Electrical properties of GaN deposited on nitridated sapphire by

Journal of Crystal Growth 201/202 (1999) 429}432
Electrical properties of GaN deposited on nitridated sapphire
by molecular beam epitaxy using NH cracked
on the growing surface
Jian-Ping Zhang*, Dian-Zhao Sun, Xiao-Bing Li, Xiao-Liang Wang,
Mei-Ying Kong, Yi-Ping Zeng, Jin-Min Li, Lan-Ying Lin
Institute of Semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, People's Republic of China
Abstract
We have found that GaN epilayers grown by NH -source molecular beam epitaxy (MBE) contain hydrogen.
Dependent on the hydrogen concentration, GaN on (0 0 0 1) sapphire can be either under biaxially compressive strain or
under biaxially tensile strain. Furthermore, we notice that background electrons in GaN increase with hydrogen
incorporation. X-ray photoelectron spectroscopy (XPS) measurements of the N1s region indicate that hydrogen is bound
to nitrogen. So, the microdefect Ga2H}N is an e!ective nitrogen vacancy in GaN, and it may be a donor partly
answering for the background electrons. 1999 Elsevier Science B.V. All rights reserved.
PACS: 71.55.Eq; 73.61.Ey; 78.30.Fs; 81.15.Hi
Keywords: GaN; Hydrogen contaminant; GSMBE; Raman spectrum
1. Introduction
Currently, metalorganic chemical vapor deposition (MOCVD) is the dominant method for the
growth of GaN, and obviously it leads to date to
the best material quality. Nevertheless, molecular
beam epitaxy (MBE), due to its capability for growing complex heterostructures, is a strong alternative to MOCVD. The commonly used plasma
source MBE still has two obstacles, i.e., low growth
rate and ion damage, to overcome. On the other
* Corresponding author. Tel.: #86-10-62339232; fax: #8610-62339266; e-mail: zhangjp@red.semi.ac.cn.
hand, using ammonia as the nitrogen precursor in
the MBE growth of GaN will not su!er from these
problems. In our lab, using NH we have demon
strated a high growth rate of over 1.0 lm/h [1].
Device structures such as modulation-doped "elde!ect transistors (MODFET) [2] and lightemitting diodes (LED) [3] have also been successfully realized by this method.
However, we "nd that GaN epilayers grown by
NH -source MBE often contain hydrogen [4], and
the in#uence of hydrogen on the epilayers needs to
be explored. In this paper, we show that hydrogen
contaminant will signi"cantly e!ect the electrical
properties of GaN. The background electrons in
0022-0248/99/$ } see front matter 1999 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 1 3 6 8 - 2
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J.-P. Zhang et al. / Journal of Crystal Growth 201/202 (1999) 429}432
GaN increase with hydrogen incorporation. Hydrogen is found to be mainly bound to nitrogen in
GaN and can relax the biaxially compressive strain
between GaN epilayers and the underlying sapphire substrates. Too much hydrogen will even
induce biaxially tensile strain in the epilayers. From
a series of experiments we propose that hydrogen
with a certain con"guration may be a donor in
GaN.
2. Experiments
All the samples used in this study were grown on
sapphire substrates by molecular beam epitaxy using NH as the nitrogen precursor. The growth
procedure can be found in Ref. [5]. We measured
hydrogen in GaN by the nuclear reaction analysis
(NRA) using F(H,ac)O [4]. The Raman shift
of the E (high) mode was used to show the strain
status of the GaN epilayers [6]. Raman spectra
were taken in backscattering geometry, with the
z(x, y)z con"guration. The 488 nm line of an Ar>
ion laser was used for excitation. The electrical
properties were measured by Hall e!ect using the
van de Pauw con"guration.
3. Results and discussion
Fig. 1. Raman spectra focused on the E (high) region for a series
of samples taken with the z(x,y)z con"guration. Upper set:
samples under biaxially compressive strain. Lower set: samples
under biaxially tensile strain.
Fig. 1 presents the Raman spectra of a series of
samples, where we only focus on the regions near
E (high) mode. All the samples were grown on
c-face sapphire substrates except that bn11a was
deposited on an A-face sapphire substrate. With the
hexagonal c-axis taken to be perpendicular to the
layer only the longitudinal-optical (LO) branches
of the A mode and the E mode are allowed [7].
The peak near the E (high) mode for bn11a is due
to E (TO) mode. For bulk GaN, the frequency of
the E (high) is equal to 568 cm\ [8]. Red shift or
blue shift to 568 cm\ will indicate that epilayers
are under biaxially tensile or compressive strain
[9]. From the upper set of curves in Fig. 1, we can
see that these samples are under biaxially compressive strain, as expected for GaN deposited on sapphire substrate [10]. However, the E (high) modes
in the lower set curves clearly show a red shift to
568 cm\. At present, we do not know exactly what
causes the biaxially tensile strains in these epilayers.
We just speculate that it may be impurity. From
XPS measurements, however, we cannot get detectable conventional impurities such as oxygen and
carbon. We then turn to hydrogen. The hydrogen
measurements imply that the strain status is well
correlated with hydrogen contaminant, as shown in
Fig. 2a. We believe that the incorporation of interstitial hydrogen may change the lattice constants of
GaN, thereby changing its strain status. As the
lattice constants expand with the hydrogen contaminant, the strain status changes from compression to tension. The hydrogen incorporation in our
experiments seems to be complex. It does not simply depend on the growth temperature or ammonia
#ux, rather, it depends on the growth rate. Higher
J.-P. Zhang et al. / Journal of Crystal Growth 201/202 (1999) 429}432
431
Fig. 3. Typical XPS spectra of GaN/sapphire epilayers. (a) N1s
region (b) G3d region.
Fig. 2. (a) The relationship between hydrogen contaminant and
strain. (b) The relationship between background electrons and
strain. (c) The relationship between background electrons
and hydrogen contaminant.
growth rate and growth temperature will induce
less hydrogen contaminant.
The background electrons measured by the Hall
e!ect increase linearly with the biaxially tensile
strain, and have an exponential decay relation with
strain when under biaxially compressive strain, as
shown in Fig. 2b. In Fig. 2c, we further notice
that the background electrons increase with hydrogen incorporation. This suggests that hydrogen
may be a donor in GaN answering for the background electrons. In order to analyze the incorporation status of hydrogen in GaN, we performed
XPS measurements on the GaN samples. Before
measurements, all samples were sputtered by Ar>
for 120 s in case of any surface contaminants. The
typical results are presented in Fig. 3. From the N1s
432
J.-P. Zhang et al. / Journal of Crystal Growth 201/202 (1999) 429}432
region, we "nd a peak at 399.72 eV belonging to the
N}H bonds. This suggests that hydrogen in GaN is
mainly bound to nitrogen. Therefore, microdefects
Ga2H}N exist in the GaN epilayers. Since this
con"guration will make the corresponding Ga
atom see insu$cient N counterpart in GaN, it
is an e!ective nitrogen vacancy, i.e., it is a
donor in GaN. So we can draw our primary conclusion as that hydrogen bound to nitrogen is a donor in GaN partly answering for the background
electrons.
4. Conclusions
In conclusion, we have found that GaN epilayers
grown by NH -source MBE contain hydrogen. De
pending on the hydrogen concentration, GaN on
(0 0 0 1) sapphire can be either under biaxially compressive strain or under biaxially tensile strain. We
further show that the background electrons in
GaN increase with hydrogen incorporation. XPS
measurements of the N1s region indicate that hydrogen is bound to nitrogen. So, the microdefect
Ga2H}N is an e!ective nitrogen vacancy in GaN,
and it may be a donor answering for the background electrons.
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