In situ spectral reflectance analysis of the early stages of GaN thermal decomposition

The decomposition of GaN layers have been in situ monitored by a spectral reflectance system. A particular interest is given to the early stages of GaN decomposition. The analysis of the spectral reflectance behavior shows a rapid reflectance increase, which is associated with the formation of gallium metallic layer. After that, reflectance oscillates and damps with the processing time indicating the decomposition of the GaN layer. The transfer matrix formalism and the effective refractive index concept have been combined to understand the experimental recorded reflectance responses during GaN decomposition under H2 flux at high temperature. We have developed an effective refractive index formula taking into account the coexistence of gallium and GaN materials for fitting experimental results. This assumption has been validated as a good tool for the interpretation of the reflectance variations. Our results are confirmed and analyzed based on scanning electron microscopic (SEM) observations and photoluminescence measurements.