Synchrotron investigations of Si/Mo/Si…c-Si (100) multilayer nanoperiodic structures

This paper presents the results of the investigation of c-Si/[Si/Mo] _n /Si multilayer nanoperiodic structures by X-ray absorption near-edge structure (XANES) spectroscopy using synchrotron radiation. Changes in the fine structure of XANES MoL _2,3 spectra confirm the formation of the silicide phase on heterophase interfaces Si/Mo/Si due to the solid-phase interactions between silicon and molybdenum layers.     

The effect of alloying on the structure and peculiarities of tribological behavior of vacuum-deposited diamond-like coatings

In this work, we report the results of a study of the atomic and crystalline structures, phase and chemical compositions and tribological properties of DLC coatings deposited by plasma-assisted (PA) CVD (a-C:H:Si and a-C:H:Si:Mo) and magnetron reactive sputtering (a-C:H:Cr). The a-C:H:Si:Mo coatings revealed the formation of ultra-dispersed inclusions of either molybdenum carbides or silicides, whereas no such inclusions were found in the a-C:H:Si compound. The a-C:H:Cr coatings that were deposited in an acetylene–nitrogen gas mixture exhibited a nanocomposite structure composed of chromium, its carbide and nitride phases. The tribological tests showed that the DLC coatings with silicon and silicon–molybdenum have a high friction coefficient and a low working performance, while the chromium-containing coatings have high levels of their mechanical and tribological characteristics, making them promising materials for operation under high contact pressures.     

MoO_3/Si界面区钼掺杂非晶氧化硅层形成的第一性原理研究

采用基于密度泛函理论的第一性原理计算方法,通过模拟MoO_3/Si界面反应,研究了MoO_x薄膜沉积中原子、分子的吸附、扩散和成核过程,从原子尺度阐明了缓冲层钼掺杂非晶氧化硅(a-SiO_x(Mo))物质的形成和机理.结果表明,在1500 K温度下, MoO_3/Si界面区由Mo, O, Si三种原子混合,可形成新的稳定的物相.热蒸发沉积初始时, MoO_3中的两个O原子和Si成键更加稳定,同时伴随着电子从Si到O的转移,钝化了硅表面的悬挂键. MoO_3中氧空位的形成能小于SiO_2中氧空位的形成能,使得O原子容易从MoO_3中迁移至Si衬底一侧,从而形成氧化硅层;替位缺陷中, Si替位MoO_3中的Mo的形成能远远大于Mo替位SiO_2中的Si的形成能,使得Mo容易掺杂进入氧化硅中.因此,在晶硅(100)面上沉积MoO_3薄膜时, MoO_3中的O原子先与Si成键,形成氧化硅层,随后部分Mo原子替位氧化硅中的Si原子,最终形成含有钼掺杂的非晶氧化硅层.     

Development of a model of silicon carbide thermodestruction for preparation of graphite layers

A three-stage scheme of the silicon carbide thermodestruction resulting in surface graphitization, which was proposed earlier (based on structural studies), is discussed. A theoretical analysis shows, however, that this process occurs in two stages, namely, thermodesorption of silicon atoms from the two outer Si-C bilayers followed by condensation of carbon atoms on the Si(0001) face of silicon carbide, thus giving rise to the formation of a two-dimensional graphite structure (graphene).     

X-Ray Photoelectron Spectroscopy and Reflection High Energy Electron Diffraction of Epitaxial Growth SiC on Si（100） Using C60 and Si

The formation of silicon carbide upon deposition of C60 and Si on Si（100） surface at 850^o C is studied via x-ray photoelectron spectroscopy and reflection high energy electron diffraction （RHEED）. The C ls, O ls and Si 2p core-level spectra and the RHEED patterns indicate the formation of 3C-SiC.     

Synthesis of epitaxial silicon carbide films through the substitution of atoms in the silicon crystal lattice: A review

A review of recent advances in the field of epitaxial growth of SiC films on Si by means of a new method of epitaxial substitution of film atoms for substrate atoms has been presented. The basic statements of the theory of the new method used for synthesizing SiC on Si have been considered and extensive experimental data have been reported. The elastic energy relaxation mechanism implemented during the growth of epitaxial SiC films on Si by means of the new method of substitution of atoms has been described. This method consists in substituting a part of carbon atoms for silicon matrix atoms with the formation of silicon carbide molecules. It has been found experimentally that the substitution for matrix atoms occurs gradually without destroying the crystalline structure of the matrix. The orientation of the film is determined by the “old” crystalline structure of the initial silicon matrix rather than by the silicon substrate surface only, as is the case where conventional methods are used for growing the films. The new growth method has been compared with the classical mechanisms of thin film growth. The structure and composition of the grown SiC layers have been described in detail. A new mechanism of first-order phase transformations in solids with a chemical reaction through an intermediate state promoting the formation of a new-phase nuclei has been discussed. The mechanism providing the occurrence of a wide class of heterogeneous chemical reactions between the gas phase and a solid has been elucidated using the example of the chemical interaction of the CO gas with the single-crystal Si matrix. It has been shown that this mechanism makes it possible to grow a new type of templates, i.e., substrates with buffer transition layers for growing wide-band-gap semiconductor films on silicon. A number of heteroepitaxial films of wide-band-gap semiconductors, such as SiC, AlN, GaN, and AlGaN on silicon, whose quality is sufficient for the fabrication of a wide class of micro- and optoelectronic devices, have been grown on the SiC/Si substrate grown by solid-phase epitaxy.     

New method for growing silicon carbide on silicon by solid-phase epitaxy: Model and experiment

A new method of solid-state epitaxy of silicon carbide (SiC) on silicon (Si) is proposed theoretically and realized experimentally. Films of various polytypes of SiC on Si(111) grow through a chemical reaction (at T = 1100–1400°C) between single-crystal silicon and gaseous carbon oxide CO (at p = 10–300 Pa). Some silicon atoms transform into gaseous silicon oxide SiO and escape from the system, which brings about the formation of vacancies and pores in the silicon near the interface between the silicon and the silicon carbide. These pores provide significant relaxation of the elastic stresses caused by the lattice misfit between Si and SiC. X-ray diffraction, electron diffraction, and electron microscopy studies and luminescence analysis showed that the silicon carbide layers are epitaxial, homogeneous over the thickness, and can contain various polytypes and a mixture of them, depending on the growth conditions. The typical pore size is 1 to 5 μm at film thicknesses of ～20 to 100 nm. Thermodynamic nucleation theory is generalized to the case where a chemical reaction occurs. Kinetic and thermodynamic theories of this growth mechanism are constructed, and the time dependences of the number of new-phase nuclei, the concentrations of chemical components, and the film thickness are calculated. A model is proposed for relaxation of elastic stresses in a film favored by vacancies and pores in the substrate.     

Amorphous SiC film formation on Si(100) using electron beam excitation

The effect of electron impact on methylsilane (CH₃SiH₃) conversion to amorphous-Si_0.5C_0.5:H (a-Si_0.5C_0.5:H) films on Si(100) has been studied by Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), temperature-programmed desorption (TPD), and low energy electron diffraction (LEED). It is found that electron impact greatly enhances CH₃SiH₃ decomposition on Si(100) at both 90 K and 300 K, resulting in a-Si_0.5C_0.5:H thin film formation. Thermal annealing of the film causes hydrogen desorption and amorphous silicon carbide (a-SiC) formation. Upon annealing to temperatures above 1200 K, the a-SiC film became covered by a thin silicon layer as indicated by AES studies. Ordered structures are not produced by annealing the a-SiC up to 1300 K.     

SiC formation on Si(100) via C60 precursors

The interaction between C₆₀ molecules and the Si(100) surface and the preparation of silicon-carbide thin films by thermal reaction of C₆₀ molecules with the Si(100) surface have been investigated using X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, reflection high-energy electron diffraction and atomic force microscopy measurements. The effects of annealing temperature and C₆₀ coverage on the SiC formation will be discussed. It is found that the C₆₀ molecules bond covalently with silicon, and the number of bonds increase upon increasing the annealing temperature. Annealing at T≥830°C entails the formation of stoichiometric silicon carbide clusters that coalesce to form a continuous SiC layer when the C₆₀ coverage is greater than one monolayer. Deep pits acting as silicon diffusion channels are present with dimensions that increase with the amounts of C₆₀.

The interaction of C₆₀ with the SiC surface was also investigated. It is found that a similar covalent interaction is present in the two systems C₆₀/Si and C₆₀/SiC.     

Mechanism of Formation of Carbon–Vacancy Structures in Silicon Carbide during Its Growth by Atomic Substitution

Physics of the Solid State - The mechanism of formation of carbon-vacancy structures in silicon carbide SiC from silicon vacancies that inevitably form during synthesizing SiC from Si by atomic...     

X-ray interface analysis of aperiodic Mo/Si multilayers

We present the non-destructive analysis of aperiodic Mo/Si multilayers by X-ray emission spectroscopy induced by electrons. The Si 3p occupied valence states of the silicon atoms present within these structures are analysed. Because of the great sensitivity of these states to the physico-chemical environment of the Si atoms, it is possible to distinguish the emission from the center of the Si layer (amorphous silicon) to that of the interfacial zones between the Mo and Si layers. Thus, the presence of molybdenum silicides is evidenced in the interfacial zones. It is also shown that the relative proportion of interfacial silicides depends on the deposition conditions.     

质子辐照下Mo/Si多层膜反射镜的微观结构状态

利用透射电子显微镜对质子辐照前后空间太阳望远镜Mo/Si多层膜的微观结构进行了表征, 并对其辐照前后反射率的变化进行了测量.研究表明, Mo/Si多层膜经质子辐照后形成了一些缺陷结构,局部区域Mo/Si的周期性遭到破坏, Mo层与Si层的宽度发生了变化,多层膜层与层之间的界面也比辐照前更为粗糙,部分层状结构由于质子辐照发生了明显的扭曲和折断等现象;此外,质子辐照导致了Mo/Si多层膜反射率的下降,这些微观缺陷的形成是光学性能降低的直接诱因. 关键词： 空间太阳望远镜 Mo/Si多层膜 微观结构 反射率     

Silica on Silicon Carbide

Silicon carbide (SiC) as both the most important non-oxide ceramic and promising semiconductor material grows stoichiometric SiO ₂ as its native oxide. During passive oxidation, a surface transformation of SiC into silica takes place causing bulk volume and bulk mass increase. This review summarizes state-of-the-art information about the structural aspects of silicon carbide, silica, and SiC–SiO ₂ interfaces and discusses physicochemical properties and kinetics of the processes involved. A special section describes the electronic properties of carbide–oxide interfaces, which are inferior compared to Si–SiO ₂ interfaces, limiting the use of SiC-based electronics. In the oxidation of SiC there is a variety of parameters (e.g., porosity, presence of sintering aids, impurities, crystallographic orientation, surface treatment, and atmospheric composition) influencing the process. Therefore, the kinetics can be complex and will be discussed in detail. Nonetheless, a general linear-parabolic time-law can be found for most SiC materials for passive oxidation, thus indicating a mainly diffusion-controlled mechanism. The pronounced anisotropy of SiC expresses itself by quite different oxidation rates for the various crystallographic faces. Manifold impact factors are reflected by oxidation rate-constants for silicon carbide that vary over orders of magnitude. The understanding of SiC oxidation and silica formation is still limited; therefore, different oxidation models are presented and evaluated in the light of current knowledge.     

Electron loss spectroscopy study of the growth by laser ablation of ultra-thin diamond-like films on Si(100)

Carbon films with thicknesses up to 10 monolayers (ML) have been grown on Si(100) substrates by means of laser ablation of graphite under ultra-high vacuum (UHV) conditions. The early stages of the growth have been characterized by Auger-electron (AES), electron-energy-loss (EELS) and ion-scattering (ISS) spectroscopies. EELS and AES can be used to qualitatively distinguish between the graphitic or diamond-like character of the films. The effect of submonolayer coverages on the surface electronic density of the silicon substrate has also been investigated. Carbon does not diffuse into silicon for room temperature depositions. Annealing at 950 °C causes graphitization and the formation of silicon carbide together with an intermixing of C and Si.     

Heat capacity of silicon carbide at low temperatures

The heat capacity of biomorphic silicon carbide, a high-porosity material with specific cellular pores, is measured in the temperature range 3.5–60 K. Biomorphic silicon carbide is prepared by the chemical removal of excess silicon from the SiC/Si biomorphic composite, a product of eucalyptus wood. It is shown that the major contribution to the heat capacity of biomorphic SiC comes from surface vibrational modes.     

Crystalline silicon films sputtered on molybdenum: A study of the silicon–molybdenum interface

Polycrystalline silicon films were grown on molybdenum (Mo)-coated substrates at high deposition rate using the pulsed magnetron sputtering technique. Our study investigates the silicon–molybdenum interface of these films to elucidate stimulating mechanisms for an ordered crystalline silicon thin film growth. Both Auger electron spectroscopy and Rutherford backscattering reveal that at a substrate temperature as low as T_S=450 °C during the deposition process intermixing of Si and Mo at the Si–Mo interface takes place leading to a compositional ratio Mo:Si of about 1:2. By Raman spectroscopy hexagonal β-MoSi₂ could be identified as the dominant phase in this intermixed region. The dependence of the resulting thickness of the reacted interface layer on the deposition conditions is not fully understood yet.     

Processes of silicide formation in the Fe/Si(111)7 × 7 system

The initial stages of the formation of iron silicides in the Fe/Si(111)7 × 7 system in the course of solid-phase epitaxy are investigated using high-resolution photoelectron spectroscopy (～100 meV) with synchrotron radiation. The spectra of the Si 2p core and valence-band electrons obtained after deposition of iron coverages of up to 28 monolayers on the surface of the sample and subsequent isochronous annealings at 650°C are measured and analyzed. It is shown that the first to form under Fe deposition is an ultrathin film of the metastable silicide FeSi with a CsCl-type structure, on which a layer of the Fe-Si solid solution with segregated silicon grows. At coverages in excess of 10 monolayers, an iron film grows on the surface of the sample. Annealing of a silicon crystal coated with a Fe layer leads to the sequential formation of two stable silicide phases, namely, the ?-FeSi and β-FeSi₂ phases, in the near-surface region of the sample. It is found that the process of solid-phase synthesis of the ?-FeSi phase passes through the stage of transformation of the iron film into the Fe-Si solid solution.     

Growth of silicon carbide films via C60 precursors

Epitaxial silicon carbide films are grown on Si(100) and Si(111) substrates at surface temperatures between 950 and 1250 K via c₆₀ precursors. Films have been grown up to thicknesses greater than 1 μm. The growth rate of the SiC film is not limited by the surface reaction rate of C₆₀ with silicon at these temperatures, rather by the arrival rate of the reactants Si (by diffusion) or C₆₀. This results in rapid film growth. Films have been characterized by low energy electron diffraction, X-ray diffraction, and Auger depth profiling. X-ray diffraction suggests the growth of β-SiC in the temperature range investigated. Auger depth profiling shows the film is stoichiometric. Selective crystalline silicon carbide growth is achieved on patterned silicon-silicon oxide samples.     

Formation of interfacial iron silicides on the oxidized silicon surface during solid-phase epitaxy

The early stages of iron silicide formation in the Fe/SiO _x /Si(100) ternary system during solid-phase epitaxy are studied by high-resolution (～100 meV) photoelectron spectroscopy using synchrotron radiation. The spectra of core and valence electrons taken after a number of isochronous heat treatments of the samples at 750°C are analyzed. It is found that the solid-phase reaction between Fe and Si atoms proceeds in the vicinity of the SiO _x /Si interface, which metal atoms reach when deposited on the sample surface at room temperature. Iron silicide starts forming at 60°C. Solid-phase synthesis is shown to proceed in two stages: the formation of the metastable FeSi interfacial phase with a CsCl-like structure and the formation of the stable β-FeSi₂ phase. During annealing, structural modification of the silicon oxide occurs, which shows up in the growth of the Si⁺⁴ peaks and attenuation of the Si⁺² peaks.     

HREELS study of the adsorption and evolution of diethylamine (DEA) on Si(1 0 0) surfaces

The adsorption of diethylamine (DEA) on Si(1 0 0) at 100 K was investigated using high-resolution electron energy loss spectroscopy (HREELS) and electron stimulated desorption (ESD). The thermal evolution of DEA on Si(1 0 0) was studied using temperature programmed desorption (TPD). Our results demonstrate DEA bonds datively to the Si(1 0 0) surface with no dissociation at 100 K. Thermal desorption of DEA takes place via a β-hydride elimination process leaving virtually no carbon behind. Electronic processing of DEA/Si(1 0 0) at 100 K results in desorption of ethyl groups; however, carbon and nitrogen are deposited on the surface as a result of electron irradiation. Thermal removal of carbon and nitrogen was not possible, indicating the formation of silicon carbide and silicon nitride.