Single-crystal transformation of sphalerite CdS to cubic phase Cu_xS

Thin films, 800-2000 Å thick, of single-crystal (100) sphalerite CdS, which were grown epitaxially on single-crystal (100) NaCl substrates and possessed a density of 10^12-13 defects/cm², were converted to Cu_xS by dipping them in a hot CuCl solution. Transmission electron microscopy of the converted samples revealed a new Cu_xS cubic phase of high quality (10^6-7 defects/cm²), while the films remained single crystal despite the transformation process. In-situ TEM studies of the CdS to Cu_xS transformation were carried out by heating CdS films with CuCl layers on their surfaces. They revealed that the initiation of transformation centres is accompanied by a stress relaxation, in which defect-free CdS grains are created, while the final film quality is determined by the elimination of these grains in the transformation process. The new Cu_xS cubic phase was found to be stable up to 330°C, even in co-existence with CdS.