Molecular orbital studies of zinc oxide chemical vapor deposition: Gas-phase radical reactions

The gas-phase reactions involved in the radical mechanism for zinc oxide chemical vapor deposition have been examined by ab initio and density functional calculations. Geometries of reactants, transition structures, intermediate complexes, and products have been optimized with B3LYP/6-31G(d), and energetics have been computed with B3LYP/6-311+G(d,p), CCSD(T)/6-311+G(d,p), and MP2/ 6-311+G(3df,2p) levels of theory. The latter two were combined to give a G2(MP2)-like estimate of the enthalpy and free energy. Initiation reactions involve the thermal dissociation of diethyl zinc. The second bond dissociation of diethyl zinc is calculated to be significantly smaller than experimental estimates. The first step in the propagation reactions is ethyl radical abstracting a hydrogen from water to form hydroxyl radical. This has the highest barrier of the reaction sequence and would appear to be the rate-limiting step. The addition of hydroxyl radical to diethyl zinc proceeds without a barrier producing ethyl zinc hydroxide and regenerates ethyl radical. A similar set of propagation reactions converts ethyl zinc hydroxide to zinc dihydroxide. Additional propagation reactions involving oxyzinc radicals were also investigated. The gas-phase intermediates can react further to produce linear and cyclic oligomers. Comparison of the gas-phase reactions in the radical and closed shell mechanisms for zinc oxide chemical vapor deposition shows that the barrier heights for the rate-limiting steps are very similar.