CHEMISTRY OF PHOTO-ASSISTED ORGANOMETALLIC VAPOR-PHASE EPITAXY OF CADMIUM TELLURIDE

A reaction mechanism is presented for photo-assisted organometallic vapor-phase epitaxy (OMVPE) of cadmium telluride. Dimethylcadmium and dimethyltellurium are photodissociated in the gas into Cd and Te atoms and methyl radicals by 248 nm photons from a continuous-wave laser. The Te atoms and methyl radicals recombine to form Te2, TeCH3, and C2H6. The Cd atoms and Te species diffuse to the substrate, adsorb, and incorporate into the cadmium telluride film. The film stoichiometry is maintained by Cd sublimation when cadmium is in excess, and by methyl radical etching of Te when tellurium is in excess. This mechanism has been incorporated into a laminar-flow model of the photo-assisted OMVPE reactor. The model predicts that at low laser power and/or high flow rates, the growth rate is controlled by the dimethylcadmium photolysis rate. This yields a linear dependence of the growth rate on the laser power density and the dimethylcadmium pressure. These results are in good agreement with experiments. The model further predicts that elemental tellurium deposits with cadmium telluride over a broad range of operating conditions. Conditions which avoid tellurium deposition are described. In addition, a simple criterion is developed for determining when an excimer laser may be modeled as a continuous light source.