Growth of CuGaS₂ Single Crystals by Chemical Vapor Transport and Characterization

Abstract

Single crystals of CuGaS2 (CGS) compounds were grown by a chemical vapor transport technique in a closed system using iodine as a transporting agent at different growth conditions. The grown crystals exhibit different colors such as yellow, green, and orange depending on the temperature of the growth zone. Single-crystal X-ray diffraction studies confirmed the chalcopyrite structure of the grown crystals. The powder X-ray diffraction showed the presence of the secondary phase in the yellow (Cu2S) and orange (Ga2S3) crystals. Scanning electron microscope analysis disclosed step growth, lateral expansion, and layer growth patterns on the yellow-, orange-, and green-colored single crystals, respectively. Energy dispersive X-ray analysis revealed the stoichiometric composition of the green crystals, whereas rich Cu and Ga were found in yellow and orange crystals, respectively. Raman spectra of green-colored CGS single crystals exhibited a high-intensity peak of the A1 mode at 315 cm-1. For yellow and orange crystals, this mode was shifted toward higher and lower frequencies, respectively. This is due to the presence of a higher amount of copper in yellow crystals and a higher amount of gallium in orange crystals. The absorption coefficient's fall is sharper of green-colored than yellow- and orange-colored CGS single crystals due to the presence of secondary phases in them. Nevertheless, the reason for the lower band gap of yellow and orange crystals might be due to the presence of defect states. Photoluminescence spectra of green and orange crystals show only one emission line each. Surprisingly, the yellow crystals now discharged two emission lines. The hole mobility and hole concentration of the green-colored CGS single crystal were found to be high when compared with those of the orange and yellow single crystals at room temperature. Because of lattice disordering, the disorder of cation vacancies was also found to be low for the green-colored single crystal. All three different-colored CGS single crystals have shown p-type conductivity.