Abstract In this paper, we have grown GaN films by metalorganic chemical vapor-phase epitaxy (MOVPE) on GaN ceramics obtained from nanocrystalline powder of different initial grain sizes. The samples have been investigated by X-ray diffraction (XRD) and photoluminescence (PL). XRD reveals that the MOVPE GaN films are of single-phase wurtzite structure. Also, it has been observed that the PL spectrum is different for the GaN films compared to that for the GaN powder and also depends on the initial grain sizes. 

A Podhorodecki

J Misiewicz

J Serafinczuk

M Nyk

R Korbutowicz

R Kudrawiec

R Paszkiewicz

W Strek

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ARTICLE IN PRESS0022-0248/$ - se
doi:10.1016/j.jc
Correspond
E-mail addr
(A. PodhorodecJournal of Crystal Growth 310 (2008) 940–943
www.elsevier.com/locate/jcrysgroGaN ceramics obtained by fusing of nanocrystalline GaN powder
at high pressures and temperatures as substrate for growth
of GaN epilayers
A. Podhorodeckia,, M. Nyka, R. Kudrawieca, J. Misiewicza, R. Paszkiewiczb,
R. Korbutowiczb, J. Serafinczukb, W. Strekc
aInstitute of Physics, Wroclaw University of Technology, Wybrzeże Wyspiańskiego 27, 50-370 Wroclaw, Poland
bFaculty of Microsystem Electronics and Photonics, Wroclaw University of Technology, Janiszewskiego 11/17, 50-372 Wroc!aw, Poland
cInstitute of Low Temperature and Structure Research, Polish Academy of Sciences, P.O. Box 1410, 50-950 Wroclaw 2, Poland
Available online 19 November 2007Abstract
In this paper, we have grown GaN films by metalorganic chemical vapor-phase epitaxy (MOVPE) on GaN ceramics obtained from
nanocrystalline powder of different initial grain sizes. The samples have been investigated by X-ray diffraction (XRD) and
photoluminescence (PL). XRD reveals that the MOVPE GaN films are of single-phase wurtzite structure. Also, it has been observed that
the PL spectrum is different for the GaN films compared to that for the GaN powder and also depends on the initial grain sizes.
r 2007 Elsevier B.V. All rights reserved.
PACS: 78.55.m
Keywords: A1. Substrates; A1. X-ray diffraction; A3. Metalorganic vapor-phase epitaxy; B1. Nanomaterials; B1. Nitrides1. Introduction
GaN and related III–V nitride materials have been
investigated for various applications in electronic and
optoelectronic devices, such as blue and green light-
emitting diodes, laser-diodes, and high-power and high-
temperature electronic devices [1,2]. Although the growth
of group III-nitride materials has progressed rapidly, it is
still difficult to obtain simultaneously cost-effective and
high-quality films. In order to obtain high-quality GaN
films an epitaxy, a growth on a substrate with the same or
very close lattice constant as GaN, seems to be necessary.
However, the lack of such a substrate limits this approach
to homoepitaxy, which needs bulk-like GaN substrate.
Usually, GaN layers are grown on sapphire, SiC or other
alternative substrates whose lattice constant is far from the
lattice constant of GaN. In the case of such a hetero-e front matter r 2007 Elsevier B.V. All rights reserved.
rysgro.2007.11.063
ing author. Tel.: +4871 320 23 58; fax: +4871 328 36 96.
ess: artur.p.podhorodecki@pwr.wroc.pl
ki).epitaxy, the optical quality of these layers is usually worse
due to the lattice mismatch, which generates structural
defects introducing numerous energy levels in the gap [3].
However, for some applications, e.g. luminophors, such
a quality of GaN layers is sufficient, but still the cost of the
substrate seems to be too high in comparison to the
obtained poor quality of the GaN layers.
Thus, in this work, we present the structural and optical
properties of metalorganic chemical vapor-phase epitaxy
(MOVPE) grown GaN films deposited onto alternative
GaN ceramic substrates obtained from the GaN nano-
crystalline powder.2. Experimental procedure
In the first step, GaN nanosized powders were synthe-
sized by the combustion method with some modifications.
The preparation procedure was divided into two main
steps—the first one is the hydrothermal processing using a
microwave reactor, and the second is the synthesizing of
ARTICLE IN PRESS
A. Podhorodecki et al. / Journal of Crystal Growth 310 (2008) 940–943 941the GaN nanocrystalline powder by using a horizontal
quartz reactor with the NH3 flow and heating the material
to the required temperatures of 850 and 1050 1C. More
details about the powder preparation can be found in our
previous works [4,5].
During the conduction of the second step, the powder
samples are fused into continuous ceramic layers. The
fabrication of the GaN nanoceramics by means of the low-
temperature–high-pressure (LTHP) technique was per-
formed under 6GPa at 800 1C for 1min. In this manner
two samples, characterized by the different sizes of the
grains, were fabricated. The samples of GaN nanoceramics
were black in color, with an average density of 5.5 g/cm3
and with surface area measured with the multipoint
Brunauer–Emmett–Teller technique JBETj10.5m2/g.
More details about substrate properties as well as about
similar substrates obtained at low temperature and at low
pressure could be found elsewhere [6,7].
In the third step, GaN layers were grown on GaN
ceramics in a horizontal flow MOVPE system. The low-
frequency (40 kHz) inductive heating method was used to
raise the temperature of the graphite susceptor up to
1020 1C. Trimethylgallium (TMGa) and ammonia (NH3)
were used with H2 carrier gas. Before the growth process,
the substrate was decreased in organic solvents and etched
in a solution of HF:H2O (1:10). The substrate was annealed
in the growth chamber for 10min at 600 1C in H2/NH3
(1:1) ambient. The NH3 was introduced at 450 1C during
heating to prevent the decomposition of the GaN
nanocrystalline powder substrate [8,9]. Then, the low-
temperature GaN layers were grown during the tempera-30
(1
0
2
)
(1
1
2
)
(1
0
3
)
(1
1
0
)
(1
0
1
)(0
0
2
)
(1
0
0
)
30 40 50 60 70
ceramic (Φ= 4 nm)
p=6GPa
powder (Φ= 11 nm)
TN=850°C
ceramic/MOVPE
 X
R
D
2Θ (deg.)
Fig. 1. X-ray diffraction patterns of (a) GaN powder obtained at 850 1C, GaN
powder obtained at 1050 1C, GaN ceramic, GaN film grown by MOVPE on Gture ramping from 600 to 850 1C for 5min with 9300 V/III
molar ratio. Next, the temperature was gradually increased
to 1020 1C for 10min. During the second temperature
ramp, the low-temperature GaN buffer layers were
annealed in NH3/H2 ambient (1:1). The high-temperature
GaN layers were grown with 2200 V/III molar ratio at
1020 1C for 60min. These experimental conditions were
selected as the optimal for GaN layers’ growth on
monocrystalline GaN substrates at our epitaxial system.
As a result, the nominally 3-mm-thick MOVPE layers were
obtained on nanocrystalline GaN powder substrates.
Overall phase compositions of the nanopowders were
determined by X-ray powder diffraction (XRD) with a
Siemens D5000 diffractometer and CuKa1 radiation,
l=0.15406 nm. The structural investigations of the GaN
thick-film and substrate have been carried out using a
Philips MRD–HRD diffractometer and the same CuKa1
radiation source.
For photoluminescence (PL), a 300 nm line of an argon
laser has been used as a pump beam with 10 kW/cm2
excitation power. An Ocean Optics HR4000 spectrometer
was applied to collect the emission signal.
3. Results and discussion
Fig. 1 shows the XRD patterns of GaN powders
obtained at two nitridation temperatures, 850 and
1050 1C. In both cases, XRD patterns show the diffraction
lines which can be attributed to the formation of a
hexagonal GaN with the wurtzite-type structure (JCPDS
file no. 02–1078). By means of the Scherrer formula we40 50 60 70
powder (Φ= 31 nm)
ceramic (Φ= 6nm)
p=6GPa
2Θ (deg.)
ceramic/MOVPE
TN=1050°C
ceramic, a GaN film grown by MOVPE on GaN ceramic and (b) GaN
aN ceramic.
ARTICLE IN PRESS
A. Podhorodecki et al. / Journal of Crystal Growth 310 (2008) 940–943942have determined that the average size of GaN grains
is 11 and 30 nm for grains obtained at 850 and 1050 1C,
respectively. In the case of the GaN powder after
the compression process, the XRD spectrum changes
significantly. Grain sizes, which are obtained using the
Scherrer formula, are comparable for both samples and
equal 571 nm. The observed size reduction of the
GaN grain after the compression process can be re-
lated to the strong deformation of GaN grains and
amorphization of the surface layer of the grains or other
phenomena.
Finally, we treat the compressed wafer as a substrate for
the MOVPE-deposited GaN layer. XRD spectra of
MOVPE GaN films are completely different from the
XRD spectra of the powder samples and the spectra of the
ceramic substrates. It has been observed that XRD peaks
related to some directions are much stronger than the
respective peaks observed for GaN powder. It is especially
visible for the MOVPE GaN film deposited on the
substrate obtained from smaller grains, where two peaks
at 34.631 and 36.991 indexed as (0 0 2) and (1 0 1),
respectively, are dominating. This indicates that the GaN
film grows better in selected directions in this case. The
other phenomenon is a narrowing of the peaks. This
behavior is related to an increase in the GaN grain size. We
suppose that during MOVPE deposition nanosize islands
appear, but with the increase in the deposition time the
size of the islands increases and they can combine to
form bigger islands. Therefore, the broadening of XRD
peaks decreases. For example, we have determined that
the average size of GaN grains for an MOVPE film is
200 nm.300 400 500 600 700 800 900 1000
~30 nm
GaN powder
GaN ceramic
TN = 850°C
GaN ceramic/GaN MOVPE
p
h
o
to
 l
u
m
in
e
s
c
e
n
c
e
 
lEXC = 300 nm
PEXC~10kW/cm
2
wavelength (nm)
Fig. 2. PL results obtained for (a) GaN powder obtained at 850 1C, GaN cera
obtained at 1050 1C, GaN ceramic, GaN film grown by MOVPE on GaN cerFig. 2 shows PL spectra measured at room temperature
for (i) GaN powder, (ii) GaN ceramic, (iii) MOVPE GaN
film, for different sizes of the initial GaN grains. PL
spectrum of GaN powder obtained at 850 1C as well as of
the ceramic obtained from this powder, exhibits a broad
yellow emission typical of poor-quality GaN layers. In
addition, no band gap-related emission is observed. This
phenomenon is associated with the very small size of GaN
grains. The surface of the GaN grain is optically active and
is probably one of the origins of the yellow emission.
However, in the case of grains obtained at 1050 1C, the
band gap-related emission is clearly observed beside the
yellow PL band. Since the band gap emission is a process
related to the crystal volume, a decrease in the crystal size
changes the surface-to-volume ratio and hence different
optical properties can be observed in different proportions.
This was already discussed in detail in our previous papers
[2,10]. Moreover, as it can be seen in Fig. 2(b), compressing
GaN grains into ceramic changes their optical properties
significantly and only the yellow emission is observed.
PL spectra for the MOVPE GaN films are different from
the PL spectrum of a GaN powder and depends on the
initial GaN grain sizes. In both cases a strong emission
band at 362 nm wavelength, which corresponds to the
band gap of a bulk GaN, has been observed. However, the
broadening of this emission in both cases is different and
equal to 30 and 15 nm for initial grains obtained at 850
and 1050 1C, respectively. Additionally, the sample ob-
tained from bigger grains exhibits additional weak emission
bands related to recombination via defect states, while in
the case of the sample obtained from smaller grains this
kind of emission does not appear at all.300 400 500 600 700 800 900 1000
GaN ceramic
TN = 1050°C
~15 nm
GaN powder
GaN ceramic/GaN MOVPE
wavelength (nm)
mic, GaN film grown by MOVPE on GaN ceramic and (b) GaN powder
amic.
ARTICLE IN PRESS
A. Podhorodecki et al. / Journal of Crystal Growth 310 (2008) 940–943 9434. Conclusion
Summarizing the above findings, we concluded that the
optical properties of MOVPE GaN films are quite satisfac-
tory for some applications, like e.g. luminophors. The
epitaxial growth of GaN films on a GaN-substrate wafer
fabricated from GaN powder seems to be a very promising
approach to obtain cheap GaN films with strong UV
emission. It is worth noticing that the process of the film
fabrication can be optimized in various ways. We can modify
the powder, as well as the parameters of epitaxial growth.
Acknowledgment
The authors would like to thank Prof. W. Łojkowski of
the Institute of High Pressure, Polish Academy of Science
for the help with the GaN ceramic preparation.References
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[6] W. Zalewski, M. Nyk, W. Strek, Appl. Phys. Lett. 90 (2007) 042511.
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GaN ceramics obtained by fusing of nanocrystalline GaN powder at high pressures and temperatures as substrate for growth of GaN epilayers

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GaN ceramics obtained by fusing of nanocrystalline GaN powder at high pressures and temperatures as substrate for growth of GaN epilayers

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