Optical properties and grain boundary effects in CuInSe2

Abstract

The optical properties of CuInSe2 thin films and single crystals are reported. The wavelength dependence of the refractive index and extinction coefficient is measured using multiple angle-of-incidence ellipsometry. Absorption coefficients as high as 6×105/cm are reported—the highest for any semiconductor. CuInSe2 is confirmed to have a room temperature, direct-bandgap transition near 0.96 eV for the single crystals, and 1.02 for the thin films. The difference is proposed to be due to nonuniformity in composition of the polycrystalline material. Additional absorption in the low absorption-coefficient regime is likely due to transitions associated with phonon absorptions for the single crystals. The effects of heat treatments in Ar, N, and O are shown to improve the optical properties of the thin films due to improvement in film compositional uniformity. Heating in high vacuum causes Se desorption near the film surface, causing a related degradation in the absorption characteristics.