Silane Radicals Research Articles

Scaling of silicon nanoparticle growth in low temperature flowing plasmas

Low temperature plasmas are an emerging method to synthesize high quality nanoparticles (NPs). An established and successful technique to produce NPs is using a capacitively coupled plasma (CCP) in cylindrical geometry. Although a robust synthesis technique, optimizing or specifying NP properties using CCPs, is challenging. In this paper, results from a computational investigation for the growth of silicon NPs in flowing inductively coupled plasmas (ICPs) using Ar/SiH4 gas mixtures of up to a few Torr are discussed. ICPs produce more locally constrained and quiescent plasma potentials. These positive plasma potentials produce an electrostatic trap for negatively charged NPs, which can significantly extend the residence time of NPs in the plasma, which in turn provides a controllable period for particle growth. The computational platforms used in this study consist of a two-dimensional plasma hydrodynamics model, a three-dimensional nanoparticle growth and trajectory tracking model, and a molecular dynamics simulation for deriving reactive sticking coefficients of silane radicals on Si NPs. Trends for the nanoparticle growth as a function of SiH4 inlet fraction, gas residence time, energy deposition per particle, pressure, and reactor diameter are discussed. The general path for particle synthesis is the trapping of small NPs in the positive electrostatic potential, followed by entrainment in the gas flow upon reaching a critical particle size. Optimizing or controlling NP synthesis then depends on the spatial distribution of plasma potential, the density of growth species, and the relative time that particles spend in the electrostatic trap and flowing through higher densities of growth species upon leaving the trap.

백금계 촉매와 Hydrosilane 개시제를 이용한 아세트산 비닐 합성

In this work, a series of hydrosilanes including phenylsilane, diphenylsilane, triethylsilane (TES), and tris(trimethylsilyl)silane combined with catalytical amount of platinum complex such as dichloro(1,5-cyclooctadiene) platinum (COD) and dichloro(dicyclopentadienyl)platinum have been used to initiate the polymerization of vinyl acetate (VAc) at 70-95 ℃. An ultrahigh molecular weight PVAc with number-average molecular weight (M<SUB>n</SUB>) up to 1.17×10<SUP>6</SUP> Da and polydispersity index (PDI) of 2.59 has been synthesized using TES as an initiator in the presence of 9.7 ppm of COD. A kinetic study indicates that the polymerization at VAc/TES/COD molar ratio = 3.0×10<SUP>5</SUP> : 60 : 1 smoothly reached 81.0% conversion in 28.0 h at 85 ℃, producing PVAc with M<SUB>n</SUB> increasing linearly from 1.41×105 to 6.01×10<SUP>5</SUP> Da with the increase of monomer conversion. A mechanistic investigation revealed that COD was firstly reduced by reductive TES, producing Pt nanoparticles and corresponding silane radicals to initiate the polymerization of VAc.

Manganese catalyzed dehydrogenative silylation of alkenes: Direct access to allylsilanes

Dehydrogenative silylation of alkenes with silanes to produce allylsilanes is achieved through manganese catalysis with a wide scope of substrate tolerance. This transformation involves silane radicals initiated by manganese complex without additional oxidant additives. It offers a general, convenient and practical protocol with excellent functional group compatibility and gram-scale capacity for the modular synthesis of allylsilanes.

Low residual stress in hydrogenated amorphous silicon-carbon films deposited by low-temperature PECVD

Low residual stress in hydrogenated amorphous silicon-carbon (a-SixC1-x:H) films prepared by plasma-enhanced chemical vapor deposition (PECVD) at temperature range of 100–200 °C was obtained. Profilometry, Fourier transform infrared (FTIR) spectroscopy and atomic force microscopy (AFM) measurements were carried out to characterize the films. The residual stress of each deposited film was calculated using profilometry measurements and the Stoney equation. The results showed that the residual stress decreases as the power density is reduced, or the temperature or the silane/methane ratio are increased. There is a deposition pressure at around 750 mTorr at which low residual stress is promoted. The residual stress showed a correlation with the carbon incorporation in the form of C–Hn molecules. The residual stress depends on the deposition regime: assisted either by silane radicals (also known as “silane starving plasma” (SSP)) or by both silane and methane radicals. Considering that the carbon incorporation under SSP regime is more controlled, there is a higher probability of having low residual stress in this regime. In agreement with the characterization, the most favorable PECVD parameters were selected to obtain a-SixC1-x:H films with low residual stress (below 100 MPa) within the temperature range (100–200 °C). These results are useful in areas such as flexible electronic devices, implantable devices, microfluidic systems, and microelectromechanical systems, among others, in which the materials and the parameters of fabrication are degraded or modified by temperature above 200 °C.

Initial growth kinetics of positive and negative hydrogenated Si cluster ions under the presence of silane radicals

We investigated the time evolution of ion populations in an ion trap filled with SiH4 and He buffer gas to elucidate the roles of silane ions and radicals generated by an electron shower. The initial growth of ions is accelerated by SiH2 until n = 3 for cations and n = 2 for anions, where the radical effect diminishes because further Si-addition is inhibited by reverse reactions. Through reactions with SiH4, the cations reach less-reactive n = 5 with large H contents and further growth significantly slows, while the anion growth completely stops at n = 2 because is not reactive with SiH4. This anion inertness is attributed to instability of the hydrogen-bonded reaction intermediates based on ab initio calculations. Hence the ionization is not a promoting factor but the SiH2 insertion into neutral SinHx is the major pathway for cluster growth in silane plasmas.

In Situ Plasma Monitoring of PECVD nc-Si:H Films and the Influence of Dilution Ratio on Structural Evolution

We report plasma-enhanced chemical vapor deposition (PECVD) hydrogenated nano-crystalline silicon (nc-Si:H) thin films. In particular, the effect of hydrogen dilution ratio (R = H2/SiH4) on structural and optical evolutions of the deposited nc-Si:H films were systematically investigated including Raman spectroscopy, Fourier-transform infrared spectroscopy (FTIR) and low angle X-ray diffraction spectroscopy (XRD). Measurement results revealed that the nc-Si:H structural evolution, primarily the transition of nano-crystallization from the amorphous state to the nanocrystalline state, can be carefully induced by the adjustment of hydrogen dilution ratio (R). In addition, an in situ plasma diagnostic tool of optical emission spectroscopy (OES) was used to further characterize the crystallization rate index (Hα*/SiH*) that increases when hydrogen dilution ratio (R) rises, whereas the deposition rate decreases. Another in situ plasma diagnostic tool of quadruple mass spectrometry (QMS) also confirmed that the “optimal” range of hydrogen dilution ratio (R = 30–40) can yield nano-crystalline silicon (n-Si:H) growth due to the depletion of higher silane radicals. A good correlation between the plasma characteristics by in situ OES/QMS and the film characteristics by XRD, Raman and FTIR, for the transition of a-Si:H to nc-Si:H film from the hydrogen dilution ratio, was obtained.

Investigation of Electron Cyclotron Resonance Chemical Vapor Deposition Process for a-Si:H Deposition, Film Characterization and In Situ Plasma Diagnostics

We present the application of quadrupole mass spectrometry (QMS) with threshold ionization mass spectrometry (TIMS), optical emission spectroscopy (OES) with an actinometry method, and Langmuir probe (LP) as an integrated technique for in situ plasma characterization of the electron cyclotron resonance chemical vapor deposition (ECR-CVD) of hydrogenated amorphous silicon (a-Si:H). The main aim of this study was to determine the relationship among the process parameters, plasma characteristics, and thin film properties, including the microstructure parameter (R*), hydrogen content (CH), and a-Si:H film growth rate. The physical properties of plasma, such as the electron density (Ne), electron energy (Te), and sheath potential (Vs), were studied using an LP and OES. In addition to the general plasma properties, the ion density (Ni) and plasma electronegativity were examined. The results indicate the ECR plasma source has high potential for producing a high-quality a-Si:H film because of the availability of few high silanes (SinH2n+2, n > 2); furthermore, the results indicate an obvious difference in the QMS analysis results between plasma and gas. Therefore, the consumption of parent molecules must be considered. The TIMS method was applied to estimate the relative concentration of ground-state silane radicals (SiHx, x < 4) and consumption of SiH4. The results indicate that, in addition to the degree of ionization, the consumption of SiH4 and Vs are key factors affecting the thin film growth rate. When microwave power density increases, the values of Ni, Ne, and Vs increase considerably, but the plasma electronegativity remains unchanged. Furthermore, we compared our results with the TIMS analysis reported by other authors and found that the ratio of SiH2 to SiH3 obtained using QMS and TIMS can be considered an indicator of film quality for R* in the ECR-CVD process.

Highly Stable Micromorph Tandem Solar Cells Fabricated by ECRCVD With Separate Silane Gas Inlets System

In this paper, hydrogenated amorphous silicon (a-Si:H) and microcrystalline silicon ( \(\mu \) c-Si:H) films are deposited by electron cyclotron resonance chemical vapor deposition with two separate silane gas inlets. One of the silane gases (S2) is introduced near the substrate region. Effects of S2 flow rate on film properties and solar cell performance are investigated in comparison to traditional plasma-enhanced chemical vapor deposition (PECVD). The results show that the introduction of S2 gas leads to: 1) significant reduction of higher order silane radicals participating film growth; 2) dense film structure with a low microstructure factor of 0.06; and 3) lower surface roughness of the interface between top a-Si:H and bottom \(\mu \) c-Si:H subcells of micromorph tandem cells, favoring bottom \(\mu \) c-Si:H deposition. Single-junction amorphous silicon solar cells show light-induced degradation (LID) of 7.8%, almost half of that observed in PECVD cells. Micromorph tandem solar cells show a 13.3% initial conversion efficiency and a 12.7% stabilized efficiency. Highly stabilized micromorph tandem solar cell with 4.7% LID can be achieved.

Integrating the Quadrupole Mass Spectrometer, Optical Emission Spectroscopy and Langmuir Probe as a Plasma Characterization Tools for ECR-CVD with a-Si:H

We present the application of quadrupole mass spectrometer (QMS), optical emission spectroscopy (OES), and Langmuir probe (LP) as an integrated technique for plasma characterization of the process during electron cyclotron resonance chemical vapor deposition (ECR-CVD) of hydrogenated amorphous silicon (a-Si:H). The main aim of this study was to investigate the use of QMS to determine the relationship between plasma and thin film characteristics, in particular microstructure parameter (R*) and hydrogen content (CH). The OES was used to assist the QMS in measuring the light and simple radicals, such as the H, Si and SiH, and the associated physical plasma properties which include electron density (Ne), electron energy (Te) and sheath potential (Vs) would be studied with the Langmuir probe. The effects of the two ECR-CVD parameters, namely the power density (1 ~ 4 w/cm2) and hydrogen dilution ratio (H2/SiH4, 0 ~ 25) were discussed. The results indicated that there is a very obvious difference in QMS analysis between plasma and gas, and it is due to the fact that the QMS analysis of plasma is accompanied with the vapor and surface reactions. Therefore, the consumption of parent molecule (SiH4) needed to be considered. The TIMS (Threshold ionization mass spectrometry) method which is usually applied to estimate the relative density trends of the ground-state silane radicals (SiHx, x < 4) was used in this study. It was found that increasing power density and hydrogen dilution ratio would lead to higher electron density and more SiH2 generated in the plasma. Furthermore, when comparing with the TIMS analysis reported by other authors, we found the ratio of SiH2 to SiH3 from QMS could be considered as a better indicator of film quality for R*.

Silane Radical Cation: A Theoretical Account on the Jahn–Teller Effect at a Triple Degeneracy

An ab initio quantum dynamics study is performed to examine the complex nuclear motion underlying the first photoelectron band of the silane molecule due to Jahn-Teller distortion via T2⊗(e+t2+t2) coupling. The problem is investigated by employing a quadratic vibronic coupling model for the Hamiltonian. All sheets of the required potential energy surface are established through extensive electronic structure calculations using the multireference configuration-interaction method. They cover at most two dimensions of the full 9D coordinate space, with the parameters defining the model Hamiltonian being determined by a least-squares fitting procedure. The results are compared with the available experimental data and discussed in relation to those obtained for the methane radical cation. The quadratic couplings of Jahn-Teller active vibrational modes are found to have a crucial role on the irregular vibronic structure, intensity of the spectral excitations, and overall width of the first photoelectron band of the title molecule. The impact of large amplitude motions on the vibronic structure and dynamics of the first photoelectron band has also been examined by varying their linear coupling parameters up to ±10%.

Optical emission spectroscopy of argon-silane plasma

Increase in the concentration of silane radicals in plasma is extremely important for deposition of thin silicon films used in industry, in particular, for manufacturing of solar elements by the method of chemical plasma deposition. The manufacture of solar elements is a direct way to the development of cheap and non-polluting energy sources. Reduction of the cost of solar elements will expand their application fields. In addition, it is important to increase the deposition area and to improve the photoelectric properties of the solar elements. This work is aimed at investigation of the influence of silicon-containing gas pressure and discharge power on the concentration of silane radicals in plasma for subsequent deposition of amorphous silicon films. The method of optical spectroscopy is conventionally used for plasma investigation. The main advantage of this method is that it does not perturb the plasma. In addition, it can be used for constant control when it is necessary to provide reproducibility of conditions in the plasma. Changes in the intensity of optical radiation of silane radical SiH* at a wavelength of 414.2 nm accompanying changes of the pressure of the argon-silane mixture are also investigated, and the radiation intensity is compared for various powers of the high-frequency discharge in the argon-silane plasma [1].

Vacuum hydride epitaxy of silicon: kinetics of monosilane pyrolysis on the growth surface

Analytical expressions relating the rate of silicon atom incorporation into a growing crystal to the typical frequency of silane molecule pyrolysis on the silicon surface in the growth temperature range are derived. Based on currently available experimental data, the range of typical decomposition frequencies of hydride molecule radicals adsorbed at the silicon wafer surface in the temperature range of 450–700°C is determined for the most widely used physicochemical models. It is shown that the most probable molecular decomposition model can be chosen based on the experimental study of the temperature dependence of the decomposition rate of adsorbed hydride molecules. A change in the silane molecule pyrolysis rate or the hydrogen desorption rate from the surface in principle makes it possible to increase the Si layer growth rate without additional substrate heating under conditions of low-temperature epitaxy (450–550°C), but no larger than by a factor of 2–3 in the former case and up to 100 times in the latter case. The analysis performed shows that physicochemical pyrolysis models in which hydrogen is trapped by the surface, mostly at the stage of decomposition of silane radicals adsorbed by the surface, are more realistic.

Hydrogen-dominated plasma, due to silane depletion, for microcrystalline silicon deposition

Plasma conditions for microcrystalline silicon deposition generally require a high flux of atomic hydrogen, relative to SiHα=0→3 radicals, on the growing film. The necessary dominant partial pressure of hydrogen in the plasma is conventionally obtained by hydrogen dilution of silane in the inlet flow. However, a hydrogen-dominated plasma environment can also be obtained due to plasma depletion of the silane in the gas mixture, even up to the limit of pure silane inlet flow, provided that the silane depletion is strong enough. At first sight, it may seem surprising that the composition of a strongly depleted pure silane plasma consists principally of molecular hydrogen, without significant contribution from the partial pressure of silane radicals. The aim here is to bring some physical insight by means of a zero-dimensional, analytical plasma chemistry model. The model is appropriate for uniform large-area showerhead reactors, as shown by comparison with a three-dimensional numerical simulations. The SiHα densities remain very low because of their rapid diffusion and surface reactivity, contributing to film growth which is the desired scenario for efficient silane utilization. Significant SiHα densities due to poor design of reactor and gas flow, on the other hand, would result in powder formation wasting silane. Conversely, hydrogen atoms are not deposited, but recombine on the film surface and reappear as molecular hydrogen in the plasma. Therefore, in the limit of extremely high silane depletion fraction (&amp;gt;99.9%), the silane density falls below the low SiHα densities, but only the H radical can eventually reach significant concentrations in the hydrogen-dominated plasma.

Laser applications in thin-film photovoltaics

We review laser applications in thin-film photovoltaics (thin-film Si, CdTe, and Cu(In,Ga)Se2 solar cells). Lasers are applied in this growing field to manufacture modules, to monitor Si deposition processes, and to characterize opto-electrical properties of thin films. Unlike traditional panels based on crystalline silicon wafers, the individual cells of a thin-film photovoltaic module can be serially interconnected by laser scribing during fabrication. Laser scribing applications are described in detail, while other laser-based fabrication processes, such as laser-induced crystallization and pulsed laser deposition, are briefly reviewed. Lasers are also integrated into various diagnostic tools to analyze the composition of chemical vapors during deposition of Si thin films. Silane (SiH4), silane radicals (SiH3, SiH2, SiH, Si), and Si nanoparticles have all been monitored inside chemical vapor deposition systems. Finally, we review various thin-film characterization methods, in which lasers are implemented.

9‐Bromoanthracene photodimers as initiators in controlled radical polymerization: Silane radical atom abstraction coupled with nitroxide mediated polymerization

AbstractSlow initiation relative to propagation has previously prevented photodimers of 9‐bromoanthracene or 9‐chloroanthracene, formed by [4 + 4] photocyclization reactions of the analogous 9‐haloanthracene, from being viable initiators in atom transfer radical polymerization (ATRP) reactions. The resulting polymers were found to possess high polydispersity index (PDI) values, much higher than expected number average molecular weight (Mn) values, with the reaction displaying a nonlinear relationship between monomer conversion and Mn. We report here the use of silane radical atom abstraction (SRAA) to create initiating bridgehead radicals in the presence of 2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPO) to mediate the polymerization. When using SRAA coupled with nitroxide mediated polymerization, a dramatic decrease in PDI values was observed compared with analogous ATRP reactions, with Mn values much closer to those anticipated based on monomer‐to‐initiator ratios. Analysis using UV‐Vis spectroscopy indicated only partial anthracene labeling (∼ 25%) on the polymers, consistent with thermolysis of the anthracene photodimer coupled with competition between initiation from the bridgehead photodimer radical and silane‐based radical. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6016–6022, 2008

Chain extension and block copolymer synthesis using silane radical atom abstraction coupled with nitroxide mediated polymerization

Silane radicals were used to abstract bromine termini from monobrominated polystyrene (PStBr) in the presence of excess monomer and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), generating polymer radicals that underwent chain extension. Typically, 70–85% of the PStBr precursors were activated by silane radical atom abstraction (SRAA) and were elongated by nitroxide mediated polymerization (NMP), shifting to higher number-average molecular weight (Mn) values as observed by gel permeation chromatography (GPC). Chain extension did not occur until the temperature was elevated to 130°C, with no increase in Mn values observed when the reaction was held at 80°C, which is the temperature of the SRAA phase. The NMP phase of the reaction showed a linear correlation between Mn values and monomer consumption, along with first order kinetics with respect to styrene. SRAA/NMP was then applied to the synthesis of polystyrene-b-poly(n-butyl acrylate) and polystyrene-b-poly(p-methylstyrene), with analysis by GPC indicating the formation of block copolymers with a similar amount of unreacted PStBr remaining. Quantitative activation and elongation of the polymer precursors were prevented due to the ability of both the t-butoxy radicals and tris(trimethylsilyl)silane radicals to add across monomeric double bonds, competing with atom abstraction. Reactions were thus performed in which the monomer was added only after the transition to the higher temperature, which resulted in improved activation of the PStBr.

Efficient metal‐free coupling of polystyrene chains using silane radical atom abstraction

Silane radicals were used to abstract bromine from the chain terminus of monobrominated polystyrene (PStBr) chains, generating PS radicals which underwent coupling reactions to produce polystyrene (PS) of controlled, predictable molecular weights and low polydispersity index (PDI) values. This method, introduced as silane radical atom abstraction (SRAA), foregoes the requirement of metal catalysts that are utilized in atom transfer radical coupling (ATRC), while giving comparable results.

The determinants of hydrogen concentrations in hydrogenated amorphous silicon films prepared using a triode deposition system

It is important to reduce the hydrogen concentration, in particular, the Si–H2 bond concentration, in a hydrogenated amorphous silicon film to improve its light-soaking stability. In a previous study, we found that a triode configuration plasma enhanced chemical vapor deposition method provides high quality and a very low hydrogen concentration film; however, the origin of the hydrogen reduction has been unknown. In this article, we investigate the essential factor causing the very low hydrogen concentrations observed in the triode system. In several experiments, we observed strong influences of deposition precursors on the resulting hydrogen concentrations. We propose that due to a steric hindrance, the hydrogen elimination process during film growth is disturbed when higher silane radicals stick to a growth surface. In a triode system, corresponding with the separation of the film growth surface from the precursor generation region, the contribution of higher silane radicals to film growth is suppressed due to their short diffusion length and frequent collisions with silane molecules.

Breakdown physics of low-temperature silicon epitaxy grown from silane radicals

We grow epitaxial silicon films on (100) Si wafers at low temperature $(l400\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C})$, from silane radicals, to understand the mechanisms of sudden epitaxy breakdown and simultaneous growth of amorphous and crystalline silicon phases. Surface roughness is below $0.7\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ before breakdown and below $1\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ in epitaxial regions persisting after breakdown, in contrast to the roughness-induced breakdown observed in molecular beam epitaxy from atomic Si. Spherical caps of hydrogenated amorphous Si (a-Si:H) breakdown cones protrude above the crystal surface, with each sphere centered on its cone apex. This means that the a-Si:H grows isotropically from impinging radicals with low surface mobility and that the a-Si:H growth rate is higher than the epitaxial growth rate. Similar physical mechanisms likely apply to nanocrystalline silicon film growth.

Effect of the synthetic method of Pt/MgO in the hydrosilylation of phenylacetylene

This work describes the synthesis and characterization of Pt/MgO (Pt/Mg(OH)2, calcined Pt/MgO and agglomerated Pt/MgO ) catalysts. The supports of the catalysts were obtained by the hydrolysis-precipitation method and the metallic phase of platinum was incorporated by impregnation. The characterization of the synthesized materials was made by means of DRX, FTIR, TGA-DTA and TPR techniques. The catalysts were applied to the hydrosilylation of PhC≡CH with three different silanes (Ph3SiH, Ph2MeSiH and PhMe2SiH) as a model reaction. The activity of the proposed catalysts Pt/Mg(OH)2, calcined Pt/MgO and agglomerated Pt/MgO had a reaction yield of 95 %. When the catalysts Pt/Mg(OH)2 and agglomerated Pt/MgO were used in the reaction with Ph3SiH, the selectivity to the trans isomer was favored (90%). A relationship was found between the increase in the reaction time and the increase of the kinetic volume of the silane radicals. The present results were compared to the previously obtained results using as a catalyst Pt/MgO obtained by the sol-gel method.