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

Quantum chemistry was adopted to study thermo-chemical properties rind reac tivity of different gas and surface species of interest for the epitaxial m etalorganic vapour phase deposition of ZnS. All calculations were performed using density functional theory methods such as the three parameters Becke -Perdew Wang hybrid DFT (B3PW91) method and different basis sets. The resul ts of these studies enabled us to have a better understanding of the fundam ental chemical steps that occur in the formation of the crystalline ZnS fil ms and to discuss the impact of gas reactions on the overall deposition che mistry. Among the different metalorganic precursors considered were Me2Zn:E t3N, H2S and (BuSH)-Bu-t. It was found, in agreement with previous experime ntal 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 t o have an activation energy of 17.4 kcal/mol. The kinetic rate is expected to be similar for the reaction between (BuSH)-Bu-t and Me2Zn, yielding (BuS ZnMe)-Bu-t and methane. Finally, by comparison between experimental and cal culated data the rate-determining step for the growth of ZnS by Me2Zn:Et3N and (BuSH)-Bu-t at low pressures and in the absence of gas phase pre-reacti ons was identified in the one site dissociative adsorption of Me2Zn or (BuS ZnMe)-Bu-t on S suface sites.