Polarization-dependent spectroscopic study of M-plane GaN on γ-LiAlO2

We investigate the polarization dependence of the absorption, reflectance, and photoreflectance spectra of a compressively strained, M-plane, wurtzite GaN(1 (1) over bar 00) film grown by molecular-beam epitaxy on a gamma-LiAlO2(100) substrate. The measurements are done with the electric-field vector (E) of the probe light being parallel (parallel to) and perpendicular (perpendicular to) to the c axis of GaN, which lies in the growth plane. We observe a significant increase in the effective optical band gap of the M-plane GaN film for Eparallel toc compared to its value for Eperpendicular toc. This result is explained by including the effect of the M-plane biaxial compressive strain on the electronic band structure of GaN. We also determine the extraordinary refractive index of GaN at energies below its band gap from the reflectance measurements. (C) 2002 American Institute of Physics.