Depth profiling of a CdS buffer layer on CuInS2 measured with X-ray photoelectron spectroscopy during removal by HCl etching

A ∼ 35 nm thick CdS buffer layer has been wet-chemically deposited on a CuInS2 thin layer solar cell absorber and subsequently removed through a stepwise HCl etch process. Sequential etching times of two and three seconds were initially performed to gradually remove the CdS top-layer and leave the CuInS2 surface intact. After each step an investigation of the sample with X-ray photoelectron spectroscopy was used to determine the stoichiometry and elemental binding energies of the sample surface. After the removal of the CdS layer longer etching times were used to study the long-term effects of the HCl. The resulting depth profile revealed Cu diffusion from the CuInS2 into the CdS, although the Cu atoms did not reach the surface of the buffer layer. In addition, a Cd containing layer was left on the sample surface after more than six hours of etching time and is apparently insoluble in HCl, showing that the Cd-S bonds in this layer differ from those in CdS. This method can also be used with other top-layers to create depth profiles.