On new two-dimensional structures produced on the Si (111) and Si (100) surface upon molecular-beam epitaxy of cobalt and silicon

Highly perfect epitaxial heterostructure CoSi₂ films have been grown on the surface of Si (111) and Si (100) single crystals by the method of molecular-beam epitaxy. The optimal regimes of the film growth with different thicknesses have been determined. It has been shown that short-term annealing of epitaxial films at T = 900–950 K leads to the formation of new CoSi₂/Si(111)-2 × 2 and CoSi₂/Si(100)−2 × 4 superstructures.     

Scanning tunneling microscopy observation of ultrathin epitaxial CoSi<Subscript>2</Subscript>(111) films grown at a high temperature

Scanning tunneling microscopy (STM) is used to study the basic laws of growth of ultrathin epitaxial CoSi₂(111) films with Co coverages up to 4 ML formed upon sequential deposition of Co and Si atoms taken in a stoichiometric ratio onto the Co–Si(111) surface at room temperature and subsequent annealing at 600–700°C. When the coverage of Co atoms is lower than ~2.7 ML, flat CoSi₂ islands up to ~3 nm high with surface structure 2 × 2 or 1 × 1 grow. It is shown that continuous epitaxial CoSi₂ films containing 3–4 triple Si–Co–Si layers grow provided precise control of deposition. CoSi₂ films can contain inclusions of the local regions with (2 × 1)Si reconstruction. At a temperature above 700°C, a multilevel CoSi₂ film with pinholes grows because of vertical growth caused by the difference between the free energies of the CoSi₂(111) and Si(111) surfaces. According to theoretical calculations, structures of A or B type with a coordination number of 8 of Co atoms are most favorable for the CoSi₂(111)2 × 2 interface.     

Composition and properties of nanoscale Si structures formed on the CoSi<Subscript>2</Subscript>/Si(111) surface by Ar<Superscript>+</Superscript> ion bombardment

The variations in the composition and structure of CoSi₂/Si(111) surface layers under Ar⁺ ion bombardment with subsequent annealing has been studied. It has been demonstrated that nanocluster phases enriched with Si atoms form on the CoSi₂ surface at low doses D ≤ 10¹⁵ cm^–2, and a pure Si nanofilm forms at high doses.     

Angle-resolved photoemission study of CoSi2 nanofilms grown on Si(111) substrates

We have carried out an angle-resolved photoemission study for CoSi₂ nanofilms grown on the Si(111)-7×7 substrates. The surface of CoSi₂(111) nanofilm changes from the bulk-truncated surface to the surface with additional Si-bilayer by annealing at higher temperature above 825 K. The angle-resolved photoemission spectra of the CoSi₂ nanofilm annealed at 853 K show the spectral features originated from the surface resonance state on the CoSi₂ surface terminated by Si-bilayer. From the detailed photoemission study, we discuss the surface electronic structure in CoSi₂(111) nanofilms grown on Si(111) substrates.     

Observation of a structural phase transition in a CoSi2 layer buried in 〈111〉 Si

Single crystalline multilayered structures of Si/CoSi₂/Si111 made by high dose implantation of Co in a Si wafer were investigated with⁵⁷Co source Mössbauer spectroscopy, channeling measurements and X-ray diffraction. The results point to a structural phase transition in the CoSi₂ buried layer between 180 and 220 K.     

Annealing studies of the Co/Si(1 1 1) interface

The behaviour of the Co/Si(1 1 1) interface upon annealing is investigated by low energy electron diffraction (LEED), angle resolved ultraviolet (ARUPS) and X-ray (XPS) photoemission spectroscopy. According to the Co thickness two regimes can be distinguished. At low coverages (≲ 8 monolayers ML) no well defined bulk silicides other than the silicon rich epitaxial CoSi₂ phase can be identified. In contrast for larger Co thickness (≳ 15–100 ML) it is found that increasing progressively the annealing temperature (up to 600°C) and time (up to ∼ 30 min) leads to the successive arrival of the following silicides phases within the probing depth of our techniques (∼ 5–20 Å): Co, Co₂Si, CoSi, CoSi₂.     

Reactive epitaxy of cobalt disilicide on Si(111)

A study of the mechanism governing the initial stages in silicide formation under deposition of 1–10 monolayers of cobalt on a heated Si(111) 7×7 crystal is reported. The structural data were obtained by an original method of diffraction of inelastically scattered medium-energy electrons, which maps the atomic structure of surface layers in real space. The elemental composition of the near-surface region to be analyzed was investigated by Auger electron spectroscopy. Reactive epitaxy is shown to stimulate epitaxial growth of a B-oriented CoSi₂(111) film on Si(111). In the initial stages of cobalt deposition (1–3 monolayers), the growth proceeds through island formation. The near-surface layer of a CoSi₂(111) film about 30 Å thick does not differ in elemental composition from the bulk cobalt disilicide, and the film terminates in a Si-Co-Si monolayer triad.     

Silicide formation in Co/Si system investigated by depth-resolved positron annihilation and X-ray diffraction

The transformation of Co/Si to CoSi₂/Si in the temperature range of 300-1170 K has been investigated using depth-resolved positron annihilation and Glancing incidence X-ray diffraction (GIXRD). The different silicide phases formed are identified from the experimental positron annihilation characteristics, which are consistent with the GIXRD results. The present study clearly indicates the absence of vacancy defects in the silicide overlayer.     

Growth of high quality CoSi2/Si - superstructures on Si (111)

Using a newly developed solid phase epitaxy technique (SPE) it is shown that ultrathin essentially pinhole-free CoSi₂ layers can be grown epitaxially on Si (111). These form the basis of a number of short period metal/semiconductor superlattices that have been grown by combining SPE-grown CoSi₂ with MBE-grown Si. Substrate temperatures for Si-MBE have to be chosen very low (≈ 350 °C) in order to avoid a roughening of the layers.     

Kinetics of CoSi2 from evaporated silicon

2 MeV⁴He⁺ backscattering spectrometry and CuK x-ray diffraction were used to study CoSi₂ formed by annealing at temperatures between 405° and 500 °C from CoSi with evaporated Si films. A laterally uniform layer of CoSi₂ forms, in contrast to the laterally nonuniform CoSi₂ layer that is obtained on single crystal Si substrates. The thickness of the CoSi₂ film formed is proportional to the square root of time at a fixed temperature. The activation energy of this reaction is about 2.3 eV.     

Studies of the promising material CoSi2 on GaAs substrates

The thermal stability of CoSi₂ thin films on GaAs substrates has been studied using a variety of techniques. The CoSi₂ thin films were formed by depositing Co(500 Å) and Si(1800 Å) layers on GaAs substrates by electron-beam evaporation followed by annealing processes, where the Si inter-layer was used as a diffusion/reaction barrier at the interface. The resistivity of CoSi₂ thin films formed is about 30 cm. The Schottky barrier height of CoSi₂/n-GaAs is 0.76 eV and the ideality factor is 1.14 after annealing at 750° C for 30 min. The CoSi₂/GaAs interface is determined to be thermally stable and the thin film morphologically uniform on GaAs after 900° C/30 s anneal. The CoSi₂ thin films fulfill the requirements in GaAs self-aligned gate technology.     

Finite Element Analysis of CoSi2 Nanocrystals on Si(001)

In this work we present a finite element analysis of pyramidal and hut-shaped CoSi₂ nanocrystals reactively deposited onto Si(001) substrates. These dots have been observed by us, as well as by other groups. Our analyses have yielded four major conclusions: (1) Elastic relaxation of CoSi₂/Si mismatch strain by three-dimensional islands drives their nucleation, rendering flat, two-dimensional, layer energetically unfavourable. (2) The effect of the nanocrystal surface and interface energies for the observed vertical aspect ratios is negligible at small nanocrystal volumes. (3) Pyramids and huts with identical vertical aspect ratios are energetically degenerate. (4) Nanocrystal growth is only energetically favourable if accompanied by an increase in vertical aspect ratio. Most of these conclusions are consistent with those found in compressively strained layers, such as Si_1?x Ge _x layers on Si.     

Schottky barrier height lowering induced by CoSi2 nanostructure

CoSi₂ nanostructures were formed through thermal agglomeration by annealing ultrathin Co film on Si substrate at high temperatures. The characteristics of the Schottky diodes with CoSi₂ nanostructures capped by a Pt layer were measured and fitted using thermionic emission theory. All the diodes have a ideality factor less than 1.1. The results show that the Schottky barrier height of these diodes significantly decreases as the annealing temperature for CoSi₂ agglomeration increases. The barrier height lowering is correlated with the agglomeration of CoSi₂ film and the formation of CoSi₂ nano-islands. The thermal field emission may be the major reason to cause barrier lowering. Although the Schottky contact interface consists of both CoSi₂ nano-islands and Pt film whose individual contact barrier height to Si is very different, the current-voltage-temperature measurements reveal that the interface homogeneity is not degraded as expected. The study demonstrates that the CoSi₂ nanostructures can both lower the Schottky barrier height and form an ideal Schottky contact with a Pt capping layer.     

Cu/SiO2/Si(111)体系中Cu和Si的扩散及界面反应

室温下利用磁控溅射在p型Si(111)衬底上沉积了Cu薄膜. 利用X射线衍射和卢瑟福背散射分别对未退火以及在不同温度点退火后样品的结构进行了表征. 在此基础上，研究了Cu/SiO₂/Si(111)体系的扩散和界面反应. 实验结果表明：当退火温度高于450℃时出现明显的扩散现象，并且随着温度的升高，体系扩散现象会更加显著. 当退火温度低于450℃时没有铜硅化合物生成，当温度达到500℃时才有铜硅化合物生成. 关键词： 薄膜 扩散 界面反应 硅化物     

Evidence of SiO at the Si-oxide interface by surface soft X-ray absorption near edge spectroscopy

Using synchrotron radiation a new surface sensitive spectroscopy has been applied to determine the local structure of the first surface oxide layer formed on the Si(111) surface. The Surface Soft X-ray Absorption (SSXA) spectra have been measured. From the analysis of the X-ray Absorption Near Edge Structures (XANES) we have extracted structural information. We have first determined that bulk amorphous SiO has a characteristic microsopic structure, which cannot be described by the random alloy or microcrystalline (Si + SiO₂) mixture models. The oxide layer formed on the Si(111) surface by ground-state molecular excitation in ultra high vacuum at temperatures (～700°C) approaching the oxide dissociation point has this unique SiO local structure. Such SiO layer not formed at room temperature is expected to be present in the SiSiO₂ interface grown at high temperature. An electronic transition to empty states at the SiSiO₂ interface has been observed.     

A new method of surface structure-analysis by medium energy electron diffraction: distinction between T4 and H3 models for the Si(111)√3 × √3 -In surface

A new method (named GB-MEED) of medium-energy electron diffraction has been invented. This method is used to measure back-scattering medium-energy electron diffraction patterns with a grazing-incident electron beam. It is demonstrated that GB-MEED is sensitive to the structural change of only a few upper layers of the Si(111) surface. A simple model of single-scattering cluster calculation similar to that for X-ray photoelectron diffraction has been applied to analyze presently measured GB-MEED patterns for the Si(111)√3 × √3-In surface. The distinction between T₄ and H₃ models for the Si(111)-√3 × √3-In surface has been made by making the best use of forward focusing of electron-scattering at a medium energy of 1 keV.     

Reactive epitaxy of cobalt disilicide on Si(100)

The growth of cobalt disilicide on the Si(100) surface by reactive epitaxy at T=350°C was studied within the 10–40 ML cobalt coverage range. A new method of mapping the atomic structure of the surface layer by inelastically scattered medium-energy electrons was employed. The films thus formed were shown to consist of CoSi₂(221) grains of four azimuthal orientations turned by 90° with respect to one another. This domain structure originates from substrate surface faceting by (111) planes, a process occurring during silicide formation. B-oriented CoSi₂(111) layers grow epitaxially on (111) facets.     

Yttrium silicide films into Si(111) — Fabrication and properties

The formation of a thin layer of hexagonal Y Si_2?x phase on a single-crystal Si(111) substrate by implantation of 195 keV Y ions with a dose of 5×10¹⁶Y ⁺/cm² at room temperature (RT) is investigated. The structural characterization of the as-implanted and annealed samples is performed using Rutherford backscattering spectrometry (RBS), X-ray diffraction (XRD) pole figure and cross-sectional transmission electron microscopy (XTEM). The results show that the orientation relationship between the Y Si_2?x layer and Si substrate is Y Si_2?x(0 0 0 1)//Si(111) and Y Si_2?x[1 1 -2 0]//Si [110].     

Band bending variation of the Si(111) surface during its thermal oxidation

During the thermal oxidation of Si(111) surfaces performed under ultra high vacuum conditions and investigated with various surface techniques such as Auger electron spectroscopy, low energy electron diffraction, ultra violet and X-ray induced photoemission spectroscopy, the Si 2p core level binding energy was measured and provided directly the band bending variation of the Si surface. A net interface charge then could be deduced. This is an elegant in situ method to have access to the electrical characteristics of the SiSiO₂ interface during its formation.     

Low temperature ion beam mixing of Co/Si systems

Co/Si systems were ion beam mixed at 77 K using a 100 keV Ar beam. The formation of different phases as a function of irradiation dose has been studied, using Mössbauer spectroscopy (MS) and Rutherford backscattering spectroscopy (RBS). It was found that Co₂Si, CoSi and CoSi₂ are formed subsequently in parallel layers. After high dose irradiation, a phase with stoichiometry Co∶Si equal to 1∶3 was observed, suggesting CoSi₃ has been formed. However, MS gave clear evidence that this phase consists of precipitates of CoSi₂ and Si. Finally, we found that the amount of mixing scales linearly with the square root of the fluence, with a mixing rate of 1.0×10⁴Å⁴.