Gas phase reactions between SiH4 and B2H6:: A theoretical study

Gas-phase reactions of silane (SiH4) and diborane (B2H6) are investigated using ab initio calculations at the MP2/6-311++g** level. Initially SiH4 and B2H6 are only weakly attracted to each other. Under thermal activation, the two molecules can overcome an energy barrier of 33.87 kcal/mol to associate into a complex SiH4-BH3-BH3 with a hydrogen-bridged Si-H-B bond. Upon bonding, one of the two hydrogen-bridged bonds B-H-B in B2H6 is broken and the other becomes polarized. Started from the SiH4-BH3-BH3 complex, three comparable fragmentation pathways involving BH3 and H-2 elimination produce several silaboranes with various silicon-boron-hydrogen ratios. A much higher barrier exists between the initial loosely bonded SiH4-B2H6 system and a direct H-2 elimination product SiH3-B2H5 with C-s symmetry. The bonding nature in, the species are further elucidated through topological analysis of electron density using the AIM theory. These intermediate silaboranes are possible precursors for chemical vapor deposition in fabricating boron doped silicon films. Their composition and polarity may determine their tendency of surface adsorption and the properties of the relevant solid. The proposed mechanisms can help to understand and control the initial gas-phase reactions in practice. Calculated IR spectra, dipole moment, and rotational constants of the species are provided to facilitate experimental investigations.