Infrared reflectance studies of GaN epitaxial films on sapphire substrate

Abstract

Infrared reflectivity measurements were performed on undoped and Si-doped GaN films grown on sapphire. After analysing the substrate reflectance accurately, a good fit to the measured reflectance has been achieved. By comparison of the measured and calculated spectra, it becomes possible to identify GaN phonons from the overlapped complicated substrate bands, and also to determine the carrier concentration even for GaN on sapphire with low and medium doping levels. In some samples, an interface layer has been demonstrated to be formed between film and substrate, which manifests itself as a damping behaviour of the interference fringes in the spectra. Detailed analysis suggests that this interface layer, with a thickness of ∼0.14 μm estimated from a two-layer model simulation, may be due to the high density of defects caused by stress relaxation.