ANODIC ELECTROSYNTHESIS OF CADMIUM SELENIDE THIN-FILMS - CHARACTERIZATION AND COMPARISON WITH THE PASSIVE TRANSPASSIVE BEHAVIOR OF THE CDS, CDTE COUNTERPARTS

Cadmium selenide thin films were electrosynthesized by an anodic route employing alkaline selenide solutions. The thermodynamic aspects of the electrodeposition chemistry were first explored via Pourbaix diagrams; the kinetic aspects were studied by linear sweep voltammetry on a Cd anode. The photoaction spectra of these anodic thin films revealed an optical gap energy (1.7 eV) in good agreement with the value known for CdSe. Their luminescence response, however, suggested a high density of carrier recombination centers located approximately 0.2 eV below the conduction band. X-ray photoelectron spectroscopy revealed that the anodic thin films were stoichiometric in composition. In situ examination of the material during thin film growth by cyclic photovoltammetry revealed anodic photoeffects at potentials just past the Cd corrosion wave. The growth kinetics in the passive region adhered to a direct logarithmic rate law, while a diffusion mechanism was seen to prevail in the transpassive regime. The morphological changes in CdSe in progressing from the passive to the transpassive regions were probed by scanning electron microscopy and energy-dispersive x-ray analysis. While CdS and CdSe present similar trends, CdTe shows a complete lack of a transpassive regime. These differences within the Group II-VI compounds are interpreted within the framework of a semiconductor model previously invoked for oxide passivation of metal surfaces.