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.     

Epitaxial growth of Al on Si(1 1 1) with Cu buffer layers

The structure of thin Al films grown on Si(1 1 1) with thin Cu buffer layers has been investigated using synchrotron radiation photoemission spectroscopy. A thin Cu(1 1 1) layer between the Si(1 1 1) substrate and an Al film may enhance quantum well effects in the Al film significantly. The strength of quantum well effects has been investigated qualitatively with respect to the thickness of the Cu buffer layer and to the Al film thickness. Deposition of Cu on Si(1 1 1)7 × 7 leads to formation of a disordered silicide layer in an initial regime before a well-ordered Cu(1 1 1) film is formed after deposition of the equivalent of 6 layers of Cu. In the regime below 6 layers of Cu the disorder is transferred to Al layers subsequently grown on top. The initial growth of up to 8 layers of Al on a well-ordered Si/Cu(1 1 1) layer leads to a disordered film due to the lattice mismatch between the two metals. When the Cu buffer layer and the Al over-layer are above 6 and 8 layers, respectively the Al film shows sharp low energy electron diffraction patterns and very sharp quantum well peaks in the valence band spectra signalling good epitaxial growth.     

Ytterbium on vicinal Si(1 0 0): Growth and properties of the 2D wetting layer and the Yb silicide phase

The Yb growth on a vicinal Si(1 0 0) surface has been studied by scanning tunneling microscopy and low energy electron diffraction in the coverage range of 1-4 ML. Two different methods of the Yb/Si(1 0 0) interface formation are applied, leading to a remarkable modification of structural and morphological properties of two-dimensional (2D) wetting layer and Yb silicide phase. In particular, the switchover of the 2D layer orientation, similar to the case of the Bi nanolines on Si(1 0 0) [J.H.G. Owen, K. Miki, D.R. Bowler, J. Mater. Sci. 41 (2006) 4568], is observed depending on the growth procedure. Moreover, the structure and morphology of the Yb silicide phase is found to depend critically on the growth conditions, and the ability to grow very long, unidirectional Yb silicide nanowires is demonstrated. The results obtained are discussed in the context of the previous studies of 1D nanowires and 3D islands of rare-earth silicides on Si(1 0 0).     

Vanadium nanoclusters on Si(1 1 1) 7 × 7 surface studied by scanning tunneling microscopy

The size distribution and shape transition of self-assembled vanadium silicide clusters on Si(1 1 1) 7 × 7 have been investigated by scanning tunneling microscopy. Nanoclusters were formed by submonolayer vanadium deposition at room temperature followed by subsequent annealing (solid phase epitaxy - SPE). At room temperature, initially V-nanoclusters are formed which occupy sites avoiding the corner hole parts of the unit cells in the Si(1 1 1) 7 × 7 surface. Upon annealing, strong metal-silicon reaction occur leading to the formation of vanadium silicide nanoclusters. As a function of temperature, both, flat (2D) and three dimensional (3D) clusters have been obtained. After annealing at temperatures around 900 K many faceted clusters are created, whereas at higher annealing temperature, around 1300 K, predominantly 3D clusters are formed. The size distribution of SPE grown clusters could be well controlled in the range of 3-10 nm. The cluster size depends on the annealing temperature as well as on the initial vanadium coverage. Based on high resolution STM images a structure model for one kind of vanadium disilicide clusters exposing atomically flat surfaces was proposed.     

Bimodal growth of manganese silicide on Si(1 0 0)

Two different growth modes of manganese silicide are observed on Si(1 0 0) with scanning tunneling microscopy. 1.0 and 1.5 monolayer Mn are deposited at room temperature on the Si(1 0 0)-(2 × 1) substrate. The as-grown Mn film is unstructured. Annealing temperatures between room temperature and 450 °C lead to small unstructured clusters of Mn or Mn_xSi_y. Upon annealing at 450 °C and 480 °C, Mn reacts chemically with the Si substrate and forms silicide islands. The dimer rows of the substrate become visible again. Two distinct island shapes are found and identified as MnSi and Mn₅Si₃.     

Initial stages of iron silicide formation on the Si(1 0 0)2 × 1 surface

The initial stages of iron silicide growth on the Si(1 0 0)2 × 1 surface during solid-phase synthesis were investigated by photoelectron spectroscopy using synchrotron radiation. The experiments were made on iron films of 1-50 monolayer (ML) thickness in the temperature range from room temperature to 750 °С. Our results support the existence of three stages in the Fe deposition on Si(1 0 0) at room temperature, which include formation of the Fe-Si solid solution, Fe₃Si silicide and an iron film. The critical Fe dose necessary for the solid solution to be transformed to the silicide is found to be 5 ML. The solid-phase reaction was found to depend on the deposited metal dose. At 5 ML, the reaction begins at 60 °С, and the solid-phase synthesis leads to the formation of only metastable silicides (FeSi with the CsCl-type structure, γ-FeSi₂ and α-FeSi₂). A specific feature of this process is Si segregation on the silicide films. At a thickness of 15 ML and more, we observed only stable phases, namely, Fe₃Si, ε-FeSi and β-FeSi₂.     

Following the oxidation of yttrium silicide epitaxially grown on Si(1 1 1) by core level photoemission spectroscopy

We have identified, by means of synchrotron radiation X-ray photoemission spectroscopy, several core-level shifted components in the Si-2p photoemission core level peak from a thin yttrium silicide layer epitaxially grown on a Si(1 1 1) surface. We have unequivocally assigned these components to different environments of the Si atoms in the silicide structure. This information has been used to monitor a surface oxidation process promoted by room temperature oxygen adsorption, identifying the final product of this reaction as a silicate-type ternary compound.     

Surface etching induced by Ce silicide formation on Si(1 0 0)

We investigate the temperature-dependent surface etching process induced by Ce silicide on Si(1 0 0) using scanning tunneling microscopy and X-ray photoelectron spectroscopy. We found that step edges on the Si(1 0 0) surface are gradually roughened due to the formation of Ce silicide as a function of substrate temperature. Unlike the Si(1 1 1) surface, however, terrace etching also occurs in addition to step roughening at 500 °C. Moreover, we found that Si(1 0 0) dimers are released and formed dimer vacancy lines because bulk diffusion of Ce silicide into Si(1 0 0) surface occurs the defect-induced strain at higher temperature (∼600 °C).     

Ca induced reconstructions of the Si(0 0 1) surface

The growth behavior of Ca on Si(0 0 1) has been studied with scanning tunneling microscopy and low energy electron diffraction. During the growth of the first atomic layer at elevated temperature, Ca induces several different ordered surface reconstructions. In order of ascending metal content, they are: a 2 × n (n = 3, 4, 5) phase that has limited long range order, a 2 × 6 striped phase, and a 1 × 3 phase. The 1 × 3 phase covers the entire surface at and beyond the point where Ca silicide island growth starts.     

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.     

Atomic structure of Bi-dimer row selectively adsorbed on Si(5 5 12)-2 × 1 surface

In order to understand the atomic structure of nanostructures self-assembled on the template with one-dimensional symmetry, Bi/Si(5 5 12) system has been chosen and Bi-adsorption steps have been studied by STM. With Bi adsorption, the clean Si(5 5 12) is transformed to (3 3 7) terraces with disordered boundary due to mismatched periodicities between (3 3 7) and (5 5 12), and Bi-dimer rows are formed inside the (3 3 7) unit as follows: Initially, when Bi atoms are deposited at the adsorption temperature of about 450 °C, they selectively replace Si-dimers and Si-adatoms and form adsorbed Bi-dimers and Bi-adatoms, respectively. If additional Bi is supplied, the Bi-dimers adsorb on the Bi-dimers and Bi-adatoms in the first layer. These adsorbed dimers in the second layer are facing each other to form a Bi-dimer pair with relatively stable p³bonding. Finally, a single Bi-dimer adsorbs above the Bi-dimer pair in the second layer, at which point the Bi layer thickness saturates. It has been concluded that the Bi-dimer pair with stable p³ bonding is the composing element in the second layer and such site-selective adsorption is possible due to the substrate-strain relaxation through inserting Bi-buffer layer limited to specific sites of the substrate.     

High-resolution photoemission studies of the interfacial reactivity and interfacial energetics of Au and Cu Schottky barriers on methyl-terminated Si(1 1 1) surfaces

The Schottky junction formation by the stepwise evaporation of gold and copper, respectively, onto methyl-terminated silicon, CH₃-Si(1 1 1), was investigated by synchrotron X-ray photoelectron spectroscopy. During the junction formation process, interface reactions occurred as revealed by the appearance of chemically shifted Si 2p components. Upon deposition of Au, the formation of about one monolayer of gold silicide, SiAu₃, with a Si 2p chemical shift of +0.75(2) eV, was observed. The SiAu₃ floated on top of the growing gold layer. Similarly, for the deposition of Cu, the methyl termination layer was partially disrupted, as indicated by the appearance of a −0.28(2) eV chemically shifted Si 2p component attributable to an interfacial copper silicide phase, SiCu₃. Hence, the termination of the Si(1 1 1) surface by methyl groups did not completely prevent interfacial reactions, but did reduce the amount interfacial reaction products as compared to bare Si(1 1 1)-(7 × 7) surfaces.Electron Schottky barrier heights of 0.78(8) eV (Au) and 0.61(8) eV (Cu) were measured. Within the experimental uncertainty the observed Schottky barriers were identical to those ones obtained on non-passivated, (7 × 7)-reconstructed Si(1 1 1) surfaces. Thus, the modification of the electronic properties of the silicon-metal contact requires the complete absence of interfacial reactions.     

Study of nickel silicide formation on Si(1 1 0) substrate

Nickel silicide formation on Si(1 1 0) and Si(1 0 0) substrate was investigated in this paper. It is confirmed that nickel monosilicide (NiSi) starts to form after 450 °C annealing for Si(1 0 0) substrate, but a higher annealing temperature is required for NiSi formation on Si(1 1 0) substrate, which is demonstrated by X-ray diffraction (XRD) and Raman scattering spectroscopy. The higher formation temperature of NiSi is attributed to the larger Ni₂Si grain size formed on Si(1 1 0) substrate. Ni silicided Schottky contacts on both Si(1 0 0) and Si(1 1 0) substrates were also fabricated for electrical characteristics evaluation. It clearly reveals that the rectifying characteristics of NiSi/n-Si(1 1 0) Schottky contacts is inferior to that of NiSi/n-Si(1 0 0) Schottky contacts, which is attributed to a lower Schottky barrier height and a rougher contact interface. The formation kinetics for nickel silicide on Si(1 1 0) substrate is also discussed in this paper.     

Mechanisms of preferential adsorption on the Si(1 1 1)7 × 7 surface

The surface relaxation mechanisms governing the preferential adsorption of metal atoms onto the faulted half-cells of a 7 × 7 reconstructed Si(1 1 1) surface are studied by rate equations and kinetic Monte Carlo simulations. The versatility of these mechanisms to control the formation of quasi-perfect 2D arrays of metal clusters is revealed via the optimization of the deposition/annealing conditions as a function of operating mechanisms, the Si(1 1 1)7 × 7 energy landscape, and the thermal stability of the created clusters. The influence on the formation process of such nanoarrays of the balance between kinetic limitations, which are especially relevant on Si(1 1 1)7 × 7, and thermodynamic tendencies is discussed.     

Atomic structure of the Si(5 5 12)-2 × 1 surface

Based on the results of scanning tunneling microscopy studies of the reconstructed Si(5 5 12)-2 × 1 surface, its atomic structure has been found. It turns out that Si(5 5 12)-2 × 1 consists of four one-dimensional structures: honeycomb (H) chain, π-bonded H′ (π) chain, dimer-adatom (D/A) row, and tetramer (T) row. Its period is composed of three subunits, i.e., (i) (3 3 7) unit with a D/A row [D(3 3 7)], (ii) (3 3 7) unit with a T row [T(3 3 7)], and (iii) (2 2 5) unit with both a D/A and a T row. Two kinds of adjacent subunits, T(3 3 7)/D(3 3 7) and D(3 3 7)/(2 2 5), are divided by H chains with 2× periodicity due to buckling, while one kind of adjacent subunits, T(3 3 7)/(2 2 5), is divided by a π chain with 1× periodicity. Two chain structures, H and π chains, commute with each other depending upon the external stresses perpendicular to the chain, which is the same for two row structures, D/A and T rows. It can be concluded that the wide and planar reconstruction of Si(5 5 12)-2 × 1 is originates from the stress balance among two commutable chains and two commutable rows.     

Study of Ni/Si(1 0 0) solid-state reaction with Al addition

The characteristics of Ni/Si(1 0 0) solid-state reaction with Al addition (Ni/Al/Si(1 0 0), Ni/Al/Ni/Si(1 0 0) and Al/Ni/Si(1 0 0)) is studied. Ni and Al films were deposited on Si(1 0 0) substrate by ion beam sputtering. The solid-state reaction between metal films and Si was performed by rapid thermal annealing. The sheet resistance of the formed silicide film was measured by four-point probe method. The X-ray diffraction (XRD) was employed to detect the phases in the silicide film. The Auger electron spectroscopy was applied to reveal the element profiles in depth. The influence of Al addition on the Schottky barrier heights of the formed silicide/Si diodes was investigated by current-voltage measurements. The experimental results show that NiSi forms even with the addition of Al, although the formation temperature correspondingly changes. It is revealed that Ni silicidation is accompanied with Al diffusion in Ni film toward the film top surface and Al is the dominant diffusion species in Ni/Al system. However, no Ni_xAl_y phase is detected in the films and no significant Schottky barrier height modulation by the addition of Al is observed.     

Pd adsorption on Si(1 1 3) surface: STM and XPS study

Pd-induced surface structures on Si(1 1 3) have been studied by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). In the initial process of the Pd adsorption below 0.10 ML, Pd silicide (Pd₂Si) clusters are observed to form randomly on the surface. By increasing the Pd coverage to 0.10 ML, the clusters cover the entire surface, and an amorphous layer is formed. After annealing the Si(1 1 3)-Pd surface at 600 °C, various types of islands and chain protrusions appears. The agglomeration, coalescence and crystallization of these islands are observed by using high temperature (HT-) STM. It is also found by XPS that the islands correspond to Pd₂Si structure. On the basis of these results, evolution of Pd-induced structures at high temperatures is in detail discussed.     

Diverse magic nanoclustering in submonolayer Tl/Si(1 1 1) system

Using scanning tunneling microscopy, ordered magic nanoclustering in the submonolayer Tl/Si(1 1 1)7 × 7 system has been studied. In contrast to the other known metal/silicon systems where ordered magic nanoclustering takes place, the Tl/Si(1 1 1) system has been found to be the only one to exhibit the diverse magic clustering. In the present study, occurrence of, at least, three distinct types of the well-defined Tl/Si(1 1 1) magic clusters has been detected.     

Growth of (√3 × √3)-Ag and (1 1 1) oriented Ag islands on Ge/Si(1 1 1) surfaces

We have studied the growth of Ag on Ge/Si(1 1 1) substrates. The Ge/Si(1 1 1) substrates were prepared by depositing one monolayer (ML) of Ge on Si(1 1 1)-(7 × 7) surfaces. Following Ge deposition the reflection high energy electron diffraction (RHEED) pattern changed to a (1 × 1) pattern. Ge as well as Ag deposition was carried out at 550 °C. Ag deposition on Ge/Si(1 1 1) substrates up to 10 ML has shown a prominent (√3 × √3)-R30° RHEED pattern along with a streak structure from Ag(1 1 1) surface. Scanning electron microscopy (SEM) shows the formation of Ag islands along with a large fraction of open area, which presumably has the Ag-induced (√3 × √3)-R30° structure on the Ge/Si(1 1 1) surface. X-ray diffraction (XRD) experiments show the presence of only (1 1 1) peak of Ag indicating epitaxial growth of Ag on Ge/Si(1 1 1) surfaces. The possibility of growing a strain-tuned (tensile to compressive) Ag(1 1 1) layer on Ge/Si(1 1 1) substrates is discussed.     

Core level photoemission and STM characterization of Ta/Si(1 1 1)-7 × 7 interfaces

We present the results of scanning tunneling microscopy (STM) and photoemission spectroscopy (PES) of the Ta/Si(1 1 1)-7 × 7 system after deposition of Ta at substrate temperatures from 300 to 1250 K. The coverage of Ta varied from 0.05 up to 2.5 of a monolayer (ML). STM shows that at 300 K and coverage less than 1 ML, a disordered chemisorbed phase is formed. Deposition on a hot surface (above 500 K) produces round 3D clusters randomly distributed on the surface. Cluster height and their diameter are found to change drastically with annealing temperature and the Ta coverage. Analysis of photoemission data of the Si 2p core levels shows that at room temperature and at coverage ?1 ML core level binding energy shifts and intensity variations of Si surface related components are observed, which clearly indicate that the reaction starts already at 300 K. Shifts in the binding energy, changes of the peak shapes and intensity of the Ta 4f doublet at higher temperatures can be explained by the formation of stable silicide on the surface.