Analysis of polarization-dependent photoreflectance studies for c-plane GaN films grown on a-plane sapphire

Abstract

The optical properties of c-plane GaN layers grown by mo lecular beam epitaxy on a-plane sapphire substrate are inves tigated. Polarization-dependent photoreflectance spectroscopy was applied with the electric field vector of theprobe beam being either parallel or perpendicular to the [10 10] direction of the GaN films. A pronounced in-plane optical anisotropy under normal incidence of light is observed which is attrib uted to anisotropic in-plane strain. It originates from the difference of the thermal expansion coefficients of the sub strate parallel and perpendicular to its c-axis. Temperature dependent studies and a comparison to photoluminescence and reflectance anisotropy studies indicate that the strain (ani sotropy) increases with decreasing sample temperature. The dependence of transition energies and oscillator strengths of GaN as a function of strain values is calculated by k·p per turbation theory for comparison. The results emphasize the experimental data. © 2009 WILEY-VCH Verlag GmbH & Co. KGaA.