Reactions in the initial stage of the CVD of BN—a quantum chemical investigation

Abstract

For a survey over the initial steps in the chemical vapor deposition of boron nitride, we have investigated the mechanism and the energetic course of the gas phase reaction of boron trichloride with ammonia at the level of second-order Møller-Plesset perturbation theory. Furthermore, we have studied dimerization and trimerization of intermediate products. The results affirm previous views that the formation of triaminoborane consists of three sequential substitutions in which a chlorine atom is exchanged for an amino group. While the overall reaction is found to be exothermic, the last substitution reaction is endothermic. In addition we have been able to show that each substitution is a two-step process: an unhindered formation of an ammonia adduct and the subsequent elimination of hydrogen chloride which proceeds through a transition state. Oligomerization and adduct formation are shown to be concurrent processes. We find that the dimerizations are also two-step reactions. At first, a van der Waals complex is formed. The following proper dimerization requires the surmounting of a barrier. From the reaction profile and the relative stabilities of oligomers and adducts we conclude that the polymerization is favored at the stage of BCl(NH₂)₂.