Large scale atomic ordering on uncovered GaAs(0 0 1) after InAs monolayer capping: Atomic structure of the (12 × 6) reconstruction

A well ordered c(8 × 2)-InAs monolayer is grown by molecular beam epitaxy (MBE) on a GaAs(0 0 1) substrate. After slow sublimation of this monolayer up to 560 °C, a homogeneously (n × 6) reconstructed GaAs surface is obtained. This surface is studied by scanning tunneling microscopy (STM) in UHV. This shows that it is well-ordered on a large scale with 200 nm long As dimer rows along and is also locally (12 × 6) reconstructed, the cell structure is proposed. We believe that this surface organization results from the specific As/Ga (0.7) surface atomic ratio obtained after the InAs monolayer growth and sublimation cycle.     

An STM study of the localized atomic reaction of 1,2- and 1,4-dibromobenzene at Si(1 1 1)-7 × 7

A comparative study is reported of the thermal reaction of 1,2- and 1,4-dibromobenzene (1,2- and 1,4-diBrPh) on Si(1 1 1)-7 × 7, investigated by STM. Some results are given for the intermediate case of 1,3-diBrPh. The STM images gave evidence of a different pattern of reaction to yield pairs of Br-Si for 1,2-, 1,3- and 1,4-diBrPh. The ratio of pairs of Br-Si to single bromination events was 1:2 for 1,2-diBrPh and 1:3 for 1,4-diBrPh. In many cases organic residue from the bromination reaction, R-Si, was evident in the STM image. The products R-Si and Br-Si were found to be bound to adjacent Si, for both 1,2- and 1,4-diBrPh. The mean Br?Br pair separation at the surface depended on the parent molecule, being 7.6 Å for 1,2-diBrPh, 10.3 Å for 1,3-diBrPh, and 11.3 Å for 1,4-diBrPh. These separations are, in each case, about 4 Å greater than the separation of the Br-atoms in the intact parent molecule, which increases systematically down the series. There was a marked decrease in the percentage of R-Si accompanying the Br-Si in going down the series, decreasing from 70% for 1,2- to 20% for 1,4-diBrPh; this was interpreted as being due to a decrease in the percentage of `benzene-mediated' reaction dynamics, in which the benzene ring was bound to the surface. At moderately increased surface temperature (45 °C) the reaction of 1,2- and also 1,4-diBrPh no longer resulted in R-Si formation, suggesting that the dynamics had altered from benzene-mediated to `bromine-mediated'.     

Corner hole adatom stacking fault structure of Bi on Au(1 1 1)

The surface atomic structure of Bi on Au(1 1 1) is studied with scanning tunneling microscopy. At about 0.5 monolayer of Bi, a well-ordered 6 × 6 atomic structure is observed. The structure has three notable features: corner holes, Bi adatoms, and stacking faults, very similar to a semiconductor surface of Si(1 1 1)-7 × 7. Out of 18 Bi surface atoms in a unit cell, six atoms are at hollow sites and are adatoms, and another six atoms are near-bridge sites. The last six atoms surround corner holes and are lower than other surface atoms by about 0.2 Å. A possible atomic model is proposed based on our observation.     

Surface reconstructions and phase transitions on the GaAs(111)B surface

The (111)B surface of GaAs has been investigated using scanning tunneling microscopy (STM) and a number of different reconstructions have been found at different surface stoichiometries. In accordance with electron diffraction studies, we find the series (2 × 2), (1 × 1)_LT, ( ) and (1 × 1)_HT with increasing annealing temperature, corresponding to decreasing surface As concentration. The (1 × 1)_LT is of particular interest, since it only occurs in a narrow temperature window between the two more established reconstructions, the (2 × 2) and the ( ). We find the (1 × 1)_LT to take the form of a mixture of the local structures of both the (2 × 2) and ( ) phases, rather than having a distinct structure. This is behaviour consistent with a kinetically limited system, dominated by the supply of As adatoms to the surface, and may be an example of a continuous phase transition. Above the (1 × 1)_LT transition, atomic resolution images of the ( ) surface reveal only a three-fold symmetry of the hexagonal structural units, brought about by inequivalent surface bonding due to the 23.4° rotation of the surface unit cell relative to the substrate. This is responsible for the disorder found in the ( ) reconstruction, since the structure may form in one of two domains. At lower surface As concentration, the (1 × 1)_HT surface adopts a structure combining small domains of a 19.1° structure and random disorder. There is no apparent similarity between the (1 × 1)_LT and (1 × 1)_HT structures, which may be due to our measurements being conducted at room temperature and without an As flux to control the surface As concentration.     

The bonding geometry of alkylsilanes on gold: Relation to surface pattern development and STM image contrast

The adsorption of methylsilane (MeSiH₃) on Au(1 1 1) was investigated using density functional theory (DFT) within a generalized gradient approximation (GGA). Two preferred chemisorption sites are identified: the hollow site and an atop site with an ejected gold substrate atom. Both of these configurations result in a tetracoordinate Si with a distorted tetrahedral geometry. The energy of adsorption is calculated allowing an analysis of the preferred binding geometry as a function of coverage. The relation of the supermolecular length scale pattern that emerges from a spinodal decomposition of the two phases arising from the two binding geometries is discussed. The pattern observed in the STM images is shown to be consistent with the local density of states calculated for the two different binding geometries.     

Scanning tunneling microscopy and molecular orbital calculation of pentacene molecules adsorbed on the Si(100)2×1 surface

Adsorption of the organic molecule pentacene on Si(100)2×1 surfaces was imaged using scanning tunneling microscopy (STM). The molecular images exhibit distinct shapes corresponding to the expected shapes for adsorption configurations. Semi-empirical molecular orbital (MO) calculations reveal a local surface density of states for the adsorbed pentacene on the Si surface. In the cases where the pentacene molecule is adsorbed on an Si dimer row, the calculated MOs are in good agreement with the molecular images observed in STM. In the case of pentacene adsorbed on two or three Si-dimer rows, however, the MOs of the pentacene do not correlate directly with the observed STM images. It is thus considered that the STM images are produced by a combination of Si dimer states and MO.     

Structural fluctuation of dimers on Ge(0 0 1) surface near transition temperature

The structural fluctuation of the orientational arrangement of buckled dimers on a Ge(0 0 1) surface near the transition temperature of the order-disorder phase transition is investigated by time-resolving dynamical Monte Carlo simulations. STM images averaged in a finite period are derived from the simulation. The coexistence of the c(4 × 2) and the apparent (2 × 1) domains in the STM images observed by experiments is reproduced in the simulated STM images. We show that the coexistence on the Ge(0 0 1) surface can be attributed to the critical slowing down near the transition temperature.     

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.     

The origin of inter-dimer-row correlated adsorption for NH3 on Si(0 0 1)

We discuss the interaction between adsorbing ammonia molecules and pre-adsorbed ammonia fragments on the Si(0 0 1) surface, searching for experimental evidence of a H-bonded precursor state predicted by modelling. While correlations along dimer rows have already been identified, these mix substrate-mediated effects due to dimer buckling with ammonia–adsorbate effects. Correlations between fragments on neighbouring dimer rows are not affected by substrate effects (in this system), allowing an analysis of direct ammonia–adsorbate effects. We present an analysis of cross-row correlations in existing high-coverage STM data which shows significant correlations between NH₂ groups on neighbouring dimer rows over a significant range, providing evidence for the H-bonded precursor state with a range of around 10 Å. We discuss implications for the interpretation of STM images of ammonia on Si(0 0 1).     

Steps as adatom sources for surface chemistry: oxygen overlayer formation on Ag(110)

Surface chemical reactions often require a ready supply of substrate atoms to occur. In principle, steps serve as an efficient source of these atoms, provided that detachment rates from the step edges are sufficiently large. In this paper, we characterize atomic detachment rates from steps on clean Ag(110) by examining step fluctuations. We show that these rates are sufficient to supply atoms to form the added-row reconstruction of oxidized Ag(110) when the oxygen partial pressure is low. For high oxygen pressures, however, we find that step detachment rates are slow compared with oxidation rates, and the step source of Ag is supplemented by vacancy-island generation on the terraces. These results are compared to those obtained for the similar O/Cu(110) and O/Ni(110) systems.     

From bilayer to monolayer growth: Temperature effects in the growth of Ru on Pt(1 1 1)

At temperatures around 373 K, Ru growth on Pt(1 1 1) proceeds via nucleation and growth of bilayer islands [H.E. Hoster et al., Phys. Chem. Chem. Phys. 3 (2001) 337]. The influence of the deposition temperature on the Ru growth behavior on Pt(1 1 1) was studied by scanning tunneling microscopy (STM) and Auger electron spectroscopy (AES) in the temperature range between 303 and 773 K. The data reveal a distinct change in the growth characteristics, most important the change from the growth of bilayer Ru islands to monolayer islands, at temperatures between 523 and 573 K. Based on AES data and on atomic resolution STM images, these changes are associated with the onset and increasing contribution of surface alloy formation via Pt–Ru exchange and, at T > 673 K, alloy formation in near surface regions. Consequences of these data for the mechanism of bilayer growth and the underlying physical origin are discussed.     

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.     

Scanning tunneling microscopy images of Al0.2GA0.8As-{1 1 0}: Influence of applied bias voltage and comparison between filled- and empty-states images

Bias-voltage dependent images of Al_0.2Ga_0.8As-{1 1 0} are presented. The images show both the filled- and empty-states of the surface. Apart from voltage-dependent changes in the apparent direction of the atomic rows also observed for binary III-V semiconductors, bright and dark areas about 2 nm in diameter appear at small voltage in filled-states images, and small ridges along [−1 1 0] appear at small voltage in empty-states images. The spatial extent of the bright and dark areas observed in filled-states images is thought to be determined by the electron-electron interaction. It is also shown that when a given patch of Al_0.2Ga_0.8As-{1 1 0} surface is imaged simultaneously in the filled- and empty-states mode, the locations and spatial extent of the alloy-related minima (the “dark patches”) do not coincide. This casts doubt on the assumption that a locally decreased tunneling probability represents an increased local content of Al.     

Structural properties of Bi-terminated GaAs(0 0 1) surface

Electronic and structural properties of Bi-terminated reconstructions on GaAs(0 0 1) surface have been studied by scanning tunneling microscopy (STM) and synchrotron radiation core-level spectroscopy. A 2-3 monolayer thick Bi-layer was evaporated on a Ga-terminated GaAs(0 0 1) surface. By heating the surface, the reconstruction changed from (2 × 1) to (2 × 4). The α2 phase with one top Bi dimer and one As or Bi dimer in the third atomic layer per surface unit cell is proposed to explain the STM images of the Bi/GaAs(0 0 1)(2 × 4) surface heated at 400 °C. Bi 5d photoemission from the Bi/GaAs(2 × 4) consisted of two components suggesting two different bonding sites for Bi atoms on the (2 × 4) surface. The variation of the surface sensitivity of the photoemission induced no changes in the intensities of the components indicating that the origins of both components lie in the first surface layer.     

Atomic description of elementary surface processes: diffusion and dynamics

A large fraction of processes which are at the foundation of our technological society involve physical and chemical properties of surfaces. Catalytic reactions and semiconductor devices production are two of the most important ones.

This paper describes a sample of some of the most relevant surface science experiments which have been recently performed, in order to understand elementary surface processes of model catalytic reactions and in semiconductor technology at the atomic level. The focus is on experiments performed with scanning tunneling microscopy and atomic force microscopy which have represented, in some cases, real breakthroughs in our understanding of these phenomena.

We then present an overview of possible experimental technique developments that can be foreseen for the future and that may give us a more in-depth understanding of the elementary processes which form the basis of important complex surface phenomena. Finally, some of the challenging tasks that lie ahead for surface scientists and the collateral opportunities are discussed.     

Scanning tunneling microscopy and abinitio studies of precursor states of Ga-induced cluster on Si(0 0 1) surface

The growth processes and structures of Ga layers formed on a Si(0 0 1) surface have been studied by scanning tunneling microscopy (STM) and abinitio calculation. The precursor states of the Ga clusters that compose the Si(0 0 1) 8 × n-Ga (n = 4, 5, 6) structures are observed in addition to the 2 × 2- and 4 × 2-Ga structures at a Ga coverage of 0.55 ML at 300 °C. There are two types of precursor clusters whose protrusions are observed as different shapes in filled-state STM images. We compare the observed STM images with the simulated ones to identify the possible structural models. From the results, we determine the structure of each precursor cluster. On the basis of the results, the formation processes of the cluster are discussed.     

Comment on ‘Step structure of vicinal Ge(001) surfaces’ by B.A.G. Kersten, H.J.W. Zandvliet, D.H.A. Blank and A. van Silfhout

We have investigated detailed structures of monatomic steps on Ge(001) at room temperature by using high-resolution scanning tunneling microscopy. Our conclusions are different from those of Kersten et al. [Surf. Sci. 322 (1995) 1] for the same system. The most crucial difference is that we have not observed the nonbonded SB step. We have pointed out a possible reason for the different conclusions.     

Time-evolution of the GaAs(0 0 1) pre-roughening process

The GaAs(0 0 1) surface is observed to evolve from being perfectly flat to a surface half covered with one-monolayer high spontaneously formed GaAs islands. The dynamics of this process are monitored with atomic-scale resolution using scanning tunneling microscopy. Surprisingly, pit formation dominates the early stages of island formation. Insight into the nucleation process is reported.     

Photoemission, NEXAFS and STM studies of pentacene thin films on Au(1 0 0)

We have investigated the initial stages of the growth of pentacene thin films on the Au(1 0 0) substrate using synchrotron radiation photoelectron spectroscopy (PES), near edge X-ray absorption fine structure (NEXAFS) and scanning tunnelling microscopy (STM). Results indicate a well-ordered structure with the pentacene molecules adopting a predominantly flat orientation with respect to the substrate for coverages of less than three monolayers. NEXAFS and photoemission data indicates the presence of a second molecular orientation for thicker films, with the introduction of a slight tilting away from planar bonding geometry at higher pentacene coverages. STM images of coverages less than three monolayers indicate a well-ordered pentacene structure allowing for the calculation of pentacene unit cell parameters. The pentacene molecular rows adopt a side-by-side bonding arrangement on the surface. For pentacene deposited at room temperature, step edges were observed to act as nucleation centres for film growth. Annealing of the substrate to 373 K was found to remove excess molecules and improve film quality, but did not otherwise change the bonding geometry of the pentacene with respect to the surface.