Browsing by Author "Lee, Yun-Min"

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Ab Initio Chemical Kinetics for Reactions of H Atoms with SiHx (x=1-3) Radicals and Related Unimolecular Decomposition Processes
(2013-06-15)
Raghunath, Putikam
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Lee, Yun-Min
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Wu, Shang-Ying
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Wu, Jong-Shinn
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Lin, Ming-Chang
Hydrogen atoms and SiHx (x = 1-3) radicals coexist during the chemical vapor deposition (CVD) of hydrogenated amorphous silicon (a-Si: H) thin films for Si-solar cell fabrication, a technology necessitated recently by the need for energy and material conservation. The kinetics and mechanisms for H-atom reactions with SiHx radicals and the thermal decomposition of their intermediates have been investigated by using a high high-level ab initio molecular-orbital CCSD (Coupled Cluster with Single and Double)(T)/CBS (complete basis set extrapolation) method. These reactions occurring primarily by association producing excited intermediates, (SiH2)-Si-1, (SiH2)-Si-3, SiH3, and SiH4, with no intrinsic barriers were computed to have 75.6, 55.0, 68.5, and 90.2 kcal/mol association energies for x = 1-3, respectively, based on the computed heats of formation of these radicals. The excited intermediates can further fragment by H-2 elimination with 62.5, 44.3, 47.5, and 56.7 kcal/mol barriers giving Si-1, Si-3, SiH, and (SiH2)-Si-1 from the above respective intermediates. The predicted heats of reaction and enthalpies of formation of the radicals at 0 K, including the latter evaluated by the isodesmic reactions, SiHx + CH4 SiH4 + CHx, are in good agreement with available experimental data within reported errors. Furthermore, the rate constants for the forward and unimolecular reactions have been predicted with tunneling corrections using transition state theory (for direct abstraction) and variational Rice-Ramsperger-Kassel-Marcus theory (for association/decomposition) by solving the master equation covering the P, T-conditions commonly employed used in industrial CVD processes. The predicted results compare well experimental and/or computational data available in the literature. (C) 2013 Wiley Periodicals, Inc.
Ab Initio Chemical Kinetics for the Unimolecular Decomposition of Si2H5 Radical and Related Reverse Bimolecular Reactions
(2014-02-15)
Wu, Shang-Ying
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Lee, Yun-Min
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Wu, Jong-Shinn
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Lin, Ming-Chang
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機械工程學系
For plasma enhanced and catalytic chemical vapor deposition (PECVD and Cat-CVD) processes using small silanes as precursors, disilanyl radical (Si2H5) 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 Si2H5 producing smaller silicon hydrides including SiH, SiH2, SiH3, and Si2H4, 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+SiH4 that proceeds via a transition state with a barrier of 33.4 kcal/mol relative to Si2H5. Additionally, the dissociation energies for breaking the SiSi and HSiH2 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, SiH2, HSiSiH2, and Si2H4(C-2h) at 0 K by using the isodesmic reactions, such as (HSiSiH2)-H-2+(C2H6Si2H6)-C-1-Si-1+(HCCH2)-H-2 and (Si2H4)-Si-1(C-2h)+(C2H6)-C-1 (Si2H6)-Si-1+(C2H4)-C-1. The results of SiH (87.2 kcal/mol), SiH2 (64.9 kcal/mol), HSiSiH2 (98.0 kcal/mol), and Si2H4 (68.9 kcal/mol) agree reasonably well previous published data. Furthermore, the rate constants for the decomposition of Si2H5 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+SiH4 product pair is most favored in the decomposition as well as in the bimolecular reactions of SiH2+SiH3, HSiSiH2+H-2, and Si2H4(C-2h)+H under T, P-conditions typically used in PECVD and Cat-CVD. (c) 2013 Wiley Periodicals, Inc.
Capturing H and H-2 by SiHx+ (x <= 4) ions: Comparison between Langevin and quantum statistical models
(2017-02)
Trong-Nghia Nguyen
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Lee, Yun-Min
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Wu, Jong-Shinn
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Lin, Ming-Chang
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機械工程學系
H, H-2, and SiHx+ (x <= 4) ions coexist under plasma-enhanced chemical vapor deposition (PECVD) conditions. We have studied the kinetics of their interactions by high-level quantum chemical and statistical theory calculations, and compared the results with classical Langevin values (similar to 2 x 10(-9)cm(3) molecule(-1) s(-1) independent of temperature). The results indicate that, for H capturing by SiHx+ (x <= 4), both theories agree within a factor of 2-4, whereas for H-2 capturing by SiHx+ (x <= 3), the modern theory gives higher and weakly temperature-dependent values by up to more than one order of magnitude, attributable to reaction path degeneracies and increased entropies of activation. The heats of formation and structural parameters of SiHx+ ions (x <= 5) in this work agree well with available experimental data. For practical applications, we have provided tables of rate constants for modeling various processes of relevance to the PECVD of a-Si: H films. (C) 2017 The Japan Society of Applied Physics
Lande subtraction method with finite integration limits and application to strong-field problems
(2012-12-11)
Jiang, Tsin-Fu
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Jheng, Shih-Da
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Lee, Yun-Min
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Su, Zheng-Yao
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物理研究所
The Lande subtraction method has been widely used in Coulomb problems, but the momentum coordinate p is an element of (0,infinity) is assumed. In past applications, a very large range of p was used for accuracy. We derive the supplementary formulation with p is an element of (0, p(max)) at reasonably small p(max) for practical calculations. With the recipe, accuracy of the hydrogenic eigenspectrum is dramatically improved compared to the ordinary Lande formula by the same momentum grids. We apply the present formulation to strong-field atomic above-threshold ionization and high-order harmonic generations. We demonstrate that the proposed momentum space method can be another practical theoretical tool for atomic strong-field problems in addition to the existing methods. DOI:10.1103/PhysRevE.86.066702