Oxygen-induced p(3 × 1) reconstruction of the W(1 0 0) surface

The oxidation of the W(1 0 0) surface at elevated temperatures has been studied using room temperature STM and LEED. High exposure of the clean surface to O₂ at 1500 K followed by flash-annealing to 2300 K in UHV results in the formation of a novel p(3 × 1) reconstruction, which is imaged by STM as a missing-row structure on the surface. Upon further annealing in UHV, this surface develops a floreted LEED pattern characteristic of twinned microdomains of monoclinic WO_x, while maintaining the p(3 × 1) missing-row structure. Atomically resolved STM images of this surface show a complex domain structure with single and double W〈0 1 0〉 rows coexisting on the surface in different domains.     

Theoretical STM images of alkaline-earth metal adsorbed Si(1 1 1)3 × 2 surfaces

We have performed total-energy calculations to study theoretical scanning tunneling microscopy (STM) images of the Si(1 1 1)3 × 2 surfaces induced by the adsorption of alkaline-earth metals (AEMs). Previously, in a series of works on Ba/Si(1 1 1) system, we have found that the observed Si(1 1 1)3 × 1-Ba LEED phase indeed has a 3 × 2 periodicity with a Ba coverage of 1/6 ML and the HCC substrate structure. Based on results of the Ba case, we proposed that the HCC structure is also adopted for other AEM atoms, which was confirmed by our recent work. In this paper, we mainly report the STM simulations for different AEM systems to compare with existing experimental data. We discuss the difference in the detailed STM images for different AEM adsorbates. Especially, the difference in filled-state images between Mg and other AEM atoms is attributed to the strong Mg-Si interaction.     

Selforganized nanopatterning of Si(0 0 1)

The (2 × n) superstructure of Si(0 0 1) consists of elongated (2 × 1) reconstructed stripes separated by a dimer-vacancy line every few nanometers, thus offering a means to obtain a nanopattered Si(0 0 1) surface. Scanning tunneling microscopy (STM) investigations of Si(0 0 1) substrates that were deoxidized at 880-920 °C reveal that the formation of the (2 × n) depends strongly on the Si coverage of the topmost surface layer. It forms only in a narrow coverage window ranging from 0.6 to 0.8 ML. Systematic Monte Carlo simulations by an algorithm that combines the diffusion of monomers and dimers with the simultaneous deposition of Si onto the Si(0 0 1) surface, corroborate the STM results and suggest Si deposition as a viable alternative for introducing dimer vacancies in a well-defined manner.     

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.     

Surface state lifetime in quasi-one-dimensional oxygen stripes on Cu(1 1 0)

Scanning tunneling spectroscopy data on the lifetime and bottom energy of a surface state band in quasi-one-dimensional stripes are presented as a function of stripe width. The adsorbate system Cu(1 1 0)-p(2 × 1)O has been used to prepare a two-dimensional adsorbate layer and almost defect free quasi-one-dimensional stripes with a width of 3-15 CuO rows, respectively. The onset of an unoccupied surface state band (at 0.56 eV above Fermi-energy on the fully covered surface) has been analyzed with respect to the quasi-particle lifetime at the crossover from two to one dimension. A drastic increase of the onset-width with decreasing stripe width is observed. It can be explained by a decrease of quasi-particle lifetime using a Fabry-Pérot-like model. We obtain a reduction of lifetime from 37 ± 8 fs on the two-dimensional adsorbate layer to 5 ± 1 fs on a three CuO rows wide stripe.     

Electron and lattice structure of ultra thin Ag films on Si(1 1 1) and Si(0 0 1)

We studied the low temperature (T ? 130 K) growth of Ag on Si(0 0 1) and Si(1 1 1) flat surfaces prepared by Si homo epitaxy with the aim to achieve thin metallic films. The band structure and morphology of the Ag overlayers have been investigated by means of XPS, UPS, LEED, STM and STS. Surprisingly a (√3 × √3)R30° LEED structure for Ag films has been observed after deposition of 2-6 ML Ag onto a Si(1 1 1)(√3 × √3)R30°Ag surface at low temperatures. XPS investigations showed that these films are solid, and UPS measurements indicate that they are metallic. However, after closer STM studies we found that these films consists of sharp Ag islands and (√3 × √3)R30°Ag flat terraces in between. On Si(0 0 1) the low-temperature deposition yields an epitaxial growth of Ag on clean Si(0 0 1)-2 × 1 with a twinned Ag(1 1 1) structure at coverage’s as low as 10 ML. Furthermore the conductivity of few monolayer Ag films on Si(1 0 0) surfaces has been studied as a function of temperature (40-300 K).     

Surface structure of the missing-row reconstruction of VC0.8(1 1 0): a scanning tunneling microscopy analysis

Scanning tunnelling microscopy (STM) was used to study the (1 1 0) surface of a VC_0.8 sample. The surface shows a missing-row reconstruction, i.e., a grating structure with ridges and valleys oriented along the [0 0 1] direction and (1 0 0) and (0 1 0) facets. We did not find unreconstructed (1 1 0) terraces. The regular spacing of the ridges corresponds to a periodicity of (3 × 1) or (4 × 1), depending on preparation, presumably related to different concentrations of carbon vacancies. In the STM images, we can also observe apparent pairing of atoms in the rows, leading to the larger c(6 × 2) and (4 × 2) superstructure cells, which also show up in LEED. We attribute these additional periodicities to ordering of carbon vacancies in the surface rows.     

Atomic surface structure of Si(1 0 0) substrates prepared in a chemical vapor environment

Subsequent III-V integration by metal-organic vapor phase epitaxy (MOVPE) or chemical vapor deposition (CVD) necessitates elaborate preparation of Si(1 0 0) substrates in chemical vapor environments characterized by the presence of hydrogen used as process gas and of various precursor molecules. The atomic structure of Si(1 0 0) surfaces prepared in a MOVPE reactor was investigated by low energy electron diffraction (LEED) and scanning tunnelling microscopy (STM) available through a dedicated, contamination-free sample transfer to ultra high vacuum (UHV). Since the substrate misorientation has a fundamental impact on the atomic surface structure, we selected a representative set consisting of Si(1 0 0) with 0.1°, 2° and 6° off-cut in [0 1 1] direction for our study. Similar to standard UHV preparation, the LEED and STM results of the CVD-prepared Si(1 0 0) surfaces indicated two-domain (2 × 1)/(1 × 2) reconstructions for lower misorientations implying a predominance of single-layer steps undesirable for subsequent III-V layers. However, double-layer steps developed on 6° misoriented Si(1 0 0) substrates, but STM also showed odd-numbered step heights and LEED confirmed the presence of minority surface reconstruction domains. Strongly depending on misorientation, the STM images revealed complex step structures correlated to the relative dimer orientation on the terraces.     

Fabrication and scanning tunneling microscopy studies of the Si(1 1 1)-(√31 × √31)-In surface

We report on the fabrication of single phase of the Si(1 1 1)-(√31 × √31)-In reconstruction surface, observed by scanning tunneling microscopy (STM) at room temperature. By depositing specific amounts of indium atoms while heating the Si(1 1 1)-(7 × 7) substrate at a critical temperature, the single phase of Si(1 1 1)-(√31 × √31)-In surfaces could be routinely obtained over the whole surface with large domains. This procedure is certified by our high-resolution STM images in the range of 5-700 nm. Besides, the high resolution STM images of the Si(1 1 1)-(√31 × √31)-In surface were also presented.     

(6 × 2) Reconstruction of the Ag/Si(1 1 1) surface at 77 K

The ground state of the Ag/Si(1 1 1)-(3 × 1) has been investigated by low temperature scanning tunneling microscopy (STM) and density-functional theory. The Fourier transform of the STM image reveals a (6 × 2) reconstruction, which is theoretically found to yield a reconstruction with lower energy than the (3 × 1). The most stable (6 × 2) structural model leads to excellent correspondence between experimental and simulated STM images, and reveals a dimerization of the silver atoms in the channels formed by neighbouring honeycomb Si chains.     

Barrier height imaging of Si(1 1 1)3 × 1-Ag reconstructed surfaces

We investigated the bias voltage polarity dependence of atomically resolved barrier height (BH) images on Si(1 1 1)3 × 1-Ag surfaces. The BH images were very similar to scanning tunneling microscopy (STM) images in both the empty and filled states. This similarity strongly supports the interpretation that the BH image reflects the vertical decay rate of the surface local density of states (LDOS). Differences in contrast and protrusion shapes between BH and STM images were observed. We attributed these differences to the geometric contribution to the STM image and to the improved spatial resolution of the BH image due to the lock-in technique.     

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.     

STM study of the Mo(1 1 2) and Mo(1 1 1) surfaces

The Mo(1 1 2) and Mo(1 1 1) surfaces have been studied by STM and DFT/GGA modeling. Due to high quality and cleanness of the surfaces, for the first time good STM images of large fragments of the Mo(1 1 2) and Mo(1 1 1) have been obtained. Lack of atomic resolution in the rows of the Mo(1 1 2) surface is attributed to flatness of distribution of density of the electronic states along the rows. This suggestion is illustrated by comparison of STM images for Mo(1 1 1) and Mo(1 1 2) and model calculations of STM pictures for these surfaces.     

Formation of oxygen-induced Si(1 1 3)-3 × 2 facets on the Si(5 5 12) surface

We report the formation of Si(1 1 3)-3 × 2 facets upon exposing oxygens on the Si(5 5 12) surface at an elevated temperature. These facets are found to form only for a limited range of oxygen exposure and exhibit a well-defined 3 × 2 LEED pattern. We also find the surface electronic state unique only to the facets in the valence band. The spectral feature of these electronic states and the behavior of a (1/3 1/2) LEED spot upon oxygen contents in the facets indicate that the formation is a heterogeneous mixture of the clean Si(1 1 3) facets free of oxygens and other facets containing oxygen atoms.     

Scanning tunneling microscopy study on a BC3 covered NbB2(0 0 0 1) surface

We have investigated a BC₃ covered NbB₂(0 0 0 1) surface using scanning tunneling microscopy (STM), scanning tunneling spectroscopy (STS), and low energy electron diffraction (LEED). The STM images reveal characteristic features of a Moiré pattern reflecting an incommensurate relation of the BC₃ sheet with the substrate: bright protrusions with the periodicity of the substrate lattice are modulated in intensity with the periodicity of the BC₃ lattice. As a result, the surface exhibits nm-scale patchy regions with either the √3 × √3 or the 1 × 1 structure of the substrate. The two-dimensional Fourier transformation pattern of the STM image is consistent with the LEED pattern proving the epitaxial and incommensurate relationship between BC₃ surface sheet and substrate. No feature of a predicted superconducting gap was found in STS spectra measured at 5 K.     

Self-assembled germanium nano-clusters on silver(1 1 0)

The adsorption of germanium on Ag(1 1 0) has been investigated by scanning tunnelling microscopy (STM), as well as surface X-ray diffraction (SXRD). At 0.5 germanium monolayer (ML) coverage, Low Energy Electron Diffraction (LEED) patterns reveals a sharp c(4 × 2) superstructure. Based on STM images and SXRD measurements, we present an atomic model of the surface structure with Ge atoms forming tetramer nano-clusters perfectly assembled in a two-dimensional array over the silver top layer. The adsorption of the germanium atoms induces a weak perturbation of the Ag surface. Upon comparison with results obtained on the (1 1 1) and (1 0 0) faces, we stress the role played by the relative interactions between silver and germanium on the observed surface structures.     

Anomalous hybridization in the In-rich InAs(0 0 1) reconstruction

The surface bonding arrangement in nearly all the confirmed reconstructions of InAs(0 0 1) and GaAs(0 0 1) have only two types of hybridization present. Either the bonds are similar to those in the bulk and the surface atoms are sp³ hybridized or the surface atoms are in a tricoordinated bonding arrangement and are sp² hybridized. However, dicoordinated In atoms with sp hybridization are observed on the InAs(0 0 1), In-rich, room temperature and low temperature surfaces. Scanning tunneling microscopy (STM) images of the room temperature (300 K) InAs(0 0 1) surface reveal that the In-rich surface reconstruction consists of single-atom rows with areas of high electron density that are separated by ∼4.3 Å. The separation in electron density is consistent with rows of undimerized, sp hybridized, In atoms, denoted as the β3′(4 × 2) reconstruction. As the sample is cooled to 77 K, the reconstruction spontaneously changes. STM images of the low temperature surface reveal that the areas of high electron density are no longer separated by ∼4.3 Å but instead by ∼17 Å. In addition, the LEED pattern changes from a (4 × 2) pattern to a (4 × 4) pattern at 77 K. The 77 K reconstruction is consistent with two (4 × 2) subunit cells; one that contains In dimers on the row and another subunit cell that contains undimerized, sp hybridized, In atoms on the row. This combination of dimerized and undimerized subunit cells results in a new unit cell with (4 × 4) periodicity, denoted as the β3(4 × 4) reconstruction. Density functional theory (DFT) and STM simulations were used to confirm the experimental findings.     

Silicon quantum wires on Ag(1 1 0): Fermi surface and quantum well states

One-dimensional Si quantum wires have been grown on silver single crystals upon deposition of ∼0.25 monolayer of Si on Ag(1 1 0) surfaces. Scanning tunneling microscopy (STM) clearly shows parallel 1D Si chains along the [−1 1 0] Ag crystallographic direction. Low Energy Electron Diffraction (LEED) confirms the massively parallel assembly of these selforganized Nanowires (NWs). We have characterized these nano-objects by measuring the dispersion of the NWs valence band at room temperature using Angle-Resolved PhotoEmission Spectroscopy (ARPES). Also, the Fermi Surface (FS) of the Ag(1 1 0) substrate has been mapped before and after the silicon deposition, trying to put in evidence the metallic or semiconductor character of the NWs silicon's states close to the Fermi level. Our results show the existence of well-defined quantum states associated to the silicon super-structure. Both LEED and ARUPS results confirm that the NWs have typical 1D features, however their metallic or semiconductor character could not be confirmed.     

Hydrogen-induced metallization on Ge(1 1 1) c(2 × 8)

We have studied hydrogen adsorption on the Ge(1 1 1) c(2 × 8) surface using scanning tunneling microscopy (STM) and angle-resolved photoelectron spectroscopy (ARPES). We find that atomic hydrogen preferentially adsorbs on rest atom sites. The neighbouring adatoms appear higher in STM images, which clearly indicates a charge transfer from the rest atom states to the adatom states. The surface states near the Fermi-level have been followed by ARPES as function of H exposure. Initially, there is strong emission from the rest atom states but no emission at the Fermi-level which confirms the semiconducting character of the c(2 × 8) surface. With increasing H exposure a structure develops in the close vicinity of the Fermi-level. The energy position clearly indicates a metallic character of the H-adsorbed surface. Since the only change in the STM images is the increased brightness of the adatoms neighbouring a H-terminated rest atom, we identify the emission at the Fermi-level with these adatom states.     

Edge-dimer row - The reason of three-bilayer steps and islands stability on Si(1 1 1)-7 × 7

A hypothesis of perpendicular dimer row formation along three-bilayer (3 BL) step was suggested. The hypothesis, explains the stability of 3 BL steps on the vicinal Si(1 1 1) surface deflected in direction as well as the limitation of Ge and Si island height by 3 BL at the initial nucleation stages on Si(1 1 1) surface. The detailed examinations of STM images of 3 BL steps were carried out. New peculiarities of atomic structure of 3 BL single step on Si(1 1 1) and 3 BL steps on Si(5 5 7) surfaces were revealed. The results of STM images examination verify the hypothesis of perpendicular dimer row formation along the boundary of the 3 BL step.