Influence of the thermal conditions of fabrication and treatment on the optical properties of In$_{2}$O$_{3}$ films

In$_{2}$O$_{3}$ films on Al$_2$O$_3$ (012) substrates are fabricated by dc magnetron sputtering at various temperatures (20–600$^{\circ}$C). The effect of annealing and the substrate temperature on the film properties are studied by the ellipsometric method and the optical transmission method. Refractive-index profiles are constructed and band gaps for direct and indirect transitions are found. It is established that annealing leads to densification of the film material and unifies the refractive index. Annealing also decreases and unifies the energies of band-to-band transitions, which can be explained by lowering the influence of barriers in annealed films. However, the band gap for direct transitions varies greater than for indirect transitions. This fact can be associated with the mechanism of indirect transitions, notably, the participation of phonons facilitates interband transitions even if they are hindered by extra barriers caused by grain boundaries. The latter can be indirect evidence of the actuality of indirect transitions in indium oxide.