Gold-free GaAs/GaAsSb heterostructure nanowires grown on silicon

Growth of GaAs/GaAsSb heterostructurenanowires on silicon without the need for gold seed particles is presented. A high vertical yield of GaAsnanowires is first obtained, and then GaAsₓSb₁ˍₓ segments are successfully grown axially in these nanowires. GaAsSb can also be integrated as a shell around the GaAs core. Finally, two GaAsSb segments are grown inside a GaAsnanowire and passivated using an AlₓGa₁ˍₓAs shell. It is found that no stacking faults or twin planes occur in the GaAsSb segments.Part of this work was funded by the Swedish Foundation for Strategic Research SSF, the Swedish Research Council VR, and the Knut and Alice Wallenberg Foundation

2D Rutherford-Like Scattering in Ballistic Nanodevices

Ballistic injection in a nanodevice is a complex process where electrons can either be transmitted or reflected, thereby introducing deviations from the otherwise quantized conductance. In this context, quantum rings (QRs) appear as model geometries: in a semiclassical view, most electrons bounce against the central QR antidot, which strongly reduces injection efficiency. Thanks to an analogy with Rutherford scattering, we show that a local partial depletion of the QR close to the edge of the antidot can counter-intuitively ease ballistic electron injection. On the contrary, local charge accumulation can focus the semi-classical trajectories on the hard-wall potential and strongly enhance reflection back to the lead. Scanning gate experiments on a ballistic QR, and simulations of the conductance of the same device are consistent, and agree to show that the effect is directly proportional to the ratio between the strength of the perturbation and the Fermi energy. Our observation surprisingly fits the simple Rutherford formalism in two-dimensions in the classical limit

On the imaging of electron transport in semiconductor quantum structures by scanning-gate microscopy: successes and limitations

This paper presents a brief review of scanning-gate microscopy applied to the imaging of electron transport in buried semiconductor quantum structures. After an introduction to the technique and to some of its practical issues, we summarise a selection of its successful achievements found in the literature, including our own research. The latter focuses on the imaging of GaInAs-based quantum rings both in the low magnetic field Aharonov-Bohm regime and in the high-field quantum Hall regime. Based on our own experience, we then discuss in detail some of the limitations of scanning-gate microscopy. These include possible tip induced artefacts, effects of a large bias applied to the scanning tip, as well as consequences of unwanted charge traps on the conductance maps. We emphasize how special care must be paid in interpreting these scanning-gate images.Comment: Special issue on (nano)characterization of semiconductor materials and structure

Formation of quantum dots in the potential fluctuations of InGaAs heterostructures probed by scanning gate microscopy

The disordered potential landscape in an InGaAs/InAlAs two-dimensional electron gas patterned into narrow wires is investigated by means of scanning gate microscopy. It is found that scanning a negatively charged tip above particular sites of the wires produces conductance oscillations that are periodic in the tip voltage. These oscillations take the shape of concentric circles whose number and diameter increase for more negative tip voltages until full depletion occurs in the probed region. These observations cannot be explained by charging events in material traps, but are consistent with Coulomb blockade in quantum dots forming when the potential fluctuations are raised locally at the Fermi level by the gating action of the tip. This interpretation is supported by simple electrostatic simulations in the case of a disorder potential induced by ionized dopants. This work represents a local investigation of the mechanisms responsible for the disorder-induced metal-to-insulator transition observed in macroscopic two-dimensional electron systems at low enough density

Scanning Gate Spectroscopy of transport across a Quantum Hall Nano-Island

We explore transport across an ultra-small Quantum Hall Island (QHI) formed by closed quan- tum Hall edge states and connected to propagating edge channels through tunnel barriers. Scanning gate microscopy and scanning gate spectroscopy are used to first localize and then study a single QHI near a quantum point contact. The presence of Coulomb diamonds in the spectroscopy con- firms that Coulomb blockade governs transport across the QHI. Varying the microscope tip bias as well as current bias across the device, we uncover the QHI discrete energy spectrum arising from electronic confinement and we extract estimates of the gradient of the confining potential and of the edge state velocity.Comment: 13 pages, 3 figure

Failing to Place Confrontation: The Car as “Void” in Jump

Whereas moments of miscommunication stud the collection Jump, this article proposes to read five such misfiring encounters between whites and non-whites, in “What Were You Dreaming?” “Comrades,” “Keeping Fit,” “The Moment Before the Gun Went Off,” and “Spoils,” through a close examination of the spatial configurations where a confrontation can happen (and fail). In all five short stories the situation of racial inequality is mediated through banal elements: the car, food/drink, and the delivering of a testimony. I will connect these elements to the unresolved opposition cast by Agamben between bios, the political life, and zoe, the biological life. I argue that in these confrontation scenes the “others” are excluded through their very inclusion, but that they are also recognized as a political presence in the privileged site of vehicles functioning in the text as correlatives of what Agamben describes as a “void” in social spaces

Inhomogeneous Si-doping of gold-seeded InAs nanowires grown by molecular beam epitaxy

We have investigated in-situ Si doping of InAs nanowires grown by molecular beam epitaxy from gold seeds. The effectiveness of n-type doping is confirmed by electrical measurements showing an increase of the electron density with the Si flux. We also observe an increase of the electron density along the nanowires from the tip to the base, attributed to the dopant incorporation on the nanowire facets whereas no detectable incorporation occurs through the seed. Furthermore the Si incorporation strongly influences the lateral growth of the nanowires without giving rise to significant tapering, revealing the complex interplay between axial and lateral growth.This work was supported by the ANR through the Project No. ANR-11-JS04-002-01, and the Ministry of Higher Education and Research, Nord-Pas de Calais Regional Council and FEDER through the “Contrat de Projets Etat Region (CPER) 2007-2013.” P.C. is the recipient of an Australian Research Council Future Fellowship (project number FT120100498)

Esthétique des marges, exploration du non-lieu : Tiepolo’s Hound de Derek Walcott

Si c’est en tant que poète et dramaturge que Derek Walcott, né à Sainte-Lucie (Petites Antilles) en 1930, s’est fait connaître, puis s’est vu décerner le Prix Nobel de littérature en 1992, il n’en a pas moins longtemps hésité entre la plume et le pinceau. Son père, mort en 1931 et qu’il n’a pas vraiment connu, était peintre lui-même, et Derek n’a cessé de dessiner, illustrant certains de ses livres et peignant les décors de ses mises en scène. Ainsi, nul mieux que lui ne sait combien l’art a ..

Scanning-gate microscopy of semiconductor nanostructures: an overview

This paper presents an overview of scanning-gate microscopy applied to the imaging of electron transport through buried semiconductor nanostructures. After a brief description of the technique and of its possible artifacts, we give a summary of some of its most instructive achievements found in the literature and we present an updated review of our own research. It focuses on the imaging of GaInAs-based quantum rings both in the low magnetic field Aharonov-Bohm regime and in the high-field quantum Hall regime. In all of the given examples, we emphasize how a local-probe approach is able to shed new, or complementary, light on transport phenomena which are usually studied by means of macroscopic conductance measurements.Comment: Invited talk by SH at 39th "Jaszowiec" International School and Conference on the Physics of Semiconductors, Krynica-Zdroj, Poland, June 201

Imaging Electron Wave Functions Inside Open Quantum Rings

Combining Scanning Gate Microscopy (SGM) experiments and simulations, we demonstrate low temperature imaging of electron probability density $|\Psi|^{2}(x,y)$ in embedded mesoscopic quantum rings (QRs). The tip-induced conductance modulations share the same temperature dependence as the Aharonov-Bohm effect, indicating that they originate from electron wavefunction interferences. Simulations of both $|\Psi|^{2}(x,y)$ and SGM conductance maps reproduce the main experimental observations and link fringes in SGM images to $|\Psi|^{2}(x,y)$.Comment: new titl