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.     

Growth mechanisms for wire-like epitaxial gold silicide islands on Si(1 1 0) surfaces

Epitaxial islands grown on various substrates are usually strained because of differences in lattice constants of the materials of the island and the substrate. Shape transition in the growth of strained islands has been proposed as a mechanism for strain relief and a way to form self-organized quantum wires. Shape transition usually leads to an elongated island growth. However, an elongated island growth may also be due to an anisotropic diffusion of material, the anisotropy being imposed by the symmetry of the substrate surface. In the present example, growth of gold silicide wire-like nanostructures on a Si(1 1 0) surface has been investigated by photoemission electron microscopy (PEEM). Growth of elongated unidirectional gold silicide islands, with an aspect ratio as large as 12:1, has been observed by PEEM following gold deposition on the Si substrate and subsequent annealing at the Au-Si eutectic temperature. Distribution of the width and the length of the gold silicide islands as a function of island area shows a feature similar to that for the shape transition. However, detailed investigations reveal that the elongated growth of gold silicide islands is rather mainly due to anisotropic diffusion of gold due to the twofold symmetry of the (1 1 0) surface of the Si substrate.     

Initial growth process and surface structure of Ag on Si(111) studied by low-energy Ion-Scattering Spectroscopy (ISS) and LEED-AES

The growth process of silver on a Si(111) substrate has been studied in detail by low-energy ion-scattering spectroscopy (ISS) combined with LEED-AES. Neon ions of 500 eV were used as probe ions of ISS. The ISS experiments have revealed that the growth at room temperature and at high temperature are quite different from each other even in the submonolayer coverage range. The following growth models have been proposed for the respective temperatures. At room temperature, the deposited Ag forms a two-dimensional (2D) island at around 2/3 monolayer (ML) coverage, where the Ag atoms are packed commensurately with the Si(111)1 substrate. One third of the substrate Si surface remains uncovered there. Then it starts to develop into Ag crystal, and at a few ML coverage a 3D island of bulk Ag crystal grows directly on the substrate. An intermediate layer, which covers uniformly the whole surface before the growth of Ag crystal, does not exist. At high temperatures (>~200°C), the well-known Si(111)√3-Ag layer is formed as an intermediate layer, which consists of 2/3 ML of Ag atoms and covers the whole surface uniformly. These Ag atoms are embedded in the first double layer of the Si substrate. It is concluded that the formation of the √3 structure needs relatively high activation energy which may originate from the large displacement of Si atoms owing to the embedment of the Ag atoms, and does not proceed below about 200°C. The most stable state of the Ag atoms on the outermost Si layer is in the shape of an island, both for the Si(111) surface and for the Si(111)√3-Ag surface.     

Simulations of atomic level stresses in systems of buried Ge /Si islands.

Stress distribution in laterally ordered arrays of coherent Ge islands on Si(001) buried in Si cap layers is examined using atomistic simulations. The obtained hydrostatic stress dependence on the spacer layer thickness shows a nearly linear inverse dependence, unlike the commonly used inverse cubic dependence derived in the framework of an isolated embedded force dipole source model. Additionally, the hydrostatic stress on the spacer surface is found to scale more closely with the area of the island rather than its volume as implicit in the use of the force dipole model.     

低温制备微晶硅薄膜生长机制的研究

采用热丝化学气相沉积技术制备了一系列处于不同生长阶段的薄膜样品，用原子力显微镜系 统地研究生长在单晶硅衬底和玻璃衬底上薄膜表面形貌的演化.按照分形理论分析得到：在 玻璃衬底上的硅薄膜以零扩散随机生长模式生长；而在单晶硅衬底上，薄膜早期以有限扩散 生长模式生长，当膜厚超过某一临界厚度时转变为零扩散随机生长模式.岛面密度与膜厚的 依赖关系表明，在临界厚度时硅衬底和玻璃衬底上的岛面密度均出现了极大值.Raman谱的测 量证实，玻璃衬底上薄膜临界厚度与非晶/微晶相变之间存在密切的关系.不同的衬底材料直 接影响反应 关键词： 生长机制 微晶硅薄膜 表面形貌 热丝化学气相沉积     

Thin film semiconductor islands and numerical experiment

The distribution functions of island nuclei of germanium melt drops on Si(100) substrate over lateral sizes and heights in the presence of a surface linear defect are obtained in a numerical experiment simulating the initial stage of the first-order phase transition. A similar model of heterogeneous condensation of silicon carbide vapor is discussed. Quasi-linear kinetic partial differential equations are solved by the efficient numerical method of stochastic analogue of nonequilibrium processes. The Volmer-Weber mechanism of cluster formation from melt islands, their crystallization, and island structure formation on the substrate are considered.     

Sculpting semiconductor heteroepitaxial islands: from dots to rods

In the Ge on Si model heteroepitaxial system, metal patterns on the silicon surface provide unprecedented control over the morphology of highly ordered Ge islands. Island shape including nanorods and truncated pyramids is set by the metal species and substrate orientation. Analysis of island faceting elucidates the prominent role of the metal in promoting growth of preferred facet orientations while investigations of island composition and structure reveal the importance of Si-Ge intermixing in island evolution. These effects reflect a remarkable combination of metal-mediated growth phenomena that may be exploited to tailor the functionality of island arrays in heteroepitaxial systems.     

Symmetry breaking in the growth of two-dimensional islands on Si(111)

We find that the shape of two-dimensional (2D) Si or Ge islands has a lower symmetry than the threefold symmetry of the underlying Si(111) substrate if Bi is used as a surfactant during growth. Arrow-shaped or rhomb-shaped 2D islands are observed by scanning tunneling microscopy. This symmetry breaking is explained by a mutual shift between the surface reconstructions present on the substrate and on the islands. Using the kinematic Wulff construction the growth velocities of the steps could be determined from the island shape if the nucleation center has been located by a marker technique.     

Probing the lateral composition profile of self-assembled islands

We apply a selective etching procedure to probe the lateral composition profile of self-assembled SiGe pyramids on a Si(001) substrate surface. We find that the pyramids consist of highly Si intermixed corners, whereas the edges, the apex, and the center of the pyramids remain Ge rich. Our results cannot be explained by existing growth models that minimize strain energy. We use a model that includes surface interdiffusion during island growth, underlining the paramount importance of surface processes during the formation of self-assembled quantum dot heterostructures in many different material systems.     

7 × 7 Si(111)Cu interfaces: Combined LEED,AES and EELS measurements

CuSi(111) interfaces have been examined as a function of substrate temperature and deposition time using different surface sensitive techniques. At room temperature a layer by layer growth was found. Strong intermixing of Cu and Si takes place and beyond 10 monolayers an ordered α Si phase in Cu was found. At high temperature, the growth follows the layer plus island or Stranski-Krastanov growth mode. Three-dimensional crystallites, probably consisting of Cu₃Si, grow epitaxially on an ordered intermediate layer.     

New surface structure analysis of Ag-Si phase by quantitative auger electron spectroscopy method

Ag or Au was deposited on a clean Si substrate at room temperature. These systems, Ag/Si and Au/Si, were annealed at various temperatures or various heating times. Due to the annealing, Ag or Au diffused into Si and/or Si diffused into the metal. The changes of the surface composition are analyzed by a quantitative Auger Electron Spectroscopy (AES) method which is newly developed as a non-destructive method. In the case of Ag/Si, Ag migrated into the Si substrate and/or Si diffused into Ag. Then, Ag-Si solid solution was produced. After the annealing, the Ag/Si system is changed into Ag/(Ag-Si)/Si of the three-phase structure. In the case of Au/Si (Au film thickness < 15 Å), the Au film thickness became thinner by annealing. The Au/Si system always keeps the Au/Si phase after annealing, while there was no Au-Si solution area. The difference between the Ag/(Ag-Si)/Si and the Au/Si structure is attributed to the reason that Au diffuses more quickly than Ag into the Si substrate. AES results after annealing cannot be explained by the model of the formation of the three-dimensional island structure which is commonly referenced.     

Au island growth on a Si(1 1 1) vicinal surface

Au island nucleation and growth on a Si(1 1 1) 7 × 7 vicinal surface was studied by means of scanning tunneling microscopy. The surface was prepared to have a regular array of step bunches. Growth temperature and Au coverage were varied in the 255-430 °C substrate temperature range and from 1 to 7 monolayers, respectively. Two kinds of islands are observed on the surface: Au-Si reconstructed islands on the terraces and three-dimensional (3D) islands along the step bunches. Focusing on the latter, the dependence of island density, size and position on substrate temperature and on Au coverage is investigated. At 340 °C and above, hemispherical 3D islands nucleate systematically on the step edges.     

Self-assembled growth of nanostructural Ge islands on bromine-passivated Si(111) surfaces at room temperature

We have deposited relatively thick (∼60 nm) Ge layers on Br-passivated Si(111) substrates by thermal evaporation under high vacuum conditions at room temperature. Ge has grown in a layer-plus-island mode although it is different from the Stranski-Krastanov growth mode observed in epitaxial growth. Both the islands and the layer are nanocrystalline. This appears to be a consequence of reduction of surface free energy of the Si(111) substrate by Br-passivation. The size distribution of the Ge nanoislands has been determined. The Br-Si(111) substrates were prepared by a liquid treatment, which may not produce exactly reproducible surfaces. Nevertheless, some basic features of the nanostructural island growth are reasonably reproducible, while there are variations in the details of the island size distribution.     

Co growth on Si(0 0 1) and Si(1 1 1) surfaces: Interfacial interaction and growth dynamics

In situ X-ray photoelectron spectroscopy (XPS) and ex situ atomic force microscopy (AFM) were used to study the growth of thin cobalt films at room temperature (RT) on both clean and H-terminated Si(0 0 1) and Si(1 1 1) surfaces. The growth proceeds by first forming an initial CoSi₂-like phase at the growth front of the Si substrate. With increasing Co coverage the interfacial layer composition becomes richer in Co and eventually a metallic Co film is formed on top. Hydrogen termination of the Si surface did not suppress the reaction of Co and Si. A pseudo-layer-by-layer growth mode is proposed to describe the growth of Co on H-terminated Si surfaces, while closed-packed small island growth occurs on clean Si surfaces. The difference in growth mode can be attributed to the increase in the surface mobility of Co adatoms in the presence of hydrogen.     

Growth dynamics studied by RHEED during Si/Ge epitaxy from gaseous hydrides

Si and Ge epitaxial growth from disilane and germane in a gas-source molecular beam epitaxy (GSMBE) system is followed in situ by reflection high-energy electron diffraction (RHEED) intensity oscillations. During Ge homoepitaxy, the growth rate on a Ge(100) substrate is found to be limited by surface hydrogen desorption below 350°C and by hydride adsorption above this temperature. Ge heteroepitaxy on Si results in incomplete layer growth leaving exposed Si at the surface during the initial stages of growth. Therefore, a gradual change in the observed Si surface concentration is seen as growth proceeds. Si heteroepitaxy on Ge follows the Volmer-Weber growth mode and proceeds via island formation. This, combined with Ge surface segregation, results in a slow decrease of the Ge surface population at the growth front. During heteroepitaxial growth, hydride reaction rates differ on Si and Ge surfaces, and therefore a changing concentration of the surface species is manifest as a gradual change in the observed oscillation frequency. This effect, observed during the early stages of growth, shows strong temperature dependence, consistent with previous observations on SiGe alloys. Following several layers of growth, however, the surfaces become rough. The influence of this roughness on the oscillation frequency is also discussed.     

Fabrication of uniform Au silicide islands on the Si(1 1 1)-(7 × 7) substrate

The Au silicide islands have been fabricated by additional deposition of Au on the prepared surface at 270 °C where the Si islands of magic sizes were formed on the Si(1 1 1)-(7 × 7) dimer-adatom-stacking fault substrate. The surface structure on the Au silicide islands shows the Au/Si(1 1 1)-√3 × √3 reconstructed structure although the substrate remains 7 × 7 DAS structure. The size of the Au silicide islands depends on the size distribution of the preformed Si islands, because the initial size and shape of the Si islands play important roles in the formation of the Au silicide island. We have achieved the fabrication of the Au silicide islands of about the same size (∼5 nm) and the same shape by controlling the initial Si growth and the additional Au growth conditions.     

Annealing of chemically vapor deposited nanoscale Ti-Si islands on Si

Small islands of Ti-Si were formed by chemical vapor deposition onto a Si substrate and subsequently annealed. The islands were deposited in the 610–730 °C temperature range by the H₂ reduction of TiCl₄, either without or with a Si-containing gas added during deposition. Subsequent annealing above 800 °C decreases the island density significantly, and the islands take characteristic shapes. After annealing, the island size depends only weakly on the amount of Ti deposited while the island density varies more significantly and depends on the addition of a Si-containing gas during deposition. Three different dominant island shapes are seen on Si(001), and different shapes are found on Si(111). PACS 68.60.Dv; 68.65.Hb; 68.35.Fx     

Positioning of strained islands by interaction with surface nanogrooves

When strained Stranski-Krastanow islands are used as "self-assembled quantum dots," a key goal is to control the island position. Here we show that nanoscale grooves can control the nucleation of epitaxial Ge islands on Si(001), and can drive lateral motion of existing islands onto the grooves, even when the grooves are very narrow and shallow compared to the islands. A position centered on the groove minimizes energy. We use as prototype grooves the trenches which form naturally around islands. During coarsening, the shrinking islands move laterally to sit directly astride that trench. In subsequent growth, we demonstrate that islands nucleate on the "empty trenches" which remain on the surface after complete dissolution of the original islands.     

Nucleation and growth of Si on Pb monolayer covered Si(111) surfaces

With a scanning tunneling microscope (STM), we study the initial stage of nucleation and growth of Si on Pb monolayer covered Si(111) surfaces. The Pb monolayer can work as a good surfactant for growth of smooth Si thin films on the Si(111) substrate. We have found that nucleation of two-dimensional (2D) Pb-covered Si islands occurs only when the substrate temperature is high enough and the Si deposition coverage is above a certain coverage. At low deposition coverages or low substrate temperatures, deposited Si atoms tend to self-assemble into a certain type of Si atomic wires, which are immobile and stable against annealing to ~ 200 °C. The Si atomic wires always appear as a double bright-line structure with a separation of ~ 9 Å between the two lines. After annealing to ~ 200 °C for a period of time, some sections of Si atomic wires may decompose, meanwhile the existing 2D Pb-covered Si islands grow laterally in size. The self-assembly of Si atomic wires indicate that single Si adatoms are mobile at the Pb-covered Si(111) surface even at room temperature. Further study of this system may reveal the detailed atomic mechanism in surfactant-mediated epitaxy.     

Influence of interfactants on thin metal film growth

The growth of Pb on Si(1 1 1) with and without Ag as an interfactant is studied in the temperature range from 270 to 375 K by microscopy and spectroscopy. Whereas Pb grows on the Si(1 1 1)-7×7 surface in the Stranski–Krastanov (SK) mode, the growth mode on the Si(1 1 1)-√3×√3-R30°-Ag surface changes from layer-by-layer below 300 K to SK mode above 300 K. Spectroscopy shows that Ag remains at the interface between the substrate and the growing Pb film. The influence of the interfactant on the growth is attributed to the increase of the island density by an order of magnitude and to the changes of the growth kinetics resulting from this increase.