STRUCTURAL AND OPTICAL-PROPERTIES OF ZN1-XCDXGA2SE4 AND ZN1-XCDXGA2SE4-CO2+ SINGLE-CRYSTALS

Single crystals of Zn1-xCdxGa2Se4 and Zn1-xCdxGa2Se4:Co2+ were grown by a chemical-transport reaction method using the ingots synthesized with 3 at.% excess of selenium as starting materials. It has been found that these single crystals have a defect chalcopyrite structure, and the tetragonal distortion factor (2-c/a) decreases due to the introduction of an excess of selenium. It has also been shown that these compounds have a direct optical-energy gap, and the direct optical-energy gap decreases when a composition X increases and when a cobalt impurity is introduced. The temperature dependence of the optical-energy gap is well given by the Varshni equation. Impurity optical absorption of the Zn1-xCdxGa2Se4:Co2+ single crystals has been investigated in the temperatures 289 and 80 K and in the wavelength region 250-2500 nm. It is concluded that a cobalt atom in the Zn1-xCdxGa2Se4:Co2+ single crystals with a small tetragonal distortion occupies nearly a cubic T-d symmetry site of the host lattice as a Co2+ ion; then the observed impurity optical-absorption peaks are attributed to the electronic transition between the energy levels of a Co2+ ion in a crystal field of a cubic T-d symmetry.