Applied Surface Science 317 (2014) 994–999
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Applied Surface Science
journal h om epa ge: www.elsevier.com/locate/apsusc
Band alignment studies of Al
2
O
3
/CuGaO
2
and ZnO/CuGaO
2
hetero-structures grown by pulsed laser deposition
R.S. Ajimsha
∗
, Amit. K. Das, M.P. Joshi, L.M. Kukreja
Laser Material Processing Division, Raja Ramanna Centre for Advanced Technology, Indore 452 013, India
a r t i c l e i n f o
Article history:
Received 14 March 2014
Received in revised form 22 August 2014
Accepted 25 August 2014
Available online 1 September 2014
Keywords:
PLD
Oxide thin film
Band offset
a b s t r a c t
We have studied the band offset and alignment of pulsed laser deposited Al
2
O
3
/CuGaO
2
and ZnO/CuGaO
2
hetero-structures using photoelectron spectroscopy. Al
2
O
3
/CuGaO
2
interface exhibited a type I band
alignment with valance band offset (VBO) of 4.05 eV whereas type II band alignment was observed in
ZnO/CuGaO
2
hetero-structure with a VBO of 2.32 eV. Schematic band alignment diagram for the interface
of these hetero-structures has been constructed. Band offset and alignment studies of these heterojunc-
tions are important for gaining insight to the design of various optoelectronic devices based on such
hetero-structures.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
CuGaO
2
is a promising p-type transparent conducting oxides
(TCO) due to it’s high transparency (∼80%) and better conduc-
tivity (∼10
-1
S cm
-1
) among p-type delafossites and its stability
towards photo corrosion [1,2]. Epitaxial growth of CuGaO
2
thin
films on Al
2
O
3
substrates was reported by K. Ueda et al. [1].
Al
2
O
3
has been identified as suitable dielectric in various optoelec-
tronic devices due to its cost effectiveness and low trap density
at the oxide–dielectric interface [3]. Recent reports have shown
that the use of Al
2
O
3
as a gate dielectric in transparent thin
film transistors (TFT’s) improved the device performance, because
of substantial reduction in leakage currents and threshold volt-
age [3]. This suggested that CuGaO
2
as p-type channel layer and
Al
2
O
3
as a gate dielectric would be possible combination for
optoelectronic devices like p-type TFT’s. Very recent report by For-
ticaux et al. [4] demonstrated diode behaviour in 3D mesoscale
hetero-structures of n-type ZnO nanowire arrays epitaxially grown
on p-type CuGaO
2
nanoplates. Even though these 3D structures
were not transparent, this could show the possibility of high
∗
Corresponding author. Tel.: +91 731 2488304; fax: +91 731 2488300.
E-mail address: ajimsha@gmail.com (R.S. Ajimsha).
quality epitaxial growth of ZnO/CuGaO
2
rectifying hetero-
structure. Above all, ZnO is an exciting multifunctional, nominally
n-type TCO with large excitonic binding energy ∼60 meV which
makes the ZnO/CuGaO
2
hetero-structure a demanding candidate
for various optoelectronic applications. Band alignment at the
interface of the hetero-structures plays a key role in carrier
transport and recombination processes in optoelectronic devices.
Valance band offsets and band alignment studies of Al
2
O
3
/CuGaO
2
and ZnO/CuGaO
2
interfaces have not been reported so far. There-
fore it becomes essential to study the band offset and hence
the band alignment of both Al
2
O
3
/CuGaO
2
and ZnO/CuGaO
2
hetero-structures. X-ray photoelectron spectroscopy (XPS) has
been demonstrated as a direct and powerful tool for determining
the valance band offset and hence the band alignment of various
hetero-structures [5–10]. In this letter, we report the band align-
ment studies of Al
2
O
3
/CuGaO
2
and ZnO/CuGaO
2
hetero-structures
using X-ray photoelectron spectroscopy. For the XPS studies of
Al
2
O
3
/CuGaO
2
hetero-structure, we used three samples (1) epi-
polished (0 0 0 1) sapphire (-Al
2
O
3
) substrate (A1), (2) 400 nm
thick CuGaO
2
thin film grown on Al
2
O
3
substrate (A2) and (3)
2 nm CuGaO
2
thin film grown on Al
2
O
3
substrates (A3). In the case
of ZnO/CuGaO
2
hetero-structure also, we used three samples (1)
aforesaid 400 nm thick CuGaO
2
film grown on Al
2
O
3
substrate (A2),
(2) 400 nm thick ZnO film grown on Al
2
O
3
substrate (B1) and (3)
http://dx.doi.org/10.1016/j.apsusc.2014.08.137
0169-4332/© 2014 Elsevier B.V. All rights reserved.