Using density functional theory to postulate a mechanism for zinc sulfide formation in a CVD reactor

Zinc sulfide has a large band gap and is widely used in optical devices, laser domes, and reflective windows. Chemical vapor deposition (CVD) is the method of choice for producing zinc sulfide from zinc vapor and hydrogen sulfide on a large scale. Here, high quality, crystallinity, and low cost are desired. As such the ability to control the CVD reaction is very important and requires the CVD reaction mechanism which is not well known. This has motivated the study of the reaction mechanism in this paper. Specifically, the reaction between zinc vapor and hydrogen sulfide in a CVD reactor has been studied using ab initio and density functional calculations. Geometry optimizations were performed using the Becke three parameter and the Lee Yang Parr functional (B3LYP)/6-31G(d) and B3LYP/6-311+G(d, p) for reactants, transition structures, intermediate complexes, and products. Enthalpy and free energy of the reaction were estimated using B3LYP/6-31G(d), B3LYP/6-311+G(d, p), Gaussian G1, Gaussian G2, and G2MP2. The rate constant in the gas phase reaction was calculated using the transition state theory (TST) and was subsequently used in a computational fluid dynamics (CFD) model including momentum, mass, and energy balance. In this paper, two different pathways have been proposed for this reaction, namely a pathway that involves one transition state, and a second pathway involving two intermediate complexes and two transition states. The calculated rate constants using the proposed pathways were employed in a simulation of the deposition process. The calculated deposition rate is in excellent agreement with experimental data for the same reactor configuration. This suggests that molecular modeling is a powerful surrogate for experiments.