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Characterization of the Crystalline Quality on GaN on Sapphire
and Ternary Alloys
Hiroshi Amano, Tetsuya Takeuchi, Shigeo Yamaguchi, Christian Wetzel, and Isamu Akasaki
Department of Electrical and Electronic Engineering, Meijo University, Nagoya, Japan 468- 8502
SUMMARY
The growth of GaN on sapphire by OMVPE using a
low-temperature deposited buffer layer was characterized
by in situ TEM observation. The crystalline quality, espe-
cially tilting and twisting of the mosaic GaN crystal, was
characterized by conventional X-ray diffraction together
with grazing-incidence X-ray diffraction. Strain and relaxa-
tion of the AlGaN and GaInN on GaN were also charac-
terized by asymmetrical X-ray diffraction. © 1998 Scripta
Technica, Electron Comm Jpn Pt 2, 81(10): 48�54, 1998
Key words: Low temperature deposited buffer
layer; mosaic structure; twist and tilt; strain and relaxation.
1. Introduction
Group III nitride-based blue and green light emitting
diodes (LED) have already been commercialized, and laser
diodes (LD) in the near ultra-violet (UV) and deep purple
region have been fabricated. Moreover, a high-power field-
effect transistor that operates in the GHz region, and also a
solar-blind high-sensitivity UV detector have been con-
structed. All of these novel devices are fabricated on highly
lattice-mismatched substrates, such as sapphire or 6H-SiC.
This is quite extraordinary if we compare it with other
systems, such as GaAs-based devices or InP-based devices,
in which lattice matching is believed to be the most impor-
tant requirement.
In this paper, we will report the growth modes, espe-
cially the initial growth stage, of GaN on sapphire at the
atomic scale, and the crystalline properties of GaN on
sapphire. Ternary alloys such as AlGaN and GaInN are also
important for the fabrication of novel devices. We also
report on the crystalline quality of ternary alloys on GaN.
2. Growth Mode of GaN on Sapphire at
Atomic Scale
When c-axis�oriented GaN is grown on (0001) sap-
phire, the in-plane lattice mismatch can be as large as 16.1%
at room temperature. Therefore, it is quite difficult to grow
high-quality GaN directly on a sapphire substrate. In the
following discussion, we assume fairly controlled condi-
tions for the growth of nitrides by organometallic vapor
phase epitaxy (OMVPE). Figure 1(a) shows schematically
the growth process of GaN directly grown on sapphire
substrate. In the initial stage, three-dimensional GaN is-
lands are partially formed. The origin of GaN island forma-
tion is either an atomic step side of sapphire or a dislocation
site. In order to relieve the large stress caused by the
difference of the in-plane lattice constants between GaN
and sapphire, the islands might be grown with slight tilting
and/or twisting compared with the crystal orientation of the
sapphire. In the next stage, due to a difference in the growth
rate of GaN on sapphire and that on GaN, or, in other words,
due to the difference in the consumption rate of the source
gases (in this case Ga-alkyls such as TMGa or TEGa),
selective growth occurs. The consumption rate of Ga-alkyls
is expected to be much higher on a GaN surface. Therefore,
CCC8756-663X/98/100048-07
© 1998 Scripta Technica
Electronics and Communications in Japan, Part 2, Vol. 81, No. 10, 1998Translated from Denshi Joho Tsushin Gakkai Ronbunshi, Vol. J81-C-II, No. 1, January 1998, pp. 65�71
48
the three-dimensional GaN islands with tilting and twisting
become larger and larger until they coalesce. Large crystal
defects are formed at the interfaces between islands. Such
GaN shows strong n-type conductivity and low electron
mobility, and p-type GaN had never previously been
achieved.
In order to improve the crystalline quality of GaN on
sapphire, first, a single crystalline AlN layer was interme-
diately grown between the GaN and sapphire substrate by
molecular beam epitaxy [1]. However, the residual donor
concentration was still in excess of 1019 cm�3, and the Hall
mobility was less than 50 cm2/Vs. By using a low-tempera-
Fig. 1. (a) Schematic viewgraph of the OMVPE growth of GaN directly on sapphire. (b) Schematic viewgraph of the
crystallization of a low-temperature AlN buffer layer deposited by OMVPE on sapphire, observed by TEM with high-
temperature sample holder. [8] (c) Schematic viewgraph of the initial growth of GaN on crystallized AlN buffer layer. [9]
49
ture deposited AlN [2] or GaN [3] buffer layer before the
growth of GaN at high temperature, the situation was
changed dramatically, and high-quality GaN with a low
residual donor concentration, a high electron mobility, a
narrow X-ray rocking curve, free exciton emission, and
stimulated emission at room temperature, among other
features, have been obtained. This finding led to the first
successful p-type GaN [4, 5], pn-junction LED [4], and
blue/UV-LED [6], and to observation of the quantum size
effect [7].
Figure 1(b) schematically shows the crystallization
process while ramping up the temperature of the low-tem-
perature deposited AlN buffer layer (LT-AlN) on sapphire,
and Fig. 1(c) shows schematically the crystal growth proc-
ess of the high-temperature grown GaN layer (HT-GaN) on
crystallized LT-AlN as observed by in situ transmission
electron microscopy (TEM) [8] and high resolution scan-
ning electron microscopy (SEM) [9], respectively. In most
cases, the thickness of the LT-AlN is about 20 to 100 nm
and the deposition temperature is about 350 to 600 °C. As-
deposited LT-AlN consists of very fine crystallites. During
ramping up from the deposition temperature to the epitaxial
temperature, about 950 to 1100 °C, the fine crystallites
merge to form large grains. This atomic rearrangement
occurs from the sapphire side and proceeds to the surface
side. When the substrate temperature reaches the epitaxial
temperature, truncated hexagonal mesa islands surrounded
by (0001) and {101_
1} facets are formed, and the whole
LT-AlN layer is then composed of such islands, forming a
brick-like structure. Compared to GaN islands grown di-
rectly on sapphire, the process of lattice mismatch relaxa-
tion should be different. In the case of HT-GaN growth on
LT-AlN, selective growth like that of GaN directly grown
on sapphire may not occur, but orientational selectivity
occurs. Largely misoriented crystallites are surrounded by
dislocations and embedded. Finally, the crystal orientation
of the whole HT-GaN layer becomes aligned.
It is well known that an HT-GaN layer grown on
sapphire using LT-buffer layer has a mosaic structure com-
posed of columns with diameters of 0.1 mm to a few mm.
Next, methods of characterizing the crystalline quality of
these layers using X-ray diffraction, and the relationship
between these properties with other properties such as
optical properties and electrical properties, are discussed.
Each column itself contains few threading dislocations.
Threading dislocations are formed around each column
because the columns are slightly misoriented with respect
to each other. The misorientation is mainly divided into two
parts, tilting and twisting. Tilting is the misorientation of
the c-axis of the columns, while twisting is the in-plane
rotation of the columns. X-ray diffraction itself is a method
of measuring the diffraction profile in the reciprocal lattice
space. By using X-ray diffraction, not only the lattice
spacing, but also the tilting and twisting of the mosaic
structure can be characterized. Figure 2(a) shows schemati-
cally the X-ray diffraction profile from GaN having a
mosaic structure around the (0002) diffraction spot in the
Fig. 2. Schematic viewgraph of the X-ray diffraction
mapping in the three-dimensional reciprocal lattice space
from the GaN having mosaic spread; (a) shows X-ray
diffraction mapping around the (0002) diffraction;
(b) shows that around (101_0) diffraction. a Shows
diffraction profile taken by w-mode scan. b Shows
diffraction profile taken by 2q/w-mode scan
with analyzer crystal.
50
three-dimensional reciprocal lattice map (RLM). In the
ordinal X-ray diffraction measurement, the w-mode and
2q/w-mode are traces of �a� and �b�, respectively. If the
dispersion of the incident X-rays is negligibly small and
thickness of GaN is thick enough, �a� and �b� reflect the
tilt profile and fluctuations in the lattice constant, respec-
tively [10, 11]. In many cases, the tilt profile is wider than
the fluctuation in the lattice constant, which results in a
disk-like profile of X-ray diffraction in RLM. Table 1 shows
one example of the Dwc and Dc of GaN grown on sapphire
by OMPVE using an LT-AlN buffer layer. The lattice con-
stant c can be precisely measured by using such symmetri-
cal diffraction, while in a plane lattice, constant a can be
measured in combination with symmetrical diffraction and
asymmetrical diffraction such as (112_
4), (202_
4), (101_5). In
order to characterize twisting of the layer, we cannot use
such asymmetrical diffraction. We need to rock the sample
perpendicular to the rocking direction of symmetrical dif-
fraction measurement. As shown in Fig. 2(b), (1010) dif-
fraction is promising for measurement of both in plane
lattice constant a and twisting. In Fig. 2(b), �a� shows the
w-mode rocking curve profile, which reflects twisting of
the film, while �b� shows the 2q/w-mode profile which
reflects fluctuations in the lattice constant a. Experimental
results on Dwa and Da are given in Table 1.
3. Hetereoepitaxy of AlGaN and GaInN
Ternary Alloys on GaN
In order to fabricate high-performance LEDs or LDs,
heteroepitaxy of ternary alloys is essential. The binary
compounds AlN, GaN, and InN are lattice-mismatched
with each other. Therefore, understanding the crystalline
quality of AlGaN and GaInN, especially the strain and
relaxation caused by lattice mismatch, is one of the most
critical issues.
The in-plane lattice mismatch between AIN and GaN
is at most 2.4%, while that of InN and GaN can be as large
as 11%. In this paper, we characterize the crystalline quality
of AlGaN and GaInN on GaN, with AlN and InN molar
fraction of less than 0.3 for both ternary alloys.
In order to characterize the macroscopic strain fea-
tures of ternary alloys or the relaxation of the dislocations,
X-ray diffraction is very useful. By measuring the X-ray
diffraction profile in the RLM around the asymmetrical
diffraction spot, for example, (112_4) or (202
_4) in the recip-
rocal lattice map, we can directly characterize the strain and
relaxation situation of the ternary alloys on binary GaN.
Figure 4 schematically shows the RLM around (0004),
(202_4) and (101
_0) diffraction spots of the GaInN and Al-
GaN grown on GaN. The abscissa is proportional to the
inverse of the lattice constant a, while the ordinate is
proportional to the inverse of the lattice constant c. In the
figure, both the fully relaxed case and the fully strained case
are shown. If the abscissas of the peak positions of the
asymmetrical diffraction of the ternary alloys are just
aligned to that of GaN, the in-plane lattice constants of the
ternary alloys are the same as that of GaN. This is the fully
strained case, sometimes called coherent growth. But if the
peak positions of the asymmetrical diffraction of the ternary
alloys are almost on the same line toward (0000) as for GaN,
the ternary alloys should be fully relaxed by dislocations,
and so on. That is sometimes called the fully relaxed, or free
standing case. It is very clear from this figure that to
determine the alloy composition of the strained ternary
alloys, special attention is necessary. When the ternary
alloys are fully relaxed, we can determine the alloy compo-
sition by precisely measuring the 2q/w-mode symmetrical
diffraction profile and assuming Vegard�s law. Otherwise,
if ternary alloys are grown coherently, we measure the
2q/w-mode symmetrical diffraction and have to calculate
Table 1. FWHMs of X-ray diffraction profiles of GaN
grown by OMVPE obtained by 2q/w-mode
and w-mode scan
[arc sec.]
(0002) (101_
0)d [mm]
Dc Dwc Da Dwa
23 406 55 599 1.81
Fig. 3. Configuration for measuring the (101_0) X-ray
diffraction from c-plane GaN having small offset.
51
Fig. 4. Two-dimensional reciprocal space mapping around (202_4) diffraction of AlGaN and GaInN grown on GaN. Both
those of coherently grown AlGaN and GaInN and perfectly relaxed AlGaN and GaInN are represented.
Fig. 5. (a) AlGaN and (b) GaInN grown coherently on GaN. Actual critical layer thickness above which ternary alloys
show lattice relaxation is thought to be much thicker than these results.
52
by assuming Hooke�s law [12]. If the layers are strained but
partially relaxed, we need to measure both a and c.
Figure 5 shows the experimentally determined thick-
ness of AlGaN and GaInN, which showed coherent growth
on GaN. Theoretical results of Matthews and Blakeslee [13]
and of Fisher and others [14], both of whose approaches
assume a balance between the strain energy and energy of
generation of misfit dislocation, are shown for comparison.
As shown in the figure, the range of thickness of ternary
alloys in which the ternary alloys show coherent growth is
much wider than that of the misfit dislocations estimated
from the above-mentioned theory, and the critical thickness
for lattice relaxation is expected to be much thicker, as
shown by the arrow.
4. Conclusions
The growth modes of GaN on sapphire, the crystal-
line quality of GaN on sapphire, and the crystalline quality
of the ternary alloys AlGaN and GaInN on GaN are sur-
veyed. The density of threading dislocations in the GaN
layer grown on sapphire by OMVPE using LT-buffer layer
ranges from 108 to 1011 cm�2. Recently, by the using the
lateral epitaxial overgrowth technique, commonly used in
silicon-on-insulator growth [15], high-quality GaN with a
dislocation density of 106 to 107 cm�2 GaN has been
achieved [16]. Lateral epitaxial growth reduces threading
of dislocations. GaN with a much lower dislocation density
or even dislocation-free GaN will surely improve the per-
formance of LEDs, LDs, or FETS. They are also important
for identification of the intrinsic properties of nitrides.
Acknowledgments. This work was partly sup-
ported by the Ministry of Education, Science, Sports and
Culture of Japan (contract nos. 07505012, 0875083,
09450133, the High-Tech Research Center Project, and the
Japan Society for Promotion of Science�s �Research for the
Future� Program in the Area of Atomic Scale Surfaces and
Interface Dynamics (project Dynamical Process and Con-
trol of Buffer Layer at Interfaces in Highly Mismatched
Systems).
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53
AUTHORS (from left to right)
Hiroshi Amano (nonmember). Education: Nagoya University, B.S., 1983, D.Eng. Academic appointments: Nagoya
University, research associate, 1988; Meijo University, lecturer, 1992. Research interests: group III nitride semiconductors.
Membership: Applied Physics Society of Japan, IEE Japan, MRS, APS.
Tetsuya Takeuchi (nonmember). Education: Nagoya University, B.S., 1990; M.S., 1992; currently doctoral candidate.
Industry positions: Hewlett-Packard Research Laboratories, 1992. Research interests: group III-V semiconductor crystal
growth. Membership: Applied Physics Society.
Shigeo Yamaguchi (nonmember). Education: Kyoto University, B.S., 1991; D.Eng., 1997. Academic appointments:
Meijo University High-Tech Research Center, 1997. Research interests: group II-VI semiconductor crystal growth, exciton
optics, group III nitride semiconductors. Membership: Applied Physics Society.
Christian Wetzel (nonmember). Education: Technical University of Munich, Diplom-Physiker, 1988; Ph.D., 1993.
Academic appointments: University of California, Berkeley; Materials Science Division, Lawrence Berkeley National Labora-
tory; Meijo University High-Tech Research Center, 1997. Research interests: electronic band and defect structure of low-di-
mensional and wide-bandgap semiconductor systems; high-sensitivity cross-modulation, external perturbation by hydrostatic
pressure; dopants and light emission in group III nitrides.
Isamu Akasaki (member). Education: Kyoto University, B.S., 1952; Nagoya University, D.Eng., 1964. Industry positions:
Kobe Industries, Ltd. (now Fujitsu, Ltd.), researcher, 1952�1959; Matsushita Research Ltd. (head of Tokyo Basic Research
Laboratory No. 4, 1964�1974; head of Semiconductor Division, 1974�1981). Academic appointments: Nagoya University,
research associate, lecturer, associate professor (1959�1964), professor, 1981�1992, professor emeritus; Meijo University,
professor, 1992. Membership: Applied Physics Society of Japan, IEE Japan, APS, MRS, ECS.
54