Polarization filtering by nonpolar M-plane GaN films on LiAlO2.

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. Since the c axis of the underlying wurtzite crystal structure lies in the film plane, its symmetry is reduced in comparison to conventional C-plane films, giving rise to an in-plane polarization anisotropy. This anisotropy is further enhanced by the anisotropic compressive in-plane strain, which occurs due to the lattice mismatch between the GaN film and the substrate LiAlO2. In addition to linear birefringence, the film also exhibits linear dichroism, resulting in a polarization filtering of an incident, linearly polarized light beam after transmission. This filtering manifests itself as a rotation of the polarization vector towards the c axis and can be as large as 80°. We compare the measured polarization rotation with calculated values taking into account the birefringence of the GaN film. In the energy range where the filtering is most effective, the birefringence almost disappears and can therefore be neglected for the determination of the rotation angle. Finally, the rotation, which is determined by the transmittance for the two orthogonal polarization directions, can be very well approximated by the product of the film thickness and the difference of the absorption coefficients for the two polarization directions. [ABSTRACT FROM AUTHOR]