
the layer 135, layer 140, and layer 150 and emits through the device substrate 100. Similar
concept is also suitable for a device consisting of a transparent insulating passivation layer,
as shown in FIG. 2.
Moreover, the device structure of the embodiments described above, for example, a LED
device having a flip-chip package structure, is only an example for describing the present
invention. The scope of the invention is not limited to the above embodiments. Moreover,
the present invention can also be provided for all of the LED devices that are formed with
an ohmic contact layer and a reflecting layer and are packaged by a process other than the
flip-chip package process for increasing the light reflecting efficiency. In addition,
although the present invention is described with a N-type doped layer being formed on the
device substrate, and a P-type doped layer being formed on the light emitting layer and, the
present invention is also applicable with the conductive type of the doped layers being
exchanged. That is, a P-type doped layer is formed on the device substrate, and a N-type
doped layer is formed on the light emitting layer. Therefore, the electrode formed on the
reflecting layer is served as a cathode, and the electrode formed on the P-type doped layer
is served as an anode.
In accordance to the present invention, the material of the ohmic contact layer includes a
- 63 -
transparent conductive metal oxide, wherein a thermal process for increasing the ohmic
contact efficiency is not required for the transparent conductive metal oxide. Therefore, the
interface between the ohmic contact layer and the SLS contact layer is smooth, and thus
the interface can be provided as a reflecting surface. Moreover, in the present invention,
the absorption to visible light of the transparent conductive metal oxide can be reduced to
less than 10% (for example, when the oxide is a indium tin oxide (ITO); therefore, the
absorption of the ohmic contact layer to the LED device is reduced drastically.
It will be apparent to those skilled in the art that various modifications and variations can
be made to the structure of the present invention without departing from the scope or spirit
of the invention. In view of the foregoing, it is intended that the present invention cover
modifications and variations of this invention provided they fall within the scope of the
following claims and their equivalents.
*****
第3筆
United States Patent
Yamamoto ,
6,809,854
et al.
Title: Image
October 26, 2004
display medium and image
forming device
Abstract
An image display medium comprising a pair of substrates disposed opposed to each other
and a group of at least two kinds of particles enclosed in the gap between the pair of
substrates, at least one of the two or more kinds of particles being capable of being
positively charged and at least the others being capable of being negatively charged and the
particles capable of being positively and negatively charged having different colors,
wherein both the particles capable of being positively and negatively charged have a shape
factor of from greater than 100 to not greater than 140 as determined by the following
equation: Shape factor=(L.sup.2 /S)/4.pi..times.100 where S is the area of particle; and L is
the perimeter of particle, and an image forming device comprising same.
Inventors: Yamamoto; Yasuo (Minamiashigara, JP); Hiraoka; Satoshi (Minamiashigara,
JP); Shigehiro; Kiyoshi (Ashigarakami-gun, JP); Machida; Yoshinori
(Ashigarakami-gun, JP); Matsunaga; Takeshi (Ashigarakami-gun, JP)
Assignee: Fuji Xerox Co., Ltd. (Tokyo, JP)
- 64 -
Appl. No.: 080689
Filed:
February 25, 2002
Foreign Application Priority Data
Aug 21, 2001[JP]
2001-250342
Current U.S. Class:
359/296; 345/31; 345/55; 345/107
Intern'l Class:
G02B 026/00; G09G 003/00; G09G 003/20; G09G
003/34
Field of Search:
359/296 345/30,31,55,84,107,105 204/450,600
252/572
References Cited [Referenced By]
U.S. Patent Documents
6113810
Sep., 2000
Hou et al.
6636186
Oct., 2003
Yamaguchi et al.
6741387
May., 2004
Shigehiro et al.
Foreign Patent Documents
A 2001-312225
Nov., 2001
JP.
Other References
Jo et al., "New Toner Display Device (1)--Image Display Using Conductive
Toner and Charge Transport Layer -", Japan Hardcopy 1999, pp. 249-252.
(w/abstract).
Jo et al. "Electronic reflective display using particle movement in electric field
(1)--Principle and characteristic of display -", Scholarly Journal of the Image
Society of Japan, vol. 39, No. 4, 2000, pp. 408-413. (w/abstract).
Primary Examiner: Ben; Loha
Attorney, Agent or Firm: Oliff & Berridge, PLC
Claims
- 65 -
252/572.
345/31.
359/296.
What is claimed is:
1. An image display medium comprising:
a pair of substrates disposed opposed to each other; and
a particle group having at least two kinds of particles enclosed in a gap between the pair of
substrates,
wherein at least one of the at least two kinds of particles can be positively charged;
wherein at least another one of the at least two kinds of particles can be negatively charged;
wherein the one and the another one have different colors from each other; and
wherein both the one and the another one have shape factors satisfying 100<the shape
factors.ltoreq.140, where the shape factor=(L.sup.2 /S)/4.pi..times.100; S is area of the
particle; and L is perimeter of the particle.
2. The image display medium according to claim 1, wherein one of the one, which can be
positively charged, and the another one, which can be negatively charged, is white.
3. The image display medium according to claim 2,
wherein the one, which is white, comprises a coloring material; and
wherein the coloring material is titanium oxide.
4. An image forming device comprising an electric field generating unit for generating an
electric field between a pair of substrates according to an image to form the image on an
image display medium;
wherein the image display medium comprising:
the pair of substrates disposed opposed to each other; and
a particle group having at least two kinds of particles enclosed in a gap between the pair of
substrates,
wherein at least one of the at least two kinds of particles can be positively charged;
wherein at least another one of the at least two kinds of particles can be negatively charged;
wherein the one and the another one have different colors from each other; and
wherein both the one and the another one have shape factors satisfying 100<the shape
- 66 -
factors.ltoreq.140, where the shape factor=(L.sup.2 /S)/4.pi..times.100; S is area of the
particle; and L is perimeter of the particle.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an image display medium using a particulate material
which allows repeated rewriting and an image forming device.
2. Description of the Related Art
As image display media enabling rewriting there have heretofore been proposed display
techniques such as twisting ball display (two-color particle rotary display), electrophoresis,
magnetophoresis, thermal rewritable medium and liquid crystal having memory properties.
These display techniques are excellent in image memory properties but are
disadvantageous in that they cannot use a white display such as paper and thus provide a
low contrast.
As a display technique using a toner which solves these problems, there has been proposed
a display technique involving the enclosure of an electrically-conductive colored toner and
a white particulate material in the gap between opposing electrode substrates. In
accordance with this display technique, electric charge is injected into the
electrically-conductive colored toner via a charge-transporting layer provided on the inner
surface of the electrode on the non-display substrate. The electrically-conductive colored
toner into which electric charge has been injected moves toward the display substrate
disposed opposed to the non-display substrate under the application of an electric field
across the electrode substrates. The electrically-conductive colored toner is then attached to
the inner side of the display substrate to make contrast from the white particulate material,
causing image display (Japan Hardcopy'99 Bulletin, pp. 249-252). In this display technique,
the image display medium is entirely composed of a solid material. Thus, this display
technique is excellent in that the display of white and black (color) can be theoretically
switched by 100%. However, this display technique is disadvantageous in that there is an
electrically-conductive colored toner which doesn't come in contact with the
charge-transporting layer provided on the inner surface of the electrode on the non-display
substrate or an electrically-conductive colored toner isolated from other
electrically-conductive colored toner. Since no electric charge is injected into these
electrically-conductive colored toners, they cannot move even under the action of an
electric field and thus remain at random on the substrates, giving a low contrast.
In order to solve these problems, Japanese Patent Application No. 2000-165138 proposes
an image display medium comprising a pair of substrates and a plurality of kinds of
particles having different colors and chargeabilities enclosed in the gap between the
substrates such that they can move between the substrates under the application of an
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electric field applied across the substrates. In accordance with this proposal, a high
whiteness degree and contrast can be attained. The applied voltage required for the display
of black-and-white image is several hundreds volt. In the constitution of the particulate
materials thus proposed, the required applied voltage is lowered, making it possible to
expand the degree of freedom of design of the driving circuit. However, under the recent
circumstances requiring further improvements of performance, further improvements of
performance have been demanded. Thus, it has been desired to lower the required driving
voltage for the purpose of further enhancing the stability and uniformity of image density,
the stability of density contrast and the degree of freedom of design of driving circuit.
The invention is intended to solve the problems of the related art and attain the following
aim. In other words, an object of the invention is to provide an image display medium
which can use a low predetermined driving voltage and shows a small change of image
density and image uniformity and a stable density contrast even after prolonged repetition
of rewriting and an image forming device therefor.
SUMMARY OF THE INVENTION
The inventors made extensive studies. The inventors paid attention to instabilization of
charged amount due to the increase of adhesion between particles and between particles
and substrate or triboelectrification of particles and deterioration of efficiency in separation
and movement of particles due to fluidity of group of particles charged by mutual friction.
As a result, it was found that the foregoing problems can be solved by eliminating these
factors. The invention has thus been worked out. According to the invention there is
provided an image display medium having a pair of substrates disposed opposed to each
other, and a particle group having at least two kinds of particles enclosed in a gap between
the pair of substrates, in which at least one of the at least two kinds of particles can be
positively charged, at least another one of the at least two kinds of particles can be
negatively charged, the one and the another one have different colors from each other, and
both the one and the another one has shape factors satisfying 100<the shape
factors.ltoreq.140, where the shape factor=(L.sup.2 /S)/4.pi..times.100; S is area of the
particle; and L is perimeter of the particle.
In the invention, it is important that the particles capable of being positively and negatively
charged have different colors. The shape factor of at least one of the particulate material is
also important. By making such an arrangement that the two particulate materials have
different colors, a density contrast can be developed across an image site having the group
of particles capable of positively charged and an image site composed of the group of
particles capable of negatively charged. Further, by setting the shape factor to the above
defined range, a proper space occur between the particles to enhance the fluidity of the
group of particles, making it possible to give a sharp distribution of triboelectricity of the
particles capable being positively and negatively charged. On the other hand, the adhesion
between the particles and the substrate due to the contact of the particles with the substrate
having the polarity being opposite to charge of the particles decreases because a proper
space exists between the positive and negative particles. In this arrangement, even
prolonged repetition of rewriting, the change of image density is small and the change of
density uniformity is small, making it possible to display an image having a stabilized
density contrast and reduce the driving voltage required for image display.
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In the image display medium of the invention, it is preferable that one of the one, which
can be positively charged, and the another one, which can be negatively charged, is white.
By making at least one of the particulate materials white, the coloring power and density
contrast of the other particulate material can be enhanced. It is also preferable that the one,
which is white, comprises a coloring material and that the coloring material is titanium
oxide. In other words, the white particulate material preferably comprises a coloring
material and the coloring material is preferably titanium oxide. The use of titanium oxide
makes it possible to enhance the opacifying power and hence further enhance the contrast
in the wavelength range of visible light.
On the other hand, the image forming device of the invention is an image forming device
for forming an image on the foregoing image display medium of the invention, the image
forming device has an electric field generating unit for generating an electric field between
the pair of substrates according to the image to be formed.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram illustrating the structure of an image forming device
according to the first embodiment of implication of the invention;
FIG. 2 is a schematic diagram illustrating the structure of an image forming device
according to the second embodiment of implication of the invention;
FIG. 3 is a diagram illustrating another example of the image display medium;
FIG. 4 is a diagram illustrating further example of the image display medium;
FIG. 5 is a diagram illustrating further example of the image display medium;
FIG. 6 is a schematic diagram illustrating the structure of an image forming device
according to the third embodiment of implication of the invention;
FIG. 7 is a diagram illustrating an electrode pattern on a print electrode;
FIG. 8 is a schematic diagram illustrating the structure of the print electrode;
FIG. 9 is a schematic diagram illustrating the structure of an image forming device
according to the fourth embodiment of implication of the invention; and
FIG. 10 is a diagram illustrating the potential of the electrostatic latent image carrier and
the counter electrode.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The invention will be further described hereinafter.
[Operating Mechanism of the Invention]
The operating mechanism of the invention will be first described.
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At least two particulate materials to be enclosed in the gap between a pair of substrates
disposed opposed to each other are mixed at a predetermined ratio in an agitating vessel
where they are then stirred. It is thought that during this mechanical agitation,
triboelectrification occurs between the particles and between the particles and the inner
wall of the vessel, causing the particles to be charged. Thereafter, the particles thus mixed
are enclosed in the gap between the pair of substrates such that a predetermined volume
packing is reached. The particles thus enclosed in the gap move back and forth between the
substrates according to the electric field when the polarity of d.c. voltage applied across the
pair of substrates is switched or an a.c. voltage is applied across the pair of substrates
(initializing step). It is thought that even during the initializing step, the particles collide
with each other and with the surface layer on the substrate to undergo triboelectrification.
Further, this initializing step makes it possible to attain desired triboelectrification.
This triboelectriciation causes at least one of the particulate materials to be positively
charged (hereinafter referred to as "first particulate material") and at least one of the others
to be negatively charged (hereinafter referred to as "second particulate material"). Thus, the
resulting Coulomb force between the first particulate material and the second particulate
material can cause these particles to be attached to each other and agglomerated. However,
if the electrostatic force acting on the individual particles which have been charged in the
electric field applied across the substrates is stronger than the Coulomb force between the
first particulate material and the second particulate material and the imaging power (mirror
image power) or the van der Waals force between the particles and the substrate, the first
particulate material and the second particulate material separate from each other and each
move toward the respective substrate having the polarity opposite to its polarity of charge.
Accordingly, it is thought that when an electric field is applied across the substrates
according to the image signal, the first particulate material and the second particulate
material move according to the electric field and are then attached to different substrates. It
is further thought that the charged particles attached to the substrates are fixed to the
substrates by the imaging power occurring between the particles and the surface layer on
the substrates or the van der Waals force between the particles and the substrate.
When each of particulate materials has a high chargeability, the cohesive force between the
first particulate material and the second particulate material is too high to cause these
particulate materials to be separated. Further, particles having a high chargeability can be
easily attached to the surface of the substrate. It is thus much likely that these particles can
stay and be fixed to the surface of the substrate even under the application of an electric
field. Moreover, when highly chargeable agglomerated particles are separated, local
discharge can occur, giving unstable chargeability. On the contrary, particles having a low
chargeability can individually be hardly affected by the external electric field and thus can
stay and keep mildly agglomerated.
As can be seen in the foregoing description, it is important for each of particulate materials
to have triboelectric properties, i.e., proper charge amount and presence of little particles
charged opposite polarity for the purpose of causing particles to have opposite polarities to
each other to be separated and moved under the application of an external electric field.
When the polarity of the electric field is then switched to move repeatedly the particles, the
resulting friction between the particles and between the particles and the surface of the
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substrates causes the increase of the chargeability of the particles, resulting in the
agglomeration of the particles or causing the particles to be fixed to the surface layer on
the substrates and hence preventing the particles from being separated therefrom. The
range of the charged amount of the particles which cause uneven image is broad from low
to high. Accordingly, it is thought important that the change of the chargeability of the
particles be small to keep the initial operating conditions.
As a method for controlling chargeability there may be used a method which comprises
allowing finely divided inorganic oxide particles or finely divided resin particles to be
present on the surface of particles to control the chargeability thereof. However, the
collision or rubbing between the first particulate material and the second particulate
material causes these finely divided particles to move toward the counterpart particles (first
particulate material or second particulate material) and/or toward the transparent electrode
substrate, resulting in the drop of charged amount. Further, the change of the fluidity of
powder causes the drop of display contrast.
The prevention of the change of the positional relationship between the surface of the first
particulate material or second particulate material and the finely divided particles is
essential for the maintenance of the chargeability and fluidity of the first particulate
material or second particulate material.
In the invention, the foregoing problems are solved by predetermining the shape factor of
both the first and second particulate materials to a specific range. In other words, by
predetermining the shape factor ((L.sup.2 /S)/4.pi..times.100) of the particulate materials
capable of being positively and negatively charged so as to meet 100<the shape
factor.ltoreq.140, the fluidity of the particles can be enhanced, making it possible to unify
the distribution of charge and improve the stability of chargeability and the speed at which
oppositely charged particles separate from each other during display (display responce) and
display contrast. Accordingly, the image display medium of the invention requires a low
driving voltage and can provide a small change of image density and density uniformity
and a stabilized density contrast even after prolonged repetition of rewriting.
While the foregoing description has been made with reference to the case where there are
one first particulate material capable of being positively charged and one second
particulate material capable of being negatively charged, there may be one or more such
first and second particulate materials. Even when there are two or more such first and
second particulate materials, a similar mechanism of operation makes the effect of the
invention possible.
[Constitution of Particulate Material of the Invention]
The particulate materials of the invention (hereinafter, "the particulate materials of the
invention" is a generic term for both the particulate materials capable of being positively
and negatively charged) have a shape factor (=(L.sup.2 /S)/4.pi..times.100, in which S is
the area of particle and L is the perimeter of particle) so as to meet 100<the shape
factor.ltoreq.140, preferably to meet 105.ltoreq.the shape factor.ltoreq.130, more preferably
to meet 110.ltoreq.the shape factor.ltoreq.125. When the shape factor of the particulate
materials is 100, there is no unevenness on the surface of the particulate materials, causing
an increase of the adhesion between the particles or between the particles and the surface
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of the substrates. Further, the resulting triboelectrification between the particles causes the
instabilization of charged amount or expansion of charge distribution (distribution of
electrification). Moreover, the fluidity of the particles charged by friction lowers,
deteriorating the efficiency in separation and movement of particles and hence raising the
required driving voltage. On the contrary, when the shape factor of the particulate materials
exceeds 140, since there are too large unevenness on the surface of the particulate
materials, the collision between the particles developed when the powder (particles) move
during repeated display causes the surface unevenness to be easily removed (destroyed),
expanding the distribution of particle size and hence the distribution of electrification is
expanded and thus the displayed image is deteriorated.
The shape factor is an index of the shape properties of a toner defined by the equation:
Shape factor=(L.sup.2 /S)/4.pi..times.100
For the determination of shape factor, the particle is observed on scanning electron
microphotograph (SEM). Using an image analyzer (Luzex, produced by Nireco
Corporation), the area (S) and perimeter (L) of the particle are then determined from the
electron microphotograph of the particle. The shape of the particle is then quantified by the
foregoing equation.
The particulate material according to the invention is normally formed by at least a
coloring material and a resin. If necessary, the particulate material of the invention may
include a charge control agent. The coloring material may also act as a charge control
agent.
Examples of the coloring material employable herein will be given below.
Examples of black coloring material include organic and inorganic dye-based and
pigment-based black coloring materials such as carbon black, titanium black, magnetic
powder and oil black.
Examples of white coloring material include white pigments such as rutile type titanium
oxide, anatase type titanium oxide, zinc white, white lead, zinc sulfide, aluminum oxide,
silicon oxide and zirconium oxide.
Other examples of chromatic coloring materials employable herein include
phthalocyanine-based, quinacridone-based, azo-based, condensed, insoluble lake pigment,
and inorganic oxide-based dye and pigments. Specific examples of these dyes and
pigments which can be preferably used herein include aniline blue, chalcoyl blue, chrome
yellow, ultramarine blue, Du Pont oil red, quinoline yellow, methylene blue chloride,
phthalocyanine blue, malachite green oxalate, lamp black, rose bengal, C.I. pigment red
48:1, C.I. pigment red 122, C.I. pigment red 57:1, C.I. pigment yellow 97, C.I. blue 15:1,
and C.I. pigment blue 15:3.
One of the two particulate materials of the invention is preferably white. In other words,
one of the two particulate materials of the invention preferably contains a white coloring
material. By making one of the two particulate materials white, the colorability and density
contrast of the other particulate materials can be improved. As the white coloring material
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for making one of the two particulate materials white there is preferably used titanium
oxide. By using titanium oxide as a coloring material, the opacifying power of the
particulate material in the wavelength of visible light can be raised to further enhance the
density contrast. As the white coloring material there is preferably used rutile type titanium
oxide in particular.
However, the invention is not limited to the case where one of the two particulate materials
of the invention is white. For example, one of the two particulate materials of the invention
may be black. This arrangement is useful particularly for the case where black letters and
other color letters or signs are exchanged for display.
Examples of the coloring material which also acts as a charge control agent include
coloring materials having an electrophilic group or electron donating group, and metal
complex. Specific examples of these coloring materials include C.I. pigment violet 1, C.I.
pigment violet 3, C.I. pigment violet 23, and C.I. pigment black 1.
The amount of the coloring material to be added is preferably from 1 to 60% by mass,
more preferably from 5 to 50% by mass based on the total mass of the particulate material
supposing that the specific gravity of the coloring material is 1.
Examples of the resin constituting the particulate material include polyvinyl resins such as
polyolefin, polystyrene, acrylic resin, polyacrylonitrile, polyvinyl acetate, polyvinyl
alcohol, vinyl chloride and polyvinyl butyral, vinyl chloride-vinyl acetate copolymers,
styrene-acrylic acid copolymers, straight silicon resins made of organosiloxane bond,
modification products thereof, fluororesins such as polytetrafluoroethyelene, polyvinyl
fluoride and polyvinylidene fluoride, polyester, polyurethane, polycarbonate, amino resins,
and epoxy resins. These resins may be used singly or in admixture. These resins may have
been crosslinked. As the resin employable herein there may be used any binder resin which
has heretofore been known as a main component for electrophotographic toner without any
problem. In particular, a resin containing a crosslinked component is preferably used.
The particulate material of the invention may comprise a charge control agent incorporated
therein to control its chargeability as necessary. As the charge control agent there may be
used any charge control agent which is used in electrophotographic toner material.
Examples of such a charge control agent include quaternary ammonium salts such as
cetylpyridyl chloride and P-51 and P-53 (produced by Orient Chemical Industries, Ltd.),
salicylic acid-based metal complexes, phenolic condensation products, tetraphenyl-based
compounds, particulate metal oxide, and particulate metal oxide surface-treated with
various coupling agents.
The charge control agent is preferably colorless or has a low coloring power or the same
color as that of the entire particulate material in which it is incorporated. When the charge
control agent to be used is colorless or has a low coloring power or the same color as that
of the entire particulate material in which it is incorporated (i.e., same as the color of the
coloring material incorporated in the particulate material), the impact on the color hue of
the particulate material selected can be reduced.
The term "colorless" as used herein is meant to indicate that the material has no color. The
term "low coloring power" as used herein is meant to indicate that the material has little
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effect on the color of the entire particulate material. The term "same color as that of the
entire particulate material in which it is incorporated" as used herein is meant to indicate
that the material itself has a color hue which is the same as or close to that of the entire
particulate material in which it is incorporated, demonstrating that it has little effect on the
color of the entire particulate material in which it is incorporated. For example, in the
particulate material containing a white pigment as a coloring material, the white charge
control agent is included in the category of the charge control agent having the "same color
as that of the entire particulate material in which it is incorporated". Anyway, the color of
the charge control agent maybe such that the color of the entire particulate material in
which it is incorporated is the same as the desired color regardless of which it is "colorless",
has a "low coloring power" or the "same color as that of the entire particulate material in
which it is incorporated".
The amount of the charge control agent to be added is preferably from 0.1 to 10% by
weight, more preferably from 0.5 to 5% by weight. The size of dispersed unit of the charge
control agent in the particulate material is preferably not greater than 5 .mu.m, more
preferably not greater than 1 .mu.m as calculated in terms of volume-average particle
diameter. The charge control agent may exist in compatibilized state in the particulate
material.
It should be adjusted that at least one of the particulate materials of the invention (two or
more particulate materials) can be positively charged while the at least the other can be
negatively charged. However, when different kinds of particles collide or rub with each
other to cause electrification, one of the particulate materials is positively charged while
the other is negatively charged due to the positional relationship between the charged
arrangement of the two particulate materials. Therefore, by properly selecting the charge
control agent, the position of the charged arrangement can be properly adjusted.
The particulate material of the invention preferably further comprises a resistivity adjustor
incorporated therein. The use of such a resistivity adjustor makes it possible to expedite the
exchange of charge between the particulate materials and hence attain early stabilization of
the device. The term "resistivity adjustor" as used herein is meant to indicate an
electrically-conductive particulate material, preferably an electrically-conductive
particulate material which causes properly charge exchange or charge leakage. The
presence of the resistivity adjustor makes it possible to avoid prolonged friction of particles
and increase of charged amount of particles due to friction between particles and substrate,
i.e., so-called charge-up.
As the resistivity adjustor there is preferably used an inorganic particulate material having
a volume resistivity of not greater than 1.times.10.sup.6 .OMEGA..cm, preferably not
greater than 1.times.10.sup.4 .OMEGA..cm. Specific examples of the inorganic particulate
material employable herein include particulate tin oxide, titanium oxide, zinc oxide, iron
oxide, and particulate material coated with various electrically-conductive oxides such as
titanium oxide coated with tin oxide. The resistivity adjustor is preferably colorless or has
a low coloring power or the same color as that of the entire particulate material in which it
is incorporated. These terms are as defined with reference to the charge control agent. The
amount of the resistivity adjustor to be added is not limited so far as it doesn't impair the
color of the colored particles but is preferably from 0.1 to 10% by weight.
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Referring to the size of the particulate material of the invention, the particle diameter and
distribution of the white particulate material and the black particulate material can be
rendered almost the same to avoid the adhesion state in which a large diameter particle is
surrounded by small diameter particles as in a so-called two-component developer, making
it possible to obtain a high white density and black density. The coefficient of variation of
particle size is preferably not greater than 15%. It is particularly preferred that the
particulate material be monodisperse. Small diameter grains can be attached to the
periphery of a large diameter grain to lower the color density characteristic of the large
diameter grain. The contrast can vary with the mixing proportion of the white and black
particulate materials. The mixing proportion of the white and black particulate materials is
preferably such that the surface of the particulate materials (two particulate materials) of
the invention are the same. When the mixing proportion of the two particulate materials
deviates greatly from the above defined range, the color of the particulate material used in
a greater mixing proportion can become loud. However, this doesn't necessarily apply in
the case where it is desired that a strong color tone display and a light color tone display be
made with the same color to make high contrast or where it is desired that the display be
made with a color obtained by mixing two kinds of colored particles.
The particle diameter of the particulate material of the invention cannot be unequivocally
defined. However, in order to obtain a good image, the volume-average particle diameter
of the particulate material is preferably from about 1 to 100 .mu.m, more preferably from
about 3 to 30 .mu.m. The distribution of particle size of the particulate material is
preferably sharp, more preferably monodisperse.
The preparation of the particulate material of the invention can be accomplished by a wet
process for preparing spherical particles such as suspension polymerization, emulsion
polymerization and dispersion polymerization, conventional grinding and classification
process for preparing amorphous particles, or the like. In order to unify the shape of the
particles, heat treatment is preferably effected.
In order to unify the distribution of particle size, the particles may be subjected to
classification. For example, various vibrational sieves, ultrasonic sieves, air type sieves and
wet sieves, rotor classifier employing the principle of centrifugal force, wind power
classifier, etc. may be used, but the invention is not limited thereto. These devices may be
used singly or in combination to provide a desired distribution of particle size. In order to
adjust the particle size distribution precisely, a wet sieve is preferably used.
As methods for controlling the shape of particle (shape factor) there are preferably used the
following methods. For example, the so-called suspension polymerization method
disclosed in Japanese Patent Laid-Open No. 1998-10775 is preferably used which
comprises dissolving a polymer in a solvent, mixing the solution with a coloring agent, and
then dispersing the mixture in an aqueous medium in the presence of an inorganic
dispersant so that it is rendered particulate wherein the step of adding a non-polymerizable
organic solvent compatible with the monomer (having little or no compatibility with the
solvent) to prepare particles which are then withdrawn is selectively followed by a drying
step of removing the organic solvent. As the drying method there is preferably used a
freeze drying method. This freeze drying method is preferably effected at a temperature of
-200.degree. C. to -10.degree. C. (preferably from -180.degree. C. to -30.degree. C.). The
freeze drying method is preferably effect at a pressure of not higher than 40 Pa,
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particularly not higher than 13 Pa. Examples of the organic solvent employable herein
include ester-based solvents such as methyl acetate and propyl acetate, ether-based solvents
such as diethyl ether, ketone-based solvents such as methyl ethyl ketone, methyl isopropyl
ketone and methyl isobutyl ketone, hydrocarbon solvents such as toluene and cyclohexane,
and halogenated hydrocarbon solvents such as dichloromethane, chloroform and
trichloroethylene. These solvents preferably can dissolve a polymer therein. These solvents
preferably a water solubility of from about 0 to 30% by weight. Cyclohexane is
particularly preferred on an industrial basis taking into account safety, cost and
productivity.
Further, a method as disclosed in Japanese Patent Laid-Open No. 2000-292971 can be used
which comprises agglomerating and coalescing small particles to provide particles having a
desired particle diameter. Moreover, a method which comprises applying a mechanical
impact (developed by Hybridizer (produced by Nara Machinery Co., Ltd.), Angmill
(produced by HOSOKAWA MICRON CORPORATION), .theta. composer (produced, by
Tokuju Kosakujo Co., Ltd.), etc.) to or heating a particulate material obtained by the
conventional known melt-kneading, crushing or classification method can be employed to
control the shape of particles.
[Structure of Substrate of the Invention]
The image display medium comprises a pair of substrates opposed to each other. The
particulate materials of the invention are enclosed in the gap between the pair of substrates.
In the invention, the substrate is an electrically-conductive sheet-like material
(electrically-conductive substrate). In order to allow the substrate to act as an image
display medium, it is necessary that at least one of the pair of substrates be a transparent
electrically-conductive substrate. In this case, the transparent electrically-conductive
substrate acts as a display substrate.
As the electrically-conductive substrate there may be used a substrate which itself is
electrically-conductive or an insulating support the surface of which has been
electrically-conducted regardless of which it is crystalline or amorphous. Examples of the
electrically-conductive substrate which itself is electrically-conductive include metal such
as aluminum, stainless steel, nickel and chromium, crystalline alloy thereof, and
semiconductor such as Si, GaAs, GaP, GaN, SiC and ZnO.
Examples of the insulating support employable herein include polymer film, glass, quartz,
and ceramics. The electrically-conduction of the insulating support can be accomplished by
subjecting the insulating support to vacuum evaporation, sputtering, ion plating or the like
with the metal described with reference to the case of the electrically-conductive substrate
which itself is electrically-conductive or gold, silver, copper or the like.
As the transparent electrically-conductive substrate there may be used an
electrically-conductive substrate having a transparent electrode formed on one side of an
insulating transparent support or a transparent support which itself is
electrically-conductive. Examples of the transparent support which itself is
electrically-conductive include transparent electrically-conductive materials such as ITO,
zinc oxide, tin oxide, lead oxide, indium oxide and copper iodide.
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Examples of the insulating transparent support employable herein include transparent
inorganic materials such as glass, quartz, sapphire, MgO, LiF and CaF.sub.2, film or sheet
of transparent organic resins such as fluororesin, polyester, polycarbonate, polyethylene,
polyethylene terephthalate and epoxy, optical fiber, SELFOC optical plate, etc.
As the transparent electrode to be provided on one side of the transparent support there
may be used a transparent layer developed by vacuum evaporation, ion plating, sputtering
or the like with a transparent electrically-conductive material such as ITO, zinc oxide, tin
oxide, lead oxide, indium oxide and copper iodide or a layer which has been developed by
vacuum evaporation or sputtering with a metal such as Al, Ni and Au to a thickness small
enough to attain semitransparency.
In a further preferred embodiment of these substrates, the opposing surface of these
substrates are provided with a protective layer having proper surface conditions because
they have effect on the polarity of charge of the particles. The protective layer can be
selected mainly from the standpoint of adhesion to the substrate, transparency, charged
arrangement and surface stainability. Specific examples of the protective layer material
employable herein include polycarbonate resin, vinyl silicone resin, and
fluorine-containing resin. The resin to be used herein may be selected from the standpoint
of the constitution of the main monomer of the particulate material. Further, a resin having
a small difference in triboelectricity from the particulate material may be selected.
[Embodiments of the Image Forming Device]
Embodiments of the image forming device of the invention using the image display
medium of the invention will be further described with reference to the attached drawings.
In the various drawings, where the parts function in the same way, the same reference
numerals are assigned. The description of these parts may be omitted.
First Embodiment
FIG. 1 illustrates an image display medium according to the present embodiment and an
image forming device for forming an image on the image display medium.
The image forming device 12 according to the first embodiment comprises a voltage
applying unit 201 as shown in FIG. 1. The image display medium 10 comprises a spacer
204, a black particulate material 18 and a white particulate material 20 enclosed in the gap
between a display substrate 14 disposed on the image display side and a non-display
substrate 16 disposed opposed to the display substrate 14. The display substrate 14 and the
non-display substrate 16 are each provided with a transparent electrode 205 as described
later. The transparent electrode 205 on the non-display substrate 16 is grounded. The
transparent electrode 205 on the display substrate 14 is connected to the voltage applying
unit 201.
The image display medium 10 will be further described hereinafter.
As the display substrate 14 and non-display substrate 16, which constitute the outside of
the image display medium 10, there are used, e.g., 7059 glass substrate with a transparent
electrode ITO having a size of 50 mm.times.50 mm.times.1.1 mm. The inner surface 206
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of the glass substrate with which the particle material comes in contact is coated with a
polycarbonate resin (PC-Z) to a thickness of 5 .mu.m. A silicon rubber plate 204 having a
size of 40 mm.times.40 mm.times.0.3 mm is cut at the center thereof by a 15 mm.times.15
mm square to form a space therein. The silicon rubber plate thus cut is disposed on the
non-display substrate 16. For example, a spherically particulate white material 20
containing titanium oxide having a volume-average particle diameter of 20 .mu.m and a
spherically particulate black material 18 containing carbon having a volume-average
particle diameter of 20 .mu.m are mixed at a weight ratio of 2:1. About 15 mg of the
mixture is sieved through a screen into the square space in the silicon rubber plate.
Thereafter, the display substrate 14 is attached to the silicon rubber plate. The two
substrates are pressed by a double clip so that the silicon rubber plate comes in contact
with the two substrates to form the image display medium 10.
Second Embodiment
A second embodiment of implication of the invention will be further described in
connection with the attached drawings.
FIG. 2 illustrates an image forming device 12 for forming an image on an image display
medium 10 comprising a simple matrix according to the present embodiment. Electrodes
403An and 403Bn (n: positive integer) form a simple matrix. A plurality of particles
having different chargeabilities are enclosed in the space formed by the electrodes 403An
and 404Bn. An electric field generator 402 comprising a waveform generator 402B and a
power supply 402A and an electric field generator 405 comprising a waveform generator
405B and a power supply 405A generates a potential on the electrodes 403An and 404Bn,
respectively. A sequencer 406 controls the electrode potential drive timing to control the
drive of voltage on these electrodes. In this arrangement, the electrodes 403A1 to An on
one side are provided with an electric field such that the particles are driven by unit of one
line at a time. The electrodes B1 to Bn on the other side are provided with an electric field
according to image data at the same time on the plane.
FIGS. 3, 4 and 5 each illustrate the view of the image forming portion of FIG. 2 on the
respective arbitrary section. The particles come in contact with the surface of the electrode
or substrate. The substrate is transparent on at least one side thereof so that the color of the
particles can be seen from outside. The electrodes 403A and 404B may be embedded in
and integrated to the respective substrate as shown in FIGS. 3 and 4 or may be separated
from the respective substrate as shown in FIG. 5.
By properly setting the electric field to the foregoing device, display is enabled by the
simple matrix. Any particles having a threshold value of movement with respect to electric
field can be driven. Thus, the drive of particles is not restricted by the color, polarity of
charging, charged amount of particles.
Third Embodiment
A third embodiment of the invention will be further described with reference to the
attached drawings. The third embodiment is an image forming device comprising a print
electrode.
- 78 -
As shown in FIG. 6 and FIG. 7A, the print electrode 11 comprises a substrate 13 and a
plurality of electrodes 15 having a diameter of, e.g., 100 .mu.m. The image forming device
12 comprises the print electrode 11, a counter electrode 26, a power supply 28, etc.
The plurality of electrodes 15 are arranged in a line at a predetermined interval according
to the image resolution in the direction (i.e., main scanning direction) almost perpendicular
to the direction of conveyance of the image display medium 10 (indicated by the arrow B)
on one surface of the display substrate 14 as shown in FIG. 7A. The electrodes 15 each
may be square as shown in FIG. 7B. Alternatively, the electrodes 15 may be arranged in
matrix as shown in FIG. 7C.
To each of the electrodes 15 are connected an AC power supply 17A and a power supply
17B through a connection controller 19. The connection controller 19 comprises a plurality
of switches composed of switches 21A each having one end connected to the electrode 15
and the other connected to the AC power supply 17A and switches 21B each having one
end connected to the electrode 15 and the other connected to the DC power supply 17B.
These switches are each on-off controlled by the controller 60 to electrically connect the
AC power supply 17A and the DC power supply 17B to the electrode 15. In this
arrangement, an a.c. voltage or d.c. voltage or an a.c. voltage having a d.c. voltage imposed
thereon can be applied to the image display medium.
The operation of the third embodiment will be described hereinafter.
Firstly, when the image display medium 10 is conveyed in the direction indicated by the
arrow B by a conveying unit (not shown) into the gap between the print electrode 11 and
the counter electrode 26, the controller 60 instructs the connection controller 19 to turn all
the switches 21A on. In this manner, an a.c. voltage from the AC power supply 17A is
applied to all the electrodes 15.
The image display medium comprises a group of two or more kinds of particles enclosed
in the space between a pair of substrates not having electrode.
When an a.c. voltage is applied to the electrode 15, the black particles 18 and the white
particles 20 in the image display medium 10 move back and forth between the display
substrate 14 and the non-display substrate 16. The resulting friction between the particles
or between the particles and the substrate causes the black particles 18 and white particles
20 to be triboelectrically charged. For example, the black particles 18 are positively
charged while the white particles 20 are not charged or negatively charged. The following
description will made with reference to the case where the white particles 20 are negatively
charged.
The controller instructs the connection controller 19 to turn on only the switch 17B
corresponding to the electrode 15 disposed according to the image data so that a d.c.
voltage is applied to the electrode 15 disposed according to the image data. For example, a
d.c. voltage is applied to the non-image area while a d.c. voltage is not applied to the
image area.
In this manner, if a d.c. voltage is applied to the electrode 15, the black particles 18 which
- 79 -
have been positively charged at the area where the print electrode 11 is disposed opposed
to the display substrate 14 move toward the non-display substrate 16 under the action of
electric field. At the same time, the white particles 20 which have been negatively charged
on the non-display substrate 16 move toward the display substrate 14 under the action of
electric field. Accordingly, only the white particles 20 appear on the display substrate 14
side. As a result, no image is displayed on the area corresponding to the non-image area.
On the other hand, if no d.c. voltage is applied to the electrode 15, the black particles 18
which have been positively charged at the area where the print electrode is disposed
opposed to the display substrate 14 remain under the action of electric field. At the same
time, the black particles 18 which have been positively charged on the non-display
substrate 16 side move toward the display substrate 14 under the action of electric field.
Accordingly, only the black particles 18 appear on the display substrate 14 side. As a result,
an image is displayed on the area corresponding to the image area.
In this manner, only the black particles 18 appear on the display substrate 14 side. As a
result, an image is displayed on the area corresponding to the image area
Thus, the black particles 18 and the white particles 20 move according to the image to
display an image on the display substrate 14 side. When the white particles 20 have not
been charged, only the black particles 18 move under the effect of electric field. The black
particles 18 on the area where no image is displayed move toward the non-display
substrate 16 and are shielded by the white particles 20 on the display substrate 14 side,
enabling the display of an image. Even after the electric field which has been generated
across the substrates of the image display medium 10 has disappeared, the displayed image
is maintained by the adhesion characteristic of particles. Since these particles can move
again when an electric field is generated across the substrates, the image forming device
can repeatedly display an image.
Thus, particles which have been charge with air as a medium move under the effect of
electric field, providing a high safety. Further, since air has a low viscosity resistance, a
high responce, too, can be attained.
Fourth Embodiment
A fourth embodiment of implication of the invention will be further described in
connection with the attached drawings. The fourth embodiment is an image forming device
comprising an electrostatic latent image carrier.
FIG. 9 illustrates an image forming device 12 according to the fourth embodiment. The
image forming device 12 comprises an electrostatic latent image forming portion 22, a
drum-shaped electrostatic latent image carrier 24, a counter electrode 26, a d.c. voltage
power supply 28, etc.
The electrostatic latent image forming portion 22 comprises a charging device 80, and a
light beam scanning device 82. In this case, as the electrostatic latent image carrier 24 there
may be used a photoreceptor drum 24. The photoreceptor drum 24 comprises a
photo-conductive layer 24B formed on a drum-shaped electrically-conductive substrate
24A made of aluminum, SUS or the like. As the photo-conductive layer there may be used
- 80 -
any known material such as inorganic photo-conductive material
(e.g., .alpha.-Si, .alpha.-Se, As.sub.2 Se.sub.3) and organic photo-conductive material (e.g.,
PVK/TNF). The formation of the photo-conductive layer can be accomplished by plasma
CVD, vacuum evaporation, dipping method or the like. If necessary, the photoreceptor
drum 24 may comprise a charge-transporting layer or overcoat layer formed thereon.
The charging device 80 uniformly charges the surface of the electrostatic latent image
carrier 24 to a desired potential. As the charging device 80 there may be used any material
which can charge the surface of the photoreceptor drum 24 to an arbitrary potential. The
present embodiment employs a corotron which applies a high voltage to an electrode wire
to generate a corona discharge between the electrode wire and the electrostatic latent image
carrier 24 so that the surface of the photoreceptor drum 24 can be uniformly charged.
Alternatively, any known charger such as electrically-conductive roll member, brush and
film member may be used. A voltage is applied to such a charger in contact with the
photoreceptor drum 24 to charge the surface of the photoreceptor drum.
The light beam scanning device 82 emits a minute spot light onto the surface of the
electrostatic latent image carrier 24 thus charged according to the image signal to form an
electrostatic latent image on the electrostatic latent image carrier 24. As the light beam
scanning device 82 there may be used any device which emits light beam onto the surface
of the photoreceptor drum 24 according to the image data to form an electrostatic latent
image on the photoreceptor drum 24 which has been uniformly charged. In the present
embodiment, a polygon mirror 84, a turning mirror 86, and an imaging optical system
comprising a light source and lens (not shown) form a laser beam having a predetermined
spot diameter which then scans on the surface of the photoreceptor drum 24 while being
turned on and off according to the image signal. In this arrangement, ROS (Raster Output
Scanner) is formed. Alternatively, an LED head comprising LED's arranged according to
desired resolution may be used.
The electrically-conductive substrate 24A on the electrostatic latent image carrier 24 is
grounded. The electrostatic latent image carrier 24 rotates in the direction indicated by the
arrow A.
The counter electrode 26 is formed by, e.g., an elastic electrically-conductive roll member.
In this arrangement, the counter electrode 26 is allowed to come in closer contact with the
image display medium 10. The counter electrode 26 is disposed with the image display
medium 10 disposed interposed between the counter electrode 26 and the electrostatic
latent image carrier 24. The image display medium 10 is conveyed in the direction
indicated by the arrow B by a conveying unit (not shown). To the counter electrode 26 is
connected a d.c. voltage power supply 28. A bias voltage VB from the d.c. voltage power
supply 28 is applied to the counter electrode 26. The bias voltage V.sub.B is in between
V.sub.H and V.sub.L wherein V.sub.H and V.sub.L are the potential of the area on the
electrostatic latent image carrier 24 which is positively charged and the potential of the
area on the electrostatic latent image carrier 24 which is not charged, respectively, as
shown in FIG. 10. The counter electrode 26 rotates in the direction indicated by the arrow
C.
The operation of the fourth embodiment will be described hereinafter.
- 81 -
When the electrostatic latent image carrier 24 begins to rotate in the direction indicated by
the arrow A, the electrostatic latent image forming portion 22 forms an electrostatic latent
image on the electrostatic latent image carrier 24. On the other hand, the image display
medium 10 is conveyed in the direction indicated by the arrow B by a conveying unit (not
shown) into the gap between the electrostatic latent image carrier 24 and the counter
electrode 26.
At this time, a bias voltage V.sub.B is applied to the counter electrode 26 as shown in FIG.
10. The potential of the electrostatic latent image carrier 24 disposed opposed to the
counter electrode 26 is V.sub.H. In this arrangement, when the area on the electrostatic
latent image carrier 24 disposed opposed to the display substrate 14 has been positively
charged (non-image area) and the black particles 18 have been attached to the area on the
display substrate 14 disposed opposed to the electrostatic latent image carrier 24, the black
particles 18 which have been positively charged move from the display substrate 14 side to
the non-display substrate 16 side so that they are attached to the non-display substrate 16.
In this manner, only the white particles 20 appear on the display substrate 14 side. As a
result, no image is displayed on the area corresponding to the non-image area.
On the other hand, when the area on the electrostatic latent image carrier 24 disposed
opposed to the display substrate 14 has not been positively charged (image area) and the
black particles 18 have been attached to the area on the non-display substrate 16 disposed
opposed to the counter electrode 26, the black particles 18 which have been charged move
from the non-display substrate 16 side to the display substrate 14 side so that they are
attached to the display substrate 14 because the potential of the electrostatic latent image
carrier 24 disposed opposed to the counter electrode 26 is V.sub.L. In this manner, only the
black particles 18 appear on the display substrate 14 side. As a result, an image is
displayed on the area corresponding to the image area.
In this manner, the black particles 18 move according to the image data to display an
image on the display substrate 14 side. Even after the electric field which has been
generated across the substrates of the image display medium 10 has disappeared, the
displayed image is maintained by the adhesion characteristic of particles and the mirror
image power between the particles and the substrate. Since these particles can move again
when an electric field is generated across the substrates, the image forming device 12 can
repeatedly display an image.
Thus, since a bias voltage is applied to the counter electrode 26, the black particles 18 can
be moved regardless of whichever the black particles 18 are attached to the display
substrate 14 or the non-display substrate 16. Therefore, it is not necessary that the black
particles 18 be previously attached to one of the substrates. Further, an image having a
high contrast and a high sharpness can be formed. Moreover, particles which have been
charge with air as a medium move under the effect of electric field, providing a high safety.
Further, since air has a low viscosity resistance, a high responce, too, can be attained.
While embodiments of the image forming device of the invention comprising the image
display medium of the invention have been described in connection with the attached
drawings, the invention should not be construed as being limited thereto except for the use
of the particles of the invention. Various structures may be employed according to the
purpose. While the foregoing embodiments have been described with reference to the case
- 82 -
where the combination of colors of particles are black and white, the invention should not
be construed as being limited thereto. Proper combinations may be selected according to
the purpose.
EXAMPLE
The invention will be further described in the following examples, but the invention should
not be construed as being limited thereto. In the following examples and comparative
examples, the effect of the invention was confirmed using the image display medium and
image forming device according to the first embodiment (image display medium and
image forming device as shown in FIG. 1) described in the foregoing paragraph
[Embodiments of image forming device of the invention] with different constitutions of
white particles 20 and black particles 18. The size, material and other factors of various
members were similar to that described in the foregoing paragraph [Embodiments of image
forming device of the invention].
(Preparation of White Particulate Material-1)
Preparation of Dispersion A
The following components were mixed, and then subjected to milling with zirconia balls
having a diameter of 10 mm.phi. for 20 hours to prepare a dispersion A.
<Formulation>
* Cyclohexyl methacrylate
* Titanium oxide
53 parts by weight
45 parts by weight
(Tipaque, produced by ISHIHARA SANGYO
KAISHA,LTD.)
* Charge control agent
(COPY CHARGE PSY VP2038, produced by
Clariant Japan Co., Ltd.)
* Cyclohexane
2 parts by weight
5 parts by weight
Preparation of Dispersion B
The following components were mixed, and then subjected to milling in the same manner
as the dispersion A to prepare a dispersion B.
<Formulation>
* Calcium carbonate
* Water
40 parts by weight
60 parts by weight
- 83 -
Preparation of Mixture C
The following components were mixed, deaerated by means of a ultrasonic device for 10
minutes, and then stirred by means of an emulsifier to prepare a mixture C.
<Formulation>
* 2% aqueous solution of CELLOGEN
* Dispersion B
* 20% aqueous solution of sodium
chloride
4.3 g
8.5 g
50 g
35 g of the dispersion A, 1 g of divinylbenzene and 0.35 g of a polymerization initiator
AIBN were measured out, thoroughly mixed, and then aerated by means of a ultrasonic
device for 10 minutes. The mixture thus obtained was put in the mixture C, and then
subjected to emulsification by means of an emulsifier. Subsequently, the emulsion thus
obtained was put in a bottle which was sealed with a silicone cover. The emulsion was
thoroughly deaerated through a syringe. The bottle was then filled with nitrogen gas.
Subsequently, the emulsion was reacted at a temperature of 60.degree. C. for 10 hours.
After cooling, the resulting dispersion containing particles was processed by a freeze dryer
at a temperature of -35.degree. C. and a pressure of 0.1 Pa for 2 days to remove
cyclohexane. The particulate material thus obtained was dispersed in ion-exchanged water.
To the dispersion was then added an aqueous solution of hydrochloric acid to decompose
calcium carbonate. The dispersion was then filtered. The dispersion was thoroughly
washed with distilled water, and then sieved through nylon sieves having a mesh size of
20 .mu.m and 25 .mu.m, respectively, to classify the particle size. The dispersion was then
dried to obtain a white particulate material-1 having an average particle diameter of
23 .mu.m. The particulate material was observed on SEM photograph. As a result, the
particles were observed to be spherical. The particles were also determined for shape factor.
The shape factor was 107.
(Preparation of Black Particulate Material-1)
A black particulate material-1 having an average particle diameter of 23.2 .mu.m was
obtained in the same manner as the white particulate material-1 except that the following
dispersion K was used instead of the dispersion A. The particulate material was observed
on SEM photograph. As a result, the particles were observed to be spherical. The particles
were also determined for shape factor. The shape factor was 110.
- 84 -
Preparation of dispersion K
The following components were mixed, and then subjected to milling with zirconia balls
having a diameter of 10 mm.phi. for 20 hours to prepare a dispersion K.
<Formulation>
* Styrene monomer
* Black pigment
(Carbon black; CF9, produced by
Mitsubishi Chemical Corporation)
* Cyclohexane
87 parts by weight
10 parts by weight
5 parts by weight
(Preparation of Black Particulate Material-2)
A black particulate material-1 having an average particle diameter of 23.3 .mu.m was
obtained in the same manner as the white particulate material-1 except that the following
dispersion K' was used instead of the dispersion A. The particulate material was observed
on SEM photograph. As a result, the particles were observed to be spherical. The particles
were also determined for shape factor. The shape factor was 102.
Preparation of Dispersion K'
The following components were mixed, and then subjected to milling with zirconia balls
having a diameter of 10 mm.phi. for 20 hours to prepare a dispersion K'.
<Formulation>
* Styrene monomer
* Black pigment
87 parts by weight
10 parts by weight
(Carbon black; CF9, produced by
Mitsubishi Chemical Corporation)
* Cyclohexane
2 parts by weight
(Preparation of Black Particulate Material-3)
A black particulate material-3 having an average particle diameter of 22.2 .mu.m was
- 85 -
obtained in the same manner as the white particulate material-1 except that the following
dispersion K" was used instead of the dispersion A and the dispersion was dried at a
temperature of 30.degree. C. and a pressure of 1.3.times.10.sup.4 for 5 hours Pa at the step
of removing cyclohexane. The particulate material was observed on SEM photograph. As a
result, the particles were observed to be spherical. The particles were also determined for
shape factor. The shape factor was 135.
Preparation of Dispersion K"
The following components were mixed, and then subjected to milling with zirconia balls
having a diameter of 10 mm.phi. for 20 hours to prepare a dispersion K".
<Formulation>
* Styrene monomer
87 parts by weight
* Black pigment
(Carbon black; CF9, produced by
Mitsubishi Chemical Corporation)
* Cyclohexane
10 parts by weight
10 parts by weight
(Preparation of Black Particulate Material-4)
100 parts by weight of a styrene-butyl acrylate copolymer resin (glass transition point:
73.degree. C.) and 10 parts by weight of carbon black (CF9, produced by Mitsubishi
Chemical Corporation) were measured out, and then melt-kneaded under heating by means
of a Banbury mixer. The mixture was roughly ground by means of a hammer mill, and
then finely ground by means of a jet mill. The material was classified by means of an
elbow jet, and then spheronized by means of Hybridizer (produced by Nara Machinery Co.,
Ltd.). The particles were then further classified to obtain a black particulate material-4
having an average particle diameter of 22.2 .mu.m. The particulate material was observed
on SEM photograph. As a result, the particles were observed to be almost spherical. The
particles were also determined for shape factor. The shape factor was 143.
(Preparation of Black Particulate Material-5)
A black particulate material-5 having an average particle diameter of 21.2 .mu.m was
obtained in the same manner as the white particulate material-1 except that the following
dispersion K'" was used instead of the dispersion A and the dispersion was dried at a
- 86 -
temperature of 30.degree. C. and a pressure of 1.3.times.10.sup.4 Pa for 5 hours at the step
of removing cyclohexane. The particulate material was observed on SEM photograph. As a
result, the particles were observed to be spherical. The particles were also determined for
shape factor. The shape factor was 120.
Preparation of Dispersion K'"
The following components were mixed, and then subjected to milling with zirconia balls
having a diameter of 10 mm.phi. for 20 hours to prepare a dispersion K'".
<Formulation>
* Styrene monomer
* Black pigment
(Carbon black; CF9, produced by
Mitsubishi Chemical Corporation)
* Cyclohexane
89 parts by weight
8 parts by weight
8 parts by weight
Examples 1 to 4; Comparative Example 1
A white particulate material and a black particulate material were mixed according to
Table 1. The mixture was then enclosed in the gap between the opposing substrates
(display substrate 14, non-display substrate 16) in the image display medium according to
the first embodiment described in the foregoing embodiments and the image forming
device for forming an image on the image display medium to obtain image display media
of examples and comparative examples. The mixing proportion of the white particulate
material to the black particulate material (by number of particles) was 2:1.
(Evaluation)
The image display media and image forming devices thus obtained were each evaluated in
the following manner.
Driving Voltage
When a d.c. voltage of 135 V was applied to the transparent electrode of the display
substrate 14 in the foregoing image display medium 10 having a predetermined amount of
a 2:1 (by weight) mixture of the white particulate material 20 and the black particulate
- 87 -
material 18 enclosed therein, the white particulate material 20 which have been negatively
charged on the non-display substrate 16 side partly begins to move toward the display
substrate 14 under the action of electric field. When a d.c. voltage (driving voltage) is then
applied to the medium, most of the white particulate material 20 move toward the display
substrate 14 to saturate substantially the display density. At this time, the black particulate
particles 18 which have positively been charged move toward the non-display substrate 16.
Even after the applied voltage was reduced to 0 V, the particles on the display substrate
didn't move, causing no change of display density. The d.c. voltage applied was used as a
driving voltage. This driving voltage is set forth in Table 1.
Uneven Image
As mentioned above, when a voltage is applied across the display substrate 14 and the
non-display substrate 16 to allow a desired electric field to act on the group of particles, the
particulate materials 18 and 20 move between the display substrate 14 and the non-display
substrate 16. By switching the polarity of the voltage applied, the particulate materials 18
and 24 move in different directions between the display substrate 14 and the non-display
substrate 16. By repeatedly switching the polarity of voltage, these particulate materials
move back and forth between the display substrate 14 and the non-display substrate 16.
During this procedure, the collision of these particles 18 and 20 and the collision of the
particles 18 and 20 and the display substrate 14 or non-display substrate 16 cause the
particles 18 and 20 to be charged to different polarities. The black particulate material 18
(black particulate material-1) was positively charged and the white particulate material 20
(white particulate material-1) was negatively charged. Thus, these particulate materials
move in opposite directions according to the electric field across the display substrate 14
and the non-display substrate 16. When the electric field is fixed to one direction, these
particulate materials 18 and 20 are each attached to the display substrate 14 or non-display
substrate 16 to display a uniform image having a high density and a high contrast free of
unevenness. The polarity of voltage was repeatedly switched at 16,000 cycles and a time
interval of 1 second and then at 5,000 cycles and a time interval of 0.1 seconds, totaling
21,000 cycles. The resulting image was then measured for reflection density contrast and
reflection density unevenness and organoleptically evaluated for uneven image.
For the organoleptical evaluation of uneven image, a densitometer X-Rite 404 was used.
The measurement was made on five points in a patch having a size of 20 mm.times.20 mm.
The dispersion of density measured at the five points was used as criterion for evaluation
of uneven density. The density value averaged over the five points was used as average
density of the test patch. For example, when the black reflection density measured at the
- 88 -
five points range within .+-.0.05 according to this criterion, it is judged that there is little
unevenness in reflection density. The results are set forth in Table 1.
Example 1
Example 2
Example 3
Example 4
Comparative
Example 1
TABLE 1
White
particulate
material
(shape
factor)
White
particulate
Black
particulate
material
Driving
(shape factor) voltage
Black
160 V
particulate
material-1 material-1
(107)
(110)
White
Black
particulate
material-1
(107)
White
particulate
material-1
(107)
particulate
material-2
(102)
Black
particulate
material-3
(135)
White
particulate
material-1
(107)
White
particulate
material-1
(107)
Black
particulate
material-5
(120)
Black
particulate
material-4
(143)
Uneven
image
.+-.0.04
170 V
.+-.0.03
150 V
.+-.0.03
140 V
.+-.0.02
150 V
.+-.0.08
As can be seen in the foregoing results, Example 1 exhibits a required driving voltage as
low as 160 V. This value was almost half that required for the case where spherical
particles having a shape factor of 100 were used as particles. Example 1 was good also in
the organoleptical evaluation of uneven image. When Example 1 was measured for density
dispersion and density change after 21,000 cycles of switching of the polarity of voltage,
the density dispersion was .+-.0.03 and the reflection density showed a change as small as
0.05 from the initial value, demonstrating that the reflection density was stable.
- 89 -
It was also made obvious that Examples 2 to 4 gave results similar to that of Example 1.
On the contrary, Comparative Example 1, which uses a black particulate material-4 having
a shape factor of not smaller than 140, required a high driving voltage and exhibited a
rough image as evaluated for uneven image, demonstrating that no good results were
obtained. When Comparative Example 1 was measured for density dispersion and density
change after 21,000 cycles of switching of the polarity of voltage, the density dispersion
was .+-.0.1 and the reflection density showed a change as great as 0.15 from the initial
value, demonstrating that the reflection density was unstable.
Similar results were obtained even when the foregoing examples and comparative
examples were applied to the image display media and image forming devices according to
the second to fourth embodiments.
As mentioned above, the invention provides an image display medium which can use a low
predetermined driving voltage and shows a small change of image density and image
uniformity and a stable density contrast even after prolonged repetition of rewriting and an
image forming device therefor.
*****
第4筆
United States Patent
Hosono ,
6,806,503
et al.
October 19, 2004
Title: Light-emitting
diode and laser
diode having n-type ZnO layer and p-type
semiconductor laser
Abstract
An ultraviolet-light-emitting semiconductor diode comprising an n-type ZnO layer with
luminous characteristics formed on a transparent substrate, and a p-type semiconductor
layer selected from the group consisting of SrCu.sub.2 O.sub.2, CuAlO.sub.2 and
CuGaO.sub.2, which is formed on the n-type ZnO layer to provide a p-n junction
therebetween. The transparent substrate is preferably a single crystal substrate having
- 90 -
atomically flat yttria-stabilized zirconia (YSZ) (III) surface. The n-type ZnO layer is
formed on the transparent substrate having a temperature of 200 to 1200.degree. C., and
the p-type semiconductor layer selected from the group of SrCu.sub.2 O.sub.2,
CuAlO.sub.2 and CuGaO.sub.2 is formed on the n-type ZnO layer. The n-type ZnO layer
may be formed without heating the substrate, and then the surface of the ZnO layer may be
irradiated with ultraviolet light to promote crystallization therein.
Inventors:
Hosono; Hideo (Yamato, JP); Ota; Hiromichi (Kawasaki, JP); Orita;
Masahiro (Funabashi, JP); Kawamura; Kenichi (Sagamihara, JP);
Sarukura; Nobuhiko (Okazaki, JP); Hirano; Msahiro (Tokyo, JP)
Assignee:
Japan Science and Technology Agency (Kawagchi, JP)
Appl. No.:
169767
Filed:
November 5, 2002
PCT Filed:
January 24, 2001
PCT NO:
PCT/JP01/00465
PCT PUB.NO.: WO01/56088
PCT PUB. Date: August 2, 2001
Foreign Application Priority Data
Jan 28, 2000[JP]
2000-024843
Current U.S. Class:
257/79; 257/103; 257/E33.037; 438/69; 438/795;
438/799
Intern'l Class:
H01L 027/15; H01L 031/12; H01L 033/00; H01L
021/00; H01L 021/26
Field of Search:
257/79,103 438/69,795,799
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5192987
Mar., 1993
Khan et al.
5228044
Jul., 1993
Ohba
372/45.
5661074
Aug., 1997
Tischler
438/32.
5739545
Apr., 1998
Guha et al.
257/40.
5834331
Nov., 1998
Razeghi
438/40.
- 91 -
257/183.
6045626
Apr., 2000
Yano et al.
6057561
May., 2000
Kawasaki et al.
6174747
Jan., 2001
Ho et al.
6190777
Feb., 2001
Asano et al.
6242761
Jun., 2001
Fujimoto et al.
257/94.
6326645
Dec., 2001
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257/94.
6366017
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Antoniadis et al.
Foreign Patent Documents
0 863 555
Sep., 1998
EP.
2000-228516
Aug., 2000
JP.
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- 92 -
148/33.
438/31.
428/447.
313/506.
Primary Examiner: Trinh; Michael
Assistant Examiner: Soward; Ida M.
Attorney, Agent or Firm: Westerman, Hattori, Daniels & Adrian, LLP
Claims
What is claimed is:
1. An ultraviolet-light-emitting diode comprising:
an n-type ZnO layer formed on a transparent substrate and exhibiting only intrinsic
luminescence in the vicinity of a band gap thereof, and
a p-type semiconductor layer selected from the group consisting of SrCu.sub.2 O.sub.2,
CUAlO.sub.2 and CuGaO.sub.2, said p-type layer being formed on said n-type ZnO layer
to provide a p-n junction therebetween,
wherein said transparent substrate is yttria-stabilized zirconia (YSZ) (111) single crystal
substrate having an atomically flat surface,
wherein said light-emitting diode further includes an indium-tin oxide (ITO) layer
heteroepitaxially grown on said transparent substrate to serve as a transparent negative
electrode layer, and
wherein the ZnO layer is heteroepitaxially grown on the ITO layer to serve as a
luminescent layer, the p-type semiconductor layer being formed on said ZnO layer to serve
as a hole-injection layer.
2. A light-emitting diode as defined in claim 1, wherein said p-type semiconductor layer is
a SrCu.sub.2 O.sub.2 layer, wherein the p-type semiconductor layer contains 20 atom % or
less of a univalent metal substituted for a Sr site of the SrCu.sub.2 O.sub.2 layer.
3. A method for producing a light-emitting diode which includes an n-type ZnO layer
formed on a transparent substrate and exhibiting only intrinsic luminescence in the vicinity
of a band gap thereof, and a p-type semiconductor layer including SrCu.sub.2 O.sub.2, said
- 93 -
p-type layer being formed on said n-type ZnO layer to provide a p-n junction therebetween,
said method comprising the steps of forming the n-type ZnO layer on the transparent
yttria-stabilized zirconia (YSZ) (111) single crystal substrate having an atomically flat
surface while keeping the substrate at a temperature in the range of 200 to 1200.degree. C.,
and forming on the ZnO layer the p-type semiconductor layer including SrCu.sub.2
O.sub.2 while keeping the substrate at a temperature in the range of 200 to 800.degree. C.
4. A method for producing a light-emitting diode which includes an n-type ZnO layer
formed on a transparent substrate and exhibiting only intrinsic luminescence in the vicinity
of a band gap thereof, and a p-type semiconductor layer including one of CuAlO.sub.2 and
CuGaO.sub.2, said p-type layer being formed on said n-type ZnO layer to provide a p-n
junction therebetween, said method comprising the steps of forming the n-type ZnO layer
on the transparent yttria-stabilized zirconia (YSZ) (111) single crystal substrate having an
atomically flat surface while keeping a substrate at a temperature in the range of 200 to
1200.degree. C., and forming on the ZnO layer the p-type semiconductor layer including
CuAlO.sub.2 or CuGaO.sub.2 while keeping the substrate at a temperature in the range of
500 to 800.degree. C.
5. A method for producing a light-emitting diode which including an n-type ZnO layer
formed on a transparent substrate and exhibiting only intrinsic luminescence in the vicinity
of a band gap thereof, and a p-type semiconductor layer selected from the group consisting
of SrCu.sub.2 O.sub.2, CuAlO.sub.2 and CuGaO.sub.2, said p-type layer being formed on
said n-type ZnO layer to provide a p-n junction therebetween, said method comprising the
steps of forming the n-type ZnO layer on the transparent yttria-stabilized zirconia (YSZ)
(111) single crystal substrate having an atomically flat surface without heating the
substrate, irradiating the surface of the ZnO layer with ultraviolet light to promote
crystallization therein, and forming on the ZnO layer the p-type semiconductor layer
selected from the group consisting of SrCu.sub.2 O.sub.2, CuAlO.sub.2 and CuGaO.sub.2
without heating the substrate, and irradiating the surface of the p-type semiconductor layer
with ultraviolet light to promote crystallization therein.
6. A method as defined in either one on claims 3 to 5, which further includes the steps of
optically polishing an yttria-stabilized zirconia (YSZ) single crystal, and heating said
polished YSZ single crystal at a temperature in the range of 1000 to 1300.degree. C. to
prepare the transparent substrate having an atomically flat surface.
7. An ultraviolet-light-emitting laser diode comprising:
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an n-type ZnO layer formed on a transparent yttria-stabilized zirconia (YSZ) (111) single
crystal substrate having an atomically flat surface and exhibiting only intrinsic
luminescence in the vicinity of a band gap thereof; and
a p-type semiconductor layer selected from the group consisting of SrCu.sub.2 O.sub.2,
CuAlO.sub.2 and CuGaO.sub.2, said p-type layer being formed on said n-type ZnO layer
to provide a p-n junction therebetween,
wherein a Mg-substituted ZnO is heteroepitaxially grown on said single crystal substrate,
wherein said n-type ZnO layer is heteroepitaxially grown on said Mg-substituted ZnO, and
wherein said p-type semiconductor layer includes; a first p-type semiconductor layer
having a relatively lower carrier concentration and serving as a hole-injection layer, and a
second p-type semiconductor layer having a relatively high carrier concentration, said
second p-type semiconductor layer being formed on said first p-type semiconductor layer.
8. An ultraviolet-light-emitting diode as defined in claim 1, which further includes a Ni
layer formed on the p-type semiconductor layer to serve as a positive electrode.
Description
TECHNICAL FIELD
The present invention relates to a light-emitting diode and a laser diode capable of emitting
ultraviolet light through current injection.
BACKGROUND ART
High-density recording media have been significantly developing in line with advances in
information technologies. For example, read-write media in an optical recording system
have been shifted from compact disks to digital video disks (DVDs) capable of recording
in higher density. The reading and writing operations in such optical disks are performed
through the medium of light. This implies the possibilities of higher recording density by
use of light having a shorter wavelength.
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From this point of view, as a semiconductor laser or laser diode (hereinafter referred to as
"LD"), GaAlAs infrared-LDs for use in compact disks and GaInAlP infrared-LDs for use
in DVDs have come into practical use. Further, various researches are carrying out toward
the practical use of other LDs, such as a GaN light-emitting diode capable of emitting blue
light having shorter wavelength.
Light-emitting diodes (hereinafter referred to as "LEDs") are predominantly used as
displays, and practical applications of GaAs, GaP and GaN LEDs have open the way for
three-color display. Researches of an ultraviolet LED are also carrying out forward
applications to a backlight for liquid crystal displays or a light source for bactericidal
devices or ultraviolet-cure resins.
Zinc oxide (hereinafter referred to as "ZnO") is known as one of luminescent materials
emitting light having a shorter wavelength than that of GaN. ZnO is widely used as
green-color fluorescent materials, for example, in low-energy-electron-impact type
electroluminescence (EL) devices, and researches are also carrying out forward application
to a transparent conductive film for solar cells by taking advantage of its high electrical
conductivity and an optical transparency in a visible wavelength range.
It is known that ZnO is a direct transition type semiconductor having a band gap of about
3.38 eV at room temperature, and exhibits fluorescence in an ultraviolet wavelength range
(about 350 nm at room temperature) by exciting with ultraviolet light. If light-emitting
diodes or laser diodes can be fabricated using ZnO, such diodes would be able to use as a
pumping source of fluorescent materials or high-density recording media.
DISCLOSURE OF INVENTION
(Problem to be Solved by the Invention)
Generally, it is required to join a p-type semiconductor to an n-type semiconductor to
fabricate a light-emitting diode or laser diode. While an n-type ZnO thin-film can be
fabricated without any difficulty, the technique of fabricating p-type ZnO thin-film
involves many challenges. In fact, the first article concerning this technique was just
reported in 1999 by Kawai, Osaka University, Japan. This article describes that a p-type
ZnO thin-film can be achieved by preparing a target made of a sintered material containing
Ga substituted for a part of Zn in ZnO and forming a film through a pulsed laser deposition
(PLD) method under N.sub.2 O gas so as to increase hole concentration of the film based
on a co-doping effect.
- 96 -
However, any other research organizations have not been able to verify that the ZnO
thin-film according to the above technique exhibits p-type semiconductor characteristics, at
the time this application was filed. ZnO inherently tends to transform readily into an n-type
semiconductor, and hardly fabricated as a stable p-type semiconductor. This complicates
fabrication of LEDs to be actuated by current injection to its p-n junction.
It has not been reported any diode formed by joining an n-type ZnO semiconductor to a
p-type ZnO semiconductor. SrCu.sub.2 O.sub.2 is one of p-type semiconductors suitable
for joining to the n-type ZnO semiconductor. SrCu.sub.2 O.sub.2 is described as an
indirect-transition type semiconductor having a band gap of about 3.2 eV at room
temperature. To the contrary, calculations of its energy band suggest that it is a
direct-transition type semiconductor. In addition, SrCu.sub.2 O.sub.2 exhibits p-type
conductivity by adding K.sup.+ ions (Kudo, Yanagi, Hosono, Kawazoe, APL, 73, 220
(1998)).
The article of Kudo et al. describes as follows.
The carrier concentration and mobility of a SrCu.sub.2 O.sub.2 thin-film fabricated
through the PLD method are 1.times.10.sup.-3 cm.sup.-3 and 0.5 cm.sup.2 /Vs,
respectively. It has a pyramidal quadratic system (space group: 141/a) and a lattice
constant of a=b=0.5480 nm and c=0.9825 nm. While the lattice matching between a ZnO
(0001) surface and a SrCu.sub.2 O.sub.2 (112) surface is 19%, SrCu.sub.2 O.sub.2 can be
heteroepitaxially grown on ZnO because quintuple of the lattice constant of SrCu.sub.2
O.sub.2 is approximately equal to sextuple of the lattice constant of ZnO. Further, they can
be formed as a single crystal phase if a substrate has a temperature of 200.degree. C. or
more.
Kudo et al. confirmed that diode characteristics were yielded by forming a n-type ZnO film
on a SrCu.sub.2 O.sub.2 film (Kudo, Yanagi, Hosono, Kawazoe, Yano, APL, 75, 2851).
However, a ZnO film having desirable crystallinity could not be obtained because in the
fabrication process of Kudo et al., the SrCu.sub.2 O.sub.2 film is formed on a substrate and
then the ZnO film is formed on the SrCu.sub.2 O.sub.2 film. Specifically, for assuring the
desirable crystallinity in a ZnO film, the substrate must be heated up to 500.degree. C. or
more, which leads to vanished diode characteristics. As a result, Kudo et al. could not
confirm any luminescence from the diode.
CuAlO.sub.2 and CuGaO.sub.2 are also p-type semiconductors suitable for joining to the
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n-type ZnO semiconductor. CuAlO.sub.2 discovered and reported by H. Kawazoe et al.
(Nature, vol.389, p.939 (1997)) is a semiconductor having a so-called delafossite-type
crystal structure and exhibiting p-type conductivity. CuAlO.sub.2 has a band gap of 3.1 eV
or more, and may provide a thin film having a resistivity of 1.OMEGA..
CuGaO.sub.2 is also a semiconductor having a so-called delafossite-type crystal structure
and exhibiting p-type conductivity. It is conceivable that these p-type transparent
semiconductors have adaptability to fabrication of diodes, but there has not been any actual
case of fabrication of diodes or light-emitting diodes from these materials.
(Means for Solving Problems)
The present invention provides a light-emitting diode comprising an n-type ZnO layer
having desirable crystallinity and a p-type semiconductor layer selected from the group
consisting of SrCu.sub.2 O.sub.2, CuAlO.sub.2 and CuGaO.sub.2. The p-type layer is
formed on the n-type ZnO layer to provide a p-n junction allowing the ZnO layer to emit
ultraviolet light.
The present invention further provides a method for producing a light-emitting diode. This
method comprises the steps of forming an n-type ZnO layer on a transparent substrate
having a temperature which allows the ZnO layer to be formed with desirable crystallinity,
and forming on the ZnO layer a p-type semiconductor layer selected from the group
consisting of SrCu.sub.2 O.sub.2, CuAlO.sub.2 and CuGaO.sub.2.
More specifically, according to a first aspect of the present invention, there is provided an
ultraviolet-light-emitting diode comprising an n-type ZnO layer formed on a transparent
substrate and exhibiting only intrinsic luminescence in the vicinity of a band gap thereof,
and a p-type semiconductor layer selected from the group consisting of SrCu.sub.2 O.sub.2,
CuAlO.sub.2 and CuGaO.sub.2. The p-type layer is formed on the n-type ZnO layer to
provide a p-n junction therebetween.
In the light-emitting diode according to the first aspect of the present invention, the
transparent substrate may be a single crystal substrate. This single crystal substrate may
have an atomically flat yttria-stabilized zirconia (YSZ) (111) surface.
The light-emitting diode according to the first aspect of the present invention may further
include a transparent electrode inserted between the transparent substrate and the ZnO
layer. The transparent electrode serves as an electrode for the ZnO layer.
- 98 -
The light-emitting diode may include a Ni layer formed on the p-type semiconductor layer.
The Ni layer serves as an electrode for the p-type semiconductor layer.
The transparent substrate may include an indium-tin oxide (ITO) layer heteroepitaxially
grown thereon to serve as a transparent negative electrode layer. In this case, the ZnO layer
is heteroepitaxially grown on the ITO layer to serve as a luminescent layer, and the p-type
semiconductor layer is formed on the ZnO layer to serve as a hole-injection layer. Further,
the p-type semiconductor layer includes a Ni layer formed thereon to serve as a positive
electrode.
In the light-emitting diode according to the first aspect of the present invention, the p-type
semiconductor layer may be a SrCu.sub.2 O.sub.2 layer containing a univalent metal
element of 20 atom % substituted for the Sr site thereof.
According to a second aspect of the present invention, there is provided a method for
producing the light-emitting diode according to the first aspect of the present invention.
This method comprises the steps of forming the n-type ZnO layer on the transparent
substrate while keeping the substrate at a temperature in the range of 200 to 1200.degree.
C., and forming on the ZnO layer the p-type semiconductor layer including SrCu.sub.2
O.sub.2 while keeping the substrate at a temperature in the range of 200 to 800.degree. C.
According to a third aspect of the present invention, there is provided a method for
producing the light-emitting diode according to the first aspect of the present invention.
This method comprises the steps of forming the n-type ZnO layer on the transparent
substrate while keeping a substrate at a temperature in the range of 200 to 1200.degree. C.,
and forming on the ZnO layer the p-type semiconductor layer including CuAlO.sub.2 or
CuGaO.sub.2 while keeping the substrate at a temperature in the range of 500 to
800.degree. C.
According to a fourth aspect of the present invention, there is provided a method for
producing the light-emitting diode according to the first aspect of the present invention.
This method comprises the steps of forming the n-type ZnO layer on the transparent
substrate without heating the substrate, irradiating the surface of the ZnO layer with
ultraviolet light to promote crystallization therein, and forming on the ZnO layer p-type
semiconductor layer selected from the group consisting of SrCu.sub.2 O.sub.2,
CuAlO.sub.2 and CuGaO.sub.2 without heating the substrate, and irradiating the surface of
the p-type semiconductor layer with ultraviolet light to promote crystallization therein.
- 99 -
Either one of the methods according to the second to fourth aspects of the present
invention may further include the steps of optically polishing an yttria-stabilized zirconia
(YSZ) single crystal, and heating said polished YSZ single crystal at a temperature in the
range of 1000 to 1300.degree. C. to prepare the transparent substrate having an atomically
flat surface.
BEST MODE FOR CARRYING OUT THE INVENTION
A light-emitting diode of the present invention can be converted into a laser diode by
forming a resonant structure therein. Specifically, each of the end faces of the
light-emitting diode along its long axis is subjected to a reactive etching to provide a
stripe-geometry structure thereon, and then total and partial reflection mirrors are
fabricated in the end faces, respectively, to form a Fabry-Perot resonator. Each of the
SrCu.sub.2 O.sub.2 and ITO layers have a wider forbidden band and a smaller refractive
index than those of the ZnO layer, and thereby acts as a current/optical confinement layer
for confining longitudinal current and light within the ZnO layer. The
stripe-geometry-structure in each of the end faces also acts as a current/optical confinement
layer for confining lateral current and light within the ZnO layer. High-efficiency
luminescence can be obtained by injecting current into this stripe-geometry-structure diode
in the forward direction, and a laser oscillation can be obtained by a threshold or more of
current injection.
The optical confinement effect is enhanced by increasing the difference between the
respective refractive indexes of the SrCu.sub.2 O.sub.2 and ITO layers, and the enhanced
optical confinement effect can provide a lowered threshold of laser oscillation. For this
purpose, CuAlO.sub.2 or CuGaO.sub.2 may be used as a substitute for SrCu.sub.2 O.sub.2.
Alternatively, a Mg-substituted ZnO layer may be inserted between the ZnO and ITO
layers.
A desirable ohmic electrode is obtained by forming a p-type semiconductor layer having a
relatively high carrier concentration on a p-type semiconductor layer having a relatively
low carrier concentration to provide a p-n junction therebetween and forming an electrode
on the former layer, and this ohmic electrode can provide a lowered current injection
threshold.
In order to allow the ZnO layer in the diode to exhibit only desirable intrinsic
luminescence in the vicinity of the band-gap thereof, the half bandwidth in the rocking
- 100 -
curve of (0002) surface of ZnO crystal phase should be sufficiently narrow, or one degree
or less, in the X-ray diffraction method. The half bandwidth is preferably 0.5 degree or less,
more preferably 0.3 degree or less. The half bandwidth correlates with the crystallinity in
the ZnO layer.
The light-emitting diode of the present invention is characterized by emitting an ultraviolet
light of 380 nm, or emitting only ultraviolet light with deleting a green color. Specifically,
for exhibiting only intrinsic luminescence in the vicinity of the band gap the crystallinity in
the ZnO layer should be sufficiently enhanced to reduce oxygen defects and excessive Zn
ion concentration in ZnO lattice.
Preferably, the transparent substrate sufficiently transmits therethrough the luminescence
of 380 nm wavelength from the ZnO layer at room temperature. The transmissivity at 380
nm is preferably in the range of 50 to 100%, more preferably 80 to 100%.
While the transparent substrate may include a plastic substrate such as polycarbonate or
poly methyl methacrylate, a glass substrate such as quartz glass or heat-resistant glass, or a
crystalline substrate such as yttria-stabilized zirconia (YSZ) (111) surface or sapphire
(0001) surface, it is required to have chemical properties sustainable to the process for
forming the ZnO layer, SrCu.sub.2 O.sub.2 layer, CuAlO.sub.2 layer or CuGaO.sub.2 layer.
Preferably, both surfaces of the glass or crystalline substrate are optically polished to
provide increase transmissivity.
If the crystalline substrate is used as the transparent substrate, the structural regularity in
the crystal faces of the substrate is reflected to the crystallinity in the ZnO layer. This
advantageously provides enhanced crystallinity in the ZnO layer and improved luminescent
characteristics. The crystalline substrate such as YSZ (111) surface or sapphire (0001)
surface preferably achieves adequate lattice matching with ZnO crystal lattice.
When a transparent negative electrode layer is inserted between the transparent substrate
and the ZnO layer as described later, the substrate is preferably made of a crystal capable
of achieving adequate lattice matching with the material of the transparent negative
electrode layer. For example, when the transparent negative electrode layer is made of
indium-tin oxide (ITO), YSZ (111) surface is particularly suitable for the substrate,
because the lattice of ITO is matched with YSZ so well.
The ZnO layer having desirable crystallinity is formed on the transparent substrate. The
carrier concentration of the ZnO layer should be in the range of 1.times.10.sup.17 to
- 101 -
1.times.10.sup.20 /cm.sup.3. If the carrier concentration is less than 1.times.10.sup.17
/cm.sup.3, the depletion layer in the p-n junction region will have an excessively increased
thickness unsuited to luminescent. If the carrier concentration is greater than
1.times.10.sup.20 /cm.sup.3, the depletion layer will have a too thin thickness unsuited to
luminescent. Preferably, the carrier concentration of the ZnO layer is in the range of
1.times.10.sup.18 to 1.times.10.sup.19 /cm.sup.3
A process for forming the SrCu.sub.2 O.sub.2 layer on the ZnO layer will be described in
detail below. The carrier concentration of the SrCu.sub.2 O.sub.2 layer is the range of
1.times.10.sup.16 to 1.times.10.sup.20 /cm.sup.3. If the carrier concentration is less than
1.times.10.sup.16 /cm.sup.3, the ZnO layer will have reduced injectable electron holes
unsuited to luminescent. The carrier concentration greater than 1.times.10.sup.20 /cm.sup.3
leads to deteriorated luminous efficiency unsuited to luminescent.
In the light-emitting diode of the present invention, luminescent can be obtained by
additionally forming negative and positive electrodes on the ZnO and SrCu.sub.2 O.sub.2
layers, respectively. The negative electrode for applying voltage may be made of a material
allowing an ohmic contact with the ZnO layer, and the positive electrode may be made of a
material allowing an ohmic contact with the SrCu.sub.2 O.sub.2 layer. Ag is typically used
as the electrode material allowing an ohmic contact with the ZnO layer.
It is necessary to use a material having a small work function, such as Ni or Pt, as the
material allowing an ohmic contact with the SrCu.sub.2 O.sub.2 layer. If a material having
a large work function such as Au or Ag, the ohmic contact cannot be achieved because of
small work function of the SrCu.sub.2 O.sub.2 layer.
Each contact surface between the layers may be formed by using these materials. For
example, a Cu wire with Ag covering the surface thereof may be used as the positive
electrode, and a Cu wire with Ni covering the surface thereof may be used as the negative
electrode. Then, these wires may be attached to the layers with solder. In this case, a notch
can be formed in the SrCu.sub.2 O.sub.2 layer to expose outside a part of the surface of the
ZnO layer so as to allow the positive electrode to be attached to the ZnO layer.
In the light-emitting diode of the present invention, a negative electrode layer may be
formed between the transparent substrate and the ZnO layer, and a positive electrode layer
may be formed on the SrCu.sub.2 O.sub.2 layer. This structure can eliminate the need for
adequately coating the lead wires to be connected to the light-emitting diode, and thereby
the lead wires such as Cu wires without coating may be connected to the negative and
- 102 -
positive electrode layers, respectively. The negative electrode layer is made of a
transparent electrode material to allow luminescence from the ZnO layer to be extracted to
outside through the negative electrode layer and the transparent substrate.
A suitable transparent electrode material for the negative electrode layer may include ITO,
AZO (Al-doped ZnO), GZO (Ga-doped ZnO), InGaO.sub.3 (ZnO).sub.m (where m is a
natural number), SnO.sub.2, and Ga.sub.3 O.sub.3. When a single crystal substrate is used
as the transparent substrate, it is preferable to use a material capable of achieving adequate
lattice matching between respective materials of the substrate and the ZnO layer. For
example, when a YSZ (111) substrate is used as the transparent substrate, a suitable
material for the negative electrode layer includes ITO, AZO, GZO, InGaO.sub.3
(ZnO).sub.m, or In.sub.2 O.sub.3 (ZnO).sub.m.
If a transparent electrode layer is applicable to the positive electrode layer, luminescence
from the ZnO layer can be extracted to outside through the SrCu.sub.2 O.sub.2 layer and
the positive electrode layer. However, any suitable transparent electrode material for the
positive electrode layer has not been discovered, and thereby the positive electrode is made
of a metal such as Ni or Pt. An additional metal layer may be formed on the positive
electrode material to provide enhanced connectivity with a lead wire or the like.
In the light-emitting diode of the present invention, a CuAlO.sub.2 layer or CuGaO.sub.2
layer may be used as a substitute for the SrCu.sub.2 O.sub.2 layer. Luminescence can be
obtained by applying a negative voltage to the ZnO layer and a positive voltage to the
CuAlO.sub.2 layer or the CuGaO.sub.2 layer In this case, any suitable material allowing an
ohmic contact with the CuAlO.sub.2 layer or the CuGaO.sub.2 layer, such as Ni or Pt
having a small work function, may be used as the positive electrode material. Further, a
positive electrode layer may be formed on the CuAlO.sub.2 layer or the CuGaO.sub.2
layer, and may be made of a metal such as Ni or Pt. An additional metal layer may be
formed on the positive electrode material to provide enhanced connectivity with a lead
wire or the like.
The term "Mg-substituted ZnO" herein means ZnO in which Zn site of ZnO crystal is
substituted with Mg ions, and can be expressed by a chemical formula (Zn.sub.1-x
Mg.sub.x)O.sub.2 where 0<x<0.2. One of p-type semiconductor layers consisting of the
SrCu.sub.2 O.sub.2, CuAlO.sub.2 and CuGaO.sub.2 layers has a lower carrier
concentration. For example, in the SrCu.sub.2 O.sub.2 layer, the lower carrier
concentration may be obtained by using SrCu.sub.2 O.sub.2 as-is or reducing down the
amount of K to be added as a dopant. The carrier concentration is arranged, for example, in
- 103 -
the range of 1.times.10.sup.16 /cm.sup.3 to 1.times.10.sup.19 /cm.sup.3. One of p-type
semiconductor layers consisting of the SrCu.sub.2 O.sub.2, CuAlO.sub.2 and CuGaO.sub.2
layers has a higher carrier concentration, and is preferably made of the same material as
that of the p-type semiconductor having the lower carrier concentration. For example, in
the SrCu.sub.2 O.sub.2 layer, the higher carrier concentration may be obtained by
increasing the amount of K to be added as a dopant. This carrier concentration is required
to be higher than that of the p-type semiconductor layer having the lower carrier
concentration, and thereby arranged, for example, in the range of 1.times.10.sup.17
/cm.sup.3 to 1.times.10.sup.20 /cm.sup.3.
The light-emitting diode of the present invention is produced through a film forming
method. The film forming method may include PLD, MBE, sputtering, vacuum
evaporation and CVD methods. It is important to select a method capable of forming a
ZnO film having desirable crystallinity without undesirable change in properties of the
substrate. Various methods such as the PLD, sputtering, CVD or MBE method may be
used to forming the SrCu.sub.2 O.sub.2 layer on the ZnO layer. While the PLD method is
suitable to form the ZnO and SrCu.sub.2 O.sub.2 layers with desirable crystallinity, it has a
problem in mass production due to a limited film area, for example, of about 20 mm
diameter. Fortunately, a PLD apparatus capable of uniformly forming a film having about
6-inch diameter has been recently placed on the market.
The sputtering method is suitable for mass production because it can form a film having a
large area. However, as compared to the PLD method, it cannot provide enhanced
crystallinity in the ZnO and SrCu.sub.2 O.sub.2 layers due to exposure of the films to
plasmas. However, some apparatus such as a Helicon sputter apparatus or ion sputter
apparatus capable of preventing exposure of the films to plasmas have been recently placed
on the market.
The CVD method is suitable to form a large film with desirable homogeneity in the ZnO
and SrCu.sub.2 O.sub.2 layers. However, impurities such as C contained in gases of
material tend to mixed into the layers. While the MBE method is suitable to form a film
with enhanced crystallinity in the ZnO and SrCu.sub.2 O.sub.2 layers as with the PLD
method, it is required to introduce oxygen gas into a film-forming vessel, and thereby the
surface of metal will be oxidized, resulting in difficulty in producing molecular beams.
While the vacuum evaporation method is one of simplest and easiest method, it has
disadvantages of difficulty in forming a large size film and controlling the chemical
composition of the SrCu.sub.2 O.sub.2 layer. As above, each of the film forming methods
- 104 -
has different features, and one suitable film forming method may be selected with focusing
on its features meeting the purpose.
The applicable film forming method can be limited by the substrate material. In a plastic
substrate used as the transparent substrate, if the substrate is heated up to a temperature, for
example, of 100.degree. C. or more, undesirable transformation is caused in the substrate.
Thus, it is required to form a film at a temperature lower than that causing such an
undesirable transformation. A process required for promoting an oxidative reaction of
materials on the surface of the substrate, such as the CVD or MBE method, is unsuitable in
this case.
The PLD or sputtering method allows the ZnO and SrCu.sub.2 O.sub.2 layers to be formed
on the plastic substrate. In this case, it is desired to promote crystallization through a
suitable method such as light irradiation because such methods themselves cannot provide
sufficiently enhanced crystallinity in the layers. For example, in the sputtering method, the
ZnO layer is formed without heating the substrate or under a film forming condition at
room temperature.
By virtue of low crystallization temperature of ZnO, the ZnO layer can be formed with
desirable crystallinity at room temperature. However, in order to obtain enhanced luminous
efficiency and produce a desirable light-emitting diode having increased brightness, the
crystallinity of the ZnO layer is preferably enhanced as much as possible. For this purpose,
it is desired to promote crystallization by irradiating the ZnO layer with ultraviolet light
such as Kr F excimer laser.
Subsequently, the SrCu.sub.2 O.sub.2 layer is formed on the ZnO layer through the
sputtering method at room temperature, and then the formed SrCu.sub.2 O.sub.2 layer is
irradiated with ultraviolet light to promote crystallization therein. A transparent negative
electrode to be sandwiched between the plastic layer and the ZnO layer can be formed in
the same way. A metal positive electrode to be formed on the SrCu.sub.2 O.sub.2 layer can
be provided only through a film forming process at room temperature. Even if the metal
layer is irradiated with ultraviolet light, the light is reflected by the metal surface and
thereby any effect of transformation cannot be expected.
In either of the film forming methods, when a glass or single crystal substrate is used as the
transparent substrate, during the formation of the ZnO layer, the substrate can be heated up
to a temperature of 1000.degree. C. Thus, the crystallinity in the ZnO layer can be
sufficiently enhanced within the temperature. The temperature for forming the ZnO layer is
- 105 -
preferably in the range of 200.degree. C. to 1200.degree. C. The crystallization is not
sufficiently promoted at a temperature less than 200.degree. C., while the components of
the ZnO layer will be vaporized at a temperature greater than 1200.degree. C.
When the transparent negative electrode is sandwiched between the transparent substrate
and the ZnO layer, the ZnO layer should be formed at a selected temperature preventing
the respective materials of the transparent negative electrode layer and the ZnO layer from
reacting with each other at the boundary face therebetween. For example, when the
transparent negative electrode layer is made of ITO, the temperature for forming the ZnO
layer is limited to the range of 200.degree. C. to 1000.degree. C. At a temperature greater
than 1000.degree. C., ITO and ZnO are reacted with each other to form another phase, and
thereby a desirable boundary face cannot be formed therebetween.
The temperature for forming the SrCu.sub.2 O.sub.2 layer may be selectively arranged in
the range of 200.degree. C. to 800.degree. C. The SrCu.sub.2 O.sub.2 layer is not
crystallized at a temperature less than 200.degree. C., and will be reacted with the
underlying ZnO layer at a temperature greater than 800.degree. C., resulting in undesirable
boundary face between the ZnO and SrCu.sub.2 O.sub.2 layers.
The method for forming the SrCu.sub.2 O.sub.2 layer may be applied to form the
CuAlO.sub.2 layer or CuGaO.sub.2 layer as a substitute for the SrCu.sub.2 O.sub.2 layer.
The temperature for forming the CuAlO.sub.2 layer or CuGaO.sub.2 layer may be
selectively arranged in the range of 500.degree. C. to 800.degree. C. The CuAlO.sub.2
layer or CuGaO.sub.2 layer is not crystallized at a temperature less than 500.degree. C.,
and will be reacted with the underlying ZnO layer at a temperature greater than 800.degree.
C., resulting in undesirable boundary face between the ZnO and CuAlO.sub.2 layers or the
ZnO and CuGaO.sub.2 layers.
Particularly, by producing the light-emitting diode of the present invention, for example,
on a YSZ (111) single crystal substrate through the PLD method used as a film forming
method, the ZnO layer can be formed with enhanced crystallinity and desirable boundary
face between the ZnO and SrCu.sub.2 O.sub.2 layers to achieve excellent luminous
efficiency in the obtained light-emitting diode.
Laser such as Kr F or Ar F excimer laser having a light energy greater than the band gap of
the ZnO and SrCu.sub.2 O.sub.2 layers is used as a light source for irradiating the target.
Laser having a light energy less than the band gap is not absorbed by the ZnO or
SrCu.sub.2 O.sub.2 target and thereby any laser ablation cannot be caused.
- 106 -
The laser having the light energy greater than the band gap is absorbed by the ZnO or
SrCu.sub.2 O.sub.2 target to cause laser ablation so that the target material is deposited on
the substrate placed opposedly to the target to form a film. Since vacuum ultraviolet light
will be inherently absorbed by oxygen in air, it is required to form vacuum along the
optical path. This leads to complicated structure, difficult management and increased cost
in the apparatus. In contrast, the Kr F excimer laser can advantageously provide
sufficiently strong light because it is not absorbed by oxygen in air, and related apparatuses
are widely place on the market.
For example, when a YSZ (111) substrate is used as the transparent substrate, the ZnO
layer can be formed with desirable crystallinity, and ITO can be used as the transparent
negative electrode. This allows a light-emitting diode to be produced with excellent
luminous efficient. Because, The YSZ (111) surface can achieve sufficient lattice matching
with the ITO (111) surface, and the ITO (111) can achieve sufficient lattice matching with
the ZnO (0001) surface. Preferably, the YSZ (111) surface is sufficiently flattened to
utilize the above feature in lattice matching.
It is known that the surface of Al.sub.2 O.sub.3 single crystal substrate, SrTiO.sub.3 single
crystal substrate or the like can be flattened by processing at high temperature under
vacuum or ambient pressure to the extent that step and terrace structure can be observed.
Such structure is generally referred to as "atomically flat surface"
The inventors has discovered that a similar atomically flat surface could be formed by
subjecting YSZ single crystal having both faces optically polished to a heat treatment at a
temperature in the range of 1000.degree. C. to 1300.degree. C., and the heat-treated YSZ
single crystal was suited to use as a substrate for the light-emitting diode of the present
invention. The substrate having the atomically flat surface is disposed opposedly to each
target with a distance, for example, in the range of 30 to 70 mm, therebetween. Preferably,
each of the target and substrate is rotated in its axis by a rotating mechanism.
It is desired to provide an ultimate vacuum of 1.times.10.sup.-5 Pa in the vacuum vessel to
remove water vapor from the vessel. The process for removing water vapor is a critical
point in the entire processes because the SrCu.sub.2 O.sub.2 readily causes chemical
reaction with water. After water vapor is removed by sufficiently increasing the vacuum in
the vessel, dry oxygen is introduced into the vessel.
In the process for forming the ITO negative electrode layer, the oxygen gas having a
- 107 -
pressure in the range of 1.times.10.sup.-4 Pa to 100 Pa is introduced into the vessel. At a
pressure less than 1.times.10.sup.-4 Pa, metal In undesirably separates out on the substrate.
At a pressure greater than 100 Pa, plume to be formed by irradiating the target with laser
become small, and thereby a film cannot be effectively formed.
The substrate can have a temperature in the range of 300.degree. C. to 1200.degree. C. At a
temperature less than 300.degree. C., the crystallization of ITC is not sufficiently promoted
and thereby desirable luminous characteristics cannot be expected. At a temperature greater
than 1200.degree. C., components of ITO will be vaporized, resulting in ineffective
formation of a film. Preferably, the temperature of the substrate is in the range of
500.degree. C. to 900.degree. C. Within this temperature range, an ITO film
heteroepitaxially grown on YSZ (111) surface can be formed.
For example, an ITO sintered body containing 10 wt % of SnO.sub.2 is used as the target.
Preferably, the target is sufficiently densified. The ITO layer preferably has a thickness in
the range of 50 nm to 2000 nm. If the ITO has a thickness less than 50 nm, this thin
thickness causes high resistance, and thereby the negative electrode cannot adequately
function. If the ITO has a thickness greater than 2000 nm, this thick thickness causes
lowered optical transmissivity, resulting in reduced amount of light to be extracted outside.
It is required to select adequate energy density of laser because the energy density has an
impact on the crystallinity, grain structure, surface flatness and transparent conductivity in
the ITO layer. The energy density of laser is a value depending on the apparatus. In the
PLD apparatus described in an example described later, a desired film could be obtained
by selecting the energy density in the range of 1 to 10 J/cm.sup.2.
In the process for forming the ZnO layer, the oxygen gas having a pressure in the range of
1.times.10.sup.-4 Pa to 100 Pa is introduced into the vessel. At a pressure less than
1.times.10.sup.-4 Pa, metal Zn undesirably separates out on the substrate. At a pressure
greater than 100 Pa, plume to be formed by irradiating the target with laser become small,
and thereby a film cannot be effectively formed.
The substrate can have a temperature in the range of 300.degree. C. to 1000.degree. C. At a
temperature less than 300.degree. C., the crystallization of ZnO is not sufficiently
promoted and thereby desirable luminous characteristics cannot be expected. At a
temperature greater than 1000.degree. C., the ITO layer will be reacted with the ZnO layer
and thereby a desirable boundary face cannot be formed between the ITO and ZnO layers.
Preferably, the temperature of the substrate is in the range of 500.degree. C. to 800.degree.
- 108 -
C. Within this temperature range, the ZnO (0001) surface can be heteroepitaxially grown
on the ITO (111) surface.
A ZnO sintered body is used as the target. Preferably, the target is sufficiently densified.
The ZnO layer preferably has a thickness in the range of 20 nm to 2000 nm. If the ZnO
layer has a thickness less than 20 nm, this thin thickness cannot cause effective
luminescence. If the ZnO layer has a thickness greater than 2000 nm, this thick thickness
causes lowered optical transmissivity, resulting in reduced intensity of light to be extracted
outside.
It is required to select adequate energy density of laser because the energy density has an
impact on the crystallinity, grain structure, surface flatness and transparent conductivity in
the ZnO layer. The energy density of laser is a value depending on the apparatus. In the
PLD apparatus described in the example, a desired film could be obtained by selecting the
energy density in the range of 1 to 10 J/cm.sup.2.
The surface of the ZnO layer should be sufficiently flattened in the step for forming the
SrCu.sub.2 O.sub.2 layer to provide a desirable boundary face between the ZnO and
SrCu.sub.2 O.sub.2 layers. It is generally known that the PLD method tends to form
semispherical protrusions, so-called droplets, on the surface of a thin film. These
protrusions undesirably form a p-n junction on the boundary face between the ZnO and
SrCu.sub.2 O.sub.2 layers. This is significantly disadvantageous to effectively inject
conductive holes from the SrCu.sub.2 O.sub.2 layer into the ZnO layer and achieve
recombination of the conductive holes and electrons.
From this point of view, the surface of the ZnO layer is preferably flattened by subjecting
to annealing at 800.degree. C. to 1200.degree. C. in a vacuum vessel, or irradiating the
surface of the ZnO layer with a gas cluster beam, or taking it out of the vacuum vessel and
polishing with a polishing agent. The insufficient flatness of the surface causes degraded
luminous efficiency or can provide a non-luminiferous diode, resulting in significantly
deteriorated yield ratio.
In the process for forming the SrCu.sub.2 O.sub.2 layer, the oxygen gas having a pressure
in the range of 1.times.10.sup.-4 Pa to 100 Pa is introduced into the vessel. At a pressure
less than 1.times.10.sup.-4 Pa, metal Sr or Cu undesirably separates out on the substrate. At
a pressure greater than 100 Pa, plume to be formed by irradiating the target with laser
become small, and thereby a film cannot be effectively formed.
- 109 -
The substrate can have a temperature in the range of 250.degree. C. to 800.degree. C. At a
temperature less than 250.degree. C., the crystallization of SrCu.sub.2 O.sub.2 is not
sufficiently promoted and thereby desirable luminous characteristics cannot be expected.
At a temperature greater than 800.degree. C., the SrCu.sub.2 O.sub.2 layer will be reacted
with the ZnO layer and thereby a desirable boundary face cannot be formed between the
ZnO and SrCu.sub.2 O.sub.2 layers. Preferably, the temperature of the substrate is in the
range of 300.degree. C. to 550.degree. C. Within this temperature range, the SrCu.sub.2
O.sub.2 layer can be formed on the ZnO (0001) surface. In particular, by selecting the
temperature around 500.degree. C., the SrCu.sub.2 O.sub.2 layer can be heteroepitaxially
grown on the ZnO (0001) surface.
A SrCu.sub.2 O.sub.2 sintered body is used as the target. A univalent metal of 20 atom %
or less as a dopant may be substituted for the Sr site thereof. For example, the hole
concentration of the film can be increased by adding 0.3 to 5 mol % of K therein. The
target is sintered under inert gas such as N.sub.2 or Ar.
Preferably, the target is sufficiently densified. The densification is relatively difficult to
achieve through an ordinary method, but a hot pressing method or a hot isostatic pressing
method is suitable for this densification. The SrCu.sub.2 O.sub.2 layer preferably has a
thickness in the range of 20 nm to 2000 nm. If the SrCu.sub.2 O.sub.2 layer has a thickness
less than 20 nm, this thin thickness cannot cause effective injection of holes to the ZnO
layer. If the ZnO layer has a thickness greater than 2000 nm, this thick thickness is
unproductive.
It is required to select adequate energy density of laser because the energy density has an
impact on the crystallinity, grain structure, surface flatness and transparent conductivity in
the SrCu.sub.2 O.sub.2 layer. The energy density of laser is a value depending on the
apparatus. In the PLD apparatus described in the example, a desired film could be obtained
by selecting the energy density in the range of 1 to 10 J/cm.sup.2.
In the process for forming the CuAlO.sub.2 layer or CuGaO.sub.2 layer, the oxygen gas
having a pressure in the range of 1.times.10.sup.-4 Pa to 100 Pa is introduced into the
vessel. At a pressure less than 1.times.10.sup.-4 Pa, metal Cu, Al or Ga undesirably
separates out on the substrate. At a pressure greater than 100 Pa, plume to be formed by
irradiating the target with laser become small, and thereby a film cannot be effectively
formed.
The substrate can have a temperature in the range of 500.degree. C. to 800.degree. C. At a
- 110 -
temperature less than 500.degree. C., the crystallization of the CuAlO.sub.2 or
CuGaO.sub.2 is not sufficiently promoted and thereby desirable luminous characteristics
cannot be expected. At a temperature greater than 800.degree. C., the CuAlO.sub.2 or
CuGaO.sub.2 layer will be reacted with the ZnO layer and thereby a desirable boundary
face cannot be formed between the ZnO layer and the CuAlO.sub.2 or CuGaO.sub.2 layer.
Preferably, the temperature of the substrate is in the range of 650.degree. C. to 750.degree.
C. Within this temperature range, the CuAlO.sub.2 or CuGaO.sub.2 layer can be formed
on the ZnO (0001) surface. In particular, by selecting the temperature around 700.degree.
C., the CuAlO.sub.2 layer or CuGaO.sub.2 layer can be heteroepitaxially grown on the
ZnO (0001) surface.
A CuAlO.sub.2 or CuGaO.sub.2 sintered body is used as the target. The hole concentration
of the film can be increased by adding a univalent metal as a dopant, for example, 0.3 to 5
mol % of K therein. The target is sintered under inert gas such as N.sub.2 or Ar. Preferably,
the target is sufficiently densified. The densification is relatively difficult to achieve
through an ordinary method, but a hot pressing method or a hot isostatic pressing method
is suitable for this densification. The CuAlO.sub.2 or CuGaO.sub.2 layer preferably has a
thickness in the range of 20 nm to 2000 nm. If the CuAlO.sub.2 or CuGaO.sub.2 layer has
a thickness less than 20 nm, this thin thickness cannot cause effective injection of holes to
the ZnO layer. If the ZnO layer has a thickness greater than 2000 nm, this thick thickness
is unproductive.
It is required to select adequate energy density of laser because the energy density has an
impact on the crystallinity, grain structure, surface flatness and transparent conductivity in
the CuAlO.sub.2 or CuGaO.sub.2 layer. The energy density of laser is a value depending
on the apparatus. In the PLD apparatus described in the example, a desired film could be
obtained by selecting the energy density in the range of 1 to 10 J/cm.sup.2.
A Ni layer is particularly suitable for the positive electrode. The Ni layer can be formed
through any film forming method. When the Ni layer is formed through the PLD method
by use of a Ni target, any additional equipment is not require for forming the Ni film.
However, the film forming efficiency of this method is relatively low because the Ni target
reflects laser. In view of the film forming efficiency, a suitable method is the sputtering or
vacuum evaporation method. Further, a suitable metal layer may be formed on the Ni layer
to provide enhanced connectivity with a connecting wire such as a Cu wire.
Ni has a significantly low etching speed. If an electrode material suitable for being etched
and capable of achieving an ohmic contact with the SrCu.sub.2 O.sub.2 layer is available,
- 111 -
it is desired to use this material as the positive electrode. The same can be applied to the
case where the CuAlO.sub.2 layer or CuGaO.sub.2 layer is used as a substitute for the
SrCu.sub.2 O.sub.2 layer.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows an ultraviolet light emitting diode of the present invention, showing YSZ
(111) substrate 1; ITO layer 2; n-type ZnO layer 3; p-type layer 4; and Ni layer 5.
FIG. 2 shows an ultraviolet light emitting diode of the present invention, showing YSZ
(111) substrate 1; ITO layer 2; n-type (ZnMg)O layer 3; n-type ZnO layer 4; p-type hole
injection layer 5; p-type semiconduction layer 6: and Ni layer 7.
FIG. 3 shows a conventional light emitting diode, showing YSZ (111) substrate 1: Ni layer
(electrode) 2; p-type layer 3; n-type ZnO layer 4; and electrode 5.
EXAMPLE
The present invention will be described in detail in conjunction with the following example.
Example 1
Fabrication of Laminated Films
Targets of an In.sub.2 O.sub.3 (herein after referred to as "ITO") sintered body containing
10 wt % of SnO.sub.2, a ZnO sintered body, a Sr.sub.1-x K.sub.x Cu.sub.2 O.sub.2
sintered body (where x is a substitution rate of K ion substituted at Sr site, and
x .quadrature. 0.2) and metal Ni were prepared. These targets were placed in a PLD
chamber, and the vacuum of 1.times.10.sup.-6 Pa was provided in the chamber.
Then, a polished substrate having YSZ (111) surface having a surface roughness of 1 nm
or less was disposed opposedly to and above the target with a distance of 30 mm
therebetween. Oxygen gas having a pressure of 2.times.10.sup.-3 Pa as atmospheric gas
was introduced in the chamber. After the substrate was heated up to 900.degree. C., the
surface of the ITO target was irradiated with Kr F (248 nm) excimer laser pulses through a
silica glass window while arranging the energy density of laser in 6 J/cm.sup.2 for each
pulse.
- 112 -
The laser was stopped when the ITO thin-film had a thickness of 800 nm, and the
temperature of the substrate was set at 800.degree. C. Then, a ZnO thin-film was formed
while arranging the energy density of laser in 5 J/cm.sup.2 for each pulse. The laser was
interrupted when the ZnO thin-film had a thickness of 400 nm, and the temperature of the
substrate was set at 350.degree. C.
Then, a SrCu.sub.2 O.sub.2 thin-film was formed while arranging the energy density of
laser in 2 J/cm.sup.2 for each pulse. The laser was interrupted when the SrCu.sub.2
O.sub.2 thin-film had a thickness of 200 nm, and the temperature of the substrate was set at
25.degree. C. Then, the Ni thin-film was formed by irradiating the Ni target with laser. The
laser was interrupted when the Ni thin-film had a thickness of 20 nm, and the laminated
film was taken out to ambient air. In order to use a W probe coated with Ag as a lead wire
for injecting current, the surface of the Ni layer in the laminated film was coated with Au
through the sputtering method. The thickness of the Au thin-film was 100 nm.
Fabrication of Mesa Structure
The surface of the laminated film was coated with a commercially available photoresist
(AZ p4620) to provide its thickness of 5 .mu.m by spin coating (2000 rpm, 20 s), and then
dried at 90.degree. C. for 30 minutes. Then, the surface of the film was irradiated with
ultraviolet light (20 mW, 10 s) through a circular photomask having a diameter of
500 .mu.m, and immersed in a commercially available developer (AZ developer) to form a
pattern. In this state, adhesiveness of the pattern and etching resistance of the film were
insufficient. Thus, the film was subjected to a heat treatment at 110.degree. C. for 30
minutes and subsequently at 200.degree. C. for 1 h, under ambient air.
Reactive Ion Etching
A mesa structure was fabricated using CF.sub.4 gas and Ar gas through a reactive ion
etching method. The Au and Ni layers were etched using CF.sub.4 gas under a gas
pressure of 5 Pa, at an RF power of 250 W. Then, the SrCu.sub.2 O.sub.2, ZnO and ITO
layers were etched using Ar gas under a gas pressure of 4.5 Pa, at an RF power of 250 W.
The ITO layer was etched to the depth of 200 nm.
Electrical and Luminous Characteristics
The W probe was brought into contact with the ITO and Au regions of the above mesa
structure device, and the negative and positive poles were connected to the ITO and Au
- 113 -
region, respectively. When current was applied thereto, the current value rapidly increased
at an applied voltage of 0.3 V or more. This is one of characteristics of p-n junction diodes.
Luminescent rapidly increased at 0.3 V or more. The wavelength of the light was about
380 nm.
Comparative Example
Fabrication of Laminated Films
The film forming process of the example was reversed. That is, a SrCu.sub.2 O.sub.2
thin-film was first formed on a substrate, and then a ZnO thin-film was formed on the
SrCu.sub.2 O.sub.2 thin-film. In this case, a p-type transparent electrode material
exhibiting high conductivity was not available, and thereby a glass substrate coated with Ni
as the electrode was used.
Targets of an In.sub.2 O.sub.3 sintered body containing 10 wt % of SnO.sub.2, a ZnO
sintered body, a Sr.sub.1-x K.sub.x Cu.sub.2 O.sub.2 sintered body and metal Ni were
prepared. These targets were placed in a PLD chamber, and the vacuum of
1.times.10.sup.-6 Pa was provided in the chamber. Then, a SiO.sub.2 glass substrate
having Ni deposited thereon was disposed opposedly to and above the target with a
distance of 30 mm therebetween. Oxygen gas having a pressure of 2.times.10.sup.-3 Pa as
atmospheric gas was introduced in the chamber. After the substrate was heated up to
350.degree. C., the surface of the SrCu.sub.2 O.sub.2 target was irradiated with Kr F (248
nm) excimer laser pulses through a silica glass window while arranging the energy density
of laser in 2 J/cm.sup.2 for each pulse.
The laser was stopped when the SrCu.sub.2 O.sub.2 thin-film had a thickness of 200 nm.
Then, a ZnO thin-film was formed while arranging the energy density of laser in 5
J/cm.sup.2 for each pulse. The laser was interrupted when the ZnO thin-film had a
thickness of 400 nm. Then, an ITO thin-film was formed while arranging the energy
density of laser in 6 J/cm.sup.2 for each pulse. The laser was interrupted when the ITO
thin-film had a thickness of 800 nm, and the laminated film was taken out to ambient air.
A mesa structure was provided to the formed laminated film. In the measurement of a
current-voltage characteristic, a non-linear characteristic indicating p-n junction could be
observed. However, no luminescent could be confirmed.
Industrial Applicability
- 114 -
The light-emitting diode of the present invention including a p-n junction yielded by
forming SrCu.sub.2 O.sub.2, CuAlO.sub.2 or CuGaO.sub.2 layer on a ZnO having a
desired crystallinity provides ultraviolet light having a wavelength of 380 nm at room
temperature without any difficulty.
The light-emitting diode of the present invention can be significantly downsized through
micro fabrication, and thereby is best suited to optical recording media. In addtion, the
light-emitting diode has a shorter wavelength than that of conventional diode. This
provides higher recording density in optical recording media.
Further, the light-emitting diode of the present invention emits ultraviolet light. Thus, this
light-emitting diode is suited to a pumping source of any visible fluorescent materials. This
makes it possible to achieve a ultra-small or super-sized and ultra-slim light source to be
applicable to illumination devices and display devices.
Further, the light-emitting diode of the present invention emitting ultraviolet light is suited
to a pumping source of a hydrogen-generating photocatalyst which are recently developing,
for example, to be applicable to hydrogen-source system for automobile hydrogen-fueled
engine. The light-emitting diode of the present invention can provide resource saving and
environmentally friendly devices, and contribute to perennial development of society.
*****
第五筆
United States Patent
Takahashi ,
6,781,648
et al.
August 24, 2004
Title: Liquid-crystal
Abstract
display device
A light source in a backlight portion is composed of red LEDs, green LEDs, and blue
LEDs. The numbers of respective kinds of LEDs used are selected so that the number of
blue LEDs is not smaller than the number of red LEDs and the number of blue LEDs is not
smaller than the number of green LEDs.
Inventors: Takahashi; Yuji (Nishikasugai-gun, JP); Matsumura; Kanae (Nishikasugai-gun,
- 115 -
JP); Kato; Hideaki (Nishikasugai-gun, JP); Kaga; Koichi (Nishikasugai-gun,
JP)
Assignee: Toyoda Gosei Co., Ltd. (Nishikasugai-gun, JP)
Appl. No.: 015624
Filed:
December 17, 2001
Foreign Application Priority Data
Dec 22, 2000[JP]
P. 2000-391713
Current U.S. Class:
349/68; 349/50
Intern'l Class:
G02F 001/133.5
Field of Search:
349/50,68 362/26,27,31 385/901
References Cited [Referenced By]
U.S. Patent Documents
5008658
Apr., 1991
Russay et al.
5008788
Apr., 1991
Palinkas
362/231.
5130828
Jul., 1992
Fergason
349/199.
5164715
Nov., 1992
Kashiwabara et al.
5377027
Dec., 1994
Jelley et al.
5835269
Nov., 1998
Natori
359/448.
6007209
Dec., 1999
Pelka
362/30.
6069676
May., 2000
Yuyama
349/62.
6104446
Aug., 2000
Blankenbecler et al.
6185016
Feb., 2001
Popovich
359/15.
6386720
May., 2002
Mochizuki
362/27.
Foreign Patent Documents
2000-241811
Sep., 2000
JP.
Primary Examiner: Parker; Kenneth
Assistant Examiner: Chung; David Y.
Attorney, Agent or Firm: McGinn & Gibb, PLLC
Claims
- 116 -
345/87.
345/4.
349/69.
349/5.
What is claimed is:
1. A color-filterless full color liquid-crystal display device, comprising:
a liquid-crystal shutter portion including a liquid crystal; and
a backlight portion including light source units and a planar light guide, wherein said light
source units comprise at least one red light-emitting device (LED), at least one green LED,
and at least one blue LED,
wherein a first number corresponding to said at least one blue LED is not smaller than a
second number corresponding to said at least one red LED, and the first number
corresponding to said at least one blue LED is not smaller than a third number
corresponding to said at least one green LED,
wherein said light source units are disposed on an edge of said planar light guide, and
wherein the number of said blue LEDs is not smaller than the number of said red LEDs
and the number of said blue LEDs is larger than the number of said green LEDs.
2. A color-filterless full color liquid-crystal display device, comprising:
a liquid-crystal shutter portion including a liquid crystal; and
a backlight portion including light source units and a planar light guide,
wherein said light source units comprise at least one red light-emitting device (LED), at
least one green LED, and at least one blue LED,
wherein a first number corresponding to said at least one blue LED is not smaller than a
second number corresponding to said at least one red LED, and the first number
corresponding to said at least one blue LED is not smaller than a third number
corresponding to said at least one green LED,
wherein said light source units are disposed on an edge of said planar light guide, and
- 117 -
wherein two red LEDs, one green LED and two blue LEDs are mounted on a substrate.
3. A color-filterless fall color liquid-crystal display device, comprising:
a liquid-crystal shutter portion including a liquid crystal; and
a backlight portion including light source units and a planar light guide,
wherein said light source units comprise at least one red light-emitting device (LED), at
least one green LED, and at least one blue LED,
wherein a first number corresponding to said at least one green LED is not larger than a
second number corresponding to said at least one red LED and the first number
corresponding to said at least one green LED is not larger than or equal to a third number
corresponding to said at least one blue LED,
wherein said light source units are disposed on an edge of said planar light guide, and
wherein two red LEDs, one green LED and two blue LEDs are mounted on a substrate.
4. A liquid-crystal display device according to claim 3, wherein said backlight portion
includes a planar light guide laminated on said liquid-crystal shutter portion so that said at
least one red LED, said at least one green LED, and said at least one blue LED are
disposed to face a side of said planar light guide.
5. A liquid-crystal display device according to claim 3, wherein a reflection layer is formed
on a surface of said planar light guide.
6. A liquid-crystal display device according to claim 3, wherein said backlight portion
includes a light emission controller for controlling light emission of each of said at least
one red LED, said at least one green LED, and said at least one blue LED, said light
emission controller applying a current to said each of said at least one red LED, said at
least one green LED, and said at least one blue LED to thereby obtain a maximum
light-emitting efficiency of said each of said at least one red LED, said at least one green
LED, and said at least one blue LED.
7. A liquid-crystal display device according to claim 3, wherein light is selectively emitted
from each of said at least one red LED, said at least one green LED, and said at least one
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blue LED in synchronization with an ON/OFF of a corresponding one of a plurality of
pixels in said liquid-crystal shutter portion.
8. A color-filterless full color liquid-crystal display device, comprising:
a liquid-crystal shutter portion including a twisted nematic (TN) liquid crystal; and
a backlight portion including light source units and a planar lightguide,
wherein said light source units comprise at least one red light-emitting device (LED), at
least one green LED, and at least one blue LED,
wherein a first number corresponds to said at least one blue LED, a second number
corresponds to said at least one red LED, and third number corresponds to said at least one
green LED, and said second number is larger than at least one of said first number and said
third number, and
wherein said light source units are disposed on an edge of said planar light guide.
9. A color-filterless full color liquid-crystal display device, comprising:
a liquid-crystal shutter portion including a super twisted nematic (STN) liquid crystal; and
a backlight portion including light source units and a planar lightguide,
wherein said light source units comprise at least one red light-emitting device (LED), at
least one green LED, and at least one blue LED,
wherein a first number corresponding to said at least one blue LED is not smaller than a
second number corresponding to said at least one red LED, and the first number
corresponding to said at least one blue LED is larger than a third number corresponding to
said at least one green LED, and
wherein said light source units are disposed on an edge of said planar light guide.
10. A liquid-crystal display according to claim 3, wherein said liquid crystal comprises a
TN liquid crystal.
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11. A liquid-crystal display according to claim 3, wherein said liquid crystal comprises an
STN liquid crystal.
12. A color-filterless full color liquid-crystal display device, comprising:
a liquid-crystal shutter portion including a liquid crystal; and
a backlight portion including light source units and a light guide,
wherein said light source units comprise at least one red light-emitting device (LED), at
least one green LED, and at least one blue LED,
wherein a first number corresponding to said at least one blue LED is not smaller than a
second number corresponding to said at least one red LED, and the first number
corresponding to said at least one blue LED larger than a third number corresponding to
said at least one green LED, and
wherein said light source units are disposed on an edge of said light guide.
13. A liquid-crystal display device according to claim 12, wherein said liquid crystal
comprises a twisted nematic (TN) liquid crystal.
14. A liquid-crystal display device according to claim 12, wherein said liquid crystal
comprises a super twisted nematic (STN) liquid crystal.
15. The color-filterless full color liquid-crystal display device according to claim 12,
wherein said light guide comprises a planar light guide.
16. A color-filterless full color liquid-crystal display device, comprising:
a liquid-crystal shutter portion including a liquid crystal; and
a backlight portion including light source units and a light guide,
wherein said light source units comprise at least one red LED, at least one green LED, and
at least one blue LED,
wherein a first number corresponding to said at least one green LED is not larger than a
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second number corresponding to said at least one red LED and the first number
corresponding to said at least one green LED is not larger than or equal to a third number
corresponding to said at least one blue LED, and
wherein said light source units are disposed on an edge of said light guide.
17. A liquid-crystal display device according to claim 16, wherein said liquid crystal
comprises a twisted nematic (TN) liquid crystal.
18. A liquid-crystal display device according to claim 16, wherein said liquid crystal
comprises a super twisted nematic (STN) liquid crystal.
19. The color-filterless full color liquid-crystal display device according to claim 16,
wherein said light guide comprises a planar light guide.
20. A liquid-crystal display device according to claim 1, wherein said backlight portion
includes a planar light guide laminated on said liquid-crystal shutter portion so that said at
least one red LED, said at least one green LED, and said at least one blue LED are
disposed to face a side of said planar light guide.
21. A liquid-crystal display device according to claim 20, wherein a reflection layer is
formed on a surface of said planar light guide.
22. A liquid-crystal display device according to claim 1, wherein said backlight portion
includes a light emission controller for controlling light emission of each of said at least
one red LED, said at least one green LED, and said at least one blue LED, said light
emission controller applying a current to said each of said at least one red LED, said at
least one green LED, and said at least one blue LED to thereby obtain a maximum
light-emitting efficiency of said each of said at least one red LED, said at least one green
LED, and said at least one blue LED.
23. A liquid-crystal display device according to claim 1, wherein light is selectively
emitted from each of said at least one red LED, said at least one green LED, and said at
least one blue LED in synchronization with an ON/OFF of a corresponding one of a
plurality of pixels in said liquid-crystal shutter portion.
24. A liquid-crystal display device according to claim 2, wherein said backlight portion
includes a planar light guide laminated on said liquid-crystal shutter portion so that said at
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least one red LED, said at least one green LED, and said at least one blue LED are
disposed to face a side of said planar light guide.
25. A liquid-crystal display device according to claim 24, wherein a reflection layer is
formed on a surface of said planar light guide.
26. A liquid-crystal display device according to claim 2, wherein said backlight portion
includes a light emission controller for controlling light emission of each of said at least
one red LED, said at least one green LED, and said at least one blue LED, said light
emission controller applying a current to said each of said at least one red LED, said at
least one green LED, and said at least one blue LED to thereby obtain a maximum
light-emitting efficiency of said each of said at least one red LED, said at least one green
LED, and said at least one blue LED.
27. A liquid-crystal display device according to claim 2, wherein light is selectively
emitted from each of said at least one red LED, said at least one green LED, and said at
least one blue LED in synchronization with an ON/OFF of a corresponding one of a
plurality of pixels in said liquid-crystal shutter portion.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a liquid-crystal display device and particularly to
improvement in a backlight portion of a color-filterless liquid-crystal display device.
2. Description of the Related Art
A field-sequential liquid-crystal display device is heretofore known as a color-filterless
liquid-crystal display device (see JP-A-2000-241811). According to the JP-A-2000-241811,
the field-sequential liquid-crystal display device has been described as follows.
That is, the field-sequential liquid-crystal display device is constituted by a backlight for
emitting light selected from light beams of three primary colors R, G and B, and a
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liquid-crystal shutter display panel. The liquid-crystal shutter display panel includes a pair
of polarizing plates, and a liquid-crystal cell sandwiched between the pair of polarizing
plates. A specific region of the liquid-crystal cell is opened as a translucent region
selectively in synchronism with light emitted from the backlight, so that the light emitted
from the backlight is displayed in the form of a predetermined display pattern. Such light
beams of three primary colors selectively emitted from the backlight and such display
patterns on the liquid-crystal shutter display panel are switched sequentially at a high speed
to overlay respective display patterns of R, G and B continuously in a time division mode
at a high speed to thereby perform color display. When, for example, only one color of R,
G and B is expressed in a specific region, the color can be displayed in the region. When
two colors of R, G and B are expressed successively in another specific region so as to be
overlaid while switched at a high speed, a mixture color of the two colors by additive color
mixture can be displayed in the region. When three colors R, G and B are expressed
successively in a further region so as to be overlaid while switched at a high speed, a
mixture color of the three colors by additive color mixture can be displayed in the region.
In the invention described in the JP-A-2000-241811, an electroluminescence (EL) was
used as the backlight of the field-sequential liquid-crystal display device. The present
inventors have investigated eagerly LEDs (LEDs) used as the backlight. As a result, the
following problem to be solved has been found.
At present, red LEDs, green LEDs and blue LEDs are available on the market. The
luminous efficiencies of these LEDs are different from one another in accordance with
emission colors. Accordingly, it is necessary to adjust power (load) applied to the LEDs of
the respective colors in accordance with colors when these LEDs form a full color
backlight. (When, for example, the blue LED is assumed to have a brightness of 1, the
brightness of the green LED and the brightness of the red LED are 6 and 3 respectively.) In
this case, degradation of a LED upon which a large load is imposed is accelerated. The
color balance emitted from the backlight may be lost with the passage of time.
According to the inventors' research, it has been found that there is a tendency that bluish
white is selected as the background color (white) of a color display.
When Twisted Nematic (TN) liquid crystal, Super Twisted Nematic (STN) liquid crystal or
the like is used as a liquid crystal material in a color-filterless liquid-crystal panel of a
field-sequential liquid-crystal display device or the like, the color of the cell is selected to
be green. Hence, green is visually recognized relatively intensively if the backlight
contains the three primary colors equal in intensity.
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SUMMARY OF THE INVENTION
The present invention is devised to solve the aforementioned problem. Configuration
according to a first aspect of the present invention is a color-filterless full color
liquid-crystal display device provided with a liquid-crystal shutter portion and a backlight
portion, the liquid-crystal shutter portion including TN liquid crystal or STN liquid crystal;
the backlight portion including red LEDs, green LEDs and blue LEDs; wherein the number
of the blue LEDs is not smaller than the number of the red LEDs; and the number of the
blue LEDs is not smaller than the number of the green LEDs.
According to the liquid-crystal display device configured as described above, the number
of the blue LEDs used is not smaller than the number of any other color type of LEDs used.
The light source color of the backlight becomes bluish, so that the power load applied to
the blue LEDs can be set to be smaller. Hence, such a backlight can be adapted to users'
needs because the backlight can be kept bluish as required in total even in the case where
light is transmitted through a greenish liquid-crystal cell made of TN or STN liquid crystal.
According to another aspect of the present invention, the numbers of the respective LEDs
used are determined, so that the number of the green LEDs is not larger than the number of
the red LEDs; and the number of the green LEDs is not larger than the number of the blue
LEDs.
When the light source of the backlight is configured as described above, the green
component contained in the backlight is weakened relatively. On the other hand, the liquid
crystal material in the liquid-crystal shutter portion is greenish. Hence, the attenuation of
the green component of backlight transmitted through the liquid crystal material becomes
the smallest, so that the green component is balanced finally.
Constituent members of the present invention will be described below.
A configuration in which a liquid-crystal cell having a shutter function is sandwiched
between a pair of polarizing plates can be used as the liquid-crystal shutter portion. On this
occasion, the liquid-crystal cell has a pair of transparent substrates, a pair of transparent
electrodes formed on surfaces of the transparent substrates facing each other, a pair of
alignment films formed on the transparent electrodes respectively, a sealing material for
joining and sealing the circumferential edges of the transparent substrates while keeping
the distance between the transparent substrates constant, and liquid crystal enclosed in an
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enclosure space formed between the transparent substrates.
Each of the polarizing plates can be provided with polarizing layers having linear
polarization axes crossing each other perpendicularly. A glass substrate or a plastic
substrate can be used as each of the transparent substrates. The liquid crystal enclosed in
the liquid-crystal cell is made of a liquid crystal material such as TN liquid crystal or STN
liquid crystal, which exhibits green.
Each of the alignment films is provided to align liquid-crystal molecules in a specific
direction on a surface and can be formed by rubbing a surface of a film of a heat-resistant
resin such as polyimide in the specific direction with a piece of cloth made of nylon or the
like.
Each of the transparent electrodes can be made of ITO, AZO (Al-added ZnO), SnO.sub.2,
or the like. Each of the transparent electrodes is constituted by a predetermined pattern
shaped like stripes. The molecular alignment of the liquid crystal is changed only in a
specific region constituted by dot units by application of a voltage to the dot units, so that
light-transmittance can be changed only in the region on the basis of the relation between a
pair of polarizing layers.
In a field-sequential type liquid-crystal display device, a specific region is selectively
opened as a translucent region in synchronism with light emitted from the backlight to
thereby display the light emitted from the backlight in the form of a predetermined display
pattern. Light beams of the three primary colors selectively emitted from the backlight and
display patterns on the liquid-crystal shutter display panel are switched successively at a
high speed to express the display patterns of R, G and B continuously in a time division
mode to thereby make color display possible.
The light source of the backlight portion is constituted by red LEDs, green LEDs, and blue
LEDs. Here, each of the red LEDs emits light with a wavelength of from 600 to 620 nm
and, for example, is made of a GaP type compound semiconductor. Each of the green
LEDs emits light with a wavelength of from 510 to 550 nm and, for example, is made of a
GaN type compound semiconductor. Each of the blue LEDs emits light with a wavelength
of from 460 to 480 nm and, for example, is made of a GaN type compound semiconductor.
The use of the LEDs as the light source permits improvement of light-emitting efficiency
and saving of consumed electric power compared with the use of a cold cathode
fluorescent tube or the like as the light source. Moreover, heat generated in the LEDs is
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small so that the influence of heat on the light guide can be reduced. In addition, the life of
the LEDs is long so that the life of the light source can be intended to become longer.
In the present invention, the numbers of the respective LEDs used in the light source are
determined, so that the number of the blue LEDs is not smaller than the number of the red
LEDs; and the number of the blue LEDs is not smaller than the number of the green LEDs.
In this configuration, the blue component is intensified in the backlight. When the
respective LEDs are switched on simultaneously, bluish white light emission is obtained.
Also when the respective LEDs are switched on in a time division mode in field-sequential
control, light is perceived as bluish white because blue light emission is intensive.
Accordingly, natural white with blue balance adjusted is obtained when the backlight is
transmitted through the greenish cell of the liquid-crystal shutter portion.
In consideration of green of the cell, the further preferred numbers of the respective LEDs
arranged are determined, so that the number of the blue LEDs is not smaller than the
number of the red LEDs; and the number of the blue LEDs is larger than the number of the
green LEDs.
According to a further aspect of the present invention, the numbers of the respective LEDs
are also determined, so that the number of the green LEDs is not larger than the number of
the red LEDs; and the number of the green LEDs is not larger than the number of the blue
LEDs.
The backlight portion includes a light guide having a surface facing the liquid-crystal
shutter portion. Light is led into the light guide from the respective LEDs described above.
In the embodiment described later, there is employed a configuration in which a planar
light guide is laminated on the liquid-crystal shutter portion so that light from the
respective LEDs is led into the light guide through a side surface of the light guide.
Examples of the translucent material constituting the light guide include: a synthetic resin
such as polycarbonate, acrylic resin, epoxy resin, etc.; and an inorganic material such as
glass, etc. It is preferable that a reflection layer is formed on any other surface of the light
guide except the surface facing the liquid-crystal shutter portion. The reflection layer can
be also formed by printing, evaporation or sputtering using light-reflective ink (for
example, white ink). Alternatively, a tape (such as a white tape) with a high light
reflectance may be pasted on the surface. Alternatively, the reflection surface may be
formed by a surface-roughening process such as etching, sandblasting, electric discharge
machining, or the like.
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Preferably, a light-diffusing layer is interposed between the light guide and the
liquid-crystal shutter portion. The light guide, the light-diffusing layer interposed thus as
occasion demands, and the liquid-crystal shutter portion are preferably stuck close to one
another.
The backlight portion includes a light emission controller by which a current is applied to
each of the LEDs to obtain the maximum light-emitting efficiency of the LED. That is, the
LEDs are operated in rated conditions respectively. As a result, the respective LEDs are
made substantially uniform in the progress of degradation thereof, so that color balance is
never lost even with the passage of time.
The light emission controller switches on the LEDs in accordance with the colors in
synchronism with the ON/OFF of the cell in the liquid-crystal shutter portion to thereby
perform a field-sequential liquid-crystal display mode.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows the configuration of a liquid-crystal display device according to an
embodiment of the present invention;
FIG. 2 is a front view of a backlight light source in this embodiment;
FIG. 3 is a sectional view taken along the line III--III in FIG. 2;
FIG. 4 is a wiring diagram of LEDs;
FIG. 5 is a front view of a light guide; and
FIG. 6 is a plan view of the light guide.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
An embodiment of the present invention will be described below.
FIG. 1 shows the configuration of a field-sequential liquid-crystal display device 1
according to this embodiment. In this embodiment, the liquid-crystal display device 1 is
constituted by a liquid-crystal shutter portion 10, a backlight portion 20, a light-diffusing
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layer 30, and a control portion 40.
The liquid-crystal shutter portion 10 has a versatile configuration. A first glass substrate 11,
a first transparent conductive film 12 made of an ITO film, liquid crystal 13 made of a TN
material, a second transparent conductive film 14 made of an ITO film and a second glass
substrate 15 are laminated successively to form the liquid-crystal shutter portion 10. Each
of the first and second transparent conductive films 12 and 14 is shaped like a matrix. A
voltage is applied to each of the first and second transparent conductive films 12 and 14 to
thereby control a corresponding region of the liquid-crystal cell to be
translucent/non-translucent.
The backlight portion 10 includes light source units 101, and a light guide 120. One of the
light source units 101 is shown in FIG. 2. FIG. 3 is a sectional view taken along the line
III--III in FIG. 2.
A green LED G1 is disposed in the center of the light source unit 101. Two blue LEDs B1
and B2 are disposed on opposite sides of the green LED G1. Two red LEDs R1 and R2 are
disposed on opposite sides of the blue LEDs. These five LEDs are aligned in a line. Wiring
for the respective LEDs is shown in FIG. 4. It is to be understood from the wiring in FIG.
4 that the first and second blue LEDs B1 and B2 are switched on simultaneously or off
simultaneously. Similarly, the first and second red LEDs R1 and R2 are switched on/off
simultaneously.
A GaN type diode is used as each of the blue and green LEDs. An AlInGaP type diode is
used as each of the red LEDs.
As shown in FIG. 3, the respective LEDs are mounted directly on a common anode (p-type
electrode) 103 in a cup-shaped window 105. An insulating substrate is used for each of the
blue and green LEDs, so that a conductive wire is laid between the common anode 103 and
a p-type layer of the common anode 103. The window 105 is surrounded by a wall which
serves as a reflection surface. The window 105 is filled with a transparent resin 107.
As shown in FIGS. 5 and 6, the light guide 120 is shaped like a plate (film). The left end of
the light guide 120 is slightly thicker than the other portions thereof as shown in FIG. 5.
Notches (light source unit mount portions) 121 are formed in the left end of the light guide
120. The light source unit 101 is mounted in each of the notches 121 so that the window
105 faces the light guide 120. The light guide 120 is made of a transparent resin (epoxy
resin). On the lower surface of the light guide 120, as shown in FIG. 1, a reflection layer
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16 is provided as a grooved surface and the upper surface (the side surface of the
liquid-crystal shutter portion) of the light guide 120 is provided as a fine hologram
processed surface.
A side surface of the light guide 120 is painted with white or a white member is provided
on a side surface of the light guide 120 so that the side surface also serves a reflection layer.
In the backlight portion 20 configured as described above, light beams emitted from the
LEDs G, R1, R2, B1 and B2 are led into the light guide 120 through the side surface of the
light guide 120. The light beams are reflected upward by the lower surface (the grooved
surface) of the light guide 120 so as to be emitted from the upper surface (the hologram
processed surface) of the light guide 120 toward the liquid-crystal shutter portion 10.
The light-diffusing layer 30 is interposed between the light guide 120 and the liquid-crystal
shutter portion 10. The light-diffusing layer 30 is made of a uniform dispersion of a
light-diffusing agent (such as mica) in a translucent resin (such as epoxy resin). The
intensity of light beams incident on the liquid-crystal shutter portion 10 is made uniform by
the light-diffusing layer 30.
In the control portion 40, a control circuit 43 sends an image signal to a liquid-crystal
driving circuit 45 in accordance with an image (of characters, graphics, etc.) formed by an
image input circuit 41. The liquid-crystal driving circuit 45 turns on/off the transparent
conductive films 12 and 14 on the basis of the input image signal to thereby drive regions
of the liquid-crystal cell corresponding to the image. The liquid-crystal driving circuit 45 is
synchronized with a light emission control circuit 46 by a synchronizing circuit 44. As a
result, the LEDs of respective colors are switched on in a time division mode, so that the
regions of the liquid-crystal cell are controlled to be switched on/off in synchronism with
the time-division switching of the LEDs. A known configuration can be used as the control
portion 40.
The light emission control circuit 46 applies a current to each of the LEDs to maximize its
light-emitting efficiency to thereby operate the LED in a rated condition.
According to the liquid-crystal display device 1 configured thus in accordance with this
embodiment, four blue LEDs, four red LEDs and two green LEDs are used in total as light
sources of the backlight portion and operated in rated conditions respectively. Because the
number of the green LEDs is smallest, green may be poor as backlight. Green TN is,
however, used in the liquid crystal 13, so that attenuation of the green component of light
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is smallest when the light passes through the liquid crystal 13. Accordingly, green is
balanced when the light is viewed finally. Because the number of the blue LEDs used is
large, bluish white in great demand can be achieved easily particularly when white is to be
generated.
The present invention is not limited to the above description of the mode for carrying out
the invention and of the embodiment thereof at all, but includes various modifications that
can be conceived easily by those skilled in the art, without departing from the description
of the scope of claim.
*****
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