Initial reactions in chemical vapor deposition of Ta₂O₅ from TaCl₅ and H₂O. An Ab initio study

Quantum chemical calculations at the Hartree-Fock, B3LYP, MP2, and CCSD(T) levels of theory were performed in order to study the mechanism of Ta₂O₅ chemical vapor deposition (CVD) from TaCl₅ and H₂O. The geometries and vibrational frequencies of reactants, products and transition states of the reactions, which are used for modeling the initial steps of the CVD process were calculated using effective core potentials for Ta and Cl atoms. The reaction between TaCl₅ and H₂O proceeds via formation of a strongly bonded six-coordinated tantalum complex (25 kcal/mol at CCSD(T)//B3LYP). The correlated methods show the transition state energy to be close to the energy of the initial reagents. Formation of TaOCla, in the unimolecular decomposition of TaCl₄OH and TaCl₃(OH)₂, has similar barriers of ∼20 kcal/mol. The catalytic effect of an assisting water molecule opens a barrierless channel for TaOCl₃ formation in the gas phase.