Spatial variations of optoelectronic properties in single crystalline CuGaSe_2 thin films studied by photoluminescence

摘要

Single crystal CuGaSe_2 (CGSe) thin films were grown epitaxially on GaAs substrates with different compositions and studied with spatially resolved photoluminescence with micrometer resolution (μPL). Polycrystalline counterparts grown on glass were studied for comparison. μPL performed at room temperature is used to analyze spatial variations of the band gap (AEg) and the splitting of quasi-Fermi levels (Δ(E_Fn —E_Fp)) of the absorber. In contrast to earlier studies on Cu(In,Ga)Se_2 (CIGSe) we have concentrated on inhomogeneities occurring in the absence of alloying effects due to the In and Ga mixture. The epitaxially grown specimen exhibited a significantly smaller amount of variations than the polycrystalline counterparts. Cu-rich samples showed higher variation of A(E_Fn — E_Fp) compared to the Cu-poor counterparts. It is suggested that this is related to formation of a secondary phase Cu_xSe under Cu-rich conditions giving rise to spatially fluctuating Cu-excess. The observed band gap variations could be attributed to strain effects in the absorber layer, and do not indicate any variations of the electronic structure of the absorber.