Anisotropy of the refractive index and absorption coefficient in the layer plane of gallium telluride single crystals

Refractive index and absorption coefficient of GaTe for light polarized in the optical axis directions in the layer plane are determined from transmission measurements. The refractive index is determined in the wavelength range from 0.7 to 25 μm and the absorption coefficient in the range of energies from 1.6 to 2 eV at room temperature and at 30 K. The optical constants are found to be anisotropic in the layer plane. The refractive index dispersion is interpreted through a Phillips-Van Vechten model and the Penn gaps for each direction are found to be EPg(⊥b) = 3.37 eV and EPg(‖b) = 3.58 eV. In the long-wavelength region a polar phonon contribution is also taken into account. The absorption edge is found to be very slightly anisotropic, which is shown to be coherent with the Se pz-like character of the valence band.