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The polarization characteristics and intrinsic photoluminescence of M-plane GaN films on LiAl[O.sub.2] substrates were studied. The results have shown that the strain relaxation can occur in an anisotropic manner allowing for a maximum of the valence band splitting at a certain film thickness.

Polarized photoreflectance (PR) spectroscopy is used to investigate a [1120]- oriented A-plane GaN film on R-plane sapphire, where the c axis of GaN lies in the film plane. The results indicate that the observed transition energies and polarization properties of the three transitions can be used to describe the presence of an overall compressive, anisotropic in-plane strain in the film.

Measured polarization rotation is compared with calculated values taking into account the birefringence almost of the GaN film. The anisotropic optical properties and polarization filtering are investigated for strained M-plane GaN films as a function of the angle of the in-plane polarization vector.