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.     

The gas-phase decomposition of methylsilane. Part II. Mechanisms of decomposition under static system conditions

The static system pyrolysis of methylsilane (T ～ 700 K, P_T ～ 150 torr), pure and in the presence of ethylene, propylene, and acetylene, has been investigated. It is proposed that in the uninhibited system, the major products (silane and dimethylsilane) are produced by free radical processes, and that the free radicals are formed at the walls from methylsilylenes. In the presence of olefins, the free radicals are trapped to form methylsilane adducts. In acetylene, trapping of methylsilylenes prevents free radical production and eliminates the free radical produced products of the pure and the olefin inhibited systems. Rates of initiation correlate with rates of reactant loss in acetylene inhibited systems, and with rates of hydrogen formation in olefin inhibited systems. Rough estimates of primary dissociation process yields give for the 1,1-H₂ elimination ?_1,1 ? 0.78, for the 1,2-H elimination ?_1,2 ? 0.16, and for the methane elimination ?CH₄ ? 0.06 at 700 K. Deuteration lowers initial step kinetics by about 15%.     

Ion and radical reactions in the silane glow discharge deposition of a-Si:H films

A mass spectrometric analysis of the positive ions and neutral products in a silane glow discharge has been performed. The active species, created by dissociation, disproportionation, and ion-molecule reactions are mainly SiH₂, SiH₃, and H. A calculation of the distribution of the SiH _n ⁺ ions shows that the silane concentration monitors the abundance of SiH ₃ ⁺ . The diffusional transport of radicals toward the discharge-tube walls can explain the observed deposition rates. The study of SiH₄-SiD₄ and SiH₄-D₂ plasmas emphasizes several reactions which modify the free-radical populations depending on the discharge conditions: disproportionation, termination, recombination, and abstraction. Heterogeneous reactions have also been observed: etching of the film by H atoms and direct incorporation of hydrogen in the growing film. A general scheme for the plasma deposition mechanism is proposed.     

CO2 laser induced reactions of chlorotrifluoroethene and 1,2-dichlorodifluoroethene with silane

Gas-phase reactions in C1FC:CF₂/SiH₄ and C1FC:CFCl/SiH₄ mixtures at total pressure arround 1 Torr induced by pulsed CO₂ laser radiation yield different products depending on whether the single laser irradiating wavelength is tuned to absorption band of olefin or silane. Channels assumed to explain variety of products are multiphoton dissociation of the olefin into carbenes, carbenes recombination and addition to parent olefin, substitution of halogen in transient carbenes by hydrogen of silane, and 1,2-migration of halogen in transient CFCl:CF^. radical.     

Bildung siliciumorganischer Verbindungen. 77. Zur Bildung von Carbosilanen aus Methylsilanen

Formation of Organosilicon Compounds. 77. Formation of Carbosilanes from Methylsilanes The products formed by pyrolysis of me₃SiH, me₂SiH₂, and meSiH₃ are reported. Sime₄ and the mentioned methylsilanes were reacted in a plasma, and the products are compared to those of the pyrolysis. The pyrolysis of me₃SiH and me₂SiH₂ essentially yields the same groups of carbosilanes which are accessible by thermal decomposition of Sime₄, if the range is restricted to compounds with 4 Si atoms at most. Cylic carbosilanes are the main products of the pyrolysis of me₃SiH, and amoung these, 1,3,5-trisilacyclohexanes and 1,3,5,7-tetrasilaadamantanes are preferrently formed. From me₂SiH₂ above all linear compounds as 1,3-disilapropanes are obtained. This is attributed to the chosen experimental procedure in which they not subject to further reaction. In the pyrolysis of meSiH₃ a yellow solid is formed besides little amounts of meH₂Si? SiH₂me. Compared to the compounds formed by pyrolysis of Sime₄, the carbosilasen obtained from me₃SiH and me₂SiH₂ possess more SiH substituents. Also the decomposition of Sime₄ in a plasma preferrently yields carbosilanes, mainly linear compounds with 2 or 3 Si atoms.     

Chemiluminescent spectra of HNO and DNO in the reaction of O(3P)/O2 with NO and hydrocarbons of aldehydes

The chemiluminescent emission from the electronically excited state of HNO (¹A″) was observed in the reactions of O(³P)/O₂ with NO and one of acetylene, ethylene, propylene, formaldehyde and acetaldehyde. The identification of the emitting species was confirmed by observing the emission from DNO in the reactin of O/O₂ with NO and ethylene-d₄. The emission spectra were different from those observed in the reaction of H(D) + NO + M.     

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.     

Mechanism and kinetics of the shock-tube decomposition of vinylsilane

The shock-induced thermal decompositions of vinylsilane and vinylsilane-d₃ (0.2% on argon) have been studied in the temperature range of 1085–1275 K, and at total pressures of about 3100 torr. In systems without silylene traps, some induced decomposition occurs which is attributed to the silylene chain sequence VSiH → C₂H₂ + SiH₂, S?iH₂ + VSiH₃ ? VSiH₂SiH₃ → VSiH₂S?iH + H₂, VSiH₂S?iH → VSiH + S?iH₂. In the presence of silylene traps (butadiene and acetylene), the overall decomposition kinetics are log k(VSiH₃, s^?1) = 14.95 ? 63,268 cal/2.303RT and log k(VSiD₃, s^?1) = 15.14 ? 64,815 cal/2.303RT. Three primary processes contribute to the decomposition: 1,1-H₂ elimination, 1,2-H₂ elimination, and ethylene elimination. Two mechanisms are proposed, one for exclusive primary process formation of C₂H₄, and the other for both primary and secondary formation routes. Modeling studies are reported which show that both mechanisms can be made compatible with the rate and product yield data within experimental errors.     

Dipole oscillator strength properties and dispersion energies for SiH4

A recommended isotropic dipole oscillator strength distribution (DOSD) has been constructed for the silane (SiH₄) molecule through the use of quantum mechanical constraint techniques and experimental dipole oscillator strength data. The constraints are furnished by experimental molar refractivity data and the Thomas–Reiche–Kuhn sum rule. The DOSD is used to evaluate a variety of isotropic dipole oscillator strength sums, logarithmic dipole oscillator strength sums, and mean excitation energies for the molecule. A pseudo-DOSD for SiH₄ is also presented which is used to obtain reliable results for the isotropic dipole–dipole dispersion energy coefficients C₆, for the interaction of silane with itself and with forty-four other species, and the triple–dipole dispersion energy coefficient C₉ for (SiH₄)₃.     

Open questions regarding the mechanism of plasma-induced deposition of silicon

Recently published values of the rate constant for the insertion of silylene into silane have been used to reevaluate our earlier estimates of the critical coil cell ration of silane, [SiH₄]_crit above which the formation of disilane dominates the plasma-induced deposition of silicon. Because the recently published values of the rate constant are significantly higher than those available at the time of writing of our earlier paper, the new values on [SiH₄]_crit are significantly lower than the earlier ones. It is shown that there is no unambiguous experimental evidence for SiH₃ to be the dominant species for the deposition of crystalline .silicon. Disilane formation and insertion of silylene into the surface o the growing filin mar explain the data as well. Several open questions are addressed.     

Reactivity of the Indenyl Radical (C9H7) with Acetylene (C2H2) and Vinylacetylene (C4H4)

The reactions of the indenyl radicals with acetylene (C₂H₂) and vinylacetylene (C₄H₄) is studied in a hot chemical reactor coupled to synchrotron based vacuum ultraviolet ionization mass spectrometry. These experimental results are combined with theory to reveal that the resonantly stabilized and thermodynamically most stable 1-indenyl radical (C₉H₇^.) is always formed in the pyrolysis of 1-, 2-, 6-, and 7-bromoindenes at 1500 K. The 1-indenyl radical reacts with acetylene yielding 1-ethynylindene plus atomic hydrogen, rather than adding a second acetylene molecule and leading to ring closure and formation of fluorene as observed in other reaction mechanisms such as the hydrogen abstraction acetylene addition or hydrogen abstraction vinylacetylene addition pathways. While this reaction mechanism is analogous to the bimolecular reaction between the phenyl radical (C₆H₅^.) and acetylene forming phenylacetylene (C₆H₅CCH), the 1-indenyl+acetylene→1-ethynylindene+hydrogen reaction is highly endoergic (114 kJ mol⁻¹) and slow, contrary to the exoergic (−38 kJ mol⁻¹) and faster phenyl+acetylene→phenylacetylene+hydrogen reaction. In a similar manner, no ring closure leading to fluorene formation was observed in the reaction of 1-indenyl radical with vinylacetylene. These experimental results are explained through rate constant calculations based on theoretically derived potential energy surfaces.     

Computational study on fluorine atom reaction with silane molecule (SiH4)

The fluorine atom’s reaction with silane molecule (SiH₄) is investigated in this work. Two reaction channels which form SiH₃+HF and SiH₃F+H are discussed in the microscopic level. The analyses of transition states show that the SiH₃+HF channel proceeds through a direct hydrogen abstract mechanism and the SiH₃F+H channel could take place via the substitution mechanism. The energetic information of the potential energy surface has been obtained using high-level ab initio molecular orbital theory. A dual-level direct dynamics method is employed to calculate the rate constants of the title reaction. The rate constants of the hydrogen abstraction channel are much larger than the substitution channel. The calculated rate constants are in best agreement with available experimental result.     

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.     

Pyrolysis of trichlorosilane in the presence of chloroform

The pyrolysis of trichlorosilane in the presence of different amounts of chloroform and the copyrolysis of HSiCl₃ with buta-1,3-diene in the presence of 1 mol.% chloroform were studied. The enthalpies of formation of products resulting from the pyrolysis of HSiCl₃ in the presence of chloroform were calculated by the quantum chemical method. Based on the thermochemical data as well as data from GLC and mass spectrometry, it was concluded from the condensate composition that introduction of chloroform into the zone of pyrolysis of HSiCl₃ favors generation of silylenes.     

Dissociative photoionization of SiH4 in the 12–19 eV region

Relative photoexcitation spectra of ionic fragments SiH⁺₃, SiH⁺₂, SiH⁺ and Si⁺ originating from photofragmentation of molecular silane (SiH₄) have been measured in the 12–19 eV region using a time-of-flight mass-analytic method combined with synchrotron radiation as a light source. No SiH⁺₄ could be detected in a measurement time of a few microseconds. The excitation spectra show a maximum photoionization efficiency around 13.3 eV for SiH⁺₂ and around 14.8 eV for SiH⁺₃. Correlations between photoionization and dissociation are discussed. The assignment of the autoionization structure between 15 and 18 eV by Börlin et al. has been positively examined and extended to include two vibrational progressions composed of the ν₁ mode with a frequency of 1690 ± 60 cm⁻¹ and the ν₂ mode with 730 ± 80 cm⁻¹.     

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).     

Directed Gas Phase Formation of Silene (H2SiCH2)

The silene molecule (H₂SiCH₂; X¹A₁) has been synthesized under single collision conditions via the bimolecular gas phase reaction of ground state methylidyne radicals (CH) with silane (SiH₄). Exploiting crossed molecular beams experiments augmented by high-level electronic structure calculations, the elementary reaction commenced on the doublet surface through a barrierless insertion of the methylidyne radical into a silicon-hydrogen bond forming the silylmethyl (CH₂SiH₃; X²A′) complex followed by hydrogen migration to the methylsilyl radical (SiH₂CH₃; X²A′). Both silylmethyl and methylsilyl intermediates undergo unimolecular hydrogen loss to silene (H₂SiCH₂; X¹A₁). The exploration of the elementary reaction of methylidyne with silane delivers a unique view at the widely uncharted reaction dynamics and isomerization processes of the carbon–silicon system in the gas phase, which are noticeably different from those of the isovalent carbon system thus contributing to our knowledge on carbon silicon bond couplings at the molecular level.     

Condensation of Argon,Monosilane and Their Mixtures in a Pulse Free Jet

Pulse supersonic outflow of Ar, SiH₄ and Ar + SiH₄ gas mixture (where monosilane is a small admixture) was studied experimentally by the method of molecular beam mass-spectrometry. Using argon as an example we have shown that condensation processes at the quasi-stationary region of a pulse flow and within a stationary jet are similar. In the flows of pure gases clusters of argon and silane (hydrogenated silicon) and in the mixture argon – silane complexes were registered. The dependencies of the intensities of monomer and cluster ions on stagnation pressure were investigated. It was shown that in the mixture jet at low stagnation pressures the condensation process with the formation of monosilane clusters takes place and at high pressures mixed argon-silane complexes are formed. The parameters of flow transition into the regime of developed condensation were determined for pure gases and their mixture.     

Formation of propylene in the reaction of methane with acetylene on a NiOx/BN heterogeneous catalyst

Reaction of methane with acetylene in the presence of a heterogeneous NiO_x/BN catalyst in the temperature range 300–450C results in the formation of propylene (1% yield). Using¹³CH₄ it was found that propylene arises both as a product of acetylene conversion and as a hydromethylation product of C₂H₂ with methane. The ratio of heavy and light C₃H₆ in the product mixture was 14.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2478–2481, November, 1990.     

On the mechanism of the OH initiated oxidation of acetylene in the presence of O2 and NO x

The mechanism of the oxidation of acetylene, in the presence of O₂ and NO _x , has been studied. Different levels of theory have been tested for the first step of the mechanism: the acetylene + OH radical reaction. Based on these results the meta-hybrid functional MPWB1K has been chosen for modeling all the other steps involved in the oxidation of acetylene. Different reaction paths have been considered and the one leading to glyoxal formation and OH regeneration is predicted to be the main channel, independently of the presence of NO _x . Two different mechanisms were modeled to account for formic acid formation, both of them involving cyclic intermediates. According to the computed activation free energies, the three-membered intermediate seems to be more likely to occur than the four-membered one. However, reaction barriers are very high and only a very small proportion of formic acid is expected to be formed through such intermediates. In the presence of NO _x , considered in this work for the first time, the main product of the tropospheric oxidation of acetylene is also expected to be glyoxal.