Optical and electrical properties of AgIn(SSe)₂ crystals

AgIn(S_XSe_1-X)₂ (X = 0-1.0) crystals were grown by a vertical gradient freezing (VGF) method. All samples indicated chalcopyrite structures and n-types by means of X-ray diffraction (XRD) and thermoprobe analysis, respectively. All crystals were slightly found to be Se- and S-rich by the electron probe microanalysis (EPMA). The samples were also In-poor in X < 0.5 and Ag-poor in X > 0.5. The AgIn(S_0.5Se_0.5)₂ (X = 0.5) crystal was mostly close to the stoichiometry. Lattice constants of a and c axes calculated by the XRD patterns were found to be proportional to X, indicating that they obeyed Vegard's law. In the photoluminescence (PL) spectra at 77K, the crystals were highly pure because a free exciton (FE) emission line was clearly observed. It was clear that the FE peak energy showed a nonlinear dependence on the composition X values in the AgIn(S_XSe_1-X)₂ crystal. The bowing parameter was estimated to be 0.25 eV. After the AgInSe₂ crystal was annealed at 640°C under a vacuum atmosphere in the AgInSe₂ crystals, the optical quality was improved because the FE peak clearly appeared and the donor-acceptor pair emission band disappeared in the PL spectra.