Characterization of interface nature and band alignment in CBD-CdS/Cu(In,Ga)Se2 bi-layer structure by photoemission and inverse photoemission spectroscopy

Depth profiles of electronic structure at the interface between Cu(In1−xGax)Se2 (CIGSe) grown by three-stage process and CdS through chemical bath deposition have been studied by ultraviolet X-ray photoelectron spectroscopy (UPS, XPS) and inverse photoemission spectroscopy (IPES). Especially, a dependence of band alignment on Ga content in the CIGSe was investigated. An intrinsic feature at an arbitrary depth was successfully exposed by etching with an Ar ion beam with a low ion energy of 330 eV. After the removal of surface contamination, the CdS layer exhibited a band gap of 2.4 eV. The band gap started to shrink when XPS core signals of CIGSe became detectable. For the interface over the CIGSe with a Ga substitution ratio x of 0.20%, valence band offset (VBO) was about 0.7 eV, and conduction band offset (CBO) looked finite. An increase of the Ga substitution ratio to 0.40 resulted in an increase of the VBO and a reduction of the CBO. An almost flat conduction band alignment was observed at the interface of CdS/Cu0.93(In0.60Ga0.40)Se2.