New facets of CdTe electrodeposition in acidic solutions with higher tellurium concentrations

Cathodic electrodeposition of cadmium telluride has been investigated in acidic aqueous solutions (pH 1- 2) to explore the effect of the tellurium (IV) concentration at values higher than in classical conditions (typically a few 10(-4) M). This concentration was increased to 1.5 x 10(-3) M. The effects of the deposition potential and the hydrodynamic regime have been studied in a systematic way. The composition of the films, as well as their optical and structural properties, have been determined as a function of deposition parameters. The deposition of stoichiometric CdTe is obtained in a potential domain, positive with respect to the cadmium deposition potential, whose width decreases from about 0.5 V to a few millivolts when the current density increases, i.e., for higher growth rates (up to 10 mum per h). A composition-potential diagram has been established. The results are explained by analysis of a general deposition mechanism of CdTe, which enables for the first time an equation of the boundary lines of the stoichiometric domain. Outside this stoichiometric window, surprisingly, the variation of the composition is not continuous between CdTe and pure Te, but presents a large domain where the atomic ratio Cd/Te of the films is fixed at 0.5. This indicates the formation of a definite compound with an apparent composition CdTe2, which can be explained by extending Kroger's theory to metastable compounds, as a result of an intermediate step in the deposition mechanism revealed by working under higher rates conditions. An optical bandgap of 1.3 eV and an amorphous structure are found for this compound. Upon thermal treatment, a phase separation takes place between Te clusters and the CdTe matrix. The formation of cadmium ditelluride, which has not been reported yet for other deposition methods, illustrates the interest of electrodeposition for obtaining low-temperature metastable phases. (C) 2004 The Electrochemical Society.