Optical Properties and Structure of In₂O₃ Films Deposited on Al₂O₃ (012) Substrates by dc-Magnetron Sputtering

Abstract

The results of studies of the optical properties and structure of In2O3 films on Al2O3 (012) substrates deposited by dc-magnetron sputtering are summarized. The investigated films differ in the deposition time, substrate temperature, and the presence of additional heat treatment in air. According to X-ray diffraction measurements, these films show a reflex, which correspond to the (222) plane of the cubic modification of In2O3. Its exact position and half-width depend on the deposition time. The optical properties of the obtained films are explained by the microstructure, which is inhomogeneous in thickness and formed during the sputtering of a target with a relatively low mechanical strength. Thus, the refractive index of films deposited on substrates at room temperature increases in the direction from the substrate to the external interface. At a substrate temperature of more than 300°C, the refractive index of the films is uniform, except for a rough layer on the surface. Heat treatment reduces the number of defects in the crystal structure of the films, and leads to densification of the film material. As a result, the inhomogeneity of the refractive index disappears and the observed band gap for direct transitions decreases. The latter result from a change in the Burstein–Moss shift in consequence of the decrease of lattice defects concentration. The band gap for “indirect” transitions (that corresponds to the true value of the band gap) is insensitive to annealing.