High resolution Si 2p gas phase photoelectron spectra of model silicon compounds: implications for hydrogen adsorbed on a silicon surface

High resolution Si 2p gas phase photoelectron spectroscopy has been used to measure the vibrational energy spacing in SiH₄ (0.295 eV) and SiD₄ (0.212 eV). These values are compared with those measured for the chemical shift (0.30 eV/hydrogen atom) of model compounds which mimic the effects of zero, one, two and three hydrogen atoms bonded to a silicon atom on a silicon surface. Implications for the interpretation of surface photoelectron spectra are discussed.     

Multiple internal reflection spectroscopy of bonded silicon wafers

Interfaces of bonded hydrophilic and hydrophobic wafer pairs are studied by multiple internal reflection spectroscopy after annealing at 1100°C. Si–H_x and SiO–H stretching modes are still present in bonded hydrophilic wafers. Interfaces of bonded hydrophobic wafers, prepared by joining HF-etched surfaces without defonized water rinsing, are characterized by the dominance of hydrides (SiH, SiH₂, SiH₃). Their concentration is about 100 times higher than for bonded hydrophilic wafers. Comparison with the ATR-spectra of HF-treated surfaces showed appreciable shifts in the peak positions indicating that Si–H bonds might be involved in the bonding process.     

The analysis of structural and electronic environments of silicon network in HWCVD deposited a-SiC:H films

Hydrogenated amorphous silicon carbon alloys (a-SiC:H) films were deposited by hot wire chemical vapour deposition (HWCVD) using SiH₄ and C₂H₂ as precursor gases. a-SiC:H films were characterized by Fourier Transform Infrared (FTIR) spectroscopy, Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Solid-state plasmon of Si network shifts from 19.2 to 20.5 eV by varying C₂H₂ flow rate from 2 to 10 sccm. Incorporation of carbon content changes the valence band structure and s orbital is more dominant than sp and p orbital with carbon incorporation.     

Site selective SiH4 adsorption on Si(111)(7 × 7) surfaces

Scanning tunneling microscopy (STM) was used to investigate room temperature adsorption and dissociation of SiH₄ 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 SiH₃ groups in the corner holes and of H atoms on one of the adjacent corner adatoms. For higher SiH₄ 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.     

The reaction of acetylene with Ni(100) and Ni(110) surfaces at room temperature

Ultraviolet photoemission spectroscopy using hv = 21.2 eV and filtered 40.8 eV radiation as well as temperature programmed thermal desorption spectroscopy are used to investigate the chemical reaction of acetylene with Ni(100) and Ni(110) surfaces at room temperature. Striking crystallographic effects and several coexisting phases are observed and found to be coverage and temperature dependent. A methodology is described and used to predict the relative energy levels for a variety of adsorbed hydrocarbon fragments on Ni surfaces. Such levels together with the thermal desorption spectra are used to identify the observed species. In particular, CH and CCH species are isolated on Ni(100) and Ni(110) surfaces, respectively, via low temperature adsorption and subsequent pulsed sample warming experiments. The room temperature adsorption phases are deduced using these ionization levels together with those of chemisorbcd acetylene, atomic hydrogen and carbon. At room temperature on Ni(100), H, C, CH and C₂H₂ species form together below 2 L exposure while CH species form thereafter, up to a saturation exposure of ~10 L. On Ni(110), H and CCH species form below 1.5 L exposure followed by the formation of CH₂ and likely CH species. The relative stabilities of these species at elevated temperatures is: C₂H₂ < CCH ? CH < CH₂. A model for the bonding of acetylene and its reaction to form CCH species on Ni(110) is proposed.     

氢稀释对纳米晶硅薄膜晶化特性的影响及薄膜生长机理

以SiH₄与H₂作为前驱气体,采用射频等离子增强化学气相沉积技术制备了纳米晶硅薄膜.利用Raman散射和红外吸收光谱等技术,对不同氢稀释比条件下薄膜的微观结构和键合特性进行了研究.结果表明,随着氢稀释比增加,薄膜的晶化率明显提高,而氢稀释比过高时,薄膜晶化率呈现减少趋势.红外吸收光谱分析表明,纳米晶硅薄膜中氢的键合模式与薄膜的晶化特性密切相关.随着氢稀释比增加,薄膜中整体氢含量和SiH₂键合密度明显减少,而在高氢稀释比条件下,氢稀释比增加导致薄膜中SiH₂键合密度和整体氢含量增加.     

The study of amorphous incubation layers during the growth of microcrystalline silicon films under different deposition conditions

<正>The structural un-uniformity of μc-Si:H films prepared using a very high frequency plasma-enhanced chemical vapour deposition method has been investigated by Raman spectroscopy,spectroscopic ellipsometer and atomic force microscopy.It was found that the formation of amorphous incubation layer was caused by the back diffusion of SiH_4 and the amorphous induction of glass surface during the initial ignition process,and growth of the incubation layer can be suppressed and uniformμc-Si:H phase is generated by the application of delayed initial SiH_4 density and silane profiling methods.     

SiH4 adsorption on SiGe(0 0 1)

The adsorption process of silane (SiH₄) on a SiGe(0 0 1) surface has been investigated by using infrared absorption spectroscopy in a multiple internal reflection geometry. We have observed that SiH₄ dissociatively adsorbs on a SiGe(0 0 1) surface at room temperature to generate Si and Ge hydrides. The dissociation of Si- and Ge-hydride species is found to strongly depend on the Ge concentration of the SiGe crystal. At a low Ge concentration of 9%, Si monohydride (SiH) and dihydride (SiH₂) are preferentially produced as compared to the higher Si hydride, SiH₃. At higher Ge concentrations of 19%, 36%, on the other hand, monohydrides of SiH and GeH and trihyderide SiH₃ are favorably generated at the initial stage of the adsorption. We interpret that when SiH₄ adsorbs on the SiGe surface, hydrogen atoms released from the SiH₄ molecule stick onto Ge or Si sites to produce Si or Ge monohydrides and the remaining fragments of -SiH₃ adsorb both on Si and Ge sites. The SiH₃ species is readily decomposed to lower hydrides of SiH and SiH₂ by releasing H atoms at low Ge concentrations of 0% and 9%, while the decomposition is suppressed by Ge in cases of 19% and 36%.     

Geometrical structure of monobridged disilyene Si(H)SiH

By means of Fourier transform microwave spectroscopy of a supersonic molecular beam, we have detected the more abundant rare isotopic species of monobridged Si(H)SiH, and determined of its geometrical structure by isotopic substitution. Unexpected abundance anomalies of the singly deuterated isotopic species with respect to normal and Si(D)SiD were observed with equal mixtures of SiH₄ and SiD₄. This nonstatistical distribution is consistent with Si(H)SiH formation which is dominated by the reaction of a bare Si atom or ion with either SiH₂ or SiH₄. Dedicated laboratory searches were also undertaken for the remaining singlet isomer, H₂SiSi, so far without success. From the absence of lines at the expected frequencies we conclude that H₂SiSi is at least 50 times less abundant than Si(H)SiH. Because it is only slightly less stable than Si(H)SiH and because the barrier to isomerization is calculated to be quite low, H₂SiSi may rearrange on the timescale of the supersonic expansion to Si(H)SiH.     

Hydrogen desorption kinetics and band bending for 6H-SiC(0 0 0 1) surfaces

The desorption kinetics of hydrogen from polished 6H-SiC(0 0 0 1) surfaces exposed to various sources of hydrogen have been determined using temperature programmed desorption (TPD). For (3 × 3) 6H-SiC(0 0 0 1) surfaces prepared via annealing and cooling in SiH₄, desorption of 0.2 ± 0.05 monolayer of molecular hydrogen was observed to occur at ≈590 °C. This β₁ H₂ desorption peak exhibited second order kinetics with an activation energy of 2.4 ± 0.2 eV. For (3 × 3) 6H-SiC surfaces exposed to atomic hydrogen generated via either a hot rhenium filament or remote hydrogen plasma, low energy electron diffraction patterns showed an eventual conversion back to (1 × 1) symmetry. Spectra acquired using Auger electron and X-ray photoelectron spectroscopies revealed that the atomic hydrogen exposure removed the excess Si. Photoelectron spectroscopy results also showed a 0.5 eV increase in binding energy for the Si2p and C1s core levels after removal of the Si-Si bilayer that is indicative of a decrease in band bending at the SiC surface. TPD from the (3 × 3) 6H-SiC(0 0 0 1) surfaces exposed to atomic hydrogen showed substantially more molecular hydrogen desorption (1-2 ML) through the appearance of a new desorption peak (β_2,3) that started at ≈200 °C. The β_2,3 peak exhibited second order desorption kinetics and a much lower activation energy of 0.6 ± 0.2 eV. A third smaller hydrogen desorption state was also detected in the 650-850 °C range. This last feature could be resolved into two separate desorption peaks (α₁ and α₂) both of which exhibited second order kinetics with activation energies of 4.15 ± 0.15 and 4.3 ± 0.15 eV, respectively. Based on comparisons to hydrogen desorption from Si and diamond surfaces, the β and α desorption peaks were assigned to hydrogen desorption from Si and C sites, respectively.     

Mechanism of H2 desorption from monohydride Si(100)2 × 1H

Most recent experimental work on H₂ desorption from the monohydride Si(100) surface seems to point to a pairwise desorption mechanism involving the concerted desorption of two hydrogen atoms on different Si atoms of a single dimer. Using ab initio SCF and CI theory and a cluster model of the surface, the present work finds that the lowest energy pathway is symmetric rather than asymmetric. The desorption energy barrier is calculated to be 3.7 eV. Compared with an experimental value of 2.6 eV, the large barrier suggests that this direct desorption mechanism is not applicable. A multi-step desorption mechanism which involves a delocalized process in the formation of dihydride SiH₂ and a localized desorption of H₂ is proposed and is shown to explain the experimental observations.     

Hydrogen vibrations on Si (111)

High resolution electron energy loss spectra are reported for atomic hydrogen and deuterium adsorption on the Si (111) 7x7 surface. The experiments were carried out for various hydrogen exposures and different substrate temperatures. Clear evidence is given for the formation of SiH₂ and/or SiH₃ complexes in the early stages of adsorption. Strong indications are also obtained for the removal of adsorbed hydrogen probably via the formation and desorption of silane during hydrogen exposure.     

Temperature dependence of the surface reactivity of SiH3 radicals and the surface silicon hydride composition during amorphous silicon growth

For hydrogenated amorphous silicon (a-Si:H) film growth governed by SiH₃ plasma radicals, the surface reaction probability β of SiH₃ and the silicon hydride (-SiH_x) composition of the a-Si:H surface have been investigated by time-resolved cavity ringdown and attenuated total reflection infrared spectroscopy, respectively. The surface hydride composition is found to change with substrate temperature from -SiH₃-rich at low temperatures to SiH-rich at higher temperatures. The surface reaction probability β, ranging from 0.20 to over 0.40 and with a mean value of β=0.30±0.03, does not show any indication of temperature dependence and is therefore not affected by the change in surface hydride composition. It is discussed that these observations can be explained by a-Si:H film growth that is governed by H abstraction from the surface by SiH₃ in an Eley-Rideal mechanism followed by the adsorption of SiH₃ at the dangling bond created.     

Correlation between optical properties and Si nanocrystal formation of Si-rich Si oxide films prepared by plasma-enhanced chemical vapor deposition

We have investigated the phase separation and silicon nanocrystal (Si NC) formation in correlation with the optical properties of Si suboxide (SiO_x, 0 < x < 2) films by thermal annealing in high vacuum. The SiO_x films were deposited by plasma-enhanced chemical vapor deposition at different nitrous oxide/silane (N₂O/SiH₄) flow ratios. The as-deposited films show increased Si concentration with decreasing N₂O/SiH₄ flow ratio, while the deposition rate and surface roughness have strong correlations with the flow ratio in the N₂O/SiH₄ reaction. After thermal annealing at temperatures above 1000 °C, Fourier transform infrared spectroscopy, Raman spectroscopy, and transmission electron microscopy manifest the progressive phase separation and continuous growth of crystalline-Si (c-Si) NCs in the SiO_x films with increasing annealing temperature. We observe a transition from multiple-peak to single peak of the strong red-range photoluminescence (PL) with increasing Si concentration and annealing temperature. The appearance of the single peak in the PL is closely related to the c-Si NC formation. The PL also redshifts from ∼1.9 to 1.4 eV with increasing Si concentration and annealing temperature (i.e., increasing NC size). The good agreements of the PL evolution with NC formation and the PL peak energy with NC size distribution support the quantum confinement model.     

Interaction of SiHx precursors with hydrogen-covered Si surfaces: Impact dynamics and adsorption sites

SiH_x (x = 1, 2, 3) ions impact on Si(0 0 1)(1 × 1):H and Si(0 0 1)(2 × 1):H surfaces has been studied by classical molecular dynamic simulations, considering an energetic range for the impinging species from 5 to 15 eV. Despite the initial full H coverage a high sticking coefficient has been obtained for all the species under investigation. A considerable fraction of adsorption events causes H removal from the surface while for other simulations a soft landing mechanism of the ions has been observed. Few representative minima for SiH₂/Si(0 0 1)(2 × 1):H were re-converged by ab initio calculations in order to check the reliability of our results.     

Chemical, energetic, and geometric heterogeneity of device-quality (1 0 0) surfaces of single crystalline silicon after HFaq etching

An analysis, based on angle-resolved X-ray photoelectron spectroscopy, multiple-internal-reflection infrared spectroscopy, and atomic force microscopy, of device-quality (1 0 0)silicon surfaces after etching in dilute aqueous solution of HF is presented. The analysis shows that the surface is mainly formed by a heterogeneous distribution of SiH, SiH₂and SiH₃ terminations, but contains sub-stoichiometric oxidized silicon. The analysis shows moreover the existence of a form of reduced silicon, not consistent with the currently accepted picture of the native HF_aq-etched surface.     

氢稀释对多晶硅薄膜结构特性和光学特性的影响

以SiCl₄和H₂为气源，用等离子体增强化学气相沉积技术，在250℃的低温下，研究氢稀释度对多晶硅薄膜结构特性的影响.实验结果表明，对于以SiCl₄和H₂组成的反应源气体，氢对薄膜生长特性的影响有异于SiH₄/H₂，在一定功率下，薄膜的晶化率随氢稀释度的减小而增加，在一定的氢稀释度下薄膜晶化度达到最大值85％；随着氢稀释度的继续减小，薄膜晶化度迅速下降，并逐渐向非晶态结构转变.随氢稀释度的减小，薄膜的光学带隙由 1.5eV减小至约1.2eV，而后增大至1.8eV.沉积速率则随氢稀释度的减小先增加后减小，在无氢条件下，无薄膜形成.在最佳氢稀释度条件下，Cl基是促进晶化度提高，晶粒长大的一个主要因素. 关键词： 多晶硅薄膜 微结构 氢稀释 4')" href="#">SiCl₄     

In situ ellipsometric characterization and process control for amorphous thin film deposition

Characterization of the growth of hydrogenated amorphous silicon (a-Si:H) and carbon (a-C:H) thin films by in situ ellipsometric analysis at 3.4 eV and 3.2 eV is reported. For a-Si:H, prepared on metal substrates from an rf discharge of SiH₄, in situ ellipsometry data are strongly influenced by the SiSi bond packing density in the growing film. Deviations in the data from model calculations assuming a thickness independent a-Si:H dielectric function, when analyzed using an effective medium approximation, reveal the geometry and scale of the initial nucleation process. Effects of the deposition conditions and substrate microstructure on the coalescence of initial nuclei are understood on the basis of new measurements. For a-C:H, prepared on c-Si substrates from CH₄ by direct ion beam deposition, ellipsometry measurements in the initial stages of growth provide monolayer sensitivity to the formation of an absorbing SiC_x layer at the substrate interface. Fits to the data in the later stages of growth establish the real and imaginary parts of the bulk dielectric function at 3.2 eV, allowing real time categorization of the nature of the bonding in such films.     

Properties of a-Si:N:H films beneficial for silicon solar cells applications

Amorphous silicon-nitride thin films a-Si:N:H were obtained by plasma enhanced chemical vapour deposition (PECVD) method from SiH₄+NH₃ at 13.56 MHz. The process parameters were chosen to obtain the films of properties suitable for optoelectronic and mechanical applications. FTIR analysis of a-Si:N:H films indicated the presence of numerous hydrogen bonds (Si-H and N-H) which passivate structural defects in multicrystalline silicon and react with impurities. The morpho-logical investigations show that the films are homogeneous. The deposition of a-Si:N:H layers leads to the decrease in friction coefficient of used substrates. Optical properties were optimised to obtain the films of low effective reflectivity, large energy gap E_g from 2.4 to 2.9 eV and refractive index in the range of 1.9 to 2.2. Reduction of friction coefficient for monocrystalline silicon after covering with a-Si:N:H films was observed: from 0.25 to 0.18 for 500 cycles.     

PHOTO- AND ELECTRO-LUMINESCENCE FROM HYDROGENATED AMORPHOUS SILICON CARBIDE FILMS PREPARED BY USING ORGANIC CARBON SOURCE

Hydrogenated amorphous silicon carbide (a-SiC:H) films were grown by using an organic source, xylene (C₈H₁₀), instead of methane (CH₄) in a conventional plasma enhanced chemical vapor deposition system. The optical band gap of these samples was increased gradually by changing the gas ratio of C₈H₁₀ to SiH₄. The film with high optical band gap was soft and polymer-like and intense photoluminescence were obtained. Room temperature electro-luminescence was also achieved with peak energy at 2.05 eV (600 nm) for the a-SiC:H film with optical band gap of 3.2 eV.