Photoluminescence properties of a near-stoichiometric CuGaSe2 film Original Research Article

Epitaxial films of CuGaSe2 were grown on (001)GaAs substrates by an MBE technique. A near-stoichiometric film with chemical compositions consistently varied from Cu- to Ga-rich was prepared by growing the film without the substrate rotation. A series of PL spectra was obtained by directing a focused laser beam point-by-point across the boundary separating the Cu- and Ga-rich regions. Distinct features of these spectra were noted. On the Cu-rich side, optical emissions peaked at 1.71, 1.67, 1.63, and 1.59 eV were observed in a PL spectrum. The peak at 1.71 eV was due to the emission of bound exciton, while the peak at 1.67 eV was caused by the free-to-bound transition. The other two peaks were identified to be the donor-to-acceptor emissions. Further annealing experiments performed in different environments suggested that the peaks at 1.67, 1.63, and 1.59 eV were associated with the optical transitions of CB→CuGa, Cui→CuGa, and Cui→VGa, respectively. On the Ga-rich side, a dominant donor-to-acceptor emission peaked at 1.62 eV was observed, which was determined to be the GaCu→VCu transition. The two defects with opposite charge states resulted in a highly compensated material with high resistivity.