"Negative" Gap in the Spectrum of Localized States of

The reflection R([??][omega]), transmission t([??][omega]), absorption [alpha]([??][omega]), and refraction n([??][omega]) spectra of polycrystalline [In.sub.2] [O.sub.3]-SrO samples with low optical transparency, which contain [In.sub.2] [O.sub.3] and [In.sub.2] Sr[O.sub.4] crystallites with [In.sub.4] Sr[O.sub.6+[delta]] interlayers, are examined. In the region of small [??][omega] values, the reflection coefficient decreases as the resistance of samples saturated with oxygen increases. Spectral dependences n([??][omega]) and [alpha] ([??][omega]) are calculated using the classical electrodynamics relations. The results are compared to the data based on the t([??][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.sub.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.sub.2] [O.sub.3] phase are crucial to tailing of the absorption edge and its shift toward lower energies. The direct gap [E.sub.gd] = 1.3 eV corresponding to the [In.sub.2] Sr[O.sub.4] phase is determined. The energy band diagram and the contribution of tunneling, which reduces the threshold energy for interband optical transitions, are discussed.