“Negative” gap in the spectrum of localized states of (In$_{2}$O$_{3})_{0.9}$(SrO)$_{0.1}$

The reflection $R(\hbar\omega)$, transmission $t(\hbar\omega)$, absorption $\alpha(\hbar\omega)$, and refraction $n(\hbar\omega)$ spectra of polycrystalline In$_{2}$O$_{3}$–SrO samples with low optical transparency, which contain In$_{2}$O$_{3}$ and In$_{2}$SrO$_{4}$ crystallites with In$_{4}$SrO$_{6+\delta}$ interlayers, are examined. In the region of small $\hbar\omega$ values, the reflection coefficient decreases as the resistance of samples saturated with oxygen increases. Spectral dependences $n(\hbar\omega)$ and $\alpha(\hbar\omega)$ are calculated using the classical electrodynamics relations. The results are compared to the data based on the $t(\hbar\omega)$ spectra. The calculated absorption spectra are interpreted within the model with an overlap of tails of the density of states in the valence band and in the conduction band. A “negative” gap $E_{gn}$ in the density of states with a width from -0.12 to -0.47 eV is formed in highly disordered samples in this model. It is demonstrated that the high density of defects and the band of deep acceptor states of strontium in the major matrix In$_{2}$O$_{3}$ phase are crucial to tailing of the absorption edge and its shift toward lower energies. The direct gap $E_{gn}$ = 1.3 eV corresponding to the In$_{2}$SrO$_{4}$ phase is determined. The energy band diagram and the contribution of tunneling, which reduces the threshold energy for interband optical transitions, are discussed.