Kinetics of metal organic-ammonia adduct decomposition: Implications for group-III nitride MOCVD

We have used infrared spectroscopy to investigate the decomposition of the gas-phase (Me)(3)M:NH3 (M = Al, Ga, In) adducts from room temperature to 573 K, at reactant concentrations in the nominal range used for AI(Ga,In)N metal organic chemical vapor deposition. At 473-523 K TMA1:NH3 decomposes quantitatively to yield (Me-2)AINH(2) and CH4. Comparison of the experimental and theoretical spectra indicates that the majority of the aluminum metal organic product exists in dimer form, i.e., [(Me-2)AINH(2)](2). The decomposition reaction exhibits unimolecular decomposition kinetics with rate constant parameters of v = 1 x 10(12) s(-1) and E-a = 25.7 kcal/mol. At temperatures <543 K, TMGa + NH3 and TMIn + NH3 mixtures are dominated by reversible adduct formation-dissociation with no detectable quantities of CH4 produced. At 574 K a small amount of decomposition is observed in TMGa + NH3 mixtures, which can be explained by a simple kinetic model that includes the effect of adduct equilibrium. Results demonstrate that the (Me)(3)Al:NH3 decomposition rate is fast enough to contribute to the early stages of a concerted parasitic chemical reaction mechanism, but the (Me)(3)Ga:NH3 decomposition rate is too slow.