Infrared optical anisotropy of diluted magnetic Ga1-xMnxN/c-sapphire epilayers grown with a GaN buffer layer by metalorganic chemical vapor deposition

Optical anisotropy of hexagonal Ga1-xMnxN (x from 0.0% to 1.5%) epitaxial films grown on c-plane sapphire substrates has been investigated using far- and mid-infrared s- and p-polarized reflectance spectra at oblique incidence (at 10 degrees, 22 degrees, and 32 degrees, respectively). The experimental data at room temperature can be well reproduced simultaneously in the measured frequency region of 200-2000 cm(-1) (5-50 mu m), which was based on a four-phase layered system using a 4x4 matrix method [M. Schubert, Phys. Rev. B 53, 4265 (1996)]. The lattice vibrations perpendicular and parallel to the optic c axis (E-1 and A(1) modes) were expressed by Lorentz oscillator dielectric function model. There was a striking absorption dip at the A(1) phonon frequency in p-polarized reflectance spectra due to the optical anisotropy. These infrared-active phonon parameters were obtained with uniaxial dielectric tensor. It was found that the A(1) longitudinal-optical phonon frequency linearly increases with the Mn composition for the diluted magnetic semiconductor epilayers. The broadening values of the A(1) phonon changed from 3.6 (+/- 1.3) to 9.0 (+/- 1.6) cm(-1), showing the high film crystal quality. Moreover, the ordinary and extraordinary dielectric functions epsilon(perpendicular to) and epsilon(parallel to) of the epilayers were determined. It indicated that epsilon(parallel to) was larger than epsilon(perpendicular to) for the Ga1-xMnxN films in the reststrahlen region, which can be ascribed to slight structural degradation of the wurtzite lattice.