Single Height Steps Research Articles

Controllable quantum point junction on the surface of an antiferromagnetic topological insulator

Engineering and manipulation of unidirectional channels has been achieved in quantum Hall systems, leading to the construction of electron interferometers and proposals for low-power electronics and quantum information science applications. However, to fully control the mixing and interference of edge-state wave functions, one needs stable and tunable junctions. Encouraged by recent material candidates, here we propose to achieve this using an antiferromagnetic topological insulator that supports two distinct types of gapless unidirectional channels, one from antiferromagnetic domain walls and the other from single-height steps. Their distinct geometric nature allows them to intersect robustly to form quantum point junctions, which then enables their control by magnetic and electrostatic local probes. We show how the existence of stable and tunable junctions, the intrinsic magnetism and the potential for higher-temperature performance make antiferromagnetic topological insulators a promising platform for electron quantum optics and microelectronic applications.

Numerical calculation of the potential distribution on the Si(111)-7 × 7 surface for scanning tunneling potentiometry

Scanning tunneling potentiometry enables us to obtain electrochemical potential distribution of the sample surface in sub-nanometer-scale spatial resolution, and provides information on electrical resistances at the surface and local defects on it. In order to evaluate the influence of the conductance through the substrate in the measurements, we performed numerical simulation using a resistance network model that consists of a conductive surface layer with one resistive site at the center, representing the metallic surface states of the Si(111)7 × 7 surface and its single height step, respectively. The calculated results demonstrate that the substrate conductance modifies the surface potential distribution, in particular around the electrodes attached at the sides of the surface layer, and reduces the amount of the potential drop at the step and the potential gradient on the terrace nearby. In spite of the modification, however, the ratio of the conductivity of the step to that of the terrace estimated from the potential profile, is not affected by the substrate conductance as far as a low-doped substrate (103 Ω · cm or larger) is used in the case of the Si(111)7 × 7 surface.

Atomic Force Microscopy Imaging of Crystalline Sucrose in Alcohols.

Imaging nanometer- or molecule-scale topography has been achieved by dynamic atomic force microscopy (AFM) when a solid object of interest is damaged by vacuum exposure or electron irradiation. Imaging in a liquid offers a means to remove contaminations from the surface scanned using the microscope tip when the object is soluble to the surrounding liquid, typically water. In the present study, we attempted to take topographic images of crystalline sucrose. A problem arose due to the high solubility of this compound to water. Cantilever oscillation could not be excited in the saturated, viscous aqueous solution. By using n-hexanol instead of water, the solubility in the solvent and thus viscosity of the solution were reduced sufficiently to excite cantilever oscillation. Single-height steps and sucrose molecules were recognized in the images and thereby recorded on the (001)-oriented facets of sucrose crystals. Furthermore, two-dimensional distribution of liquid-induced force pushing or pulling the tip was mapped on planes perpendicular to the hexanol–sucrose interface. Observed uneven force distributions indicated liquid hexanol structured on the corrugated surface of sucrose. The viscosity tuning demonstrated here, which is not limited to hexanol instead of water, extends the range of liquid–solid interfaces to be probed by dynamic AFM.

Surface reconstructions of (0001) AlN during metal‐organic vapor phase epitaxy

AbstractWe observed (0001) AlN wurzite surfaces by atomic force microscopy after 500 nm regrowth in metal‐organic vapor phase epitaxy. The steps changed from double to single height with decreasing V/III ratio. The single height step edges were alternating smooth and rough due to the two different step types on (0001) wurzite surfaces. By reducing the V/III ratio, the widths equalize for terraces with smooth and rough edges, until Al terminated steps start to dominate, and thus promote again double height steps. Using in situ ellipsometry at nm under static conditions, we could directly identify three different surface reconstructions at high , low , and without which correlates with the different step terminations.

Computational study of a multistep height model

An equilibrium random surface multistep height model proposed in [Abraham and Newman, EPL 86, 16002 (2009)] is studied using a variant of the worm algorithm. In one limit, the model reduces to the two-dimensional Ising model in the height representation. When the Ising model constraint of single height steps is relaxed, the critical temperature and critical exponents are continuously varying functions of the parameter controlling height steps larger than one. Numerical estimates of the critical exponents can be mapped via a single parameter, the Coulomb gas coupling, to the exponents of the O(n) loop model on the honeycomb lattice with n ≤ 1.

STM imaging of a model surface of Ru(4,4′-dicarboxy-2,2′-bipyridine) 2(NCS) 2 dye-sensitized TiO 2 photoelectrodes

A TiO 2(1 1 0)-(1 × 1) surface was prepared in an ultra-high vacuum, transported in laboratory air, and observed with a scanning tunneling microscope (STM) operated in a vacuum of 10 −4 Pa. Empty state images showed atomically flat terraces separated by single-height steps, on which 5-fold-coordinated surface Ti atoms were observed as spots arranged in a rectangular lattice. The Ru(4,4′-dicarboxy-2,2′-bipyridine) 2(NCS) 2 (N3) dye was adsorbed on the TiO 2 surface by immersing the TiO 2 wafer into an acetonitrile solution of the dye. In the empty state images, individual N3 molecules were observed as oval particles protruding by 0.6 nm from the TiO 2 surface. The oval shape elongated to the [1 1 ¯ 0] directions was attributed to electron tunneling from tip to unoccupied states localized at the two carboxyl groups bound to the TiO 2 surface.

Structure and morphology of the tenfold surface of decagonalAl71.8Ni14.8Co13.4in its low-temperature random tiling type-I modification

We have investigated the structure and morphology of the tenfold surface of decagonal ${\mathrm{Al}}_{71.8}{\mathrm{Ni}}_{14.8}{\mathrm{Co}}_{13.4}$ by highly surface sensitive He atom scattering (HAS), high resolution low energy electron diffraction(SPA-LEED), and low temperature scanning tunneling microscopy (STM). The SPA-LEED patterns reveal more than 500 individual diffraction spots in the $\mathbf{k}$-vector range of $\ensuremath{\mid}{\mathbf{k}}_{\ensuremath{\Vert}}\ensuremath{\mid}l3\phantom{\rule{0.3em}{0ex}}{\mathrm{\AA{}}}^{\ensuremath{-}1}$. The positions of all observed diffraction spots agree with the surface projections of the reciprocal lattice structure of the type-I bulk phase. HAS shows identical spot positions as SPA-LEED, thus demonstrating a top surface layer with long range quasicrystalline order and a reciprocal lattice structure consistent with that of a bulk truncated surface. SPA-LEED peak widths are found to vary between different diffraction orders. Based on an analysis of a randomized Fibonacci sequence, this is linked to the random nature of the tiling of the type-I structure. STM measurements reveal a surface morphology characterized by rough single-height steps separating terraces with widths on the order of $100\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. Two different surface terminations are observed, a coarse and a fine one, frequently coexisting on single terraces. The fine structure termination directly reflects the atomic structure of a bulk truncated surface, allowing a random rhombic tiling to be identified. In order to compare diffraction, real-space data, and atomic structure models, the Patterson function and autocorrelation of the surface structure, respectively, are studied. This allows an understanding of the coarse structure termination as consisting of subunits of a few atoms each arranged statistically on sites defined by the atomic tiling of the bulk tenfold planes.

Bismuth nanolines on Si(001) and their influence on mesoscopic surface structure

Experimental studies of Bi heteroepitaxy on Si(001) have recently uncovered a self-organised nanoline motif which has no detectable width dispersion. The Bi lines can be grown with an aspect ratio that is greater than 350 : 1. This paper describes a study of the nanoline geometry and electronic structure using a combination of scanning tunneling microscopy (STM) and ab initio theoretical methods. In particular, the effect that the lines have on Si(001) surface structure at large length scales, l > 100 nm, is studied. It has been found that Bi line growth on surfaces that have regularly spaced single height steps results in a 'preferred' domain orientation.

Single2×1domain orientation on Si(001) surfaces using aperiodic Bi line arrays

Studies of Bi heteroepitaxy on Si(001) have shown that lines grow to lengths of up to $500\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ if the substrate is heated to above the Bi desorption temperature $(500\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C})$ during or after Bi deposition. Unlike many other nanoline systems, the lines formed by this nonequilibrium growth process have no detectable width dispersion. Although much attention has been given to the atomic geometery of the line, in this paper, we focus on how the lines can be used to create a majority $2\ifmmode\times\else\texttimes\fi{}1$ domain orientation. It is demonstrated that the Bi lines can be used to produce a single-domain orientation on Si(001) if the lines are grown on Si(001) surfaces with a regular distribution of single height steps. This is a compelling example of how a nanoscale motif can be used to modify mesoscopic surface structure on Si(001).

Atom-Resolved Surface Structures and Molecular Adsorption on TiO2(001) Investigated by Scanning Tunneling Microscopy

Well-ordered surface structure was prepared on a rutile TiO2(001) surface and investigated on the atomic scale by scanning tunneling microscopy (STM). The surface had a latticework-like structure with rows running along the [110] and [110] directions. Each row consisted of narrow terraces separated by single-height steps on their slopes at both sides. An added-row structural model that consists of Ti7O12 suboxide row added on each narrow terrace of a {114}-microfacet structure was proposed on the basis of atom-resolved STM images as well as local coordination considerations. The added row had 4-fold coordinated and 5-fold coordinated Ti atoms, each of which was observed as a bright spot in STM images. Formic acid molecules adsorbed on both Ti sites, while methanol adsorbed preferentially on the 4-fold coordinated Ti atoms, which may support the validity of the structural model. The latticework structure was proved to be stable against thermal reactions of formic acid and methanol.

Ab initiodensity functional studies of stepped TaC surfaces

We report on density functional total energy calculations of the step formation and interaction energies for vicinal TaC(001) surfaces. Our calculations show that double and triple-height steps are favored over single-height steps for a given vicinal orientation, which is in agreement with recent experimental observations. We provide a description of steps in terms of atomic displacements and charge localization and predict an experimentally observable rumpled structure of the step-edges, where the Ta atoms undergo larger displacements compared to the C atoms.

Local work function of a rutile TiO 2( [formula omitted])-(1×1) surface observed by Kelvin probe force microscopy

The work function of an atomically flat TiO 2(1 1 0) surface was mapped with a lateral resolution of several nanometers by using a Kelvin probe force microscope. Islands crystallized on an extended terrace exhibited larger work functions than those of the terrace itself. The increment of the work function was dependent on the size of each island; a small island presented an enhanced increment. Laterally oriented electric dipole moments are proposed on single-height steps terminating the islands to interpret the size-dependent increment of the work function.

Strain induced stabilization of stepped Si and Ge surfaces near (001)

We report on calculations of the formation energies of several [100] and [110] oriented step structures on biaxially stressed Si and Ge (001) surfaces. It is shown that a novel rebonded [100] oriented single-height step is strongly stabilized by compressive strain compared to most well-known step structures. We propose that the side walls of “hut”-shaped quantum dots observed in recent experiments on SiGe/Si films are made up of these steps. Our calculations provide an explanation for the nucleationless growth of shallow mounds, with steps along the [100] and [110] directions in low- and high-misfit films, respectively, and for the stability of the (105) facets under compressive strain.

Chemical order driven morphology of a vicinal surface of Fe 3Al(1 1 1): an He diffraction and STM study

The morphology and atomic structure of a vicinal surface of Fe 3Al(1 1 1) has been studied by thermal energy helium atom scattering and scanning tunneling microscopy. The second order B2→DO 3 structural phase transition in the chemically ordered alloy induces a morphological transition at its surface: single-height steps, which are evenly distributed above the bulk critical temperature T c (≈820 K), bunch four by four below T c. The latter morphology is related to bulk layer stacking and appears driven by minimization of terrace energy. Terraces are pure Al and exhibit a (√3×√3) R30° surface reconstruction, corresponding to 2/3 Al atoms and 1/3 vacancies in the terminal layer. The first sub-surface layers are enriched in Al by surface segregation. Surface vicinality makes possible the emergence of bulk DO 3 domain boundaries, where four-steps bunches separate in two pairs of steps, which allows following chemical ordering in the near-surface region. The results point out the strong influence of long range chemical order on surface morphology.

Role of surface steps in the arrangement of silicon nano-dots on a vicinal Si(111) surface: Scanning tunneling microscopy investigation

We have investigated the role of surface steps in the arrangement of silicon nano-dots on a vicinal Si(111) surface by scanning tunneling microscopy. Nanometer sized silicon nitride islands were formed on a vicinal Si(111) surface, which was 1° off toward [1̄1̄2] direction, via thermal nitridation using N2 gas. On the nitrided surface, oxygen gas was dosed to induce a local selective etching of bare silicon using the silicon nitride islands as masks. The resultant surface showed one-dimensional arrangement of silicon nano-dots along the step edges of silicon surface. The lateral size of the dot in the direction perpendicular to the step edges was restricted to the terrace width of the stepped Si(111) surface. We consider that the preferential growth of silicon nitride islands on the edges of single height steps is responsible for the arrangement of silicon nano-dots along the step edges of the 1° off vicinal Si(111) surface.

Roughening and preroughening transitions in crystal surfaces with double-height steps

We investigate phase transitions in a solid-on-solid model where double-height steps as well as single-height steps are allowed. Without the double-height steps, repulsive interactions between up-up or down-down step pairs give rise to a disordered flat phase. When the double-height steps are allowed, two single-height steps can merge into a double-height step (step doubling). We find that the step doubling reduces repulsive interaction strength between single-height steps and that the disordered flat phase is suppressed. As a control parameter a step doubling energy is introduced, which is assigned to each step doubling vertex. From transfer matrix type finite-size-scaling studies of interface free energies, we obtain the phase diagram in the parameter space of the step energy, the interaction energy, and the step doubling energy.

The Ten-Fold Surface of The Decagonal Al72Ni11Co17 Quasicrystal Studied by Leed, Spa-Leed, Aes and Stm.

AbstractThe ten-fold surface of the decagonal Al72Ni11Co17 (d-Al-Ni-Co) quasicrystal has been investigated using low energy electron diffraction (LEED), spot profile analysis LEED (SPA- LEED), Auger electron spectroscopy (AES) and scanning tunnelling microscopy (STM). This was done as a function of both annealing temperature and annealing time. The long-range order of the surface, as indicated by LEED, increases both as a function of annealing time and temperature. STM shows the surface to be rough and cluster-like at low annealing temperatures (≤725 K), whilst annealing to temperatures in excess of 725 K results in the formation of terraces. These terraces are small (≤ 100 Å width) at lower annealing temperatures and increase in size (100 Å ≤ x ≤ 500 Å) as the annealing temperature is increased (≥ 850 K). They are characterised by the presence of three-fold protrusions which align preferentially. STM images show single height steps as expected due to the periodicity of d-Al-Ni-Co in the z direction. To date it has not been possible to obtain atomic resolution, although this work is continuing.

Theory of adsorption and diffusion of Si adatoms on H/Si(1 0 0) stepped surface

First-principles calculations on the formation energy of single height steps on the hydrogen terminated Si(1 0 0) surface and the adsorption and diffusion of Si adatoms on the stepped surfaces are reported. It is found that the formation energy of the S A step is lower than that of the S B step, leading to a relatively straight S A step and a relatively rough S B step. The S A step acts as a poor sink for Si adatoms whereas the S B step acts as a strong sink. In the initial stage of the homoepitaxial growth of the hydrogen terminated Si(1 0 0) surface, one-dimensional islands may be formed similarly to on the bare Si(1 0 0) surface.

Evolution of surface morphology of vicinal Si(111) surfaces after aluminum deposition

We have studied changes in surface morphology of vicinal Si(111) surfaces with a miscut of 1.3° in the [ 2 ̄ 11] direction after Al deposition at elevated temperatures. The clean surface phase separates into a (111)-oriented phase and a stepped phase. Submonolayer Al deposition at 650°C, the normal preparation temperature of the Al/Si(111)- ( 3 × 3 ) R30° structure, only induces minor changes in the surface morphology. However, after Al deposition at temperatures above the order–disorder phase transition temperature, the step bunches break apart into a uniform array of single height steps with an average step–step separation determined by the macroscopic miscut. From a quantitative analysis of the amount of meandering of steps and the terrace width distribution, we determined the diffusivity of steps and the strength of the repulsive step–step interaction. The repulsive interaction between steps is enhanced by the Al adsorption compared to both the high-temperature (1×1) and (7×7) phases of the clean surface.

In-situ study of gold-induced surface structures and step rearrangements on the Si(001) surface by high-temperature STM

The formation of Au-induced surface structures and step arrangements on the Si(001) surface at temperatures between 500 and 700°C were observed in situ by UHV high-temperature STM. At 500–600°C, various kinds of stripe structures with two-fold periodicity along the stripe ([110] direction) were formed after about 1 ML deposition, and their widths depended on the substrate temperature. At 700°C, the stripes were well ordered and a c(18 × 2) structure was formed. Stripes were elongated along the dimer rows of the original substrate surface, independent of the substrate temperature. During the formation of the stripe structure, an array of single-height steps was rearranged to form straight 2 n-height steps parallel to the stripes, and a single-domain structure was formed at high temperatures.