Understanding GaN nucleation layer evolution on sapphire and its impact on GaN dislocation density

Abstract

For improved GaN films on sapphire, GaN nucleation layers (NLs) are typically grown prior to high temperature growth. Using optical reflectance and AFM image analysis we have uncovered mechanistic details of GaN NL evolution during the ramp to high temperature. As the temperature is increased the NL decomposes and GaN nuclei form. We will demonstrate that the GaN nuclei are formed from gas phase Ga atoms generated during the NL decomposition which recombine with ambient NH 3 . Continued GaN growth on these nuclei results in GaN films with dislocation densities as low as 4x10 8 cm -2 . We will show how the NL decomposition kinetics can be extracted from the optical reflectance waveforms and used to control the nuclei formation and growth. More recently we have investigated possible correlations between the GaN nucleation density and the resultant film dislocation density. We have initiated studies of ultra-low ( 7 cm -2 ) nucleation densities on sapphire using multi-step NL growth and annealing schemes. We find that over a wide range of nucleation densities that the nucleation density scales quadratically with the NL thickness. The dependence of the dislocation density on the nucleation density is currently being explored.