Spectroscopic Ellipsometry in-situ Monitoring/Control of GaN Epitaxial Growth in MBE and MOVPE

In-situ spectroscopic ellipsometry (SE) studies on MBE and MOVPE growth of GaN were carried out to elucidate the film growth behaviors and the critical issues in epilayer quality improvement. The whole RF-MBE growth process of GaN was monitored and evaluated by SE and reflection high-energy electron diffraction (RHEED). Two relaxation processes in RF-MBE growth of GaN were identified and found to play important roles in epilayer quality improvement. The first relaxation took place in the very initial buffer layer growth stage, during which GaN started to relax on sapphire substrate by Stranski-Krastanov mode (S-K) growth after several MLs of GaN deposition, and self-organized quantum dots were formed during this stage. The second one was the surface roughening, taking place during the temperature ramping. Isolated islands were formed during the temperature ramping and annealing, but the islands coalescences were scarce in the buffer layer. The dominant pseudo-2D growth mode in the initial stage of epilayer growth and the limited surface migration were thought to be the bottleneck for quality improvement in RF-MBE grown GaN films. In comparison to RF-MBE growth, SE signal evolution in MOVPE growth of GaN and the related mechanisms were also discussed. The growth behavior differences between MOVPE and MBE were clarified.