A density functional theory study of surface and gas phase processes occurring during the MOCVD of ZnS

Abstract
Quantum chemistry was adopted to study thermo-chemical properties and reactivity of different gas and surface species of interest for the epitaxial metalorganic vapour phase deposition of ZnS. All calculations were perfomed using density functional theory methods such as the three parameters Becke-Perdew Wang hybrid DFT (B3PW91) method and different basis sets. The results of these studies enabled us to have a better understanding of the fundamental chemical steps that occur in the formation of the crystalline ZnS films and to discuss the impact of gas reactions on the overall deposition chemistry. Among the different metalorganic precursors considered were Me₂Zn : Et₃N, H₂S and ^tBuSH. It was found, in agreement with previous experimental studies, that the bond energy between Me₂Zn and Et₃N is very low. The kinetic constant for the reaction between H₂S and Me₂Zn to yield HSZnMe and methane was determined by locating the transition state and it was found to have an activation energy of 17.4 kcal/mol. The kinetic rate is expected to be similar for the reaction between ^tBuSH and Me₂Zn, yielding ^tBuSZnMe and methane. Finally, by comparison between experimental and calculated data the rate-determining step for the growth of ZnS by Me₂Zn : Et₃N and ^tBuSH at low pressures and in the absence of gas phase pre-reactions was identified in the one site dissociative adsorption of Me₂Zn or ^tBuSZnMe on S suface sites.