Journal of Crystal Growth 404 (2014) 199–203
Contents lists available at ScienceDirect
Journal of Crystal Growth
journal homepage: www.elsevier.com/locate/jcrysgro
Effect of additional hydrochloric acid flow on the growth of non-polar
a-plane GaN layers on r-plane sapphire by hydride vapor-phase epitaxy
Moonsang Lee a,d, Dmitry Mikulik a, Sungsoo Park a, Kyuhyun Im b, Seong-Ho Cho a,
Dongsu Ko c, Un Jeong Kim b,n, Sungwoo Hwang b,n, Euijoon Yoon d,e,n
a
Compound Semiconductor Lab., Samsung Advanced Institute of Technology, Samsung Electronics, Yongin, Kyunggi-do 446-712, Republic of Korea
Nano-electronics Lab., Samsung Advanced Institute of Technology, Samsung Electronics, Yongin, Kyunggi-do 446-712, Republic of Korea
c
Analytical Engineering Group, Samsung Advanced Institute of Technology, Samsung Electronics, Yongin, Kyunggi-do 446-712, Republic of Korea
d
Department of Materials Science and Engineering, Seoul National University, Daehak-dong, Gwanak-gu, Seoul 151-742, Republic of Korea
e
Energy Semiconductor Research Center, Advanced Institutes of Convergence Technology, Seoul National, University, Suwon 443-270, Republic of Korea
b
art ic l e i nf o
a b s t r a c t
Article history:
Received 7 April 2014
Received in revised form
23 June 2014
Accepted 1 July 2014
Communicated by: T. Paskova
Available online 11 July 2014
The effect of additional HCl flow on the growth of a-plane GaN layers on r-plane sapphire by hydride
vapor-phase epitaxy was investigated. Upon increasing the additional HCl flow rate, the surface
roughness of the a-plane GaN layers, as measured by atomic force microscopy, reduced. The crystal
quality of a-plane GaN, however, deteriorated, as confirmed by high stacking fault density observed by
transmission electron microscopy, relating to the increased nuclei density and mosaicity and a high full
width at half maximum in the ω-scan X-ray rocking curve. These observations were attributed to the
large difference of growth rate and etch rate along c-direction, and m-direction of a-plane GaN, which
were originated from the surface energetics of the crystallographic planes of GaN.
& 2014 Elsevier B.V. All rights reserved.
Keywords:
A1. Single crystal growth
A3. Hydride vapor-phase epitaxy
B1. Nitrides
B2. Semiconducting III–V materials
1. Introduction
Although great device performance has been achieved for
GaN-based light-emitting diodes (LEDs), conventional GaN-based
devices experience a quantum-confined Stark effect along the
c-axis that is induced by spontaneous polarization and a piezoelectric field. This leads to a large spatial separation between
electron and hole wavefunctions, resulting in the loss of internal
quantum efficiency in quantum well layers [1–3]. The use of nonpolar GaN layers can circumvent such a negative effect of polar
GaN-based devices along the c-axis; therefore, the research on
non-polar GaN has gained considerable attention nowadays [4,5].
One of the well-known methods for fabricating non-polar GaN
substrates is to slice bulk polar GaN along the non-polar direction [6].
This method, however, has some serious disadvantages such as
low productivity and size limitation. the most feasible, popular,
and commercially available way is non-polar GaN growth on
foreign substrates (e.g., a-plane GaN on r-plane sapphire [7], and
m-plane GaN on m-plane SiC or LiAlO2 [8–10]) by using metal
organic chemical vapor deposition (MOCVD), molecular beam
epitaxy (MBE), or hydride vapor-phase epitaxy (HVPE). Among these
methods, HVPE is the most promising one because it provides a high
growth rate and relatively good crystal quality. In a conventional
HVPE system using Ga metal and HCl gas as GaCl sources for GaN
growth, the occurrence of an unintentional flow of HCl gas, which
has not reacted with Ga metal, is inevitable. Xiua et al. [11] described
the effect of HCl on the surface morphology of c-plane GaN grown on
sapphire by HVPE; however, the effect of HCl on non-polar GaN has
not been clarified until now.
In this study, we investigated the effect of HCl gas flow on nonpolar a-plane GaN grown on r-plane sapphire by flowing additional
HCl gas directly to the growth zone in an HVPE reactor. Owing to the
unique growth characteristics and surface energetics of non-polar
GaN, the excess HCl gas significantly affected the growth characteristics of non-polar GaN, such as surface morphology and crystal
quality. With an increase in the additional HCl flow, the surface
roughness of a-plane GaN was found to be reduced. However,
transmission electron microscopy (TEM) observations and ω-scan
X-ray rocking curves revealed that the crystal quality of a-plane GaN
deteriorated with an increase in the additional HCl flow.
2. Experimental
n
Corresponding authors.
E-mail addresses: [email protected] (U.J. Kim),
[email protected] (S. Hwang), [email protected] (E. Yoon).
http://dx.doi.org/10.1016/j.jcrysgro.2014.07.002
0022-0248/& 2014 Elsevier B.V. All rights reserved.
Non-polar a-plane GaN layers were grown on 2-in. (1 1 0 2)
r-plane sapphire substrates by using a vertical-type downstream
200
M. Lee et al. / Journal of Crystal Growth 404 (2014) 199–203
HVPE system with a reactor diameter of 6 in. To prevent significant
stress evolution during GaN growth on the r-plane sapphire
substrates, the surface treatment of the sapphire substrates
was employed before non-polar GaN growth. It consisted of HCl
etching (flow rate: 70 sccm) and nitridation with NH3 gas
(flow rate: 2500 sccm) for 5 min. This treatment resulted in the
formation of a discontinuous AlN buffer layer on the substrates.
Scanning electron microscopy (SEM) analysis revealed the discontinuous layer to be composed of AlN nanodots (not shown in
this paper). Subsequently, GaN was grown on the AlN buffer layer
under atmospheric pressure and a temperature of 1080 1C. HCl
gas (flow rate: 40 sccm) was reacted with liquid Ga metal to form
GaCl gas (conversion efficiency from HCl to GaCl was 15%). Then,
GaCl was transported to the growth zone where it reacted with
NH3, leading to GaN layer growth. The total flow rate of N2 used as
the carrier gas was 15 slm and the V/III ratio was approximately 6.
The thickness of the grown non-polar a-plane GaN layers was
3 mm for all samples. To investigate the effect of HCl on non-polar
GaN, the HVPE system was modified by adding another special
channel for flowing additional HCl gas (at flow rates of 0, 100, 150,
and 200 sccm) directly to the substrate without reaction with
liquid Ga metal or NH3.
The surface morphology of the grown non-polar a-plane GaN
layers was evaluated by atomic force microscopy (AFM). The
structural properties of the grown layers were analyzed using
double-crystal X-ray diffraction and transmission electron microscopy (TEM) measurements. X-ray rocking curves (ω-scan) were
measured using a Cu Kα line (λ¼0.154060 nm) from a Bede D3
system. Plan-view TEM samples were prepared by grinding and
mechanical polishing followed by ion milling. The TEM observation was carried out using a FEI F20 microscope with a field
emission gun operating at 200 kV.
3. Results and discussion
The growth rate and surface roughness of the grown a-plane
GaN layers as a function of the additional HCl flow rate are listed in
Table 1. As shown in the table, upon increasing the additional HCl
flow rate, the growth rate decreased owing to the etching of GaN
by HCl. Moreover, AFM analysis revealed that the surface roughness (Rq) also decreased, ranging from 57.3 nm to 37.2 nm, upon
increasing the additional HCl flow rate, as shown in Fig. 1.
In the absence of the additional HCl flow, the surface of a-plane
GaN layers showed a typical anisotropic morphology with longitudinal stripe patterns along the [0 0 0 1] direction, as illustrated
in Fig. 1(a). With increasing additional HCl flow rate, the longitudinal stripe patterns on the surface started to fade gradually and
completely disappeared when the additional HCl flow rate became
more than 150 sccm, as can be seen in Fig. 1(c) and (d).
This behavior can be attributed to the difference in the etch
rates of a-plane GaN along the m- and c-directions, resulting from
the different surface energetics for each crystallographic plane.
Jindal et al. calculated the surface energies for the crystallographic
planes of GaN [12]. They found that the surface energy of
N-face c-plane GaN (387 meV/Å2) is much higher than that of
m-plane (137 meV/Å2), a-plane (159 meV/Å2), and Ga-face c-plane
GaN (129 meV/Å2). It is well known that higher surface energy
leads to higher surface instability, resulting in high reactivity with
other materials. This implies that N-face c-plane GaN with higher
surface energy could be more readily etched by HCl gas than mplane and Ga-face c-plane GaN. We consider that this gave rise to
the aforementioned gradual suppression of the stripe pattern
along the c-axis direction. Therefore, the surface morphology of
a-plane GaN can be improved by increasing the additional HCl
flow rate. Furthermore, the enhancement in surface diffusion and
coalescence over the (1 1 2 0) plane of GaN nuclei due to a lower
growth rate may influence the surface morphology. It should be
noted here that the growth rate of a-plane GaN decreases owing to
HCl etching, which promotes the coalescence of GaN nuclei.
Fig. 2(a)–(c) exhibits the plan-view TEM images of a-plane GaN
with a g-vector [1 1 0 0], showing basal stacking faults for
additional HCl flow rates of 0, 100, and 200 sccm, respectively. It
can be seen that the stacking fault density is strongly affected by
the additional HCl flow rate, as depicted in Fig. 2(d). We speculate
that the dependence of stacking fault density on the additional HCl
flow rate is related to GaN nucleation. Vennéguès et al. reported
that basal stacking faults are mainly induced by the coalescence of
3D nuclei [13]; this indicates that the density of basal stacking
faults in non-polar films depends on the density of GaN nuclei.
Considering GaN nucleation on the discontinuous AlN buffer layer,
it can be concluded that the density of GaN nuclei can increase
owing to the increase in the number of nucleation sites on the rplane sapphire surface by HCl treatment during the initial stage of
growth. Consequently, we consider that the density of basal
stacking faults is proportional to the additional HCl flow rate.
Basal stacking faults result in anisotropic scattering of electrons
and holes in non-polar GaN films. This scattering originates at the
interface between basal stacking faults and matrix wurtzite GaN,
and orients the band-edge discontinuities and local electric fields
along the [0 0 0 1] direction of GaN. This orientation restricts the
in-plane transportation of carriers along the c-axis direction, thus
generating junction leakage currents [14–17]. Therefore, we predict that the optical properties of a-plane GaN grown by HVPE
could be significantly improved by minimizing the excess HCl
amount on the a-plane GaN surface.
The structural quality of a-plane GaN on r-plane sapphire was
evaluated from the full width at half maximum (FWHM) of
(1 1 2 0) X-ray rocking curves along the c-direction as a function
of the additional HCl flow rate during growth, and the results are
shown in Fig. 3.
The additional HCl flow increases the FWHM in the (1 1 2 0)
X-ray rocking curve aligned along the c-axis direction, indicating
the deterioration in the crystal quality of a-plane GaN. As mentioned above, the additional HCl flow etches the GaN layers at
different rates depending on the crystal planes of a-plane GaN. Lee
et al. [18] showed that the mosaic structure of a-plane GaN layers
grown on r-plane sapphire results from the difference between the
growth rates of the Ga- and N-face c-plane GaN, and that between
the growth rates of a-plane GaN along the c- and m-directions.
Typically, longitudinal stripe-shaped grains of a-plane GaN are
stacked such that they are tilted up by neighboring grains owing to
the difference between the growth rates of the Ga- and N-face
c-plane GaN before lateral coalescence with adjacent grains.
Table 1
Characteristics of non-polar a-plane GaN layers at different additional HCl flow rates during growth.
Template
A
B
C
D
Additional HCl gas flow rate (sccm) during growth
Growth rate (mm/h)
Surface roughness (nm)
0
44
57.3
100
43
50.9
150
39
44.2
200
31
37.2
M. Lee et al. / Journal of Crystal Growth 404 (2014) 199–203
201
Surface roughness (nm)
65
60
55
50
45
40
35
30
0
50
100
150
200
Additional HCl flow rate (sccm)
Fig. 1. AFM images of a-plane GaN grown on r-plane sapphire with additional HCl flow of (a) 0 sccm, (b) 100 sccm, (c) 150 sccm and (d) 200 sccm. (e) Surface roughness as a
function of additional HCl flow rate.
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M. Lee et al. / Journal of Crystal Growth 404 (2014) 199–203
Stacking fault density (/cm)
6
1.6x10
6
1.4x10
6
1.2x10
6
1.0x10
5
8.0x10
5
6.0x10
0
50
100
150
200
Additional HCl flow rate (sccm)
Fig. 2. (a)–(c) Plan-view TEM images of a-plane GaN and (d) stacking fault density as a function of additional HCl flow rate. (a) 0 sccm, (b) 100 sccm and (c) 200 sccm.
1050
1000
FWHM (arcsec)
We consider that the difference between the etch rates of N- and
Ga-face c-plane GaN in longitudinal GaN grains causes a significant difference between the growth rates of Ga- and N-face of
GaN. Moreover, this etch rate difference increases the density of
longitudinal stripe-shaped grains upon accelerating the mosaicity.
The values of FWHM in the X-ray rocking curve aligned along the
m-direction of a-plane GaN, i.e., [1 1 0 0] and [ 1 1 0 0], were
measured (not shown in this paper). No dependence, however,
was found between the FWHM and additional HCl flow rate. This
result presumably originates from the isotropic characteristics of
the etch rate along the m-direction of a-plane GaN as confirmed by
no directionality of the surface energy along the m-direction of
GaN [12].
FWHM of (11-20) X-ray rocking curve
along c-axis direction
950
900
850
800
750
700
650
600
0
50
100
150
200
Additional HCl flow rate (sccm)
Fig. 3. FWHM of ω-scan in (1 1 2 0) X-ray rocking curve for a-plane GaN along caxis at different additional HCl flow rates.
Therefore, these grains would agglomerate with disordered grains
and tilt along the c-axis, increasing the density of disordered
grains and thus deteriorating the crystal quality of a-plane GaN.
4. Conclusions
We studied the effect of additional HCl flow on the growth of
a-plane GaN by HVPE. With additional HCl flow at rates up to
200 sccm, the surface roughness largely reduced but the crystal
quality was found to be deteriorated. These observations were
attributed to the large growth rate difference and etch rate
M. Lee et al. / Journal of Crystal Growth 404 (2014) 199–203
difference induced by the additional HCl flow, originating from the
surface energetics of the crystallographic directions of a-plane
GaN. Higher stacking fault density due to increased nuclei density
and mosaicity was induced by the additional HCl gas flow and was
expected to deteriorate the crystal quality of a-plane GaN. We
expect that the surface roughness and crystal quality of non-polar
a-plane GaN could be controlled through additional HCl gas flow
during HVPE growth.
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