Ab Initio Chemical Kinetics for the Unimolecular Decomposition of Si_2H_5 Radical and Related Reverse Bimolecular Reactions

摘要

For plasma enhanced and catalytic chemical vapor deposition (PECVD and Cat-CVD) processes using small silanes as precursors, disilanyl radical (Si_2H_5) is a potential reactive intermediate involved in various chemical reactions. For modeling and optimization of homogeneous a-Si:H film growth on large-area substrates, we have investigated the kinetics and mechanisms for the thermal decomposition of Si_2H_5 producing smaller silicon hydrides including SiH, SiH_2, SiH_3, and Si_2H_4, and the related reverse reactions involving these species by using ab initio molecular-orbital calculations. The results show that the lowest energy path is the production of SiH + SiH_4 that proceeds via a transition state with a barrier of 33.4 kcal/mol relative to Si_2H_5. Additionally, the dissociation energies for breaking the Si-Si and H-SiH_2 bonds were predicted to be 53.4 and 61.4 kcal/mol, respectively. To validate the predicted enthalpies of reaction, we have evaluated the enthalpies of formation for SiH, SiH_2, HSiSiH_2, and Si_2H_4(C_(2h)) at 0 K by using the isodesmic reactions, such as ~2HSiSiH_2 + ~1C_2H_6→~1Si_2H_6 + ~2HCCH_2 and ~1Si_2H_4(C_(2h)) + ~1C_2H_6 →~1Si_2H_6 + ~1C_2H_4. The results of SiH (87.2 kcal/mol), SiH_2 (64.9 kcal/mol), HSiSiH_2 (98.0 kcal/mol), and Si_2H_4 (68.9 kcal/mol) agree reasonably well previous published data. Furthermore, the rate constants for the decomposition of Si_2H_5 and the related bimolecular reverse reactions have been predicted and tabulated for different T, P-conditions with variational Rice-Ramsperger-Kassel-Marcus (RRKM) theory by solving the master equation. The result indicates that the formation of SiH + SiH_4 product pair is most favored in the decomposition as well as in the bimolecular reactions of SiH_2 + SiH+3, HSiSiH_2 + H_2, and Si_2H_4(C_(2h))+H under T, P-conditions typically used in PECVD and Cat-CVD.