Optical Properties of Ultrathin Electrodeposited CdS Films Probed by Resonance Raman Spectroscopy and Photoluminescence

We report resonance Raman and photoluminescence data from ultrathin films of CdS grown on Au substrates using electrochemical atomic layer epitaxy (ECALE). Samples ranged in coverage from 25 to 200 monolayers. Quantitative analysis of resonance Raman intensities leads to several important conclusions. First, ECALE does not involve growth by random precipitation of CdS onto the Au surface; contiguous thin layers of material are deposited. Second, the electronic structure of the films in this coverage regime corresponds to that of bulk CdS. Finally, charge carriers are rapidly trapped at the surface of the semiconductor on the time scale of the Raman scattering event; the trapping rate decreases linearly as a function of CdS coverage.