Mechanistic aspects of the electrodeposition of stoichiometric CdTe on semiconductor substrates

The kinetics of the electrodeposition of CdTe from an aqueous solution containing HTeO2+ and Cd2+ have been studied using metallic (glassy carbon) and semiconductor (GaAs, CdS and Si) substrates. At metal electrodes, an almost diffusion-limited deposition current is often observed, and the deposit generally contains substantial amounts of elemental Te. At semiconductor electrodes, on the other hand, the deposition current is kinetically limited under identical conditions, and the CdTe film can be of very good quality. The origin of this marked difference has been investigated by comparing the influence of pH and the Cd2+/HTeO2+ concentration ratio at both types of substrates. At semiconductor electrodes, it appears that a low surface electron concentration leads to kinetically limited Te(IV) reduction if the concentrations of SO42– and Cd2+ are high and the proton concentration is low. This low surface electron concentration cannot be obtained at metallic substrates. Combination of optimised solution composition and correct choice of semiconductor substrate allows definition of conditions for the growth of stoichiometric CdTe.