Ultraviolet electroluminescence from n-ZnO/p-NiO heterojunction
light-emitting diode
R. Deng
a,b
, B. Yao
a,
n
, Y.F. Li
a
,Y.Xu
a
, J.C. Li
a
, B.H. Li
c
, Z.Z. Zhang
c
, L.G. Zhang
c
, H.F. Zhao
c
, D.Z. Shen
c
a
State Key Lab of Superhard Materials, and Department of Physics, Jilin University, Changchun 130023, People’s Republic of China
b
School of Materials Science and Engineering, Changchun University of Science and Technology, Changchun 130022, People’s Republic of China
c
Laboratory of Excited State Processes, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130033, People’s Republic of China
article info
Article history:
Received 22 June 2012
Received in revised form
3 August 2012
Accepted 17 August 2012
Available online 28 August 2012
Keywords:
Thin films
Heterojucntions
Light-emitting diode
Electroluminescence
abstract
The n-ZnO/p-NiO heterojunction was prepared by depositing a p-type NiO film on a c-plane sapphire by
rf magnetron sputtering and then growing a n-type ZnO film on the NiO film by plasma-assisted
molecular beam epitax y. The heterojunction shows a diode-like rectification characteristic with a turn-
on voltage of 3.6 V and emits UV light upon putting a forward bias. The intensity of the UV emission
increases as injection current increases from 0.5 to 3.5 mA, but the wavelength of the UV emission
decreases from 404 to 387 nm. It is demonstrated that the UV emission comes from near band-edge
radiative recombination of electron and hole in the ZnO layer. The mechanism of the UV electro-
luminescence is discussed in the present work.
& 2012 Elsevier B.V. All rights reserved.
1. Introduction
Zinc oxide, with a wide direct band gap of 3.37 eV and an exciton
binding energy of 60 meV, is one of the most promising candidates
for preparation of short-wavelength light emitting diodes (LEDs) and
lasing devices (LDs) as well as photodiode devices [1–3]. Upto now,
although electroluminescence (EL) of ZnO p–n homojunction LEDs
has been realized, it is still difficult for further commercial applica-
tion because of the lack of reproducible and stable p-type ZnO thin
films [1]. On the other hand, heterojunction-based LEDs are
expected to exhibit improved current confinement comparing to
homojunction LEDs, for this reason, considerable efforts have been
denoted to fabricating ZnO-based heterojunction by employing
n-type ZnO active region and other p-type semiconductors, such
as GaN, Si, AlGaN, Cu
2
O, SrCu
2
O
2
,SiC,ZnTeandNiO[1,4–15].
Moreover, EL is observed in some of these heterojunction devices.
However, the EL spectrum consists of UV and visible emission bands
in many cases, and the intensity of the visible emission is usually
stronger than that of UV emission, [13,16], which is related to not
only lattice mismatch and band alignment at the interface of the
heterojunction but also electronic structure and electrical properties
of the p-type semiconductors. Hence, it is important to select
suitable p-type semiconductor for realizing single and strong UV
EL in the heterojunction. It is known that NiO is a natural p-type
semiconductor w ith direct wide band gap of 3.7 eV and c an be
fabricated by simple technique, the former usually make the hetero-
junction have high emission efficiency, and the latter leads to low
cost. Recently, a single UV EL was observed in ZnO/NiO based
heterojunction with a complex structure of n
þ
-GaN/n-ZnO/i-
Mg
1 x
Zn
x
O/p-NiO [15], indicating that the NiO is good p-type
semiconductor suitable to preparation of UV ZnO-based heterojunc-
tion. However, such complex structure including GaN is dependent
on complex fabrication technique, leading to difficulty in fabrication
and expensive cost. Thus, it is necessary to design and produce
ZnO/NiO based heterostructure with a simple structure for research
and application of UV ZnO/NiO based heterojunction LED. In the
present work, fabrication and EL of an n-ZnO/p-NiO heterojunction
with simple structure are investigated.
2. Experimental details
P-type NiO (p-NiO) thin film with a hole concentration of
2 10
16
cm
3
and a Hall mobility of 6 cm
2
/V s was deposited on
c-plane sapphire substrate by rf magnetron sputtering a Ni target
with the mixture of Ar and O
2
gases. For depositing n-type ZnO
(n-ZnO) layer with high crystal quality on the NiO film, an n-ZnO
layer was deposited on the as-deposited p-type NiO film by
plasma-assisted molecular beam epitaxy (P-MBE). The electron
concentration and mobility of the n-type ZnO layer are deter-
mined to be 4 10
17
cm
3
and 8 cm
2
/V s, respectively. The Ni/Au
alloy and indium metal were used as electrodes of the p-type NiO
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journal homepage: www.elsevier.com/locate/jlumin
Journal of Luminescence
0022-2313/$ - see front matter & 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jlumin.2012.08.039
n
Corresponding author.: Tel: þ 86 431 8516 8857.
E-mail address: binyao@jlu.edu.cn (B. Yao).
Journal of Luminescence 134 (2013) 240–243