PHOTOASSISTED ORGANOMETALLIC VAPOR-PHASE EPITAXY OF CDTE

A mathematical model has been developed which describes the transport phenomena and elementary reaction kinetics encountered during photoassisted organometallic vapor-phase epitaxy of cadmium telluride. Dimethylcadmium and dimethyltellurium in helium carrier gas are fed to a horizontal, square duct containing a substrate heated to 165-degrees-C. The organometallic compounds are photodissociated in the gas by illumination with 248 nm photons from a continuous-wave laser. Average deposition rates predicted by the model agree to within 10% of those measured experimentally. The numerical simulations reveal that the growth rate is controlled by the photolysis rate of the organometallic compounds. This results in a linear dependence of the growth rate on dimethylcadmium pressure and laser power. The simulations further indicate that elemental tellurium deposits with cadmium telluride over a wide range of operating conditions. The elemental tellurium is avoided by operating below 20 Torr total pressure, by illuminating the gas directly above the substrate, and by feeding two-four times more dimethylcadmium than dimethyltellurium. Under these conditions, the flux of methyl radicals to the surface is sufficient to etch away the excess tellurium. The model has also been used to identify an illumination geometry suitable for obtaining uniformly thick CdTe films.