The Role of Multifunctional Kinetics During Early-Stage Silicon Hydride Pyrolysis: Reactivity of Si2H2 Isomers with SiH4 and Si2H6

The four lowest energy Si₂H₂ isomers on the singlet potential energy surface, i.e., trans-HSiSiH, SiSiH₂, Si(H)SiH, and Si(H₂)Si, have been suggested as precursor molecules to silicon wafer growth during silicon hydride pyrolysis.  Pyrolysis of the feed gas, typically SiH₄ or Si₂H₆, is a standard protocol to create polycrystalline or amorphous silicon nanoparticles in the gas phase or controlled growth of silicon wafers at a gas-solid interface.  Si₂H₂ isomers are the simplest multifunctional silicon hydrides, i.e., these molecules contain multiple functionality by way of divalent silicon centers and/or Si-Si bonds with bond orders greater than one.  A detailed understanding of the microkinetics for the gas-phase nucleation of silicon hydrides will allow for the improvement of applications in which silicon nanoparticles are desired or side products such as biological imaging and chemical vapor deposition (CVD), respectively. 

Kinetic parameters for the dominant pathways during the addition of the four lowest energy Si₂H₂ isomers to monosilane, SiH₄, and disilane, Si₂H₆, were calculated using G3//B3LYP, statistical thermodynamics, conventional and variational transition state theory, and internal rotation corrections.  These simple multifunctional silicon hydrides were used as model chemical species to probe the nature of the reactive center with multifunctionality to determine if multifunctional kinetics follow the reaction family concept.  Each of the four isomers was found to be reactive with monosilane and disilane under pyrolytic conditions.  These relatively unstable and unsaturated silicon hydride species can potentially contribute to gas-phase silicon nanoparticle nucleation due to high reactivity.  Si₂H₂ isomers may play a critical role in the early stages of silicon nanoparticle nucleation, and filling the gap in knowledge for the reactivity of multifunctional silicon hydrides under pyrolytic conditions is needed.

Keywords: kinetics, nanoparticles, intermediates, G3//B3LYP, chemical vapor deposition