Scanning Tunneling Micrographs Research Articles

Suppressing Segregation in Highly Phosphorus Doped Silicon Monolayers.

Sharply defined dopant profiles and low resistivity are highly desired qualities in the microelectronic industry, and more recently, in the development of an all epitaxial Si:P based quantum computer. In this work, we use thin (monolayers thick) room temperature grown silicon layers, so-called locking layers, to limit dopant segregation in highly phosphorus doped silicon monolayers. We present secondary ion mass spectroscopy and atom probe tomography measurements that demonstrate the effectiveness of locking layers in suppressing P segregation. Scanning tunneling micrographs of the surface of the locking layer show that the growth is epitaxial, despite the low growth temperature, while magnetotransport measurements reveal a 50% decrease in the active carrier density. We show that applying a finely tuned rapid thermal anneal can restore the active carrier density to 3.4 × 10(14) cm(-2) while maintaining ultra sharp dopant profiles. In particular, 75% of the initial deposited P is confined in a layer with a full width at half-maximum thickness of 1.0 nm and a peak P concentration of 1.2 × 10(21) cm(-3) (2.5 atom %).

Self-Assembled p-Carborane Analogue of p-Mercaptobenzoic Acid on Au{111}

The p-carborane cluster analogue of p-mercaptobenzoic acid, 1-HS-12-COOH-1,12-C2B10H10, has been synthesized and characterized using nuclear magnetic resonance spectroscopy, single-crystal X-ray diffraction analysis, quantum-chemical calculations, and scanning tunneling microscopy. The single-crystal structure and selected packing aspects are discussed and presented in comparison with the two-dimensional periodic arrangements. Scanning tunneling micrographs, recorded under ambient conditions, are used to compare pure monolayers of 1-HS-1,12-C2B10H11 to coadsorbed monolayers of both the parental precursor and carboxyl-functionalized p-carboranethiolate on Au{111}. Monolayers of both constituents are further characterized by X-ray photoelectron spectroscopy, which shows good agreement between the stoichiometry of each pure monolayer and the nominal stoichiometries of the respective molecules. Results indicate that most of the molecules of both derivatives adsorb as thiolates but that a small fraction of eac...

Phase stability analysis of the InAs/GaAs (001) wetting layer from first principles

The large atomic-size mismatch between In and Ga and the large lattice-mismatch strain between InAs and GaAs make the InAs/GaAs (001) growth interface a complex alloy system, the understanding of which can enhance control of device synthesis and nanostructure self-assembly. We present a detailed first-principles analysis of the full progression of surface reconstructions observed on the InAs/GaAs(001) wetting layer during early stages of In deposition. We use systematic techniques to identify the most likely surface reconstruction prototypes of the InAs wetting layer on GaAs(001) using density functional theory (DFT) and then develop several cluster expansion Hamiltonians in order to thoroughly explore surface alloy disorder due to species substitution of In, Ga, and As at the surface. We use these results to construct a first principles 0-K surface phase diagram of the InAs wetting layer on GaAs(001) and test the sensitivity of our predicted phase diagram to DFT approximations and convergence errors. We find two alloy configurations of the $(4\ifmmode\times\else\texttimes\fi{}3)$ structural prototype that are likely ground-state surface reconstructions, and simulated scanning tunneling micrographs (STM) of these reconstructions indicate that they can explain prominent features of experimentally obtained STM of the InAs/GaAs $(4\ifmmode\times\else\texttimes\fi{}3)$ surface.

C(4 × 2) and related structural units on the SrTiO3(001) surface: Scanning tunneling microscopy, density functional theory, and atomic structure

Density functional theory is used to simulate high-bias, constant-current scanning tunneling micrographs for direct comparison with experimental images. Coupled to previous spectroscopic data, these simulations are used to determine the atomic structure of Ti-rich nanostructures on strontium titanate (001) surfaces. These nanostructures have three consecutive TiO(x) surface layers and exploit the distinctive structural motif of the c(4 × 2) reconstruction as their main building block. A structural model of a characteristic triline defect is also proposed.

Water adsorption on SrTiO3(001): I. Experimental and simulated STM

Density functional theory-based simulations of scanning tunneling micrographs were used for comparison to published experimental images of reconstructed SrTiO3(001) surfaces. It was found that the addition of dissociatively adsorbed H2O to the presently accepted structural solution of the 2×1 reconstruction is more consistent with the experimental data. A proposed model for the c(4×4) reconstruction, based on the hydrated 2×1 structure, agrees well with experiment and is consistent with a formation process consisting of the simple dehydration of a wet 2×1 structure.

Influence of wetting layers and quantum dot size distribution on intermediate band formation in InAs/GaAs superlattices

We examine the influence of the wetting layers (WLs) and the quantum dot (QD) size distribution on the sub-bandgap external quantum efficiency (EQE) of QD solar cells. We use a finite-element Schrödinger-Poisson model that considers QD and wetting layer shapes, sizes, and spacings from cross-sectional scanning tunneling and atomic force micrographs. A comparison between experiments and computations reveals an insignificant contribution of the WL to the sub-bandgap EQE and a broadening of sub-bandgap EQE associated with a variation in QD sizes in the growth direction.

Scanning tunneling microscopy of cauliflower-like DNA nanostructures synthesised by loop-mediated isothermal amplification

DNA nanotechnology is a novel approach for synthesis of DNA-based nanostructures. Stem-loops, nanojunctions, sticky-ends and periodic lengths of DNA are the most essential nanostructures in DNA nanofabrications. Loop-mediated isothermal amplification (LAMP) is a powerful technology for repetitive synthesis of double-stranded and cauliflower-like DNAs. The process leads to long and repetitive sequences of DNAs, which are fabricated via loop primers. The authors demonstrate here scanning tunneling micrographs of LAMP-synthesised DNAs deposited on highly ordered pyrolytic graphite. The scans are compared with natural DNAs. Scanning tunneling microscopy (STM) images indicated the creation of periodic long DNAs, stem-looped DNAs and three-way DNA nanojunctions. It is also suggested that such nanomaterials could be promising candidates for use in DNA-based nanodevices.

Tunnel coupled dangling bond structures on hydrogen terminated silicon surfaces

We study both experimentally and theoretically the electronic behavior of dangling bonds (DBs) at a hydrogen terminated Si(100)-2×1 surface. Dangling bonds behave as quantum dots and, depending on their separation, can be tunnel coupled with each other or completely isolated. On n-type highly doped silicon, the latter have a net charge of -1e, while coupled DBs exhibit altered but predictable filling behavior derived from an interplay between interdot tunneling and Coulomb repulsion. We found good correlation between many scanning tunneling micrographs of dangling bond structures and our theoretical results of a corresponding extended Hubbard model. We also demonstrated chemical methods to prevent tunnel coupling and isolate charge on a single dangling bond.

Understanding the Contrast Mechanism in Scanning Tunneling Microscopy (STM) Images of an Intermixed Tetraphenylporphyrin Layer on Ag(111)

The appearance of tetraphenylporphyrins in scanning tunneling micrographs depends strongly on the applied bias voltage. Here, we report the observation and identification of certain features in scanning tunneling microscopy (STM) images of intermixed layers of tetraphenylporphyrin (2HTPP) and cobalt-tetraphenylporphyrin (CoTPP) on Ag(111). A significant fraction of an ordered monolayer of commercially available CoTPP appears as "pits" at negative bias voltages around -1 V. The obvious possibility that these pits are missing molecules within the ordered layer could be ruled out by imaging the molecules at reduced bias voltages, at which the contrast of the pits fades, and at positive bias voltages around +1 V, at which the image contrast is inverted. With the investigation of the electronic structure, in particular the density of states (DOS) close to the Fermi level, of CoTPP and 2HTPP layers by means of ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling spectroscopy (STS), the contrast mechanism was clarified. The correlation of the bias dependent contrast with the UPS data enabled us to interpret the "pits" as individual 2HTPP molecules. Additional evidence could be provided by imaging layers of different mixtures of 2HTPP and CoTPP and by high-resolution STM imaging of the features in CoTPP.

Reflectance difference spectroscopy of mixed phases of indium phosphide (001)

Reflectance difference spectra of mixed (2×1) and (2×4) phases of indium phosphide (001) have been recorded and benchmarked against scanning tunneling micrographs of the surface. The line shapes are found to be linear combinations of the spectra of the pure (2×1) and (2×4) structures, Δr/rmixed=xΔr/r(2×4)+(1−x)Δr/r(2×1), where x is the weighting factor. Thus, in the absence of adsorbates, the reflectance difference spectra can be used to estimate the surface composition, i.e., the fractional coverage of phosphorous is ΘP=1−0.81x±0.06x.

Reflectance-difference spectroscopy of mixed arsenic-rich phases of gallium arsenide (001)

The relationship between the reflectance difference spectra and the atomic structure of arsenic-rich reconstructions of GaAs(001) has been investigated. Scanning tunneling micrographs reveal that a roughening process occurs as the surface structure changes with decreasing arsenic coverage from 1.75 to 0.75 monolayers (ML). At 1.65 ML As, small pits, one bilayer in depth and having the same $c(4\ifmmode\times\else\texttimes\fi{}4)$ reconstruction as the top layer, form in the terraces. At the same time, gallium atoms are liberated to the surface, disrupting the $c(4\ifmmode\times\else\texttimes\fi{}4)$ ordering. At about 1.4 ML As, $(2\ifmmode\times\else\texttimes\fi{}4)$ domains nucleate and grow on top of the $c(4\ifmmode\times\else\texttimes\fi{}4).$ Further desorption of arsenic causes the underlying layer to gradually decompose into a metastable $(2\ifmmode\times\else\texttimes\fi{}n)$ phase $(n=2,$ 3, or 4), and finally into the $(2\ifmmode\times\else\texttimes\fi{}4).$ In the reflectance difference spectra, negative peaks at 2.25 and 2.8 eV correlate with the $c(4\ifmmode\times\else\texttimes\fi{}4)$-type arsenic dimers. However, the intensity of the latter feature strongly depends on the presence of adsorbates, such as alkyl groups and gallium adatoms. By contrast, the intensity of the positive peak at 2.9 eV is directly proportional to the density of $(2\ifmmode\times\else\texttimes\fi{}4)$-type dimers.

Atomic control of TiO 2 (110) surface by oxygen plasma treatment

Segregation of impurities in TiO 2 substrates impacts the local atomic structure and stoichiometry of the native TiO 2 surfaces. We investigated the effects of oxygen plasma on the TiO 2 surfaces. It was found that annealing TiO 2 crystals in oxygen plasma at above 500°C effectively removed calcium and carbon contaminants, and dramatically improved the surface morphology and stoichiometry. Scanning tunneling micrographs revealed atomically smooth surfaces with well-defined step-terrace morphology after the plasma treatment, while X-ray photoelectron spectra showed that all the Ti cations on the resulting surfaces were fully coordinated.

Preparation and Characterization of Highly Transparent Conducting Composites Consisting of Acid-Doped Poly(o-toluidine) and Poly(methyl methacrylate)

Composites consisting of camphor sulfonic acid (CSA)-doped poly(o-toluidine) (POT) and insulating poly(methyl methacrylate) (PMMA) show percolation behavior and the value of percolation threshold to be 4 wt% of conducting CSA-doped POT. The composite was considerably transparent, e.g., the transmittance of the composite (∼15um) with 4% of CSA-doped POT and 96% of PMMA was about 70% at 540nm. Conductivity at the percolation threshold point is attributed to the interconnected network, which was revealed by scanning tunneling micrographs. Thermogravimetric analysis demonstrates four step decomposition and phase separated morphology of CSA-doped POT is found to be thermodynamically stable up to 250°C. This was supported by mass spectra simultaneously recorded in thermal analysis.

Electron diffraction and scanning tunneling microscope studies of TaCl 5-graphite intercalation compounds

Tantalum(V) chloride (TaCl 5) was intercalated into highly oriented pyrolytic graphite and into natural graphite flakes. X-ray diffraction measurements show that second stages were obtained. The fresh intercalation compounds were investigated by scanning tunneling microscopy in air and by electron diffraction. The commensurate ( 7 × 7 ) superlattice of tantalum(V) chloride shows a rotation of ±19.1° against the graphite lattice. Additional superstuctures of longer dimensions were detected by scanning tunneling microscopy. A Moiré pattern and some bands were observed. The graphite intercalation compounds were immersed in water for two hours at room temperature and dried at 100°C in air. X-ray diffraction showed that the original stages were preserved after exposure to water. The scanning tunneling micrographs of samples exposed to water show a pattern similar to that of pristine graphite, the superlattice had disappeared. Electron diffraction studies on such samples gave evidence that the intercalate is rearranged and forms amorphous layers. In the case of water-immersed samples, no surface structures of the intercalate were observable with scanning tunneling microscopy.

Surface phases of GaAs and InAs (001) found in the metalorganic vapor-phase epitaxy environment

We have characterized the (2×4) and (4×2) reconstructions of GaAs and InAs (001) that are present in a metalorganic vapor-phase epitaxy (MOVPE) reactor. Scanning tunneling micrographs show that these surfaces are terminated with arsenic and gallium (or indium) dimers. The (2×4) dimer row exhibits a mottled appearance, which is ascribed to the adsorption of alkyl groups on some of the sites. On the (4×2), <10% of the surface is covered with small (2×4) islands. These results show that, in the MOVPE environment, the GaAs and InAs surface structures are nearly the same as those found in ultrahigh vacuum molecular beam epitaxy.

Site-specific chemistry of carbon tetrachloride decomposition on GaAs(001)

, it was discovered that this molecule dissociatively adsorbs onto Ga dimers and second-layer Ga atoms at 473 K. Upon heating to 673 K, the chlorine desorbs from these sites as GaCl, thereby reducing the Ga coverage. Scanning tunneling micrographs reveal that GaCl desorption produces a highly disordered (2×4)surface, and transforms the (4×2) into a new (3×2) reconstruction. The (3×2) surface appears to be stabilized by carbon incorporation.

The reaction of carbon tetrachloride with gallium arsenide (001)

Carbon tetrachloride dissociatively adsorbs on the Ga-rich (4×2) reconstruction of GaAs (001) at 200 °C. Upon heating to 440 °C, the chlorine desorbs as GaCl, which etches the surface. Scanning tunneling micrographs reveal that this reaction transforms the (4×2) into a Ga-rich (3×2) structure that is interlaced with As-rich (2×4) phases. The (3×2) is well ordered, while the (2×4) phases exhibit a high degree of disorder. This work establishes the surface reaction pathway for carbon doping of GaAs with CCl4.

Observation of the atomic surface structure of GaAs(001) films grown by metalorganic vapor-phase epitaxy

We present atomically resolved scanning tunneling micrographs of the surfaces of GaAs(001) films grown by metalorganic vapor-phase epitaxy (MOVPE). Thin films deposited in an MOVPE reactor were transferred to an (ultra high) vacuum system without air exposure. After heating the samples from 480 to 580°C, high-quality images of the (2 × 4)/c(2 × 8), (1 × 6)/(2 × 6) and (4 × 2)/c(8 × 2) reconstructions were obtained. In addition, a new Ga-rich (3 × 2)/(3 × n) phase was observed that forms during annealing at 540°C. This structure consists of single dimer rows running along the [110] direction with a spacing of 12 Å. The rows vary in length, and are separated by line defects which occur on average every 20 Å ( n = 5). A model is proposed for the (3 × 2) which consists of rows of Ga dimers alternating between the first and third layers. Since this structure exhibits a deficit of one electron, line defects are required to expose As dimers in the second layer and neutralize the surface charge.

O Binding Sites on Stepped Pt(111) Surfaces.

First-principles calculations show that O adatoms are attracted to step edges on Pt(111), gaining 0.2 to 0.3eV per nearest neighbor step-edge Pt atom, and that they favor {open_quote}{open_quote}fcc-like{close_quote}{close_quote} over {open_quote}{open_quote}hcp-like{close_quote}{close_quote} sites by {approximately}0.4 eV. These preferences account for the different appearances, in scanning tunneling micrographs, of O-saturated, {ital A}- and {ital B}-type island edges on Pt(111). They imply that O binds in a three-fold, fcc hollow site behind a {ital B}step, while twofold edge bridging is favored for {ital A} steps. {copyright} {ital 1996 The American Physical Society.}

Scanning tunneling microscopy: Energetics from statistical analysis.

The attraction between two Fe atoms adsorbed on Fe(100) should be much too weak to produce the 0.5--0.7-eV bond that has been deduced by analyzing scanning tunneling micrographs. The assumption that adatom diffusion proceeds by the same mechanism at high and low temperatures may be the source of the discrepancy.