Optical polarization anisotropy in strained A-plane GaN films on R-plane sapphire

We have used polarized photoreflectance (PR) spectroscopy to study [1120]-oriented A-plane GaN films on R-plane sapphire substrates. For this type of films, the c-axis of GaN lies in the film plane. When the probe beam is polarized perpendicular to the c-axis, we observe a single feature in the PR spectrum, while two features at higher energies are observed for parallel polarization. Comparing the transition energies with the ones obtained by electronic band-structure calculations, we identify the observed PR spectral features as the three possible transitions around the fundamental band gap of GaN. However in comparison to unstrained GaN, their polarization properties and energies have been significantly modified by the inplane anisotropic strain in the film. Finally, we can also determine the individual in-plane anisotropic strain components