applied
sciences
Article
Investigations on the Cosputtered ITO-ZnO
Transparent Electrode Ohmic Contacts to n-GaN
Wei-Hua Hsiao 1 , Tai-Hong Chen 2 , Li-Wen Lai 2 , Ching-Ting Lee 3 , Jyun-Yong Li 1 ,
Hong-Jyun Lin 1 , Nan-Jay Wu 1 and Day-Shan Liu 1, *
1
2
3
*
Institute of Electro-Optical and Materials Science, National Formosa University, Yunlin 632, Taiwan;
s706802000213@gmail.com (W.-H.H.); 10476110@gm.nfu.edu.tw (J.-Y.L.); n125343@yahoo.com.tw (H.-J.L.);
tsaicc1221@gmail.com (N.-J.W.)
Industrial Technology Research Institute South, Tainan 734, Taiwan; a0922639175@gmail.com (T.-H.C.);
ting3561387@yahoo.com.tw (L.-W.L.)
Institute of Microelectronics, Department of Electrical Engineering, National Cheng Kung University,
Tainan 701, Taiwan; tsaicc@ee.ncku.edu.tw
Correspondence: dsliu@nfu.edu.tw; Tel.: +886-5-6315665
Academic Editors: Chien-Hung Liu and Huei-Chu Weng
Received: 29 December 2015; Accepted: 16 February 2016; Published: 22 February 2016
Abstract: Transparent indium tin oxide (ITO) and cosputtered ITO-zinc oxide (ZnO) films’ contacts to
an n-GaN epilayer were investigated. Both of these electrodes’ contact to the n-GaN epilayer showed
Schottky behavior, although the contact resistance of the ITO-ZnO/n-GaN system was lower than
that of the ITO/n-GaN system. By placing a thin Ti interlayer between the ITO-ZnO/n-GaN interface,
nonalloyed ohmic contact was achieved. The inset Ti interlayer was both beneficial both for enhancing
the outdiffusion of the nitrogen atoms at the surface of the n-GaN and suppressing the indiffusion of
oxygen atoms from the surface of the ITO-ZnO to n-GaN. The figure-of-merit (FOM), evaluated from
the specific contact resistance and optical property of the Ti/ITO-ZnO system’s contact to the n-GaN
epilayer, was optimized further at an adequate thickness of the Ti interlayer.
Keywords: cosputtered ITO-ZnO; n-GaN; transparent electrode; Ti interlayer; ohmic contact;
figure-of-merit
1. Introduction
Group III-nitride semiconductors are crucial materials for the fabrication of optoelectronic
devices [1,2]. Among them, gallium nitride (GaN) has been the most attractive material during
the past decades due to its mature fabrication using epitaxial and doping technologies. One of the
important requirements for developing high-quality, GaN-based optoelectronic devices is the formation
of ohmic contact with low specific contact resistance and/or Schottky contact with quality rectifying
behavior. In regard to the material’s contact to GaN, opaque or semitransparent single, bilayer, and
multi-metal structural layers have been used extensively as ohmic contact electrodes in contact with
GaN. Moreover, since the n-GaN epilayer has the advantage of a high carrier concentration, which is
beneficial for current flow tunneling the space charge region, and a low work function of about 4.1 eV,
many common metals, such as Ti, Al, In, Cr, and their combined structures have been reported to be
good contact electrodes [3–5]. However, although a very low contact resistance was achieved from
the above-mentioned electrodes, the resulting optoelectronic devices have very low external quantum
efficiency because most of the light that is transmitted is blocked by these electrodes. Accordingly,
the development of a transparent conductive oxide (TCO) film for ohmic contact to the n-GaN was
essential to improve the performance of the device by enhancing the optical transmittance as well
Appl. Sci. 2016, 6, 60; doi:10.3390/app6020060
www.mdpi.com/journal/applsci
Appl. Sci. 2016, 6, 60
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as spreading the current transmission. To date, only a few papers have reported the preparation
of the TCO electrode contact to n-GaN and the structures showed ohmic contact behavior only
through an additional process on the contact’s structure or using a highly-doped n-GaN epilayer [6–9].
In addition, some researchers have used a thin Ti or In interlayer inset between the TCO/n-GaN
interface to provide a nonalloyed ohmic contact system [10,11]. For example, Sheu et al. reported
that ohmic contact behavior with a specific contact resistance of 5.1 ˆ 10´4 Ω¨ cm2 was only obtained
when the ITO contacted a highly doped n-GaN (n = 1 ˆ 1019 cm´3 ) [6]. Tang et al. demonstrated
an ITO/ZnO/n-GaN ohmic contact system with a specific contact resistance of 3.0 ˆ 10´4 Ω¨ cm2
after annealing at 500 ˝ C in H2 ambient for 5 min [7]. Hwang et al. proposed a nonalloyed Ti/ITO
system ohmic contact to a plasma treatment n-GaN surface with a specific contact resistance of
3.2 ˆ 10´6 Ω¨ cm2 [8]. Kang et al. used a thin In interlayer inset between the ITO/N-face n-GaN
interface to realize a specific contact resistance of 1.8 ˆ 10´5 Ω¨ cm2 after annealing at 300 ˝ C in N2
ambient for 60 s [10]. Guo et al. used a thin Ti interlayer to optimize the ITO/n` -GaN ohmic contact
behavior with a specific contact resistance of 4.2 ˆ 10´6 Ω¨ cm2 after annealing at 600 ˝ C in N2 ambient
for 5 min [11].
In this work, a cosputtered indium tin oxide (ITO)-zinc oxide (ZnO) film, which had a much
lower resistivity than the ITO film (as described elsewhere [12]), was deposited onto the n-GaN
epilayer. In order to produce the nonalloyed ohmic contact system, a thin interlayer of Ti was inset
between the cosputtered ITO-ZnO film and the n-GaN epilayer. The thickness of the Ti interlayer in
the Ti/ITO-ZnO system affected the optical transmittance, and the resulting specific contact resistance
was systematically investigated. An optimal ITO-ZnO/Ti/n-GaN contact structure was obtained by
considering both the behavior of the ohmic contact and the optical transmittance.
2. Experimental
A 300-nm-thick ITO film and cosputtered ITO-ZnO film were respectively deposited onto the
Si-doped n-GaN epilayer using a radio-frequency (rf) magnetron cosputtering system. The n-GaN
epilayer with a thickness of 2 µm was grown after the growth of an undoped GaN buffer layer on
c-plane sapphire using the metal organic chemical vapor deposition technology. Patterns for the
current-voltage (I-V) measurements using the transmission-line method (TLM) were prepared through
the standard photolithography and lift-off technology. ITO target was employed as the source for the
ITO film deposition, while an additive ZnO target cosputtered with the ITO target was used to prepare
the cosputtered ITO-ZnO film at an theoretical atomic ratio of 33% (Zn/(Zn + In) at. %). The detailed
deposition parameters for preparing the cosputtered ITO-ZnO film are reported elsewhere [12]. These
ITO/n-GaN and ITO-ZnO/n-GaN contact systems were then rapidly annealed at 100, 200, and 300 ˝ C,
respectively, for 1 min under vacuum ambient. Another set of the n-GaN epilayer surface was deposited
by a Ti interlayer with the thickness ranging from 2 nm to 10 nm using the electron beam evaporation
system prior to the cosputtered ITO-ZnO film deposition. The metallic Ti (20 nm)/Al (200 nm) system
prepared by using the electron beam evaporation was also deposited onto n-GaN epilayer as the
controlled sample.
Film thicknesses were measured using a surface profile system (Dektak 6M, Veeco, New York, NY,
USA). Resistivity, carrier concentration, and Hall mobility of the ITO and cosputtered ITO-ZnO films
as well as the n-GaN epilayer were conducted using the van der Pauw method (Ecopia HMS-5000,
Ecopia Anyang, Korea) at room temperature. Optical transmittance of the ITO and cosputtered
ITO-ZnO film as well as the Ti/ITO-ZnO structures was measured by a UV-VIS spectrophotometer
(Labomed UVD-3500, Labomed, Inc. Los Angeles, CA, USA). Auger electron spectroscopy (AES) depth
profiles of the Ti/Al, Ti/ITO-ZnO structures and the ITO-ZnO film directly contact to n-ZnO film were
performed on a scanning Auger nanoprobe (Ulvac-PHI, PHI 700, ULVAC, Kanagawa, Japan). The
I-V characteristics of these contact systems were measured by a semiconductor parameter analyzer
(HP4156C, Aglient, Santa Clara, CA, USA).
Appl. Sci. 2016, 6, 60
3 of 9
3. Results and Discussions
Table 1 provides the electrical properties of the ITO and cosputtered ITO-ZnO films as well as3 of 9 the
n-GaN epilayer examined from the Hall-effect measurements at room temperature. Both the ITO and
cosputtered
ITO-ZnO
films were
n-type
degenerated
semiconductors
with high electron
and cosputtered ITO‐ZnO films were n‐type degenerated semiconductors with concentrations
high electron 20 cm´3 and 1.54
21 cm´3 , respectively.
of
2.59
ˆ
10
ˆ
10
In
company
with
the
carrier
mobility
of the
20
−3
21
−3
concentrations of 2.59 × 10 cm and 1.54 × 10 cm , respectively. In company with the carrier 2 V´1 s´1 ), which2 was
cosputtered
ITO-ZnO
film
(10.6
cm
about
two-fold
greater
than
that
of
the
ITO
−1
−1
mobility of the cosputtered ITO‐ZnO film (10.6 cm V s ), which was about two‐fold greater than 2
´
1
´
1
´
4
2 V−1s−1), the cosputtered ITO‐ZnO film (~3.82 × 10
−4 Ω∙cm) possessed a film (5.4 cm V s ), the cosputtered
ITO-ZnO film (~3.82 ˆ 10 Ω¨ cm) possessed
a much lower
that of the ITO film (5.4 cm
´3 Ω¨ cm). −3Figure 1 shows the optical transmittance spectra
resistivity
than
the
ITO
film
(~2.46
ˆ
10
much lower resistivity than the ITO film (~2.46 × 10 Ω∙cm). Figure 1 shows the optical transmittance of the ITO and cosputtered ITO-ZnO films. The incorporation of the Zn atoms into the ITO film led
spectra of the ITO and cosputtered ITO‐ZnO films. The incorporation of the Zn atoms into the ITO to the apparent red-shift of the absorption edge in the near UV wavelengths, and the optical energy
film led to the apparent red‐shift of the absorption edge in the near UV wavelengths, and the optical band gap of the cosputtered ITO-ZnO film was decreased to 3.41 eV, as evaluated from the curves of
energy band gap of the cosputtered ITO‐ZnO film was decreased to 3.41 eV, as evaluated from the 2
the (αhν)
the2photon
(eV) shown
the inset
figure
[13].inset Although
absorption
onset
curves of versus
the (αhν)
versus energy
the photon energy in(eV) shown in the figure the
[13]. Although the of
the
cosputtered
ITO-ZnO
film
occurred
at
a
longer
wavelength,
it
still
had
good
transparency
at
absorption onset of the cosputtered ITO‐ZnO film occurred at a longer wavelength, it still had good the
visible
wavelengths,
with
an
average
transmittance
at
wavelengths
of
400–700
nm
of
about
84.6%
transparency at the visible wavelengths, with an average transmittance at wavelengths of (Table 1), even a little better than that of the ITO film (~81.1%).
400–700 nm of about 84.6% (Table 1), even a little better than that of the ITO film (~81.1%). Appl. Sci. 2016, 6, 60 Table 1. Electrical properties and average optical transmittance of the ITO and cosputtered ITO-ZnO
Table 1. Electrical properties and average optical transmittance of the ITO and cosputtered ITO‐ZnO films as well as the electrical properties of the n-gallium nitride (GaN) epilayer.
films as well as the electrical properties of the n‐gallium nitride (GaN) epilayer. Sample
Sample
ITO ITO
ITO‐ZnO ITO-ZnO
n-GaN
n‐GaN ´3
))
n (cm
n (cm−3
20
2.59 × 10
2.59 ˆ 1020 1.54 × 10
1.54 ˆ 102121 4.59 ˆ 101818 4.59 × 10
μµnn(cm
(cm22/V
/VSS) )
5.4 5.4
10.6 10.6
156.1
156.1 ρ (Ω
∙cm)
ρ (Ω¨
cm)
−3
2.46 × 10
2.46 ˆ 10´3
−´4
4
3.82 × 10
3.82 ˆ 10 3 8.71 ˆ 10−´3
8.71 × 10
TTave
(%) (%)
ave
81.1 81.1
84.6 84.6
‐ -
Figure 1. Optical transmittance spectra of the ITO and cosputtered ITO‐ZnO films (the corresponded Figure 1. Optical transmittance spectra of the ITO and cosputtered ITO-ZnO films (the corresponded
optical energy band gaps determined from the (αhν)
optical energy band gaps determined from the (αhν)2 versus the photon energy (eV) also are provided versus the photon energy (eV) also are provided
in the inset figure). in the inset figure).
The I‐V curves of the as‐deposited ITO and cosputtered ITO‐ZnO films’ contact to the n‐GaN The I-V curves of the as-deposited ITO and cosputtered ITO-ZnO films’ contact to the n-GaN
epilayer, respectively, measured from the contact spacing of 15 μm are shown in Figure 2. Both of epilayer, respectively, measured from the contact spacing of 15 µm are shown in Figure 2. Both of
these contact systems exhibited nonlinear characteristics even though the cosputtered ITO‐ZnO film’s these contact systems exhibited nonlinear characteristics even though the cosputtered ITO-ZnO film’s
contact to the
the n-GaN
n‐GaN epilayer a lower resistance an applied of 2 Ω)
V contact to
epilayer
hadhad a lower
seriesseries resistance
at an at applied
voltage voltage of 2 V (~485
(~485 Ω) than the ITO/n‐GaN contact system (~784 Ω). The rectifying properties with significant than the ITO/n-GaN contact system (~784 Ω). The rectifying properties with significant leakage
leakage currents measured from these contact systems occurred because of the large discrepancy in currents measured from these contact systems occurred because of the large discrepancy in work
work functions between the transparent electrode the epilayer
n‐GaN [14].
epilayer [14]. A post‐annealing functions
between
the transparent
electrode
and the and n-GaN
A post-annealing
treatment
treatment was conducted on these contact systems for the purpose of making these structures become was conducted on these contact systems for the purpose of making these structures become ohmic
ohmic contacts. 3a,b, respectively, the I‐V ofcurves of ITO/n‐GaN and ITO‐ZnO/n‐GaN contacts.
Figure Figure 3a,b, respectively,
give thegive I-V curves
ITO/n-GaN
and ITO-ZnO/n-GaN
contact
contact systems processed by a rapid thermal annealing (RTA) treatment at various temperatures for 1 min under vacuum ambient, measured from the contact spacing of 15 μm. The currents of both of these contact systems were apparently increased as the annealing temperature was increased. The largest current at a bias of 2 V was obtainable from the 200 °C–annealed ITO/n‐GaN contact system, while that of the ITO‐ZnO/n‐GaN contact system occurred at an annealing temperature of 300 °C. Appl. Sci. 2016, 6, 60
4 of 9
systems processed by a rapid thermal annealing (RTA) treatment at various temperatures for 1 min
under vacuum ambient, measured from the contact spacing of 15 µm. The currents of both of these
contact systems were apparently increased as the annealing temperature was increased. The largest
current at a bias of 2 V was obtainable from the 200 ˝ C–annealed ITO/n-GaN contact system, while
Appl. Sci. 2016, 6, 60 4 of 9 that
of the ITO-ZnO/n-GaN contact system occurred at an annealing temperature of 300 ˝ C. However,
Appl. Sci. 2016, 6, 60 4 of 9 ˝
although the series resistance of the 300 C–annealed ITO-ZnO/n-GaN contact system was decreased
decreased significantly to about 200 Ω at a bias of 2V, its I‐V curve still was somewhat insufficient for decreased significantly to about 200 Ω at a bias of 2V, its I‐V curve still was somewhat insufficient for significantly
to about 200 Ω at a bias of 2V, its I-V curve still was somewhat insufficient for achieving
achieving ohmic contact. In contrast to the ITO‐ZnO/n‐GaN contact system, the as‐deposited metallic achieving ohmic contact. In contrast to the ITO‐ZnO/n‐GaN contact system, the as‐deposited metallic ohmic
contact. In contrast to the ITO-ZnO/n-GaN contact system, the as-deposited metallic Ti/Al
Ti/Al contact to the n‐GaN epilayer exhibited a good, linear I‐V characteristic without the requirement Ti/Al contact to the n‐GaN epilayer exhibited a good, linear I‐V characteristic without the requirement contact
to the n-GaN epilayer exhibited a good, linear I-V characteristic without the requirement of
of a post‐annealing process, as shown in Figure 4a. The specific contact resistance derived from the aof a post‐annealing process, as shown in Figure 4a. The specific contact resistance derived from the post-annealing process, as shown in Figure 4a. The specific contact resistance derived from the
resistance of the Al/Ti/n‐GaN contact system as a function of the contact spacing in the set figure was resistance of the Al/Ti/n‐GaN contact system as a function of the contact spacing in the set figure was resistance
of the Al/Ti/n-GaN contact system as a function of the contact spacing in the set figure was
−55 Ω∙cm22. The mechanism responsible for the achievement of the nonalloyed 2 . The mechanism responsible for the achievement of the nonalloyed
−5
approximately 5.1 × 10
approximately 5.1 × 10
Ω∙cm
approximately
5.1 ˆ 10´
Ω¨ cm. The mechanism responsible for the achievement of the nonalloyed ohmic contact behavior using the metallic Ti/Al structure mainly was due to the elemental Ti, which ohmic contact behavior using the metallic Ti/Al structure mainly was due to the elemental Ti, which ohmic
contact behavior using the metallic Ti/Al structure mainly was due to the elemental Ti, which
possessed a much lower work function (ϕ = 4.3 eV). This value was only a little higher than the work possessed a much lower work function (ϕ = 4.3 eV). This value was only a little higher than the work possessed
a much lower work function (φ = 4.3 eV). This value was only a little higher than the
function of n‐GaN (ϕ = 4.1 eV) as compared to that of the transparent ITO (ϕ = 5.1 eV) [15,16]. In function of n‐GaN (ϕ = 4.1 eV) as compared to that of the transparent ITO (ϕ = 5.1 eV) [15,16]. In work
function of n-GaN (φ = 4.1 eV) as compared to that of the transparent ITO (φ = 5.1 eV) [15,16].
addition, since the electron carriers correlated with the nitrogen vacancies (V
) at the n‐GaN surface addition, since the electron carriers correlated with the nitrogen vacancies (V
In
addition, since the electron carriers correlated with the nitrogen vacancies (VNNN) at the n‐GaN surface ) at the n-GaN surface
would be increased as a consequence of the Ti atom, Ti was beneficial for inducing the outdiffusion would be increased as a consequence of the Ti atom, Ti was beneficial for inducing the outdiffusion would
be increased as a consequence of the Ti atom, Ti was beneficial for inducing the outdiffusion
of the
the nitrogen
nitrogen atoms
atoms at
at the
the n-GaN
n‐GaN surface.
surface. Figure
Figure 4b
4b shows
shows the
the elemental
elemental distributions
distributions over
over the
the of the nitrogen atoms at the n‐GaN surface. Figure 4b shows the elemental distributions over the of
Ti/n‐GaN interface conducted from the AES depth profile. The outdiffusion of the N atoms was Ti/n‐GaN interface depth profile.
profile. The N atoms
atoms was
was Ti/n-GaN
interface conducted conducted from from the the AES AES depth
The outdiffusion outdiffusion of of the the N
observed clearly at the Ti/n‐GaN interface, resulting in the increase in the electron concentration at observed clearly at the Ti/n‐GaN interface, resulting in the increase in the electron concentration at observed
clearly at the Ti/n-GaN interface, resulting in the increase in the electron concentration at the
the n‐GaN surface associated with large amounts of the V
donors [17,18]. the n‐GaN surface associated with large amounts of the V
NN donors [17,18]. n-GaN
surface associated with large amounts of the VN donors
[17,18].
Figure 2.2. 2. I-V
I‐V curves of the the as‐deposited ITO cosputtered
and cosputtered cosputtered ITO‐ZnO films’ contact contact to n‐GaN Figure
curves
of of the
as-deposited
ITO ITO and
ITO-ZnO
films’ contact
to
n-GaNto epilayer,
Figure I‐V curves as‐deposited and ITO‐ZnO films’ n‐GaN epilayer, respectively, measured from the contact spacing of 15 μm. respectively,
measured from the contact spacing of 15 µm.
epilayer, respectively, measured from the contact spacing of 15 μm. 22
(a)
(a)
ITO/n-GaN
ITO/n-GaN
spacing: 15
15mm
spacing:
44
22
00
00
as
as
o
100oCC
100
o
o
200CC
200
o
o C
300
300 C
-1-1
-2-2
-2-2
spacing: 15
15mm
spacing:
Current
Current(mA)
(mA)
Current
Current(mA)
(mA)
11
ITO-ZnO/n-GaN
(b) ITO-ZnO/n-GaN
(b)
-1-1
0
0
Voltage
(V)
Voltage
(V)
11
as
as
o
100oCC
100
o
o
200CC
200
o
o C
300
300 C
-2-2
-4-4
22
-2-2
-1-1
0
0
Voltage
(V)
Voltage
(V)
11
22
Figure
contact
systems
Figure 3.
3. I-V
I‐V curves
curves of
of the
the (a)
(a) ITO/n-GaN
ITO/n‐GaN and
and (b)
(b) ITO-ZnO/n-GaN
ITO‐ZnO/n‐GaN contact contact systems systems processed
processed by
by Figure 3. I‐V curves of the (a) ITO/n‐GaN and (b) ITO‐ZnO/n‐GaN processed by various
annealing
temperatures
for
1
min
under
vacuum
ambient,
measured
from
the
contact
spacing
measured from from the the contact contact various annealing annealing temperatures temperatures for for 1 1 min min under under vacuum vacuum ambient, ambient, measured various of 15 µm.
spacing of 15 μm. spacing of 15 μm.
Appl. Sci. 2016, 6, 60 Appl. Sci. 2016, 6, 60
(a) Al/Ti/n-GaN
(b) Al/Ti/n-GaN
10
0
95
90
-10
-20
Ti
N
Ga
Al
O
Intensity (arb. unit)
15 m
25 m
35 m
50 m
Resistance (ohms)
Current (mA)
20
5 of 9 5 of 9
=5.09E-04 cm2
85
80
75
70
65
60
55
10
15
20
25
30
35
40
45
50
55
Effective Spacing(m)
-2
-1
0
Voltage (V)
1
2
0.0
0.5
1.0
1.5
2.0
2.5
Sputter time (min)
3.0
Figure 4. (a) I-V curve of the as-deposited metallic Ti/Al system’s contact to n-GaN epilayer
Figure 4. (a) I‐V curve of the as‐deposited metallic Ti/Al system’s contact to n‐GaN epilayer and (b) and (b) the elemental distribution of the Al/Ti/n-GaN contact system conducted from AES depth
the elemental distribution of the Al/Ti/n‐GaN contact system conducted from AES depth profile measurement.
profile measurement. The
structures’
contact
to the to n-GaN
that the transparent
The above-mentioned
above‐mentioned structures’ contact the epilayer
n‐GaN demonstrated
epilayer demonstrated that the electrodes,
such as ITO and cosputtered ITO-ZnO films, have a nonlinear I-V curve even when
transparent electrodes, such as ITO and cosputtered ITO‐ZnO films, have a nonlinear I‐V curve even annealed
at an elevated
temperature,
whereas whereas the opaque
system
had
ohmichad contact
behavior
when annealed at an elevated temperature, the Ti/Al
opaque Ti/Al system ohmic contact when
contacting
the n-GaN the epilayer
without
a post-annealing
treatment. Therefore,
employed
behavior when contacting n‐GaN epilayer without a post‐annealing treatment. we
Therefore, we aemployed a thin Ti interlayer which was deposited between the interface of ITO‐ZnO and n‐GaN to thin Ti interlayer which was deposited between the interface of ITO-ZnO and n-GaN to achieve a
true
ohmic contact system with optical transparency at the visible wavelengths. Figure 5a shows the
achieve a true ohmic contact system with optical transparency at the visible wavelengths. Figure 5a I-V
characteristics of the ITO-ZnO/Ti/n-GaN contact systems as a function of the thickness of the Ti
shows the I‐V characteristics of the ITO‐ZnO/Ti/n‐GaN contact systems as a function of the thickness interlayer,
measured from the contact spacing of 15 µm (the I-V curve of the Al/Ti/n-GaN contact
of the Ti interlayer, measured from the contact spacing of 15 μm (the I‐V curve of the Al/Ti/n‐GaN system
also is shown for comparison). The cosputtered ITO-ZnO film directly contacted the n-GaN
contact system also is shown for comparison). The cosputtered ITO‐ZnO film directly contacted the epilayer,
which resulted in a nonlinear I-V property that became an ohmic contact behavior when a
n‐GaN epilayer, which resulted in a nonlinear I‐V property that became an ohmic contact behavior thin
Ti
interlayer
was deposited prior to the deposition of the cosputtered ITO-ZnO film. The current
when a thin Ti interlayer was deposited prior to the deposition of the cosputtered ITO‐ZnO film. The flow
of the
nonalloyed
ITO-ZnO/Ti/n-GaN
contact system
was
increased
initially asinitially the thickness
current flow of the nonalloyed ITO‐ZnO/Ti/n‐GaN contact system was increased as the of
the
Ti
interlayer
increased,
but
it
decreased
as
the
thickness
of
the
Ti
interlayer
reached
10
nm.
thickness of the Ti interlayer increased, but it decreased as the thickness of the Ti interlayer reached It10 was
worth
noting
thatnoting the current
of the
ITO-ZnO/n-GaN
contact structure
insetstructure by an 8-nm-thick
nm. It was worth that the current of the ITO‐ZnO/n‐GaN contact inset by Ti
an interlayer
at a bias of 2 V obviously was higher than that of the metallic Ti/Al structure’s contact to
8‐nm‐thick Ti interlayer at a bias of 2 V obviously was higher than that of the metallic Ti/Al structure’s the
n-GaN epilayer. The evolution of the specific contact resistance as a function of the Ti interlayered
contact to the n‐GaN epilayer. The evolution of the specific contact resistance as a function of the Ti thickness
in the
ITO-ZnO/Ti/n-GaN
contact systemcontact is illustrated
inis Figure
5b. The
interlayered thickness in the ITO‐ZnO/Ti/n‐GaN system illustrated in specific
Figure contact
5b. The resistances
of these ITO-ZnO/Ti/n-GaN contact systems as well as the Ti/Al ohmic contact to the
specific contact resistances of these ITO‐ZnO/Ti/n‐GaN contact systems as well as the Ti/Al ohmic n-GaN
summarized
Table
2. A specific
resistance
of 2.5
ˆ 10´3resistance Ω¨ cm2 was
contact epilayer
to the are
n‐GaN epilayer inare summarized in contact
Table 2. A specific contact of obtainable
from2 was obtainable from the cosputtered ITO‐ZnO film’s contact to the n‐GaN epilayer the cosputtered ITO-ZnO film’s contact to the n-GaN epilayer with an additional
2.5 × 10−3 Ω∙cm
2-nm-thick
Ti interlayer inset between the interface of ITO-ZnO and n-GaN. As the thickness of the Ti
with an additional 2‐nm‐thick Ti interlayer inset between the interface of ITO‐ZnO and n‐GaN. As interlayer
reached 8 nm, the specific contact resistance was optimized further to 4.8 ˆ 10´5 Ω¨ cm2 , a
the thickness of the Ti interlayer reached 8 nm, the specific contact resistance was optimized further −5 Ω∙cm
2, a value that was a little better than that of the metallic Ti/Al contact to the n‐GaN value
that was
a little
better than that of the metallic Ti/Al contact to the n-GaN epilayer. The optical
to 4.8 × 10
transmittances
of
these
Ti/ITO-ZnO systems
with
various Tisystems interlayered
given in
epilayer. The optical transmittances of these Ti/ITO‐ZnO with thicknesses
various Ti are
interlayered Figure
6 (the optical transmittance of the cosputtered ITO-ZnO film also is shown for comparison).
thicknesses are given in Figure 6 (the optical transmittance of the cosputtered ITO‐ZnO film also is The
thin Ti interlayer did not change the onset absorption of the cosputtered ITO-ZnO film at the
shown for comparison). The thin Ti interlayer did not change the onset absorption of the cosputtered near-UV
wavelength.
the optical transparency
ofoptical the Ti/ITO-ZnO
systems
was
reduced
ITO‐ZnO film at the However,
near‐UV wavelength. However, the transparency of the Ti/ITO‐ZnO gradually
as the
thickness
of the Ti as interlayer
increased.
TheTi average
transmittances
the average visible
systems was reduced gradually the thickness of the interlayer increased. atThe wavelengths
of
these
Ti/ITO-ZnO
systems
are
listed
in
Table
2.
It
can
be
found
that
the
average
transmittances at the visible wavelengths of these Ti/ITO‐ZnO systems are listed in Table 2. It can be transmittances
of these Ti/ITO-ZnO systems decreased monotonously from 83.2% to 48.8% as the
found that the average transmittances of these Ti/ITO‐ZnO systems decreased monotonously from thickness
the Ti
was increased
from 2 to 10
nm.
Accordingly,
the Accordingly, cosputtered
83.2% to of
48.8% as interlayer
the thickness of the Ti interlayer was increased from although
2 to 10 nm. ITO-ZnO
film’s
ohmic
contact
to
the
n-GaN
epilayer
can
be
optimized
when
an
8-nm-thick
Ti interlayer
although the cosputtered ITO‐ZnO film’s ohmic contact to the n‐GaN epilayer can be optimized when isan inset
between the
interface,
the structural
transmittance atinterface, the visiblethe wavelengths
8‐nm‐thick Ti ITO-ZnO/n-GaN
interlayer is inset between the ITO‐ZnO/n‐GaN structural apparently
decreased
63.2%wavelengths as comparedapparently to the cosputtered
ITO-ZnO
film
transmittance at the to
visible decreased to 63.2% as (~84.6%).
compared Herein,
to the we
proposed ITO‐ZnO the figure film of merit
(FOM),Herein, Φ, to quantitatively
theof performance
of these
cosputtered (~84.6%). we proposed determine
the figure merit (FOM), Φ, to quantitatively determine the performance of these transparent electrodes’ ohmic contact to the Appl. Sci. 2016, 6, 60
Appl. Sci. 2016, 6, 60 6 of 9
6 of 9 Appl. Sci. 2016, 6, 60 6 of 9 10
5
(a) ITO-ZnO/Ti/n-GaN
spacing : 15 m
0
-5
-5
-10
-2
-1
-10
-2
-1
(b) ITO-ZnO/Ti/n-GaN contact system
10
5
0
(b) ITO-ZnO/Ti/n-GaN contact system
2
Current
(mA)(mA)
Current
(a) ITO-ZnO/Ti/n-GaN
spacing : 15 m
10
Contact
resistance
( cm() cm2)
Contact
resistance
transparent
electrodes’
contact
to the according n-GaN epilayer.
value isexpression calculated which according
to into the
n‐GaN epilayer. This ohmic
value is calculated to the This
following takes following
expression
which
takes
into
consideration
both
the
specific
contact
resistance,
ρ
,
and
the
consideration both the specific contact resistance, ρc, and the average optical transmittance, c
n‐GaN epilayer. This value is calculated according to the following expression which takes into average
optical
Tave [19,20]: transmittance, Tave [19,20]:
consideration both the specific contact resistance, ρc, and the average optical transmittance, Tave [19,20]: T 10 (1) ΦΦ“
(1)
ρc
(1) Φ
10
Al/Ti/n-GaN
0nm
2nm
Al/Ti/n-GaN
5nm
0nm
8nm
2nm
10nm
5nm
8nm1
0
Voltage (V)10nm
0
1
-3
10
10
2
2
-3
-4
-4
2
2
4
6
8
Thickness of Ti layer (nm)
4
6
8
10
10
Voltage (V)
Thickness
offunction Tiaslayer
(nm)
Figure
5.I‐V (a)characteristics I-V characteristics
ofITO‐ZnO/Ti/n‐GaN the ITO-ZnO/Ti/n-GaN
system
a function
of interlayered the Ti
Figure 5. (a) of the contact contact
system as a of the Ti interlayered thickness, measured from the contact spacing of 15 µm (the I-V curve of the Al/Ti/n-GaN
thickness, measured from the contact spacing of 15 μm (the I‐V curve of the Al/Ti/n‐GaN contact system also is Figure 5. (a) I‐V characteristics of the ITO‐ZnO/Ti/n‐GaN contact system as a function of the Ti interlayered contact system also is shown for comparison) and (b) the corresponding specific contact resistance as a
shown for comparison) and (b) the corresponding specific contact resistance as a function of the Ti layered thickness. thickness, measured from the contact spacing of 15 μm (the I‐V curve of the Al/Ti/n‐GaN contact system also is function of the Ti layered thickness.
shown for comparison) and (b) the corresponding specific contact resistance as a function of the Ti layered thickness. Figure 6. Optical transmittance of the Ti/ITO‐ZnO systems as a function of the Ti interlayered thickness (the optical transmittance of the pure ITO‐ZnO also is shown for comparison). Figure Optical transmittance of the Ti/ITO‐ZnO systems as a function the Ti interlayered Figure 6.6. Optical
transmittance
of the
Ti/ITO-ZnO
systems
as a function
of the Tiof interlayered
thickness
thickness (the optical transmittance of the pure ITO‐ZnO also is shown for comparison). (the
optical
transmittance
of
the
pure
ITO-ZnO
also
is
shown
for
comparison).
Table 2. Specific contact resistance, optical average transmittance, and FOM of the Ti/ITO‐ZnO systems as a function of the thickness of the Ti interlayer (the specific contact resistance of the TI/Al Table 2.2. Specific
Specific contact resistance, optical average transmittance, and ofFOM of the Ti/ITO‐ZnO Table
contact
resistance,
optical
average
transmittance,
and FOM
the Ti/ITO-ZnO
systems
contact to the n‐GaN epilayer also is given for comparison). systems as a function of the thickness of the Ti interlayer (the specific contact resistance of the TI/Al as a function of the thickness of the Ti interlayer (the specific contact resistance of the TI/Al contact to
contact to the n‐GaN epilayer also is given for comparison). ρc (Ω∙cm2)
Tave (%)
FOM (Ω−1∙cm−2) Thickness of Ti Interlayer
the n-GaN epilayer
also is given for comparison).
−5
Ti/Al 5.1 × 10 ‐ ‐ ρc (Ω∙cm2)
Tave (%)
FOM (Ω−1∙cm−2) Thickness of Ti Interlayer
2
´1
0 nm ‐ 84.6 Thickness of Ti/Al Ti Interlayer
T ave (%)
ρc (Ω¨
cm ) −5 FOM (Ω ‐ ‐ ¨ cm´2 )
5.1 × 10
‐ −3 83.2 49 2 nm 2.5 × 10
Ti/Al
-84.6 - ‐ 0 nm ‐ 5.1 ˆ 10´5
5 nm 3.8 × 10−3−4 75.1 144 0 nm
84.6
-49 83.2 2 nm 2.5 × 10
−5
63.2 203 8 nm 4.8 × 10
´3 −4 2 nm
83.2
49
2.5
ˆ
10
5 nm 3.8 × 10 −4 75.1 144 10 nm 7.8 × 10
48.8 <1 ´4
5 nm
75.1
144
3.8 ˆ4.8 × 10
10
−5 63.2 203 8 nm ´5
8 nm
63.2
203
4.8 ˆ7.8 × 10
10
−4 10 nm 48.8 <1 The higher the FOM of the Ti/ITO‐ZnO system obtained, the better the quality of the transparent 10 nm
48.8
<1
7.8 ˆ 10´4
electrode contacted to the n‐GaN epilayer. Table 2 gives the FOM of the Ti/ITO‐ZnO systems as a The higher the FOM of the Ti/ITO‐ZnO system obtained, the better the quality of the transparent The higher the FOM of the Ti/ITO-ZnO system obtained, the better the quality of the transparent
function of the thickness of the Ti interlayer. A highest FOM of 203 was achieved from the electrode contacted to the n‐GaN epilayer. Table 2 gives the FOM of the Ti/ITO‐ZnO systems as a electrode contacted to the n-GaN epilayer. Table 2 gives the FOM of the Ti/ITO-ZnO systems as a
Ti/ITO‐ZnO system with an 8‐nm‐thick Ti interlayer contacting the n‐GaN epilayer. Moreover, the function of the thickness of the Ti interlayer. A highest FOM of 203 was achieved from the FOM obviously was reduced to a value lower than unity for the cosputtered ITO‐ZnO film’s contact Ti/ITO‐ZnO system with an 8‐nm‐thick Ti interlayer contacting the n‐GaN epilayer. Moreover, the FOM obviously was reduced to a value lower than unity for the cosputtered ITO‐ZnO film’s contact Appl. Sci. 2016, 6, 60
7 of 9
function of the thickness of the Ti interlayer.
Appl. Sci. 2016, 6, 60 A highest FOM of 203 was achieved from the Ti/ITO-ZnO
7 of 9 system with an 8-nm-thick Ti interlayer contacting the n-GaN epilayer. Moreover, the FOM obviously
to the n‐GaN to
epilayer a 10‐nm‐thick Ti interlayer. This film’s
was the result both the was
reduced
a valuewhile lowerinset thanby unity
for the cosputtered
ITO-ZnO
contact
toof the
n-GaN
specific contact resistance and average optical transmittance being apparently inferior to that of the epilayer while inset by a 10-nm-thick Ti interlayer. This was the result of both the specific contact
sample inset a Ti interlayer with the thickness of 8 nm. inferior
Figure to
7a,b elemental resistance
andby average
optical transmittance
being apparently
thatshow of thethe sample
inset
distributions of the cosputtered ITO‐ZnO film directly contacting the n‐GaN epilayer and an by a Ti interlayer with the thickness of 8 nm. Figure 7a,b show the elemental distributions of the
8‐nm‐thick interlayer between the the
interface ITO‐ZnO and n‐GaN, respectively. The cosputtered Ti ITO-ZnO
film inset directly
contacting
n-GaN of epilayer
and an
8-nm-thick
Ti interlayer inset
cosputtered ITO‐ZnO film’s contact to the n‐GaN epilayer had significant interdiffusion at the between the interface of ITO-ZnO and n-GaN, respectively. The cosputtered ITO-ZnO film’s contact
ITO‐ZnO/n‐GaN interface, especially for the O and N atoms. The outdiffusion of the N atoms at the to the n-GaN epilayer had significant interdiffusion at the ITO-ZnO/n-GaN interface, especially for
n‐GaN surface led to an increase in the electron carriers at the n‐GaN surface, whereas the indiffusion the O and N atoms. The outdiffusion of the N atoms at the n-GaN surface led to an increase in the
of the O atoms to the n‐GaN surface from the ITO‐ZnO film was proven to result in the compensation electron carriers at the n-GaN surface, whereas the indiffusion of the O atoms to the n-GaN surface
of the the
carriers and film
impede carrier thereby exhibiting Schottky as from
ITO-ZnO
wasthe proven
to transmission, result in the compensation
of thethe carriers
andproperty, impede the
shown in Figure 3. In contrast to the cosputtered directly n‐GaN carrier transmission,
thereby
exhibiting
the SchottkyITO‐ZnO property,film as shown
incontacting Figure 3. the In contrast
epilayer, a thin Ti interlayer inset between the interface of ITO‐ZnO and n‐GaN resulted in more N to the cosputtered ITO-ZnO film directly contacting the n-GaN epilayer, a thin Ti interlayer inset
atoms being outdiffused from the n‐GaN surface, as shown in Figure 7b. In addition, the indiffusion between the interface of ITO-ZnO and n-GaN resulted in more N atoms being outdiffused from the
of the O atoms to the n‐GaN epilayer, which was harmful to the carriers’ transmission, was depressed n-GaN surface, as shown in Figure 7b. In addition, the indiffusion of the O atoms to the n-GaN
by the additive Ti interlayer. Accordingly, this nonalloyed ITO‐ZnO/Ti/n‐GaN contact system had epilayer, which was harmful to the carriers’ transmission, was depressed by the additive Ti interlayer.
excellent ohmic contact behavior with optical transmittance. Accordingly, this nonalloyed ITO-ZnO/Ti/n-GaN contact system had excellent ohmic contact behavior
with optical transmittance.
Ga
(b) IT O -ZnO /T i/n -G aN
Intensity (arb. unit)
Intensity (arb. unit)
(a) IT O -Z nO /n -G aN
O
N
In
Zn
N
In
Zn
Sn
0
Ga
O
Ti
Sn
1
2
3
Sputter tim e(m in)
4
5
0
1
2
Sputter tim e(m in)
3
4
Figure 7. Elemental distributions of (a) the cosputtered ITO-ZnO film directly contacting the n-GaN
Figure 7. Elemental distributions of (a) the cosputtered ITO‐ZnO film directly contacting the n‐GaN epilayer and (b) the ITO-ZnO/n-GaN contact system inset by an 8-nm-thick Ti layer conducted from
epilayer and (b) the ITO‐ZnO/n‐GaN contact system inset by an 8‐nm‐thick Ti layer conducted from AES depth profile measurements.
AES depth profile measurements. 4. Conclusions
4. Conclusions Transparent conductive
ITO
andand cosputtered
ITO-ZnO
films were
as used ohmicas contact
electrodes
Transparent conductive ITO cosputtered ITO‐ZnO films used
were ohmic contact to an n-GaN epilayer. Both of these TCO films had an average transmittance higher than 80%
electrodes to an n‐GaN epilayer. Both of these TCO films had an average transmittance higher than at the visible wavelengths. However, although the cosputtered ITO-ZnO film had a resistivity
80% at the visible wavelengths. However, although the cosputtered ITO‐ZnO film had a resistivity ´4 Ω¨ cm) that was almost an order of magnitude lower than that of the ITO film, the
(~3.82
ˆ 10
−4 Ω∙cm) that was almost an order of magnitude lower than that of the ITO film, the obvious (~3.82 × 10
obvious indiffusion of the O atoms into the n-GaN epilayer was observed from the AES depth
indiffusion of the O atoms into the n‐GaN epilayer was observed from the AES depth profile. This profile.
This led to the ITO-ZnO/n-GaN contact system having Schottky contact behavior even when
led to the ITO‐ZnO/n‐GaN contact system having Schottky contact behavior even when processed by processed by an RTA treatment. The nonlinear I-V characteristic of the ITO-ZnO/n-GaN contact
an RTA treatment. The nonlinear I‐V characteristic of the ITO‐ZnO/n‐GaN contact system evolved system evolved into a true ohmic contact when a thin Ti interlayer was introduced between the
into a true ohmic contact when a thin Ti interlayer was introduced between the interface of ITO‐ZnO interface
of ITO-ZnO
andcontact n-GaN.resistance The specific
resistance of the ITO-ZnO/Ti/n-GaN
contact
and n‐GaN. The specific of contact
the ITO‐ZnO/Ti/n‐GaN contact system initially was system initially
with
the
ofincreasing the Ti interlayer
increasing
andas then
degraded
decreased with was
the decreased
thickness of the Ti thickness
interlayer and then degraded the inset Ti as the inset Ti interlayer reached 10 nm, whereas the optical transmittance at the visible wavelengths
interlayer reached 10 nm, whereas the optical transmittance at the visible wavelengths monotonously monotonously decreased as the interlayer thickness increased. Combined with the specific contact
decreased as the interlayer thickness increased. Combined with the specific contact resistance and resistance and the average optical transmittance at the visible wavelengths, the highest FOM was
the average optical transmittance at the visible wavelengths, the highest FOM was obtainable from obtainable from the ITO-ZnO/Ti/n-GaN contact system with the 8 nm thickness of the Ti interlayer,
the ITO‐ZnO/Ti/n‐GaN contact system with the 8 nm thickness of the Ti interlayer, which exhibited which exhibited a specific contact resistance
of 4.8
ˆ 10´5 Ω¨ cm2 and an average optical transmittance
−5 Ω∙cm
2 and an average optical transmittance of 63.2%. This a specific contact resistance of 4.8 × 10
ohmic contact behavior achieved from the nonalloyed ITO‐ZnO/Ti/n‐GaN contact system was achieved by this mechanism: the inset Ti interlayer functioned to effectively induce the outdiffusion of the N atoms at the n‐GaN surface and to suppress the indiffusion of the O atoms from the cosputtered ITO‐ZnO film to the n‐GaN epilayer. Appl. Sci. 2016, 6, 60
8 of 9
of 63.2%. This ohmic contact behavior achieved from the nonalloyed ITO-ZnO/Ti/n-GaN contact
system was achieved by this mechanism: the inset Ti interlayer functioned to effectively induce the
outdiffusion of the N atoms at the n-GaN surface and to suppress the indiffusion of the O atoms from
the cosputtered ITO-ZnO film to the n-GaN epilayer.
Acknowledgments: This work was supported by Ministry of Science and Technology under Ministry of Science
and Technology 103-2221-E-150 -067 and Industrial Technology Research Institute South under FY104-AE2.
Author Contributions: Day-Shan Liu organized and designed the experiment procedures; Tai-Hong Chen,
Li-Wen Lai, and Ching-Ting Lee supported the measurement results and wrote this paper; Wei-Hua Hsiao,
Jyun-Yong Li, Hong-Jyun Lin, and Nan-Jay Wu executed the film depositions, measurements and resulting
structures analysis. All authors read and approved the final version of the manuscript to be submitted.
Conflicts of Interest: The authors declare that there is no conflict of interests regarding the publication of
this paper.
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