Ab initio chemical kinetics for the unimolecular decomposition of Si2H5 radical and related reverse bimolecular reactions

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, SiH_3, 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(C_2h) at 0 K by using the isodesmic reactions, such as ²HSiSiH₂ + ¹C₂H₆→¹Si₂H₆ + ²HCCH₂ and ¹Si₂H₄(C_2h) + 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(C_2h) + H under T, P‐conditions typically used in PECVD and Cat‐CVD. © 2013 Wiley Periodicals, Inc.     

An iets study of alumina and magnesia supported —sihx and their use as substrates for inorganic vibrational spectroscopy

Tunneling spectra of Al₂O₃/—SiH_x, MgO/—SiH_x, Al₂O₃/—SiD_x, and Al₂O₃/—SiH_x + NCS^? are reported. Analysis of the vibrational spectra observed from isotopic substitution of the barriers obtained by deposition of a thin film of SiO onto alumina and magnesia indicate that the supported species is —SiH. The Al₂O₃/—SiH barrier can be used as a support for studying inorganic ions by IETS.     

The reactions between negative hydrogen ions and silane

Contracted CI-calculations have been performed in order to find out the mechanisms of the reactions involved when negative hydrogen ions react with silane. There were initially severe problems to find a balanced basis set to describe the reactions including correlation, particularly for the choice of diffuse functions. Finally, in agreement with earlier calculations, SiH ₅ ^– was found to be more stable than SiH₄+H^– by 21 kcal/mol but less stable than SiH ₃ ^– and H₂ by 6 kcal/mol. A barrier in the S _N2 reaction SiH₄+H^– SiH₅ has previously been predicted by calculations, which was not confirmed by the present CI calculations. The lack of a barrier is in agreement with experimental evidence. Contrary to what is expected from the orbital symmetry rules, which predict two allowed pathways, SiH ₅ ^– does not dissociate easily to the lower lying SiH ₃ ^– + H₂. A barrier of 57 kcal/mol, which was very difficult to locate, was finally found. In order to explain the experimental observation of SiH ₃ ^– and the lack of observation of SiH ₅ ^– a different mechanism for the reaction SiH₄+H^– SiH ₃ ^– + H₂ is suggested. For a direct proton transfer a barrier of less than 10 kcal/mol is predicted.     

Emission cross sections from fragments produced by electron impact on silane

Optical emission spectroscopy in the visible and near UV of a silane plasma was performed in a low pressure hot cathode glow discharge bounded into a magnetized multipolar wall. Emissions from Si, Si⁺, SiH, SiH⁺ and H are shown to originate from the dissociative excitation of silane molecules by electron impact. The absolute cross sections for the various photoemissive processes were measured in the 17–68 eV range. The relevance of optical emission spectroscopy to silane plasma diagnostics is discussed.     

Ab initio chemical kinetics for reactions of H atoms with SiHx (x = 1–3) radicals and related unimolecular decomposition processes

Hydrogen atoms and SiH_x (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 SiH_x 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, ¹SiH₂, ³SiH₂, 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 H₂ elimination with 62.5, 44.3, 47.5, and 56.7 kcal/mol barriers giving ¹Si, ³Si, SiH, and ¹SiH₂ 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, SiH_x + CH₄ = SiH₄ + CH_x, 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. © 2013 Wiley Periodicals, Inc.     

Stationary and non-stationary etching of Si(100) surfaces with gas phase and adsorbed hydrogen

Stationary and non-stationary etching of Si(100) surfaces by hydrogen were studied between 200 K and 800 K using direct product detection and thermal desorption spectroscopy. Silane was the only etch product observed. The rates of silane SiD_nH_4−n isotopes measured during etching D-saturated Si(100) surfaces with gaseous H illustrate that the etch reaction proceeds between surface silyl and incoming H in a direct (Eley–Rideal or hot-atom) reaction step: H(g)+SiD₃(ad)→SiD₃H(g). Non-stationary etching via silane desorption occurs through disproportionation between surface dihydride and silyl groups, SiH₂(ad)+SiH₃(ad)→SiH₄(g).     

Neutral and ion chemistry in a C2H4-SiH4 discharge

This paper reports on a mass spectrometric study of the neutral and ionic species in a low-pressure rf discharge sustained in a C₂H₄-SiH₄ mixture diluted in helium. It is shown that C₂H₄ is readily decomposed into C₂H ₂ ^* and C₂H₃. The formation of secondary products such as C₄H₂, C₄H₄, and C₄H₆ is observed and confirms the presence of C₂H₂ in the discharge. Methylsilane (CH₃SiH₃) and ethylsilane (C₂H₅SiH₃) are also synthesized in this discharge. It is also observed that the major ions C₂H ₄ ⁺ , C₃H ₅ ⁺ , SiH ₃ ⁺ , Si₂H ₄ ⁺ , SiCH ₃ ⁺ , SiC₂H ₃ ⁺ , and SiC₂H ₇ ⁺ are not representative of the direct ionization of neutral species. Their formation is thus interpreted on the basis of ion-molecule reactions.     

Kinetics and mechanism of the silane decomposition

The homogeneous gas-phase decomposition kinetics of silane has been investigated using the single-pulse shock tube comparative rate technique (T = 1035–1184?K, P_total ≈? 4000 Torr). The initial reaction of the decomposition SiH₄ \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm SiH}_{\rm 4} \mathop \to \limits^1 {\rm SiH}_{\rm 2} + {\rm H}_{\rm 2} $\end{document} SiH₂ + H₂ is a unimolecular process in its pressure fall-off regime with experimental Arrhenius parameters of logk₁ (sec^?1) = 13.33 ± 0.28–52,700 ± 1400/2.303RT. The decomposition has also been studied at lower temperatures by conventional methods. The results confirm the total pressure effect, indicate a small but not negligible extent of induced reaction, and show that the decomposition is first order in silane at constant total pressures. RRKM-pressure fall-off calculations for four different transition-state models are reported, and good agreement with all the data is obtained with a model whose high-pressure parameters are logA₁ (sec^?1) = 15.5, E_1(∞) = 56.9 kcal, and ΔE^0±₀(1) = 55.9 kcal. The mechanism of the decomposition is discussed, and it is concluded that hydrogen atoms are not involved. It is further suggested that silylene in the pure silane pyrolysis ultimately reacts with itself to give hydrogen: 2SiH₂ → (Si₂H₄)* → (SiH₃SiH)* → Si₂H₂ + H₂. The mechanism of H ? D exchange absorbed in the pyrolysis of SiD₄-hydrocarbon systems is also discussed.     

Ion/Molecule reactions in SiH4/H2S and GeH4/H2S mixtures

The gas phase ion chemistry of silane/hydrogen sulfide and germane/hydrogen sulfide mixtures was studied by ion trap mass spectrometry (ITMS), in both positive and negative ionization mode. In positive ionization, formation of X/S (X = Si, Ge) mixed ions mainly takes place via reactions of silane or germane ions with H₂S, through condensation followed by dehydrogenation. This is particularly evident in the system with silane. On the other side, reactions of H_nS₂⁺ ions with XH₄ (X = Si, Ge) invariably lead to formation of a single X? S bond. In negative ionization, a more limited number of mixed ion species is detected, but their overall abundance reaches appreciable values, especially in the SiH₄/H₂S system. Present results clearly indicate that ion processes play an important role in formation and growth of clusters eventually leading to deposition of amorphous solids in chemical vapor deposition (CVD) processes. Copyright © 2009 John Wiley & Sons, Ltd.     

Modellrechnungen an einigen Siliziumwasserstoffverbindungen vom Typ SiHn,SiH n + und SiH n − nach der Einzentrenmethode

Zusammenfassung Es werden molekulare Struktur, Energie des Grundzustandes, Bindungsabstände, Bindungsenergie, Ionisierungsenergie und Protonenaffinität der Siliziumwasserstoffverbindungen SiH _n , SiH _n ⁺ und SiH_n ^– (n=3, 4 oder 5) nach der Einzentrenmethode berechnet.

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OCE-calculations on some silicon hydrides of the type SiH _n , SiH _n ⁺ and SiH _n ^–

OCE-Calculations are reported for molecular structures, ground state energies, bond distances, binding energies, ionization potentials and proton affinities of the silicon hydrides SiH _n , SiH _n ⁺ and SiH _n ^– (n=3, 4 or 5).

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Résumé Calcul par la méthode monocentrique de la structure moléculaire, de l'énergie de l'état fondamental, des longueurs de liaison, des énergies de liaison, des potentiels d'ionisation et des affinités protoniques pour les hydrures de silicium SiH _n , SiH _n ⁺ et SiH _n ^– (n=3, 4 ou 5).

     

Thermal decomposition kinetics of polysilanes: Disilane,trisilane, and tetrasilane

The decomposition kinetics of disilane with added butadiene, trisilane both neat and with added butadiene, trimethylsilane or H₂, and normal and iso-tetrasilane both neat and in the presence of added butadiene are reported. Arrhenius parameters of the primary dissociation reactions are determined: A-factors suggest that polysilane decompositions (1) have similar intrinsic activation entropies (ΔS? ≈? 6.2 ± 5 e.u.) and (2) have activation energies which increase with increasing reaction endothermicities. Relative trapping efficiencies of SiH₄, Si₂H₆, Si₃H₈, C₄H₆, Me₃SiH, and H₂ toward SiH₂ and SiH₃SiH are also determined. Other results include the heat of formation of silylsilylene, ΔH ${}_{\text{°}}^{\text{f}}$ (SiH₃SiH) = 75.3 Kcal/mol, and the activation energy for 1,1-H₂ elimination from disilane (EH₂ = 57.8 kcal/mol).     

Reactions of trimethylgallium with silicon hydrides,germane and diborane

Methylation of SiH₄, MeSiH₃, Si₂H₆, GeH₄ and B₂H₆, but not of PH₃ or AsH₃, was observed during reaction (230–324°C) with GaMe₃. The products from the SiH₄ and Si₂H₆ reactions were MeSiH₃, Me₂SiH₂ and Me₃SiH. The GeH₄-derived products were similar, with Me₄Ge also being formed. The only methylated products from B₂H₆ was BMe₃. The silane reactions were surface-catalyzed (presumably by surface hydroxyl groups), while those of GeH₄ and B₂H₆ may have occurred via gas-phase free radical processes.     

Spectroscopic detection of particles from shock-wave-induced decomposition of SiH4

Particle formation from silane pyrolysis was studied in a shock tube. Molecular and atomic species (SiH₂, SiH, Si₂, H₂, Si, H) were identified by thermal induced fluorescence at temperatures exceeding 3000 K. At temperatures below 2500 K, silicon cluster formation was detected by optical extinction measurements.     

Facile hydrosilylation of norbornadiene by silanes R3SiH and R2SiH2 with molybdenum catalysts

Photochemically activated [Mo(CO)₆] and [Mo(CO)₄(η⁴-nbd)] have been demonstrated to be very effective catalysts for hydrosilylation of norbornadiene (nbd) by tertiary (Et₃SiH, Cl₃SiH) and secondary (Et₂SiH₂ and Ph₂SiH₂) silanes to give 5-silyl-2-norbornene, which under the same reaction conditions transform in ring-opening metathesis polymerization (ROMP) to unsaturated polymers and to a double hydrosilylation product, 2,6-bis(silyl)norbornane. The yield of a particular reaction depends very strongly on the kind of silane involved. The reaction products were identified by means of chromatography (GC–MS) and ¹H and ¹³C NMR spectroscopy. In photochemical reaction of [Mo(CO)₄(η⁴-nbd)] and Ph₂SiH₂ in cyclohexane-d₁₂, η²-coordination of the SiH bond to the molybdenum atom is supported by ¹H NMR spectroscopy due to the detection of two equal-intensity doublets with ²J_HH = 5.4 Hz at δ 6.12 and −5.86 ppm.     

Molecular Beam Mass Spectrometry System for Characterization of Thermal Plasma Chemical Vapor Deposition

A molecular beam mass spectrometry system for in situ measurement of the concentration of gas phase species including radicals impinging on a substrate during thermal plasma chemical vapor deposition (TPCVD) has been designed and constructed. Dynamically controlled substrate temperature was achieved using a variable thermal contact resistance method via a backside flow of an argon/helium mixture. A high quality molecular beam with beamtobackground signal greater than 20 was obtained under film growth conditions by sampling through a small nozzle (75 m) in the center of the substrate. Mass discrimination effects were accounted for in order to quantify the species measurements. We demonstrate that this system has a minimum detection limit of under 100 ppb. Quantitative measurements of hydrocarbon species (H, H₂, C, CH₃, CH₄, C₂H₂, C₂H₄) using Ar/H₂/CH₄ mixtures and silicon species (Si, SiH, SiH₂, SiCl, SiCl₂, Cl, HCl) using Ar/H₂/SiCl₄ mixtures were obtained under thermal plasma chemical vapor deposition conditions.     

Hydrosilylation reactions catalysed by decacarbonyldimanganese(O)

Decacarbonyldimanganese(O) complex, Mn₂(CO)₁₀, has been evaluated as a catalyst for hydrosilylation reactions of 1-hexene with tertiary silanes, Et₃SiH and (EtO)₃SiH. The reaction of Et₃SiH appears to be first order with respect to the catalyst, to the hexene and to the silane, although catalyst deactivation occurs when relatively high silane concentrations are used. The reaction rate is slightly affected by varying the type of the silane used. The rate of disappearance of the tertiary silane is consistent with that of the 1-hexene, which means that the catalyst is selective to hydrosilylation reactions. This was confirmed by following the rates of disappearance of Si-H and CC IR bands at 2210, 2100 and 1650 cm⁻¹ for (EtO)₃SiH, Et₃SiH and 1-hexene respectively. A comparison of the behaviour of Mn₂(CO)₁₀ with that of Co₂(CO)₈ is reported here, together with a suggested mechanism for the manganese catalyst.     

Photochemical reactions of [W(CO)4(η-nbd)] with hydrosilanes: Generation of new hydrido complexes of tungsten and their reactivity

Photolysis of the norbornadiene (nbd) complex [W(CO)₄(η⁴-nbd)] (1) creates a coordinatively unsaturated d⁶ species which interacts with the Si-H bond of tertiary and secondary silanes (Cl₃SiH, Et₃SiH, Et₂SiH₂, Ph₂SiH₂) to yield hydride complexes of varying stability. In reaction of complex 1 with Cl₃SiH, oxidative addition of the Si-H bond to the tungsten(0) center gives the seven-coordinate tungsten(II) complex [WH(SiCl₃)(CO)₃(η⁴-nbd)], which has been fully characterized by NMR spectroscopic methods (¹H, ¹³C{¹H}, 2D ¹H-¹H COSY, 2D ¹³C-¹H HMQC and ²⁹Si{¹H}). Reaction of 1 with Et₃SiH leads to the hydrosilylation of the η⁴-nbd ligand to selectively yield endo-2-triethylsilylnorbornene (nbeSiEt₃). The latter silicon-substituted norbornene gives the unstable pentacarbonyl complex [W(CO)₅(η²-nbeSiEt₃)], whose conversion leads to the initiation of ring-opening metathesis polymerization (ROMP). Reaction of secondary silanes (Et₂SiH₂ and Ph₂SiH₂) with 1 leads to the hydrosilylation and hydrogenation of nbd and the formation of bis(silyl)norbornane and silylnorbornane as the major products. In reaction of 1 and Et₂SiH₂, the intermediate dihydride complex [WH(μ-H-SiEt₂)(CO)_x(η⁴-nbd)] was detected by ¹H and ¹³C NMR spectroscopy. As one of the products formed in photochemical reaction of W(CO)₆ with Ph₂SiH₂, the dinuclear complex [{W(μ-η²-H-SiPh₂)(CO)₄}₂] was identified by NMR spectroscopic methods.     

Mathematical simulation of gas-phase deposition of epitaxial silicon films in the Si-C-H and Si-H systems

The effect of experimental conditions on the magnitude and uniformity of the deposition rate of epitaxial silicon obtained by chemical deposition from the gas phase in the SiCl₄-H₂, SiHCl₃-H₂, and SiH₄-H₂ systems (in the temperature ranges from 1300 to 1520 K for the chloride and 1270 to 1370 K for the silane systems) has been examined. Chloride and silane processes are compared.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1217–1222, July, 1995.     

SIMS for hydrogen quantification and structural analysis of amorphous silicon germanium compounds

Summary Hydrogen has been analysed quantitatively in a-Si_1–xGe_x:H alloys by SIMS with H variation from 1×10¹⁹ –1.3×10²² atoms/cm³ and x between 0 and 1. To quantify the absolute H concentration, SIMS measurements have been calibrated with nuclear reaction analysis, which exhibits excellent agreement with SIMS data for the total range of H and Ge variance. From abundances of the molecule ions SiH⁺ and GeH⁺ the fractions H bound to Si or Ge can be discerned and are in good accordance with quantification of SiH and GeH stretching modes in IR spectroscopic measurements. Preferential attachment of H to Si compared to Ge by a factor of 3.5 is determined for glow discharge a-SiGe:H samples; during annealing up to 900 K only small changes of this factor are observed.     

Computational investigation of the adsorption and reactions of SiHx (x = 0–4) on TiO2 anatase (101) and rutile (110) surfaces

The adsorption and reactions of the SiH_x (x = 0–4) on Titanium dioxide (TiO₂) anatase (101) and rutile (110) surfaces have been studied by using periodic density functional theory in conjunction with the projected augmented wave approach. It is found that SiH_x (x = 0–4) can form the monodentate, bidentate, or tridentate adsorbates, depending on the value of x. H coadsorption is found to reduce the stability of SiH_x adsorption. Hydrogen migration on the TiO₂ surfaces is also discussed for elucidation of the SiH_x decomposition mechanism. Comparing adsorption energies, energy barriers, and potential energy profiles on the two TiO₂ surfaces, the SiH_x decomposition can occur more readily on the rutile (110) surface than on the anatase (101) surface. The results may be used for kinetic simulation of Si thin‐film deposition and quantum dot preparation on titania by chemical vapor deposition (CVD), plasma enhanced CVD, or catalytically enhanced CVD. © 2013 Wiley Periodicals, Inc.