In situ investigation of as grown Cu(In,Ga)Se2 thin films by means of photoemission spectroscopy

Abstract

A UHV-transferred Cu(In,Ga)Se2 (CIGSe) thin film prepared by a three-stage process on an Mo-coated float glass was successfully investigated by detailed UPS/XPS and directly compared with air-transferred and cyanide etched samples from the same batch. Besides the fact that the valence band maximum of the UHV-transferred sample is located 1.19 eV below the Fermi level, pointing to a widened bandgap and type inversion, almost no oxygen or sodium was detected on the surface. After air transfer, Na- and O-containing species were found on the CIGSe surface that could be only partially removed upon cyanide etching. The residual and very low Na1s signal is slightly shifted, which is attributed to a modified band bending at the grain boundaries with Ga-, In-, and Se-related oxides still present on the CIGSe surface after the etching process. Thickness analysis reveals that both the sodium containing surface layer (after air-transfer) and the oxygen containing termination layer (after cyanide etching) are restricted to a few monolayers only. Finally, the topmost CIGSe layer appears to be Cu-free due to the fact that no oxidized Cu was found after air exposure. In addition, the Cu core level does not follow the surface termination induced shift of the valence band after the KCN treatment, pinning its maximum 0.77 eV below the Fermi level. These findings are ascribed to a Cu(In,Ga)7Se11 surface as deduced from XPS core level analysis.