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



The 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 [10equation image0] 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)