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.     

Atomic structure of the carbon induced Si(0 0 1)-c(4 × 4) surface

The atomic and electronic structures of the Si(0 0 1)-c(4 × 4) surface have been studied by scanning tunneling microscopy (STM) and density functional theory (DFT). To explain the experimental bias dependent STM observations, a modified mixed ad-dimer reconstruction model is introduced. The model involves three tilted Si dimers and a carbon atom incorporated into the third subsurface layer per c(4 × 4) unit cell. The calculated STM images show a close resemblance to the experimental ones.     

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.     

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 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.     

First-principles study of oxidized Nb(1 0 0) surface structures

The atomic positions of the oxygen-induced c(2 × 2)-O, (3 × 1)-O and (4 × 1)-O surface structures on Nb(1 0 0) are determined by first-principles electronic structure calculations within the density functional theory comparing experimentally observed scanning tunneling microscopy (STM) images. STM images of these surfaces are calculated on the basis of the theory of Tersoff and Hamann. The theoretical and experimental STM images of the oxygen-chemisorbed c(2 × 2)-O structural model agree well. However, only the oxide-covered (3 × 1)-O and (4 × 1)-O structural models with two layers of NbO and contraction of the unit length along longitudinal 〈1 0 0〉 direction by 10% result in the theoretical STM images that agree with the experimental ones.     

Quasi-one-dimensional structures on the Si(1 1 1) surface induced by Ba adsorption

Ba-induced quasi-one-dimensional reconstructions of the Si(1 1 1) surface have been investigated by low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). While the 3 × ‘2’ surface shows double-periodicity along the stripes in STM images consistent with half-order streaks observed in LEED patterns, no sign of the double-periodicity along the chain direction was detected for the 5 × 1 surface. The 5× stripes in STM images show internal structures with multiple rows. The two rows comprising the boundaries of a 5× stripe in the filled-state STM image are found to have 3a × √3/2 spacing across the stripe. The observation of the successive 3× and 2× spacings between the boundary rows supports a structural model proposed for the Ba-induced 5 × 1 Si reconstruction composed of honeycomb chains and Seiwatz chains. The highest coverage 2 × 8 surface does not reveal a quasi-1D row structure in STM images.     

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.     

Adsorption of cobalt phthalocyanine on a Si(1 0 0) surface with Bi-line structures as evaluated by scanning tunneling microscopy

We report the reaction dynamics of cobalt phthalocyanine (CoPc) molecules with Bi-line structures (BLSs) on a Si(1 0 0) surface, investigated using scanning tunneling microscopy (STM). When CoPc molecules were deposited on a Si(1 0 0) surface with BLSs at room temperature, single-spot protrusions were observed in the STM image instead of four-spot images corresponding to CoPcs flat molecular structure. Moreover, domains with a c(4 × 4) periodicity appeared on the terraces of the Si(1 0 0) surface. This indicates that CoPc molecules may have decomposed on the surface by catalytic reaction with Bi atoms.     

Nano-crater formation on a Si(1 1 1)-(7 × 7) surface by slow highly charged ion-impact

Using scanning tunneling microscopy (STM) and time of flight secondary ion mass spectrometry (TOF/SIMS), we observed radiation effects on a Si(1 1 1)-(7 × 7) surface in the collision of a single highly charged ion (HCI) with a charge state q up to q = 50. The STM observation with atomic resolution revealed that a nanometer sized crater-like structure was created by a single HCI impact, where the size increased rapidly with q. The secondary ion yields also increased with q in which multiply charged Si ions (Siⁿ⁺) were clearly observed in higher q HCI-collisions. The sputtering mechanism is briefly discussed, based on the so-called Coulomb explosion model.     

Addimer chain structures: Metastable precursors to island formation on Ge-Si(0 0 1)-(2 × n) alloyed surface

We have identified addimer chain structures as metastable precursors to compact epitaxial islands on the (2 × n) reconstructed SiGe wetting layer, using polarity-switching scanning tunneling microscopy (STM). These chain structures are comprised of 2-12 addimers residing in the troughs of neighboring substrate dimer rows. The chain structures extend along equivalent 〈1 3 0〉 directions across the substrate dimer rows in a zigzag fashion, giving rise to kinked and straight segments. We measure a kink-to-straight ratio of nearly 2:1. This ratio corresponds to a free energy difference of 17 ± 4 meV, favoring the formation of kinked segments. The chain structures convert to compact epitaxial islands at elevated temperatures (?90 °C). This conversion suggests that the chain structures are a precursor for compact island formation on the SiGe wetting layer. We digitally process filled- and empty-state STM images to distinguish chain structures from compact islands. By monitoring the populations of both species over time, the chain-to-island conversion rates are measured at substrate temperatures ranging from 90 to 150 °C. The activation energy for the conversion process is measured to be 0.7 ± 0.2 eV with a corresponding pre-exponential factor of 5 × 10^4±2 s⁻¹.     

The chemisorption of pentacene on Si(0 0 1)-2 × 1

Adsorption structures of the pentacene (C₂₂H₁₄) molecule on the clean Si(0 0 1)-2 × 1 surface were investigated by scanning tunneling microscopy (STM) in conjunction with density functional theory calculations and STM image simulations. The pentacene molecules were found to adsorb on four major sites and four minor sites. The adsorption structures of the pentacene molecules at the four major sites were determined by comparison between the experimental and the simulated STM images. Three out of the four theoretically identified adsorption structures are different from the previously proposed adsorption structures. They involve six to eight Si-C covalent chemical bonds. The adsorption energies of the major four structures are calculated to be in the range 67-128 kcal/mol. It was also found that the pentacene molecule hardly hopped on the surface when applying pulse bias voltages on the molecule, but was mostly decomposed.     

A scanning tunneling microscopy study of PH3 adsorption on Si(1 1 1)-7 × 7 surfaces, P-segregation and thermal desorption

PH₃ adsorption on Si(1 1 1)-7 × 7 was studied after various exposures between 0.3 and 60 L at room temperature by means of scanning tunneling microscopy (STM). PH₃-, PH₂-, H-reacted, and unreacted adatoms can be identified by analyzing empty-state STM images at different sample biases. PH_x-reacted rest-atoms can be observed in empty-state STM images if neighboring adatoms are hydrogen terminated. Most of the PH₃ adsorbs dissociatively on the surface, generating H- and PH₂-adsorbed rest-atom and adatom sites. Dangling-bonds at rest-atom sites are more reactive than adatom sites and the faulted half of the 7 × 7 unit cell is more reactive than the unfaulted half. Center adatoms are overwhelmingly preferred over corner adatoms for PH₂ adsorption. The saturation P coverage is ∼0.18 ML. Annealing of PH₃-reacted 7 × 7 surfaces at 900 K generates disordered, partially P-covered surfaces, but dosing PH₃ at 900 K forms P/Si(1 1 1)- surfaces. Si deposition at 510 K leaves disordered clusters on the surface, which cannot be reordered by annealing up to 800 K. However, annealing above 900 K recreates P/Si(1 1 1)- surfaces. Surface morphologies formed by sequential rapid thermal annealing are also presented.     

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.     

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.     

Interpretation of initial stage of 3C-SiC growth on Si(1 0 0) using dimethylsilane

The initial stage of cubic silicon carbide (3C-SiC) growth on a Si(0 0 1) surface using dimethylsilane (DMS) as a source gas was observed using scanning tunneling microscopy (STM) and reflection high-energy electron diffraction (RHEED). It was found that the dimer vacancies initially existing on the Si(0 0 1)-(2 × 1) surface were repaired by the Si atoms in DMS molecules, during the formation of the c(4 × 4) surface. From the STM measurement, nucleation of SiC was found to start when the Si surface was covered with the c(4 × 4) structure but before the appearance of SiC spots in the RHEED pattern. The growth mechanism of SiC islands was also discussed based on the results of RHEED, STM and temperature-programmed desorption (TPD).     

Scanning tunneling microscopy study of growth of Pt nanoclusters on thin film Al2O3/NiAl(1 0 0)

The growth of Pt nanoclusters on thin film Al₂O₃ grown on NiAl(1 0 0) was studied by using scanning tunneling microscopy (STM). The samples were prepared by vapor depositing various amounts of Pt onto the Al₂O₃/NiAl(1 0 0) at different substrate temperatures in ultra high vacuum (UHV). The STM images show that sizeable Pt nanoclusters grow solely on crystalline Al₂O₃ surface. These Pt clusters appear to be randomly distributed and only a few form evident alignment patterns, contrasting with Co clusters that are highly aligned on the crystalline Al₂O₃. The size distributions of these Pt clusters are rather broader than those of the Co clusters on the same surface and the sizes are evidently smaller. With increasing coverage or deposition temperature, the number of larger clusters is enhanced, while the size of the majority number of the clusters remains around the same (0.4 nm as height and 2.25 nm as diameter), which differs drastically from the Pt clusters on γ-Al₂O₃/NiAl(1 1 0) observed earlier. These Pt cluster growth features are mostly attributed to smaller diffusion length and ease to form stable nucleus, arising from strong Pt-Pt and Pt-oxide interactions and the peculiar protrusion structures on the ordered Al₂O₃/NiAl(1 0 0). The thermal stability of Pt nanoclusters was also examined. The cluster density decreased monotonically with annealing temperature up to 1000 K at the expense of smaller clusters but coalescence is not observed.     

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.     

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.     

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.