Production and characterization of nitrided CVD-SiC films

Silicon Carbide (SiC) and SiC with free silicon [SiC(Si)] thin films were prepared by chemical vapor deposition (CVD) using a CH₃SiCl₃-H₂-Ar gas mixture at a temperature of 1223 K. Afterwards these layers were gas nitrided in an ammonia-hydrogen-argon mixture at 1273 K. The solid product is an extremely thin film of silicon nitride on SiC or SiC(Si)-basic layers. These ultra thin silicon nitride films were investigated by glow discharge optical spectroscopy (GDOS) and x-ray photoelectron spectroscopy (XPS). The thickness of the layers was determined to a maximum value of 30 nm.Dedicated to Professor Dr. rer. nat. Dr. h.c. Hubertus Nickel on the occasion of his 65th birthday     

The Combustion of SiCl4 in Hot O2/H2 Flames

A simple kinetic model describing the molecular gas phase reactions during the formation of fumed silica (AEROSIL®) was developed. The focus was on the formation of molecular SiO₂, starting from SiCl₄, hydrogen and oxygen. Wherever available, kinetic and thermodynamic parameters were taken from the literature. All other parameters are based on quantum chemical calculations. From these data, an adiabatic model for the combustion reaction has been developed. It was found that a significant amount of molecular SiO₂ forms after about 0.1 and 0.6 ms at starting temperatures between 1000 and 2000 K. The initial reaction of the SiCl₄ combustion in a hydrogen/oxygen flame was found to be different from the combustion in air: The high reactivity of SiCl₄ towards water is favored over the SiCl₄ dissociation, which is the initial and rate‐determining step during the combustion of SiCl₄ in air.     

Behavior of carbon-containing impurities during plasma synthesis of trichlorosilane

The behavior of CCl₄ and CHCl₃ admixtures during the plasma synthesis of trichlorosilane via the hydrogenation of SiCl₄ in a capacitively coupled radiofrequency (40.68 MHz) discharge was studied. It was shown that the main portion of the impurities undergo chemical transformations yielding silicon carbide and carbon. The degree of conversion determined from the impurity concentrations in the reactant SiCl₄ and its hydrogenation products is strongly dependent on the processing parameters and reaches 99.9% for CCl₄ and 96% for CHCl₃.     

Kinetics of the initiation step of the thermal decomposition of SiCl4

The initiation reaction of the thermal decomposition of silicon tetrachloride was studied behind reflected shock waves at temperatures between 1550 K and 2370 K and pressures between 1 and 1.5 bar. Atomic resonance absorption spectrometry (ARAS) was applied for time-resolved measurements of H atoms at the L_α-line in SiCl₄/H₂/Ar systems. Additional experiments were performed in the SiCl₄/Ar system following the absorption of SiCl₄ at the L_α-line. Rate coefficients for the reaction (RI) were determined to be: The choice between two possible alternatives of the first decomposition step, namely elimination of either Cl₂ or Cl, has been made in favor of the second reaction on the basis of kinetic and energetic considerations. © 1997 John Wiley & Sons, Inc. Int J Chem Kinet 29: 415–420, 1997.     

A new solution route for the synthesis of silicon nanoparticles presenting different surface substituents

This paper describes a new solution route for the preparation of silicon nanoparticles functionalized at the surface using potassium incorporated in graphite (C₈K) as reducing agent of silicon tetrachloride (SiCl₄). In a first step, chloride,capped silicon nanoparticles were obtained. They allowed an easy access, in a second step, to a range of surface functionalities by simple nucleophilic substitutions: hydroxy, alkyl, alkoxy or amino groups. They were characterized by FTIR, ¹H, ¹³C and ²⁹Si NMR. A mean diameter of 13 nm was found for these particles.     

Adsorption and surface diffusion of silicon growth species in silicon carbide chemical vapour deposition processes studied by quantum-chemical computations

The effect chlorine addition to the gas mixture has on the surface chemistry in the chemical vapour deposition (CVD) process for silicon carbide (SiC) epitaxial layers is studied by quantum-chemical calculations of the adsorption and diffusion of SiH₂ and SiCl₂ on the (000-1) 4H–SiC surface. SiH₂ was found to bind more strongly to the surface than SiCl₂ by approximately 100 kJ mol^?1 and to have a 50 kJ mol^?1 lower energy barrier for diffusion on the fully hydrogen-terminated surface. On a bare SiC surface, without hydrogen termination, the SiCl₂ molecule has a somewhat lower energy barrier for diffusion. SiCl₂ is found to require a higher activation energy for desorption once chemisorbed, compared to the SiH₂ molecule. Gibbs free energy calculations also indicate that the SiC surface may not be fully hydrogen terminated at CVD conditions since missing neighbouring pair of surface hydrogens is found to be a likely type of defect on a hydrogen-terminated SiC surface.     

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.     

Modeling of the elementary gas-phase reaction during chemical vapor deposition of silicon carbide from CH3SiCl3/H2

We established a gas-phase, elementary reaction model for chemical vapor deposition of silicon carbide from methyltrichlorosilane (MTS) and H₂, based on the model developed at Iowa State University (ISU). The ISU model did not reproduce our experimental results, decomposition behavior of MTS in the gas phase in an environment with H₂. Therefore, we made several modifications to the ISU model. Of the reactions included in existing models, 236 were lacking in the ISU model, and thus were added to the model. In addition, we modified the rate constants of the unimolecular reactions and the recombination reactions, which were treated as a high-pressure limit in the ISU model, into pressure-dependent rate expressions based on the previous reports (to yield the ISU+ model), for example, H₂(+M) → H + H(+M), but decomposition behavior remained poorly reproducible. To incorporate the pressure dependencies of unimolecular decomposition rate constants, and to increase the accuracies of these constants, we recalculated the rate constants of five unimolecular decomposition reactions of MTS using the Rice-Ramsperger-Kassel-Marcus method at the CBS-QB3 level. These chemistries were added to the ISU+ model to yield the UT2014 model. The UT2014 model reproduced overall MTS decomposition. From the results of our model, we confirmed that MTS mainly decomposes into CH₃ and SiCl₃ at the temperature around 1000°C as reported in the several studies.     

Mechanisms and kinetics of the thermal decompositions of trichlorosilane,dichlorosilane, and monochlorosilane

Decomposition studies of trichlorosilane, dichlorosilane, and monochlorosilane at 921 K, 872 K, and 806 K, respectively, are reported. The studies were made at fixed reactant pressures over a range of total pressures in a wall conditioned, quartz reactor connected to a quadrupole mass-spectrometer. Products were monitored sequentially and continuously in time. The dichlorosilane decomposition was also studied by the comparative-rate single-pulse shock-tube method at temperatures around 1250 K. Two mechanisms of decomposition are considered: a silylene based mechanism initiated by molecular elimination reactions (Scheme I), and a free radical based mechanism initiated by bond fission reactions (Scheme V). Modeling tests of these mechanisms show that only the former is consistent with the experimental data. The decompositions are shown to be essentially nonchain processes initiated by the following pressure dependent reactions: HSiCl₃(SINGLEBOND)4→ SiCl₂+HCl, H₂SiCl₂(SINGLEBOND)1→ SiCl₂+H₂ and H₃SiCl(SINGLEBOND)5→ HSiCl+H₂. High pressure Arrhenius parameters recommended for these reactions are A_4,∞=A_1,∞=A_5,∞=10^14.5±0.5 s⁻¹, E_4,∞=71.9±2.1 kcal/mol, E_1,∞=69.2±2.0 kcal/mol, and E_5,∞=60.6±1.8 kcal/mol. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet: 30: 69–88, 1998.     

Silazanes plus MCl4-substitution vs. rearrangement reactions

Novel silicon and tin compounds were synthesized by the reaction of lithium salts of 1,3-di-phenyl-1,1,3,3-tetramethyldisilazane H(DPTMDS) and 1,1,3,3,5,5-hexamethylcyclotrisilazane H₃(HMCTS) with SiCl₄ and SnCl₄, respectively. The reactions with H(DPTMDS) yield the substitution products (DPTMDS)₂SiCl₂ and (DPTMDS)₂SnCl₂. In contrast, the reactions with the cyclic silazane H₃(HMCTS) lead to a large variety of products due to the competition of substitution reactions and ring contractions. The resulting new compounds were characterized by elemental analyses, NMR spectroscopy, and single crystal X-ray structure analyses. Dedicated to Prof. Edmunds Lukevics on the occasion of his 70th birthday __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 12, pp. 1845–1856, December, 2006.     

Beiträge zur Chemie der Halogensilan-Addukte. VIII. Darstellung und Eigenschaften der kationischen Bis-2,2′-Bipyridin-Silicium-Komplexe, [SiCl2b[py2]2+ und [SiF2bipy2]2+

Contributions to the Chemistry of Halogenosilane Adducts. VIII. Preparation and Properties of the Cationic Bis-2,2′-Bipyridinesilicon Complexes, [SiCl₂bipy₂]²⁺ and [SiF₂bipy₂]²⁺ The reactions of SiCl₂bipy₂ (green isomer) and SiF₂bipy₂ with chlorine yield the new ionic complexes of Si, [SiX₂bipy₂]Cl₂ (X = Cl, F). Bromine and iodine react similarly. With these reagents, however, formation of insoluble polyhalides of the complex cations inevitably occurs rendering further investigations difficult. The compounds contain the cis-octahedral cation [SiX₂bipy₂]²⁺. They are soluble in methanol and water and are unusually stable in these solvents. [SiCl₂bipy₂]Cl₂ starts to react observably with methanol (substitution of SiCl) only after weeks. Thus reactions may be performed in this solvent. It follows from the investigation that the green isomer of SiCl₂bipy₂ is a cis-octahedral molecular complex of silicon. Two other green isomers of SiCl₂bipy₂ are shown to exist. The reactions of chlorine with these isomers yield products different from the cis-octahedral complex reported above. Ion-exchange and metathetical reactions of [SiCl₂bipy₂]Cl₂ yield the new compounds [SiCl₂bipy₂]X₂ (X = Br^?, J^?, NO₃^?, ClO₄^?, [Cr(NH₃)₂(NCS)₄]^?, PtCl₆^2?/2). All compounds contain the [SiCl₂bipy₂]²⁺-cation which is investigated in detail (¹H-, ²⁹Si-NMR, IR, UV, ESCA, conductivity, molecular weight). The use of AgF for the synthesis of ionic SiF-complexes (X = F) gives rise to more complicated reactions.     

SCF Xα SW calculations of chemical shifts of X-ray and electron emission lines and relaxation properties of SiX4 molecules

SCF Xα SW calculations of the 1s and 2p binding energies, KLL Auger energies and Kα transition energies for the molecules SiH₄, SiCl₄ and SiF₄ and the corresponding atomic Xα calculations for charged free silicon ions have been carried out. The results provide information about relaxation properties and anomalous chemical Kα shifts in hydrides.     

Tetrachlorosilane Consumption in Radio Frequency Glow Discharge

Time-resolved mass spectrometry was used for analysis of the plasma reactions in radio frequency (RF) SiCl ₄ and SiCl ₄ –O ₂ discharges as functions of starting partial pressure and electrical power. Molecular concentrations of the reactants and products from SiCl ₄ alone and with O ₂ were obtained from the mass spectra and used for plotting the kinetic curves. The SiCl ₄ and O ₂ consumption rates were calculated from the kinetic curves and compared with results of theoretical simulation of the reaction. Direct electron impact decomposition was found to be the main pathway for pure SiCl ₄ conversion. On the contrary, the consumption of SiCl ₄ in the SiCl ₄ +O ₂ mixtures was largely chemical. The experimental macrokinetics are in agreement with a model in which oxidation is caused by the atomic oxygen.     

Kinetic behavior of benzene hydrogenation and thiophene hydrogenolysis on sulfide Ni-Mo/Al2O3 catalyst

Summary An unsteady-state kinetic model of both benzene hydrogenation (HDA) and thiophene hydrogenolysis (HDS) on a sulfide hydrotreating catalyst Ni-Mo/Al₂O₃ has been developed. The model adequately describes experimental data obtained at the pressure 2 MPa, temperature 573 K and at various contact times and ratios of benzene/thiophene. The model is based on the assumption that the catalyst surface contains only one type of active sites, i.e., Ni atoms in the sulfide bimetallic species, which are responsible for both hydrogenolysis and hydrogenation reactions.     

Thermodynamics investigation of the gas-phase reactions in the chemical vapor deposition of silicon borides with BCl3–SiCl4–H2 precursors

The gas-phase reaction thermodynamics in the chemical vapor deposition (CVD) process of preparing silicon borides with the precursors of BCl₃–SiCl₄–H₂ is investigated with a relatively complete set of 220 species, in which the thermochemistry data are calculated with accurate model chemistry at G3(MP2) and G3//B3LYP levels combined with standard statistical thermodynamics. The data include the heat capacities, entropies, enthalpies of formation, and Gibbs free energies of formation. Based on these data, the distribution of the equilibrium concentration of the 220 species is obtained with the principle of chemical equilibrium. BHCl₂, SiHCl₃, and BH₂Cl are found to be the crucial intermediates. This work provides fundamental data for analyzing the thermochemistry of the CVD process of the BCl₃–SiCl₄–H₂ system, which is instructive to optimize the input precursors and temperatures for controlling the composition of the condensed phase B, SiB₆, and SiB₁₄.     

Mechanism of thermal decomposition of allyltrichlorosilane with formation of three labile intermediates: dichlorosilylene,allyl radical,and atomic chlorine

It is experimentally found that allyltrichlorosilane dissociates under vacuum pyrolysis (~10^–2 Torr) at temperatures above 1100 K to form three labile intermediates: allyl radical, dichlorosilylene, and monoatomic chlorine. On the basis of experimental and theoretical data obtained, it is shown that the decomposition reaction proceeds in two steps. The first step is a typical reaction of homolytic decomposition to two radicals (C₃H₅ and SiCl₃) at the weakest Si—C bond. Due to weakness of the Si—Cl bond in the SiCl₃ radical, the energy of which is even somewhat lower than the dissociation energy of the Si—C bond in starting AllSiCl₃, this radical undergoes further dissociation to SiCl₂ and Cl, thus resulting in three intermediates of different classes of highly reactive species formed from AllSiCl₃.     

Thermodynamics and kinetics of initial gas phase reactions in chemical vapor deposition of titanium nitride. Theoretical study of TiCl4 ammonolysis

Thermodynamic equilibrium and kinetics of the gas‐phase reaction between TiCl₄ and NH₃ have been studied computationally using results from recent quantum mechanical calculations of titanium tetrachloride ammonolysis. 1 These calculations were based upon the transition state theory for the direct reactions and RRKM theory for the reactions proceeding via intermediate complex. Rate constants for the barrierless reactions were expressed through the thermodynamic characteristics of the reagents and products using a semiempirical variational method. The kinetic simulation of the gas‐phase steps of CVD was performed within a model of a well‐stirred reactor at temperatures 300–1200 K and residence times between 0.1–2 s. At temperatures below 450 K formation of donor–acceptor complexes between TiCl₄ and NH₃ is the dominating process. At higher temperatures sequential direct ammonolysis takes place. At typical LPCVD conditions the only product of the first step of ammonolysis, TiCl₃NH₂, is formed in substantial amount. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1366–1376, 2001     

Dichlorosilylene–hydrogen chloride complex: direct IR spectroscopic detection in argon matrix

A 1: 1 donor–acceptor complex between SiCl₂ and HCl was detected by matrix IR spectroscopy. The existence of the complex was previously predicted theoretically in the course of analysis of mechanisms of chemical vapor deposition (CVD) processes involving chlorosilanes. The quantum chemical calculations at the G4(MP₂) level confirmed the possibility of formation of only one stable complex upon the reaction of SiCl₂ with HCl. In addition to the complex of the simplest composition, complexes of SiCl₂ with HCl associates were observed upon matrix annealing. The only product formed upon the photolysis of the complexes of all types was trichlorosilane, a product of silylene insertion into the H–Cl bond.

     

Controlling Ethylene Hydrogenation Reactivity on Pt13 Clusters by Varying the Stoichiometry of the Amorphous Silica Support

Ethylene hydrogenation was investigated on size‐selected Pt₁₃ clusters supported on three amorphous silica (a‐SiO₂) thin films with different stoichiometries. Activity measurements of the reaction at 300 K revealed that on a silicon‐rich and a stoichiometric film, Pt₁₃ exhibits a similar activity to that of Pt(111), in line with the known structure insensitivity of the reaction. On an oxygen‐rich film, a threefold increased rate was measured. Pulsing ethylene at 400 K, then measuring the activity at 300 K, resulted in complete loss of activity on the silicon‐rich surface compared to only marginal losses on the other surfaces. The measured reactivity trends correlate with charging characteristics of a Pt₁₃ cluster on the SiO₂ films, predicted through first‐principle calculations. The results reveal that the stoichiometry‐dependent charging by the support can be used to tune the selectivity of reaction pathways during a catalytic hydrogenation reaction.     

The Insertion of EII and EIV Chlorides (E=Si,Ge) into the Si−Si Bond of Disilylene

Reactions of silicon and germanium dichlorides L ⋅ ECl₂ (E=Si, L=IPr; E=Ge, L=dioxane) with the phosphinoamidinato-supported disilylene ({κ²(N,P)-NNP}Si)₂ resulted in formal tetrylene insertions into the Si−Si bond. In the case of the reaction with silylene, two products were isolated. The first product ({κ²(N,P)-NNP}Si)₂SiCl₂, is the formal product of direct SiCl₂ insertion into the Si−Si bond of ({κ²(N,P)-NNP}Si)₂ and thus features two separated silylamido silylene centers. Over time, migration of the SiCl₂ group to a lateral position afforded the second product, the disilylene {κ²(Si,P)−SiCl₂NNP}Si−Si{κ²(N,P)-NNP}. In contrast, insertion of GeCl₂ resulted only in the isolation of the germanium analogue of {κ²(Si,P)−SiCl₂NNP}Si−Si{κ²(N,P)-NNP}, containing a Ge atom in the central position namely, compound {κ²(Si,P)−SiCl₂NNP}Ge−Si{κ²(N,P)-NNP}, which is a rare example of a silylene-germylene. Finally, reaction of disilylene ({κ²(N,P)−NNP}Si)₂ with SiCl₄ and SiHCl₃ led to the formation of the new bis(silyl)silylene, ({NNP}SiCl₂)₂Si:. All four new products from these insertion reactions have been characterized by multinuclear NMR and single-crystal X-ray diffraction studies.