High pressure direct synthesis of III-V nitrides

Article abstract of DOI:10.1016/S0921-4526(98)01300-3

The nitrides of group III metals: (AlN, GaN, InN) are very important materials due to their applications for short wavelength optoelectronics (light-emitting diodes and laser diodes). In this paper, the results of high-pressure direct synthesis of AlN, Ga

Full text of DOI:10.1016/S0921-4526(98)01300-3

Physica B 265 (1999) 1—5
High pressure direct synthesis of III—V nitrides
M. Boc´ kowski*
High Pressure Research Center, Soko!owska 29/37, 01-142 Warsaw, Poland
Abstract
The nitrides of group III metals: (AlN, GaN, InN) are very important materials due to their applications for short
wavelength optoelectronics (light-emitting diodes and laser diodes). In this paper, the results of high-pressure direct
synthesis of AlN, GaN and InN are presented. The conditions for the thermodynamical stability for AlN, GaN and InN
are discussed. The influence of the kinetic barrier on the synthesis of InN, GaN and AlN is considered using the results of
quantum mechanical calculations of the dissociative adsorption of N on the liquid metal surface. It is shown that the
ꢀ
kinetic barrier for InN synthesis is very high, preventing direct synthesis of this compound. This barrier is lower for GaN,
which allows to grow GaN crystals from the solution of atomic nitrogen in liquid gallium. The lowest barrier is for AlN,
which leads to the combustion reaction of liquid aluminum in nitrogen atmosphere, in the pressure range from 10 to
6
50 MPa. At higher pressure, the extinction of combustion synthesis takes place. ( 1999 Published by Elsevier Science
B.V. All rights reserved.
Keywords: AlN; GaN; InN; Combustion synthesis; Crystal growth
due to the excellent thermal conductivity of AlN,
high electrical resistivity, and low thermal expan-
sion coefficient, similar to that of silicon.
1
. Introduction
Semiconducting III—V nitrides such as indium
In this paper, we analyze the behavior of the
systems consisting of Al, Ga and In metals and
nitrogen under pressure up to 2 GPa, at high tem-
peratures, from the point of view of direct synthesis
of the corresponding nitrides.
nitride (InN), gallium nitride (GaN) and aluminum
nitride (AlN) are currently considered as the most
promising optoelectronic materials in the green,
blue and ultraviolet spectral region. This is due to
their thermal stability and large direct energy gap
[1]. The nitrides technology is developed to the
point that commercial blue light-emitting diodes
GaN/GaInN are commercially available [2]. On
the other hand, from few years the ceramics of AlN
are applied in electronics as heat sinks [3]. This is
2
. Thermal stability of InN, GaN and AlN
The III—V nitrides are crystals with high bonding
energies. The consequence of these are their high
melting temperatures (see Table 1) and good thermal
stability. On the other hand, the strong triple bond
in nitrogen molecule lowers the thermodynamical
*
Fax: #48-22-632-42-18; e-mail: bocian@unipress.waw.pl.
0
921-4526/99/$ — see front matter ( 1999 Published by Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 1 3 0 0 - 3

2
M. Bo c& kowski / Physica B 265 (1999) 1—5
Table 1
melting point (see Table 1). On the contrary, GaN
is stable up to 2000 K. AlN is stable to its melting
point. In the case of this compound, high pressure is
not necessary to extend its region of stability.
Melting conditions for III-N nitrides compounds
Nitride
Melting
temperature
Decomposition
N pressure at
melting point
ꢀ
(K)
AlN
GaN
InN
&3500 [4]
&2800 [4]
&2200 [4,7]
20 MPa [5]
4.5 GPa [6]
6 GPa [7]
3
. Kinetic aspect of direct synthesis of AlN,
GaN and InN
The first stage of direct synthesis of nitrides is the
dissolution of nitrogen in liquid metal. This process
was analyzed by quantum mechanical calculations
[
9]. The nitrogen molecule approaching the metal
surface is repelled by the metal that results in a po-
tential barrier. If the molecule has enough energy to
overcome the potential barrier, it gets closer to the
metal and dissociates into two atoms forming new
bonds with the metal. The potential barriers are
lower than the bonding energy in the nitrogen
molecule. However, their values are quite high (see
Table 2), which suggests, that the dissociation pro-
cess can be kinetically controlled even for relatively
high temperatures. Such barrier is the lowest for Al
and the highest for In. We can conclude that effi-
cient N dissociation requires higher temperatures
ꢀ
Fig. 1. Gibbs free energy change with temperature for GaN and
for In than for Al.
its constituents for nitrogen pressure of 10ꢁ Pa and 1.1 GPa.
Fig. 2 shows the rate of dissociation process
estimated for nitrogen pressure of 2 GPa. The
horizontal dotted line is the practical limit for the
effective synthesis of nitrides. In this case it is 10 mg
potential of the nitrides constituents: Me#1/2N .
Since the free energy of its constituents decreases
ꢀ
of nitride at 1 cm during 100 h. It follows from Fig.
ꢀ
with temperature faster than the free energy of the
crystal, nitrides lose their stability at high temper-
ature. At atmospheric pressure AlN is stable up
2
that the synthesis of InN at 2 GPa N pressure is
ꢀ
impossible. This synthesis, to be reasonably fast,
requires at least 1800 K, which is quite far above
the thermal stability limit for indium nitride at
2
500 K. GaN loses the stability at 1200 K. InN is
stable only up to 600 K. On the other hand, the
application of pressure allows to increase the free
energy of nitrogen and therefore the free energy of
its constituents. As a consequence, at high pressure
the stability range is extended. This is clearly shown
in Fig. 1. This figure presents the change in Gibbs
free energy with temperature for GaN and its con-
2
GPa (900 K). In contrast to that, the synthesis of
AlN at high pressure should be very fast already at
temperatures slightly exceeding 1000 K. For GaN
Table 2
Potential barriers for N adsorption on Al, Ga, In metal surface.
The triple bond in the N molecule is equal to 9.8 eV/molecule
ꢀ
ꢀ
stituents for N pressure of 10ꢁ Pa and 1.1 GPa [8].
ꢀ
We can see that the stability region of GaN is
Nitride
Potential barrier (eV)
extended at high nitrogen pressure. At pressure up
to 2 GPa and temperature up to 2000 K (our ex-
perimental conditions), InN is thermodynamically
stable up to 900 K which is very far from the
InN
GaN
AlN
5.8
4.8
3.2

M. Bo c& kowski / Physica B 265 (1999) 1—5
3
observe combustion synthesis of AlN [11]. This
observation is in agreement with our theoretical
considerations. Due to the low kinetic barrier for
synthesis, the formation of AlN at high-pressure
should be very fast. However, we observe low- and
high-pressure limits for the combustion process.
The lower limit can be simply explained by the
access of nitrogen molecules into the reaction zone,
which is pressure dependent. The synthesis rate is
proportional to pressure [9]. Up to 100 MPa, in-
creasing nitrogen pressure leads to the increase of
nitrogen density and the amount of nitrogen gas at
the reactional interface. However, at pressures
higher than 100 MPa the combustion reaction be-
gins to be controlled mainly by the thermal con-
ductivity of nitrogen. Fig. 3 shows the typical
time—temperature (t—¹) profile measured during
combustion synthesis. The analysis of the difference
between the maximal temperature of combustion
(¹ ) and ignition temperature (¹ ) (see Fig. 3) as
Fig. 2. Nitrogen dissociation rate on Al, Ga, In metal surface at
GPa.
2
the rate of dissociation is average. However, for the
effective synthesis the temperature must be higher
than 1500 K, which indicates that the application
#
*'ꢀ
a function of pressure is presented in Fig. 4. As we
can see this difference decreases for N pressure
ꢀ
of high N pressure allows direct synthesis of GaN.
higher than 100 MPa [11]. At high pressure the
ꢀ
thermal conductivity and the thermal capacity of
nitrogen increase and the heat losses during reac-
tion become significant. Therefore, one can observe
the extinction of the combustion process [10,11].
This is an interesting agreement between this obser-
vation and a theoretical result obtained by Saur
et al. [12] for a totally different reaction: combus-
tion of methane in argon—oxygen mixture. Saur et
al. have shown that the maximal combustion tem-
perature can be reached at 100 MPa. For higher
pressure, due to the high thermal conductivity and
high thermal capacity of gas, the combustion tem-
perature decreases and the extinction of combus-
tion is to be expected.
4
. Experimental procedure
Direct synthesis experiments were carried out in
the high- pressure high-temperature equipment de-
scribed elsewhere [10]. Inside the chamber hydro-
static N pressure up to 2 GPa can be obtained.
A one-, two-, or three-zone graphite furnace, ca-
pable of reaching temperatures up to 2000 K was
used in order to heat the metal samples. The metal
ꢀ
(Al, Ga, In) was placed into the crucible, in the
furnace, and in the high-pressure chamber. The
sample was heated with a constant rate to the given
temperature and annealed at this temperature, at
high nitrogen pressure. Then the sample was cooled
down at a constant rate, the system was decom-
pressed and the sample removed.
As the result of AlN combustion synthesis, we
have obtained very pure, low oxygen content, alu-
minum nitride powder. This powder was sintered
to the ceramic form of high thermal conductiv-
ity—200 W/m K [13].
5
. Results and discussion
5
.2. GaN synthesis
5
.1. AlN synthesis
The heating of gallium sample does not lead to
During the heating of the Al sample at N pres-
sure, in the pressure range from 10 to 650 MPa, we
the combustion reaction. Annealing of gallium at
high pressure and high temperature allows to grow
ꢀ

4
M. Bo c& kowski / Physica B 265 (1999) 1—5
Fig. 3. Typical time—temperature (t—¹) profile measured during combustion of aluminum in nitrogen gas.
Fig. 4. Difference between combustion temperature and temperature of ignition as a function of N pressure.
ꢀ
quite big GaN crystals. Nitrogen molecules disso-
ciate on gallium surface and dissolve in the metal.
Therefore the crystals are grown from the solution
of atomic nitrogen in liquid gallium. Supersatura-
tion, that is the driving force for the crystallization
process, is created by the application of temper-
ature gradients along the gallium samples. Usually,
the pressure of the experiments, 1.5—2 GPa, is
higher than the equilibrium one for high temper-
ature end of the solution. Therefore pressure and
temperature in the whole sample correspond to
gallium nitride stability range. Since the gallium is

M. Bo c& kowski / Physica B 265 (1999) 1—5
5
liquid metal. The rate of nitrogen dissociation on
the liquid metal surface is kinetically controlled and
it is proportional to N pressure. For AlN the
ꢀ
nitrogen pressure allows to control the mechanism
of synthesis. By combustion of bulk Al, high
purity AlN can be synthesised. For GaN, pressure
allows to extend the stability range up to the tem-
perature where effective crystallization is possible.
Dislocation-free high-quality single crystals can be
grown.
Acknowledgements
Fig. 5. Typical GaN platelet.
The research reported in this paper was sup-
ported by Committee for Scientific Research grant
no. 7783495 C/2399.
in the temperature gradient, the nitrogen from the
hot end of the sample is transported by convection
and diffusion to the cooler part, where the solution
becomes supersaturated and crystallisation of GaN
is possible. The mechanism of GaN crystal growth
is described in details elsewhere [14]. The GaN
crystals are of wurzite structure, in the form of
hexagonal platelets. Fig. 5 shows typical GaN
platelet, grown with the rate of 0.1 mm/h into
References
[1] J.H. Edgar, J. Mater. Res. 7 (1) (1992) 235.
[
2] S. Nakamura, T. Mukai, M. Senoh, N. Iwasa, T. Yamada,
T. Matsushita, H. Kiyoku, Y. Sugimoto, Jpn. J. Appl.
Phys. 35 (1996) L74.
[3] L.M. Sheppard, Ceram. Bull. 69 (11) (1990) 1801.
+
1 0 !1 0, direction, perpendicular to the c-axis.
[4] J.A. Van Vechten, Phys. Rev. B 7 (1973) 1479.
[5] W. Class, Contract Rep., NASA-Cr-1171, (1968).
The platelets are single crystals of perfect morpho-
logy suggesting stable layer by layer growth. Pres-
ently the maximum size of crystals exceeds 15 mm.
The GaN pressure grown crystals are of the highest
structural quality and the lowest dislocation den-
sity (10ꢀ—10ꢂ) [15], ever reported for this material.
[
[
6] J. Karpin
7] I. Grzegory, S. Krukowski, J. Jun, M. Boc
M. Wroblewski, S. Porowski, AIP Conf. Proc. vol. 309,
995, pp. 565.
[8] I. Grzegory, J. Jun, M. Boc
Wroblewski, B. |ucznik, S. Porowski, J. Phys. Chem.
´
ski, S. Porowski, J. Crystal Growth 66 (1984) 11.
´
kowski,
´
1
´
kowski, S. Krukowski, M.
´
Solids 56 (1995) 639.
[
9] S. Krukowski, Z. Romanowski, I. Grzegory, S. Porowski,
5
.3. InN synthesis
J. Crystal Growth 175 (1998) 155.
[
[
[
10] S. Porowski, I. Grzegory, J. Jun, in: J. Jurczak,
B. Baranowski (Eds.), High Pressure Chemical Synthesis,
Elsevier, Amsterdam, 1989, p. 21.
After heating and annealing of indium at high
nitrogen pressure (1—2 GPa) we have never ob-
served the combustion of liquid indium or crystal
growth of indium nitride. This is due to the high
kinetic barrier for indium nitride synthesis at tem-
11] M. Bo c´ kowski, A. Witek, S. Krukowski, M. Wr o´ blewski,
S. Porowski, R.-M. Marin-Ayral, J.-C. Tedenac, J. Matter.
Synth. Process. 5 (6) (1997) 449.
12] M. Saur, F. Behrendt, E.U. Frank, Berichte der Bunsen-
Gesselshaft fur Physicalische Chimie 97 (1993) 900.
perature where InN is stable at N pressure of
ꢀ
2
GPa (only 900 K).
[13] A. Witek, M. Bockowski, A. Presz, M. Wro
Krukowski, W. W"osinski, K. Jab"onski, J. Mater. Sci. 33
1998) 3321.
14] I. Grzegory, M. Boc
M. Wroblewski, S. Porowski, MRS Internet J. Nitrides
´ blewski, S.
´
´
(
[
[
´ kowski, B. |ucznik, S. Krukowski,
6
. Conclusions
´
Semicond. Res. 1 (1996) 20.
The main barrier preventing the direct synthesis
of nitrides is the interface between nitrogen and
15] J.L. Weyher, S. Mueller, I. Grzegory, S. Porowski, J. Crys-
tal Growth 182 (1—2) (1997) 17.