Effect of Fermi Level Motion on ESR and Optical Properties of CuAlS2

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Abstract
Optical absorption and electron spin resonance (ESR) spectra have been studied in as-grown, Cr-doped, and annealed in different atmospheres CuAlS2crystals. It has been found that the major influence on the optical and ESR properties of CuAlS2crystals, obtained by chemical vapour transport, arises from the transition ions Fe3+and Cr2+, which are present in the crystal lattice as residual impurities. Thermal treated crystals exhibit drastic changes in their absorption and ESR spectra depending on the annealing atmosphere, which is explained by the process of the Fermi level motion in the vicinity of the Fe2+/Fe3+and Cr+/Cr2+demarcation levels. The results obtained show that the valence states of Fe and Cr ions and their relative amounts in each valence state are controlled by the position of the Fermi level in the band gap of the host crystal, and are stoichiometry on dependent.

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