XPS analysis of p-type Cu-doped CdS thin films

X-ray photoelectron spectroscopy (XPS) was used to examine the chemical states and depth profiles of Cu, Cd and S in p-type Cu-doped CdS (CdS(Cu)) thin films. An Auger parameter was used to examine the chemical states of the elements. Auger parameters of Cu and Cd were similar to those of Cu2S and CdS, respectively. Therefore, it is thought that the doped Cu was Cu+ and Cd was Cd2+ in the CdS(Cu) films. Depth profiles of the elements in the CdS(Cu) films showed that the concentration of S was about 50% and constant, that Cu was diffused throughout the films, but rich at the surface and interface of CdS and the substrate, and that the concentration of Cd decreased with Cu doping, although the sum of the concentrations of Cd and Cu was also about 50%. Since Cu–S compounds such as Cu2S were not detected by XRD and EDX analyses using TEM in CdS(Cu) films. It is thought that the p-type characteristics of CdS(Cu) films are due to a Cu acceptor in which some of the Cd2+ are substituted by Cu+ in unit cells of CdS, not to the formation of p-type Cu–S compounds.