Vacancy Lines Research Articles

Adsorbate-mediated step transformations and terrace rearrangement ofSi(100)−(2×1)

Scanning tunneling microscopy and density-functional theory have been combined to demonstrate structural transformations of steps of $\text{Si}(100)\text{\ensuremath{-}}(2\ifmmode\times\else\texttimes\fi{}1)$ induced by nondangling bond-terminated Cl adsorbates. We identify a stable, bridge-bonded step adsorption site and show that supersaturation facilitates the population of those sites, leading to rebonded atom etching, step retreat, and extensive terrace rearrangement from the diffusion of resultant atomic vacancy lines across the supersaturated surface. Similarities to H-Si(100) are briefly discussed.

Out-of-equilibrium dynamics of the spiral model

We study the relaxation of the bi-dimensional kinetically constrained spiral model. Weshow that, due to the reversibility of the dynamic rules, any unblocked state fullydecorrelates in finite times irrespective of the system being in the unjammed or thejammed phase. As a consequence, the evolution of any unblocked configurationoccurs in a different sector of phase space from the one that includes the blockedequilibrium configurations at criticality and in the jammed phase. We argue that suchout-of-equilibrium dynamics share many points in common with coarsening in theone-dimensional Ising model and we identify the coarsening structures that are,basically, lines of vacancies. We provide evidence for this claim by analyzing thebehavior of several observables including the density of particles and vacancies,the spatial correlation function, the time-dependent persistence and the linearresponse.

Laser-induced defect insertion in DNA-linked 2D colloidal crystal array

Insertion of vacancies at predetermined sites within the lattice of colloidal crystals is a prerequisite in order to realize high-quality, opaline-based photonic devices. In this contribution, we demonstrate a novel methodology to afford controlled insertion of vacancies within two-dimensional (2D) opaline arrays. These 2D opaline arrays have been substrate-anchored with the help of DNA hybridization. This provides a heat-sensitive ‘adhesive’ between substrate and microspheres within a surrounding aqueous medium that enables tuning the hybridization strength of DNA linker as well as a mechanism to facilitate the removal of unbound microspheres. Focusing a laser beam onto the substrate/microsphere interface induces a localized heating event that detaches the irradiated microspheres, leaving behind vacancies. By repeating this process, line vacancies were successfully obtained. The effects of salt concentration, laser power, light-absorbing dyes, DNA length and refractive-index mismatch were investigated and found to correlate with heat-induced microsphere release.

Influence of defects on mechanical properties of bicrystal copper grain boundary interfaces

Defects play a key role in the determination of material properties, especially at small scales. The influence of several kinds of defects (point vacancies, line vacancies and cracks) on the deformation and fracture characteristics of a planar copper grain boundary interface with a 45° lattice misorientation is explored using molecular dynamics simulations and the embedded-atom method. Both tensile and shear modes of interfacial separation are considered. The results show that the crystalline defects can have a strong influence on the interfacial behaviours. The sensitivity of the mechanical properties of the interface to a defect type may be different under tension than under shear. It is found that some defect topologies can improve certain properties (e.g. strength and fracture strain) of the bicrystal interface system.

Controlled Self-Organization of Atom Vacancies in Monatomic Gallium Layers

Ga adsorption on the Si(112) surface results in the formation of pseudomorphic Ga atom chains. Compressive strain in these atom chains is relieved via creation of adatom vacancies and their self-organization into meandering vacancy lines. The average spacing between these line defects can be controlled, within limits, by adjusting the chemical potential mu of the Ga adatoms. We derive a lattice model that quantitatively connects density functional theory (DFT) calculations for perfectly ordered structures with the fluctuating disorder seen in experiment and the experimental control parameter mu. This hybrid approach of lattice modeling and DFT can be applied to other examples of line defects in heteroepitaxy.

First-principles study of adsorption and diffusion on Ge/Si(0 0 1)-(2 × 8) and Ge/Si(1 0 5)-(1 × 2) surfaces

Using first-principles total-energy calculations, we have investigated the adsorption and diffusion of Si and Ge adatoms on Ge/Si(0 0 1)-(2 × 8) and Ge/Si(1 0 5)-(1 × 2) surfaces. The dimer vacancy lines on Ge/Si(0 0 1)-(2 × 8) and the alternate S A and rebonded S B steps on Ge/Si(1 0 5)-(1 × 2) are found to strongly influence the adatom kinetics. On Ge/Si(0 0 1)-(2 × 8) surface, the fast diffusion path is found to be along the dimer vacancy line (DVL), reversing the diffusion anisotropy on Si(0 0 1). Also, there exists a repulsion between the adatom and the DVL, which is expected to increase the adatom density and hence island nucleation rate in between the DVLs. On Ge/Si(1 0 5)-(1 × 2) surface, the overall diffusion barrier of Si(Ge) along ( 5 ¯ 0 1 ) direction is relative fast with a barrier of ∼0.83(0.61) eV, despite of the large surface undulation. This indicates that the adatoms can rapidly diffuse up and down the (1 0 5)-faceted Ge hut island. The diffusion is also almost isotropic along [0 1 0] and [ 5 ¯ 0 1 ] directions.

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).

Molecular dynamics simulations of crack propagation in Ni with defects

A series of molecular dynamics simulations using the embedded atom method is conducted to investigate crack propagation under mode I loading in a Ni single crystal with and without defects. The crack system (0 0 1)[1 0 0] in a slab of 160 000 atoms was studied. Defects consisting of lines of vacancies were introduced near the crack tip. Critical loads and strain energy distributions around the crack tip are obtained. Our results indicate that the critical strain necessary for crack propagation is dependent on the defect configuration and can either increase or decrease relative to the defect-free system.

Metal-semiconductor (semimetal) superlattices on a graphite sheet with vacancies

It has been found that periodically closely spaced vacancies on a graphite sheet cause a significant rearrange-ment of its electronic spectrum: metallic waveguides with a high density of states near the Fermi level are formed along the vacancy lines. In the direction perpendicular to these lines, the spectrum exhibits a semimetal or semiconductor character with a gap where a vacancy miniband is degenerated into impurity levels.

Defective Structure of BN Nanotubes: From Single Vacancies to Dislocation Lines

A combination of electron microscopy and theoretical calculations provides new insights into the structure, electronics, and energetics of point defects and vacancy lines in BN single-wall nanotubes (SWNT). We show that the point defects forming under electron irradiation in the BN SWNTs are primarily divacancies. Due to the partially ionic character of the BN bonding, divacancies behave like an associated Schottky pair, with a dissociation energy of around 8 eV. Clustering of multiple vacancies is energetically favorable and leads to extended defects which locally change the nanotube diameter and chirality. Nevertheless these defects do not alter significantly the band gap energy, and all of them have electronic structure similar to that of BN divacancies. We thus conclude that under irradiation BN SWNT may have a very stable alteration of its electronic and optical properties.

Self-assembly of steps and vacancy lines during the early stages of Ge∕Si(001) heteroepitaxy

The wetting layer formed during the early stages of Ge∕Si(001) growth has been found in recent experiments to undergo a roughening process, where the SA surface steps affect the spatial organization of vacancy lines (VLs) by increasing (stretching) or decreasing (squeezing) their average spacing. Using a combination of atomistic simulations and elastic theory of surface defects, we have computed the interaction energy of the SA steps and VLs for each of the observed defect configurations. We find that the repulsive SA-VL interactions lead to an increase in the spacing of the VLs in the “stretch” arrangement, but do not significantly affect the VL spacing in the “squeeze” configuration, providing an explanation for the observed correlations in the wetting layer roughness.

Ga-induced atom wire formation and passivation of stepped Si(112)

We present an in-depth analysis of the atomic and electronic structure of the quasi one-dimensional (1D) surface reconstruction of Ga on Si(112) based on Scanning Tunneling Microscopy and Spectroscopy (STM and STS), Rutherford Backscattering Spectrometry (RBS) and Density Functional Theory (DFT) calculations. A new structural model of the Si(112)6 x 1-Ga surface is inferred. It consists of Ga zig-zag chains that are intersected by quasi-periodic vacancy lines or misfit dislocations. The experimentally observed meandering of the vacancy lines is caused by the co-existence of competing 6 x 1 and 5 x 1 unit cells and by the orientational disorder of symmetry breaking Si-Ga dimers inside the vacancy lines. The Ga atoms are fully coordinated, and the surface is chemically passivated. STS data reveal a semiconducting surface and show excellent agreement with calculated Local Density of States (LDOS) and STS curves. The energy gain obtained by fully passivating the surface calls the idea of step-edge decoration as a viable growth method toward 1D metallic structures into question.

Dynamic Ad-Dimer Twisting Assisted Nanowire Self-Assembly on Si(001)

Based on ab initio total energy calculation, we show that a dynamic ad-dimer twisting assisted (DATA) process plays a crucial role in facilitating a novel structural reconstruction involving surface and subsurface atoms on Si(001). It leads to self-assembly of long nanowires of group-V elements (Bi, Sb) in the trenches of surface dimer vacancy lines (DVLs) with a characteristic double-dimer configuration. The key to this is the lowering of the kinetic barrier by the DATA process in conjunction with a favorable interaction between ad-dimers and step edges in DVLs. The present results provide an excellent account for experimental observations and reveal the atomistic origin and the dynamic transformation path for nanowire self-assembly on Si(001).

Interactions of Carbon Atoms and Dimer Vacancies on the Si(001) Surface

Abstract We investigate the interactions between substitutional carbon atoms on the defect free, (2×1) reconstructed Si(001) surface, and bring evidence that the interaction energy differs significantly from the inverse-cube distance dependence that is predicted by the theory of force dipoles on an elastic half-space. Based on Tersoff potentials, we also calculate the interactions between carbon atoms and dimer vacancies. The calculations indicate that dimer vacancies (DVs) are strongly stabilized by fourth-layer C atoms placed directly underneath them. By use of simple model Monte Carlo simulations, we show that the computed interactions between carbon atoms and DVs lead to self-assembled vacancy lines, in qualitative agreement with recent experimental results.

Crossover energetics for halogenated Si(100): Vacancy line defects, dimer vacancy lines, and atom vacancy lines

Crossover energetics for halogenated Si(100): Vacancy line defects, dimer vacancy lines, and atom vacancy lines

Intrinsic Vacancy-Induced Nanoscale Wire Structure in HeteroepitaxialGa2Se3/Si(001)

A highly anisotropic growth morphology is found for heteroepitaxial gallium sesquiselenide (Ga2Se3) on the lattice matched substrate, arsenic-terminated Si(001). Scanning tunneling microscopy of Ga2Se3 films reveals nanoscale, wirelike structures covering the surface in parallel lines, less than 1 nm wide and up to 30 nm long. Core-level photoemission spectroscopy and diffraction reveals the local structure of buried Ga and Se atoms to reflect the bulk, defected zinc-blende structure of beta-Ga2Se3, which contains ordered 110 arrays of Ga vacancies. These ordered vacancy lines are proposed to be responsible for the observed growth anisotropy in heteroepitaxial Ga2Se3.

Surface energetics and structure of the Ge wetting layer on Si(100)

Ge deposited on Si(100) initially forms heteroepitaxial layers, which grow to a critical thickness of $\ensuremath{\sim}3$ MLs before the appearance of three-dimensional strain relieving structures. Experimental observations reveal that the surface structure of this Ge wetting layer is a dimer vacancy line (DVL) superstructure of the unstrained Ge(100) dimer reconstruction. In the following, the results of first-principles calculations of the thickness dependence of the wetting layer surface excess energy for the $c(4\ifmmode\times\else\texttimes\fi{}2)$ and $4\ifmmode\times\else\texttimes\fi{}6$ DVL surface reconstructions are reported. These results predict a wetting layer critical thickness of $\ensuremath{\sim}3$ MLs, which is largely unaffected by the presence of dimer vacancy lines. The $4\ifmmode\times\else\texttimes\fi{}6$ DVL reconstruction is found to be thermodynamically stable with respect to the $c(4\ifmmode\times\else\texttimes\fi{}2)$ structure for wetting layers at least 2 ML thick. A strong correlation between the fraction of total surface induced deformation present in the substrate and the thickness dependence of wetting layer surface energy is also shown.

Formation of atom wires on vicinal silicon.

The feasibility of creating atomic wires on vicinal silicon surfaces via pseudomorphic step-edge decoration has been analyzed for the case of Ga on Si(112). Scanning tunneling microscopy and density functional theory calculations indicate the formation of Ga zigzag chains intersected by quasiperiodic vacancy lines or "misfit dislocations." This structure strikes a balance between the system's drive towards chemical passivation and its need for strain relaxation in the atom chains. Spatially fluctuating disorder, intrinsic to the reconstruction, originates from the two symmetry-degenerate orientations of the zigzag chains on vicinal Si.

Atomic scale control and understanding of cubic silicon carbide surface reconstructions,nanostructures and nanochemistry

The atomic scale ordering and properties of cubic silicon carbide(β-SiC) surfaces and nanostructures are investigated by atom-resolved room andhigh-temperature scanning tunnelling microscopy (STM) and spectroscopy (STS),synchrotron radiation-based valence band and core level photoelectron spectroscopy(VB-PES, CL-PES) and grazing incidence x-ray diffraction (GIXRD). In this paper, wereview the latest results on the atomic scale understanding of (i) the structure ofβ-SiC(100) surface reconstructions, (ii) temperature-induced metallic surface phasetransition, (iii) one dimensional Si(C) self-organized nanostructures havingunprecedented characteristics, and on (iv) nanochemistry at SiC surfaces withhydrogen. The organization of these surface reconstructions as well as the 1Dnanostructures’ self-organization are primarily driven by surface stress. In thispaper, we address such important issues as (i) the structure of the Si-rich3 × 2, the Si-terminatedc (4 × 2), the C-terminatedc (2 × 2) reconstructionsof the β-SiC(100) surface, (ii) the temperature-induced reversible metallic phase transition, (iii) the formation of highly stable (up to900 °C) Si atomic and vacancy lines, (iv) the temperature-induced sp tosp3 diamondlike surface transformation, and (v) the first example of H-induced semiconductor surface metallizationon the β-SiC (100) 3 × 2 surface. The results are discussed and compared to other experimental and theoreticalinvestigations.

Comparative study of dimer-vacancies and dimer-vacancy lines on Si( [formula omitted]) and Ge( [formula omitted

Although the clean Si(0 0 1) and Ge(0 0 1) surfaces are very similar, experiments to date have shown that dimer-vacancy (DV) defects self-organize into vacancy lines (VLs) on Si(0 0 1), but not on Ge(0 0 1). In this paper, we perform empirical-potential calculations aimed at understanding the differences between the vacancies on Si(0 0 1) and Ge(0 0 1). We identify three energetic parameters which characterize the DVs on the two surfaces: the formation energy of single DVs, the attraction between two DVs in adjacent dimer rows, and the strain sensitivity of the formation energy of DVs and VLs. At the empirical level of treatment of the atomic interactions (Tersoff potentials), all three parameters are favorable for the self-assembly of DVs on the Si(0 0 1) surface, but not on Ge(0 0 1). The most significant difference between the defects on Si(0 0 1) and on Ge(0 0 1) concerns the formation energy of single DVs, which is three times larger in the latter case. By calculating the strain-dependent formation energies of DVs and VLs, we propose that the experimental observation of self-assembly of vacancies on clean Ge(0 0 1) could be achieved by applying compressive strains of the order of 2%.