MODELING OF THE COUPLED KINETICS AND TRANSPORT IN THE ORGANOMETALLIC VAPOR-PHASE EPITAXY OF CADMIUM TELLURIDE

A catalytic reaction mechanism is proposed for the organometallic vapor-phase epitaxy of cadmium telluride. Dimethylcadmium and diethyltellurium dissociatively adsorb onto exposed metal atoms on the cadmium telluride surface. The sticking probability is given by S = 1.5 x 10(-4)(1-theta-R), where theta-R is the methyl and ethyl coverage. Subsequently, methyl and ethyl radicals desorb at a rate given by r(d) = 1.5 x 10(9) exp(-25 [kcal/mol]/RT)theta-R(s-1). This mechanism is consistent with surface science studies of the adsorption and decomposition of organometallic molecules on semiconductor surfaces. A boundary-layer model incorporating the catalytic reaction is used to simulate the kinetics of cadmium telluride deposition in a horizontal, cold-wall reactor. The simulations show the same dependence of the growth rate on temperature as was observed by I.B. Bhat et al. [J. Electrochem. Soc. 134 (1987) 195] on sapphire substrates. The growth rate increases exponentially with temperature below 350-degrees-C, but remains constant above 400-degrees-C. At low temperatures, the reaction is controlled by the desorption of alkyl radicals. At high temperatures, the reaction is mainly controlled by the adsorption of organometallic molecules. The growth rate changes when different substrates are used in the experiments. This suggests that the kinetics of alkyl radical desorption are influenced by the cadmium telluride surface structure.