Zn-doped CuAlS
2
transparent p-type conductive thin films deposited
by pulsed plasma deposition
Ming Yang, Yinghua Wang, Guifeng Li, Zhan Shi, and Qun Zhang
a兲
Department of Materials Science, Fudan University, 220 Han-Dan Road, Shanghai 200433,
People’s Republic of China
共Received 27 March 2009; accepted 14 September 2009; published 14 October 2009兲
CuAl
0.90
Zn
0.10
S
2
thin films were deposited by pulsed plasma deposition. The dependence of
structural, surface morphology, electrical, and optical properties of the films on substrate
temperature was investigated. X-ray diffraction patterns reveal that the film be amorphous structure.
The electrical properties are sensitive to the substrate temperature. A typical sample with
conductivity of 50.9 S cm
−1
, carrier mobility of 3.13 cm
2
V
−1
s
−1
, carrier concentration of 1.41
⫻10
19
cm
−3
, and average transmission of 74% in visible range of 400–700 nm was obtained. A
transparent p-CuAlS
2
:Zn/ n-In
2
O
3
:W heterogeneous diode was also fabricated and exhibits
rectifying current-voltage characteristics. The ratio of forward current to the reverse current exceeds
80 within the range of applied voltages of −3.0–+3.0 V and the turn-on voltage is approximately
0.5–0.8 V. © 2009 American Vacuum Society. 关DOI: 10.1116/1.3244565兴
I. INTRODUCTION
p-type transparent conductive oxide 共 p-TCO兲 thin films
have attracted interest in the emerging field of “transparent
electronics.”
1
Recently, a series of p-TCOs such as delafos-
site CuMO
2
共M =Al, Cr, Ga, In, Sc, Y, B兲,
2–8
SrCu
2
O
2
,
9
ox-
ysulfides of LaCuOCh 共Ch= S, Se兲,
10,11
binary oxides of
NiO,
12
codoped p-ZnO,
13–15
etc., have been reported with
various deposition techniques. However, although p-type
conductivity has already been demonstrated in these wide
band gap oxide semiconductors, the rather low value 共mostly
⬃1Scm
−1
兲 is still not high enough to match that of com-
mercially used n-TCOs 共
⬎10
3
Scm
−1
兲 nowadays, which
greatly limits their application in active semiconductor de-
vices, where the pn junction is a key structure. It is needed to
explore novel material systems other than the oxides to get
improved conductivity. Sulfides have been studied as a
promising candidate in recent years. Park et al.
16
found the
higher covalency of the Cu–S interactions in BaCu
2
S
2
rela-
tive to the Cu–O interactions in the oxides. Higher covalency
is expected to lead to broader bands, smaller effective
masses, hence higher mobility and smaller dopant ionization
energies, which could effectively improve the hole conduc-
tivity. Huang et al. focused on another kind of sulfide
CuAlS
2
, which has a chalcopyrite structure and a superlattice
of the zinc blende, with a wide band gap of ⬃3.5 eV. It
contains an ordered tetrahedral array of flattened CuS
4
and
undistorted AlS
4
. This tetrahedral symmetry and the associ-
ated crystal-field splitting impart a strong Cu 3d and S 3p
hybridization at the valence band maximum 共VBM兲. Mean-
while, the distortion of CuS
4
causes dispersion at the VBM,
which facilitates hole conduction.
17
The bulk of Zn-doped
CuAlS
2
had been studied and the thin film had been tried to
be prepared.
18
For further research, the detailed investiga-
tions on the film properties are required.
In this article, Zn-doped CuAlS
2
thin films were prepared
using the pulsed plasma deposition
19
共PPD兲 method. The de-
pendence of structural, surface morphology, electrical, and
optical properties of the films on the substrate temperature
was studied. A transparent heterojunction diode of
p-CuAlS
2
:Zn/ n-In
2
O
3
:W 共IWO兲 was also fabricated and
the current-voltage 共I-V兲 curve was characterized.
II. EXPERIMENTAL DETAILS
CuAl
0.90
Zn
0.10
S
2
ceramic target was provided by Liu et
al.
18
The films were prepared using a PPD system 共previ-
ously called CSA,
19
Organic Spintronics Corporation, Italy兲.
The vacuum chamber was evacuated down to a pressure of
1⫻10
−2
Pa prior to deposition. The films with about 200 nm
were deposited on glass substrates in Ar 共99.99%兲 ambient.
The glass substrate was cleaned in pure water, alcohol, and
acetone for 20 min in an ultrasonic bath, respectively. The
working pressure was maintained at 2.4 Pa during film depo-
sition. The discharge gas type was chosen as Ar. High volt-
age dc power supply 共HCL 140-20000兲 was used to provide
the high accelerating voltage. The voltage and current were
set at 18.0 kV and 4.8 mA, respectively. Thus the strength of
electrical field between the trigger 共⬃0V兲 and hollow cath-
ode 共⬃18 kV兲 is estimated to be 10
5
V/ m, while that be-
tween the cathode and target is also about 2⫻10
5
V/ m. The
acceleration quartz tube has a diameter of 2 mm and a length
of 10 cm. The extracted electron current density was in the
order of 1 A / cm
2
. The distance between the end of tube and
surface center of target was about 4 mm.
20
The full width at
half maximum of the pulse was about 100 ns.
21
The substrate
was heated from 300 to 500 °C by an infrared lamp and the
temperature was measured by a thermocouple 共Eliwell
EWTQ 915, Invensys, Italy兲. More details about PPD work-
ing principles could reference early reports by Witke et al.
20
X-ray diffraction 共XRD兲 patterns of the samples were ob-
tained 共Rigaku D/Max–2550 V兲 using Cu K
␣
radiation 共
=0.154 18 nm兲. The surface morphology was measured us-
a兲
Electronic mail: zhangqun@fudan.edu.cn
1316 1316J. Vac. Sci. Technol. A 27„6…, Nov/Dec 2009 0734-2101/2009/27„6…/1316/4/$25.00 ©2009 American Vacuum Society
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