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Growth and decomposition of bulk GaN: role of the ammonia/nitrogen ratio

2002; Elsevier BV; Volume: 236; Issue: 4 Linguagem: Inglês

10.1016/s0022-0248(02)00825-4

1873-5002

Heenae Shin, Erdem Arkun, Darren B. Thomson, P.Q. Miraglia, Edward A. Preble, R. Schlesser, Scott D. Wolter, Zlatko Sitar, R. F. Davis,

ZnO doping and properties

Gallium nitride crystals grown via vapor-phase transport processes that incorporate ammonia as the only source of nitrogen below atmospheric pressures experience significant surface roughening and the eventual cessation of growth. Investigations of these phenomena in this research, and in the context of the discovery of a non-ceasing process route to larger GaN crystals, showed that the RMS values of the surface roughness of single crystal GaN (0 0 0 1) films exposed to pure ammonia flowing at 60 sccm for 2 h at 1130°C increased from the as-received value of 3.7–6.8 nm, 21.4 and 32.6 nm at 100, 430 and 760 Torr, respectively. Quadrupole mass spectrometry revealed that the concentrations of H2 and N2 measurably increased at pressures above 400 Torr. The primary reason for the increased roughness above 430 Torr was the enhanced etching of GaN via reaction with atomic and molecular hydrogen derived from the dissociation of the ammonia. At lower pressures, the decomposition of the GaN via the formation and evaporation of N2 and Ga increased in importance relative to etching for enhancing surface roughness. Dilution with nitrogen reduced the amount of hydrogen generated from the dissociation of the ammonia. The GaN surface annealed at 1130°C and 430 Torr in ammonia diluted with 33 vol% N2 maintained the smoothest surface with a nominal RMS value of 10.4 nm. Mixtures with higher and lower percentages of N2 showed enhanced roughness under the same conditions. Use of this optimum gas mixture also allowed the seeded growth of a 1.5×1.5×2.0 mm3 GaN crystal and a 2.3×1.8×0.3 mm3 thick platelet with neither observable decomposition nor cessation of the growth over periods of 36 and 48 h, respectively.