Prediction of silicon nanoparticle formation: Thermochemistry and kinetics generalized from quantum chemistry.

摘要

Pyrolysis of the feed gas, typically SiH4 or Si2H 6, 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 to form semiconductor-grade materials through chemical vapor deposition (CVD) methods. Polymerization of silicon hydrides in the gas phase causes deposits on a growing semiconductor surface forming point defects. A detailed understanding of the microkinetics for the gas-phase formation of hydrogenated silicon nanoparticles will allow for the improvement of applications in which silicon nanoparticles are desired or side products such as biological imaging and CVD, respectively. While a limited number of computational studies of hydrogenated silicon nanoparticle formation have been carried out to address these concerns at the elementary step level, augmentation of these models to address multifunctionality, more accurate treatment of kinetics, and the complex, polycyclic nature of silicon hydrides is warranted.Using quantum chemical calculations, statistical thermodynamics, conventional and variational transition state theory, and internal rotation corrections, accurate rate coefficients were calculated for over 130 reactions involving 1,2-hydrogen shift, substituted silylene addition/elimination, cyclization/ring opening, H2 addition/elimination, and multifunctional silicon hydride addition/elimination. These reaction classes have been observed experimentally, yet rate coefficients cannot be measured directly for all possible reactions of silicon hydrides of relevant sizes and substituents. Thus, hydrides containing up to 10 silicon atoms, a variety of acyclic and cyclic substituents about the reactive center, and polycyclic nature were explored. Benson's group additivity model was extended to transition states to predict Arrhenius parameters, i.e., the pre-exponential factor and activation energy, for the major monofunctional reaction classes during silicon hydride pyrolysis. The Evans-Polanyi correlation was revised for multifunctional kinetics, and representative pre-exponential factors were calculated. Additionally, thermochemical properties for 175 hydrogenated clusters containing up to 13 silicon atoms were calculated to analyze the polycyclic and multifunctional nature of complex species. Primarily, this research serves to further the understanding of hydrogenated silicon nanoparticle formation chemistry at the molecular level. However, the kinetic correlations governing hydrogenated silicon nanoparticle formation also have practical value to engineers designing new reactors for semiconductors or tailored nanoparticles.