Bi Nanolines Research Articles

One-dimensional topological phase and tunable soliton states in atomic nanolines on Si(001) surface

Formation of exotic topological states on technologically important semiconductor substrate is significant from the aspects of both fundamental research and practical implementation. Here, we demonstrate one-dimensional (1D) topological phase and tunable soliton states in atomic nanolines self-assembled on Si(001) surface. By first-principles calculations and tight-binding modeling, we reveal that Bi nanolines provide an ideal system to realize a multi-orbital Su–Schrieffer–Heeger (SSH) model, and the electronic properties can be modulated by substrate-orbital-filtering effect. The topological features are confirmed by nontrivial end states for a finite-length nanoline and (anti-)soliton states at the boundary of two topologically distinct phases. We demonstrate that solitons are highly mobile on the surface, and their formation could be controlled by surface B/N doping. As these nanolines can extend several micrometers long without kinks, and quantum transport simulations suggest clear signatures of topological states characterized by transmission resonance peaks, our work paves an avenue to achieve 1D topological phase compatible with semiconductor technology and to engineer the properties with high tunability and fidelity for quantum information processing.

Electronic structure of Bi nanolines on InAs(100)

Self-assembled nanolines are attractive to build the technological devices of next generation, but characterizing their electronic properties is often difficult to achieve. In this work we employ angle-resolved photoemission spectroscopy and density functional theory to clarify the electronic structure exhibited by self-assembled Bi nanolines grown on the InAs(100) surface. A surface resonance associated to the reconstructed ζ(4 × 2) surface is visible in the photoemission spectra before and after the formation of the Bi nanolines. This demonstrates that Bi deposition does not necessarily drive a transition to an unreconstructed surface in the substrate, which is contrary to what was reported in previous studies. In addition, experiment and theory show the presence of a flat band located in the band gap of InAs, just above the valence band maximum. This flat band is associated to the Bi nanolines and possesses a strong orbital character, consistent with its unidimensional nature. These spectral features suggest that Bi nanolines on InAs(100) may have a strongly polarized conductivity, which makes them suitable to be exploited as nanowires in nanotechnology. The coexistence with an accumulation layer suggests an even farther functionalization.

Activation of two dopants, Bi and Er in δ-doped layer in Si crystal

Conventional doping processes are no longer viable for realizing extreme structures, such as a δ-doped layer with multiple elements, such as the heavy Bi, within the silicon crystal. Here, we demonstrate the formation of (Bi + Er)-δ-doped layer based on surface nanostructures, i.e. Bi nanolines, as the dopant source by molecular beam epitaxy. The concentration of both Er and Bi dopants is controlled by adjusting the amount of deposited Er atoms, the growth temperature during Si capping and surfactant techniques. Subsequent post-annealing processing is essential in this doping technique to obtain activated dopants in the δ-doped layer. Electric transport measurement and photoluminescence study revealed that both Bi and Er dopants were activated after post-annealing at moderate temperature.

Evolution of lattice defects upon Bi-doping of epitaxial Si overlayers on Si(1 0 0)

Single Bi dopants in Si exhibit promising properties for quantum information science. Here we study, as a function of Bi-doped Si film thickness, the evolution and ultimate removal of lattice defects that are associated with a recently demonstrated viable route to precisely incorporate Bi dopants in homoepitaxial Si thin films. Scanning tunneling microscopy imaging reveals how the elongated defect structures in the Si lattice, originating from prefabricated Bi nanolines on the substrate surface that are the source of the Bi dopants, evolve with increasing Si overlayer thickness. Moreover, we demonstrate that a prolonged low-temperature annealing is able to annihilate these defect structures while leaving a significant Bi dopant concentration in the Si films.

Electronic coupling between Bi nanolines and the Si(001) substrate: An experimental and theoretical study

Atomic nanolines are one dimensional systems realized by assembling many atoms on a substrate into long arrays. The electronic properties of the nanolines depend on those of the substrate. Here, we demonstrate that to fully understand the electronic properties of Bi nanolines on clean Si(001) several different contributions must be accounted for. Scanning tunneling microscopy reveals a variety of different patterns along the nanolines as the imaging bias is varied. We observe an electronic phase shift of the Bi dimers, associated with imaging atomic p-orbitals, and an electronic coupling between the Bi nanoline and neighbouring Si dimers, which influences the appearance of both. Understanding the interplay between the Bi nanolines and Si substrate could open a novel route to modifying the electronic properties of the nanolines.

Subatomic electronic feature from dynamic motion of Si dimer defects in Bi nanolines on Si(001)

Scanning tunneling microscopy (STM) reveals unusual sharp features in otherwise defect free bismuth nanolines self-assembled on Si(001). They appear as subatomic thin lines perpendicular to the bismuth nanoline at positive biases and as atomic size beads at negative biases. Density functional theory (DFT) simulations show that these features can be attributed to buckled Si dimers substituting for Bi dimers in the nanoline, where the sharp feature is the counterintuitive signature of these dimers flipping during scanning. The perfect correspondence between the STM data and the DFT simulation demonstrated in this study highlights the detailed understanding we have of the complex Bi-Si(001) Haiku system.

Dopant activation mechanism of Bi wire-δ-doping into Si crystal, investigated with wavelength dispersive fluorescence x-ray absorption fine structure and density functional theory

We successfully characterized the local structures of Bi atoms in a wire-δ-doped layer (1/8 ML) in a Si crystal, using wavelength dispersive fluorescence x-ray absorption fine structure at the beamline BL37XU, in SPring-8, with the help of density functional theory calculations. It was found that the burial of Bi nanolines on the Si(0 0 1) surface, via growth of Si capping layer at 400 °C by molecular beam epitaxy, reduced the Bi–Si bond length from to Å. We infer that following epitaxial growth the Bi–Bi dimers of the nanoline are broken, and the Bi atoms are located at substitutional sites within the Si crystal, leading to the shorter Bi–Si bond lengths.

Bi nanolines characterization by linear optical methods

Bi nanolines characterization by linear optical methods

Degenerate electronic structure of reconstructed MnSi1.7 nanowires on Si(001)

Higher manganese silicide nanowires have been grown on the Si(001)-2 × 1 surface by the pre-growth of Bi nanolines. Scanning tunnelling microscope (STM) observations show that the nanowire has a linear surface reconstruction with a periodicity of 0.56 nm, and we propose a reconstruction on their surface to reduce the density of dangling bonds, which forms linear structures matching the dimensions from STM. Scanning tunnelling spectroscopy (STS) data agree with previous calculation results and reveal that the nanowires are degenerate semiconductors, with potential application for spintronics.

Properties of self-assembled Bi nanolines on InAs(100) studied by core-level and valence-band photoemission, and first-principles calculations

We have studied self-assembled bismuth (Bi) nanolines on the Bi-terminated InAs(100) surface by core-level and valence-band photoelectron spectroscopy, and ab initio first-principles calculations. A structural model for this intriguing surface is suggested based on the comparison of the measured and calculated core-level shifts. Also, the atomic origins for the core-level shifts are proposed based on the calculations. A clear peak related to this surface was observed in the valence band 0.34 eV below the Fermi level, which can be used as a fingerprint of a well-ordered Bi/InAs(100) nanoline surface.

The interaction of Cu with Bi nanolines on H-passivated Si(001): an ab initioanalysis

In spite of the known tendency for Cu to be a fast diffuser inside bulk Si, it was recentlyshown that it presents a clear preference for migrating towards a hydrogen terminatedSi(001) surface along the dimer rows; on a surface with Bi nanolines, this will drive the Cutowards the Bi nanolines. In this paper, we present a density functional theory study of thebehaviour of Cu atoms near self-assembled Bi nanolines on the hydrogen-passivated Si(001)surface, predicting that Cu will attack the nanolines and insert in the Bi–Si bonds. Themigration routes from subsurface locations and surface deposition are found and thepairing tendency for Cu is examined and compared to that for Ag on Bi nanolines.

Atomic-scale nanowires on Si(001): Cu on Bi

The fabrication of nanowires on the atomic scale is an extremely challenging process, typically only achieved via self-assembly or nanoscale lithography. We present density-functional theory results which show that a sequence of different nanowires one atom wide can be grown by deposition of copper on a nanoscale template: the Bi nanolines which self-assemble on Si(001). The resulting nanowires are characterized and show strong dispersion along the wire, as well as isolation from the substrate provided by the nanoline template.

Electronic structure of Bi lines on clean and H-passivated Si(100)

By means of scanning tunnelling microscopy and spectroscopy, we have investigated theelectronic structure of Bi nanolines on clean and H-passivated Si(100) surfaces. Maps of thelocal density of states (LDOS) images of the Bi nanolines are presented for the first time.The spectra obtained for nanolines on a clean Si surface and the LDOS imagesagree with ab initio predicted spectra for the Haiku structure. For nanolines ona H-passivated surface, the spectra obtained suggest that the Bi nanoline maylocally pin the surface Fermi level, and the LDOS images taken at low bias show adistribution of states different to what was expected at the Bi nanolines. The results arediscussed with respect to use of the nanolines as atomic wire interconnections.

Density-functional calculations for self-assembled Bi-nanolines on the InAs(100) surface

We have performed an ab initio investigation of the stability, atomic geometry, and electronic properties of the self-assembled bismuth (Bi)-nanolines on the Bi-stabilized indium arsenide (InAs)(100) surface. Our calculations were performed within the local density approximation of the density functional theory, using pseudopotentials to describe the electron-ion interactions. We have examined several metastable Bi nanolines arrangements on the top of the Bi-stabilized InAs(100) surface. Our total energy calculations suggest that the most stable configuration of the Bi nanolines is formed by Bi dimers parallel to the Bi dimers on the Bi/InAs(100) surface. We have found that the structure is metallic with several occupied and unoccupied surface states within the bulk InAs gap region. These states are mainly due to the top and first sublayer Bi atoms. Our theoretically simulated scanning tunneling microscope (STM) image shows a very bright line along the [01¯1] direction, which is consistent with the experimental STM images.

Self-assembled Bi nanolines on the InAs(100) surface: a theoretical study

The authors have performed an ab initio theoretical investigation of the stability, atomic geometry and electronic properties of the self-assembled Bi nanolines on the Bi-stabilised InAs(100) surface. The calculations were performed within the local density approximation of the density functional theory, using pseudopotentials to describe the electron-ion interactions. The authors have examined several metastable Bi nanolines arrangements on the top of the Bi-stabilised InAs(100) surface. The authors' total energy calculations suggest that the most stable configuration of the Bi nanolines is formed by Bi dimers parallel to the Bi dimers on the Bi/InAs(100) surface.

Ytterbium on vicinal Si(1 0 0): Growth and properties of the 2D wetting layer and the Yb silicide phase

The Yb growth on a vicinal Si(100) surface has been studied by scanning tunneling microscopy and low energy electron diffraction in the coverage range of 1–4ML. Two different methods of the Yb/Si(100) interface formation are applied, leading to a remarkable modification of structural and morphological properties of two-dimensional (2D) wetting layer and Yb silicide phase. In particular, the switchover of the 2D layer orientation, similar to the case of the Bi nanolines on Si(100) [J.H.G. Owen, K. Miki, D.R. Bowler, J. Mater. Sci. 41 (2006) 4568], is observed depending on the growth procedure. Moreover, the structure and morphology of the Yb silicide phase is found to depend critically on the growth conditions, and the ability to grow very long, unidirectional Yb silicide nanowires is demonstrated. The results obtained are discussed in the context of the previous studies of 1D nanowires and 3D islands of rare-earth silicides on Si(100).

Iron silicide wires patterned by Bi nanolines on the H/Si(001) surface: Spin density functional calculations

R. H. Miwa,1 W. Orellana,2 and G. P. Srivastava3 1Instituto de Fisica, Universidade Federal de Uberlândia, CP 593, CEP 38400-902, Uberlândia, MG, Brazil 2Departamento de Ciencias Fisicas, Facultad de Ingenieria, Universidad Andres Bello, Av. Republica 252, 837-0134, Santiago, Chile 3School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, United Kingdom Received 3 October 2007; revised manuscript received 19 May 2008; published 11 September 2008

Theoretical investigation of Mn adsorbates aside self-organised Bi nanolines on hydrogenated Si(0 0 1) surface

We have theoretically investigated the atomic structure, magnetic behaviour, and electronic properties of Mn adsorbates on hydrogen passivated self-organised Bi nanolines on the Si(0 0 1)surface. It is found that the most stable geometry for 1 6 monolayer (ML) coverage of Mn is just underneath the first Si(0 0 1) surface layer. The Mn atoms in the optimised configuration are seven-fold coordinated with their neighbouring Si atoms. Total energy calculations suggest that the Mn adsorbates form a degenerate state of ferromagnetic and anti-ferromagnetic lines parallel and adjacent to the self-assembled Bi lines. The density functional band structure calculation within the local-spin density approximation shows that the ferromagnetic system behaves like a metal in both spin channels. On the other side, the anti-ferromagnetic phase exhibits a half-metallic phenomenon with semiconducting character for the majority spin channel and semi-metallic character for the minority spin channel.

Nanostructure formation aided by self-organised Bi nanolines on Si(0 0 1)

We provide a mini review of recent theoretical investigations of nanostructure formation aided by self-organised Bi nanolines on the Si(0 0 1) surface. It is suggested that hydrogen-passivated single-domain Si(0 0 1) produced by the formation of defect-free, hundreds of nm long, and 1.2–1.5 nm wide Bi nanolines provides an appealing template with preferential sites for adsorption of other elements. Based on ab initio pseudopotential calculations it is suggested that using the Bi nanoline template it should be possible to grow the following structures: mixed Ge–Si dimer structures on the Si(0 0 1) terrace between two neighbouring Bi nanolines; small In nanoclusters along the nanoline; and line and cluster structures of Fe atoms with novel electronic and magnetic properties.

Formation of an ordered pattern of Bi nanolines on InAs(100) by self-assembly

Self-assembly of uniform patterns of nanolines over large surface areas has been proven to be difficult. The authors report that bismuth (Bi) adsorbate self-assembles into an ordered pattern of Bi nanolines separated by 4.3nm on the Bi-stabilized InAs(100)(2×1). The resulted nanoline surface is studied by scanning tunneling microscopy (STM) and low-energy electron diffraction. The plausible atomic models for the Bi nanolines are proposed on the basis of the STM results. The Bi lines are suggested to consist of two chains of adjacent Bi dimers positioned parallel to the chain and parallel to the Bi dimers of the (2×1) substrate.