Growth of GaN from elemental Gallium and Ammonia via Modified Sandwich Growth Technique
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E11.38.1
Growth of GaN from elemental Gallium and Ammonia via Modified Sandwich Growth Technique E. Berkman, R. Collazo, R. Schlesser and Z. Sitar, North Carolina State University, Department of Materials Science and Engineering, Raleigh, NC 27695-7919
ABSTRACT Gallium nitride (GaN) films were grown on (0001) sapphire substrates at 1050°C by controlled evaporation of gallium (Ga) metal and reaction with ammonia (NH3) at a total reactor pressure of 800 Torr. Pure nitrogen (N2) was flowed directly above the molten Ga source to prevented direct reaction between the molten Ga and ammonia, which causes Ga spattering and GaN crust formation. At the same time, this substantially enhanced the Ga transport to the substrate. A simple mass-transport model based on total reactor pressure, gas flow rates and source temperature was developed and verified. The theoretical calculations and growth rate measurements at different ammonia flow rates and reactor pressures showed that the maximum growth rate was controlled by transport of both Ga species and reactive ammonia to the substrate surface. INTRODUCTION GaN and related alloys have a tremendous potential for electronic and optoelectronic applications. In fact, due to their direct energy gaps covering the whole visible spectrum and a large part of the UV range, GaN and its alloys with AlN and InN recently became the basic materials for short-wavelength optoelectronics. However, the development of GaN-based technology is still strongly limited by difficulties in obtaining high quality, large-area native substrates in large quantities. High quality, transparent and colorless platelets were grown from the solution in liquid gallium at high temperatures and high N2 pressures [1]. Since Johnson et al. [2] synthesized polycrystalline GaN by the reaction of NH3 with molten Ga, several researchers have employed similar methods to grow GaN single crystals. Ejder [3] studied the growth of GaN from the vapor phase by the reaction of Ga and NH3 to investigate the effect of the Ga vapor concentration on crystal morphology. Balkas et al. [4] were successful in obtaining high quality GaN crystals up to 1 mm in length at a growth rate of 0.5 mm/h in the direction perpendicular to the basal plane via evaporation of Ga in ammonia. Growth by a modified vapor phase transport process called the sublimation sandwich method (SSM) was reported by Wetzel and co-workers [5, 6] where GaN films were grown from metallic Ga and ammonia on (0001) 6H–SiC. Vodakov et al [7] reported growth rates of 200–1100 µm/h using the same technique. In the sandwich technique, the evaporated Ga metal or sublimed GaN powder is transported in a NH3 atmosphere to the substrate where deposition occurs. It was found that the use of pure GaN powder led to partial decomposition of the source material into liquid Ga and N2 vapor at the onset of the growth process [8, 9]. Therefore, the process has an inherent difficulty in that pre-synthesized GaN powder or crystals must be continuously supplied. The use of pure Ga as the source m
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