Infrared optical anisotropy of diluted magnetic Ga_1−xMn_xN∕c-sapphire epilayers grown with a GaN buffer layer by metalorganic chemical vapor deposition

Optical anisotropy of hexagonal Ga_1−xMn_xN (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°, 22°, and 32°, respectively). The experimental data at room temperature can be well reproduced simultaneously in the measured frequency region of 200–2000 cm⁻¹ (5–50 μm), which was based on a four-phase layered system using a 4×4 matrix method [ M. Schubert Phys. Rev. B 53 4265 (1996)]. The lattice vibrations perpendicular and parallel to the optic c axis (E₁ and A₁ modes) were expressed by Lorentz oscillator dielectric function model. There was a striking absorption dip at the A₁ 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 A₁ longitudinal-optical phonon frequency linearly increases with the Mn composition for the diluted magnetic semiconductor epilayers. The broadening values of the A₁ 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 ε_⊥ and ε_‖ of the epilayers were determined. It indicated that ε_‖ was larger than ε_⊥ for the Ga_1−xMn_xN films in the reststrahlen region, which can be ascribed to slight structural degradation of the wurtzite lattice.