Optimization of the growth of CdTe thin films formed by electrochemical atomic layer epitaxy in an automated deposition system

This paper describes the deposition of CdTe thin films by electrochemical atomic layer epitaxy (ALE). ALE involves the formation of compounds an atomic layer at a time, using surface-limited reactions. That is, atomic layers of the elements making up a compound are deposited in a cycle, where each cycle produces a monolayer of the compound. In electrochemical ALE, the surface-limited reactions that produce the atomic layers are referred to as underpotential deposition (UPD). This article describes the dependence of the deposit structure, morphology and composition on a number of the steps in the deposition cycle. Separate optimized solutions and potentials are used to deposit each of the elements. Specifically, a variety of deposition and stripping potentials have been examined, resulting in a broad range of deposit compositions and morphologies. The dependence of the deposits on the potential used to form Cd atomic layers is a good example. If the potential was too positive, no CdTe deposits were formed, as no Cd was deposited, so there was nothing for Te UPD to form on. If the potentials were too far negative, bulk Cd began to deposit, and Cd-rich three-dimensional growth predominated. There was a 0.2 V plateau for the Cd deposition potential where stoichiometric films were deposited; however, the highest quality films were formed within a 0.1 V wide plateau, between -0.55 and -0.65 V. The optimal Te deposition potentials appear to be between -0.7 and -0.8 V. At more positive potentials, the Cd atomic-layers stripped, while at more negative potentials Te dendrites were formed and the surface roughened badly. Potentials of -1.2 V and below should be used for the Te stripping step, in order to remove all excess Te, above an atomic layer. Lf more positive potentials were used, some excess Te remained and three-dimensional growth resulted. Te stripping should be performed for at least 20 s to completely remove the excess Te. Neither substrate orientation, nor annealing to 300 degrees C had much effect on the quality of the deposited films.