A Novel Approach for CdS Thin-Film Deposition: Electrochemically Induced Atom-by-Atom Growth of CdS Thin Films from Acidic Chemical Bath

Abstract

Thin films of cadmium sulfide (CdS) have been electrochemically deposited in acidic chemical baths (pH between 1.4 and 4.6) containing cadmium chloride and thioacetamide (TAA). Although chemical formation of CdS particles continuously occurs in the solution phase, no chemical growth of CdS thin film takes place under the acidic conditions. Electroreduction of protons at conductive substrates at small current densities (several tens of μA/cm2) was found to trigger a growth of highly crystallized hexagonal CdS thin films. The resultant film consists of particles having hexagonal cylindrical shape. The individual particles are made up of single crystals of CdS. The film growth is achieved by the atom-by-atom growth of individual crystallites and not by the agglomeration of fine particles formed in the solution phase. The film could become as thick as 500 nm without suffering from disordering of the film structure. Kinetic analysis was made for the formation of CdS particles in the solution phase, the film growth, and the electroreduction of protons at the substrate. Different activation energies of 75.3, 56.7, and 21.0 kJ/mol were found for these processes, respectively. The deposition mechanism is different from the ordinary electrodeposition process, since the film growth is not under control of the electrode kinetics. It is supposed that the electroreduction of protons imposes a pH gradient at the vicinity of the substrate to reduce the activation barrier for the decomposition of Cd−TAA complex at the surface of the substrate. The electrochemical reaction is therefore used only to maneuver the chemical formation of CdS to take place preferentially at the substrate surface, while prohibiting the contribution of the solution phase reaction to the film growth.