CuInS2–CdS heterojunction valence band offset measured with near-UV constant final state yield spectroscopy

The valence band offset of the heterojunction between CuInS2 (CIS) and chemical bath deposited CdS has been determined both by means of combined x-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) and by near-UV constant final state (CFS) yield spectroscopy. The use of the latter shows that this spectroscopic method is indeed suitable for the investigation of chalcopyrite thin films. The higher information depth due to the low excitation energies (7.5–4.0 eV) used in CFS makes it possible to obtain a signal from the valence band of the CIS substrate even after a relatively thick (∼5 nm) CdS layer has been deposited. The number of samples needed to determine the band offset is reduced from three to one and the effects of chemical changes on the CIS surface can be accounted for because the valence band edges of both materials are measured directly. The valence band offsets with the XPS/UPS and CFS methods were determined to be 1.25±0.20 and 1.45±0.20 eV, respectively.