OPTICAL-PROPERTIES OF UNDOPED AND COBALT-DOPED IB-GA-IVA-SE4 SINGLE-CRYSTALS

Single crystals of undoped and Co-doped I(B)-Ga-IV(A)-Se4 quaternary compounds, i.e., CuGaGeSe4, CuGaSnSe4, AgGaGeSe4, AgGaSnSe4, CuGaGeSe4:Co2+, CuGaSnSe4:Co2+, AgGaGeSe4:Co2+, and AgGaSnSe4:Co2+ have been grown by the chemical-transport reaction method. The temperature dependence of the optical energy gaps for these compounds is investigated in the temperature range 10-300 K. Also, the impurity optical absorption of Co2+ in these quaternary semiconductors is studied in the visible and near-infrared regions. The optical energy gap of I(B)-Ga-IV(A)-Se4 single crystals as doped with 2 mol % of cobalt is smaller by about 4-6 % than that of undoped ones. The temperature dependence of the optical energy gap is well fitted with a Manoogian and Leclerc equation. We observed the impurity optical-absorption bands of Co2+ ions in these quaternary semiconductors. The observed optical-absorption bands are well explained due to the electronic transitions between the split energy levels of Co2+ with T(d) symmetry of the I(B)-Ga-IV(A)-Se4 host lattice.