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

The results of studying the optical properties and structure of In2O3 films on Al2O3 (012) substrates deposited by dc-magnetron sputtering are summarized. The films investigated differ in terms of the deposition time, substrate temperature, and the presence of additional heat treatment in air. According to X-ray diffraction measurements, these films show a reflection, 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 resulting 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 the films deposited onto substrates at room temperature increases in the direction from the substrate to the external interface. At a substrate temperature of more than 300 degrees 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, inhomogeneity of the refractive index disappears and the observed band gap for direct transitions decreases. The latter results from a change in the Burstein-Moss shift as a consequence of a decrease in the lattice-defect concentration. The band gap for indirect transitions (corresponding to the true value of the band gap) is insensitive to annealing.