Recent Results in the Crystal Growth of GaN at High N 2 Pressure
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Internet Journal o f
Nitride S emiconductor Research
Volume 1, Article 20
Recent Results in the Crystal Growth of GaN at High N2 Pressure I. Grzegory, M. Bockowski, B. Lucznik, S. Krukowski, M. Wroblewski, S. Porowski High Pressure Research Center This article was received on May 31, 1996 and accepted on October 23, 1996.
Abstract We present recent results on bulk GaN crystallization. The best quality GaN crystals grown from the solution at high N2 pressure without an intentional seeding are single crystalline platelets of stable morphology reaching dimensions up to 10 mm. The fastest growth direction for such crystals is [1 0 0], perpendicular to the GaN c-axis. The maximum stable growth rate perpendicular to crystal c-axis is determined from the experiment and used for an estimate of the effective supersaturation for the {10 0} face assuming two dimensional layer growth. The heat of GaN disssolution, determined from experimental solubility data, is used for the estimation of the edge energy of 2-D nuclei on the growing {10
0} face. Bulk crystal growth
seeded by a single hexagonal needle with well developed {10 0} faces is also reported. The crystallization mechanisms and morphological stability in seeded growth of GaN are discussed on the basis of experimental results. The physical properties of the GaN crystals and homoepitaxial layers grown on them are briefly reviewed.
1. Introduction GaN is a strongly bonded compound well suited for applications such as high power, high temperature electronics. The bonding energy of GaN is 8.9 eV/atom whereas for GaAs it is only 6.5 eV/atom [1]. Consequently, the free energy G(T) of the crystal is very low in relation to the reference state of free N and Ga atoms. On the other hand, the N2 molecule is also strongly bonded (4.9eV/atom). Therefore, the free energy of the constituent Ga and N2 becomes quite close and surpasses that of GaN as the temperature is raised. Figure 1 shows the free energy of GaN (1 mole) versus its constituents (Ga + 1/2N2) as a function of temperature. The G(T) of the constituents decreases faster than that of the GaN crystal and at higher temperatures GaN become thermodynamically unstable. The crossing of G(T) curves determines the equilibrium temperature where GaN coexists with its constituents at a given N2 pressure. The application of pressure allows us to increase the free energy of the constituents much more than that of the crystal shifting the equilibrium point to higher temperature and extending the stability range of GaN. The equilibrium pN2 - T conditions for GaN determined experimentally by Karpinski et al. [2] [3] are shown in Figure 2. Most of crystallization processes discussed in this paper have been carried-out in large volume gas pressure reactors at an N2 overpressure of up to 15 kbar, which corresponds to a GaN stability limit at 1850K. The maximum temperature of 1850 K is quite far from the melting temperature of GaN. According to the theoretical estimation of Van Vechten [4] the melting point of GaN is ~3000K. The N2 equil
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