Photoluminescence of CuGaS₂ epitaxial layers grown by metalorganic vapor phase epitaxy

High quality epitaxial layers of CuGaS₂ have been grown on GaAs and GaP substrates by low-pressure metalorganic vapor phase epitaxy using cycropentadienylcoppertriethylphosphine, normal-tripropylgallium, and ditertiallybutyl sulphide precursors. Photoluminescence (PL) studies at 8 K have been carried out, and exciton-related PL peaks at 2.489 and 2.477 eV have been observed in addition to PL peaks at 2.43 (free-to-bound), 2.39 (donor-acceptor pair), and broad band at 1.65 eV. Temperature and excitation intensity dependences of the PL peaks have been studied in detail, and PL properties are compared with those for the bulk CuGaS₂ crystals grown by the chemical vapor transport (CVT) method. PL spectra for nearly stoichiometric CuGaS₂ epilayers exhibited the intense free exciton-related PL peak over the entire temperature range (8-300 K), which are in contrast to PLs dominated by defect-related deep peaks for Ga-rich or Cu-rich epilayers and the impurity related near-band edge PL for the CVT bulk crystal. These PL results show that the nearly stoichiometric CuGaS₂ epilayers grown in this study are the highest quality reported so far.