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

Abstract

AbstractThe optical properties of c ‐plane GaN layers grown by molecular beam epitaxy on a ‐plane sapphire substrate are investigated. Polarization‐dependent photoreflectance spectroscopy was applied with the electric field vector of the probe beam being either parallel or perpendicular to the [10$ \bar 1 $0] direction of the GaN films. A pronounced in‐plane optical anisotropy under normal incidence of light is observed which is attributed to anisotropic in‐plane strain. It originates from the difference of the thermal expansion coefficients of the substrate parallel and perpendicular to its c‐axis. Temperature‐dependent studies and a comparison to photoluminescence and reflectance anisotropy studies indicate that the strain (anisotropy) 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 perturbation theory for comparison. The results emphasize the experimental data. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)