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.     

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.     

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.     

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.     

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.     

Room-Temperature Silicon Nitrides Prepared with Very High Rates (>50 nm/s) in Atmospheric-Pressure Very High-Frequency Plasma

We have investigated the structure and stability of SiN _x films deposited with very high rates (>50 nm/s) in atmospheric-pressure (AP) He-based plasma excited by a 150 MHz very high-frequency (VHF) power using a cylindrical rotary electrode at room temperature. The SiN _x films are prepared on Si(001) substrates with varying VHF power density (P _VHF), H₂ concentration and source ratio (NH₃/SiH₄). Infrared absorption spectroscopy is used to analyze the bonding configurations in the films. The results show that increasing H₂ concentration under the supply of a moderately large P _VHF, together with the adjustment of NH₃/SiH₄ ratio, enables us to prepare SiN _x showing reasonable stability against a buffered hydrofluoric acid solution in spite of the very high deposition rate of 130 nm/s. The achievement of such a high-rate deposition at room temperature is primarily due to the significant enhancement of both gas-phase and surface-phase reactions in AP-VHF plasma.     

Mechanism of plasma polymerization ofN-silyl-substituted cyclodisilazane: Structure and properties of polymer film

NN-Bis(dimethylsilyl)tetramethylcyclodisilazane (NSCDSN) was selected for plasma polymerization as a model monomer representing N-silyl-substituted cyclodisilazanes. Owing to the aromatic character resulting from the strong (d-p) coupling between silicon and nitrogen atoms, these compounds manifest the highest thermal stability among the organosilicones. GC/MS examination of a low-molecular-weight fraction evaporated from the plasma polymer revealed the presence of various monomer derivatives having mostly the general structure of N-silyl-substituted cyclodisilazane (Si₄N₂) units. GC/MS and ATR/IR studies have shown that the Si₄N₂ skeleton displays a high resistance to plasma fragmentation and that it was incorporated as such into the polymer film. A radical mechanism for plasma polymerization was postulated assuming the formation of and propagation precursors. The low value found for the polar component of the surface free energy confirmed the contribution of Si₄N₂ units to the polymer film. TGA investigation showed the rate and degree of thermal decomposition of plasma-polymerized (PP) NSCDSN to be lower than those of plasma polymers from N-hydrogen-substituted silazanes. Vacuum pyrolysis, at 800°C, converted the polymer film into a glassy, dense, and almost inorganic material with a strong adhesion to the metal substrates.     

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.     

Ab Initio Chemical Kinetics for SiHx Reactions with Si2Hy (x = 1,2,3,4; y = 6,5,4,3; x + y = 7) under a‐Si:H CVD Condition

Gas‐phase reactions of SiH_x with Si₂H_y (x = 1,2,3,4; y = 6,5,4,3) species, respectively, which may coexist under chemical vapor deposition (CVD) conditions, have been investigated by means of ab initio molecular orbital and statistical theory calculations. Potential energy surface (PES) predicted at the CCSD(T)/CBS//B3LYP/6–311++G(3df,2p) level shows that these reactions take place primarily via trisilany radicals, n‐Si₃H₇ and i‐Si₃H₇. For example, SiH₂ can associate with Si₂H₅ producing n‐H₂SiSiH₂SiH₃ exothermically by 55.8 kcal/mol; SiH₃ can undergo addition to H₂SiSiH₂ to produce n‐Si₃H₇ or associate with H₃SiSiH barrierlessly forming i‐Si₃H₇; whereas SiH can insert into one of the Si─H bonds of Si₂H₆ to give excited n‐Si₃H₇. Similarly, H₂SiSiH and SiSiH₃ can undergo insertion reactions with SiH₄ producing n /i‐Si₃H₇ intermediates, respectively, to be followed by fragmentation to various smaller species. These processes are fully depicted in the complete PES. The predicted heats of formation of various species agree well with available thermochemical data. The rate constants and product branching ratios for the low‐energy channel products have been calculated for the temperature range 300–1000 K by variational RRKM (Rice–Ramsperger–Kassel–Macus) theory with Eckart tunneling corrections. The results may be employed for realistic kinetic modeling of the plasma‐enhanced chemical vapor deposition growth of a‐Si:H thin films under practical conditions.     

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.     

Convenient approach to Si–H-containing polymers

New synthetic routes to organosilicon polymers containing SiH₂ groups and organic -electron units in the polymer main chain are described. These polymers are expected to be useful precursors for ceramics. The polymer backbone is formed by condensation of ,-bis(trifluoromethylsulfonyloxy)-substituted organo-silicon compounds containing SiH₂ groups with the organometallic dinucleophiles Li₂C₂, Li₂C₄, and 1,4-BrMg–C₆H₄–MgBr. We could confirm the formation of the silicone polymers at low temperatures, in short reaction times, and with high yields. The structural characterization is based on ²⁹Si, ¹³C, and ¹H NMR spectroscopy. The narrow ²⁹Si NMR signals of the products indicate the regular alternating arrangement of the building blocks in the polymer backbone resulting from the fact that the condensation reactions are not accompanied by exchange processes analogous to metal halogen exchange. Weight-average molecular weights in the range of M_w = 10000–20000, relative to polystyrene standards, were found by GPC. The pyrolytic behaviour of [H₂Si–H₂Si–C C–]_n 2a is compared with the behaviour of the methylated derivative [Me₂Si–Me₂Si–C C–]_n.     

Theoretical Study of Interaction Between S2 and SiHx (x=1, 2, 3) in Porous Silicon

The interaction between S₂ molecule and SiH_x (x=1, 2, 3) in porous silicon is investigated using the B3LYP method of density functional theory with the lanl2dz basis set. The model of porous silicon doped with CH₃,Si-O-Si and OH species is built. By analyzing the binding energy and electronic transfer, we conclude that the interaction of S₂ molecule with SiH_x (x=1, 2, 3) is much stronger than the interaction of S₂ molecule with CH₃ and OH, as S₂ molecule is located in different sites of the model. Using the transition state theory, we study the Si₂H₆+S2→H₃SiH₂SiS+HS reaction, and the reaction energy barrier is 50.2 kJ/mol, which indicates that the reaction is easy to occur.     

Theoretical study on the substitution reactions of fluorosilylenoid H2SiLiF with SiH3XH n−1 (X = F,Cl, Br,O, N; n = 1, 1, 1, 2, 3)

The substitution reactions of H₂SiLiF (A) with SiH₃XH _n?1 (X = F, Cl, Br, O, N; n = 1, 1, 1, 2, 3) have been studied using DFT and ab initio methods. The results indicate that the substitution reactions of A with SiH₃XH _n?1 proceed via two reaction paths, I and II. The following conclusions emerge from this work. (i) The substitutions of A with SiH₃XH _n?1 are nucleophilic reactions and occur in a concerted manner. Path I is more favorable than path II. The substitution barriers of A with SiH₃XH _n?1 for path I decrease with the increase of the atomic number of X for the same period systems, whereas the barriers increase with the increase of the atomic number of X for the same family systems. (ii) The substitution products are H₂SiFSiH₃ and LiXH _n?1. If the H atoms in SiH₃ of SiH₃XH _n–1 are substituted by different atoms or groups, silanes H₂SiFSiH₃ obtained via paths I and II would be enantiomers. (iii) All the substitution reactions of A with SiH₃XH _n?1 are exothermic.     

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.     

The mechanism of plasma-induced deposition of amorphous silicon from silane

Time-resolved mass spectrometric data show that the concentration of di- and trisilane, which are formed from monosilane under discharge conditions typical for the deposition of high electronic quality amorphous silicon, correlate with the measured deposition rate of a-Si. The data can be quantitatively and self-consistently described by a simple set of consecutive reactions:

1. SiH₄ →-SiH₂ + H₂
2. SiH₂ + SiH₄ → Si₂H₆
3. SiH₂ + Si₂H₆ → Si₃H₈
4. Si _n H_{2(n + 1)} →n ·a-Si:H+(n+1)H₂,n=2,3

The only fitting parameter necessary for an excellent fit of the measured data over a wide range of experimental parameters is the value of the reactive sticking coefficient .for the decomposition of di- and trisilane (reaction 3). The resultant value agrees well with the published data of other authors and with those calculated from the measured deposition rate and Si₂H₆, concentration. We did not find and physically meaningful way to lit the measured data with the various “SiH₃ models” proposed by other authors who assumed that the dominant species responsiblefor the deposition of a-Si: H is the SiH₃, radical. For this and some additional reasons mentioned in the present paper. the SiH₃ model is in disagreement with available experimental data.     

Self-Organization and Dynamics of Nanoparticles in Chemically Active Plasmas for Low-Temperature Deposition of Silicon and Carbon-Based Nanostructured Films

Self-organization and dynamic processes of nano/micron-sized solid particles grown in low-temperature chemically active plasmas as well as the associated physico-chemical processes are reviewed. Three specific reactive plasma chemistries, namely, of silane (SiH₄), acetylene (C₂H₂), and octafluorocyclobutane (c—C₄F₈) RF plasma discharges for plasma enhanced chemical vapor deposition of amorphous hydrogenated silicon, hydrogenated and fluorinated carbon films, are considered. It is shown that the particle growth mechanisms and specific self-organization processes in the complex reactive plasma systems are related to the chemical organization and size of the nanoparticles. Correlation between the nanoparticle origin and self-organization in the ionized gas phase and improved thin film properties is reported. Self-organization and dynamic phenomena in relevant reactive plasma environments are studied for equivalent model systems comprising inert buffer gas and mono-dispersed organic particulate powders. Growth kinetics and dynamic properties of the plasma-assembled nanoparticles can be critical for the process quality in microelectronics as well as a number of other industrial applications including production of fine metal or ceramic powders, nanoparticle-unit thin film deposition, nanostructuring of substrates, nucleating agents in polymer and plastics synthesis, drug delivery systems, inorganic additives for sunscreens and UV-absorbers, and several others. Several unique properties of the chemically active plasma-nanoparticle systems are discussed as well.     

Molecular structures and infrared spectra of SiZrH4: A theoretical study

Molecular structures of the SiZrH₄ complex were investigated at BPW91, BPW91/IEF-PCM, B3LYP, and MP2 levels of theory with substantial basis sets. Relative stability of the stable conformers is fairly dependent on the methods, solvent effects, and zero-point energy corrections. All the four levels of calculations indicated that the singlet HSi(μ-H)ZrH₂, trans-Si(μ-H)₂ZrH₂, cis-Si(μ-H)₂ZrH₂, and the triplet Si(μ-H)₃ZrH are stable and comparable in energy. The energy of these four isomers is well below that of the Zr(³F₂) + SiH₄ system. The trans-dibridged, rather than the tribridged, isomer was always predicted to be the most stable one by all the four levels of calculations. For the two dibridged isomers, the two SiH₂ stretching modes are highly coupled with the two ZrH₂ stretching modes. And such coupling cannot be removed by the full deuteration.     

Surface characterization of plasma‐modified poly(ethylene terephthalate) film surfaces

Poly(ethylene terephthalate) (PET) film surfaces were modified by argon (Ar), oxygen (O₂), hydrogen (H₂), nitrogen (N₂), and ammonia (NH₃) plasmas, and the plasma‐modified PET surfaces were investigated with scanning probe microscopy, contact‐angle measurements, and X‐ray photoelectron spectroscopy to characterize the surfaces. The exposure of the PET film surfaces to the plasmas led to the etching process on the surfaces and to changes in the topography of the surfaces. The etching rate and surface roughness were closely related to what kind of plasma was used and how high the radio frequency (RF) power was that was input into the plasmas. The etching rate was in the order of O₂ plasma > H₂ plasma > N₂ plasma > Ar plasma > NH₃ plasma, and the surface roughness was in the order of NH₃ plasma > N₂ plasma > H₂ plasma > Ar plasma > O₂ plasma. Heavy etching reactions did not always lead to large increases in the surface roughness. The plasmas also led to changes in the surface properties of the PET surfaces from hydrophobic to hydrophilic; and the contact angle of water on the surfaces decreased. Modification reactions occurring on the PET surfaces depended on what plasma had been used for the modification. The O₂, Ar, H₂, and N₂ plasmas modified mainly CH₂ or phenyl rings rather than ester groups in the PET polymer chains to form C? O groups. On the other hand, the NH₃ plasma modified ester groups to form C? O groups. Aging effects of the plasma‐modified PET film surfaces continued as long as 15 days after the modification was finished. The aging effects were related to the movement of C?O groups in ester residues toward the topmost layer and to the movement of C? O groups away from the topmost layer. Such movement of the C?O groups could occur within at least 3 nm from the surface. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3727–3740, 2004     

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.     

TiOx Films Deposited by Plasma Enhanced Chemical Vapour Deposition Method in Atmospheric Dielectric Barrier Discharge Plasma

The plasma enhanced chemical vapour deposition method applying atmospheric dielectric barrier discharge (ADBD) plasma was used for TiO_x thin films deposition employing titanium (IV) isopropoxide and oxygen as reactants, and argon as a working gas. ADBD was operated in the filamentary mode. The films were deposited on glass. The films?? chemical composition, surface topography, wettability and aging were analysed, particularly the dependence between precursor and reactant concentration in the discharge atmosphere and its impact on TiO_x films properties. Titanium in films near the surface area was oxidized, the dominating species being TiO₂ and substoichiometric titanium oxides. The films exhibited contamination with carbon, as a result of atmospheric oxygen and carbon dioxide reactions with radicals in films. No relevant difference of the film surface due to oxygen concentration inside the reactor was determined. The films were hydrophilic immediately after deposition, afterwards their wettability diminished, due to chemical reactions of the film surface and chemical groups involved in the atmosphere.