Effects of Ligand Exchange Reactions on the Composition of Cd1-yZnyTe Grown by Metalorganic Vapor-Phase Epitaxy

Abstract

The metalorganic vapor-phase epitaxy (MOVPE) of cadmium zinc telluride (Cd1-yZnyTe) from dimethylcadmium (DMCd), dimethylzinc (DMZn), diethylzinc (DEZn), and diisopropyltelluride (DIPTe) was studied using on-line infrared spectroscopy to monitor the feed and effluent gases. The film composition was measured by X-ray diffraction. No zinc was incorporated into the film when DMCd and DMZn were used due to the very low reactivity of the latter compound. When DMCd and DEZn are tried, the films were nonuniform with Cd-rich films deposited at the reactor inlet and Zn-rich films deposited near the reactor outlet. This film profile was due to alkyl ligand exchange reactions between the group II precursors in the feed, producing DMZn, methylethylzinc (MEZn), methylethylcadmium (MECd), and diethylcadmium (DECd). The decomposition rates of these precursors vary over a wide range with DECd reacting at a 250 K lower temperature than DMZn. Since the organocadmium compounds were consumed at a much faster rate, CdTe was deposited first, while ZnTe was deposited downstream. The ligand exchange reactions explain why previous workers found it difficult to grow Cd1-yZnyTe alloys of uniform composition by MOVPE.