SiH3 Radicals Research Articles

Reactive Force Field Molecular Dynamics Study of the Effects of Gaseous Species on the Composition and Crystallinity of Silicon–Germanium Thin Films

We simulated the growth of a silicon–germanium (SiGe) film using reactive force field molecular dynamics (ReaxFF MD) in combinations of SiH3, SiH2, GeH3, and GeH2 radicals to evaluate the effects of gaseous species on thin-film composition and crystallinity and to understand the growth mechanisms. The film compositions could be estimated in these combinations because of the linear increase in the Ge content of the films. The average crystallinity grown by SiH3 was higher than that by SiH2 radicals. The crystallinity of the film grown by SiH3 radicals tends to be drastically decreased by GeH2 radicals. The growth mechanisms for XH3 and XH2 (X = Si or Ge) radicals were compared. XH3 radicals abstracted surface H atoms, and then more XH3 radicals chemisorbed onto the formed dangling bonds, resulting in film growth through a two-step reaction known as the Eley–Rideal-type (ER-type) mechanism. The ER-type mechanism grows the film with a low hydrogen content and high crystallinity. In contrast, XH2 radicals displayed not only the ER-type mechanism but also a one-step reaction, the H-capturing mechanism, which incorporates surface H atoms into the gaseous species. The H-capturing mechanism results in film growth with high hydrogen content and low crystallinity. The growth mechanisms are influenced by high/low H-coverage. The surface H atoms thermally move around the bonded atoms and give their kinetic energy to the diffusing gaseous species. Excess surface H atoms promote desorption. Our results from the ReaxFF MD suggested experimental settings and conditions that would enable the growth of high-quality films. Our results also suggested that SiH3 and GeH3 radicals should be mainly generated in the gas phase for high-quality SiGe film growth.

Gas-phase kinetics in atmospheric-pressure plasma-enhanced chemical vapor deposition of silicon films

Atmospheric-pressure (AP) plasma-enhanced chemical vapor deposition of silicon (Si) films was numerically simulated. The AP plasma used for Si depositions was excited by a 150-MHz very high-frequency (VHF) electric power, which was capable of generating continuous glow discharges covering the entire electrode surface. The experimental film thickness profiles could be well fitted by the simulations by adjusting the electron density in the plasma. The results showed that, although neutral–neutral reactions proceed very rapidly due to the frequent collisions between the gas species, the dissociation of the source SiH4 molecules by electron impact is the key factor that governs the chemistry occurring in the AP-VHF plasma and promotes the film growth on the substrate. The input power dependences of electrical property of the Si films could be explained by the contribution of SiH3 radical to the deposition. It was also shown that, even though the plasma was continuous glow, the electron density changed in the direction of gas flow, suggesting that the very rapid nucleation of clusters and their growth into nanoparticles were occurring in the AP-VHF plasma.

Role of H3 + ions in deposition of silicon thin films from SiH4/H2 discharges: modeling and experiments

A 1D fluid model including a detailed chemistry for silane-hydrogen discharges as well as surface reactions to account for deposition and etching processes has been implemented to study the effects of gas pressure (1 to 3.5 Torr) and silane concentration (2 to 10%) on the deposition rate of silicon thin films in a standard RF-PECVD reactor. The thickness of the films and their deposition rate as functions of the process conditions were determined from optical modelling of UV-visible spectroscopic ellipsometry measurements. The experimental values of the deposition rate were compared with results from the 1D fluid model. SiH3 radicals are found to be the main contributor to the computed deposition rates, while H3 + ions play the main role in the etching process. The study reveals that etching by hydrogen ions must be taken into account to reproduce properly the experimental deposition rate. In particular etching by H3 + ions must be taken into account to achieve a good agreement between the experimental and modelled values of the deposition rate as a function of the total gas pressure and the silane fraction in the discharge.

Monte Carlo Simulation of Chemical Reactions in Plasma Enhanced Chemical Vapor Deposition: from Microscopic View to Macroscopic Results

We propose in the present work a Monte Carlo Simulation (MCS) of chemical reactions occurring in Plasma Enhanced Chemical Vapor Deposition (PECVD) reactor during the a-Si:H growth. From a microscopic view of chemical reactions, this MCS allowed to obtain macroscopic results. In gas phase, important reactions have been identified that contribute to the production of H, SiH2 and SiH3. We found that SiH4 →SiH2 + 2H is the dominant silane electron-impact dissociation. We found that the reaction SiH4 + H →SiH3 + H2 plays a central role in the production of SiH3 radicals. At the surface, the microscopic view allowed us to calculate site and surface reaction probabilities of SiH3 radicals. Results at macroscopic level were consistent with other works.

Two-dimensional simulations of multi-hollow VHF SiH4/H2 plasma

A triode multi-hollow VHF SiH4/H2 plasma (60 MHz) was examined at a pressure of 20 Pa by two-dimensional simulations using the fluid model. In this study, we considered the effect of the rate constant of reaction, SiH3 + SiH3→SiH2 + SiH4, on the plasma characteristics. A typical VHF plasma of a high-electron density with a low-electron temperature was obtained between two discharge electrodes. Spatial profiles of SiH3+, SiH2+, SiH3- and SiH3 densities were similar to that of the electron density while the electron temperature had a maximum value near the two discharge electrodes. It was found that the SiH3 radical density did not decrease rapidly near the substrate and the electron temperature was lower than 1 eV, suggesting that the triode multi-hollow plasma source can provide high quality amorphous silicon with a high deposition rate.

Possible scenarios for SiS formation in the interstellar medium: Electronic structure calculations of the potential energy surfaces for the reactions of the SiH radical with atomic sulphur and S2

In this Letter we report on the first characterization of the reactions SiH + S and SiH + S2 by means of electronic structure calculations of the stationary points along the reactive potential energy surfaces. According to our calculations, both reactions are barrierless and can lead to the formation of SiS (a species observed in interstellar objects) for which there are no convincing formation routes in current astrochemical models. Furthermore, we have verified that SiS2 cannot be considered an interstellar reservoir of sulphur because it is easily attacked by the abundant H atoms.

Effect of incident kinetic energy on a-Si:H structure: A molecular dynamics simulation study

This paper utilized molecular dynamics simulation to investigate the influences of the incident kinetic energy on the structure of hydrogenated amorphous silicon (a-Si:H) thin films. The SiH3 radical impinged on the Si (001) surface with substrate temperature of 500 K in the incident kinetic energy range from 0.04 to 5.81 eV. The results showed that high incident kinetic energy could smooth the surface, promote the densification of internal structure, and increase the adsorption rate of SiH3 radical and the crystalline volume fraction. The SiHx and dangling bonds, which are the key factors that affecting the film quality, have been further analyzed, the results reflected that the increase of the incident kinetic energy will increase the content of SiH combination pattern and decrease the dangling bonds in the films, thus improving the quality of the thin films. There is a critical value, 0.64 eV, 1.45 eV or 4.04 eV, of this variation rate. Steep variation trends is corresponding to the incident kinetic energy less than this critical value, and variation trends is gentle for that above it.

Absolute density of precursor SiH3 radicals and H atoms in H2-diluted SiH4 gas plasma for deposition of microcrystalline silicon films

Microcrystalline hydrogenated silicon films were produced at a high deposition rate of about 2 nm/s by using a capacitively coupled plasma under a practical pressure of around 1 kPa. The SiH4 source gas was almost fully dissociated when highly diluted with H2 gas, and the dominant species in the gas phase were found to be SiH3 radicals, which are film-growth precursors, and H atoms. The absolute density of these species was measured as the partial pressure of SiH4 gas was varied. With the increasing SiH4 gas flow rate, the SiH3 radical density, which was on the order of 1012 cm−3, increased linearly, while the H-atom density remained constant at about 1012 cm−3. The film growth mechanism was described in terms of precursors, based on the measured flux of SiH3 radicals and H atoms, and the relative fraction of higher-order radicals.

Collisional rate coefficients of SiH(X[formula omitted]) open-shell molecule induced by its collision with He([formula omitted

New adiabatic potential energy surfaces (PESs) for 2A′ and 2A″ states of SiH(X2Π)-He(1S) complex have been calculated at the restricted coupled cluster level of theory including single, double and triple excitation [RCCSD(T)]. The RKHS method was used for the analytic fit of these PESs. The resulting PESs were employed later in the close-coupling approach for the computation of the inelastic integral cross sections which take into account the fine-structure of the SiH radical. Scattering calculations, are done on a grid of collision energies large enough to ensure converged state-to-state rate coefficients for the temperature ranging from 8K up to 300K.

A computational study on the adsorption configurations and reactions of SiHx(x = 1-4) on clean and H-covered Si(100) surfaces

Possible adsorption configurations of H and SiHx (x=1−4) on clean and H-covered Si(100) surfaces are determined by using spin-polarized DFT calculations. The results show that, on the clean surface, the gas-phase hydrogen atom and SiH3 radicals effectively adsorb on the top sites, while SiH and SiH2 prefer the bridge sites of the first layer. Another possibility for SiH is to reside on the hollow sites with a triple-bond configuration. For a partially H-coverd Si(100) surface, the mechanism is similar but with higher adsorption energies in most cases. This suggests that the surface species become more stable in the presence of surface hydrogens. The minimum energy paths for the adsorption/migration and reactions of H/SiHx species on the surfaces are explored using the climbing image-nudged elastic band method. The competitive surface processes for Si thin-film formation from SiHx precursors are also predicted. The study reveals that the migration of hydrogen adatom is unimportant with respect to leaving open surface sites because of its high barriers (>29.0kcal/mol). Alternatively, the abstraction of hydrogen adatoms by H/SiHx radicals is more favorable. Moreover, the removal of hydrogen atoms from adsorbed SiHx, an essential step for forming Si layers, is dominated by abstraction rather than the decomposition processes.

Exploring the Mechanism of Reactions of SiX3 and CX3 Radicals with Si20X20 Fullerenes (X = H, F): A Density Functional Study

We have applied density functional calculations to study reactions SiX3 + Si20X20, and CX3 + Si20X20 (X = H, F) based on two reaction channels (H- or F-abstraction and H-or F-displacement from Si20H20 and Si20F20). Our results show that SiX3 radicals prefer the hydrogen or halogen atom abstraction from Si20H20 and Si20F20 fullerenes. The exothermic reaction channels are proceed via reactant-like transition states i.e. the elongation of the breaking bonds of transition states is smaller than that of their forming bonds. Among the mentioned reaction channels, the reactions of SiF3 radical with Si20F20 and CF3 radical with Si20H20 are most favorable both thermodynamically and kinetically with the lowest barrier height exothermic character. Generally, the reactions of Si20H20 and Si20F20 with SiF3 radical are more favorable than SiH3 radical and the reaction of CH3 and CF3 radicals with Si20H20 fullerenes is more favorable than SiH3 and SiF3 radicals. These results are in agreement with the electrostatic surface potentials of reactants.

Origin of Chemical Order in a-SixCyHz: Density-Functional Tight-Binding Molecular Dynamics and Statistical Thermodynamics Calculations

We investigate the growth mechanisms and structures of hydrogenated amorphous silicon carbide (a-SixCyHz) during chemical vapor deposition (CVD) by using density-functional tight-binding molecular dynamics (DFTB MD) and statistical thermodynamics (ST) calculations. Our multiscale modeling from an atomic to an experimental scale allows us to bridge the gap between micro- and macroscopic knowledge. As in any compound, the degree of chemical order in a-SixCyHz is of practical importance. However, the origin of chemical order and effects of composition on the degree of chemical order remain unknown. First, CVD simulations are performed by the impingement of CH3 and SiH3 radicals on a Si(001)-(2 × 1):H surface with DFTB MD. The initial growth process consists of an abstraction-adsorption mechanism, where a CH3 or SiH3 radical abstracts a H atom and forms a dangling bond (DB) on the surface, and a subsequent CH3 or SiH3 radical is adsorbed on the DB. A surface-adsorbed CH2 species with a DB is inserted into a n...

Radio frequency plasma deposited boron doped high conductivity p-type nano crystalline silicon oxide thin film for solar cell window layer

Wide band gap p-type nano crystalline silicon oxide (p-nc-SiO:H) window layer is useful for multi junction solar cells. We prepared such a material by using high hydrogen dilution and low plasma power that shows high optical gap as well as high conductivity. By varying CO2 flow rates of from 0.3 to 2.5 sccm we obtained the films with crystallite volume fraction (Xc) ranging from 43.7% to 4.5%, while by decreasing the plasma power density from 426, to 28 mW/cm2 the Xc was observed to increase from 14.0% to 45.8%. At a high plasma power the hydrogen dilution did not show significant change in film properties. The electrical conductivity and activation energy was observed to be favorable at low plasma power density, which was 3.4 × 10−2 S cm−1 and 0.102 eV respectively at 28 mW/cm2 plasma power density. It is expected that at a low plasma power a large number of SiH3 radicals actively take part in film deposition, leading to an improved film properties. Solar cells fabricated with the selected p-nc-SiO:H, shows improved short wavelength response of the quantum efficiency, indicating the advantage of using such p-type materials. The optical gap of these p-nc-SiO:H films were in the range of 1.784 to 2.130 eV.

The reason why thin-film silicon grows layer by layer in plasma-enhanced chemical vapor deposition.

Thin-film Si grows layer by layer on Si(001)-(2 × 1):H in plasma-enhanced chemical vapor deposition. Here we investigate the reason why this occurs by using quantum chemical molecular dynamics and density functional theory calculations. We propose a dangling bond (DB) diffusion model as an alternative to the SiH3 diffusion model, which is in conflict with first-principles calculation results and does not match the experimental evidence. In our model, DBs diffuse rapidly along an upper layer consisting of Si-H3 sites, and then migrate from the upper layer to a lower layer consisting of Si-H sites. The subsequently incident SiH3 radical is then adsorbed onto the DB in the lower layer, producing two-dimensional growth. We find that DB diffusion appears analogous to H diffusion and can explain the reason why the layer-by-layer growth occurs.

Plasma diagnostic approach for high rate nanocrystalline Si synthesis in RF/UHF hybrid plasmas using a PECVD process

Hydrogenated nanocrystalline silicon (nc-Si : H) films intended for efficient nc-Si : H solar cells are usually made at the transition to the nanocrystalline regime using the plasma-enhanced chemical vapor deposition (PECVD) process. This change occurs within a sensitive process window and is affected by various deposition parameters. This paper reports a study of nc-Si : H films' fabrication by utilizing systematic plasma diagnostics. This work presents a novel approach for plasma processing using radio frequency (RF), ultra high frequency (UHF) and RF/UHF hybrid plasmas. Using careful analysis, efforts are made to investigate the radicals and plasma formation by changing the operating source power and silane (SiH4) concentration. The aim of this work is also to investigate the PECVD process and conditions favorable for the synthesis of nc-Si : H film. For the present study, we systematically use the optical emission spectroscopy (OES), normal, and RF-compensated Langmuir probe (LP) and vacuum ultraviolet absorption spectroscopy diagnostics. Measurements reveal that the OES diagnostic is consistent with the LP measurements. Investigation reveals that UHF power in addition to RF enables higher dissociation of H or SiH radicals and the production of higher plasma density. The combined effect of both RF and UHF sources is used as the hybrid plasma source. Measurements also reveal that inbetween SiH4 flow rates ∼20–30 sccm, there is significant change in the plasma characteristics that denotes the nc-Si : H−a-Si : H transition region. An atomic hydrogen density (nH) in the range ≈(8 − 10) × 1011 cm−3 and plasma density n0 ≈ (2 − 3) × 1011 cm−3 with a silane to hydrogen ratio of 1–2% with high crystallinity has been obtained. Along with the discussion on the effect of frequency on plasma chemistry, this explains the RF power coupling and the role of electrons and ions in plasmas with increasing frequency.

Investigation of structural disorder using electron temperature in VHF-PECVD on hydrogenated amorphous silicon films for thin film solar cell applications.

Electrode distances and gas flow ratios are important parameters for fabricating intrinsic (i-type) layers of hydrogenated amorphous silicon (a-Si:H) films using a very high frequency plasma-enhanced chemical-vapor deposition (VHF-PECVD) system. In this work, we investigated the relationship between the electrode distances and gas flow ratios on the properties of i-type a-Si:H films. The electrical, chemical and structural properties are improved with decreasing electrode distances (20-40 mm) at a hydrogen ratio [R (H2/SiH4) = 4], due to the low electron temperature and heating effect. A low electron temperature generates silane-related-reactive species (SiH3) and decreases structural disorder resulting in high quality i-type a-Si:H films. The electrical, chemical and structural properties of the a-Si:H films are confirmed using Al coplanar electrodes, FTIR, Raman spectroscopy, and spectroscopy ellipsometry (SE). When a solar cell is fabricated using the a-Si:H film, J(sc) of 13.2-14.8 mA/cm2, photoconductivity of 1.5 x 10(-5)-8.6 x 10(-6) S/cm, Si--H2 content of 0-1.24 at.%, and hydrogen content of about 10 at.% are obtained. These results together with a model of the plasma chemistry indicate that H atoms and SiH3 radicals play an important role in the deposition process.

Control of micro void fraction and optical band gap in intrinsic amorphous silicon thin films (VHF-PECVD) for thin film solar cell application

Plasma parameters are important factors for fabricating intrinsic (i-type) layers of hydrogenated amorphous silicon (a-Si:H) films using a very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD) system. In this work, the effects of hydrogen ratio (R), power density, pressure, electrode instances (Ed) and substrate temperature (Ts) on the growth and the properties of intrinsic amorphous silicon (i-type a-Si:H) thin films are investigated. The structural defect (micro void fraction: R*) and optical band gap (Eg) can be controlled by changing the plasma conditions. High quality i-type a-Si:H is obtained with low power, pressure, moderate hydrogen ratio (R&9552;H2/SiH4: 4) and short electrode distance conditions at which a lot of SiH3 radicals are generated. When the substrate temperate is above 200°C, hydrogen effusion decreases SiH bonding and optical band gap of 1.74eV is obtained.The properties of i-type a-Si:H depending on plasma conditions show hydrogen content (C(H))=6–11%, R*=0–0.2 and Eg=1.74–1.86eV. The properties of region of low structural defect and good passivation represent C(H)=about 10 at.%, R*=0 and Eg=1.77eV.

Computational Study for Reactions of H Atoms with Adsorbed SiH3 and Si2H5 on H-Covered Si(100)-(2 × 1) Surface

Density functional theory calculations with spin-polarization effect were employed to illustrate the adsorption of SixHy (x = 1–2; y = 1–5) species and the reactions of H atoms with adsorbed SiHx (x = 1–3) and Si2H5 species on the H-covered Si(100)-(2 × 1) surface. The configurations and energies of these adsorbates were elucidated. It was found that H vacancy sites can be easy created by SiH3 and Si2H5 radicals with small barriers (2.1 and 1.6 kcal/mol for SiH3 and Si2H5, respectively). The Si2H5, Si2H4, SiHSiH3, SiH3, SiH2, SiH, and Si radials interact with the surface more forcefully than the Si2H6 and SiH4 molecules. Potential energy surfaces for the reaction mechanisms of H + SiHx(a) and H + Si2H5(a) were mapped by using the nudged elastic band method. The calculation results demonstrate that the most favorable pathway is hydrogen abstraction leading to the production of H2 and SixHy (x = 1–2; y = 0–4) because of their low barriers and high exothermicities. Rate constant calculations were performed to study the kinetic behavior for simulation of silicon thin-film growth by chemical vapor deposition processes.

Excitation energies, photoionization cross sections, and asymmetry parameters of the methyl and silyl radicals.

Vertical excitation energies of the methyl and silyl radicals were inferred from ab initio electron propagator calculations on the electron affinities of CH3(+) and SiH3(+). Photoionization cross sections and angular distribution of photoelectrons for the outermost orbitals of both CH3 and SiH3 radicals have been obtained with the Molecular Quantum Defect Orbital method. The individual ionization cross sections corresponding to the Rydberg channels to which the excitation of the ground state's outermost electron gives rise are reported. Despite the relevance of methyl radical in atmospheric chemistry and combustion processes, only data for the photon energy range of 10-11 eV seem to be available. Good agreement has been found with experiment for photoionization cross section of this radical. To our knowledge, predictions of the above mentioned photoionization parameters on silyl radical are made here for the first time, and we are not aware of any reported experimental measurements. An analysis of our results reveals the presence of a Cooper minimum in the photoionization of the silyl radical. The adequacy of the two theoretical procedures employed in the present work is discussed.

Features of the two-component decomposition of monosilane molecules on a silicon surface under epitaxial-process conditions

Using the results of technological experiments based on a two-component kinetic model (SiH4 → SiH3 + SiH), the range of characteristic decomposition frequencies of silicon-hydride molecule radicals adsorbed by the layer growth surface in the temperature range of 450–700°C is determined; the degree of silicon surface coverage by SiH radicals under epitaxial-growth conditions is estimated. The behavior of the temperature dependences of the factor of surface filling by individual molecule fragments and the rate of their decomposition on the silicon surface are determined under various conditions corresponding to a constant concentration ratio of monosilane radicals (SiH = gSiH3) or constant rates of their decomposition \(\left( {v_{SiH_3 } = \xi v_{SiH} } \right)\) in the entire temperature range. It is shown that the observed shape of the temperature dependence of the molecule decomposition rate on the growth surface is not described by simple activation-type curves, which is associated with features of the interaction of the molecular hydride beam with the Si surface under conditions of low and high levels of surface-bond saturation with hydrogen. The effect of the mechanism of the adsorption of hydrogen atoms and various conditions of their transfer from the molecule to the growth surface on the temperature dependences is considered.