Photoluminescence spectra of CuGaSe₂ crystals

CuGaSe₂ (CGS) crystals (Cu/Ga = 0.9-1.4) which have been expected as photovoltaic applications were successfully grown by a vertical gradient freezing method. All samples were of the chalcopyrite structures, nearly stoichiometric and showed p-type conductivities. The samples became poor crystallinity with increasing enclosed Cu/Ga ratios in the Cu-rich region because the values of full-width at half-maximum at (112) diffraction lines increased with increasing enclosed Cu/Ga ratios. Lattice constants of a and c axes in the Cu-rich regions were larger than those in the Cu-poor regions, indicating that Cu atoms played an important role with the lattice constants. The grown stoichiometric CGS sample was of high quality since the free-exciton emission line was clearly observed in photoluminescence (PL) spectrum at 77K. The CGS samples turned out to have many donor- and acceptor-type defects because the donor-acceptor pair emission bands had considerable intensities in the PL spectra.