Theoretical study on the SiH4−nCln (n=0–4) reaction mechanisms for polysilicon production process

Theoretical Investigations for Kinetics of the Chemical Reactions: H + SiClx (x = 1, 2, 3)

Hydrogen atoms and SiCl_x (x = 1, 2, 3) radicals coexist during the hydrogenation of silicon tetrachloride (STC, SiCl₄), an important process in the fabrication of industrial polysilicon. In this work, the mechanisms and kinetics of the reactions between H and SiCl_x (x = 1, 2, 3) were studied by theory. The structures and vibrational frequencies of reactants, products, intermediates, and transition states (TSs) were determined at the B2PLYP/may-cc-pVTZ level. The single-point energies of minima and saddle points were refined using the coupled-cluster single-double with triple perturbative (CCSD(T)) with the complete basis set extrapolation method. Some special treatments were designed to obtain reliable wave functions for unimolecular reactions without tight TSs by the density functional theory. Subsequently, Lennard-Jones (L-J) parameters between each intermediate (SiHCl_x) and bath gas (He) were obtained at the MP2/jul-cc-pVTZ level to derive reliable temperature- and pressure-dependent rate coefficients for unimolecular reactions according to the variational Rice-Ramsperger-Kassel-Marcus theory. For bimolecular reactions, rate coefficients were determined by the variational transition-state theory. The rate coefficients of barrierless reactions were derived based on the loose TSs with the maximum free energy. Finally, the master equation analysis was used to investigate the variation of the rate coefficients with pressure and temperature in the activated paths.

Revealing the Chemical Reaction Properties of a SiHCl3 Pyrolysis System by the ReaxFF Molecular Dynamics Method

The pyrolysis kinetics of SiHCl₃ and its reaction mechanism are essential for the chemical vapor deposition process in polysilicon industries. However, due to the high temperature and lack of in situ experimental detection technology, it is difficult to carry out experimental research on the pyrolysis kinetics of SiHCl₃. In this work, reactive force field molecular dynamics simulations of SiHCl₃ pyrolysis were performed to investigate the effect of temperature on the pyrolysis kinetics of SiHCl₃ at the atomistic scale in a wide temperature range (1000-2000 K). The lumped Si clusters containing more than five Si atoms tended to appear at the later period of the reaction under a temperature lower than 1300 K, some of which even possessed polycyclic structures; nevertheless, small ones with less than two Si atoms such as SiHCl₂ and HCl tended to emerge under a high temperature. The changes of partial energy terms with time evolution under various temperatures were proved to be rooted in the distribution of intermediates based on the momentary simulation period. In general, the reaction network at a low temperature was more complicated than that at a high temperature, resulting from the fact that more chemical events and intermediates came into existence, and the maximum number of Si atoms in one single molecule/radical was observed under a low temperature than that under a high temperature. As to the variation of SiHCl₃ with the progress of the reaction, the linear fitting tendency disappeared under the temperature above 1300 K, which changed in fluctuation with the further elevation of temperature, elucidating the fact that SiHCl₃ can act as a product and not just as a reactant to participate in elementary chemical events frequently.