Growth of AlN bulk crystals from the vapor phase
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Growth of AlN bulk crystals from the vapor phase Raoul Schlesser1,2, Rafael Dalmau1, Rositza Yakimova3, and Zlatko Sitar1,2 1 Dept. of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695-7919, U.S.A. 2 HexaTech, Inc., Raleigh, NC 3 on Sabbatical leave from Linkoping University, Sweden ABSTRACT AlN single crystals were grown by two different vapor phase processes: by vaporization of metallic Al in a nitrogen atmosphere, or by sublimation of an AlN source. Growth experiments were carried out under quasi-stagnant flow conditions, with typical flow rates of 100 sccm at reactor pressures ranging from 300 to 700 Torr. Growth temperatures ranged from 1800 to 2300°C. In Al vaporization experiments, the crystal shape and fastest growth direction was found to strongly depend on the growth temperature: at relatively low temperatures (1800-1900°C) long needles were grown, temperatures around 1900-2000°C yielded twinned platelets, while cplatelets were formed at temperatures above 2100°C. These c-plates grew at a rate of 5 mm/hr in the c-plane. When using AlN as a source material, growth rates were considerably slower, however, long-term stability of the Al flux was greatly improved. Seeded growth was demonstrated under these conditions. All grown single crystals were transparent and virtually colorless. Analytical results indicated very high crystalline quality and dislocation densities lower than 104 cm-3. INTRODUCTION Wide bandgap nitride semiconductors have been identified as promising materials for a broad range of applications in electronics and optoelectronics. At present, heteroepitaxial thin films of nitride materials are being grown by various thin film techniques. Blue, III-nitride LEDs grown on sapphire and SiC substrates have become commercially available; solid state lasers and high-frequency/high-power amplifiers have been demonstrated. However, the lifetime and performance of most nitride devices are limited by the relatively poor quality of heteroepitaxial material grown on non-lattice-matched substrates. AlN (or GaN) single crystalline substrates with low dislocation densities are expected to drastically decrease the defect density in overgrown III-nitride thin film structures and thus greatly improve the characteristics of nitride devices. Bulk growth of single crystalline AlN has been attempted in several laboratories via sublimation [1-5], vaporization [6] and solution routes [7]. To date, no AlN single crystals of high quality have been produced in considerable volume. In this paper we experimentally investigate the growth of AlN single crystals from the vapor phase. We will compare two different growth techniques: growth by vaporization of metallic Al in a nitrogen atmosphere and growth by sublimation of AlN. EXPERIMENT The growth of AlN crystals by vaporization of Al or sublimation of AlN requires elevated process temperatures, as the vapor pressure of Al above metallic Al, and particularly AlN, remains low, and Al reacts only insignificantly with nitrogen at temperatures
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