Optical properties of undoped and Co2+-doped CaS, CaSe, BaS, and BaSe single crystals

Undoped and Co2+-doped CaS, CaSe, BaS, and BaSe single crystals were grown by using the chemical transport reaction method. These single crystals crys tallized into a cubic structure. When Co2+ was doped into the single crysta ls, their lattice constant a increased and their optical energy gap decreas ed. Impurity optical absorption peaks due to Co2+ were observed in the Co2-doped single crystals. Within the framework of the crystal field theory, w e identified these peaks as originating from the electron transitions betwe en the energy levels of Co2+ sited at the octahedral (O-h) symmetry point.