For improved GaN films on sapphire, GaN nucleation layers were typically grown prior to high temperature growth. Using optical reflectance and atomic force microscopy image analysis, mechanistic details of GaN nucleation layer evolution during ramping to high temperatures was uncovered. As the temperature was increased the nucleation layer decomposed and GaN nuclei form. It was demonstrated that the GaN nuclei were formed from gas phase Ga atoms generated during the nucleation layer decomposition which recombine with ambient NH3.

Continued GaN growth on these nuclei results in GaN films with dislocation densities as low as 4 x 108/cm2. It was shown how the nucleation layer decomposition kinetics could be extracted from the optical reflectance waveforms and used to control nuclei formation and growth. Possible correlations between the GaN nucleation density and the resultant film dislocation density were investigated. Studies of ultra-low (< 107/cm2) nucleation densities on sapphire were conducted using multi-step nucleation layer growth and annealing schemes. It was found that over a wide range of nucleation densities that the nucleation density scales quadratically with the nucleation layer thickness. The dependence of the dislocation density on the nucleation density was currently being explored.

Understanding GaN Nucleation Layer Evolution on Sapphire and Its Impact on GaN Dislocation Density. D.D.Koleske, M.E.Coltrin, S.R.Lee, G.Thaler, K.C.Cross, M.J.Russell: Proceedings of the SPIE, 2007, 6841, 68410H