Optical properties of undoped and Co-doped CuAlGeSe4 and CuAlSnSe4 single crystals

Abstract

CuAlGeSe4, CuAlSnSe4, CuAlGeSe4:Co2+, and CuAlSnSe4:Co2+ single crystals were grown by the chemical transport reaction method. These single crystals had a defect chalcopyrite structure and a direct band structure. Optical energy gaps at 286 K were found to be 2.394, 1.874, 2.302, and 1.794 eV for CuAlGeSe4, CuAlSnSe4, CuAlGeSe4:Co2+, and CuAlSnSe4:Co2+ single crystals, respectively. Impurity optical-absorption peaks due to cobalt dopants were observed at 12 243, 7002, and 3890 cm−1 in CuAlGeSe4:Co2+ single crystal and 11 949, 7056, and 3920 cm−1 in CuAlSnSe4:Co2+ single crystal at 286 K. Each of these peaks split into four peaks due to the second-order spin-orbit coupling effect at 30 K. The peaks are assigned to the electron transition between the energy levels of a Co2+ ion sited at the cubic Td symmetry point in the host lattice. Then, for CuAlGeSe4:Co2+ and CuAlSnSe4:Co2+ single crystals, the crystal-field parameter Dq is found to be 389 and 392 cm−1, respectively, the first-order spin-orbit coupling parameter λ −194 and −202 cm−1 the second-order spin-orbit coupling parameter p 238 and 242 cm−1, and the Racah parameter B 505 and 483 cm−1.