Effects of grain size on the mosaic tilt and twist in InN films grown on GaN by metal-organic chemical vapor deposition

Article abstract of DOI:10.1063/1.2345224

The structural property of InN films grown on Ga-face GaN layers by metal-organic chemical vapor deposition has been studied by high-resolution x-ray diffraction. The mosaic tilt and twist are found to be strongly dependent on the surface lateral grain size. The twist decreases with increasing grain size and finally approaches to a constant level. On the other hand, the mosaic tilt increases substantially when the grain size becomes large enough and exceeds the width of step terraces on the GaN surface, showing an important mechanism for the defect generation in the InN/GaN system with large out-of-plane lattice mismatch.

Full text of DOI:10.1063/1.2345224

Effects of grain size on the mosaic tilt and twist in InN films grown on GaN
by metal-organic chemical vapor deposition
H. Wang, Y. Huang, Q. Sun, J. Chen, L. L. Wang et al.
Citation: Appl. Phys. Lett. 89, 092114 (2006); doi: 10.1063/1.2345224
View online: http://dx.doi.org/10.1063/1.2345224
View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v89/i9
Published by the American Institute of Physics.
Additional information on Appl. Phys. Lett.
Journal Homepage: http://apl.aip.org/
Journal Information: http://apl.aip.org/about/about_the_journal
Top downloads: http://apl.aip.org/features/most_downloaded
Information for Authors: http://apl.aip.org/authors
Downloaded 08 Mar 2013 to 128.143.22.132. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

APPLIED PHYSICS LETTERS 89, 092114 ͑2006͒
Effects of grain size on the mosaic tilt and twist in InN films grown on GaN
by metal-organic chemical vapor deposition
a͒
H. Wang, Y. Huang, Q. Sun, J. Chen, L. L. Wang, J. J. Zhu, D. G. Zhao, S. M. Zhang,
D. S. Jiang, Y. T. Wang, and H. Yang
State Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy
of Sciences, P.O. Box 912, Beijing 100083, China
͑
Received 19 May 2006; accepted 5 July 2006; published online 30 August 2006͒
The structural property of InN films grown on Ga-face GaN layers by metal-organic chemical vapor
deposition has been studied by high-resolution x-ray diffraction. The mosaic tilt and twist are found
to be strongly dependent on the surface lateral grain size. The twist decreases with increasing grain
size and finally approaches to a constant level. On the other hand, the mosaic tilt increases
substantially when the grain size becomes large enough and exceeds the width of step terraces on
the GaN surface, showing an important mechanism for the defect generation in the InN/GaN system
with large out-of-plane lattice mismatch. © 2006 American Institute of Physics.
͓
DOI: 10.1063/1.2345224͔
During the past decade, InN has excited more research
interest than before for its newly discovered small band gap
value ͑ϳ0.7 eV͒ ͑Refs. 1 and 2͒ and its excellent electrical
crease in tilt as InN islands grow large enough to cross the
step terraces on the GaN surface.
The growth of InN was conducted in a conventional
horizontal metal-organic chemical vapor deposition
3
,4
properties predicted by theoretical calculations. However,
the growth of InN is more difficult than that of other group
III nitride semiconductors due to its low growth temperature
required and the lack of suitable lattice-matched substrate
materials. Currently, the growth of InN is carried out mainly
on as-grown epitaxial GaN layers. The crystalline quality of
InN grown on GaN is much better than that directly grown
͑
MOCVD͒ reactor operated at atmospheric pressure with N2
as the carrier gas. The 4 ␮m-thick Ga-face GaN layers
grown on c-plane sapphire substrate via a two-step procedure
by MOCVD served as templates for the InN growth. Prior to
the growth, the as-grown GaN templates were thermally
cleaned at 700 °C in NH atmosphere. Trimethyl indium and
3
5
,6
NH3 were used as precursors for In and N, respectively.
X-ray measurement was performed using a Rigaku SLX-1A
x-ray diffractometer equipped with Si ͑220͒ analyzer. A
NanoScope IIIa atomic force microscope ͑AFM͒ was em-
ployed for surface morphology observation. Besides, trans-
mission electron microscopy ͑TEM͒ experiments were car-
ried out in JEM 2010 electron microscope operating at
on sapphire substrates. Moreover, InN/GaN material sys-
tem also facilitates the study of InN/GaN based heterostruc-
7
ture device applications. On the other hand, however, the
huge lattice mismatch ͑ϳ10%͒ between InN and GaN poses
a great difficulty to the InN researchers. For example, it has
been found that three-dimensional ͑3D͒ InN island growth is
8
,9
preferred through the Stranski-Krastanov growth mode,
2
00 kV.
A series of InN films with a nominal thickness about
which often results in an undesired rough surface morphol-
ogy. Furthermore, threading dislocations ͑TDs͒ can evolve
2
3
00–400 nm were deposited at a temperature ranging from
50 to 500 °C. As shown by AFM images in Fig. 1, the
1
0
from the coalescence of neighboring misoriented islands.
This affects the microstructure and crystalline quality of the
InN epilayer.
surface morphology is markedly characterized by the lateral
A mosaic model is widely used to describe the epilayer
microstructure, in which the in-plane and out-of-plane rota-
tions of grains are quantified by the mosaic twist and tilt,
respectively. The extent of tilt and twist can be directly re-
lated to the full width at half maximum ͑FWHM͒ of the
͑
0002͒ symmetric and the ͑hkil͒ skew symmetric x-ray rock-
11
ing curves with either h or k being nonzero. A detailed
characterization of the extent of tilt and twist in heteroepi-
taxial InN films can be conveniently implemented by virtue
of the well-established high-resolution x-ray diffraction ͑HR-
XRD͒ technology.
In this letter, we have made a systematic study of the
structural property of InN epilayers grown on GaN by HR-
XRD. It is found that the extent of mosaic tilt and twist is
strongly dependent on the average grain size as they ap-
proach coalescence. Particularly, there is a remarkable in-
FIG. 1. 5ϫ5 ␮m2 AFM images of InN samples grown at 350 °C ͑a͒,
400 °C ͑b͒, 450 °C ͑c͒, and 500 °C ͑d͒, respectively.
a͒Electronic mail: wangh@red.semi.ac.cn
0003-6951/2006/89͑9͒/092114/3/$23.00
89, 092114-1
© 2006 American Institute of Physics
Downloaded 08 Mar 2013 to 128.143.22.132. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

092114-2
Wang et al.
Appl. Phys. Lett. 89, 092114 ͑2006͒
FIG. 2. Cross-sectional bright-field TEM image of InN film observed along
¯
¯
the ͓1120͔ direction with g=͑1010͒.
grains with various diameters dependent on the growth tem-
perature. When the growth is carried out between 350 and
450 °C, the films take the form of small and close packed
InN grains with diameters less than 250 nm because the mo-
1
2
bility of the source atoms is too low to form larger islands.
At an elevated temperature ͑Ͼ475 °C͒, InN can grow into
larger islands and the average surface lateral grain diameter
varies from about 200 to 1000 nm by adjusting V/III ratio
during the growth.
It is noted that the AFM measurement can only reveal
the surface morphology of InN and the grain size might
change significantly during epigrowth. In order to check the
evolution of the grain size, a bright-field cross-section micro-
graph of InN film was measured with the operative
¯
g=͑1010͒, as shown in Fig. 2. It is found that InN film has
nearly straight grain boundaries which arise near the
InN/GaN interface and extend up to the top of the InN layer
͑
a͒. It is also observed that these boundaries are connected
with the “grooves between islands” emerging on the InN
surface. It suggests that the average InN grain size starting
from the coalescence can be quite well characterized by “lat-
eral surface grain size” determined by AFM. In fact, such
kind of microstructures has been reported as InN columnar
FIG. 3. ͑a͒ FWHM of x-ray rocking curves measured in a skew symmetric
geometry as a function of the lattice plane inclination angle with respect to
9
,13
structure when the growth was conducted below 500 °C.
͑
0002͒. The dashed lines are fitting curves. ͑b͒ FWHM of ͑0002͒ rocking
To evaluate the twist of InN films by the x-ray diffrac-
tion, we adopt the method presented in Ref. 14. The mea-
¯
curves ͑filled squares͒ and the extrapolated FWHM of ͑1010͒ rocking
curves ͑filled circles͒ as a function of average grain size.
¯
sured FWHMs of symmetric ͑0002͒ and skew ͑101l͒ reflec-
tion rocking curves of InN films and the as-grown GaN
template are plotted in Fig. 3͑a͒ in the logarithmic scale as a
function of the inclination angle ͑␹͒ between ͑0002͒ and
¯
tonically when the grain size increases and finally reaches to
nearly a constant.
It is known that the density of TDs with edge and screw
components in principle can correspond to the twist and tilt
in the layers, respectively. The dependence of twist on the
grain size, as shown in Fig. 3͑b͒, is in agreement with the
study on the mosaic structure of GaN grown on sapphire
where the in-plane misorientation between the grains is com-
͑
1
101l͒ for the InN films with average grain diameters of 35,
20, 250, 265, 500, and 1000 nm, respectively. The dashed
lines in the figure are fitting curves using the formula pro-
posed in Ref. 15. From these fitting curves, the FWHM of
¯
the ͑1010͒ rocking curves can be extrapolated to ␹=90°. The
16
pensated by edge TDs ͑a type͒ located at grain boundaries.
¯
FWHMs of ͑0002͒ and ͑1010͒ rocking curves reflect the
In InN/GaN system there is a large in-plane lattice mis-
match, which is mostly accommodated by a network of
“geometrical misfit dislocations ͑MDs͒” on the InN/GaN
mean tilt and twist, respectively. From these curves, it is
evident that the FWHM monotonically increases with the
inclination angle for all InN films, but the evolution strongly
1
7
interface. Since the growth mode in such a large lattice-
mismatched system involves the independent formation of
3D InN nuclei, each nucleus should keep a special registry
with the GaN layer by the periodical MDs network defined
depends on the average grain diameter d . For InN films with
0
small grain diameters ͑d ഛ250 nm͒, the FWHM of ͑0002͒
0
rocking curve remains below 5 arc min which is comparable
to that of the GaN template, whereas the FWHM increases
dramatically with the plane inclination ␹. This indicates a
significant larger twist in these films. In comparison with the
small grain films, a larger tilt but a less twist is found in the
films with larger grains.
¯
in ͕1100͖ planes. The occurrence of a-type TDs can be at-
tributed to the discontinuity of the MD networks between the
adjacent islands during coalescence. Thus, it is reasonable
that the mosaic twist associated with the a-type TD density is
predominantly influenced by the average grain diameter as
they approach the coalescence, i.e., decreases with increasing
grain size.
In order to clarify the correlation between mosaic struc-
¯
ture and grain size, the FWHMs of ͑0002͒ and ͑1010͒ rock-
ing curves are plotted against the average grain diameter in
Fig. 3͑b͒. It is found that the tilt increases substantially as the
grain size exceeds 250 nm while the twist decreases mono-
To shed a light on the nature of the mosaic tilt in InN
grown on GaN, two thin InN films with a nominal thickness
of about 40 nm were grown at 400 and 500 °C, respectively.
Downloaded 08 Mar 2013 to 128.143.22.132. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions

092114-3
Wang et al.
Appl. Phys. Lett. 89, 092114 ͑2006͒
In conclusion, we have studied the structural property of
InN films with various grain sizes grown on Ga-face GaN
templates with a stepped surface. It is found that the twist
decreases with increasing grain size and approaches to a con-
stant level, while the mosaic tilt increases dramatically as the
grain size is large enough and exceeds 250 nm. The critical
grain size is dependent on the average width of step terraces
on GaN surface and is attributed to the generation of mixed
dislocations, showing an important mechanism for the defect
generation in the InN/GaN system with large out-of-plane
lattice mismatch.
2
FIG. 4. 5ϫ5 ␮m AFM images of the surfaces of the as-grown GaN layer
͑
4
a͒ and the thin InN films with a nominal thickness of about 40 nm grown at
00 °C ͑b͒, and 500 °C ͑c͒, respectively.
The authors acknowledge the support from the National
Natural Science Foundation of China ͑Grant Nos. 60506001
and 60576003͒.
Their surface AFM images are shown in Figs. 4͑b͒ and 4͑b͒.
The InN film grown at 400 °C exhibits a special morphology
characteristic, i.e., small InN nuclei are aligned in lines on
GaN surface ͓see Fig. 4͑b͔͒. Comparing this surface mor-
phology with that of the GaN layer shown in Fig. 4͑a͒, we
tentatively attribute such alignment of islands to the prefer-
ential nucleation of InN along the surface steps on the GaN
surface due to the Ehrlich-Schwoebel barrier effect which is
against the diffusion of adatoms towards the descending
1
J. Wu, W. Walukiewicz, K. M. Yu, J. W. Ager III, Hai Lu, William J.
Schaff, Yoshiki Saiko, and Yasushi Nanishi, Appl. Phys. Lett. 80, 3967
͑2002͒.
V. Y. Davydov, A. A. Klochikhin, V. V. Emtsev, D. A. Kurdyukov, S. V.
2
Ivanov, V. A. Vekshin, F. Bechstedt, J. Furthmuller, J. Aderhold, J. Graul,
A. V. Mudryi, H. Harima, A. Hashimoto, A. Yamamoto, and E. E. Haller,
Phys. Status Solidi B 234, 787 ͑2002͒.
1
8,19
steps.
For the films grown at 500 °C, on the other hand,
3
S. K. O’Leary, B. E. Foutz, M. S. Shur, U. V. Bhapkar, and L. F. Eastman,
large and separated InN islands are observed with the grain
size of about 750 nm which is close to that of thicker films
grown under the same condition, as shown in Fig. 1͑d͒. We
believe that the existence of atomic steps on the GaN surface
can give rise to the mosaic tilt in the InN films when the
average grain size is large enough, for example, exceeds
J. Appl. Phys. 83, 826 ͑1998͒.
B. E. Foutz, S. K. O’Leary, M. S. Shur, and L. F. Eastman, J. Appl. Phys.
4
8
5, 7727 ͑1999͒.
5
6
7
R. S. Qhalid Fareed, R. Jain, R. Gaska, M. S. Shur, J. Wu, W.
Walukiewicz, and M. Asif Khan, Appl. Phys. Lett. 84, 1892 ͑2004͒.
P. Singh, P. Ruterana, M. Morales, F. Goubilleau, M. Wojdak, J. F. Carlin,
M. Ilegems, and D. Chateigner, Superlattices Microstruct. 36, 497 ͑2004͒.
Song-Bek Che, Wataru Terashima, Takayuki Ohkubo, Masayoshi
Yoshitani, Maoki Hashimoto, Kouichirou Akasaka, Yoshihiro Ishitani, and
Akihiko Yoshikawa, Phys. Status Solidi C 1, 2559 ͑2005͒.
Y. F. Ng, Y. G. Cao, M. H. Xie, X. L. Wang, and S. Y. Tong, Appl. Phys.
Lett. 81, 3960 ͑2002͒.
E. Dimakis, E. Iliopoulos, K. Tsagaraki, Th. Kehagias, Ph. Komninou, and
A. Georgakilas, J. Appl. Phys. 97, 113520 ͑2005͒.
X. J. Ning, F. R. Chien, P. Pirouz, J. W. Yang, and M. Asif Khan, J. Mater.
Res. 11, 580 ͑1996͒.
250 nm, which is very close to the average width
͑
200–250 nm͒ of step terraces on GaN surface determined
by AFM. That is, when the grain size is small enough to be
accommodated on one step terrace, no remarkable tilt will
occur in the film since these InN grains are well-oriented
along the c axis. While islands grow larger and exceed the
width of the surface step terrace on the GaN surface, they
will tilt with respect to the c axis.
8
9
0
1
11
B. Heying, X. H. Wu, S. Keller, Y. Li, D. Li, D. Kapolnek, B. P. Keller,
S. P. DenBaars, and J. S. Speck, Appl. Phys. Lett. 68, 643 ͑1996͒.
E. Dimakis, E. Iliopoulos, K. Tsagaraki, and A. Georgakilas, Appl. Phys.
Lett. 86, 133104 ͑2005͒.
There has been a general model accounting for crystal-
lographic tilt in relation with the lattice elastic distortion ex-
1
2
2
0
erted by the substrate surface steps ͑so-called Nagai’s tilt͒.
However, this model is valid only for the systems with small
13
A. Yamamoto, Y. Murakami, K. Koide, M. Adachi, and A. Hashimoto,
2
1
mismatch. In our case, the heterosystem involves a large
out-of-plane lattice mismatch of about 10% between InN and
GaN, therefore the role of MDs and the island growth mode
becomes dominant in the formation of the tilt. Here, we pro-
pose that the tilt observed in our films is a result of the
preferential occurrence of defects. The presence of steps on
the substrate surface modifies the uniform biaxial stress field
of the films near the steps. The change in stress distribution
favors the generation of mixed dislocations with a Burgers
Phys. Status Solidi B 228, 5 ͑2001͒.
H. Heinke, I. Kirchner, S. Einfeldt, and D. Hommel, Appl. Phys. Lett. 77,
2145 ͑2000͒.
J. C. Zhang, D. G. Zhao, J. F. Wang, Y. T. Wang, J. Chen, J. P. Liu, and H.
Yang, J. Cryst. Growth 268, 24 ͑2004͒.
R. Chierchia, T. Böttcher, H. Heinke, S. Einfeldt, S. Figge, and D.
Hommel, J. Appl. Phys. 93, 8918 ͑2003͒.
1
1
1
1
4
5
6
7
Th. Kehagias, A. Delimitis, Ph. Komninou, E. Iliopoulos, E. Dimakis,
A. Georgakilas, and G. Nouet, Appl. Phys. Lett. 86, 151905 ͑2005͒.
R. L. Schwoebel and E. J. Shipsey, J. Appl. Phys. 37, 3682 ͑1966͒.
G. Ehrlich and F. G. Hudda, J. Chem. Phys. 44, 1039 ͑1966͒.
H. Nagai, J. Appl. Phys. 45, 3789 ͑1974͒.
1
1
2
8
9
0
2
2
vector of c+a by gliding. In fact, the dislocation generation
and development mechanisms in the lattice-mismatched sys-
tem depend upon the specific growth conditions. However,
the mechanism presented in this letter should be one of the
important mechanisms for the defect creation.
21
A. Pesek, K. Hingerl, F. Riesz, and K. Lischka, Semicond. Sci. Technol.
6, 739 ͑1991͒.
2
2
S. Srinivasan, L. Geng, R. Liu, F. A. Ponce, Y. Narukawa, and S. Tanaka,
Appl. Phys. Lett. 83, 5187 ͑2003͒.
Downloaded 08 Mar 2013 to 128.143.22.132. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissions