Potential-pH diagram of the Cd-Te-NH3-H2O system and electrodeposition behavior of CdTe from ammoniacal alkaline baths

A potential-pH diagram of the Cd-Te-NH3-H2O system was constructed based on diagrams of the Cd-NH3-H2O and Te-H2O systems and discussed in connection with the redox behavior of an ammonia-alkaline CdTe electrolytic bath with pH 10.7. CdTe has a wide domain of stability throughout the acidic and alkaline regions, and the redox behavior was well explained with the diagram. The diagram indicated that the cathodic electrodeposition of CdTe occurs across a domain of stability of tellurium metal, i.e., at lower potentials than the deposition potential of bulk Te and higher than that of bulk Cd, with respect to the bath with pH < ca. 11.5, while in the higher pH region, CdTe is expected to deposit directly from Te(IV) and Cd(II) ions. The deposition mechanism is considered as follows: (i) deposition of tellurium layer followed by (ii) an immediate underpotential deposition of Cd on it, which prevents the bulk deposition of tellurium. It can be considered that the stoichiometric CdTe is more easily electrodeposited from alkaline baths, since the domain for tellurium metal is narrower in the alkaline region compared to the conventionally employed acidic region with pH 0-2. Therefore, the bulk deposition of elemental tellurium is less apt to occur from an alkaline bath. (C) 1999 The Electrochemical Society. S0013-4651(98)07-007-4. All rights reserved.