Showing papers by "Peter H. Beton published in 1996"

Resonant magnetotunneling through individual self-assembled InAs quantum dots.

Abstract: A single-barrier GaAs/AlAs/GaAs heterostructure, with self-assembled In-based quantum dots incorporated in the AlAs tunnel barrier, exhibits a series of resonant peaks in the low temperature current‐voltage characteristics. We argue that each peak arises from singleelectron tunneling through the discrete zero-dimensionalstate of an individual InAs dot. We use the tunneling for fine probing of the local density of states in the emitter-accumulation layer. Landau-quantized states are resolved at magnetic field Bk I as low as 0.2 T. Spinsplitting of the dot electron states has been observed for B? I . c 1997 Academic Press Limited

Adsorbed and substituted Sb dimers on GaAs(001).

Abstract: Department of Physics and Astronomy, University of Wales College of Cardiff, Cardiff CF1 3AT, United Kingdom~Received 1 February 1996!The surface reconstructions formed on Sb-terminated GaAs~001! have been investigated using scanningtunneling microscopy ~STM!. Following Sb deposition on the ~234! reconstructed GaAs~001! surface, ~134!,~133!, and ~234! phases are observed in order of increasing annealing temperature. Structural models forthese phases are discussed based on an examination of the STM images and previously published core-levelphotoemission data. Considerable differences exist between the unit cell structure and ordering on the cleanGaAs~001! and Sb-induced~234! surfaces. Following desorption of Sb at 580 °C, a~432! structure is ob-served that appears to be identical to the reconstruction produced by annealing the clean GaAs~001! surface atthis temperature. This result shows that a previously proposed model for the clean GaAs~001!-~432! phase isincorrect. @S0163-1829~96!52624-X#

(2×4)/c(2×8) to (4×2)/c(8×2) transition on GaAs(001) surfaces

Abstract: We present scanning tunneling microscopy data illustrating the evolution of the decapped GaAs(001) surface following annealing in stages from 450 to 540 °C. After annealing at 450 °C a (2×4) reconstruction is formed by kinked rows of two As dimer unit cells. Following annealing in the 475–500 °C range small isolated regions of (4×2) reconstruction are visible, with a considerable increase in disorder of the remaining (2×4) reconstructed areas. Annealing at higher temperatures causes the (4×2) structure to become increasingly dominant. We have noted significant differences in the surface morphology as a function of annealing time. Our images of the (4×2) surface are similar to those recently reported by other groups but we propose a new structural model.

Passivation of Si(111)‐7×7 by a C60 monolayer

Abstract: C60 monolayers are formed on a Si(111)‐7×7 surface under ultrahigh vacuum (UHV) conditions. The effects of exposure to atmosphere (for 30 min) and water (for 30 s) are assessed by comparing images of the surface acquired using an UHV scanning tunneling microscope. Following exposure and/or immersion we are able to resolve the C60 molecules exhibiting hexagonal order in an arrangement which is essentially identical to that formed prior to withdrawal from the UHV system. Our results clearly show that deposition of one monolayer of C60 on a Si surface can inhibit chemical attack by water and atmospheric oxygen.

C60 manipulation and cluster formation using a scanning tunneling microscope

Abstract: We have used the tip of an ultrahigh vacuum scanning tunneling microscope to induce displacements of C60 molecules on the Si(111)‐7×7 surface at room temperature. The manipulation is achieved by using a sweeping procedure we have developed which moves the tip closer to the surface and sweeps it across in a predetermined direction. Feedback control of the tunnel current is maintained throughout and the tip‐surface separation is adjusted by changing the sample bias and tunnel current. For a 0.007 monolayer (ML) coverage of C60, a sweeping area of 60 A×60 A was used to move individual C60 molecules, while for higher coverages (0.05–0.2 ML) a sweeping area of 216 A×216 A was used to move large groups of C60 molecules. We show an example at 0.2 ML coverage where we have removed C60 over an area 110 A×370 A resulting in the formation of a line of C60 molecules 20–30 A in width.

Island, trimer, and chain formation on the Sb‐terminated GaAs(111)B surface

Abstract: The surface structures resulting from the deposition of various coverages of Sb on the GaAs(111)B‐(2×2) surface at room temperature, followed by annealing in the 100–375 °C temperature range, have been investigated using scanning tunneling microscopy. At low annealing temperatures Sb islands are observed displaying no ordered atomic structure, between which the As trimer based (2×2) reconstruction of the clean GaAs surface is visible. Annealing above 250 °C causes the formation of Sb trimers which are distinguishable from the remaining As trimers via contrast differences in filled‐ and empty‐state images. Annealing at still higher temperatures leads to the creation of Sb chain pairs oriented along the substrate 〈110〉 directions, coexisting with regions of Sb trimer based reconstruction between the chain pairs. The local periodicity of the patterns of Sb trimers between the chain pairs is dependent on the separation between chain pairs.

STM investigation and manipulation of molecules adsorbed on an Si(111) surface

Abstract: We have used an ultra-high-vacuum (UHV) scanning tunnelling microscope (STM) to image molecules adsorbed on a Si(111) surface. At low coverage ( monolayers) molecules are adsorbed at random sites. For coverages close to a monolayer they are partially ordered in a hexagonal arrangement. Second- and higher-layer islands, in which the molecules are clearly resolved, are observed at higher coverage. These islands may be desorbed by annealing in the range , leaving an Si surface terminated by a monolayer. This surface is stable to exposure to air and immersion in water. In addition, recent work on manipulation of molecules at various coverages is reviewed and results relating to tip alignment in UHV are discussed.

Fabrication of Si nanostructures by controlled sidewall oxidation

Abstract: We have fabricated Si dots and wires with widths as small as 2 nm by thermally oxidizing an etched mesa in a controlled manner. Electrical contacts are deposited on the Si nanostructures so that their current-voltage characteristics may be measured. For dots fabricated from wafers with a n-i-n doping profile we observe alinear dependence of the room temperature conductance on the device diameter, which we explain in terms of the profile of the SiSiO2 interface. We are able to extract the width of the unoxidized Si from this dependence and find that the I(V) of devices with active widths less than 10 nm measured at 77 K is non-linear. I(V) measurements are also presented for bar shaped devices fabricated from pn material which clearly show that rectification occurs, even for active widths less than 10 nm.