Surface morphology of a Si(310) substrate used for molecular beam epitaxy of CdHgTe: II. Si(310) surface annealed in As4 vapors

The morphology of a hydrogenated Si(310) surface annealed in As₄ vapors is studied by fast electron diffraction and scanning tunneling microscopy. It is established that, at annealing temperatures above 700°C, the surface morphology is changed; namely, (311) facets and steps with heights equal to several interplanar distances are formed. At temperatures below 600°C, there is no surface faceting and the height of steps is equal to two interplanar distances. This makes this surface suitable for growing A²B⁶-based heterostructures.     

Surface micromorphology of CdTe(310) layers grown by molecular beam epitaxy

Surface morphology of CdTe(310) buffer layers grown by molecular beam epitaxy has been investigated by the method of reflection of high-energy electron diffraction. It was established that a clean CdTe(310) surface is atomically flat. Its reconstruction can be described by a unit cell coinciding with the unit cell of the unreconstructed (310) surface. It is determined that Te₂ adsorption in amounts of less than 0.2 monolayers results in the surface reconstruction with the formation of terraces parallel to the (100) plane and are 3/2a long. A system of (100) + (210) facets develops on the CdTe(310) surface with the increase in the Te adsorption layer’s thickness up to 0.3 monolayers and more.     

Observation of Si(111) and gold-deposited Si(111) surfaces using micro-probe reflection high-energy electron diffraction

Observations of clean Si(111) and gold-deposited Si(111) surfaces have been performed using micro-probe reflection high-energy electron diffraction. It was found that many atomic steps on a Si(111) surface run in nearly the same direction, about 9° off the [1̄1̄2] direction. When gold was deposited on this surface at a substrate temperature of about 800°C, 5 × 1, diffuse √3 × √3R30°, sharp √3 × √3 R30° structures and Au clusters appeared on the surface with continuation of the deposition. During the deposition process, it was found that one kind of Si(111) 5 × 1 Au domain grew selectively along these atomic steps and nearly covered the entire surface. A phenomenon of gold clusters moving during the deposition was also observed. These clusters all moved in nearly the same direction so as to climb the atomic steps.     

Structural and phase transformations during initial stages of copper condensation on Si(001)

Auger-electron spectroscopy, electron-energy loss spectroscopy, low-energy electron diffraction, and atomic-force microscopy are employed to investigate the growth mechanism, composition, structural and phase states, and morphology of Cu films (0.1–1 nm thick) deposited on a Si(001)-2 × 1 surface at a lower temperature of Cu evaporation (900°C) and room temperature of a substrate. The Cu film phase is shown to start growing on the Si(001)⁻² × 1 surface after three Cu monolayers (MLs) are condensed. It has been revealed that atoms of Cu and Si(001) are mixed, a Cu₂Si film phase is formed, and, thereafter, Cu₃Si islands arise at a larger coating thickness. Annealing of the first Cu ML leads to reconstruction of the Si(001)-1 × 1-Cu surface layer, thereby modifying the film growth mechanism. As a consequence, the Cu₂Si film phase arises when the thickness reaches two to four MLs, and bulk Cu₃Si silicide islands begin growing at five to ten MLs. When islands continue to grow, their height and density reach, respectively, 1.5 nm and 2 × 10¹¹ cm⁻² and the island area is 70% of the substrate surface at a thickness of ten MLs.     

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.     

Si(113)表面原子结构的低能电子衍射研究

利用低能电子衍射(LEED)研究了离子轰击加退火处理的和淀积外延的两种Si(113)表面的原子结构。发现对于经750—800℃退火后的两种Si(113)表面,当其温度高于600℃时存在1×1非再构表面相。随着样品温度缓慢地冷却至室温,Si(113)-1×1表面经过3×1(约600—400℃)最后转变为3×2再构。当退火温度为600℃时,则只出现3×1再构,室温下的3×2和3×1表面都是很稳定的。讨论了表面杂质对Si(113)表面原子结构的影响。在衬底温度为580℃的Si(113)表面上进行淀积生长,当外延 关键词：     

Passivation of Si (111) surfaces by ordered metal layers

The properties of √3 × √3 ordered gold and silver monolayers on a Si(111) substrate have been investigated by Auger, low energy electron diffraction and photo-emission analysis. It has been found that oxygen adsorption on these surfaces is considerably weaker than on clean Si surfaces. This new result clearly emphasizes the correlation between the oxidation properties of Si atoms and their local environment. A comparison is made with previous results concerning Au-Si amorphous metallic alloys where gold atoms act as a catalyst for the oxidation.     

Formation of ordered arrays of Ag nanowires and nanodots on Si(5 5 7) surface

The ordered arrays of Ag nanowires and nanodots have been grown in ultra-high vacuum on the Si(5 5 7) surface containing regular steps of three bilayer height. Formation of Ag nanostructures have been studied by scanning tunneling microscopy, low energy electron diffraction and Auger electron spectroscopy at room temperature. It was shown that a sample exposure in the vacuum before Ag growth affects the shape of the forming Ag islands. This effect is caused by oxygen adsorption on the silicon surface from the residual atmosphere in the vacuum chamber. When Ag is deposited on the clean silicon surface the islands, overlapping several (1 1 1) neighboring terraces, form. The arrays of silver nanowires elongated along steps and silver nanodots, arranged in lines parallel to the steps, can be formed on the Si(5 5 7) surface depending on the amount of adsorbed oxygen.     

Structure of Si(111)−7 × 7

A model of the Si(111)?7 × 7 surface atomic arrangement is put forward on the basis of results already established for Si(111) and Si(100) surfaces. The unit mesh contains a triangular double-layer island with 21 first-layer atoms. The island is laterally expanded and is bounded by [1̄1̄2] steps with second-layer edge atoms forming asymmetric dimers. It is shown that salient features of low energy electron diffraction (LEED) patterns for Si(111)?7 × 7 can be explained by the model. The LEED patterns are interpreted qualitatively by a double-diffraction mechanism involving forward diffraction in the selvedge. It is shown that the patterns contain characteristic formations of fractional-order spots attributable to the dimers at the island boundaries. The best agreement with observed patterns is obtained with the following parameter values: dimer bond length 2.5 ± 0.2 Å, island lateral expansion 3 ± 2%. Some of the implications of the model for the chemical reactivity and electronic properties of the Si(111)?7 × 7 surface are discussed.     

Electronic properties of cleaved Si(111) upon room-temperature deposition of Au

Low energy electron diffraction (LEED), Auger electron spectroscopy (AES), and photoemission yield spectroscopy (PYS) measurements are performed on a set of 2 × 1 reconstructed silicon (111) surfaces with different bulk dopings as a function of gold coverage θ, from zero to a few hundred monolayers, obtained by UHV evaporation on a sample kept at room temperature. Our measurements show the formation of an Au-Si alloy with the first two monolayers of gold deposit which induces a decrease of the ionization energy Φ by about 0.15 eV while no variation of the work function is observed. In the effective density of states, the double structure related to the 2 × 1 reconstruction is then replaced by a single peak at ? 0.4 eV below the valence band edge. At larger coverages, the Au-Si alloy remains on top of a gold layer which forms an abrupt interface with the Si substrate.     

Comparison of the Si(111) (7 × 7) and (1×1)surfaces

High-resolution electron energy-loss spectroscopy and monochromatic low-energy electron diffraction have been applied to the study of the Si(111)(7 × 7) surface and the thermally-quenched Si(111) (1 × 1) surface. For the (1 × 1) surface, the inelastic continuum, observed for the (7 × 7) surface, due to the Drude absorption of electrons in the dangling-bond surface states is not existent, which indicates that the surface-state band associated with the dangling-bonds of the (1 × 1) surface is insulating. The observed electronic transitions indicate that the (7 × 7) and (1 × 1) surfaces have similar local band geometries and that they differ only in long-range order. The (1 × 1) surface is considered to have a disordered structure. The defect model is favored for the (7 × 7) structure.     

RHEED studies of Si(100) surface structures induced by Ga evaporation

Si(100) surface structures induced by Ga molecular beam deposition in an ultra-high vacuum have been investigated using a reflection high-energy electron diffraction system (RHEED). It has been found that the Ga evaporation of submonolayer thickness on a clean Si(100) 2 × 1 surface produces surface structures of Si(100) 3 ×2, 5 × 2, 2 ×2 and 8 × 1 sequentially in the temperature range from 350 to 680° C. The RHEED patterns and a two-dimensional phase diagram including five superstructures are presented.     

Properties of the interaction of europium with Si(111) surface

Formation of the Eu/Si(111) system as the metal layer thickness gradually increases from 0.5 to 60 monolayers (ML) deposited on the silicon surface at room temperature, and after heating at up to 900 °C, has been studied by Auger electron spectroscopy, electron-energy-loss spectroscopy, and low-energy-electron diffraction. It is shown that room-temperature film growth passes through three stages, depending on the Eu layer thickness: metal chemisorption, interdiffusion of the metal and substrate atoms, and buildup of the metal on the surface of the system. Heating of ultrathin (about one ML) Eu films deposited at room temperature results in ordering of metal atoms on the silicon surface with only weak interaction. Heating thick (above 15 ML) Eu layers on the silicon surface produces silicides whose structure depends on the heating temperature. Fiz. Tverd. Tela (St. Petersburg) 40, 562–567 (March 1998)     

Surface diffusion of Ni on Si(111) with coadsorption of Co

The effect of the coadsorption of Co and Ni on an Si(111) surface structure and on the diffusion of adsorbed atoms is investigated by low-energy electron diffraction and Auger electron spectroscopy. It is established that surface structures similar to those formed with the adsorption of Co alone are formed with the Ni and Co coadsorption on an Si(111) surface. It is found that the contribution of surface diffusion to the transport of Ni atoms is sharply higher on an Si(111) surface with submonolayer Co concentrations in the temperature range 500–750 °C than for a pure surface, where the main mechanism of Ni transport along the surface is diffusion of Ni atoms through the bulk of Si. Fiz. Tverd. Tela (St. Petersburg) 41, 1489–1494 (August 1999)     

UHV-SEM observations of cleaning process and step formation on silicon (111) surfaces by annealing

The cleaning process and step formation by high temperature annealing up to 1250°C on the Si(111) surface are observed by an ultra-high-vacuum scanning electron microscope (UHV-SEM). The clean surface is composed of alternate planes of terraces and step bands with widths of several μm and 1 μm, respectively, in the 〈1?1?2〉 direction. Both planes are inclined by about 10° to each other. The surface steps are not only monolayer steps, but also higher steps comprising several monolayers. Monolayer steps join to form a high step, and 70–80 steps of several monolayers high form a step band by bunching in an average distance of several hundred A toward the 〈1?10〉 direction. The step structure depends on the annealing temperature and on the angle at which the cutting plane is off from the exact 〈111〉 orientation. In several studies of high energy reflection electron microscopy under small grazing angle incidence monolayer steps were observed on the terrace, but no rough structures like the step bands and high steps could be discerned. The step structure observed by the present experiment is compared with those observed by previous workers.     

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.     

Nucleation and growth during molecular beam epitaxy (MBE) of Si on Si(111)

The nucleation of Si on Si(111) has been studied during deposition in UHV by spot profile analysis of low energy electron diffraction (SPA-LEED). A new method of evaluation is developed by separating the measured spot profile into a central spike and a broad shoulder. The energy dependence of the fraction of the central spike out of the total diffracted intensity provides the vertical distribution of surface atoms over different levels. With this method it is shown that the first nucleation occurs in islands of double height. Only after deposition of several layers a layer-by-layer growth is found with a well defined nucleation of a new layer before the former one is completed.     

A structure analysis of Ag-adsorbed Si(111) surface by LEED/CMTA

The Ag induced superstructures on the Si(111) surface have been studied by low energy electron diffraction constant momentum transfer averaging (LEED/CMTA) technique. The vertical displacements of the atoms are determined from the analysis of the specularly reflected (00) beam intensities. Unexpected behavior of the Ag atoms is clarified: For the √3 × √3-Ag surface it is verified that the Ag atoms are embedded in the first double layer of Si, leading to a considerable rearrangement of the substrate. In contrast, for the 3 × 1-Ag surface, the Ag atoms are riding on the Si surface and the reconstruction of the substrate is small.     

Chemisorption of Si on Al(111) surfaces: A local-chemical-bond analysis from Auger transition density of states

Auger and electron loss spectroscopies have been used to study the local chemical bond between Si and Al, in the first stages of growth of Si deposited at room temperature on Al(111) surfaces. Si follows a layer-by-layer mechanism up to 2 monolayers with the formation of an Al(111)-3 × 3-Si structure at about 0.44 monolayers. A detailed analysis of the L_2,3VV Auger spectra for this structure allows to interpret the Si and Al Auger transition density of states (TDOS) in terms of the actual p-like partial DOS centered on the Si and Al sites. The experimental results indicate a strong SiAl interaction with the formation of a p-type local covalent bond between the Si and Al surface atoms.