NAN O E X P R E S S Open Access
Band offsets of non-polar A-plane GaN/AlN and
AlN/GaN heterostructures measured by X-ray
photoemission spectroscopy
Ling Sang
1*
, Qin Sheng Zhu
2
, Shao Yan Yang
2
, Gui Peng Liu
2
, Hui Jie Li
2
, Hong Yuan Wei
2
, Chun Mei Jiao
2
,
Shu Man Liu
2
, Zhan Guo Wang
2
, Xiao Wei Zhou
3
, Wei Mao
3
, Yue Hao
3
and Bo Shen
1*
Abstract
The band offsets of non-polar A-plane GaN/AlN and AlN/GaN heterojunctions are measured by X-ray photoemission
spectroscopy. A large f orward-backward asymmetry is observed in the non-polar GaN/AlN and AlN/GaN heterojunctions.
The valence-band offsets in the non-polar A-plane GaN/AlN and AlN/GaN heterojunctions are determined to be 1.33 ±
0.16 and 0.73 ± 0.16 eV, respectively. The large valence-band offset difference of 0.6 eV between the non-polar GaN/AlN
and AlN/GaN heterojunctions is considered to be due to piezoel ectric strain effect in the non-p olar heterojunction
overlayers.
Keywords: GaN/Al N; Heterostructur e; X-ray photoemission sp ectroscopy; Non -polar
Background
During the last decade, group III-V nitrides are very
promising semiconductor materials for application in
high frequency heterojunction field-effect transistors
(HFETs) [1-8]. Large band offsets at the heterojunctions
are important to realize these device applications. To
understand the band offsets between nitride materials at
heterointerface is requested for fabricating devices. The
heterojunction formed between GaN and AlN is particu-
larly application-oriented because of their large band
gap difference induced by the polarization properties of
nitride materials [9-11]. Several groups have reported
ΔE
V
values for GaN/AlN heterojunctions fabricated by
different growth techniques and determined the value of
ΔE
V
in a range from 0.5 to 1.4 eV [12]. However, these
GaN/AlN heterojunctions reported were nearly depos-
ited all on (0001) orientation substrates. Espec ially,
Martin et al. reported valence-band offsets of GaN/AlN
and AlN/GaN heterojunctions on C-plane sapphire sub-
strates were 0.60 ± 0.24 and 0.57 ± 0.22 eV, respe ctively,
both values were almost the same to each other [13].
In recent years, non-polar nitride heterostructure ha s
drawn great interest owing to its potential applications
in normally-off HE MT, high-efficiency field-free deep-
ultraviolet (UV) l ight emitting diodes (LEDs) with
wavelengths of 200 to 300 nm or sensors and so on.
Non-polar nitride can eliminate internal polarization fields
because of the absence of spontaneous polarization in the
non-polar materials. However, the valence-band offset of
non-polar A-plane GaN/AlN heterostructures ha s been
studied by few. In this paper, we s tudied valence-band
offsets of non-polar A-plane GaN/AlN and AlN/GaN
heterostructures deposited on R-plane sapphire substrates
measured by X-ray photoemission spectroscopy (XPS).
Methods
The samples investigated were grown on R-plane sapphire
substrates by metal-organic chemical vapor deposition
(MOCVD). Four s amples were used in our XPS experi-
ments, namely, a 1.5-μm-thick GaN layer, 250-nm AlN
layer, 5-nm GaN/250-nm AlN heterojunction, and 5-nm
AlN/1.5-μm GaN heterojunction. Triethylgallium (TEGa),
trimethylaluminum (TMAl), and ammonia (NH
3
)were
used as the sources of Ga, Al, and N, respectively. The car -
rier gas was high-purity hydrogen. Before growing GaN
and AlN layers, R-plane (10
1 2) sapphire substrates were
thermally cleaned in H
2
ambientat1,000°Cfor10minto
remove the adsorbed water molecules and activate sapphire
* Correspondence: lingsang@pku.edu.cn; bshen@pku.edu.cn
1
State Key Laboratory of Artificial Microstructure and Mesoscopic Physics,
School of Physics, Peking University, Beijing 100871, China
Full list of author information is available at the end of the article
© 2014 Sang et al.; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons
Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction
in any medium, provided the original work is properly credited.
Sang et al. Nanoscale Research Letters 2014, 9:470
http://www.nanoscalereslett.com/content/9/1/470
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