Ab initiochemical kinetics for the unimolecular decomposition of Si₂H₅radical and related reverse bimolecular reactions

Abstract

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 SiSi and HSiH2 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(C2h) at 0 K by using the isodesmic reactions, such as 2HSiSiH2 + 1C2H6→1Si2H6 + 2HCCH2 and 1Si2H4(C2h) + 1C2H6 → 1Si2H6 + 1C2H4. 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 + H2, and Si2H4(C2h) + H under T, P‐conditions typically used in PECVD and Cat‐CVD. © 2013 Wiley Periodicals, Inc.

Ab Initio Chemical Kinetics for SiHxReactions with Si2Hy(x= 1,2,3,4;y= 6,5,4,3;x+y= 7) under a-Si:H CVD Condition

“…Table S2 in the Supporting Information shows that the theoretical and experimental frequencies and rotational constants of SiH 4 agrees within 2%. For the H 2 SiSiH and the H 3 SiSi isomers, our structural parameters are very consistent with previous theoretical predictions .…”

“…One can see that our predicted enthalpies of formation at 0 K of SiH (89.9 kcal/mol), SiH 2 (65.3 kcal/mol), Si 2 H 5 (59.5 kcal/mol), n ‐Si 3 H 7 (69.0 kcal/mol), and i ‐Si 3 H 7 (67.0 kcal/mol) are in good agreement with experimental results, 89.6 ± 2.0, 65.6 ± 0.7, 59.2, 69.3, and 67.8 kcal/mol, respectively (see Table ). For H 3 SiSiH and H 2 SiSiH, no experimental data are available; our predicted enthalpies of formation at 0 K, 77.4 and 97.8 kcal/mol, are also very close to 77.3 and 98.0 kcal/mol calculated at the QCISD(T)/CBS//QCISD/6–311++G(d,p) and CCSD(T)/6–311++G(3df,2p)//CCSD (T)/6–311+G(d,p) levels of theory, respectively. These results reflect the reliability of the computational scheme aided by CBS extrapolation used in this work.…”

Capturing H and H2 by SiHx + (x ≤ 4) ions: Comparison between Langevin and quantum statistical models

“…We have recently established the kinetic database for many neutral SiH x radical reactions on the basis of such an effort. [7][8][9][10][11][12][13] Under a typical PECVD experimental condition with SiH 4 as a precursor gas, its molecular ions, such as SiH x + (x = 1-4), often coexist with neutral species including H and H 2 in the PECVD process. The ionic species including SiH 4 + , SiH 3 + , SiH 2 + , SiH + , and Si + generated in a silane plasma have been studied by mass spectroscopy.…”