Deep defect levels in CuGaSe2 investigated with photoluminescence

We present a comprehensive study of photoluminescence spectra of the CuGaSe₂ (CGS) thin films, with a focus on the influence of variation in the copper content on optical transitions involving deep defects. The CGS samples investigated here were grown via the three-stage co-evaporation method, and exhibited varying [Cu]/([Cu]+[Ga]) ratios of 1.01, 0.98, and 0.92. We analyzed only the part of the spectrum below 1.3 eV, and carefully considered the interference effects in the analysis. Two prominent peaks were identified in this low-energy region. The first peak (A), located at 0.98 eV, was present in all samples, regardless of their stoichiometry. The second peak (B) at 1.12 eV emerged exclusively in Cu-poor samples, indicating its dependence on copper stoichiometry. Based on an analysis of the PL spectra, ab initio calculations, and literature data, peaks were tentatively assigned to donor-acceptor radiative recombination involving native defects. We proposed two alternative models to explain the nature of peak A at 0.98 eV. The first model assumed a nonintuitive recombination mechanism due to the V_Se charge state change by capturing a hole located on V_Cu. In the second model, we proposed a transition between a deep donor Ga_Cu(2+/+) and a deep acceptor, which is probably Cu_Ga(0/-). Peak B at 1.12 eV, within both models, is explained as a recombination between a deep donor, i.e. a substitutional defect Ga_Cu (2+/+), and a shallow acceptor V_Cu.