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(11̄00) film grown by molecular-beam epitaxy on a γ-LiAlO2(100) substrate. The measurements are done with the electric-field vector (E) of the probe light being parallel (∥) and perpendicular (⊥) 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 E∥c compared to its value for E⊥c. 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.