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.     

Highly resolved scanning tunneling microscopy study of Si(0 0 1) surfaces flattened in aqueous environment

A surface preparation method with fine SiO₂ particles in water is developed to flatten Si(0 0 1) surfaces on the nanometer scale. The flattening performance of Si(0 0 1) surfaces after the surface preparation method is investigated by scanning tunneling microscopy. The observed surface is so flat that 95% of the view area (100 × 100 nm²) is composed of only three atomic layers, namely, one dominant layer occupying 50% of the entire area and two adjacent layers. Furthermore, a magnified image shows the outermost Si atoms regularly distributed along the 〈1 1 0〉 direction on terraces.     

Study of Si(0 0 1)4 × 2-Ga structure by scanning tunneling microscopy and ab initio calculation

We have studied Si(0 0 1)-Ga surface structures formed at Ga coverages of slightly above 0.50 monolayer (ML) at 250 °C by scanning tunneling microscopy (STM). 4 × 2-, 5 × 2-, and 6 × 2-Ga structures were observed in a local area on the surface. The 4 × 2-Ga structure consists of three protrusions, as observed in filled- and empty-state STM images. The characters of these structures are clearly different from those of other Si(0 0 1)-Ga structures. We also performed an ab initio calculation of the energetics for several possible models for the 4 × 2-Ga structure, and clarified that the three-orthogonal-Ga-dimer model is the most stable. Also, the results of comparing the simulated STM images and observation images at various bias voltages indicate that this structural model is the most favorable.     

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.     

Diffusion of Pb adatom and ad-dimer on Si(1 0 0) from ab initio studies

The diffusion pathways of Pb adatoms and ad-dimers on Si(1 0 0) are investigated by first-principles calculations. Pb adatoms are found to diffuse on top of the Si(1 0 0) dimer row with an energy barrier of 0.31 eV. However, Pb dimers are energetically more stable. Pb dimers on top of the dimer row have a high energy barrier (0.95 eV) to rotate from the lowest energy configuration to the orientation parallel to the underlying Si(1 0 0) dimer row. Once the ad-dimer is oriented parallel to Si(1 0 0) dimer row, they can diffuse along the dimer row with an energy barrier of only 0.32 eV.     

Structural and statistical analysis of Yb/Si(1 1 1) and Eu/Si(1 1 1) reconstructions

We have performed the structural and statistical analysis of Yb/Si(1 1 1) and Eu/Si(1 1 1) surfaces in the submonolayer regime utilizing low-energy electron diffraction and scanning tunneling microscopy (STM). The almost identical series of one-dimensional chain structures (e.g., 3 × 2/3 × 1, 5 × 1, 7 × 1, 9 × 1, and 2 × 1 phases) are found in order of increasing metal coverage for both adsorbed systems, however, only the Eu/Si system reveals the ‘√3’-like reconstruction before the 2 × 1 endpoint phase. The atomic models of chain structures are proposed and discussed. In particular, our results suggest the odd-order n×1 (n=5,7,9,…) intermediate reconstructions to incorporate the Seiwatz chains and honeycomb chains with the proportion of m:1, where . The statistical analysis of STM images is carried out to examine the correlation of atomic rows on Eu/Si and Yb/Si surfaces. It is found that Eu stabilizes more ordered row configuration compared to Yb, which can be explained in terms of indirect electronic interaction of atomic chains or/and different magnetic properties of adsorbed species.     

Theoretical study on the temperature-induced structural transition of the Si(1 1 3) surface

The temperature-induced structural transition of the Si(1 1 3) surface is investigated by ab initio calculations. In this study, it is found that the room-temperature phase and the high-temperature phase have the 3 × 2 interstitial structure and the 3 × 1 interstitial structure, respectively. The existence of the 3 × 2 and 3 × 1 interstitial structures is supported by the analysis of scanning tunneling microscopy (STM) images and the calculation of surface core level shifts using final state pseudopotential theory. The theoretical STM images of interstitial structures are in good agreement with the STM images suggested by experiments. The analysis of STM images provides an insight into the characteristics of domain boundaries observed frequently in experiments. It is also found that the domain boundary can be formed by local 3 × 1 interstitial structures on the 3 × 2 interstitial surface. The theoretical analysis of the surface core level shifts reveals that the surface core levels in experiment originate from the interstitial structures. The lowest values in the surface core level shifts are found to be associated with the 2p core level shifts of the interstitial atoms.     

Scaling behavior of island density in submonolayer growth of CaF2 on vicinal Si(1 1 1)

We have studied the scaling behavior of two-dimensional island density during submonolayer growth of CaF₂ on vicinal Si(1 1 1) surfaces using scanning tunneling microscopy. We have analyzed the morphology of the Si(1 1 1) surfaces where CaF₂ partial monolayers with coverages of about 0.1 monolayer are deposited at ∼600 °C. The number density of terrace nucleated islands increases with substrate terrace width l as ∼l⁴ in a low island density regime. This scaling behavior is consistent with predictions for the case of the irreversible growth of islands.     

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.     

Reflection high-energy positron diffraction pattern from a Si(1 1 1)-(7 × 7) surface

We have investigated the feature of reflection high-energy positron diffraction (RHEPD) pattern from a Si(1 1 1)-(7 × 7) surface. The RHEPD pattern observed in the total reflection condition is quite different from the conventional reflection high-energy electron diffraction (RHEED) pattern. This fact is attributed to the different penetration depths of positrons and electrons. We show that the intensity distribution of RHEPD pattern is reproduced considering the dimer-adatom-stacking fault (DAS) model with optimized atomic positions and scattering potentials of adatoms and rest atoms.     

STM study of Si(1 1 3) 3 × 2-Ga surface

The Ga-adsorbed structure on Si(1 1 3) surface at low coverage has been studied by scanning tunneling microscopy (STM). The bright protrusion corresponding to the position of the dimer without the interstitial Si atom of the clean surface disappeared in the filled-state STM image after Ga adsorption, although the protrusion due to the Si adatom still remained. On the basis of the adatom-dimer-interstitial (ADI) model, this result indicates that the Ga atom is adsorbed interstitially at the center of another pentamer that does not have the interstitial Si atom. An ab initio calculation was performed and STM images were simulated.     

Formation mechanism of the Si(1 1 1)7×7 reconstruction studied by scanning tunneling microscopy: Zipper-like restructuring in the sequential size changes of isolated single faulted-halves

The formation mechanism of the 7×7 reconstruction on annealed Si(1 1 1) surfaces has been demonstrated at the atomic level. In situ observations of unreconstructed regions (‘1×1’) on terraces after rapid quenching to 380 °C were done using scanning tunneling microscopy (STM) with a scanning speed of 1.7 s per frame. In the narrow ‘1×1’ regions, we imaged isolated single-faulted (F) halves of the dimer-adatom-stacking-fault (DAS) structure from “birth” to “death”. During “life”, the isolated single F-halves frequently changed their size. The size changes between odd-sized F-halves always took place through even-sized F-halves of intermediate sizes: 5×5-F^′↔6×6-F↔7×7-F^′↔8×8-F↔9×9-F^′↔10×10-F↔11×11-F^′↔12×12-F↔13×13-F^′, where the 5×5-F^′, 7×7-F^′ and so forth are irregular-type structures of the odd-sized F-halves. Even-sized F-halves and the irregular-type structures are necessary in the size changes, whereas the regular-type structures have never been involved. Lifetimes of the 10×10-F, 8×8-F, and 6×6-F at 380 °C are about 10.5, 6, and 2-3 s, respectively, which are much shorter than those of the isolated irregular-type structures of the odd-sized F-halves. With the aid of room temperature STM images of a rapidly quenched surface, we determined the atomic structures of the even-sized F-halves. We have proposed a sequential size change (SSC) model, including undiscovered parts of the size changes ‘1×1’ ↔2×2-F↔3×3-F^′↔4×4-F↔5×5-F^′, as the formation and decay mechanism of isolated single F-halves in the ‘1×1’ region. The SSC model has the following sequence: ‘1×1’ ↔ 2×2-F↔3×3-F^′↔4×4-F↔5×5-F^′↔6×6-F↔7×7-F^′↔8×8-F↔9×9-F^′↔10×10-F↔11×11-F^′↔12×12-F↔13×13-F^′. It was found by collecting statistics of size-change directions that one of two equivalent sides of the irregular-type structures, which have a mirror symmetry, is involved in the size changes thus indicating that other parts of the F-halves remain unchanged. Based on such findings, we have proposed the atomic processes for bond-rearrangements in the SSC model. The bond-rearrangements proceed along one side of a triangular F-half by breaking the existing dimers and forming new dimers like a “zipper”. Proposed atomic processes of the zipper-like restructuring are illustrated by a ball-and-stick model. The reason for the appearance of the even-sized F-halves and the irregular-type structures of the odd-sized ones is discussed in terms of the energy barrier heights along a reaction path in the size change of single F-halves.     

SrTiO3(0 0 1) reconstructions: the (2 × 2) to c(4 × 4) transition

Scanning tunneling microscopy (STM) is used to investigate the (0 0 1) surface structure of Nb doped SrTiO₃ single crystals annealed in ultra high vacuum (UHV). Atomically resolved images of the (2 × 2) reconstructed surface are obtained after annealing a chemically etched sample. With further annealing dotted row domains appear, which coexist with the (2 × 2) reconstruction. The expansion of these domains with further annealing gives rise to the formation of a TiO₂ enriched c(4 × 4) reconstruction.     

Impact of interface relaxation on the nanoscale corrugation in Pb/Si(1 1 1) islands

The nanoscale hexagonal pattern observed in scanning tunneling microscopy (STM) for 3-layer and 4-layer Pb islands on Si(1 1 1) is studied theoretically. We found that besides thickness the atomic rearrangement at the Pb/Si interface plays an important role in determining the STM patterns. Electronic structures of the Pb film on Si(1 1 1) obtained from fully relaxed and unrelaxed Pb films are qualitatively different. Simulated STM images for Pb films with different stacking also show that the corrugation patterns are sensitive to the buried Pb-Si interfacial structure.     

Eu- and Yb-induced reconstructions on a vicinal Si(1 0 0) surface

Early stages of rare-earth metal (Yb and Eu) growth on a vicinal, single-domain Si(1 0 0)2 × 1 surface have been studied in the coverage range of 0.1-0.3 monolayer (ML) by low energy electron diffraction, scanning tunneling microscopy, and synchrotron radiation photoemission spectroscopy. We show that Yb induces the 2 × 3 periodicity in the whole range of coverage studied. The 2 × 3 reconstruction coexists with the local 3 × 2/4 × 2 structure at about 0.2 ML of Yb. In contrast, Eu forms the 3 × 2 periodicity at 0.1-0.2 ML, whereas this structure is converted into the 2 × 3 phase at about 0.3 ML. The atomic arrangement and electronic properties of these reconstructions and the adsorbate-mediated modification of surface morphology are investigated.     

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.     

Theoretical investigation of the Au/Si(1 1 1)-(5 × 2) surface structure

The atomic structure of the Au/Si(1 1 1)-(5 × 2) surface has been studied by density-functional theory calculations. Two structure models, proposed experimentally by Marks et al. and Hasegawa et al., have been examined on an equal ground. In our total-energy calculations, both models are found to be locally stable and energetically comparable. In our electronic-structure analyses, however, both models fail to reproduce the key features of angle-resolved photoemission spectra and scanning-tunneling-microscopy images, indicating that the considered models need to be modified. Suggestions for the modification are given based on the present calculations.     

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).     

Formation and stabilization of Fe-induced magic clusters on Si(1 1 1)-(7 × 7) template

We demonstrate the growth of Fe-induced magic clusters on Si(1 1 1)-(7 × 7) template by in situ scanning tunneling microscopy (STM). These clusters form near a dimer row at one side of the half-unit cell (HUC); and with three different equivalent orientations. A cluster model comprising three top layer Si atoms bonded to six Fe atoms at the next layer in the 7 × 7 faulted-half template is proposed. The optimized cluster structure determined by first-principles total-energy calculation shows an inward-shifting of the three center Fe atoms. The clusters and the nearby center-adatoms of the next HUCs appear with a significantly reduced height below bias voltages 0.4 V in high resolution empty-state STM images, suggesting an energy gap opening near the Fermi level at these localized cluster and adatom sites. We explain the stabilization of the clusters on the 7 × 7 template using the gain in electronic energy as the driving force for cluster formation.     

Aromatic interactions in the close packing of phenyl-imides at Cu(1 1 0) surfaces

The oxidation of aniline at Cu(1 1 0) surfaces at 290 K has been studied by XPS and STM. A single chemisorbed product, assigned to a phenyl imide (C₆H₅N(a)), is formed together with water which desorbs. Reaction with preadsorbed oxygen results in a maximum surface concentration of phenyl imide of 2.8 × 10¹⁴ mol cm⁻² and a surface dominated by domains of three structures described by , and unit meshes. However, concentrations of phenyl imide of up to 3.3 × 10¹⁴ mol cm⁻² were obtained from the coadsorption of aniline and dioxygen (300:1 mixture) resulting in a highly ordered biphasic structure with and domains. Comparison of the STM and XPS data shows that only half the phenyl imides at the surface are imaged. Pi-stacking of the phenyl rings is proposed to account for this observation.