All Inorganic Colloidal Quantum Dot LEDs
by
Vanessa Claire Wood
Submitted to the Department of Electrical Engineering and Computer
Science
in partial fulfillment of the requirements for the degree of
Masters of Science in Computer Science and Engineering
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
June 2007
© Massachusetts Institute of Technology 2007. All rights reserved.
A u th or .....................
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Department of Electrical Engineering and Computer Science
May 2007
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Certified by...... . 1
Accepted by
...................................
Vladimir Bulovid
Associate Professor
Thesis Supervisor
....
c .
.
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- .............. ....
Arthur C. Smith
Chairman, Department Committee on Graduate Students
MASSACHUSETTS INSTnTUTE
OF TECHNOLOGY
AUG 16 2007
LIBRARIES
ARCHNVES
2
All Inorganic Colloidal Quantum Dot LEDs
by
Vanessa Claire Wood
Submitted to the Department of Electrical Engineering and Computer Science
on May 11, 2007, in partial fulfillment of the
requirements for the degree of
Masters of Science in Computer Science and Engineering
Abstract
This thesis presents the first colloidal quantum dot light emitting devices (QD-LEDs)
with metal oxide charge transport layers. Colloidally synthesized quantum dots (QDs)
have shown promise as the active material in optoelectronic devices because of their
tunable, narrow band emission. To date, the most efficient QD-LEDs involve a monolayer of closely packed QDs sandwiched between organic charge transport layers.
However, these organic materials are subject to degradation due to atmospheric oxygen and water vapor. In contrast, metal-oxide films used in this work are chemically
and morphologically stable in air and can withstand numerous organic solvents, which
increases the flexibility of device processing. Furthermore, they can sustain higher
carrier injection rates needed to realize an electrically pumped colloidal QD laser.
This thesis details the characterization techniques, such as Atomic Force Microscopy,
photoluminescence spectroscopy, Hall Effect measurements, X-Ray Diffraction, and
Ultraviolet Photoelectron Spectroscopy, used to design efficient QD-LEDs. It reviews
the steps used to optimize device performance and obtain a transparent device architecture with external quantum efficiency of 0.15% and a peak luminance of 7000
Cd/m 2 . This manifests a 100-fold improvement in efficiency over any previously reported all inorganic QD-LED structure.
Thesis Supervisor: Vladimir Bulovid
Title: Associate Professor
3
4
Acknowledgments
Many thanks to my advisor, Professor Vladimir Bulovid for his guidance, enthusiasm,
and encouragement.
Many coworkers deserve recognition. Special thanks goes to Jean Michel Caruge
for teaching me about RF sputtering and for our brainstroming sessions, to Jonathan
Halpert for synthesizing the quantum dots used in this work, and to Dr. Peter
Mardilovich for his insights on metal oxides. I would also to thank Alexi Arango,
Polina Anikeeva, Gerry Chen, Kaveh Milaninia, James Perkins, and Venda Porter for
their assistance with various aspects of this work. And thanks to all in the Bulovi
group for making lab such a fun place to be.
This research was supported by NSF-MRSEC, NSF-NIRT, a Presidential Early
Career Award for Science and Engineering, the Institute for Soldier Nanotechnologies,
and a National Defense Science and Engineering Graduate Fellowship.
5
6
Contents
15
1 Introduction
1.1
. . . . . . . . . . . . . . . . . . . . . . . . . . .
15
1.1.1
Limits of Organic QD-LED Technology . . . . . . . . . . . . .
19
1.1.2
QD-LED function .........................
20
Quantum Dot LEDs
1.2
All-inorganic QD-LEDs ..........................
21
1.3
Q D-Lasers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
26
27
2 Material Properties and Growth
2.1
M etal Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
27
2.2
Radio Frequency Magnetron Sputtering . . . . . . . . . . . . . . . . .
32
2.3
Quantum Dots
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
35
2.4
Colloidal Quantum Dot Synthesis . . . . . . . . . . . . . . . . . . . .
38
43
3 Design of All-Inorganic QD-LEDs
3.1
3.2
3.3
Characterization of Sputtered Films . . . . . . . . . . . . . . . . . . .
43
3.1.1
Atomic Force Microscopy . . . . . . . . . . . . . . . . . . . . .
43
3.1.2
X-Ray Diffraction . . . . . . . . . . . . . . . . . . . . . . . . .
45
3.1.3
Hall Effect Measurements
. . . . . . . . . . . . . . . . . . . .
49
The Quantum Dot-Metal Oxide Interface . . . . . . . . . . . . . . . .
51
3.2.1
Atomic Force Microscopy . . . . . . . . . . . . . . . . . . . . .
51
3.2.2
Quantum Dot Luminescence Quenching
. . . . . . . . . . . .
53
3.2.3
Summary of Sputtering Parameters of Ceramic Materials . . .
54
Ultraviolet Photoelectron Spectroscopy . . . . . . . . . . . . . . . . .
55
7
3.3.1
M etal Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . .
59
3.3.2
Quantum Dots . . . . . . . . . . . . . . . . . . . . . . . . . .
63
67
4 All Inorganic QD-LEDs
67
4.1
Measurement Techniques .........................
4.2
All Inorganic QD-LEDs
. . . . . . . . . . . . . . . . . . . . . . . . .
68
4.3
Improving Efficiency with ZnS . . . . . . . . . . . . . . . . . . . . . .
72
4.4
Improving Efficiency with ZnO
. . . . . . . . . . . . . . . . . . . . .
73
4.5
Transparent QD-LEDs . . . . . . . . . . . . . . . . . . . . . . . . . .
77
4.6
Toward Green and Blue All Inorganic QD-LEDs . . . . . . . . . . . .
80
4.7
Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
84
8
List of Figures
1-1
Schematic of liquid crystal cell [1]. . . . . . . . . . . . . . . . . . . . .
1-2
Room temperature optical absorption spectra of CdSe QDs dispersed
16
in hexane [2]. Varying the size of the QD, tunes its optical properties
continuously through the visible spectrum. This is not possible for any
type of organic lumophore. The photograph of the QDs excited by
UV light reveals their high photoluminescence efficiencies through the
visible region of the spectrum. . . . . . . . . . . . . . . . . . . . . . .
18
1-3 a) Normalized electroluminescence spectra for QDs showing their narrow band emission throughout in the visible and infrared region of the
spectra [3], [4]. b) White QD-LED made by sandwiching a monolayer of
mixed red, green, and blue QDs between organic charge transport layers [5]. c) RGB pixels, 25 by 25 pm, featuring electroluminescence from
red and green QDs and blue emission from the organic hole transport
layer, N,N'-Bis(3-methylphenyl)-N,N'-bis(phenyl)benzidine (TPD) [6].
19
1-4 A schematic of the (a) structure and (b) band diagram of a typical QDLED structure. The schematic in (c) depicts the injection of charge
into device and the formation of excitons across the QD layer for a
QD-LED under forward bias.
. . . . . . . . . . . . . . . . . . . . . .
9
21
1-5 a) An SEM image showing the device structure composed of a colloidal CdSe/ZnS QD layer sandwiched between a p-type, MOCVD,
GaN layer grown on a sapphire substrate and a n-type GaN layer grown
using ENABLE. b) Photographs of the device electroluminescence at
10 and 30 V for which non-uniform emission is evident. The white
arrow points to blue emission from the GaN. [7] . . . . . . . . . . . .
1-6
23
a) EL spectrum for hybrid organic-inorganic device made with QDs
sandwiched between a sputtered NiO HTL and an organic (Alq 3 ) ETL
biased at 9 V. b) i-v and EQE curves for device. . . . . . . . . . . . .
2-1
Disordered Kronig Penny model (a) potential and (b) resulting wavefunction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2-2
25
28
Schematic of the density of states for an amorphous solid semiconduc. . . . . . . . . . . . . . . . . . . . . . . . .
29
Localized electron hopping model. . . . . . . . . . . . . . . . . . . . .
31
2-4 Cross sectional view of electrode assembly for RF sputtering. . . . . .
34
tor at different energies.
2-3
2-5
Schematic depicting synthesis procedure for CdSe as first described in [2]. 39
2-6
Schematic of the structure of a) ZnCdSe, b) (CdSe)ZnS, c) ZnSe/CdSe/ZnS,
and d) ZnCdS QDs . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-1
40
Atomic Force Microscopy surface topography images of a) ITO RFsputtered on glass, and b) NiO RF-sputtered onto the ITO shown in
(a) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3-2
Atomic Force Microscopy surface topography images showing smooth
ZnO : SnO 2 on top of the ITO in Figure 3-la . . . . . . . . . . . . . .
3-3
45
X-ray diffraction spectra for a) RF-sputtered ITO, b) RF-sputtered
ITO in a heated chamber, and c) commercially purchased ITO.
3-4 X-ray diffraction spectra for NiO, ZnS, ZnO, and ZnO : SnO 2.
3-5
44
. .
. . . ..
47
48
AFM images of a) QDs stamped onto NiO, b) QDs in chloroform spun
onto NiO, and c) QDs in a 9:1 hexane to octane solution spun onto NiO. 52
10
3-6
PL spectrum of 30 nm thick CdZnSe QD layer on glass (solid red
line) and 30 nm thick CdZnSe QD layer between NiO and ZnO:SnO 2
(dotted red line). The two samples, shown schematically to the left,
were excited within the same optical geometry using a UV lamp. We
measured a 40% drop in the PL intensity on average. . . . . . . . . .
3-7
Schematics explaining key features of the UPS spectrum of a semiconductor...........................................
3-8
54
57
Schematics explaining key features of the UPS spectrum of a semiconductor...........................................
58
UPS spectra for ITO. . . . . . . . . . . . . . . . . . . . . . . . . . . .
60
3-10 UPS spectra for ZnO, SnO 2 , and ZnO : Sn02 . . . . . . . . . . . . . .
61
3-9
3-11 a) The UPS spectrum for ZnS on an ITO substrate at 6V reserve bias.
b) A close up of spectrum for low kinetic energy electrons reveals an
onset at 4.87 eV. c) The derivative of the intensity spectrum for large
kinetic energy electrons reveals a 2.55 eV gap between the Fermi level
and the valence band . . . . . . . . . . . . . . . . . . . . . . . . . . .
62
3-12 Absorption spectra of QDs following different treatments for removing
the ligands. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
64
3-13 UPS spectra for CdSe QDs. The spectrum in (a) gives the number
of electrons hitting the detector each second as a function of electron
energy. The spectrum was taken with the sample reversed biased at
6V to create a sharper turn on. The plot in (b) is the derivative of
the signal at higher electron energies. No bias was applied for this
measurem ent. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
65
3-14 AFM of a CdSe QD film on ITO after soaking it in a solution of 0.1M
butylyamine in acetonitrile for 5 minutes and baking at 70 C for 30
m inutes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4-1
66
Device schematic and approximate band structure as determined by
UPS measurements. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11
69
4-2 The J-V curve for the first all-inorganic QD-LED and band diagram
schematic under forward bias
4-3
. . . . . . . . . . . . . . . . . . . . . .
70
EL spectrum showing emission entirely from QD layer. Photograph of
emission from the device at 6 V applied bias. . . . . . . . . . . . . . .
71
4-4 EQE as a function of current density for the first metal oxide based
QD-LED. Maximum EQE for this device is 0.09%. The inset shows
that a maximum luminance of 7000 Cd/M 2 is reached at 3.5 A/M 2 . .
72
4-5 Schematic of device structure and proposed band diagram, under forward bias, containing ZnS electron blocking layer. . . . . . . . . . . .
73
4-6
J-V and QE of all inorganic QD-LED with a ZnS electron blocking layer. 74
4-7
EL of all inorganic QD-LED with a ZnS electron blocking layer.
4-8
Schematic of device structure with an insulating ZnO layer, and EL
.
. .
74
spectrum of device biased at 10 V and showing emission entirely from
75
QDs. ...........................................
4-9 J-V and EQE plots for device with an insulating ZnO layer. . . . . .
75
4-10 Schematic of device and EL spectrum at 10 V showing emission entirely
from Q D s. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
76
4-11 J-V and EQE for ITO/ZnO : SnO 2 /ZnO/QD/ZnO/ZnS/ZnO : SnO 2 /Ag
structure. ......
.................................
77
4-12 A schematic of the structure of the first transparent QD-LED. A SEM
cross sectional image is shown to the right. . . . . . . . . . . . . . . .
78
4-13 Photographs of a transparent all inorganic colloidal QD-LED. .....
79
4-14 J-V and EQE characteristics for a transparent QD-LED. . . . . . . .
79
4-15 Band diagram showing reserve and forward bias applied to the transparent QD-LED structure. . . . . . . . . . . . . . . . . . . . . . . . .
81
4-16 CIE chromaticity diagram showing the current NTSC standard and
location of QDs on the CIE diagram. . . . . . . . . . . . . . . . . . .
82
4-17 J-V plots comparing green and red QD devices. . . . . . . . . . . . .
83
12
4-18 Band diagram in forward bias for proposed device structure. EL spectra for green and blue QD devices. The broad band emission is indicative of W 0 3 emission . . . . . . . . . . . . . . . . . . . . . . . . . . .
13
84
14
Chapter 1
Introduction
In this thesis, I report the development of thin film light emitting devices (LEDs)
with metal oxide charge transport layers and colloidal quantum dots (QDs) as the active emissive layer. Chapter 1 reviews the operation of organic light emitting devices
(OLEDs) and the benefits of integrating of colloidal QD into the OLED structure. It
also motivates the development of all-inorganic LEDs with QDs as the luminescent
medium. Chapter 2 explores the basic physical properties and growth techniques
associated with QDs and metal oxides that make them an attractive material set
with which to fabricate the all-inorganic QD-LEDs. Chapter 3 discusses characterization techniques, such as Atomic Force Microscopy, Hall Effect measurements, X-ray
diffraction, and Ultraviolet Photoelectron Spectroscopy, used to design efficient QDLEDs. Chapter 4 presents the first examples of all-inorganic QD-LEDs.
1.1
Quantum Dot LEDs
There are several technologies that must be discussed in order to illustrate the potential benefits of using QD-LEDs for displays. Liquid crystal displays (LCDs) currently
dominate the flat panel display market. LCDs are essentially voltage driven light
switches that allow white light from a back plane to either be transmitted or absorbed. The schematic in Figure 1-1 can aid in understanding their operation. Light
from the backplane passes through a polarizer and then through the liquid crystals,
15
INCIDENT
UNPOL.ARISED
LIGHT
INCIDEDU
POLARISER
UNPOLARISED
LIGHT
SUBSTRATE
ELECTRODE
+ALIGNMENT
ElECTRODE
+ALIGNMENTf
SUBSTRATE
LIGHT ABSORBED
IN THE OFF-STATE
POLARISER
POLARISED LIGHT
TRANSMITED
IN THE ON-STATE
Figure 1-1: Schematic of liquid crystal cell [1].
which can be aligned with application of an electric field. Liquid crystals are birefringent, which means that depending on their orientation relative to the incoming light,
the polarization of the light can be rotated. This light then impinges on a second
polarizer oriented perpendicular to the first polarizer. The amount the polarization of
the light is rotated while traversing the liquid crystals determines the amount of light
transmitted through the liquid crystal cell. Any transmitted light then passes through
a color filter to provide red, green, and blue (R.GB) pixels. However, LCDs have several important limitations including narrow viewing angle, limited color range, low
power efficiency because of the use of filters, and slow switching speeds between the
on and off state of each pixel.
Inorganic LEDs (ILEDs) are used primarily for indicator lights and back plane
sources for LCD displays. Because ILEDs involve epitaxial growth of single crystals of
semiconductor on lattice-matched substrates, their cost has made them prohibitively
expensive for most large area lighting applications.
In 1987, Kodak published papers announcing the use of amorphous film of small
16
organic molecules for light emitting devices [8]. In contrast to ILEDs, organic LEDs
(OLEDs) offer extremely cheap production. Furthermore, unlike LCDs, OLED displays maintain color verity at any viewing angle. OLED displays are now commercially available in cell phones, digital cameras, and PDAs, and flat panel screens.
However, OLEDs have been slow to enter the market because of some fundamental
physical limitations for which solutions are not immediately obvious. For one, the
human eye is most sensitive to green light, and finding red and blue emitters efficient
enough to match the perceived brightness of green lumophores has been a challenge.
Compounding this problem is the fact that organic materials are broad band emitters,
so blue emitters typically appear blue-green or emit excessive amounts of UV light.
Secondly, an efficient process for laterally patterning red, green, and blue emitters has
yet to be developed. Standard lithographic procedures, such as photoresist spinning,
baking, and stripping, are not compatible with organic materials.
Using colloidal quantum dots (QDs) as luminescent dopants in OLEDs offers an
attractive solution to the challenges described above. QDs are nanoparticles that
can be synthesized to emit anywhere from the ultraviolet to the infrared regions of
the spectrum by changing their size and chemical composition [2]. For example, as
shown in Figure 1-2, varying the size of CdSe QDs between 17 to 120
A tunes
them
to absorb and emit at colors ranging from blue to red. The typical emission spectrum
of a QD has a full width half maximum of less than 40 nn, meaning it has excellent
color saturation. Colloidal QDs routinely have high photoluminescence efficiencies
of 40-60%) that can reach 90 % in synthetic procedures. Furthermore, QDs do not
photobleach as do organic lumophores.
QDs were first integrated into a polymer LEDs in 1994 [9]. Inserting a single
close-packed monolayer of QDs into an OLED structure yielded a device with a peak
external quantum efficiency (EQE) of 0.52% [10]. Narrowband electroluminescnece,
shown in Figure 1-3a, has been observed in QDs throughout the visible and the
infrared [3], [4]. QD-LEDs with organic charge transport layers now boast EQEs of
about 2% in the red, 0.5% in the green, and 0.2% in the blue [5]. Figure 1-3b displays
a picutre of a white LED with an EQE of 0.36% under 10 V applied bias, which was
17
D Di
eter=
120A
80A
72A
C
0
55A
0.
45A
33A
29A
20A
1 7A
400
500
600
700
Wavelength (nm)
Figure 1-2: Room temperature optical absorption spectra of CdSe QDs dispersed
in hexane [2]. Varying the size of the QD, tunes its optical properties continuously
through the visible spectrum. This is not possible for any type of organic lumophore.
The photograph of the QDs excited by UV light reveals their high photoluminescence
efficiencies through the visible region of the spectrum.
18
I
a.
I
*
I
I
*
I
E
2
-j
w
7I
40
N
44
400
500600
1200
Wavelength (nm)
1400
i6o
Figure 1-3: a) Normalized electroluminescence spectra for QDs showing their narrow
band emission throughout in the visible and infrared region of the spectra [3], [4]. b)
White QD-LED made by sandwiching a monolayer of mixed red, green, and blue QDs
between organic charge transport layers [5]. c) RGB pixels, 25 by 25 pm, featuring
electroluminescence from red and green QDs and blue emission from the organic hole
transport layer, N,N'-Bis(3-methylphenyl)-N,N'-bis(phenyl)benzidine (TPD) [6].
created by sandwiching a monolayer of mixed red, green, and blue QDs between
organic hole and electron transporting layers [5]. Deposition of this QD monolayer
was enabled by a microcontact printing technique [6]. By patterning the stamp used
in this printing process, RGB pixels 25 by 25 gm were achieved. Figure 1-3c shows
such a display, created by stamping perpendicular lines red and green QDs. The
blue emission comes from the organic hole transport layer, N,N'-Bis(3-methylphenyl)N,N'-bis(phenyl)benzidine (TPD). These results demonstrate that QD-LEDs are a
promising technology for displays and large area lighting.
1.1.1
Limits of Organic QD-LED Technology
Hybrid organic-inorganic QD-LEDs combine the benefits of two material sets: organics offer ease of fabrication while the inorganic nanocrystals provide efficient, tunable,
19
narrow-band emission. However, hybrid QD-LEDs also retain one of the major problems facing organic LEDs (OLEDs).
Namely, the organic charge transport layers
of hybrid QD-LEDs are susceptible to photooxidation from self-emitting light, thermal and electrochemical degradation, and deterioration from atmospheric oxygen and
water vapor [11, 12, 13, 14]. A variety of packaging techniques, including the simple
solution of a cover glass epoxied to the sample [15], have successfully increased the
operating lifetime and allowed OLEDs and hybrid QD-LEDs to become commercially
viable. However, packaging comprises a significant fraction of the device cost making
it difficult for QD-LEDs to compete with the already established technology of LCDs.
Furthermore, organic materials undergo a change in morphology and decompose at
high drive currents, which frustrates development of an electrically pumped colloidal
QD laser.
My thesis explains how the organic charge transport layers in the QD-LED structure can be replaced with chemically and morphologically stable metal oxides to
achieve robust and efficient devices that can operate unpackaged in air and sustain
high current densities.
1.1.2
QD-LED function
Before discussing the prior work on QD-LEDs with inorganic charge transport layers,
it is important to review basic structure and functioning of a QD-LED. As depicted
in Figure 1-4, the simplest QD-LED consists of a hole transport layer (HTL), a QD
layer, and an electron transport layer (ETL) sandwiched between two electrodes.
Typically the HTL and ETL are each approximately 50 nm thick, and the QD region
ranges from a monolayer to several monolayers of QDs. A band diagram for such
a device is shown schematically in Figure 1-4b. When the device is in forward bias
(See Figure 1-4c), the anode injects holes into the HTL while the cathode injects
electrons into ETL. The field across the device carries the holes and electrons to the
QD layer, where the electrons and holes can form bound pairs, known as excitons, on
the QDs. Excitons can dissipate their energy by emitting light characteristic of the
optical bandgap of the QDs.
20
b.
J__
ETL
a..
s
Q~~spotons
Figure 1-4: A schematic of the (a) structure and (b) band diagram of a typical QDLED structure. The schematic in (c) depicts the injection of charge into device and
the formation of excitons across the QD layer for a QD-LED under forward bias.
1.2
All-inorganic QD-LEDs
There are two types of QDs: epitaxial and colloidal. Colloidal QDs are synthesized
from organometallic precursors injected into an organic solvent at high temperature.
The temperature activates nucleation of small crystallites, which continue to grow
from the unreacted precursors until stopped by cooling. This procedure allows for
fine control over QD size and results in a very monodisperse solution. The synthesis
also leaves colloidal QDs coated in organic ligands so that the QDs can be made
soluble in a diverse set of solvents including chloroform, hexane, ethanol, and even
water. Colloidal QDs are therefore solution processable and can be patterned on
substrates using large scale techniques such as spin coating, microcontact-printing,
and inkjet deposition. Epitaxial QDs, often known as self-assembled QDs (SAQDs),
form when a semiconducting material is deposited, generally through molecular beam
epitaxy (MBE), on a substrate having a different lattice constant or at a very high
rate. Strain causes the top material buckle and form QDs, in what is referred to as
Stranski-Krastanow growth. MBE can then be used to deposit a capping layer and
the top charge injection layer [16]. As grown, SAQDs are already integrated into a
21
monolithic semiconductor structure making them a seemingly obvious choice for a
robust and stable all-inorganic LED structure.
However, colloidal QDs possess properties that make them superior to epitaxial
QDs for optoelectronic applications. Epitaxial QDs do not offer the monodispersity,
the high photoluminescence intensities, the low cost, or the easy fabrication advantages of colloidally grown QDs. Also, the size of an epitaxial QD is determined by the
material set available. This prevents integration of more than one color QD into the
simple three-layer structure described in Section 1.1.2, making it difficult to realize
RGB pixels or white light LEDs. For these reasons, my thesis focuses on the development of LEDs with colloidal QDs sandwiched between inorganic charge transport
layers.
So far, several methods for integrating colloidal QDs into inorganic heterostructure have been proposed, but have only met with limited success. Early attempts
to use inorganic transport materials in QD-LEDs placed QDs between indium tin
oxide (ITO) and silver electrodes [171. The low efficiency of about 10-3 cd/A in these
devices is probably due to quenching of the QD luminescence by the highly conductive electrodes [18]. A novel fabrication technique, which was used to build the first
all inorganic colloidal QD device exhibiting narrow-band electroluminescence (EL),
is known as energetic neutral atom beam lithography/epitaxy (ENABLE) [7]. Thin
films grown using ENABLE are similar to those grown with metal-organic chemical
vapor deposition (MOCVD), but ENABLE does not require elevated substrate temperatures and organometallic precursors, which destroy the luminescent properties of
QDs. ENABLE involves a chemical reaction on the sample surface between a beam
of neutral atoms, such as nitrogen, and a metal, like gallium, that is simultaneously
deposited via e-beam evaporation. Low temperature ENABLE (300 C) allows deposition of GaN on the QDs without compromising their performance; in fact, the GaN
acts as an encapsulating layer, reducing the photo-oxidation of the QDs. As depicted
in Figure 1-5a, the devices themselves consist of a Langmuir-Blodgett film of colloidal
CdSe/ZnS QDs sandwiched between a p-type, MOCVD, GaN layer grown on a sapphire substrate and a n-type GaN layer grown using ENABLE. The device is reported
22
a.
SEM image
Figure 1-5: a) An SEM image showing the device structure composed of a colloidal
CdSe/ZnS QD layer sandwiched between a p-type, MOCVD, GaN layer grown on a
sapphire substrate and a n-type GaN layer grown using ENABLE. b) Photographs
of the device electroluminescence at 10 and 30 V for which non-uniform emission is
evident. The white arrow points to blue emission from the GaN. [7]
to exhibit no degradation after several months; however, as shown in Figure 1-5b, its
operating voltages are fairly large and there is non-uniform illumination of each pixel.
At 30 V, blue GaN emission is observed. Furthermore, the EQE was small (0.001 to
0.01%).
Integration of colloidally and epitaxially grown QDs has been proposed as another
strategy for obtaining an all inorganic device although demonstration of EL has yet
to be realized in such a structure. One study describes a hybrid structure where
colloidally grown InAs QDs are deposited on top of a GaAs SAQD structure grown
using MBE [19]. Migration enhanced epitaxy (MEE), which is a low temperature
(350 C) technique similar to MBE, is then used to overcoat the colloidally grown InAs
QDs with epitaxial GaAs. This design combines the structural benefits of SAQDs
with the highly photoluminescent (PL) efficiencies of colloidal QDs. Another study
23
investigated the reverse process of growing epitaxial QDs on colloidal QDs to eliminate
nonuniformities in QD size at their interface with the charge transport layers [20].
Specifically, CdSe(ZnS) core shell QDs are overcoated with ZnSe deposited via MEE.
Previous research in our group has shown that replacing the organic charge transport layers with metal oxides is a viable approach to creating a more chemically and
electrically stable QD-LED. Uniform EL was achieved by replacing an organic hole
transport material with p-type nickel oxide (NiO) and continuing to use the standard
organic material, Tris-(8-hydroxquinoline) aluminum (Alq 3 ) as the electron transport
layer [21]. To fabricate this device, NiO was radio-frequency magnetron sputtered
in an argon and oxygen environment onto conductive indium tin oxide (ITO), which
serves as the anode. The oxygen content of the plasma during sputtering as well
as the deposition rate determined the number of excess hole donor sites (the extent
of the p-type doping) of the NiO. To complete the device, which is depicted in the
inset of Figure 1-6a, QDs were spin coated on top of the NiO, and Alq 3 and a silver
electrode were thermally evaporated on the QDs. The spectrum in Figure 1-6a shows
EL characteristic of the QDs, and a peak external quantum efficiency (EQE) of 0.18%
was observed (See Figure 1-6b).
This work demonstrated three areas where, if metal oxides are to be used as the
charge transport layers of a QD-LED. considerable improvement is needed. First, the
device yield of 10% and the low EQE suggest that much of the injected current is
shunted through the device structure due to ITO and NiO surface roughness. Second,
QD luminescence is most likely quenched by free carriers in the NiO layer. Third,
differences in carrier mobilities in the Alq3 and NiO layers can cause either electron or
hole pile up at the QD layer. This can result in charging of the QDs and subsequent
Auger processes, which can contribute to diminished device EL.
The goal of my thesis work was to address these three areas limiting the integration
of metal oxides into the QD-LED structure and to investigate whether metal oxides
can be successfully deposited onto top of QDs to form a high efficiency, all-inorganic
QD-LED.
24
a.
I
.
*
I
I
*
12 -
C
8
QDs
4
-l
-p
200
500
400
300
600
700
800
Wavelength (nm)
b.
0.1
140
12
wj 0.01
0101
1E-3
8
S61r
S4.
2
-
. ,i
6
8
-4
i
10
6
,
8 10 12 14
i V ltag (V
12
Voltage (V)
14
16
16
18-
18
Figure 1-6: a) EL spectrum for hybrid organic-inorganic device made with QDs sandwiched between a sputtered NiO HTL and an organic (Alq 3 ) ETL biased at 9 V. b)
i-v and EQE curves for device.
25
1.3
QD-Lasers
An ideal QD is a 0-dimensional structure; an electron in it is confined in all-directions
as if in a box. The QD density of states given by a series of 6 - functions at the energies corresponding to the discreet levels allowed in the "box". These quantized
levels give the QD atom-like electronic and optical properties. Because of these characteristics, a semiconductor laser with a QD active region promises, among other
advantages, low and temperature-independent threshold current and high-frequency
modulation with negligible chirping effects (small linewidth enhancement). [22]
In 1993, a laser based on epitaxially QDs was realized [23]; however, a QD laser
operating at room temperature has yet to exhibit a better modulation bandwidth and
a smaller linewidth enhancement factor than a quantum well lasers [24]. A large modulation bandwidth is possible if the structure has large differential gain [25]. A large
differential gain is predicted for a structure with a high density of QDs [24]. However,
multilayered epitaxial QD growth is challenging and leads to nonuniformities in QD
size. This causes an inhomogeneous broadening of about 10 to 30 meV [26], which in
turn results in a reduced differential gain [24].
In contrast to epitaxial QDs, colloidal QDs have better size distribution and can
be easily deposited in densely packed multilayers. To date, however, the only efficient
electroluminescent colloidal QDs structures involve a monolayer of close-packed QDs
sandwiched between organic layers for charge transport and injection. As discussed in
Section 1.1.1, the Van der Waals-bonds of organic materials can not support the high
current densities required for laser operation. Metal oxides have primarily covalent
and ionic bonds, suggesting that they can sustain much higher current fluxes than
organics. Data presented in Chapter 4 indicates that colloidal QD and metal oxide
structures may be the key to a high-frequency modulation and small chirp laser.
26
Chapter 2
Material Properties and Growth
2.1
Metal Oxides
Metal oxides, and ceramic materials in general, are attractive charge transport layers because of their great range of electrical properties. Ceramics typically have the
chemical formulas: MaXc or MaNbXc, where M and N are metals, and X is a nonmetallic element [27]. They can be metallic (e.g. TiO2), insulating (e.g. ZnO or
NiO), or semiconducting (e.g. nonstoichiometric ZnO or NiO). In this thesis, I will
discuss the electronic ceramics indium oxide (In 2 0 3 ), nickel oxide (NiO), tin oxide
(SnO 2 ), zinc oxide (ZnO), tungsten oxide (W0 3 ), and zinc sulfide (ZnS).
There are two types of bonding in these materials: ionic and covalent bonds. Ionic
bonds are defined by the Coulombic attraction between oppositely charged ions and
result in insulating materials with very strong, non-directional bonds. Ionic bonding
occurs if the energy of the molecular unit is lower than the energy of the separated
ions. Covalent bonding, on the other hand, is the sharing of electrons. The electrons
reside somewhere between the two atoms, in a configuration that depends on which
electronic orbitals are sharing the electrons. This can lead to directional bonds. In
ceramic materials, the valence electrons of the metal ions are shared with the valence
shell of the nonmetal ion, which leads to bonds that range from highly covalent to
highly ionic. [28]
Depending on the growth process and thermal treatment of the electronic ceramic,
27
a.
x
b.
x
Figure 2-1: Disordered Kronig Penny model (a) potential and (b) resulting wavefunction.
its microstructure can range from crystalline to amorphous. In this work, we aim for
amorphous ceramic charge transport layers so that charge travels through the bulk
of the materials instead of along grain boundaries or defects in a crystalline or polycrystalline layers. This assures more uniform charge transport that will be immune
to defects in the charge transport layers. While the band theory and consequently,
electronic transport, of crystalline ceramics is well understood, amorphous materials
lack periodic boundary conditions and long range order. In an amorphous material,
the wavefunction loses phase coherence over several atomic spacings. Some understanding can be gained from modeling an amorphous material with one-dimensional
Kronig-Penny model where the finite square well potentials are spaced at random
intervals as depicted in Figure 2-1a. This potential has a wave packet solution, shown
in Figure 2-1b, and given by:
c e^Ixasin(kx)
(2.1)
This model implies that all electronic states in an amorphous solid are localized [29].
Extending the finite square well problem to two or three dimensions however
indicates that there can be both localized and extended electronic states. Figure 2-2
shows a typical electronic density of states. The extended states (or Bloch states)
resemble conduction and valence bands in a crystalline material. Disorder in the solid
leads to exponential band tails [30]. Defects in the crystal result in mid-gap states.
28
Extended
States
...........
EB
Diffusion
Ec .States
Mobility Gap
C
E>
(Localized
States)
Ev .....
Density of States
Figure 2-2: Schematic of the density of states for an amorphous solid semiconductor
at different energies.
The band tail and mid-gap states are localized states and lie in a region of the band
diagram known as the mobility gap.
A quantity known as the Anderson parameter quantifies the degree of electron
localization of a particular state. The Anderson parameter, A, is given by:
A =
AE
(2.2)
d.''
J is the overlap integral of periodic basis states, coupled by nearest neighbor interactions. Disorder is introduced into the model by assuming that each basis states
resides at a different site energy spanning a width, AE:
ZJ
J= E
0,Vn,kidr3
(2.3)
n~i
where
i electron wavefunction of the ith state, Vn is the interaction potential of
the nth site.
When the Anderson parameter is small (about 0.2), all states can
be considered localized. In metal oxides, there is typically some critical energy, Ec,
below which all states are localized. This energy is typically slightly less than EB,
the mobility edge. [29]
The regimes of the band structure in an amorphous ceramic discussed above ex-
29
hibit different mechanisms for charge transport. Above the mobility edge (EB), there
is band conduction as in crystalline semiconductor, although with considerably lower
mobility because of scattering events from the disorder in the lattice. Between EB
and Ec, electrons move via diffusion, and below E, via thermally activated hopping.
In the diffusion transport regime between EB and Ec, the electronic wavefunctions
are highly modulated, meaning that there are alternating regions of high and low
probabilities of finding an electron. Ignoring the possible interaction of the electron
and the lattice and the subsequent formation of polaron states, the mobility of an
electron is:
ea =
(2.4)
Tg
=FVel
where a is the distance between the electronic wavefunction maximum and minimum
and vij is the frequency that an electron is at a maximum. [27]
The classical picture of the "hopping" transport mechanism is shown schematically
in Figure 2-3. Assume two localized sites, separated by energy AE (Figure 2-3a). An
electron localized on site 1, can, with thermal energy, polarize the lattice(Figure 23b). The binding energy of the electron (Eb), which is the sum of energy required
to polarize the lattice and energy the energy the electron gains from polarizing the
lattice, is given by:
e2 1
-(
2ro k.
Eb=
1
)
k,
(2.5)
where ro is the radius of the electronic state, and k, and k, are the infinite-frequency
and static dielectric constants, respectively. For the electron to hop to site 2, the
electronic energies of the states must be equal(Figure 2-3c). The difference in energy
between this equal energy configuration and the original energy configuration with
the sites separated by AE is called the hopping activation energy and is given by:
AE
2
Eb
2
(AE) 2
(2.6)
8Eb
When the energy of the two sites are equal, the electron can tunnel from site 1 to site
30
Ef
-..--.--
E..-.-f)
-.......-
AE
...-..
2Eb
.....
...
Ef .......
b)
Site 2
Site 1
a)
-.....
Eb -A
.
Eb6+AE
2
2
........................
2Eb
Figure 2-3: Localized electron hopping model.
2 with a probability, p = exp(-2ad). This gives a transition rate of:
W=pvexp
-EH
'
(2.7)
where v is the phonon frequency. If two sites are close enough together, adiabatic
hopping can take place, in which an electron tunnels repeatedly between site 1 and
2, aided by a resonance process. [27].
This "hopping" conduction is the primary mode of charge transport in the midgap defect states. In metal oxides, defects can arise from dopants, vacancies, or
structural variations, and are extremely important in defining the electrical properties
of the material. In this work, the most important electronic defects result from the
reduction or oxidation of the metal ions, which can be controlled with the oxygen
partial pressure during the oxide growth [31]. At low oxygen concentrations, material
can lose oxygen and generate electrons that contribute to n-type conductivity:
00
--
-02 +V
2
31
+ 2e//
(2.8)
The nomenclature indicates that when a vacancy V'* is created at a previously occupied oxygen site (O.), free oxygen molecules (On) and electrons 2e// result. If there
is instead a surplus of oxygen, it can fill an oxygen vacancy, leaving two holes and
contributing to p-type conduction:
1
-02 + V * -- O0 0 + 2h'
2
(2.9)
We note that while ZnO and NiO are both insulating metal oxides, oxygen depletion
leads to n-type ZnO (Zni+,O.) and oxygen doping results in p-type NiO (Ni,0 1+,).
As will be discussed in Chapters 3 and 4, this fine tuning of the conductivity offered
by metal oxide proves crucial to the development of efficient all-inorganic QD-LEDs.
2.2
Radio Frequency Magnetron Sputtering
We choose radio-frequency (RF) magnetron sputtering to deposit the ceramic thin
films. RF magnetron sputtering rates are similar to those of thermal evaporation,
making it a fast deposition technique compared, for example, to MBE, MEE, or
ENABLE that have been used in previous attempts to make all-inorganic QD-LEDs.
In the sputtering process, energetic ions bombard a target, causing atoms to be ejected
from the target and land on a substrate, which, in our system, is located above the
target. The ions are provided by a plasma, which is generated by flowing gas, typically
Argon, between two electrodes. An electron. accelerated by the potential between
the electrodes, collides with an Argon atom resulting in an Argon ion (Ar+), a free
electron (ei 0,), and the now much slower original electron (e owed). Symbolically, this
reaction, can be summarized by:
efast
+ Ar -+ Ar++e-
+ e-wed
e~owe- and e- 0 are accelerated by the field between the electrodes.
(2.10)
When these
electrons attain 15.76 eV of energy, which is the ionization energy of Ar, they can
each cause the reaction of Eqn. (2.10). The creation of Ar ions is therefore an avalanche
32
process. [32]
To understand RF sputtering, it is useful to review the mechanism for gas discharge in DC sputtering. In DC sputtering, a voltage is applied between an anode and
a cathode, and ions are attracted to the cathode while electrons are repelled. This
surplus of ions screens the negative charge of the cathode, causing most of the voltage
to be dropped across an area close to the cathode, which is known as Crook's Dark
space. In this space, electrons are accelerated away from the cathode to ionize Ar
atoms as described previously. Meanwhile, this voltage drop accelerates ions toward
the cathode. The cathode is also the sputtering target, so ions etch microscopic bits
of material from target. In this work, we consider relatively low energy sputtering
so the process can be modeled as momentum transfer through elastic collisions. The
maximum energy transferred in a collision between the ion (with mass M1 and energy
E) and a target atom (with mass M2 ) is:
4M A1 2 E
(Al 1 + M 2 )2
(2.11)
This corresponds to a sputtering yield:
S 0C
1
A
A(E)cosO (M1 +1 A2 2 )2
E
(2.12)
where 9 is angle between the target surface normal and the incident ions and A(E)
is the mean free path for ions near the target surface [33].
These calculations in-
dicate that sputtered atoms leave the target with kinetic energies of 3-10 eV. Our
rf-sputtering is done at pressures of 4-6 mTorr, and the sputtered atoms undergo
collisions with gas atoms in the chamber. When they arrive at the substrate, which
is located between 5 and 6 inches from the target, the atoms have approximately 1-2
eV of energy.
The process described above is DC sputtering; in RF sputtering, the anode and
cathode are reversed at radio-frequencies. In our chamber, the polarity is alternated
at a standard 13.65 MHz. RF sputtering gives a greater range of material choices than
DC sputtering. In DC sputtering, if a target made of an electrically insulating oxides
33
Ar ions
ta rget
RF electrode
grounded
neshield
watercooling
to RF power
supply
Argon
gas
Figure 2-4: Cross sectional view of electrode assembly for RF sputtering.
were placed at the cathode, positive charge would accumulate on the target during
the ion bombardment. This would neutralize the cathode voltage, gradually impeding
the acceleration of the electrons and the subsequent ionizing collisions, and prevent
DC sputtering. In contrast, with RF sputtering, the anode and cathode are reserved
during each cycle, and the target is alternatively bombarded with ions and electrons.
The electrons neutralize the positive charge. Electron mobility is several orders of
magnitude larger than ion mobility so the target will self-bias itself negatively with
respect to the plasma. This negative bias attracts ions and causes the formation of
the Crook's dark space as described previously in the case of DC sputtering. The
choice of the 13.56 MHz frequency is important; if the polarity switching is too slow,
not enough ions will not reach the cathode area to form the dark space. Alternatively,
if the switching frequency is increased, the negative-self bias will become larger and
the sputtering rate will grow. [34]
Magnetron sputtering refers to superimposing a static magnetic field on the electrode to confine the electrons to the plasma where they have a higher probability of
ionizing an Ar atom [35]. This increases sputtering rates and prevents the high en34
ergy electrons from hitting the sample surface and heating the substrate. Meanwhile,
the magnetic field does not significantly effect the path of the Ar gas ions, because
their mass is orders of magnitude larger than that of the electrons. The schematic
in Figure 2-4 presents a cross sectional view of the electrode in an RF magentron
sputtering system.
The structure of sputtered thin films depends on the deposition rate, the pressure
of the working gas, the temperature of the substrate, and the substrate surface roughness. Optimization of these parameters for construction of an all-inorganic colloidal
QD-LED will be discussed in Chapters 3 and 4 of this thesis.
2.3
Quantum Dots
As discussed in Section 1.2, QDs are attractive lumophores for thin film LEDs because
their emission is continuously tunable across the visible and infrared wavelengths. In
this section, I follow Ref. [36] to explain the origin of the unique optical properties of
QDs.
The absorption and emission properties of a QD are governed by the quantum
size effect. The material of a QD is structurally identical to that of the bulk crystal,
but, in a QD, the electron and hole pairs are confined by boundaries of the QD.
This confinement leads to quantization of the bulk energy levels, resulting in atomiclike absorption and emission spectra for QDs. These quantum size effects become
important when the size of the QD becomes less than the Bohr radius:
aB= ~~-na
M*
(2.13)
where f is the dielectric constant of the material, m* is the effective mass of the
particle (electron or hole), and m is the rest mass of the electron, and a0 is the Bohr
radius of the hydrogen atom.
When the radius of a QD is smaller than the Bohr radius of the hole, electron, and
exciton, we can approximate the QD with the particle in a sphere model. That is,
electrons and holes can be considered to be in a hollow sphere enclosed by an infinite
35
potential. The energy of a particle in this potential is given by:
h2 k2
En=
n-
2mo
h2
-
2
(2.14)
'
2moa 2
where a 2 , is the nth zero of the lth-order spherical Bessel function. These energies
resemble the the kinetic energy of a free particle, except that in this case the wave
vector, kn,, is quantized. From Eqn. (2.14), it follows that as the confinement increases
(a decreases), the energy of the electron or hole wavefunctions is increased, and an
electron-hole pair recombination event will release more energy. Therefore, as CdSe
QDs are reduced in size, the emission becomes bluer.
In bulk crystalline semiconductor theory, it is assumed that every particle in the
solid experiences some average potential that is periodic, such that its wavefunctions
can be described by Bloch functions:
XIn,k(T) =
(2.15)
Un,k(Tr)exp(k -T)
where un,I(f) has the same periodicity as the crystal potential and exp(k .f) describes
the wavefunction phase shift between the atoms of the crystal. The wavefunctions are
indexed by the wavevector k and the band index n. Furthermore, the effective mass
approximation considers the conduction and valence bands of the semiconductor to
be parabolic and uses the effective mass to account for the complexities of the band
structure.
If the QD diameter is larger than the lattice constant of the QD material, it is
possible to use the effective mass approximation and write the single particle wave
function as a linear combination of Bloch functions:
=jS
C,,,u.,I(T) e xp
36
T)
(2.16)
By separating terms with strong and weak k dependence, Eqn.(2.16) becomes:
Cexp(k
j
- ) = u,,,o(7;)fp (f)
'1'p (r) = Un,O( )
(2.17)
k
where f8p(r) is the envelope wavefunction and un,O (r) can be determined from the
tight-binding approximation by summing over the atomic wavefunctions, e0,k. With
these approximations, it is possible to write the energy of the electron-hole pair states
as:
h2
P2lhkh
Pne ke~
E=
Eg + 22( m*,
M'* +
±*eM''
E~2ya2
1.8e2
- .E
(.8
(2.18)
where
1.8e 2
Ea
(2.19)
is the first order Coulombic correction factor that accounts for the attraction between
the electron and the hole in the QD.
Given Eqn. (2.17), the absorption coefficient, a(w), is derived as for the bulk crystal
case.
a(w) = mrw Pcvp(hW
mown
-
Eg)
(2.20)
The overlap of the electron and hole wavefunctions is given by the dipole matrix
element
peC = (uc.
P0uv)
(2.21)
and is related to the probability that an electronic transition will occur. p is the
density of states, which as discussed in Section 1.3, is 6-functions in the case of the
0-dimensional QD. Therefore, the absorption spectrum of a QD will have a distinct
peak. In reality, there is inhomogeneous broadening of the absorption spectrum due
to size distribution of a QD sample. This non-uniformity in QD size also accounts
for the broadening of the emission peak.
While this simplistic particle in a sphere model explains the basic absorption
and emission properties of QDs, it does not account for complexity of the valence
band, mixing of the three valence subbands, or electron-hole exchange interaction.
37
Much theoretical work has been done developing more advanced models that take
into account these effects.
2.4
Colloidal Quantum Dot Synthesis
The first examples of colloidal QDs were CdSe, CdS, and CdTe nanocrystals [2].
Figure 2-5 shows the basic steps of colloidal QD synthesis. A reaction vessel containing
a coordinating solvent for colloidal dispersion and electronic surface passivation is
preheated to 300 C and then removed from the heat. Organometallic precursors are
then are injected into the reaction vessel which results in homogeneous nucleation
of nanocrystals until the reagents are depleted. This nucleation reaction causes the
temperature of the vessel to drop to approximately 180 C. The reaction vessel is then
gradually reheated to approximately 300 C to allow for slow growth and annealing
of the QDs.
The absorption spectrum of reaction solution is monitored and the
growth temperature modulated to precisely control the size of the QDs. Less than
5% deviation in QD size is obtained. The QDs are precipitated and centrifuged dry
to separate them from the reaction byproducts. They are then redispersed in the
solvent of choice. Since this process was first developed in 1993, techniques have
been developed to improve the photoluminescent efficiencies of the QDs. While CdSe
and CdTe core QDs can provide tuned emission throughout the visible and infrared,
there are specific techniques and material sets that work well for particular parts
of the spectrum. The following paragraphs briefly describe the techniques Jonathan
Halpert in the group of Professor Moungi Bawendi at MIT employed to synthesize
the QDs used in this work.
Red QDs discussed here are either CdSe core QDs overcoated with ZnS ((CdSe)Zns)
or ZnCdSe alloyed QDs, shown schematically in Figure 2-6a and b. (CdSe)ZnS QDs
are synthesized following the method of Dabbousi et.al. [37]. The CdSe cores are
made as described above. The ZnS shell is grown on the cores using a technique
similar to that of the core synthesis. CdSe cores, capped in the coordinating ligands
trioctylphosphine oxide (TOPO), are placed in a reaction vessel, which is heated to
38
1.
2.
V--?
r"
or,
Preheat coordinating ligands
(-300 C)
3.
Remove coordinating ligands from heat;
inject organometallic precursors.
Nucleation reaction turns clear ligand
solution yellow-green.
Heat solution (200-300 C) for several
hours until QDs reach desired size
Trioctylphosphine oxid
(TOPO) capping ligand
Figure 2-5: Schematic depicting synthesis procedure for CdSe as first described in [2].
39
b)
a)
\\\
ZnS
///1A
shell
d)
C)
~2V~ ~
CdSe
d
ZnS
shell
w
0
/
Figure 2-6: Schematic of the structure of a) ZnCdSe, b) (CdSe)ZnS, c)
ZnSe/CdSe/ZnS, and d) ZnCdS QDs.
a temperature between 200 and 300 C. The organometallic precursors, diethyl zinc
and hexamethyldisilathiane, for the ZnS shell are slowly injected into the vessel and
stirred. The solution is cooled to 90 C and left stirring for several hours. Alloyed
ZnCdSe QDs are synthesized following the work of Zhong et.al. [38]. Trioctylphosphine selenide is injected into a pot of ZnO, CdO, oleic acid, and 1-octadecene at 310
C. The QDs were precipitated twice with acetone and redispersed in chloroform.
The green QDs used in this work are "triple-decker" structures since it is difficult to synthesize alloyed ZnCdSe QDs below 540 nm. "Triple decker" synthesis
consists of making double shells of ZnSe and CdSe as described by Ivanov et.al. [39]
and overcoating these cores with ZnS 2-6. ZnSe cores were prepared by injecting a
solution of diethyl zinc, trioctylphosphine selenide (TOP-Se) and TOP into a flask of
hexadecylamine (HDA) at 310 C. The cores were then grown at 270 C for 2 hours.
The solution was cooled to 150 C injected into a degassed solution of TOPO and
40
hexylphosphonic acid (HPA) while a solution of dimethyl cadmium, TOP-Se and
TOP was added dropwise. The solution was heated at 150 C for 19 hours. These
cores, made of ZnSe and CdSe, were separated from their growth byproducts and prepared in a reaction vessel at 150 C. Two solutions, one of dimethyl cadmium, diethyl
zinc and TOP, the other of TMS2-S and TOP were added slowly to the vessel after
which the solution was cooled to room temperature. The QDs were then precipitated
using methanol/butanol as above and redispersed in hexane. This process was performed three times, filtering after each dispersion with a 0.2m filter, and redissolving,
in the final step, in chloroform. The quantum yield of the ZnSe/CdSe/ZnS QDs was
measured to be approximately 65% using coumarin 540 as a standard (89% QY in
ethanol).
Blue emitting QD are ZnCdS (See Figure 2-6) and are prepared by injecting a
degassed solution of oleylamine and elemental sulfur into a flask under argon, which
contained a clear solution of CdO and ZnO dissolved into oleic acid and octadecene
at 310 C, similar to that reported by Zhong et.al. [38]. After cooling, the ZnCdS
cores were then precipitated by the addition of acetone and separated from the supernatant by centrifugation. The QDs were then precipitated a second time using
methanol/butanol, centrifuged and redispersed in chloroform. The quantum yield for
the ZnCdS QDs was found to be about 48% using coumarin 480 as a dye standard
(99% QY in ethanol).
41
42
Chapter 3
Design of All-Inorganic QD-LEDs
In order to first design and then make systematic improvements to a metal oxide
based all-inorganic QD-LED, we characterize the electrical and structural properties
of the individual layers and the interfaces of the structure. The conclusions of the
work described in this chapter are perhaps best summarized in tables. Table 3.2.3
lists the growth parameters for the metal oxides that make them compatible with
QDs, and Table 3.3.1 provides the approximate electronic states for the metal oxides
and the QDs.
3.1
3.1.1
Characterization of Sputtered Films
Atomic Force Microscopy
Atomic Force Microscopy (AFM) is used to study the surface quality of sputterdeposited metal oxides. In AFM, a sharp Si tip, with a radius of curvature of several
nanometers and attached to the end of a cantilever, is brought in close proximity
with the sample surface. The cantilever is oscillated near its resonant frequency.
Forces between the sample surface and the tip alter the oscillation frequency of the
cantilever. Measuring this change in the oscillation frequency provides an image of
the sample surface topography.
As discussed in Section 1.2, surface roughness of the ITO layer translated into
43
b.
a.
5 nm
2.5 nm
Onm
5pmx5pm
10pmx10pm
Figure 3-1: Atomic Force Microscopy surface topography images of a) ITO RFsputtered on glass, and b) NiO RF-sputtered onto the ITO shown in (a)
roughness of the NiO deposited on top of the ITO, causing current shunts through
the device structure and device yields of only about 50%. One of the first steps to
designing efficient devices with large yields is therefore to develop consistent procedures for sputter depositing smooth metal oxide layers. Sputtering a series of films
with different growth conditions and characterizing their surfaces with AFM revealed
that depositing ITO on glass at a rate of 0.06 A/s using a power of 12 W in an
inert Ar atmosphere at 4 mTorr achieves a surface roughness of less than 1 nm rms
(Figure 3-1a). Heating of the substrate during growth was required to control the
ITO resistivity. NiO sputtered onto this ITO at a deposition rate of 0.2 A/s in a
1:100 02 to Ar atmosphere at 6 mTorr using 210 W of RF power maintains the film
smoothness (Figure 3-1b).
Co-sputtered ZnO and SnO 2 (ZnO: SnO 2 ) can also be used as a hole transport
layer (See Section 4.4). Figure 3-2 shows a ZnO: Sn0 2 on ITO surface with only
0.56 nm rms roughness. The ZnO: SnO 2 was deposited by simultaneous sputtering
ZnO at 15 W and SnO 2 at 9 W RF power in an argon environment at 4mTorr. This
corresponds to a 0.2 A/s deposition rate. These deposition powers were selected to
tune the conductivity of the film; as long as the power for each deposition remains
under 20 W, the film surface is smooth.
44
7 nm
2.5 nm
O nm
10 pm x 10 pm
Figure 3-2: Atomic Force Microscopy surface topography images showing smooth
ZnO: SnO 2 on top of the ITO in Figure 3-1a
3.1.2
X-Ray Diffraction
X-Ray Power Diffraction (XRPD) can be used to analyze the crystalline structure
of materials. X-rays incident on a crystal with lattice planes spaced a distance d
apart are reflected off the crystal ions. The path difference between reflections from
neighboring lattice planes is equal to 2dsin(6). If this path difference is equal to an
integer number of wavelengths, constructive interference will result. This is commonly
referred to as Bragg's condition. If the material is crystalline (has good periodicity),
at certain values of 6, all the reflections will be in phase and result in discrete, sharp
peaks in a collected intensity vs. 9 plot. In amorphous solids, which lack a periodic
structure, these peaks are not present. [28]
The ceramic materials used in our devices (See Table 3.2.3 for information on
deposition parameters) were measured using a Rigaku Powder Diffractometer, which
consists of a high-powered rotating anode generator that supplies X-rays to a 250mm
Bragg-Brentano diffractometer. Figure 3-3 shows the X-ray diffraction spectra for
the different types of ITO used in our structure. Often the top electrode of our
devices is ITO deposited as described in Section 3.1.1 but without heat, which would
45
damage the QD layer. Figure 3-3a shows this RF sputtered ITO to be amorphous.
When ITO is deposited under the same conditions and with heat, estimated to be
about 250 C at the substrate surface, the film becomes polycrystalline (Figure 3-3b).
Commercially purchased ITO exhibits even more crystalline features (Figure 3-3c).
Either the commercially purchased ITO or the ITO sputtered in a heated chamber
can be used as the bottom electrode.
Figure 3-4 shows the X-ray diffraction spectra for the charge transport layers.
Figure 3-4a indicates that the NiO we grow is in its natural occurring from, bunsenite, which has a cubic structure that exhibits predominately (111) faces with an
octahedral morphology. The ZnS film appears completely amorphous. The ZnO is
polycrystalline. Crystalline wurtzite ZnO has unit cell dimensions a = 3.250A and
c = 5.207A. S1n
2
is a tetragonal rutile structure with unit cell dimensions of a
4.737A and c = 3.186A [40]. As expected for a film that is blend of two materials
with different crystal structures and sizes, ZnO : SnO 2 is amorphous.
This XRPD data highlights several aspects of all inorganic QD-LED design that
are not yet fully understood. First, the devices with any of the three types of ITO
electrodes turn on, indicating that a smooth surface - not the crystal morphology of
the film - is the critical parameter. However, it is unclear whether different degrees
of crystallinity lead to non-uniformities in lateral conductivity that may effect device
yields, performance, and stability. Furthermore, we do not know whether the degree
of ordering in the ITO layer alters the subsequent material deposition. Secondly, we
aim to deposit amorphous charge transport layers, reasoning that defects or grain
boundaries in a polycrystalline material could result in unwanted current pathways
and a shunted device. While ZnS and ZnO : Sn0 2 are amorphous, ZnO and NiO show
some crystal structure. Further experiments will be done to determine whether more
crystalline films improve the device performance. For example, crystalline ZnO is a
wurzite structure, which does not possess an inversion center, so polar or nonpolar
surface states can develop depending on which index face is exposed [40]. Control
over the ZnO surface could perhaps be used to facilitate deposition and stability of
QDs next to ZnO.
46
a.
180-
I k.
160-
U
14012010080
b.
iL.I~
.
1k .L.I
I
~
60
40)
-I
20
10
70
0
0
30
2
a
20 (deg)
250
200150"TIM-
100.
50II
20
10
o
do
50
40
30
D
20 (deg)
C.
.
.
.
.
.
.
.
.
.
.
.
.
.
350300250200.
8
150
1005010
20
30
40
50
20(deg)
60
70
80
Figure 3-3: X-ray diffraction spectra for a) RF-sputtered ITO, b) RF-sputtered ITO
in a heated chamber, and c) commercially purchased ITO.
47
NiO
ZnO
180
200-
160140-
150-
120
100-
100-
80-60
50-
40
20
I
20
10
30
60
D
40
h
8o
9bo
3
'0
3
20(deg)
ZAIV I
.
.
.
.
.
.
.
.
.
2 50
20 (deg)
* 70
4'
'
.
ZnS
S 2
180-
200-
16014D-
150-
120100-
10050-
2010
20
30
50
20 (deg)
40
60
70
Eb
i
20
30
40
50
60
70
20 (deg)
Figure 3-4: X-ray diffraction spectra for NiO, ZnS, ZnO, and ZnO : SnO 2 .
48
do
3.1.3
Hall Effect Measurements
Hall Effect measurements provide a quick method for determining the carrier density, electrical resistivity, and mobility of carriers in a semiconducting material. Hall
Effect measurements cannot be assumed to be reliable for amorphous materials [29].
However, in metal oxides with high enough carrier densities, it may be possible to
determine approximate values for resistivity and mobility.
A combination of resistance and Hall measurements is needed to calculate the
mobility. A square sample is contacted at each of its corners (1, 2, 3, and 4). First, the
sheet resistance is determined using the Van der Pauw technique. Two characteristic
sample resistances, RA and RB, are measured by applying a dc current (I) between
two contacts and measuring the voltage across the other two. Mathematically, this
can be expressed as:
(3.1)
RA =
112
RB =
(3.2)
123
RA and RB can be used to solve for the sheet resistance, Rs, with the van der Pauw
equation:
-IrRA
-7r RB
)=1(3.3)
exp( Rs ) + exp(
Rs
If the thickness, d, of a sample is known, the bulk electrical resistivity, p can be
calculated using p = Rsd. The next step is determining the Hall voltage. When a
conducting material carrying a constant current i is placed in a magnetic field, B.
electrons (or holes) will follow a curved trajectory according to the Lorentz force,
Fm = qv x B, and pile up on one side of the resulting material. Uncompensated
electrical charge will be left on the other side of the material, resulting in the buildup
of an electrical field, E. Quickly equilibrium will be reached such that the force the
electric field FE = qE on a carrier completely counteracts the force of the magnetic
field FM = qv x B on the carrier. After this equilibrium condition is met, E will
49
Table 3.1: Hall Effect data
Sample
ZnO:Sn0
Sheet Resistivity (Q/sq)
Bulk Resistivity (Q-cm)
Sheet Carrier Concentration (/cn 2 )
Bulk Carrier Concentration (/cm3 )
Mobility (cm 2 /(V - s))
Hall Coefficient (M2 /C)
6.98 * 107
405
-1.63 * 1010
-2.88 * 1010
6.04
-4.38
2
ITO (no heat)
ITO (with heat)
232
1.05 * 10--
46
2.49 * 10--4.65 * 1015
-8.61 * 1020
291
-0.13
-5.76 * 1014
-1.28 * 1020
46.7
-1.08
remain constant. The Hall voltage, VH, is equal to:
VH= Ed = vBd
(3.4)
where v is the drift velocity. Since
i
=
(3.5)
rqA
where n is the bulk carrier density and A is the cross-sectional area. Consequently,
the sheet carrier density, n, is:
n, = nd =
iB
qI H1
(3.6)
Now, with both the sheet resistance and the Hall voltage, the mobility of the carriers
can be calculated using:
1
qnsRs
_
VH
RsiB
(3.7)
Results of the Hall Effect measurements for our metal oxides are summarized in
Table 3.1.3. These results should be regarded as approximate although the trends
they display are correct. ZnO:SnO 2 , and ITO were deposited on glass with the same
sputter deposition conditions used during device fabrication (See Table 3.2.3). ITO is
indeed n++, and, as expected, the conductivity of heated ITO was greater than that
of unheated ITO. ZnO:SnO 2 is a n-type semiconductor. Hall Effect measurements
50
for the other ceramic layers of our devices are not reported. ZnO and ZnS samples
are too resistive. Measurements on NiO exhibit the anomalous Hall Effect, which is
characterized by sign reversal of the Hall Coefficient and excessively large values for
the carrier density [29]. The origin of the anomalous Hall Effect is still under debate,
but it is well documented in magnetic metal oxides such as NiO [41].
3.2
3.2.1
The Quantum Dot-Metal Oxide Interface
Atomic Force Microscopy
AFM also enables investigation of QD deposition onto metal oxides. In QD-LEDs
with organic charge transport layers, simple spin casting of QDs is not possible as
the solvent in which the colloidal QDs are dispersed can damage or dissolve the
organic layers. The current technique of choice for depositing QDs onto organics is
microcontact printing, which uses a perelyne coated PDMS stamp. This technique
repeatedly produces a complete, hexagonally closed packed monolayer [6]. Stamping
of QDs was attempted on NiO. Figure 3-5a shows that QDs do not transfer from the
stamp to the metal oxide. While it may be possible to functionalize the metal oxide
surface or stamp surface to allow for efficient transfer of the QDs from the stamp to
the metal oxides, metal oxides are chemically robust enough to withstand a variety of
printing techniques including spin coating and inkjet printing. With the former, the
solvent as well as spin speed and acceleration can dramatically affect the deposition of
QDs, and different metal oxides exhibit affinities for different solvents. In the case of
NiO, QDs dissolved in a 10% octane, 90% hexane solution gave better coverage than
QDs dissolved in chloroform (Figures 3-5b and 3-5c). As seen in Figure 3-5c, one
monolayer, closed-packed coverage is difficult to achieve; typically, a concentration
large enough to give several layers of QDs is needed for complete coverage of the
metal oxide.
51
a.
50*
250
1 pm x 1 pm
C.
b.
1 pmx1 pm
1 pmx1 pm
Figure 3-5: AFM images of a) QDs stamped onto NiO, b) QDs in chloroform spun
onto NiO, and c) QDs in a 9:1 hexane to octane solution spun onto NiO.
52
3.2.2
Quantum Dot Luminescence Quenching
QDs possess the unique feature of fluorescence intermittency, commonly referred to
as the "blinking" phenomenon. When a disperse film of QDs is optically excited,
the QD photoluminescence is observed to turn on and off. This blinking is explained
by Auger photoionization [42]. Light incident upon the QDs generates electron-hole
pairs. When an electron-hole pair recombines, energy from this annihilation process
can transfer energy to another electron-hole pair, causing the pair to split and ejecting
either the electron or hole into an electronic state of the surrounding medium. The
type of carrier that is ejected depends on the band offset between the QDs and the
surrounding film. After the QD ejects the carrier, it is charged. When a new electronhole pair is photogenerated on the charged QD, it tends to recombine non-radiatively
because of the Coulomb potential. A QD will begin fluorescing again when the free
carrier returns to the QD. The dark or "off" state of a QD is therefore associated
with a charged QD, while the fluorescing or "on" state is associated with a neutral
QD. In practice, the blinking phenomenon of QDs is only observed in films where
individual QDs are separated from each other; however, Auger photoionization still
occurs when multiple layers of QDs are optically excited.
Luminescence quenching of QDs occurs for electrically excited QDs as well. Both
photolumiencense (PL) and electrolumiencense (EL) quenching is enhanced by the
presence of an electrically conductive film, which has multiple free states to which
a carrier can move when the QD is photoionized. Indeed, a layer of QDs on metal
will not luminescence. To minimize EL quenching in the all inorganic QD-LEDs, we
use three to four layers of QDs for the emissive layer. The QDs touching the charge
transport layers experience luminescence quenching, but the middle layers of QDs,
surrounded by insulating QDs, luminesce. To determine whether the metal oxides
touching the QDs in our devices are too conductive and cause too much luminescence
quenching, we perform PL quenching measurements. Using the same UV source, we
optically excite a film of QDs on glass and a film of QDs embedded in the proposed
structure and compare the PL intensity from the QDs. As shown in Figure 3-6, in
53
UV excitation
20 -
-QDs
16-
c
12-
-
8
4
QDs
0
540
560
580
600 620 640
Wavelength (nm)
660
680
700
Figure 3-6: PL spectrum of 30 nm thick CdZnSe QD layer on glass (solid red line) and
30 nm thick CdZnSe QD layer between NiO and ZnO:SnO 2 (dotted red line). The
two samples, shown schematically to the left, were excited within the same optical
geometry using a UV lamp. We measured a 40% drop in the PL intensity on average.
the case of QDs sandwiched between NiO and ZnO : SnO 2 , we observe a 40 percent
decrease in PL intensity. As discussed in Section 4.4, we can further decrease the
amount of luminescence quenching in QDs by placing an insulting metal oxide layer
next to the QDs.
3.2.3
Summary of Sputtering Parameters of Ceramic Materials
Table 3.2.3 summarizes the sputtering conditions that yield effective electron and
hole transport layers. It is extremely importantly that the targets be stoichiometric
prior to deposition. For example, the ZnO target becomes easily 02 depleted when
sputtered in an Argon environment, and nonstoichiometric ZnO will result in an ntype oxide. This can be corrected by sputtering with 02, or, if 02 sputtering is not
possible during the actual layer deposition, presputtering the target in an a 10:1.5
54
Table 3.2: Electronic Properties of Metal Oxides
Material
RF Power (W)
ITO
12-15
NiO
200
ZnO
18
ZnS
20
ZnO: SnO 2
15:9
Pressure (mTorr)
4-8
4
4
4
4
Ar (ccm)
10
40
10
10
10
02
(ccm)
0
0.4
0
0
0
Heat (C)
0-500
no
no
no
no
Ar: 02 ratio every 10 im of growth.
3.3
Ultraviolet Photoelectron Spectroscopy
To design an efficient optoelectronic device, it is important to understand the energetics of its component materials. The work function 0, the electron affinity x,
and the ionization energy (IE) are all important parameters for determining how the
energy levels of materials will align at an interface and indicate how charge will be
transported between them.
The electron affinity is the energy needed to excite an electron from the conduction
band minimum. The ionization energy is the energy needed to excite an electron from
the top of the valence band. The workfunction is energy between the Fermi level and
the vacuum level [43]. The workfunction and ionization energy can be calculated
using a technique known as Ultraviolet Photoelectron Spectroscopy (UPS). In UPS,
photons of a fixed energy (hv) in the deep UV (typically between 10 and 45 eV)
bombard a sample, ejecting electrons from electronic states with different binding
energies. This leads to ejected electrons with a range of kinetic energies given by:
Ek= hv - Eb- o,
(3.8)
where Eb is binding energy of the electron, and 0, is the sample workfunction.
In our case, the UV source is a cold cathode capillary discharge lamp. Helium gas
55
is leaked into the capillary, which kept under high vacuum using differential pumping.
Ignition is achieved by running 1000 V and 100 mA across the capillary. We optimize
the discharge current for a Helium I resonance, which provides photons of 21.21
eV. These photons are focused onto the sample. Electrons ejected from the sample
are collected with a hemispherical deflection analyzer. The analyzer consists of two
concentric hemispheres across which voltage is applied, causing electrons that pass
between them to deflect. The particular kinetic energy of the electron determines how
it much it is deflected. Electron multipliers are located across the exit plane of the
analyzer to detect the number of electrons deflected by each amount. The detector
for our system provides 0.05 eV resolution.
Figure 3-7 explains a UPS spectrum for a semiconductor. Figure 3-7a shows that
the highest energy electrons have energies equal to the photon energy 21.21 eV. This
means that the electron experiences no binding energy. In other words, it is located
at the Fermi level. In a metal, a significant electron count is observed at an electron
kinetic energy of 21.21 eV. However, in a non-degenerate semiconductor, electrons
are not located at the Fermi level and the counts are essentially zero. Slightly lower
electron kinetic energies (higher binding energies) corresponds to electron emission
from the band gap of the semiconductor. For a perfect crystalline semiconductor, the
number of electrons emitted should be zero. In practice, however, electrons emitted
from gap states, dangling bonds, and adsorbed atoms are observed. A significant
increase in electron counts is observed when electrons begin to be ejected from the
valence band (See Figure 3-7c). The peaks in the UPS spectrum, indicated in Figure 38d, are due discrete electronic states within the valence band of the semiconductor.
The sharp cutoff (See Figure 3-8e) occurs when electrons can no longer escape from
the semiconductor. This energy corresponds to the workfunction of the material.
I calibrated the UPS system using Au, Ag, and Zn, which have well known workfunctions. From these measurements, it was possible to determine that the workfunction of the detector
(Odet)
was 4.3 eV. As shown in Figure 3-7, the true kinetic energy
of the emitted electrons cannot be measured. Rather, the analyzer software records
the kinetic energy, offset by the workfunction of the analyzer.
56
a)
e-
21.21 eV
IE
Osample
EF
~ ~
Ev-T -- ~ ~ ~ ~~
Semiconductor
sE
no
C
(det
C
Detector "
Kinetic Energy
o eV
4...
21.2 1 eV
b)
21.2 1 eV
Binding Energy
(relative to EF
21.21 eV
eEF
Ev.
samplede
...
.
E
-GE
.a-
M
Semiconductor
Detector
oeV
Kinetic Energy
. ........................................................
21.2 1 eV
Binding Energy
(relative to EF)
21.2 1 eV
0 eV
c)
'A'''
21.21 eV
e.
EF
Ev- -
-
Ek
.bJ
sample
---
Semiconductor
(det
C
a)
-J
C
4
Dto
Detector
21.21 eV
Kinetic Energy .
< .....
I........................................................ 0 eV
21.21 eV
Binding Energy
oev
(relative to EF)
Figure 3-7: Schematics explaining key features of the UPS spectrum of a semiconductor.
57
d)
21.21 eV
e.
Ek
4I-J
Osample
EF
Semiconductor
(det
Detector
oe\
I
........
21.2 1I eV
21.211eV
Kinetic Energy
.......................................... 0 eV
Binding Energy
(relative to EF)
e)
t
Ek
21.21 eVE
E
Osample
4det
J-
C
Sve-
Semicondiuctor
Detector
o e's
Kinetic Energy
. ........................................................
At....
21.21 eV
Binding Energy
1.21 eV
21
u
(relative to EF
Figure 3-8: Schematics explaining key features of the UPS spectrum of a semiconductor.
58
Before discussing the results of the UPS measurements on metal oxides and QDs,
it is important to recognize that UPS presents several important limitations and
is most valuable as a tool for looking at trends between similar material sets. As
described above, UPS assumes that each incident photon transfers all its energy to
one electron. This picture is naive. The one electron picture is applicable only in
cases where the ejection of one electron from a band will not substantially effect the
other electrons in the band. [40] This is an acceptable assumption for post-transition
metal oxides such as ZnO and SnO 2 , but not for transition metal oxides such as TiO 2 ,
organic materials, or QDs, where the removal of an electron can substantially change
the electronic structure.
Furthermore, many of the metal oxides we use in our devices are insulating. UPS
measurements on insulating samples can be difficult if not impossible. The incident
photons are neutral particles, but they produce electrons which leave the sample
surface. In an insulating sample, a positive surface charge builds up and causing
the electrons emitted from the sample to slow down due to the Coulomb attraction.
This shifts the UPS spectrum [40]. To minimize the likelihood of this happening,
we deposited the materials of interest in thin layers ( 20nm) on highly conductive
ITO. Ultimately, we would like to perform other measurements of the workfunction
of our materials, such as Kelvin probe, to compare with the UPS measurements and
to paint a more complete picture of the electronic structure.
3.3.1
Metal Oxides
We begin by performing UPS measurements on commercially obtained indium tin
oxide (ITO), which is indium oxide (In 2 0 3 ) doped with Sn, and is used as the anode
in our devices. The ITO was cleaned following the standard procedure of sonicating
in Micro90 detergent, rinsing with DI water, sonicating in acetone, and washing with
boiling isopropanol prior to insertion into the ultrahigh vacuum analysis chamber.
UPS measurements gave a workfunction of 3.9 eV (seen as the sharp onset of the
dotted line in Figure 3-9a). The sample was then in situ Argon sputter cleaned for
10 minutes and UPS measurements were taken again. This time a workfunction of
59
a)
1.50M
,
,
-
1.25M
b)
I
,
- -
Ar sputter deaned
- .not Arsputter deaned
_
1.00M
750.00k
-3.71
500.00k
-
250.00k 4''
0.04.2
0.0)
eV
-Ar sputter deaed
- _ Ar Sputtdened
...
-X
_
_
.
~
4
3.9
3.9eV
0
468
Kinectic Energy (eV)
._
2
6
7
18
19
20
Kinetic Energy (eV)
21
Figure 3-9: UPS spectra for ITO.
4.2 eV (the onset of the solid curve in Figure 3-9a) was found. Figure 3-9b shows the
derivative of the intensity curves in Figure 3-9a. There is a depression at the Fermi
edge (21.21 eV) indicating that electronic levels at the Fermi level are occupied, and
that the ITO sample is n++ doped. As expected, this is not visible in the sample that
has riot been sputtered cleaned. The pronounced minimum of the difference curve
(Figure 3-9b) 3.71 eV away from the Fermi level, visible for both the as-cleaned and
sputtered cleaned samples, indicates that the IE for ITO is 7.91 eV (3.71 + 4.2 =
7.91 eV).
In Figure 3-10, we show the UPS spectra for ZnO, SnO 2 , and cosputtered ZnO : SnO 2
The deposition conditions are listed in Table 3.2.3. Each film was deposited on ITO,
sputtered cleaned in situ with an Ar plasma for 10 minutes, and bias at -6V during
the UPS measurements.
The plot in Figure 3-10a reveals the unique valence band structure of each film.
The ZnO and ZnO : SnO 2 show similar signatures. We attribute this to the fact that
the ZnO : SnO 2 grown as described above is predominately ZnO. The aspects of the
ZnO stemming from Zn can be understood by superimposing the UPS spectra of Zn
and ZnO shown in Figure 3-10b. The peak in the UPS spectrum for ZnO at 11 eV
comes from the filled Zn 3d band. 0 2p orbitals are primarily responsible for the
valence band peak from 13 to 18 eV. The hump on the left side of the peak at 15 eV
comes from a combination of the 0 2p and Zn 4s orbitals. These similarities between
the ZnO and Zn spectra stem from that fact that there is very little hybridization of
60
a)
b)
'M
....
Zn
-ZnO-
4M-
41
3M-
3
M_
Zn 3d
2M-
S21vi
Zn 4s
1M0-
0
81
46
0
12 14
KE (eV)
16
18
20
4
22
d)
c)
8
10
16
12
14
KE (eV)
18
20
2
I
-1M_
2M
-3M
4M
~--znO
4.02 eV
.5M_
IMv
nOS
3.72 eV
3.0
3.5
-6M-
4.12 reV
4.0
4.5
KE (eV)
-JM.
5.0
5.5
1177-
3.61 eV
3.70 eV
3.96 eV
--
nsno2
q
6.0
17.0 17.5 18.0 185 1.0 19.5 20.0 20.5 210 21.5 22.0
KE (eV)
Figure 3-10: UPS spectra for ZnO, SnO 2 , and ZnO: SnO 2
electronic orbitals in ZnO. Indeed, Zn and
o
have very different electronegativities,
and ZnO approaches the limit of an ionic bonded solid where the bonding orbitals
are concentrated on the 0 atoms and the antibonding ones on Zn. The data from
Figure 3-10c and d can be combined to determine an IE of 7.33 eV (3.72 + 3.61 =
7.33 eV) for ZnO, 8.08 eV (4.12 + 3.96 = 8.08 eV) for SnO 2 , and 7.72 eV (4.02 +
3.70 = 7.72 eV) for ZnO : SnO 2. We note that the IE for the cosputtered ZnO : SnO 2
is between the IEs for the separate films, consistent with expectations.
UPS measurement were also performed on ZnS, which we use as an electron blocking layer in our devices. As shown in Figure 3-11, the work function is measured to
be 4.87 eV and the energy difference between the Fermi edge and the valence band
is 2.55 eV, indicating an IE equal to 7.42 eV.
To summarize these results, we list the approximate electronic states for the electronic ceramics we use in our all inorganic QD-LEDs in Table 3.3.1.
The values
for electronic affinities are found from literature. These values allow us to develop
61
a)
b)
4M-
4M-
3M.
3M-
2M-
2M~ M-
r
-1M-
4
6
4.81 eV
8
10 12 4
16
Kinetic Energy (e%)
18
20
.5
2
4.0
4.5
5.0
5.5
Kinetic Energy (eV)
60
6.5
7.0
L)
0.0-
-500.0kb
e
2.55 eV
-M--
-1.5M
-2.M
1
17
18
19
20
Kinectic Energy (eV)
22
Figure 3-11: a) The UPS spectrum for ZnS on an ITO substrate at 6V reserve bias.
b) A close up of spectrum for low kinetic energy electrons reveals an onset at 4.87 eV.
c) The derivative of the intensity spectrum for large kinetic energy electrons reveals
a 2.55 eV gap between the Fermi level and the valence band.
62
Table 3.3: Electronic Properties of Metal Oxides
Material
Electron Affinity, x (eV)
NiO
1.46
ZnO
4.03
ZnS
3.7
4.48
SnO 2
ZnO: SnO 2
4.32
Ionization Energy (eV)
5.33
7.33
7.42
8.08
7.72
tentative band diagrams of the devices.
3.3.2
Quantum Dots
Knowledge of the band structure of QDs would greatly aid in the design of QD-LEDs.
We eventually hope to measure the valence band of the ZnCdSe and CdSe:ZnS QDs,
but we begin with CdSe QDs because their band structure is simpler than that of
multi-layer, alloyed, or overcoated QDs.
Initial UPS measurements on three to four monolayers of CdSe core QDs spincoated on a conductive ITO substrate were dominated by signal from organic ligands
surrounding the QDs. Venda Porter from the group of Professor Bawendi has developed several techniques for removing the ligands from the QDs to improve the
conductivity of QD films. The first method (Method 1) is to heat the QD film to 150
C in a vacuum oven for 30 minutes so that the organic ligands such as TOP, TOPO,
or oleic acid can evaporate. The second method (Method 2) is to soak the QDs in a
solution comprised of 0.1M butylamine in acetonitrile for 5 minutes. The acetonitrile
dissolves the TOPO ligands and butylamine replaces them. Butylamine is a shorter
ligand with a lower evaporation point than TOPO. The QD film is then heated to 70
C in a vacuum oven for 30 minutes to allow evaporation of the butylamine. In the
third method (Method 3), the QD film is again soaked in the acetonitrile-butylamine
solution for 5 minutes. The film is then rinsed with methanol for 2 minutes to dissolve
the butylamine and heated at 70 deg to evaporate any remaining methanol. These
three methods have the potential to damage the QDs and alter their band structure.
63
0.05-
1
-No treatment
Method 1
-(thod
2 teMnthod 3
0.040.03-d
0.020.01-
0.00 -,
560
,
580
.
.
600
620
640
Wavergth (nmn)
Figure 3-12: Absorption spectra of QDs following different treatments for removing
the ligands.
To determine which process had the least effect on the energy levels, we compare the
absorbance data of each film to that of an untreated film of QDs on ITO. As shown
in Figure 3-12, the QD film treated using Method 1 shows red-shifting of the energy
spectra. When heated to 150 C, the QDs partially anneal together, resulting in less
quantum confinement and consequently less-energetic optical transition levels. Films
treated with Method 3 exhibited blue-shifting of the absorption spectra, indicating
that methanol stripped off not only butylamine from the QDs but also some Cd and
Se atoms. An alternative explanation is that the methanol soak leaves the QDs unpassivated and subject to oxidation, which would also cause a blue shift. Method
2 showed the least change in absorption spectra so we use this treatment prior to
performing UPS measurements on the QDs.
A UPS measurement of the CdSe QDs treated with Method 2 is shown in Figure 313. As explained above, electrons excited from the Fermi level of the sample have an
energy of 21.21 eV. The minimum in the plot of the signal derivative (Figure 3-13b)
corresponds to the energy of the electrons that escape from the top of the valence
band. Therefore the energy spacing between the Fermi level and the top of the valence
band is 21.21 eV - 18.53 eV = 2.68 eV. The onset point of the plot in Figure 3-13a gives
the workfunction of the material. The sum of these two values is the ionization energy
64
0.0.
4M-
-200.0k.
3M-
-
400.Ok.
-600.Ok
2M-
-800.Ok
M-A0
3.0
3.5
4.0
4.5
d.0
4.5
Nnetic Energy (eV)
18.0
18.5
19.0
19.5
Nnetic
20.0 20.5
Energy (eV)
21.0
21.5
22.0
Figure 3-13: UPS spectra for CdSe QDs. The spectrum in (a) gives the number
of electrons hitting the detector each second as a function of electron energy. The
spectrum was taken with the sample reversed biased at 6V to create a sharper turn
on. The plot in (b) is the derivative of the signal at higher electron energies. No bias
was applied for this measurement.
(IE) of the QDs, which, in this case, is 2.68 eV + 3.75 eV = 6.43 eV. This value for
the IE is about 0.4 eV smaller than theoretical calculations based on bulk CdSe. This
discrepancy could be due to a dipole layer or remnant organic ligands. Alternatively,
the AFM (see Figure 3-14) of the QD film following treatment with Method 2 reveals
incomplete QD coverage of the ITO. In subsequent studies, a full monolayer of CdSe
QDs on substrates such as Au or highly oriented pyrolitic graphite (HOPG) should
be tested as well. Furthermore, QDs of different sizes should be investigated to see if
the quantum size effect is visible.
With this UPS data, it is possible to construct tentative band diagrams, which will
be used in Chapter 4 to aid in understanding the mechanism of our device operation.
65
15 nm
7.5 nm
Onm
I pm x 1 pm
Figure 3-14: AFM of a CdSe QD film on ITO after soaking it in a solution of 0.1M
butylyamine in acetonitrile for 5 minutes and baking at 70 C for 30 minutes.
66
Chapter 4
All Inorganic QD-LEDs
4.1
Measurement Techniques
When testing QD-LEDs, three important data sets - the current-voltage characteristics, the external quantum efficiency (EQE), and the electroluminescence (EL) spectrum - are always recorded. All the measurements discussed below were taken in air
on unpackaged devices. The current-density-voltage (J-V) characteristic was recorded
using a LabView controlled Keithley 2612 current/voltage source-meter. The EQE,
measured at the same time as the J-V curve, is defined as the ratio between the
number of photons emitted per second and the number of electrons consumed per
second. Practically the EQE is calculated using:
QE(%) = (I I.-
Ri
Id) qA
g
hc
(4.1)
where I, is the photocurrent detected using a calibrated Newport 2112 silicon photodetector from the device EL, Id is the dark current registered by the photodetector
when the device is not operating, and R is the responsivity of the detector for the
peak wavelength of the device, A. i is the current through the device, q is the electron charge, h is Planck's Constant, and c is the speed of light. g is a configuration
factor that accounts for the geometry of the light collection setup in which the LED
is not flush with the photodetector face. Given that thin film LEDs are assumed to
67
be Lambertian emitters, with an emission intensity, I = Iocos(6), we can calculate the
ratio of the power measured to the total power emitted from the device to be:
a2
S='R2+a2
a 2(4.2)
where R is the perpendicular distance between the photodetector and the LED, and
a is the radius of the photodetector.
From the EQE. it is possible to calculate the device brightness. Brightness is
given in units of candelas per square meter. A candela (Cd) is a, measure of luminous
intensity, LI, and is calculated at each wavelength using:
LI(A)
=
683.002 * EL(A) * CIE(A) * i * EQE
where EQE and i are the device efficiency and current.
(4.3)
EL(A) * CIE(A) gives a
typical human response to the device EL spectrum. LI must be integrated over all
wavelengths to get the total luminous intensity. Brightness is then found by dividing
this total by the active device area and 27.
For devices that showed appreciable efficiencies, we also measured the evolution
of the EQE over several minutes of continual operation.
This was done by fixing
the current through the device to the value where the EQE was a measured to be a
maximum during a J-V sweep. The enitted light was then measured by the Newport
2112 photodetector and the EQE calculated as described above.
The EL spectrum is taken with a Ocean Optics spectrometer while biasing the
device with the Keithley 2612. To increase the signal to noise ratio, the integration
time was set to 1000 ms.
4.2
All Inorganic QD-LEDs
As discussed in Section 1.2, NiO can serve as the hole transport layer in a QD-
LED [21], and the AFM analysis detailed in Section 2.1 demonstrated that we can
deposit smooth ITO and NiO layers. which will eliminate electrical shorts. One of
68
Vacuum level
2
3
Ag
ITO
QDs
v
5.33
LU
6
7
8
6.8
7.72
Figure 4-1: Device schematic and approximate band structure as determined by UPS
measurements.
the major breakthroughs in this work was the idea to co-sputter ZnO and SnO 2
(ZnO:SnO 2 ) for the electron transport layer (ETL). Whereas pure SnO 2 films tend to
be polycrystalline with grain boundaries, AFM and X-ray diffraction measurements
reveal that ZnO:SnO 2 films are relatively smooth (less than 0.6 nm rms roughness)
and amorphous, reducing the likelihood of morphologically-induced electrical shorts.
Since the ZnO to Sn0 2 ratio determines the film conductivity, the excess oxygen is not
needed in the sputtering process as it is for NiO. This drastically decreases potential
damage to the QD layer and allows for direct deposition of the metal oxide onto the
QDs. If 02 were needed for the growth, it would oxidize the organic ligands, which
cap the QDs, and produce trap sites that facilitate non-radiative recombination of
QD excitons. Finally, the tentative energy band alignments of the materials (See
Figure 4-1b), determined based on UPS measurements, indicate that the conduction
band level of the ZnO:SnO 2 allows injection of electrons into the QD layer.
The procedure for growing the first all inorganic QD-LED (See structure in Figure 4-1) is described below. Sputter deposition parameters were given in Table 3.2.3,
but are briefly reviewed here with emphasis on deposition rates. Clean glass sub-
69
turn on
1
Ag
1
a) 0.1
C
ITO
L1E-3
1 ~ V2
1
Voltage (V) 10
Figure 4-2: The J-V curve for the first all-inorganic QD-LED and band diagram
schematic under forward bias
strates were patterned with a 60 nm thick ITO anode deposited via RF sputtering
in an inert Ar environment at a rate of 0.06 A/s. The chamber was heated to 500
C, which corresponds to a substrate temperature of approximately 250 C. The HTL
QD-LED is a 20 nm thick layer of p-type NiO, sputtered onto the ITO at a deposition rate of 0.2 A/s at room temperature. The luminescent layer consists of ZnCdSe
alloyed QDs with an emission peak at A = 638nm and a full width at half-maximum
(FWHM) of 40 nm. This solution was concentrated enough so that when spin-coated
in a nitrogen atmosphere onto the NiO substrate, it formed 3 to 4 close-packed lay-
ers in a 30 ± 5 nm thick film. A 50 nm thick film of ZnO:SnO 2 was then deposited
directly onto the QDs by simultaneously sputtering ZnO and SnO 2 for a combined
deposition rate of 0.2 A/s. The structure, shown in Figure 4-1, was completed by
thermally evaporating a 40nm thick silver cathode onto the ZnO:SnO 2 at a rate of
1.5 A/s.
Figure 4-2 shows the typical forward biased J-V characteristic of the QD-LED
described above. Between 1 and 3 V applied bias, a slope of about 2 is observed on
the log-log plot. This is indicative of space charge limited conduction. We hypothesize
that the current at these voltages is due to only one carrier, which is consistent with
the band diagram schematic in forward bias that shows predominately one type of
70
4000
9 Volt bias
350030002 250020001500
10005000
400
.
450
500
550
600
Wavelength (nm)
650
700
750
Figure 4-3: EL spectrum showing emission entirely from QD layer. Photograph of
emission from the device at 6 V applied bias.
carrier build up at the QD layer. The inflection point of the graph, which signals
injection of two types of carriers into the QD layer, also corresponds to the voltage
at which the device turns on. The electroluminescence (EL) spectrum in Figure 4-3
shows emission exclusively from the QD layer. In this spectrum, taken at 9 V, the
QD EL peak is centered at A = 642 nm and has a FWHM of 38 nm, which is similar
to the PL spectrum of these QDs. Figure 4-4 shows EQE as a function of current
density, which reaches a maximum of 0.09%, almost two orders of magnitude higher
than previous reports of devices embedding QDs between doped inorganic transport
layers [7]. Furthermore, our device has bright, uniform pixels even at the low voltage
of 6V (Figure 4-3). The brightness is measured to be 7000 Cd/m 2 at current density
of 3.5 A/m 2 . This is 70 times typical video brightness. Furthermore, comparable
brightness was observed when the devices were tested after being stored in air for four
days, in contrast to unpackaged organic QD-LEDs that can not withstand prolonged
71
I
I
I
I
I
0.1
0.01104
E103
1E-3
w
%102
UiU
W
10
1E-4
39 10'
1E-5-
1011.0
1E-6
I
1.0
1.5
1.5
2.0
2.5 3.0 3.5
Current Density (A/cm2)
4.0
,
2.0
2.5
Current Density
3.0
3.5
4.0
(A/cm2 )
Figure 4-4: EQE as a function of current density for the first metal oxide based QDLED. Maximum EQE for this device is 0.09%. The inset shows that a maximum
luminance of 7000 Cd/n 2 is reached at 3.5 A/1n 2
atmospheric exposure.
4.3
Improving Efficiency with ZnS
The IV curve and band diagram in Figure 4-2 indicate that one carrier type is more
easily injected into the QD layer. This can result in charging of the QDs,. which
decreases device efficiency. One approach to solving this problem is to replace the
NiO with a different metal oxide hole injection layer that has better band alignment
with the QDs. The other approach, which we pursued, is to insert an electron blocking
layer into the electron transport layer (ETL) that will slow the injection of electrons
into the device. Since QDs often incorporate ZnS as a wide band gap shell, we try ZnS
as an electron blocking layer. The device structure and its tentative band diagram
under forward bias are shown in Figure 4-5.
72
0
Ag
QDs
Figure 4-5: Schematic of device structure and proposed band diagram, under forward
bias, containing ZnS electron blocking layer.
As predicted, the EQE of the device increases to 0.096%; however, we note that
the insertion of the insulating ZnS layer increases the turn on of the device to 7.5 V
from 4.5 V (Figure 4-6). Prior to turn on, there is again evidence of space charge
limited condution. The EL spectrum (Figure 4-7) shows emission exclusively from
the QD layer.
4.4
Improving Efficiency with ZnO
As discussed in Section 3.2.2, QD luminescence can be quenched by free carriers in
the neighboring oxide layers. To reduce the amount of this luminescence quenching,
we deposit a layer of insulating ZnO on top of the QDs as shown in Figure 4-8a.
The ZnO is deposited at the parameters given in Table 3.2.3. Figure 4-9b reveals
a 22% increase in EQE over the device with only the ZnS electron blocking layer.
Furthermore, even though ZnO is an insulating layer, the turn on voltage remains
7.5 V (Figure 4-9a). We attribute this to the fact that the ZnO does not completely
cover the QD layer. Again, we note that all the EL is from the QDs (Figure 4-8b).
To decrease luminescence quenching from the other side of the QD layer we tried
73
0.1.
0.11
0.011
' 0.01
1 E-3
tur on
1E-3
IE-41
V
1E-5
'i
2
Voltag (V)
1E-4
10
Curent Density (Ndm
Figure 4-6: J-V and QE of all inorganic QD-LED with a ZnS electron blocking layer.
200-
. 150-
100-
id
50-
5W - 560
60
60
760
750
800
Wavelength (nm)
Figure 4-7: EL of all inorganic QD-LED with a ZnS electron blocking layer.
74
200-
150 -
100-
Ed
50-
n.
- 1o
5 ~ 0 6 60 6 g0
750
700
80
Wavelength (ni
Figure 4-8: Schematic of device structure with an insulating ZnO layer, and EL
spectrum of device biased at 10 V and showing emission entirely from QDs.
0.
0.10
W0.01
0.01-
tn
turn on
IE4
1 E-
i
0.01
.1
2
Current Density (Ncm)
Voltage (V)
Figure 4-9: J-V and EQE plots for device with an insulating ZnO layer.
75
-140
I
12G
i 10a
(U
b80
QDs
-
60
40
20
0
50
700
650
550600
Wavelength (nm)
750
Figure 4-10: Schematic of device and EL spectrum at 10 V showing emission entirely
from QDs.
sputtering a layer of more resistive NiO prior to deposition of the QD layer. While
devices grown with this insulating NiO layer turn on, they exhibit EQEs of only about
0.001%. We also tried depositing a ZnO layer on top of the p-type NiO prior QD
deposition. This again yielded low device efficiencies.
We therefore decided to build a symmetric structure with ZnO : SnO 2 serving as
both the ETL and HTL. As shown in Figure 4-10a, ZnO layers were placed on either
side of the QD layer in order to minimize luminescence quenching. The EL is entirely
from the QD layer.
Figure 4-11 shows that the EQE, as compared to the device in Figure 4-8, increases
only slightly from 0.11% to 0.12%. The turn on voltage, however, increases from 7.5
V to 10 V. While the ZnO layer on the top of the QDs does not form a complete layer,
the ZnO layer underneath the QDs does. Significant voltage is dropped across this
insulating layer. As constructed, this device does not seem to give any substantial
benefits over the ITO/ NiO/ QD/ ZnO/ ZnS/ ZnO: SnO 2/ Ag structure. However
in the next section, we will described how the device structure can be used to make
a transparent QD-LED and an AC driven structure.
76
0.1
S000.01
1E-3
a
1E 4
0.01
turn on
1E-3
..
.
.
.
Voltage (V)
0.1
Current Density (A/cm2 )
10
1
Figure 4-11: J-V and EQE for ITO/ZnO : SnO 2 /ZnO/QD/ZnO/ZnS/ZnO: SnO 2 /Ag
structure.
4.5
Transparent QD-LEDs
Transparent light emitting devices offer the potential for displays with two way viewing capabilities and stacked red, green, and blue emission layers [44]. OLEDs were
the first technology to show promise as transparent displays. Because of the Frank
Condon shift, many organic materials absorb in the UV and emit in the visible. In
contrast, standard inorganic semiconducting materials such as GaAs and InP absorb
in the visible making them opaque. Currently transparent inorganic LEDs are made
using p-i-n structures, with wide band gap p and n-type layers and a thin intrinsic
region that absorbs in the visible. However, this complicates growth since lattice
matching between the p, i, and n-type materials is needed.
As in the first TOLED structures [44], we choose to replace the Ag electrode in the
ITO/ ZnO: SnO 2 / ZnO/ QD/ ZnO/ ZnS/ ZnO: SnO 2 / Ag structure described in
Figure 4-10 with conductive ITO. Unlike TOLEDs, which require a thin layer of Mg
doped Ag to improve the band level offsets and aid in the injection of electrons into
the ETL, we RF-sputter 60 nm of ITO directly onto the ZnO : SnO 2 ETL. The ITO
77
b)
a)
0QDs
Figure 4-12: A schematic of the structure of the first transparent QD-LED. A SEM
cross sectional image is shown to the right.
is deposited in an inert Ar environment at a power of 14 W and a pressure of 4 mT,
which corresponds to a rate of 0.6 A/s. The conductivity of this ITO electrode is not
as large as the conductivity of the bottom electrode because, during growth of this
top layer, the chamber was not heated to avoid annealing the QD layer. A schematic
of the final structure is shown in Figure 4-12. A scanning electron micrograph (SEM)
cross-section image of the same structure deposited on Si instead of glass is also
shown. Si was used as the substrate to allow for easier sample cleaving; however
ITO deposition onto the Si introduced surface roughness that is not present in our
devices grown on glass substrates. None the less, the SEM shows an intact QD layer
sandwiched between oxide layers.
A photograph of MIT written on a sheet of white paper in 14 point as seen through
the device (See Figure 4-13) demonstrates that the QD layer and the electrodes are
hardly visible. The pictures on the right show the device off and on, under 20 V
applied bias. In this case, MIT is written in white on a blue background.
The J-V characteristics and EQE of this device are shown in Figure 4-14. The
device operates in both forward and reverse bias. We note a 0.15% E.Q.E. in reverse
bias, implying a 0.3% EQE from both faces. The EQE for forward bias is 0.07% from
one facet. We can begin to understand device operation by looking at the tentative
78
Figure 4-13: Photographs of a transparent all inorganic colloidal QD-LED.
1
-
E
C
(D
(D
- Forward bias
- Reverse bias
. Forward bias
Reverse bias
0.1-
0.1 -
-.
0.01-
w 0.010
w
1 E-3
1E-3-
1 E-4
0.1
1
Voltage (V)
Ir--t+ 1
0.01
10
0.1
Current Density (Nacm2 )
Figure 4-14: J-V and EQE characteristics for a transparent QD-LED.
79
band diagrams for the device structure under reverse and forward bias 4-15. In both
cases, the ZnS acts as an electron blocking layer. However, in forward bias, the
ZnS slows electron injection into the device whereas, in reverse bias, the ZnS keeps
electrons at the QD layer.
4.6
Toward Green and Blue All Inorganic QD-LEDs
One of the main attractions of using QDs as the active emissive layer in LEDs is
their continuous tunability across the visible spectrum. A standard RGB display
cannot cover the range of human color perception, which is quantified on a Cornmission International de l'Eclairage (CIE) chromaticity diagram (See Figure 4-16).
Coordinates on the CIE diagram are calculated by integrating the product of emission
spectrum (P(A)) and the CIE color matching functions (x(A), y(A), z(A)), which were
determined by measuring the mean color perception of a sample of human observers:
X=
P(A)x(A)M
(4.4)
Y=
P(A)y(A)OA
(4.5)
Z=
P(A)z(A)OA
(4.6)
A point on the CIE diagram gives the color saturation of an emitter and indicates
how pure the color appears to the human eye. A display with three different emitters
forms what is referred to as a color triangle on the CIE diagram. The color saturation
of QDs is high, which means that QD CIE coordinates lie closer to the edge of the
CIE diagram. As shown in Figure 4-16, the color triangle that can be defined by QDs
emitters is larger than that of the current National Television System Committee
(NTSC) standard.
If metal oxide based QD-LEDs are to be marketable as a display technology, it is
necessary to develop efficient green and blue emitting devices. Our first attempt at
a green device consisted of integrating "triple decker" QDs (discussed in Section 2.4)
tuned to emit at 550 nmn into the structure described in Section 4.2. In other words,
80
Forward bias
ITO
ITO
0
Reverse bias
ITO
ITO
Figure 4-15: Band diagram showing reserve and forward bias applied to the transparent QD-LED structure.
81
-
4
-
i
0.9
-
....... .
........
Potential
QD- LED colors
0.8
-- 0.7
0.6
0.5
-
- 0.12
0
NTSC
0.0
color triangle
X 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Figure 4-16: CIE chromaticity diagram showing the current NTSC standard and
location of QDs on the CIE diagram.
the green QDs were spun at 1200 rpm onto a structure consisting of a 60 nm ITO
electrode topped with 20 nm of RF sputtered NiO with 1% 02 doping. The QDs
were then covered with 45 nm of codeposited ZnO and SnO 2 followed by a 40 nm
silver electrode. A device with CdSe:ZnS QDs tuned to 617 nm was grown in parallel
with the green device. The J-V plots are shown in Figure 4-17. We note that the
turn on voltage for the green device is 4 V higher than for the red device but that
the operational resistance of the two devices are similar. This indicates that there is
a barrier to charge injection from the metal oxides into the green QDs that does not
exist for the red QDs. After the devices turn on, both show evidence of space charge
limited conduction as emphasized by the dotted line. Both devices show low EQEs
of 10-4%.
These results suggest that metal oxides with different band alignments are needed
to improve injection into green and blue QDs. We note that the location of the
valence band of QDs is believed to be approximately the same for different size QDs.
This is because the effective mass of a hole is much heavier than that of an electron,
82
-- green
0.1
/
/
1E-3
1E-4 1E49
1E-5*
..
1
Voltage (V)
10
Figure 4-17: J-V plots comparing green and red QD devices.
so the band bending that occurs due to quantum confinement is less for the valence
band than for the conduction band. It is therefore approximated that the valence
band of a QD is close to the bulk CdSe value of 6.8 eV and the conduction band is
at approximately 4.1 eV for blue QDs and 4.6 eV for green QDs. One metal oxide
that seemed to be a logical option to improve charge injection into the QDs is W0 3 ,
which is quoted in literature as having an electron affinity of 3.33 eV and a band gap
of 2.6 eV ??. The band structure schematic in Figure 4-18 seems to indicate that
this design will facilitate charge injection into the QDs and minimize the possibility
of charge trapping at the QD layer. The devices were grown by sputtering 20 nm of
W0 3 onto a conductive ITO electrode. To prevent photoluminescence quenching of
the QDs explained in Section 3.2.2, a 10 nm layer of resistive W0 3 doped with 18% 02
was deposited on the first WO3 layer doped with 6% 02. These layers were grown by
sputtering in a Ar:0
2
environment at 20 W of RF power at 4 mTorr. Green and blue
QDs were spun cast onto separate substrates. A 10 nm layer of insulating ZnO was
sputtered followed by 10nm of ZnS and 40 nm of cosputtered ZnO : SnO 2. In place of
Ag, an Au anode was deposited on top to reduce oxidation of the electrode. Very low
EQEs of approximately 104% were measured, and emission spectra of the green and
the blue devices, shown in Figure 4-18, are indicative of broadband W0
3
emission.
W0 3 does not appear to be the solution to efficient green and blue device, but there
83
green
-- blue
Au
00
ITO
00
0
400
0
500
700
600
Wavlenoh (nm)
800
Figure 4-18: Band diagram in forward bias for proposed device structure. EL spectra for green and blue QD devices. The broad band emission is indicative of W0 3
emission.
exist many other metal oxides that can be characterized and tried in devices.
4.7
Future Work
In this thesis, I have explained the creation of transparent all-inorganic colloidal QDLEDs made with metal oxide charge transport layers. The devices exhibit 0.3% EQE,
have uniform pixel illumination, and show emission exclusively from the QD layer.
There are three main areas on which my future research will focus. The first is
to achieve efficient devices in the green and the blue. Success in this area may come
from the use of techniques such as UPS to gain a better understanding of the band
structure in QDs and metal oxides. The second research area is to investigate the
possibility of making colloidal QD lasers for the reasons described briefly in Section
1.3. Currently the exciton density on our QDs during device operation is only 0.2
excitons per QD, which is 10 to 50 times smaller than the needed density. This means
that to achieve lasing, higher current fluxes through the devices will be necessary.
Third, these devices exhibit photovoltaic properties and should be optimized for such
applications.
84
900
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