Structural and optical properties of single-phase ZnO
1x
S
x
alloy films
epitaxially grown by pulsed laser deposition
Yunbin He
a,
⇑
, Lei Zhang
a
, Liangheng Wang
a
, Mingkai Li
a
, Xunzhong Shang
a
, Xiong Liu
a
, Yinmei Lu
a,b
,
Bruno K. Meyer
b
a
Key Laboratory of Green Preparation and Application for Functional Materials, Ministry of Education of China, Faculty of Materials Science & Engineering, Hubei University,
Wuhan 430062, China
b
I. Physikalisches Institut, Justus-Liebig-Universität Gießen, Heinrich-Buff-Ring 16, 35392 Gießen, Germany
article info
Article history:
Received 1 October 2013
Accepted 25 October 2013
Available online 5 November 2013
Keywords:
ZnOS alloy films
Pulsed laser deposition
Epitaxial growth
Lattice parameters
Band gap
abstract
We report on a detailed investigation of the structural and optical properties of single crystalline ZnO
1x
S
x
thin films, placing emphasis on the elucidation of the correlation of the band gap and lattice parameters,
particularly the lattice constant a, with the S content in the alloy films. High-quality ZnO
1x
S
x
thin films
with different S concentrations Xs(06 Xs 6 0.18) were grown epitaxially on c-plane sapphire substrates
by pulsed laser deposition using a ZnS ceramic target with varying O
2
partial pressures. X-ray diffraction
studies revealed that all grown ZnO
1x
S
x
thin films have a single-phase wurtzite structure. With increas-
ing Xs value from 0 to 0.18, both lattice constants c and a expand monotonically from 5.204 to 5.366 Å
and from 3.255 to 3.329 Å, respectively, while the optical band gap shrinks from 3.27 to 2.92 eV with
a bowing parameter of 2.91 eV. Based on these information, ZnOS/MgZnO heterostructures that have a
perfect in-plane lattice match and a maximum barrier height can be proposed, which might eventually
lead to new optoelectronic devices with superior performance.
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
ZnO is a typical wide band gap semiconductor (E
g
3.3 eV at
room temperature) that holds great promise for applications in
optoelectronic devices operating in the blue to ultraviolet spectral
region. Among the other advantages, its large exciton binding en-
ergy (60 meV) and oscillator strength could lead to laser action
based on exciton recombination at or even above room tempera-
ture [1,2]. Since the exciton binding energy and oscillator strength
can be further boosted at least twice in quantum confined struc-
tures, ZnO based devices shall take advantage of quantum-well
(QW) structures to optimize the device performance [3]. Following
extensive investigations on the band gap engineering of ZnO by
isovalent cationic substitution (e.g. MgZnO, BeZnO and CdZnO al-
loys), constructing and fabricating single and multiple ZnO-based
QWs from ZnO/ZnMgO [4], ZnO/ZnBeO [5], CdZnO/ZnO [6], and
CdZnO/MgZnO [7] on various substrates have already been demon-
strated with different growth techniques. On the other hand, alloy-
ing of ZnO by isovalent anionic substitution [8], e.g. replacing
oxygen by sulfur, is far less explored, and documentation of ZnO-
based heterostructures involving ZnOS alloys is not available in
the literature up to date.
Yoo et al. [9] first successfully doped S into ZnO with pulsed la-
ser deposition (PLD) by alternatively ablating a ZnO and a ZnS tar-
get. They found that the sulfur concentration in single-phase
wurtzite ZnOS films deposited on c-plane sapphire at 700 °C was
limited to 0.13 and both the c-axis lattice constant and the band
gap showed a nonlinear correlation with the sulfur content in
the films. Applying radio-frequency (RF) reactive sputtering and
using a ZnS ceramic target with Ar/O
2
as working/reactive gases,
Meyer et al. [10] synthesized single-phase wurtzite ZnO
1x
S
x
alloy
films in the whole composition range (0 6 x 6 1) on both glass and
c-plane sapphire substrates at a relatively low substrate tempera-
ture of 340 °C, and found that the c-axis lattice constant changed
linearly while the band gap varied nonlinearly with the sulfur con-
tent x in the ZnOS alloys, and an optical bowing parameter of
3.0 eV was determined. With similar technique, Pan et al. [11]
deposited ZnO
1x
S
x
alloy films on glass substrates at 300 °C with
post-annealing at 500–600 °C. They noticed a phase separation oc-
curred in the composition range of 0.23 < x < 0.77, and a non-linear
variation of the c-axis lattice constant and band gap with respect to
the S content in both the wurtzite O-rich (0 6 x 6 0.23) and
zincblende S-rich (0.77 6 x 6 1) films. In a recent study, we used
the PLD technique, employing also a ZnS ceramic target with O
2
as the reactive gas to grow ZnOS alloy films [12]. High-quality
epitaxial films allowed us to determine the S solubility (up to
0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jallcom.2013.10.201
⇑
Corresponding author. Tel./fax: +86 27 88661803.
E-mail addresses: ybhe@hubu.edu.cn, yunbin.he@physik.uni-giessen.de (Y. He).
Journal of Alloys and Compounds 587 (2014) 369–373
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
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