Reaction kinetics and transport phenomena underlying the low-pressure metalorganic chemical vapor deposition of GaAs

A kinetic model describing the growth of GaAs films from triethyl-gallium (TEGa) and arsine by low-pressure metalorganic chemical vapor deposition (LP-MOCVD) has been developed. This precursor combination produces GaAs films with very low carbon contamination, when compared to films grown from trimethyl-gallium (TMGa) and arsine. The kinetic model includes both gas-phase and surface reactions based on reported decomposition mechanisms of the two precursors. Growth experiments were performed in a LP-MOCVD reactor operating at 3 Torr, a pressure that minimizes background levels of both ionized donors and acceptors in the film. The kinetic model was coupled to a transport model describing flow, heat and mass transfer in the experimental reactor. Finite element simulations were performed to estimate unknown rate parameters of surface growth reactions by fitting predicted to observed growth rates. The model predicts that the growth rate is limited by site-blocking effects due to slow desorption of adsorbed ethyl radicals at low temperatures and by the fast desorption of adsorbed gallium at high temperatures. The predicted Ga* coverage is high at intermediate temperatures suggesting the absence of a diffusion-limited growth regime, which is common in atmospheric pressure MOCVD. It appears that in LP-MOCVD, the temperature uniformity of the susceptor (and not gas-phase transport processes) controls the thickness uniformity of the film at operating conditions maximizing the growth rate. The proposed process model can evolve into a useful tool for reactor design and scale-up by minimizing the large number of experimental trial and error runs typically required to identify optimal operating conditions in MOCVD reactors.