Formation Of Disilane Research Articles

Formation of Higher Silanes in Low-Temperature Silane (SiH4) Ices.

A novel approach for the synthesis and identification of higher silanes (SinH2n+2, where n ≤ 19) is presented. Thin films of (d4-)silane deposited onto a cold surface were exposed under ultra-high-vacuum conditions to energetic electrons and sampled on line and in situ via infrared and ultraviolet-visible spectroscopy. Gas phase products released by fractional sublimation in the warm-up phase after the irradiation were probed via a reflectron time-of-flight mass spectrometer coupled with a tunable vacuum ultraviolet photon ionization source. The formation mechanisms of (higher) silanes were investigated by irradiating codeposited 1:1 silane (SiH4)/d4-silane (SiD4) ices, suggesting that both radical-radical recombination and radical insertion pathways contribute to the formation of disilane along with higher silanes up to nonadecasilane (Si19H40).

Analysis of the Gas Phase Reactivity of Chlorosilanes

Trichlorosilane is the most used precursor to deposit silicon for photovoltaic applications. Despite of this, its gas phase and surface kinetics have not yet been completely understood. In the present work, it is reported a systematic investigation aimed at determining what is the dominant gas phase chemistry active during the chemical vapor deposition of Si from trichlorosilane. The gas phase mechanism was developed calculating the rate constant of each reaction using conventional transition state theory in the rigid rotor-harmonic oscillator approximation. Torsional vibrations were described using a hindered rotor model. Structures and vibrational frequencies of reactants and transition states were determined at the B3LYP/6-31+G(d,p) level, while potential energy surfaces and activation energies were computed at the CCSD(T) level using aug-cc-pVDZ and aug-cc-pVTZ basis sets extrapolating to the complete basis set limit. As gas phase and surface reactivities are mutually interlinked, simulations were performed using a microkinetic surface mechanism. It was found that the gas phase reactivity follows two different routes. The disilane mechanism, in which the formation of disilanes as reaction intermediates favors the conversion between the most stable monosilane species, and the radical pathway, initiated by the decomposition of Si2HCl5 and followed by a series of fast propagation reactions. Though both mechanisms are active during deposition, the simulations revealed that above a certain temperature and conversion threshold the radical mechanism provides a faster route for the conversion of SiHCl3 into SiCl4, a reaction that favors the overall Si deposition process as it is associated with the consumption of HCl, a fast etchant of Si. Also, this study shows that the formation of disilanes as reactant intermediates promotes significantly the gas phase reactivity, as they contribute both to the initiation of radical chain mechanisms and provide a catalytic route for the conversion between the most stable monosilanes.

Interactions between radical growth precursors on plasma-deposited silicon thin-film surfaces

We present a detailed analysis of the interactions between growth precursors, SiH3 radicals, on surfaces of silicon thin films. The analysis is based on a synergistic combination of density functional theory calculations on the hydrogen-terminated Si(001)-(2x1) surface and molecular-dynamics (MD) simulations of film growth on surfaces of MD-generated hydrogenated amorphous silicon (a-Si:H) thin films. In particular, the authors find that two interacting growth precursors may either form disilane (Si2H6) and desorb from the surface, or disproportionate, resulting in the formation of a surface dihydride (adsorbed SiH2 species) and gas-phase silane (SiH4). The reaction barrier for disilane formation is found to be strongly dependent on the local chemical environment on the silicon surface and reduces (or vanishes) if one/both of the interacting precursors is/are in a "fast diffusing state," i.e., attached to fivefold coordinated surface Si atoms. Finally, activation energy barriers in excess of 1 eV are obtained for two chemisorbed (i.e., bonded to a fourfold coordinated surface Si atom) SiH3 radicals. Activation energy barriers for disproportionation follow the same tendency, though, in most cases, higher barriers are obtained compared to disilane formation reactions starting from the same initial configuration. MD simulations confirm that disilane formation and disproportionation reactions also occur on a-Si:H growth surfaces, preferentially in configurations where at least one of the SiH3 radicals is in a "diffusive state." Our results are in agreement with experimental observations and results of plasma process simulators showing that the primary source for disilane in low-power plasmas may be the substrate surface.

Formation of Disilanes in the Reaction of Stable Silylenes with Halocarbons

Reactions of stable silylenes 1 and 2 with a variety of halogenated organic compounds have been studied. Depending on the nature of the halocarbon, the products can be disilanes, LSiX-LSiR, or monosilanes, LSiXR, or a mixture of both types of products. Hexachloroethane reacts with the silylenes to give mainly the dichlorodisilane, LSiX-LSiX. The experimental results are rationalized in terms of several related free-radical chain mechanisms.

Atomistic calculation of the SiH 3 surface reactivity during plasma deposition of amorphous silicon thin films

We report a direct, statistically significant calculation of the surface reactivity of the SiH 3 radical on hydrogenated amorphous silicon (a-Si:H) using molecular-dynamics simulations of repeated impingement of SiH 3 radicals on growth surfaces of smooth a-Si:H films over the temperature range 475–800 K. SiH 3 can either incorporate into the film by adsorbing onto a surface Si dangling bond or inserting into Si–Si bonds (sticking), or abstract surface H through Eley–Rideal (ER) or Langmuir–Hinshelwood (LH) pathways to produce SiH 4 gas, or react with another surface SiH 3 to desorb as Si 2H 6 (recombination), or leave the film by reflection or desorption. The overall surface reaction probability, β, includes both radical sticking and recombination. In agreement with experimental measurements, β is almost constant over the temperature range studied, as are the probabilities for sticking and recombination, s and γ, respectively; the calculated mean value of β is 0.47 ± 0.03. Energetic analysis of the various surface reactions shows that radical adsorption, radical insertion, and ER abstraction are barrierless processes, which explains the measured temperature independence of β. LH abstraction is activated, but competes with disilane formation, yielding a temperature-independent γ. Also, LH abstraction leads to H elimination from a-Si:H during growth and can partly explain the experimentally measured temperature dependence of the H content in the a-Si:H film.

Gas‐Phase Reaction Pathways from SiH4 to Si2H6, Si2H4 and Si2H2: A Theoretical Study

AbstractFor Abstract see ChemInform Abstract in Full Text.

Density functional study of the mechanism of halophilic reactions of some silylenes

The hybrid density functional B3LYP method has been used to study the mechanism of disilane formation from silylenes. The main findings can be summarized as follows: (a) Lewis acid–base complexes between silylenes and halocarbons only play a limited role in silylene insertions, and therefore the acid–base complex mechanism proposed by West et al. does not apply to disilane formation. (b) Regardless of whether transient or stable, all silylenes follow the energetically favorable general reaction pathway: (I) Y 2Si:+ClCR 3 → TS1 → Y 2ClSi–CR 3. (II) Y 2Si:+Y 2ClSi–CR 3 → TS2 → Y 2ClSi–SiY 2CR 3.

Gas-Phase Reaction Pathways from SiH4to Si2H6, Si2H4and Si2H2: A Theoretical Study

Initial gas-phase reactions for a system containing silane (SiH4) and hydrogen (H2) were investigated using ab initio calculations at the MP2/6-311++g** level. Twenty-four minimum and 19 transition states were located on the potential energy surface of the neutral species containing one or two silicon atoms. The bonding nature of each species was elucidated by topological analyses of electron density using Bader's AIM theory. The reaction pathways were classified into three parts: Part (I) shows elimination of the first H2 and formation of disilane (Si2H6). Part (II) shows elimination of the second H2 and formation of five disilene (Si2H4) isomers. Part (III) shows elimination of the third H2 and formation of four disilyne (Si2H2) isomers. More energy is required to obtain species with less hydrogen content, and such species are more likely to be stabilized by a hydrogen bridged bond (Si−H−Si). Calculated IR spectra, dipole moments, and rotational constants were provided for comparison with the experiments.

Theoretical study of halophilic reactions of stable silylenes with chloro- and bromocarbons.

A theoretical study of the mechanism of the reaction of stable silylenes with halocarbons has been carried out using the B3LYP density functional method. The main findings are as follows: (1) Lewis acid-base complexes formed between silylenes and halocarbons do not play a role in silylene insertion chemistry into halocarbons; therefore, the acid-base complex mechanism proposed by West et al. (J. Am. Chem. Soc. 2002, 124, 4186) is not appropriate to describe the disilane formation reaction. (2) The disilane formation reactions follow the energetically favorable general reaction pathway (X = halogen): (i) Y2Si: + HCX3 --> TS1 --> Y2XSi-CHX2. (ii) Y2Si: + Y2XSi-CHX2 --> TS2 --> Y2XSi-SiY2CHX2. (3) The observed preference of stable silylenes to undergo C-X bond insertion rather than C-H bond has been investigated. The theoretical findings suggest that this preference is a result of the thermodynamic factor. (4) Stable silylenes prefer to insert into a C-Br rather than a C-Cl bond because the energy barrier to insertion is lower, and the reaction is more exothermic.

The direct synthesis of methylchlorosilanes: New aspects concerning its mechanism

New results are given regarding the mechanism of the chemical process of copper alloyed silicon with methyl chloride (the `direct process'). As indicated by Photo-EMF measurements, carried out with doped silicon samples the reactivity of silicon significantly depends on the type of the doping with elements like phosphorus (n-type) tin, boron, indium (p-type). In-situ trapping experiments with 2,3-dimethylbutadiene are consistent with the creation of silylene intermediates SiMeCl and SiCl2 . Theselectivity of their competitive insertion steps can be controlled by the doping type and concentrations of the doping elements, especially the phosphorus/tin ratio criterion. n-Type doping favors the silylene insertion into the C-Cl bond due to the electronic silylene stabilization on the silicon surface. In case of p-type doping silylene insertion into Si-Cl bond is favored more intensively leading to the formation of disilanes.

Halophilic reactions of a stable silylene with chloro and bromocarbons.

A number of disilanes have been synthesized from a stable silylene, 1 (N,N'-di-tert-butyl-1,3-diaza-2-silacyclopent-4-en-2-ylidene), and a variety of halocarbons. It is proposed that disilane formation is a result of an initial halophilic interaction between the silylene and halocarbon. Formation of disilanes from 1 and CCl4, 2a, CHCl3, 2b, CH2Cl2, 2c, benzyl chloride, 2d, and bromobenzene, 5, are described here. An X-ray crystal structure of 2b was determined.

The Use of a Fixed-Bed Reactor to Evaluate the Interactions of Catalysts and Promoters in the Methyl Chlorosilane Reaction and to Determine the Effect of Cu in the Form of the Eta Phase on This Reaction

A fixed-bed reactor was employed to study the methylchlorosilane (MCS) reaction, also called the direct process. We used a copper−silicon contact mass as the main MCS catalyst. The copper−silicon contact mass was prepared by the reaction of CuCl with silicon. The effect of added zinc and phosphorus to the MCS reaction was explored, and we found that, at Cu/Zn ratios > 30, phosphorus addition resulted in an increase in selectivity for dimethyldichlorosilane (Di) at the expense of methyltrichlorosilane (Tri) and residue. Addition of tin to the MCS reaction resulted in an increased overall rate but with a decrease in Di and an increase in disilane formation. Addition of phosphorus and high tin levels resulted in a high rate but with a high selectivity for Di; phosphorus negated the selectivity penalty caused by the addition of tin alone. Phosphorus appeared to cause an increase in formation of the eta phase (Cu3Si), as determined by analysis of MCS beds formed under different conditions. Previously reported ...

In-situ Raman spectroscopy and laser-induced fluorescence during laser chemical vapor precipitation of silicon nanoparticles

Raman scattering in combination with laser induced fluorescence (LIF) has been used to identify molecules and intermediates during the nucleation and growth of nanosized silicon particles by CO 2 laser excitation of silane. Upon switching on the CO 2 laser, the silane Raman peak height decreases due to a decrease in species number density and LIF peaks emerge due to presence of SiH 2 radicals. The dissociation temperature has been determined inside the reaction zone and a value of 605 K is obtained. The nucleation and growth mechanism starts with the insertion of the SiH 2 radicals into the Si-H bond of silane which is demonstrated by the formation of disilane. Amorphous nanosized silicon particles are obtained at reactor pressures of 50 torr and are collected on a cold finger.

Studying of silane thermal decomposition mechanism

The mechanism of silane thermal decomposition is investigated in a flow reactor. The time dependencies of silane consumption and disilane formation were compared with those parameters of solid product (aerosol particles) such as concentration, total hydrogen content in solid product, and fraction of hydrogen contained in solid product as polyhydride groups (SiH2)n. Silane loss and gaseous product formation were analyzed using a mass spectrometer. The hydrogen content in solid product was analyzed by the methods of IR-spectroscopy and hydrogen evolution. Based on a simple kinetic scheme we qualitatively explained the experimental dependencies of silane conversion and disilane formation, the effective activation energy of the decomposition process, and the amount of polyhydride groups in the solid product on reaction time and initial silane concentration. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 99–110, 1998.

Studying of silane thermal decomposition mechanism

The mechanism of silane thermal decomposition is investigated in a flow reactor. The time dependencies of silane consumption and disilane formation were compared with those parameters of solid product (aerosol particles) such as concentration, total hydrogen content in solid product, and fraction of hydrogen contained in solid product as polyhydride groups (SiH2)n. Silane loss and gaseous product formation were analyzed using a mass spectrometer. The hydrogen content in solid product was analyzed by the methods of IR-spectroscopy and hydrogen evolution. Based on a simple kinetic scheme we qualitatively explained the experimental dependencies of silane conversion and disilane formation, the effective activation energy of the decomposition process, and the amount of polyhydride groups in the solid product on reaction time and initial silane concentration. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 99–110, 1998.

Mechanisms and rate determining steps in plasma induced high rate CVD of silicon an germanium: similarities and differences

The recent progress in the understanding of the reaction mechanism of plasma induced CVD of silicon from silane and the identification of the rate determining steps enabled us to significantly increase the deposition rate of device quality a-Si, μc-Si and epi-Si. The mechanism is similar to that found for thermal CVD. The first step is the fragmentation of monosilane into SiH 2 and H 2 followed by fast insertion and formation of disilane and trisilane. The higher silanes represent the reactive intermediates for the silicon deposition because their reactive sticking coefficients are orders of magnitude larger than that of monosilane. All these features of the mechanisms are well documented by experimental data from our and several other laboratories and by theoretical calculations which will be summarized here. The understanding of plasma CVD of geranium from monogermane is less complete. Our recent data show that there are at least two different kinetic regimes with a different reaction mechanism. the data show that, unlike in the case of silicon, higher germanes are probably not involved in the process. A very preliminary interpretation of the data suggests that divalent fragments and possibly other redical species are the dominant precursors for the deposition of germanium. Significant differences between silicon and germanium are found also regarding the amorphous-to-crystalline transition

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.

Controversies in the suggested mechanisms of plasma-induced deposition of silicon from silane

The originally proposed mechanism of plasma-induced deposition of silicon from silane via the SiH3 radical, as suggested by several research groups, was based on speculation rather than on facts. Critical consideration of the experimental facts which were published recently shows that there is no substantial evidence for this reaction channel, and that the possible contribution of the SiH3 radical to the generally observed deposition rate of amorphous silicon (a-Si) and crystalline silicon (c-Si) is negligible.Based on the experimental data available so far it is shown that the most probable dominant reaction channel involves, as a first step, the fragmentation of silane into SiH2 and H2. In the case of a-Si deposition, this is followed by rapid formation of disilane (and to lesser extent higher silanes) which then decomposes at the surface of the growing film. The deposition of c-Si involves a direct insertion of SiH2 into the surface of the growing film. The ion-impact-induced dehydrogenation of the surface is the rate-determining step for fast growth rates.

Optical emission and mass spectroscopic studies of the gas phase during the deposition of SiO2 and a-Si:H by remote plasma-enhanced chemical vapor deposition

This paper will present mass spectrometric and optical emission spectroscopic studies of the deposition process of amorphous hydrogenated silicon (a-Si:H) and silicon dioxide (SiO2) by remote plasma-enhanced chemical vapor deposition (remote PECVD). We have established that the silane reactant, which is not directly exposed to a rf plasma in either of the deposition processes, is not fragmented or chemically combined in the gas phase. Specifically there is no evidence for the formation of disilane, Si2H6, or siloxanes or silanols in the gas phase, as in the direct PECVD process. In the case of the a-Si:H depositions, the silane is excited in the gas phase and the excited species, SiH*4 , is the deposition precursor. In the case of the SiO2 depositions, the active species promoting deposition is an O2 metastable neutral molecule. The by-products of the respective reactions are H2 and H2O.

Tricyclic heterocycles with bifunctional silicon centers

Condensation of the diorganometallic reagents ( o-MC 6H 4) 2 X (M  Li, MgCl) with HSiCl 3 followed by reduction with LiAlH 4 provides dibenzosilacycles, I (a, X  -; b, X NMe; c, X  CH 2; d, X  CH 2CH 2) with two exocyclic H-substituents, ▪. Conditions for the stepwise conversion of I to mixed bifunctional systems, ▪ (II, X  Cl, Br; III, X  OR), and bifunctional derivatives, ▪ (IV, X  Cl; V, X  OR) were determined. Controlled halogenation of I to II was accomplished with one molar equivalent of SO 2Cl 2 or NBS although CCl 4 in the presence of ClRh(PPh 3) 3 or PdCl 2 results in slow monochlorination. The reaction of I with excess SOCl 2 or SO 2Cl 2 converts I to III but the latter is faster and provides fewer side reactions. Conversion of I to IV with excess alcohols occurs in high yield with ClRh(PPh 3) 3 but in low yield with H 2PtCl 6. Controlled alcoholysis of I to III could not be achieved except with tBuOH. The dichlorides, IV, are methylated in high yield to VII, ▪. Reaction of Ib with ClRh(PPh 3) 3 results in elimination of H 2 and formation of disilanes as indicated by trapping reactions with alcohols (formation of Vb).