Dissociation Of SiH4 Research Articles

Silicon Hydride Clusters Si5Hn (n = 3−12) and Their Anions: Structures, Thermochemistry, and Electron Affinities

The molecular structures, electron affinities, and dissociation energies of the Si(5)H(n)/Si(5)H(n)(-) (n = 3-12) species have been calculated by means of three density functional theory (DFT) methods. The basis set used in this work is of double-zeta plus polarization quality with additional diffuse s- and p-type functions, denoted DZP++. The geometries are fully optimized with each DFT method independently. Three different types of the neutral-anion energy separations presented in this work are the adiabatic electron affinity (EA(ad)), the vertical electron affinity (EA(vert)), and the vertical detachment energy (VDE). The first Si-H dissociation energies for neutral Si(5)H(n) and its anion have also been reported.

The Influence of B2H6 on the Growth of Silicon Nanowire

The un-doped and boron-doped silicon nanowires (SiNWs) were grown via vapor–liquid–solid (VLS) mechanism by low pressure chemical deposition (LPCVD). The diameters of un-doped and boron-doped SiNWs varied from 18.5 to 75.3 nm and 26.6 to 66.1 nm, respectively. The critical growth temperature of boron-doped SiNWs is 10°C lower than that of un-doped ones and the diameters of the boron-doped SiNWs is always larger than that of the un-doped ones under different growth temperatures. This is because that the introduction of diborane enhanced the dissociation of SiH4 which determines the growth process of SiNW. A growth process of silicon nanowire is proposed to describe the influence of B2H6.

High pressure regime of plasma enhanced deposition of microcrystalline silicon

An investigation of the effect of the total gas pressure on the deposition of microcrystalline thin films form highly diluted silane in hydrogen discharges was carried out at two different frequencies. The study was performed in conditions of constant power dissipation and constant silane partial pressure in the discharge while using a series of plasma diagnostics as electrical, optical, mass spectrometric, and in situ deposition rate measurements together with a simulator of the gas phase and the surface chemistry of SiH4∕H2 discharges. The results show that both the electron density and energy are affected by the change of the total pressure and the frequency. This in turn influences the rate of high energy electron–SiH4 dissociative processes and the total SiH4 consumption, which are favored by the frequency increase for most of the pressures. Furthermore, frequency was found to have the weakest effect on the deposition rate that was enhanced at 27.12MHz only for the lowest pressure of 1Torr. On the other hand, the increase of pressure from 1to10Torr has led to an optimum of the deposition rate recorded at 2.5Torr for both frequencies. This maximum is achieved when the rate of SiH4 dissociation to free radical is rather high; the flux of species is not significantly hindered by the increase of pressure and the secondary gas phase reactions of SiH4 act mainly as an additional source of film precursors.

The Silicon Hydride Clusters Si3Hn (n ≤ 8) and Their Anions: Structures, Thermochemistry, and Electron Affinities

AbstractFor Abstract see ChemInform Abstract in Full Text.

The Silicon Hydride Clusters Si3Hn (n ≤ 8) and Their Anions: Structures, Thermochemistry, and Electron Affinities

The molecular structures, electron affinities, and dissociation energies of the Si 3 H n /S1 3 H n - (n ≤ 8) species have been examined using five hybrid and pure density functional theory (DFT) methods. The basis set used inthis work is of double-ζ plus polarization quality with additional diffuse s- and p-type functions, denoted DZP++. These methods have been carefully calibrated (Chem. Rev. 2002, 102, 231). The geometries are fully optimized with each DFT method independently. Three different types of the neutral-anion energy separations presented in this work are the adiabatic electron affinity (EA a d ), the vertical electron affinity (EA v e r t ), and the vertical detachment energy (VDE). The first Si-H dissociation energies D e (Si 3 H n →Si 3 H n - 1 +H) for the neutral Si 3 H n and D e (Si 3 H n - →Si 3 H n - 1 - +H) for the anionic Si 3 H n - species have also been reported. In the prediction of bond lengths, the BHLYP predicts the most reliable Si-Si bond lengths and the B3LYP predicts the most reliable Si-H bond lengths. The most reliable EA a d , obtained at the B3LYP and BPW91 levels of theory, are 2.34 or 2.32 eV (Si 3 ), 2.56 eV (Si 3 H), 1.73 or 1.74 eV (Si 3 H 2 ), 2.46 or 2.45 eV (Si 3 H 3 ), 1.95 or 1.93 eV (Si 3 H 4 ), 2.24 or 2.23 eV (Si 3 H 5 ), 1.41 or 1.30 eV (Si 3 H 6 ), and 2.14 eV (Si 3 H 7 ). For Si 3 H 8 , there are no reliable EA a d but there are reliable VDE. The values of VDE for Si 3 H 8 are 1.03 eV (B3LYP) or 1.10 eV (BPW91). The first dissociation energies (Si 3 H n →Si 3 H n - 1 +H) predicted by all of these methods are 2.65∼2.74 eV (Si 3 H), 2.82∼3.12 eV (Si 3 H 2 ), 2.13∼2.23 eV (Si 3 H 3 ), 2.92∼3.08 eV (Si 3 H 4 ), 2.63∼2.95 eV (Si 3 H 5 ), 3.29∼3.53 eV (Si 3 H 6 ), 2.19∼2.54 eV (Si 3 H 7 ), and 3.45∼3.62 eV (Si 3 H 8 ). For anion clusters (Si 3 H n - → Si 3 H n - 1 - +H), the dissociation energies predicted are 2.89∼2.95 eV (Si 3 H - ), 2.01∼2.27 eV (Si 3 H 2 - ), 2.83∼2.96 eV (Si 3 H 3 - ), 2.37∼2.68 eV (Si 3 H 4 - ), 2.96∼3.10 eV (Si 3 H 5 - ), 2.36∼2.74 eV (Si 3 H 6 - ), 3.03∼3.16 eV (Si 3 H 7 - ), and 1.44∼1.54 eV (Si 3 H 8 - ).

Dissociative Adsorption of Methylsilane on the Si(100)-2 × 1 Surface

Density functional theory calculations have been used to investigate the mechanisms of dissociative adsorption of methylsilane (CH3SiH3) on the Si(100)-2 x 1 surface. Three different reaction pathways via Si-H, C-H, and Si-C dissociation are used to describe the formation of -SiH2CH3, -CH2SiH3, -CH3, -SiH3, and -H fragments adsorbed on the dimer dangling bonds. The geometry, energetics, and vibration properties of the critical points along the potential energy surface using the Si9H12 one-dimer and Si15H16 two-dimer cluster models are investigated. Our results indicate that the product of Si-H dissociation is the dominating dissociative product. The results also show that methylsilane is a good candidate for the growth of SiC films.

First principles calculations of the local arrangement of silicon hydrides on the Si( [formula omitted])-c(2×4) surface

Using first principles total energy calculations, we have studied the energetics and structural properties of silicon trihydride (SiH 3) and dihydride (SiH 2) species adsorbed on the Si(0 0 1)-c(2 × 4) surface. These are fragments produced in the chemical vapor deposition of silane and disilane on Si(0 0 1). It is found that in the absence of hydrogen atoms, the SiH 2 subunit prefers to be adsorbed in the on-dimer site (on top of a Si dimer) rather than in the intra-layer site (between two Si dimers of the same row). However, if we consider the two hydrogen atoms produced in the dissociation of the SiH 4 molecule, the relative stability of the two sites is reversed. These results are in good agreement with experiments that found the SiH 2 fragment to be located at the intra-row site only. We have also considered the dissociation of SiH 4 into a SiH 3 fragment and a hydrogen atom. It is found that the SiH 3 fragment and the hydrogen atom are both attached to Si dangling bonds. The lowest energy configuration involving a trihydride radical is metastable with respect to the lowest energy configuration involving a dihydride radical. A similar situation is found for the adsorption of a Si 2H 6 molecule. The configuration with trihydride fragments is less stable than the configuration with dihydride fragments.

DECOMPOSITION OF SiH4 ON THE SA TYPE STEPPED Si(100) SURFACE

The density functional theory method is used to explore the mechanism of dissociative adsorption of silane (SiH4) on the SA type stepped Si(100) surface. Two reaction paths are described that produce silyl (SiH3) and hydrogen atom fragments adsorbed on the dimer bonds present on each terrace. It has been found that the initial stage of the dissociation of SiH4 on the SA type stepped Si(100) surface shows similarity to the dissociation of SiH4 on the flat Si(100) surface; SiH3 and hydrogen fragments bond to the Si dimer atoms by following the first reaction path.

Structural evaluation of polycrystalline silicon thin films by hot-wire-assisted PECVD

Tungsten wires were introduced into a plasma-enhanced chemical vapor deposition (PECVD) system as a catalyzer: we name this technique ‘hot-wire-assisted PECVD’ (HW-PECVD). Under constant deposition pressure (pg), gas flow ratio and catalyzer position, the effects of the hot wire temperature (Tf) on the structural properties of the poly-Si films have been characterized by X-ray diffraction (XRD), Raman scattering and Fourier-transform infrared (FTIR) spectroscopy. Compared with conventional PECVD, the grain size, crystalline volume fraction (Xc) and deposition rate were all enhanced when a high Tf was used. The best poly-Si film exhibits a preferential (220) orientation, with a full width at half-maximum (FWHM) of 0.2°. The Si-Si TO peak of the Raman scattering spectrum is located at 519.8 cm−1 with a FWHM of 7.1 cm−1. The Xc is 0.93. These improvements are mainly the result of promotion of the dissociation of SiH4 and an increase in the atomic H concentration in the gas phase.

Hydrogenated amorphous silicon deposited at very high growth rates by an expanding Ar–H2–SiH4 plasma

The properties of hydrogenated amorphous silicon (a-Si:H) deposited at very high growth rates (6–80 nm/s) by means of a remote Ar–H2–SiH4 plasma have been investigated as a function of the H2 flow in the Ar–H2 operated plasma source. Both the structural and optoelectronic properties of the films improve with increasing H2 flow, and a-Si:H suitable for the application in solar cells has been obtained at deposition rates of 10 nm/s for high H2 flows and a substrate temperature of 400 °C. The “optimized” material has a hole drift mobility which is about a factor of 10 higher than for standard a-Si:H. The electron drift mobility, however, is slightly lower than for standard a-Si:H. Furthermore, preliminary results on solar cells with intrinsic a-Si:H deposited at 7 nm/s are presented. Relating the film properties to the SiH4 dissociation reactions reveals that optimum film quality is obtained for conditions where H from the plasma source governs SiH4 dissociation and where SiH3 contributes dominantly to film growth. Conditions where ion-induced dissociation reactions of SiH4 prevail and where the contribution of SiH3 to film growth is much smaller lead to inferior film properties. A large contribution of very reactive (poly)silane radicals is suggested as the reason for this inferior film quality. Furthermore, a comparison with film properties and process conditions of other a-Si:H deposition techniques is presented.

Film growth precursors in a remote SiH4 plasma used for high-rate deposition of hydrogenated amorphous silicon

The SiH4 dissociation products and their contribution to hydrogenated amorphous silicon (a-Si:H) film growth have been investigated in a remote Ar–H2–SiH4 plasma which is capable of depositing device-quality a-Si:H at 10 nm/s. SiH3 radicals have been detected by means of threshold ionization mass spectrometry for different fractions of H2 in the Ar–H2-operated plasma source. It is shown that at high-H2 flows, SiH4 dissociation is dominated by hydrogen abstraction and that SiH3 contributes dominantly to film growth. At low-H2 flows, a significant amount of very reactive silane radicals, SiHx(x⩽2), is produced, as concluded from threshold ionization mass spectrometry on SiH2 and optical emission spectroscopy on excited SiH and Si. These radicals are created by dissociative recombination reactions of silane ions with electrons and they, or their products after reacting with SiH4, make a large contribution to film growth at low-H2 flows. This is corroborated by the overall surface reaction probability which decreases from ∼0.5 to ∼0.3 with increasing H2 fraction. The film properties improve with increasing H2 flow and device-quality a-Si:H is obtained at high H2 fractions where SiH3 dominates film growth. Furthermore, it is shown that at high-H2 flows the contribution of SiH3 is independent of the SiH4 flow while the deposition rate varies over one order of magnitude.

Absolute cross section for the formation of Si(1S) atoms following electron impact dissociation of SiH4

A combination of electron scattering and laser-induced fluorescence (LIF) techniques was used in the experimental determination of the absolute cross section for the formation of Si(1S) ground-state atoms following the neutral molecular dissociation of SiH4 by electron impact for energies from 20 eV to 100 eV. Electron impact on SiH4 produces—among other species—Si(1S) ground-state atoms which are detected by pumping the Si(3p)2 1S→(3p)(4s)1P transition at 390 nm with a tunable dye laser and recording the subsequent Si(3p)(4s)1P→(3p)2 1D fluorescence at 288 nm. We found a peak cross section for the formation of Si(1S) atoms from SiH4 of 4.5×10−17 cm2 at an impact energy of 60 eV. When compared to the previously determined total SiH4 neutral dissociation cross section obtained from measurements in a constant-flow plasma reactor [Perrin et al., Chem. Phys. 73, 383 (1982)], we find a branching ratio of about 0.037 for the formation of Si(1S) atoms in the electron-impact induced neutral dissociation of SiH4. The absolute calibration of our measured dissociation cross section was made relative to the cross section for the formation of N2+(X) ground-state ions produced by electron impact on N2 which was previously measured in the same apparatus using the same experimental technique. This cross section is known to within ±10% and can serve as a benchmark for the calibration of neutral dissociation cross sections as discussed previously [Abramzon et al., J. Phys. B 32, L247 (1999)]. © 2000 American Institute of Physics.

Electron-induced dissociation of SiH complexes in hydrogenated Si-doped GaAs. Application to the fabrication of microstructures

We study the role of hot electron injection in the dissociation of the SiH complexes which appear in n-type Si-doped GaAs epilayer exposed to a hydrogen or deuterium plasma. Firstly, the results recently obtained in room-temperature aging experiments on hydrogenated or deuterated Schottky diodes submitted to high bias voltage are summarized and the role of hot carriers in the dissociation of donors and the observed isotope effect are described. Then, it is shown that SiH dissociation can also be achieved using hot carriers injected into the semiconductor by electron beam. Such electron-beam effects are finally used for the fabrication and characterization by cathodoluminescence of micronic conductive GaAs structures.

Decrease in Deposition Rate and Improvement of Step Coverage by CF4 Addition to Plasma-Enhanced Chemical Vapor Deposition of Silicon Oxide Films

The low dielectric constant of F-doped silicon dioxide film makes it suitable for use as an intermetal film to improve the performance of ultra-large scale integrated circuits (ULSIs). One of the properties required by an intermetal film is good gap filling. It is known that fluorine addition to silicon oxide decreases the film deposition rate and improves the step coverage. In order to investigate these phenomena, we study the reaction mechanism of plasma-enhanced chemical vapor deposition (PECVD) silicon oxide film and its change by fluorine addition. Using a two film-forming species model, we explain the dependence of the deposition rate and the step coverage on the residence time for a SiH4/N2O-based PECVD system. The precursor produced by the dissociation of SiH4 has a relatively high sticking probability (≈0.5), while an intermediate species has a low (<10-4) sticking probability. The concentration of each species for deposition is changed by the residence time of the gas, thus the deposition rate and the step coverage show dependence on the residence time. The deposition rate of silicon oxide films is decreased and the step coverage is improved by CF4 addition during SiH4/N2O-based PECVD. From the estimation of the sticking probability, we suggest that the reason for the improvement of the step coverage by fluorine addition is not the etching effect by CF4 addition, but the decrease in sticking probability of the precursor produced by the dissociation of SiH4.

Formation of cationic silicon clusters in a remote silane plasma and their contribution to hydrogenated amorphous silicon film growth

The formation of cationic silicon clusters SinHm+ by means of ion–molecule reactions in a remote Ar–H2–SiH4 plasma is studied by a combination of ion mass spectrometry and Langmuir probe measurements. The plasma, used for high growth rate deposition of hydrogenated amorphous silicon (a-Si:H), is based on SiH4 dissociation in a downstream region by a thermal plasma source created Ar–H2 plasma. The electron temperature, ion fluence, and most abundant ion emanating from this plasma source are studied as a function of H2 admixture in the source. The electron temperature obtained is in the range of 0.1–0.3 eV and is too low for electron induced ionization. The formation of silicon containing ions is therefore determined by charge transfer reactions between ions emanating from the plasma source and SiH4. While the ion fluence from the source decreases by about a factor of 40 when a considerable flow of H2 is admixed in the source, the flux of cationic silicon clusters towards the substrate depends only slightly on this H2 flow. This implies a strong dissociative recombination of silicon containing ions with electrons in the downstream region for low H2 flows and it causes the distribution of the cationic silicon clusters with respect to the silicon atoms present in the clusters to be rather independent of H2 admixture. The average cluster size increases, however, strongly with the SiH4 flow for constant plasma source properties. Moreover, it leads to a decrease of the ion beam radius and due to this, to an increase of the ion flux towards the substrate, which is positioned in the center of the beam. Assuming unity sticking probability the contribution of the cationic clusters to the total growth flux of the material is about 6% for the condition in which solar grade a-Si:H is deposited. Although the energy flux towards the film by ion bombardment is limited due to the low electron temperature, the clusters have a very compact structure and very low hydrogen content and can consequently have a considerable impact on film quality. The latter is discussed as well as possible implications for other (remote) SiH4 plasmas.

Atomistic simulation study of the interactions of SiH3 radicals with silicon surfaces

SiH 3 radicals created by electron impact dissociation of SiH4 in reactive gas discharges are widely believed to be the dominant precursor for plasma deposition of amorphous and nanocrystalline silicon thin films. In this article, we present a systematic computational analysis of the interactions of SiH3 radicals with a variety of crystalline and amorphous silicon surfaces through atomistic simulations. The hydrogen coverage of the surface and, hence, the availability of surface dangling bonds has the strongest influence on the radical–surface reaction mechanisms and the corresponding reaction probabilities. The SiH3 radical reacts with unit probability on the pristine Si(001)-(2×1) surface which has one dangling bond per Si atom; upon reaction, the Si atom of the radical forms strong Si–Si bonds with either one or two surface Si atoms. On the H-terminated Si(001)-(2×1) surface, the radical is much less reactive; the SiH3 radical was reflected back into the gas phase in all but two of the 16 simulations of radical impingement designed to sample the high-symmetry adsorption sites on the surface. When SiH3 reacts on the H-terminated surface, it either inserts into the Si–Si dimer bond or returns to the gas phase as SiH4 after abstracting H from the surface. The insertion into the Si–Si bond occurs through a dissociative adsorption reaction mechanism that produces two surface SiH2 species after transfer of one of the H atoms from SiH3 to one of the dimer Si atoms. The energetics and dynamics of the surface reactions are analyzed in detail. During simulations of a-Si:H film growth, adsorption onto a dangling bond, dissociative insertion, and H abstraction reactions also were observed to occur with similar energetics as the corresponding reactions on crystalline surfaces. The radical is much more mobile on surfaces of a-Si:H films than crystalline surfaces, especially when the hydrogen concentration in the amorphous film and, thus, on the surface is high.

Structure, energetics, and vibrational properties of Si-H bond dissociation in silicon

We investigate hydrogen dissociation from an isolated Si-H bond in bulk silicon, using ab initio density- functional total-energy calculations. From the bonding site, we find that hydrogen needs to overcome a barrier of less than 2.0 eV in order to reach the next lowest local minimum in the energy surface. This minimum occurs at the antibonding site and is 1.2 eV higher in energy than the ground state. In addition, we consider the role of lattice relaxations and free carriers during the dissociation process. We discuss the relevance of our results for Si-H dissociation in several systems, including the Si-SiO 2 interface. @S0163-1829~99!05919-6# passivating the dangling bond, forming a strong Si-H bond. An additional local minimum is found for hydrogen at the antibonding site ~i.e., behind the Si atom, on the other side of the dangling bond!. Compared to hydrogen in the bonding site, hydrogen in the antibonding site has an energy 1.2 eV higher, an Si-H bond length ;3% longer and an Si-H stretch-mode frequency which is 180 cm 21 lower. When hy- drogen is trapped in the antibonding site, electronic levels are introduced into the gap allowing for carrier-enhanced Si-H dissociation. The saddle point for migration from the dangling bond to the antibonding site occurs at the bond- center site, with a migration barrier of 1.75 eV. Large lattice relaxations are required for the H atom to approach the bond- center site; the kinetics of Si-H dissociation through the bond-center site will thus be tied to silicon-silicon vibra- tional modes. An alternative pathway from the ground state to the antibonding site exists, which does not go through the bond-center site, but still requires sizable displacements of the Si atoms; this pathway has a slightly higher barrier ~1.9 eV!. The rest of this paper is organized as follows. In Sec. II, we discuss details of the theoretical approach. Our results are reported in Sec. III. In Sec. IV we discuss the implications of these results for device degradation and for experiments per- formed on hydrogen-passivated Si-SiO2 interfaces. Sec. V summarizes the paper.

Laser-Induced-Fluorescence Study of SiH2 in a RF SiH4/SiH2Cl2 Plasma.

The behavior of the SiH2 radical in a RF (13.56 MHz) SiH4/SiH2Cl2 mixture plasma wasinvestigated usinglaser-induced fluorescence spectroscopy with pulsed as well as CW laser excitation. Itwas shown that SiH2 inthe mixture plasma is mainly produced via dissociation of SiH4 and it is less reactivewith SiH2Cl2 than with SiH4. The SiH2 density in the mixture plasma increased slightly with increasing SiH2Cl2 fraction, despite thedecrease in the SiH4 density. This enhanced decomposition of SiH4 as a result of SiH2Cl2 admixing was foundto be caused by the increase in the electron temperature, which resulted from the increase in the density ofelectron-attaching species produced from SiH2Cl2 in the plasma.

A low-temperature ion vapor deposition technique for silicon and silicon–germanium epitaxy

Ion beam vapor deposition is a new technique to grow Si and SiGe layers on Si substrates at low temperatures. The growth of Si and SiGe layers reported in this paper involves ion-assisted dissociation of SiH4 into Si and H2 where Si is deposited. The Ge incorporation is achieved through thermal co-evaporation of elemental Ge. The SiGe films deposited at temperatures below 350 °C are planar and epitaxially grown, as confirmed by Rutherford back scattering, transmission electron microscopy, and electron diffraction analysis. The in situ cleaning is achieved by Ar ion bombardment followed by an in situ thermal annealing to repair the damage. Polycrystalline SiGe films are also deposited on substrate surfaces coated with a thin oxide.

Site selective SiH 4 adsorption on Si(111)(7 × 7) surfaces

Scanning tunneling microscopy (STM) was used to investigate room temperature adsorption and dissociation of SiH 4 on Si(111)(7 × 7) surfaces. The data show a pronounced site selectivity for this process. Initially the reaction involves exclusively the corner holes and the adjacent Si adatoms of the (7 × 7) reconstruction, with preferential adsorption of SiH 3 groups in the corner holes and of H atoms on one of the adjacent corner adatoms. For higher SiH 4 exposures the reactivity of the corner adatoms is significantly reduced, hydrogen adsorption occurs preferentially on the center adatoms. Deposited SiH x groups ( x = 2, 3) nucleate now in small clusters on the terraces. A higher density of these SiH x clusters on domain boundaries or at steps indicates a higher reactivity of these defect sites.