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.     

Porous silicon: repeatability of generation?

A systematic investigation of the preparation of porous silicon layers (PSLs) on silicon wafers by anodic treatment in hydrofluoric acid/water/ethanol mixtures under various conditions was carried out with the aim to develop a repeatable process for homogeneous layers. The preparations were controlled by FTIR spectroscopy supported by TOF-SIMS measurements of the uppermost surface area. The repeatability of PLS generation was proved on the grounds of the IR-absorption of SiH_x and O_xSiH_y surface groups with respect to their frequencies and intensities. Uniform properties of PSLs are an essential assumption for defined chemical modifications.     

XPS,AFM, ATR and TPD evidence for terraced,dihydrogen terminated, 1×1 (100) silicon

Heating (100) silicon at high temperature (say, higher than 850 °C) in H₂, cooling to 670–700 °C in the same ambient, and quenching to room temperature in N₂ results in environmentally robust, terraced 1 × 1 (100) SiH₂. Evidence for this conclusion is based on angle‐resolved x‐ray photoelectron spectroscopy, atomic force microscopy, infrared absorption spectroscopy in the attenuated total reflection mode, thermal programmed desorption, and reflection high‐energy electron diffraction. Copyright © 2005 John Wiley & Sons, Ltd.     

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.     

Surface hydrogen and growth mechanisms of synchrotron radiation-assisted silicon gas source molecular beam epitaxy using disilane

Surface hydrogen and growth mechanisms are investigated for synchrotron radiation (SR)-assisted gas source molecular beam epitaxy (SR-GSMBE) using Si₂H₆ on the Si(100) surface in the low-temperature region. The surface silicon hydrides (deuterides) are monitored in situ during the epitaxial growth by means of infrared reflection absorption spectroscopy with a Si(100) substrate and a CoSi₂ buried metal layer. It is concluded that the chemisorption of gas-phase reactive species such as SiH_n and H generated by SR irradiation and the subsequent hydrogen desorption are the key mechanisms of SR-GSMBE at low substrate temperatures. © 1998 John Wiley & Sons, Ltd.     

(Aminocarbene)(Divinyltetramethyldisiloxane)Iron(0) Compounds: A Class of Low‐Coordinate Iron(0) Reagents

The combined use of aminocarbene and divinyltetramethyldisiloxane (dvtms) as supporting ligands enables the access of unprecedented low‐coordinate iron(0) alkene compounds [L_nFe(η²:η²‐dvtms)] (L=N‐heterocyclic carbene (NHC) or cyclic (alkyl)(amino)carbene (CAAC), n=1 or 2) from the reactions of FeCl₂ with alkali‐metal reducing agents, free aminocarbene ligands, and dvtms. The iron(0) species deliver their {L_nFe⁰} fragments to perform redox reactions with Ph₂SiH₂, S₈, Se, and DippN₃, furnishing novel aminocarbene‐supported iron(IV) silylene, all‐ferrous iron–sulfur/selenium cubanes, and bis(imido)iron(IV) compounds. These conversions demonstrate the potential synthetic utility of the carbene‐supported iron(0) complexes as a valuable class of low‐coordinate iron(0) reagents.     

A study of the electronic structure of phenylsilanes by X-ray emission spectroscopy and quantum chemical calculation methods

The electronic structure of a series of phenylsilanes Ph_4?n SiH _n (n = 0?C3) is studied by X-ray emission spectroscopy and quantum chemical calculations by the density functional theory method. Based on the calculations theoretical X-ray emission SiK??₁ spectra of phenylsilanes Ph_4?n SiH _n (n = 0?C4) are constructed and their energy structure and shape turn out to be well consistent with experiment. The distribution of the electron density of states with different symmetry of Si, C, H atoms are also constructed. An analysis of the obtained X-ray fluorescent SiK??₁ spectra and the distribution of the electron density of states in Ph₄Si and Ph₃SiH compounds shows that their energy structure is mainly determined by a system of the energy levels of phenyl ligands weakly perturbed by interactions with valence AOs of silicon. In the energy structure of MOs of the PhSiH₃ compound, energy orbitals related to t ₂ and a ₁ levels of tetrahedral SiH₄ are mainly presented.     

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.     

Gas‐Phase Formation of the Disilavinylidene (H2SiSi) Transient

The hitherto elusive disilavinylidene (H₂SiSi) molecule, which is in equilibrium with the mono‐bridged (Si(H)SiH) and di‐bridged (Si(H₂)Si) isomers, was initially formed in the gas‐phase reaction of ground‐state atomic silicon (Si) with silane (SiH₄) under single‐collision conditions in crossed molecular beam experiments. Combined with state‐of‐the‐art electronic structure and statistical calculations, the reaction was found to involve an initial formation of a van der Waals complex in the entrance channel, a submerged barrier to insertion, intersystem crossing (ISC) from the triplet to the singlet manifold, and hydrogen migrations. These studies provide a rare glimpse of silicon chemistry on the molecular level and shed light on the remarkable non‐adiabatic reaction dynamics of silicon, which are quite distinct from those of isovalent carbon systems, providing important insight that reveals an exotic silicon chemistry to form disilavinylidene.     

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.     

IR Spectrum and Structure of Protonated Monosilanol: Dative Bonding between Water and the Silylium Ion

We report the spectroscopic characterization of protonated monosilanol (SiH₃OH₂⁺) isolated in the gas phase, thus providing the first experimental determination of the structure and bonding of a member of the elusive silanol family. The SiH₃OH₂⁺ ion is generated in a silane/water plasma expansion, and its structure is derived from the IR photodissociation (IRPD) spectrum of its Ar cluster measured in a tandem mass spectrometer. The chemical bonding in SiH₃OH₂⁺ is analyzed by density functional theory (DFT) calculations, providing detailed insight into the nature of the dative H₃Si⁺‐OH₂ bond. Comparison with protonated methanol illustrates the differences in bonding between carbon and silicon, which are mainly related to their different electronegativity and the different energy of the vacant valence p_z orbital of SiH₃⁺ and CH₃⁺.     

Electronic structure of silanes in the semi-empirical equivalent orbital method

The problem of the prediction of the valence IPs for silanes is considered. It is shown that the data on the silicon band structure combined with the photoelectron spectra of SiH₄, and Si₂H₆ permit to obtain the parameter scale, which includes all the nearest neighbour, second neighbour and the main third neighbour interaction parameters. Using the derived parameter scale the vertical ionization potentials of Si₃H₈, SiH(SiH₃)₃, Si(SiH₃)₄, the infinite polysilane valence band structure and the inner a _1g level for disilane are calculated. All the calculated levels are located above ? 20 eV and are expected to be measurable by the He (I) photoelectron spectroscopy.     

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.     

Plasma Composition by Mass Spectrometry in a Ar-SiH<Subscript>4</Subscript>-H<Subscript>2</Subscript> LEPECVD Process During nc-Si Deposition

Mass spectrometry has been used to assess plasma composition during a low-energy plasma-enhanced chemical vapor deposition (LEPECVD) process using argon-silane-hydrogen (Ar-SiH₄-H₂) gas mixtures with input flows of 50 sccm Ar, 2–20 sccm SiH₄ and 0–50 sccm H₂ at total pressures of 1–4 Pa. Energy-integrated ion densities, residual gas analysis and threshold ionization mass spectrometry have been used to characterize the transition from amorphous (a-Si) to nano-crystalline silicon (nc-Si) deposition at constant LEPECVD operating parameters. While relative ion densities have a marked decrease with H₂ input, the densities of SiH_x (x < 4) radicals show evolution trends depending on the SiH₄ and H₂ input. For conditions leading to nc-Si growth a turning point is reached above which SiH is the main radical. Observed SiH_x density trends with H₂ input are explained based on kinetic reaction rates calculated from previously obtained Langmuir probe data.     

Preparation of a liquid boron‐modified polycarbosilane and its ceramic conversion to dense SiC ceramics

A boron‐modified ethynylhydridopolycarbosilane (B‐EHPCS) was successfully prepared via the hydroboration reaction of ethynylhydridopolycarbosilane (EHPCS) with 9‐borabicyclo‐[3.3.1]nonane (9‐BBN). The as‐synthesized B‐EHPCS with a branched structure was characterized by means of gel permeation chromatography (GPC), Fourier transform infrared spectroscopy (FTIR), and nuclear magnetic resonance (NMR). The structural evolution of ceramic conversion of B‐EHPCS was investigated by solid‐state NMR. The ¹³C magic angle spinning (MAS) NMR results indicated that the C?C and C?C groups of B‐EHPCS take part in the hydrosilation cross‐linking at a relatively low temperature (170°C). According to the ²⁹Si MAS NMR analysis, the CSiH₃ end groups are most reactive hydride functionality involved in the hydrosilation cross‐linking. With increasing curing temperature, the C₂SiH₂ and CSiH₃ units are completely consumed, while C₃SiH units remain even after curing at 600°C. The TGA results show the 1200°C ceramic yield of B‐EHPCS reaches 86%, which is 10% higher than that of the parent EHPCS (76%). At high temperatures, the introduction of <1 wt% boron significantly inhibits silicon carbide (SiC) crystallization. The 1800°C ceramics derived from B‐EHPCS are found to be significantly denser than that from EHPCS. Copyright © 2010 John Wiley & Sons, Ltd.     

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).     

Frustrated Lewis Pair Chelation as a Vehicle for Low‐Temperature Semiconductor Element and Polymer Deposition

The stabilization of silicon(II) and germanium(II) dihydrides by an intramolecular Frustrated Lewis Pair (FLP) ligand, PB , ⁱPr₂P(C₆H₄)BCy₂ (Cy=cyclohexyl) is reported. The resulting hydride complexes [PB{SiH₂}] and [PB{GeH₂}] are indefinitely stable at room temperature, yet can deposit films of silicon and germanium, respectively, upon mild thermolysis in solution. Hallmarks of this work include: 1) the ability to recycle the FLP phosphine‐borane ligand ( PB ) after element deposition, and 2) the single‐source precursor [PB{SiH₂}] deposits Si films at a record low temperature from solution (110 °C). The dialkylsilicon(II) adduct [PB{SiMe₂}] was also prepared, and shown to release poly(dimethylsilane) [SiMe₂]_n upon heating. Overall, this study introduces a “closed loop” deposition strategy for semiconductors that steers materials science away from the use of harsh reagents or high temperatures.     

Reaction of 1-(dimethylsilyl)-2-silylbenzene with platinum(0) phosphine complex: Isolation and characterization of the Si3-Pt-H complex

Reaction of two equivalents of 1,2-C₆H₄(SiMe₂H)(SiH₃) with Pt(depe)(PEt₃)₂ (depe = Et₂PCH₂CH₂PEt₂) in toluene at room temperature afforded two novel isomeric {1,2-C₆H₄ -(SiMe₂H)(SiH₂)}{1,2-C₆H₄(SiMe₂)(SiH₂)}(H)Pt^IV (depe) complexes 1 and 2 in 5:1 ratio among eight possible isomers. Complex 1 is one of the few examples of tris(silyl)(hydrido)platinum(IV) complexes structurally characterized by single crystal X-ray analysis. The structure of complex 1 was unambiguously determined by multinuclear NMR and single crystal X-ray analysis.     

Synthesis,Structural Characterization and Reactivity of a Bis(phosphine)(silyl) Platinum(II) Complex

Treatment of 1,2‐C₆H₄(SiH₃)(SiH₃) ( 1 ) with Pt(dmpe)(PEt₃)₂ (dmpe=Me₂PCH₂CH₂PMe₂) in the ratio of 1:1 leads to the complex {1,2‐C₆H₄(SiH₂)(SiH₂)}Pt^II (dmpe) ( 2 ), which can react with proton organic reagent bearing hydroxy group with low steric hindrance to form a tetra‐alkoxy substituted silyl platinum(II) compound ( 3 ). Compounds 2 and 3 are the very rare examples of silyl transition‐metal complexes derived from this chelating hydrosilane ligand. To the best of our knowledge, there are only 6 examples of silyl metal complexes prepared from this ligand with such structural features registered in the Cambridge Structural Database, among them, only one silyl platinum(II) compound is presented. The structures of complexes 2 and 3 were unambiguously determined by multinuclear NMR spectroscopic studies and single crystal X‐ray analysis.     

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.