Influence of Alkalinity and Steric Hindrance of Lewis-Base Catalysts on Atomic Layer Deposition of SiO2

Abstract

The effects of different Lewis-base catalysts, NC5H5, N(CH3)3, NH3, NH2CH3, and NH(CH3)2, on the possible reaction pathways in atomic layer deposition (ALD) of SiO2 are investigated by density functional theory (DFT) calculations on both single and double −OH* surface models. The ALD of SiO2 undergoes successive self-terminating SiCl4 (A) and H2O (B) half reactions, respectively. On the basis of the double −OH* surface model, the rate-limiting step is the first SiCl4 half reaction with an activation energy of about 29.0 kcal/mol for the formation of the bridged −SiCl2*– group in the absence of catalysts. The activation energy of the subsequent H2O half reaction is slightly lower than that of the first half reaction. The selected Lewis-base catalysts can effectively catalyze both half reactions in the ALD growth of SiO2 through strong hydrogen bonding interaction with reaction substrates, with the activation energies of both half reactions reduced to about 1.3–14.6 kcal/mol. However, the desorption of byproduct in the H2O half reaction becomes the rate-determined step in the catalytic ALD process. The effects of alkalinity and steric hindrance of various catalysts on reaction pathways are tested. The catalytic activities of these Lewis bases are qualitatively correlated to their alkalities. As the pKa value increases, the adsorption of precursors becomes stronger and the activation energy decreases in both half reactions except N(CH3)3 with pKa = 9.8. The reactions catalyzed by N(CH3)3 with the largest spacing size are slightly blocked by its steric hindrance relative to those reactions catalyzed by NH3. In consideration of the neighboring −OH* groups, the Lewis bases with relatively large steric hindrance could not effectively catalyze half reactions to form the bridged intermediates between two nearest-neighboring −OH* groups. The compromise between alkalinity and steric hindrance of Lewis-base catalysts may be important for the rational design of effective catalysts for the low-temperature growth of metal oxides.