Growth and decomposition of bulk GaN: Role of the ammonia/nitrogen ratio
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-ceasin 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 seem for 2 h at 11 30 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 H-2 and N-2 measurably increased at pressures above 400Torr. The primary reason for the increased roughness above 430Torr 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 N-2 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 1130degreesC and 430 Torr in ammonia diluted with 33 vol% N-2 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 x 1,5 x 2.0mm(3) GaN crystal and a 2.3 x 1.8 x 0.3 mm(3) thick platelet with neither observable decomposition nor cessation of the growth over periods of 36 and 48h, respectively. (C) 2002 Elsevier Science B.V. All rights reserved.