Polarized photoreflectance spectroscopy of strained A -plane GaN filmson R -plane sapphire

We have investigated a [ 11 2 ¯ 0 ] -oriented A -plane GaN film on R -plane sapphire, where the c axis of GaN lies in the film plane, by polarized photoreflectance (PR) spectroscopy. Near the fundamental energy gap of GaN, the PR spectrum with the probe light polarized perpendicular to the c axis exhibits one feature corresponding to a single transition labeled T 1 . For a polarization parallel to c axis, two different features labeled T 2 and T 3 are observed at higher energies than the transition T 1 . In order to explain the origin of these three features, we compare the measured energies with calculations of the transition energies and oscillator strengths of the three band-to-band transitions of GaN near its fundamental gap for an anisotropic in-plane strain in the A plane. The analysis shows that the observed transition energies and polarization properties of the three transitions can be explained by the presence of an overall compressive, anisotropic in-plane strain in the film.