Optical and photoelectrical properties of AgCd2GaS4 single-crystal compounds

The poorly known AgCd2GaS4 single-crystal compounds that crystallize in a rhombic structure (space group Pmn21) are studied. Deviations from the stoichiometric composition of the samples and random occupation of the cation sublattice sites by Ag and Ga ions result in violation of long-range order in the atomic arrangement and make the AgCd2GaS4 compounds structurally closer to disordered systems. In this case, it is found that the fundamental optical-absorption edge is smeared and shifted to longer wavelengths, and is adequately described by the Urbach rule. In addition, a broadening of the spectral peaks of photoconductivity and luminescence is observed. The concentration of charged point defects responsible for the smearing of the absorption edge is calculated. It is found to be 1.2×1020 cm−3. The AgCd2GaS4 single crystals are photosensitive semiconductors. From the position of the absorption edge, the optical band gap of the compound is estimated (Eg0 = 2.28 eV at T = 297 K). The photoluminescence spectra of the AgCd2GaS4 single crystals are similar to the spectra of defect-containing CdS single crystals; for the AgCd2GaS4 crystals, the emission peaks are shifted to longer wavelengths with respect to the peaks for CdS crystals by Δλ = 0.06−0.1 μm. From the analysis of the experimental data, some conclusions on the nature of photoactive centers in AgCd2GaS4 compounds are drawn.