Browsing by Author "Nguyen, T. N."

Now showing 1 - 2 of 2

Ab Initio Chemical Kinetics for the Thermal Decomposition of SiH2+ and SiH3+ Ions and Related Reverse Ion-Molecule Reactions of Interest to PECVD of alpha-Si:H Films
(2019-11-01)
Nguyen, T. N.
;
Lee, Y. M.
;
Wu, J. S.
;
Lin, M. C.
;
交大名義發表
The mechanisms and kinetics for the thermal decomposition of SiH2+ and SiH3+ ions, and related reverse reactions involving their ion fragments have been investigated for the first time by using ab initio molecular orbital and variational RRKM calculations. Geometries of all species involved the title reactions have been optimized by the CCSD(T), CCSD, and MP2 methods with the Aug-CC-pVTZ basis set while their single point energies have been refined by the CCSD(T)/CBS calculation for all the three levels for comparison. The barrierless processes involved have been calculated at the CASPT2//CASSCF/Aug-CC-pVTZ level of theory. The potential energy surfaces indicate that the SiH2+ and SiH3+ ions can eliminate an H atom via barrierless processes with dissociation energies of 48.4 and 91.5 kcal mol(-1), respectively, or eliminate an H-2 molecule via tight transition states at TS1 (38.5 kcal mol(-1)) and TS3 (59.9 kcal mol(-1)) giving H-2(+) Si+ (21.3 kcal mol(-1)) and H-2(+) SiH+ (36.7 kcal mol(-1)), respectively, at the CCSD(T)/CBS//CCSD(T)/Aug-CC-pVTZ level of theory. The rate constants for the reactions predicted for a wide range of T,P-conditions reveal that they increase with rising temperature and pressure and H-2 elimination is the major channel in each of the decomposition reactions. The relative energies predicted by different methods agree closely; the predicted heats of formation for various species are in good agreement with available experimental values. The predicted rate constants for the forward and reverse reactions may be employed for kinetic modeling of PECVD processes.
Ab Initio Chemical Kinetics for the Thermal Decomposition of SiH4 (+) Ion and Related Reverse Ion-Molecule Reactions of Interest to PECVD of a-Si:H Films
(2017-07-01)
Nguyen, T. N.
;
Lee, Y. M.
;
Wu, J. S.
;
Lin, M. C.
;
機械工程學系
The thermal unimolecular decomposition of SiH4 (+) ion and its related reverse reactions, SiH3 (+) + H and SiH2 (+) + H-2, have been investigated by ab initio molecular orbital and quantum statistical variational RRKM theory calculations. The potential energy surface has been calculated at different levels of theory; the results at the highest level, CCSD(T)/CBS//CCSD(T)/6-311++G(3df,2p), show that the decomposition of SiH4 (+) can mainly occur via a barrierless channel giving SiH2 (+) + H-2 lying 11.8 kcal/mol above the reactant, or via a transition state forming SiH3 (+)center dot center dot center dot H complex to be followed by a barrierless decomposition yielding SiH3 (+) + H lying 23.5 kcal/mol above the reactant. Barrierless processes were calculated using the CASPT2 and CASSCF methods with the 6-311++G(3df,2p) basis set and fitted with Morse potentials. The rate constants were predicted by solving master equations based on the RRKM theory at the E,J-resolved level; the results show that the channel SiH4 (+) -> SiH2 (+) + H-2 is predominate under PEVCD conditions. For H- and H-2-capturing by SiH3 (+) and SiH2 (+) ions, respectively, the rate constants were found to be weakly dependent on temperature at the high-pressure limit and decrease rapidly with pressure. The calculated heats of formation of the SiH (x) (+) (x = 2-4) ions are in close agreement with available thermochemical data.