Dynamic observations of interface motion during the oxidation of silicon

We describe real time observations of the behaviour of the silicon-oxide interface during oxidation in situ in an ultrahigh vacuum transmission electron microscope. We have formed clean, flat Si(111) surfaces by heating under UHV and allowed oxidation or oxygen etching to proceed in the microscope. We have examined the kinetics of both the oxidation and etching reactions using an imaging technique based on the use of forbidden reflections in silicon. We find that oxidation to form SiO₂ occurs by the reaction of discrete monolayers with no flow of surface steps. This is in dramatic contrast to oxygen etching, during which the volatile oxide SiO evaporates preferentially from step edges.     

Reaction of the oxygen molecule at the Si(100)-SiO2 interface during silicon oxidation

Using constrained ab initio molecular dynamics, we investigate the reaction of the O2 molecule at the Si(100)-SiO2 interface during Si oxidation. The reaction proceeds sequentially through the incorporation of the O2 molecule in a Si-Si bond and the dissociation of the resulting network O2 species. The oxidation reaction occurs nearly spontaneously and is exothermic, irrespective of the O2 spin state or of the amount of excess negative charge available at the interface. The reaction evolves through the generation of network coordination defects associated with charge transfers. Our investigation suggests that the Si oxidation process is fully governed by diffusion.     

Electron-active silicon oxidation

Visible and ultraviolet light was employed in a series of refining experiments to reveal the kinetics of photon-enhanced oxidation of silicon. The experimental evidence then gave rise to an electron-active silicon oxidation process involving electron emission from Si into SiO₂ moderating the dissociation of molecular oxygen near the interface. Photonicallystimulated oxidation enhancement then straightforwardly derives from an enhanced level of electron emission. This simple model helps to clarify normal dry thermal oxidation growth kinetics, as well as thin oxide rapid growth behavior and the charged-versus-neutral oxidant controversy. Finally, attention is called to the not generally recognized but likely role that secondary electron emission from surfaces may play in moderating the chemistry of various electron, ion and photon beam-induced surface reactions.     

Microscopic dynamics of silicon oxidation

We study the silicon oxidation process and the dynamic structure of the SiO2-Si (001) interface using a grand canonical Monte Carlo approach. We find that Si-O-Si bridge bonds are the main building blocks of the advancing interface, and we identify a kinetic pathway that continually creates new bridge bonds. Oxidation proceeds by local events, with little evidence of "step flow" in the simulation. Yet the interface remains remarkably smooth and abrupt as it advances.     

Early stages of silicon oxidation

Experiments have shown that the early stages of silicon oxidation proceed layer by layer, so that one layer is essentially complete before another develops. Other experiments show that the mechanism does not involve step growth, the most obvious mechanism. We use a new approach to modelling the growth to show that these two observations can be understood when there is a rate-determining step which depends strongly on the local oxide thickness. The rate in question might be the sticking probability, or the rate of incorporation of adsorbed oxygen species into the oxide network. Such mechanisms are possible when transport by an ionic species dominates, contrary to the situation for thicker films. Our modelling suggests the mechanisms are driven by the image interaction, as in earlier suggestions by Stoneham and Tasker, rather than an effect of the electric field central to the Mott-Cabrera mechanism.     

Dynamic light scattering at the ice water interface during freezing

The 488 nm radiation of an argon laser is scattered quasi-elastically at the [0001]-face (basal plane) of a growing ice crystal. The scattering plane is the basal plane. The Rayleigh-linewidth is proportional to the square of the scattering vector. One measures about 2 krad/s at a scattering angle of 90°. The linewidth does not depend on the growth rate. Scattering is only observed once a critical growth rate ν_crit = 1,5 μ m?s has been exceeded. Then the scattering intensity depends linearly on the growth rate in the range between 0,03 μ m?s and 2,5 μ m?s. Once the surface is molten scattering vanishes and does not reappear until the growth rate has again exceeded the threshold ν_crit. The coherence properties of the scattered light indicate that the thickness of the scattering layer is less than 6 μ m. The observations are interpreted in terms of a fluctuating interface. The decay time of the fluctuations has been calculated. Satisfactory agreement with the observed Rayleigh linewidth is obtained.     

Quasi-soliton propagation in dispersion-engineered silicon nanowires

Soliton-like propagation of ultra-short pulses in dispersion-engineered silicon photonic wires is theoretically investigated via the nonlinear Schrödinger equation. It is shown that by proper patterning of silicon waveguides, the engineering of group velocity dispersion can effectively compensate for both linear and two-photon absorption-induced nonlinear losses. Quasi-soliton propagation is demonstrated for 100-fs pulses over large propagation lengths for a realistic silicon wire of optimally patterned waveguide width.     

Aes study of silicon bonding states during oxidation of Si(111)

During thermal oxidation of Si(111) the fine structure of the Si LVV spectrum is correlated with the ratio of SiO₂ to Si peak heights and with the attenuation of the Si peak. Criteria are given for recognizing the onset of silica formation. The width of the transition region, characterized by a precursor of silica, is evaluated as 6–7 Å. Above this thickness, SiO₂ appears to grow layer by layer. Comparison between low-pressure oxidation of clean Si and Auger sputter profiling of thick oxides suggests that the interface has a similar structure.     

Aes study of the segregation behaviour of silicon in high-silicon steel during oxidation

The segregation behaviour of silicon during oxidation of a high-silicon steel has been investigated by AES. The results show that silicon seems to have two states of oxidation: one leading to the formation of SiO_x and iron oxides when the oxidation and the following heat treatments in vacuum are performed below 500°C and the other occurring at temperatures higher than 500°C, leading to the formation of SiO₂ and segregation of this species toward the surface without oxidation of iron.     

Anodic oxidation of porous silicon bilayers

This paper focuses on the study of the effect of anodic oxidation in porous silicon bilayers composed of two porous layers of different porosities. The order of the two types of layers has been alternated, and the thicknesses and refractive indices have been optically characterized by Fourier transform infrared spectroscopy. The results show that the refractive index of anodic oxidized porous silicon is reduced significantly with respect to just formed porous silicon. It is also observed that the quality of the oxidation is related to the porosity of the inner porous layer of the silicon bilayer structure. This effect is interpreted in terms of quantum size effects.     

Metal atom catalyzed oxidation of silicon

The kinetics of oxygen adsorption/oxidation of Co(poly), CoSi(100) and Si(111) has been measured using UPS and AES techniques. At CoSi oxygen adsorption proceeds via dissociative sticking at Co and oxygen bonding to Si, i.e. the metallic component at the silicide acts as a catalyst. Blocking the metal with molecular CO prevents this catalytic action. Even if the surface is saturated with oxygen, with the thickness of the oxidised layer being ∼15 Å, Co metal is still present in the topmost layer. The unability of the metallic component at the CoSi surface to establish metal-oxygen bonds is explained by a structural effect.     

The oxidation characteristics of nitrogen-implanted silicon

The trapping of 5 keV deuterium in ～ 165 and ～ 4000 nm thick BeO films grown by thermal oxidation on Be substrates was investigated at different temperatures using the D(³He,H)⁴He nuclear reaction. The ratio of implanted D to BeO molecules obtained at saturation is 0.24 to 0.34. The D migrates from its end of range location and distributes itself uniformly in the BeO film. With increasing implant temperature the BeO layer flakes from the Be substrate. The distribution of D in BeO films at high concentrations is not consistent with diffusivity measurements at low concentrations of T in BeO.     

Soliton propagation in tapered silicon core fibers

Numerical simulations are used to investigate soliton-like propagation in tapered silicon core optical fibers. The simulations are based on a realistic tapered structure with nanoscale core dimensions and a decreasing anomalous dispersion profile to compensate for the effects of linear and nonlinear loss. An intensity misfit parameter is used to establish the optimum taper dimensions that preserve the pulse shape while reducing temporal broadening. Soliton formation from Gaussian input pulses is also observed--further evidence of the potential for tapered silicon fibers to find use in a range of signal processing applications.     

Dynamic cracking resistance of metallic materials during rapid propagation of a self-similar crack

A model is proposed to determine the dynamic cracking resistance K _ID of metals and alloys for the case of a rapidly moving fractal or self-affine crack. The values of this characteristic correlate with the fractal dimension D _f of the future contour of a crack surface profile. K _ID is lower or higher than K _IC depending on the fractal dimension.     

Application of crystalline silicon surface oxidation to silicon heterojunction solar cells

We study the effect of ultra-thin oxide (SiO_x) layers inserted at the interfaces of silicon heterojunction (SHJ) solar cells on their open-circuit voltage (V_OC). The SiO_x layers can be easily formed by dipping c-Si into oxidant such as hydrogen peroxide (H₂O₂) and nitric acid (HNO₃). We confirm the prevention of the undesirable epitaxial growth of Si layers during the deposition of a-Si films by the insertion of the ultra-thin SiO_x layers. The formation of the SiO_x layers by H₂O₂ leads to better effective minority carrier lifetime (τ_eff) and V_OC than the case of using HNO₃. c-Si with the ultra-thin SiO_x layers formed by H₂O₂ dipping, prior to deposition of a-Si passivation layers, can have high implied V_OC of up to ∼0.714 V.     

Embedded silicon nanocrystal interface structure and strain

The structure of nanocrystal-matrix interface and strain in embedded nanocrystals are studied using large-scale atomistic simulations, with the examples of Si nanocrystal embedded in amorphous matrix of SiO₂. Photoluminescence from silicon nanocrystals embedded in a dielectric matrix like SiO₂ and Si₃N₄ are promising for Si-based optical devices. The nanocrystal-matrix interface plays a crucial role in understanding its optical and electrical properties. Nanocrystals with diameters varying from 2.17 to 4.56 nm are studied. A detailed quantitative analysis of the variation of Si/SiO₂ interface structure and strain distribution with nanocrystal diameter is reported. A linear variation of the interface width with nanocrystal diameter is observed with thinner interfaces for larger nanocrystals. Local deformation analysis reveals that the smaller nanocrystals are highly strained, whereas the strain in the larger ones shifts to the interface. This is in accordance with observed increase in total percentage of defect states in the interface from 39 to 70% for diameter increasing from 2.17 to 4.56 nm. Moreover, based on the atomic arrangements at the interface, optically active defects like Pb centres, E centres and non-bridging oxygen centres are identified and a dominance of Pb centres is observed for all the nanocrystals. The detailed structural characterization-related investigations using the proposed simulation approach will find useful application in designing system-level response of embedded nanocrystals and also to correlate various experimental observations.     

Analysis of rapid dry oxidation of silicon

By analyzing the most recent models on rapid initial oxidation and the experimental data at low temperatures we prove unambiguously that neither enhanced nor retarded oxygen diffusion nor any kind of additional oxygen transport flux can account for anomalous initial regime of silicon dry oxidation.The rapid growth is mainly due to the enhanced oxygen solubility and partly to the enhancement of the reaction rate constantk _s. We argue that the reaction rate depends linearly on the oxygen solubility for low solubilities pertinent to dry oxidation but that it saturates at high solubilities characteristic for the wet oxidation.     

Magnetic effects in the oxidation of silicon

The mass spectrometry study has indicated that the magnetic field accelerates the oxidation of the surface of silicon crystals. The oxidation rate also depends on the nuclear spin of silicon: the oxidation rate of atoms with magnetic nuclei (²⁹Si) is almost twice as high as that of atoms with spinless, unmagnetized nuclei (²⁸Si and ³⁰Si). Both effects—magnetic field and magnetic isotope—reliably prove that the oxidation of silicon is a spin-selective reaction involving radicals and radical pairs as intermediate paramagnetic particles. A spin-selective magnetic sensitive oxidation mechanism is discussed.