Oliver Warschkow

A single-atom transistor

Over the past decade we have developed a radical new strategy for the fabrication of atomic-scale devices in silicon [1]. Using this process we have demonstrated few electron, single crystal quantum dots [2], conducting nanoscale wires with widths down to ~1.5nm [3] and most recently a single atom transistor [4]. We will present atomic-scale images and electronic characteristics of these atomically precise devices and demonstrate the impact of strong vertical and lateral confinement on electron transport. We will also discuss the opportunities ahead for atomic-scale quantum computing architectures and some of the challenges to achieving truly atomically precise devices in all three spatial dimensions.

The structure and chemistry of the TiO2-rich surface of SrTiO3 (001)

Oxide surfaces are important for applications in catalysis and thin film growth. An important frontier in solid-state inorganic chemistry is the prediction of the surface structure of an oxide. Comparatively little is known about atomic arrangements at oxide surfaces at present, and there has been considerable discussion concerning the forces that control such arrangements. For instance, one model suggests that the dominant factor is a reduction of Coulomb forces; another favours minimization of ‘dangling bonds’ by charge transfer to states below the Fermi energy. The surface structure and properties of SrTiO3—a standard model for oxides with a perovskite structure—have been studied extensively. Here we report a solution of the 2 × 1 SrTiO3 (001) surface structure obtained through a combination of high-resolution electron microscopy and theoretical direct methods. Our results indicate that surface rearrangement of TiO6-x units into edge-sharing blocks determines the SrO-deficient surface structure of SrTiO3. We suggest that this structural concept can be extended to perovskite surfaces in general.

Defect structure studies of bulk and nano-indium-tin oxide

The defect structure of bulk and nano-indium-tin oxide was investigated by a combination of experimental techniques, including high-resolution synchrotron x-ray diffraction, extended x-ray absorption fine structure, and time-of-flight neutron diffraction on powder specimens. The structural results include atomic positions, cation distributions, and oxygen interstitial populations for oxidized and reduced materials. These structural parameters were correlated with theoretical calculations and in situ electrical conductivity and thermopower measurements as well as existing defect models, with special reference to the model of Frank and Kostlin [G. Frank and H. Kostlin, Appl. Phys. A 27, 197 (1982)].

A comprehensive survey of M2AX phase elastic properties

M(2)AX phases are a family of nanolaminate, ternary alloys that are composed of slabs of transition metal carbide or nitride (M(2)X) separated by single atomic layers of a main group element. In this combination, they manifest many of the beneficial properties of both ceramic and metallic compounds, making them attractive for many technological applications. We report here the results of a large scale computational survey of the elastic properties of all 240 elemental combinations using first-principles density functional theory calculations. We found correlations revealing the governing role of the A element and its interaction with the M element on the c axis compressibility and shearability of the material. The role of the X element is relatively minor, with the strongest effect seen in the in-plane constants C(11) and C(12). We identify several elemental compositions with extremal properties such as W(2)SnC, which has by far the lowest value of C(44), suggesting potential applications as a high-temperature dry lubricant.

Atomic-scale structure of the SrTiO3(001)-c(6×2) reconstruction : Experiments and first-principles calculations

The c(6×2) is a reconstruction of the SrTiO3(001) surface that is formed between 1050 and 1100 °C in oxidizing annealing conditions. This work proposes a model for the atomic structure for the c(6×2) obtained through a combination of results from transmission electron diffraction, surface x-ray diffraction, direct methods analysis, computational combinational screening, and density functional theory. As it is formed at high temperatures, the surface is complex and can be described as a short-range-ordered phase featuring microscopic domains composed of four main structural motifs. Additionally, nonperiodic TiO2 units are present on the surface. Simulated scanning tunneling microscopy images based on the electronic structure calculations are consistent with experimental images.

Solution of the p(2 2) NiO(1 1 1) surface structure using direct methods

A solution for the p(2 2) NiO(1 1 1) surface reconstruction was obtained using direct methods applied to X-ray diAraction data. The solution was refined with 296 data points and 21 parameters using v 2 minimization Ov 2 a 1:82; Ra 0:17U. The surface atoms showed very small relaxation from the bulk interatomic distances (Ni‐Ni distances are 2:9 0:1 A; Ni‐Oˇ2:0 0:1 A). The solution can be characterized by alternating close-packed layers of oxygen and nickel atoms: the top surface layer is nickel terminated with 3/4 of the nickel atoms missing, the next oxygen layer is completely full, and the third, nickel layer, has 1/4 of the nickel atoms missing. The structure is consistent with theoretical predictions of octopolar termination of the surface and exhibits the features observed by previous STM studies. In addition, local density functional calculations have been carried out in this work in order to gain insights into the surface charge distribution and electronic structure of the proposed reconstruction. Calculated partial atomic charges and magnetic moments as well as densities of state are reported. The cation deficient nature of the surface requires the presence of electron holes for charge compensation, which we find mainly located on second layer oxygen atoms. The structure diAers from that recently reported for the same surface, and we are not able to reproduce the reported good fit to the (same) experimental data. ” 2000 Elsevier Science B.V. All rights reserved.

The effect of argon on the structure of amorphous SiBCN materials: an experimental and ab initio study

It has previously been noted that the implantation of argon atoms into amorphous SiBCN materials (prepared by magnetron sputtering in various N2+Ar mixtures) leads to variations in a number of material properties; for example an increase in compressive stress. Little is known about the mechanism by which Ar incorporation affects structural and mechanical properties of these materials. Here, we report ab initio molecular dynamics simulations of amorphous SiBCN materials. Using liquid-quench simulations, we investigate how the presence of Ar atoms in the sample affects the liquid quench and the final structure of the material. In the absence of Ar, we find that the final structures are homogenous. The presence of Ar, however, leads to the formation of Si-enriched regions in the vicinity of implanted Ar atoms. This result provides new insight into the role of implanted Ar in the formation of structures in amorphous SiBCN materials. It can also explain the ability of Si to relieve stress generated by these implanted Ar atoms.

Evolution of classical/quantum methodologies: applications to oxide surfaces and interfaces

Abstract First-principles quantum chemical approaches have evolved in the direction of greater precision for describing properties of small molecules, and with reduced precision, to the description of macromolecules and extended systems. However, traditional methodologies are inadequate to meet the increasing demands for time- and temperature-dependent analyses of molecular and particulate structure–function relations. We describe several of the extant hybrid classical/quantum schemes which have been evolving to meet the challenge of bridging size scales from 1 to 1000 A and time scales from 1 to 107 fs. The current state of affairs is illustrated with examples of applications to metal oxide surfaces and interfaces, and future trends are discussed.

Phosphorus δ-doped silicon: mixed-atom pseudopotentials and dopant disorder effects

Within a full density functional theory framework we calculate the band structure and doping potential for phosphorus δ-doped silicon. We compare two different representations of the dopant plane; pseudo-atoms in which the nuclear charge is fractional between silicon and phosphorus, and explicit arrangements employing distinct silicon and phosphorus atoms. While the pseudo-atom approach offers several computational advantages, the explicit model calculations differ in a number of key points, including the valley splitting, the Fermi level and the width of the doping potential. These findings have implications for parameters used in device modelling.

Bottom-up assembly of metallic germanium

Extending chip performance beyond current limits of miniaturisation requires new materials and functionalities that integrate well with the silicon platform. Germanium fits these requirements and has been proposed as a high-mobility channel material, a light emitting medium in silicon-integrated lasers, and a plasmonic conductor for bio-sensing. Common to these diverse applications is the need for homogeneous, high electron densities in three-dimensions (3D). Here we use a bottom-up approach to demonstrate the 3D assembly of atomically sharp doping profiles in germanium by a repeated stacking of two-dimensional (2D) high-density phosphorus layers. This produces high-density (1019 to 1020 cm−3) low-resistivity (10−4Ω · cm) metallic germanium of precisely defined thickness, beyond the capabilities of diffusion-based doping technologies. We demonstrate that free electrons from distinct 2D dopant layers coalesce into a homogeneous 3D conductor using anisotropic quantum interference measurements, atom probe tomography, and density functional theory.