Optical properties of undoped and Co2+- doped Cu2ZnGeSe4 crystals

The optical properties of Cu2ZnGeSe4 crystals, a type I-2-II-IV-VI4 compoun d, grown by using the chemical transport reaction method were investigated. The effects of Go-doping in the crystals were also studied. It was shown f rom the X-ray pou der diffraction measurement that these compounds had a te tragonal superstructure of sphalerite, the so-called stannite, with lattice constants of a=5.618 Angstrom and c=11.04 Angstrom for the undoped Cu2ZnGe Se4 and a=5.626 Angstrom and c=11.05 Angstrom for the Go-doped compound. Th e direct band gap at 298 K, obtained from the optical absorption measuremen t, was found to be 1.518 eV for the undoped Cu2ZnGeSe4 and 1.456 eV for the Go-doped Cu2ZnGeSe4. The shrinkage of the band gap due to Go-doping was ab out 62 meV. We observed several absorption bands of Co2+ ions in two near i nfrared regions of the optical absorption spectra of Cu2ZnGeSe4:Co2+ These absorption bands were assigned as due to electronic transitions between the split energy levels of Co2+ ions in a T-d crystal field under spin-orbit i nteraction.