Optical properties of undoped and Co2+-doped Cd2GeSe4 single crystals

Cd2GeSe4 and Cd2GeSe4:Co2+ (Co: 0.2, 0.3, 0.4, and 0.5 mol %) single crystals with a rhombohedral (hexagonal) structure were grown by the vertical Bridgman method. The structural and optical properties of Cd2GeSe4:Co2+ (Co: 0.2, 0.3, 0.4, and 0.5 mol %) were characterized by x-ray diffraction and optical absorption measurements at room temperature (298 K). The lattice constants, a0 and c0, increased when increasing Co2+ in the Cd2GeSe4:Co2+ crystal to 0.3 mol %. They increased sharply when increasing Co2+ to 0.4 and 0.5 mol %. The optical energy band gap of the Cd2GeSe4:Co2+ crystal decreased when increasing Co2+ in the Cd2GeSe4:Co2+ crystal to 0.3 mol % and it also decreased sharply when increasing the amount of Co2+ to 0.4 and 0.5 mol %. Two impurity optical absorption bands were observed at wavelength ranges of 1300–1900 nm and 700–900 nm. These bands originated from the Co2+ ions located at the C3v symmetry point in the Cd2GeSe4:Co2+ crystal.