The Rashba Hamiltonian and electron transport

The Rashba Hamiltonian describes the splitting of the conduction band as a result of spin-orbit coupling in the presence of an external field and is commonly used to model the electronic structure of confined narrow-gap semiconductors. Due to the mixing of spin states some care has to be exercised in the calculation of transport properties. We derive the velocity operator for the Rashba-split conduction band and demonstrate that the transmission of an interface between a ferromagnet and a Rashba-split semiconductor does not depend on the magnetization direction, in contrast with previous assertions in the literature.Comment: one tex file, two figures; paper to appear in this form in PRB (RC

Magnetotunneling Between Two-dimensional Electron Gases in InAs-AlSb-GaSb Heterostructures

We have observed that the tunneling magnetoconductance between two-dimensional (2D) electron gases formed at nominally identical InAs-AlSb interfaces most often exhibits two sets of Shubnikov-de Haas oscillations with almost the same frequency. This result is explained quantitatively with a model of the conductance in which the 2D gases have different densities and can tunnel between Landau levels with different quantum indices. When the epitaxial growth conditions of the interfaces are optimized, the zero-bias magnetoconductance shows a single set of oscillations, thus proving that the asymmetry between the two electron gases can be eliminated.Comment: RevTeX format including 4 figures; submit for publicatio

Radiation-induced oscillatory magnetoresistance as a sensitive probe of the zero-field spin splitting in high mobility GaAs/AlGaAs devices

We suggest an approach for characterizing the zero-field spin splitting of high mobility two-dimensional electron systems, when beats are not readily observable in the Shubnikov-de Haas effect. The zero-field spin splitting and the effective magnetic field seen in the reference frame of the electron is evaluated from a quantitative study of beats observed in radiation-induced magnetoresistance oscillations.Comment: 4 pages, 4 color figure

Spin transport of electrons through quantum wires with spatially-modulated strength of the Rashba spin-orbit interaction

We study ballistic transport of spin-polarized electrons through quantum wires in which the strength of the Rashba spin-orbit interaction (SOI) is spatially modulated. Subband mixing, due to SOI, between the two lowest subbands is taken into account. Simplified approximate expressions for the transmission are obtained for electron energies close to the bottom of the first subband and near the value for which anticrossing of the two lowest subbands occurs. In structures with periodically varied SOI strength, {\it square-wave} modulation on the spin transmission is found when only one subband is occupied and its possible application to the spin transistor is discussed. When two subbands are occupied the transmission is strongly affected by the existence of SOI interfaces as well as by the subband mixing

Coherent spin valve phenomena and electrical spin injection in ferromagnetic/semiconductor/ferromagnetic junctions

Coherent quantum transport in ferromagnetic/ semiconductor/ ferromagnetic junctions is studied theoretically within the Landauer framework of ballistic transport. We show that quantum coherence can have unexpected implications for spin injection and that some intuitive spintronic concepts which are founded in semi-classical physics no longer apply: A quantum spin-valve (QSV) effect occurs even in the absence of a net spin polarized current flowing through the device, unlike in the classical regime. The converse effect also arises, i.e. a zero spin-valve signal for a non-vanishing spin-current. We introduce new criteria useful for analyzing quantum and classical spin transport phenomena and the relationships between them. The effects on QSV behavior of spin-dependent electron transmission at the interfaces, interface Schottky barriers, Rashba spin-orbit coupling and temperature, are systematically investigated. While the signature of the QSV is found to be sensitive to temperature, interestingly, that of its converse is not. We argue that the QSV phenomenon can have important implications for the interpretation of spin-injection in quantum spintronic experiments with spin-valve geometries.Comment: 15 pages including 11 figures. To appear in PR

High-GF planar aluminium-silicon hybrid strain transducers

We demonstrate here a simple planar aluminium-silicon strain sensor incorporating an external aluminium ohmic shunt (metal-semiconductor hybrid) which exhibits a geometrically enhanced room-temperature gauge factor (GF) of up to 843 under uni-axial tensile strains of up of 8 times 10 -5 for a silicon p-type doping level of 1 times 10 17 cm -3 . We also show that the GF is dependent on the silicon doping density; a GF of 317 being demonstrated for a p-type doping density of 1 times 10 20 cm -3 . Moreover a GF well above 1000 is possible in more lightly doped samples. The observed behaviour, to be contrasted with the gauge factor of -93 observed in homogeneous p-type silicon, is the result of the stress-induced anisotropy in the silicon conductivity that acts to deflect the current away from the metallic shunt for tensile strains

Piezoresistance in Silicon at Uniaxial Compressive Stresses up to 3 GPa

The room-temperature longitudinal piezoresistance of n-type and p-type crystalline silicon along selected crystal axes is investigated under uniaxial compressive stresses up to 3 GPa. While the conductance (G) of n-type silicon eventually saturates at about 45% of its zero-stress value (G0) in accordance with the charge transfer model, in p-type material G=G0 increases above a predicted limit of about 4.5 without any significant saturation, even at 3 GPa. Calculation of G=G0 using ab initio density functional theory reveals that neither G nor the mobility, when properly averaged over the hole distribution, saturate at stresses lower than 3 GPa. The lack of saturation has important consequences for strained-silicon technologies

Giant, Anomalous Piezoimpedance in Silicon-on-insulator

International audienceA giant, anomalous piezoresponse of fully depleted silicon-on-insulator devices under mechanical stress is demonstrated by impedance spectroscopy. This piezoresponse strongly depends on the measurement frequency, ω, and consists of both a piezoresistance (PZR) and a piezocapacitance, whose maximum values are −1100 × 10−11 and −900 × 10−11 Pa−1, respectively. These values should be compared withthe usual bulk PZR in p-type silicon, 70 × 10−11 Pa−1. The observations are well described by models of space-charge-limited hole currents in the presence of fast electronic traps having stress-dependent capture rates (ωc) and emission rates. Under steady-state conditions (i.e., when ω ωc), where the impedancespectroscopy measurements yield results that are directly comparable with those of previously published reports of PZR in depleted, silicon nano-objects, the overall piezoresponse is just the usual, bulk silicon PZR. Anomalous PZR is observed only under non-steady-state conditions when ω ≈ ωc, with a symmetry suggesting that the electromechanically active fast traps are native Pb0 interface defects. The observations suggest new functionalities for fully depleted silicon-on-insulator, and shed light on the debate over the PZR of carrier-depleted nanosilicon

Mechanical stress dependence of the Fermi level pinning on an oxidized silicon surface

International audienceA combination of micro-Raman spectroscopy and micro-XPS (X-ray photo-electron spectroscopy) mapping on statically deflected p-type silicon cantilevers is used to study the mechanical stress dependence of the Fermi level pinning at an oxidized silicon (001) surface. With uniaxial compressive and tensile stress applied parallel to the ⟨110⟩ crystal direction, the observations are relevant to the electronic properties of strain-silicon nano-devices with large surface-to-volume ratios such as nanowires and nanomembranes. The surface Fermi level pinning is found to be even in applied stress, a fact that may be related to the symmetry of the Pb0 silicon/oxide interface defects. For stresses up to 240 MPa, an increase in the pinning energy of 0.16 meV/MPa is observed for compressive stress, while for tensile stress it increases by 0.11 meV/MPa. Using the bulk, valence band deformation potentials the reduction in surface band bending in compression (0.09 meV/MPa) and in tension (0.13 meV/MPa) can be estimated