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We report ongoing experiments on the growth of GaN by hydride vapor phase epitaxy (HVPE), using a newly designed Aixtron horizontal reactor. Growth was carried out on c-plane Al2O3 substrates on which a thin GaN layer had been predeposited by MOVPE and patterned using a dielectric mask. The mask pattern was designed to give information on the growth rate and morphology along different directions, and contained both a star-shaped pattern and arrays of parallel stripes of various widths and orientations. All growths were performed at atmospheric pressure and ~1050°C deposition temperature. For the range of experimental conditions investigated the maximum ratios of lateral to vertical growth velocities of around 2 and coalescence of the layer after approximately 10 μm of growth were observed for stripes oriented along the <100> direction.