Formation of In$_2$O$_3$ films by magnetron sputtering on Al$_2$O$_3$ (012) substrates

The results of studies of the microstructure and optical characteristics of In$_2$O$_3$ films deposited by the dc-magnetron sputtering of a polycrystalline target onto single-crystal sapphire substrates are summarized. The technological regimes of films preparation differed in the deposition time, substrate temperature, and the presence of additional heat treatment of film structures in air. It has been established that the optical refractive index of films deposited on a “cold” substrate increases in the direction from the substrate to the external interface. Heat treatment of the films eliminates the inhomogeneity of the refractive index and leads to a decrease in the band gap. The observed optical properties are explained by the thickness inhomogeneous microstructure of the films, which is formed during sputtering of a target with a relatively low mechanical strength.