Phase composition of CuxS thin films: spectroscopic evidence of covellite formation

Transition metal sulphides, grown in alternate layers by Electrochemical Atomic Layer Deposition (ECALD) to form thin films, are relevant candidates as photovoltaic materials for solar-cell production. However, the knowledge of the phase composition of the deposited sulphide, which is a fundamental information to assign and predict its effective semiconducting properties, is often hindered by the limited amount of material and by its morphological constraints. In the present study, an X-ray Photoelectron Spectroscopy (XPS) and Atomic Force Microscopy (AFM) investigation of a copper sulphide thin film, synthesised according to the ECALD method, is proposed as an alternative and efficient way to identify the deposited semiconductor. The detailed XPS analysis of the Cu and S contents in the film points to a 1:1 stoichiometric ratio, corresponding to the CuS compound. Moreover, Cu is observed only as monovalent, and the possible co-existence of sulphide ions and of disulphide units is inferred. Accordingly, the attribution of the structure of the thin film to covellite, CuS, is proposed, as being the only one able to account for all experimental evidences. We suggest that the covellite structure can be set up by spontaneous rearrangement starting from a wurtzite-type CuS deposit, driven by the negative Gibbs energy involved in the formation of the compound from the elements. Finally the morphological characterisation of the film confirms its spatial homogeneity, thus making the covellite film a potential candidate for solar-cell applications.