Showing papers on "Quantum tunnelling published in 1989"

Conductance and exchange coupling of two ferromagnets separated by a tunneling barrier.

Abstract: A theory is given for three closely related effects involving a nonmagnetic electron-tunneling barrier separating two ferromagnetic conductors. The first is Julliere's magnetic valve effect, in which the tunnel conductance depends on the angle \ensuremath{\theta} between the moments of the two ferromagnets. One finds that discontinuous change of the potential at the electrode-barrier interface diminishes the spin-polarization factor governing this effect and is capable of changing its sign. The second is an effective interfacial exchange coupling -J cos\ensuremath{\theta} between the ferromagnets. One finds that the magnitude and sign of J depend on the height of the barrier and the Stoner splitting in the ferromagnets. The third is a new, irreversible exchange term in the coupled dynamics of the ferromagnets. For one sign of external voltage V, this term describes relaxation of the Landau-Lifshitz type. For the opposite sign of V, it describes a pumping action which can cause spontaneous growth of magnetic oscillations. All of these effects were investigated consistently by analyzing the transmission of charge and spin currents flowing through a rectangular barrier separating free-electron metals. In application to Fe-C-Fe junctions, the theory predicts that the valve effect is weak and that the coupling is antiferromagnetic (Jl0). Relations connecting the three effects suggest experiments involving small spatial dimensions.

Self-consistent study of the resonant-tunneling diode

Abstract: Quantum transport in the resonant-tunneling diode (RTD) is modeled here with the Wigner formalism including self-consistent potentials for the first time. We examine the computational aspects of the Wigner-function approach and the boundary conditions for the model. The calculated I-V characteristics show an intrinsic bistability in the negative-differential-conductivity region of the curve. Intrinsic bistability results from charge storage and the subsequent shifting of the internal potential of the device. The cathode region of the RTD shows a strong depletion and quantization of electrons in a deep triangular potential well, which reduces the barrier height to a ballistic electron injected from the cathode, enhancing the valley current and reducing the peak-to-valley ratio. Undoped spacer layers prevent the formation of a deep quantum well at the cathode barrier, and the distribution does not deplete as sharply as without the spacer layer. The I-V curve with the spacer layers shows a much lower negative resistance, and a sharper bistable region. A finite relaxation time for the electrons increases the negative resistance, reduces the peak-to-valley ratio of the current, and causes a ``soft'' hysteresis in the bistable region. A zero-bias anomaly is found to result from high-momentum tails in the distribution at the barrier interface. These high-momentum tails contribute a small high-conductance current. The transient current during switching from the peak to the valley of the I-V curve shows inductive behavior and negative resistance for frequencies below 2 THz.

Oscillations up to 420 GHz in GaAs/AlAs resonant tunneling diodes

Abstract: We report room‐temperature oscillations up to frequencies of 420 GHz in a GaAs resonant tunneling diode containing two 1.1‐nm‐thick AlAs barriers. These results are consistent with a recently proposed equivalent circuit model for these diodes in which an inductance accounts for the temporal delay associated with the quasibound‐state lifetime. They are also in accordance with a generalized impedance model, described here, that includes the effect of the transit time delay across the depletion layer. Although the peak‐to‐valley ratio of the 420 GHz diode is only 1.5:1 at room temperature, we show that its speed is limited by the parasitic series resistance rather than by the low negative conductance. A threefold reduction in this resistance, along with a comparable increase in the peak‐to‐valley ratio, should allow oscillations up to about 1 THz.

Determination of the local electronic structure of atomic‐sized defects on Si(001) by tunneling spectroscopy

Abstract: Tunneling spectroscopy and voltage‐dependent scanning tunneling microscopy have been used to study the geometry and electronic properties of atomic‐sized defects on the Si(001) surface. Individual dimer vacancies are shown to be semiconducting, consistent with the π‐bonded defect model of Pandey. Another type of characteristic defect is found which gives rise to strongly metallic tunneling I–V characteristics, demonstrating that it has a high density of states at the Fermi level and is likely active in Fermi level pinning on Si(001). Spatially dependent I–V measurements and tunneling barrier height measurements also directly reveal the spatial extent of this metallic character and provide direct measures of the ‘‘size’’ of the defects.

High‐resolution near‐infrared spectroscopy of water dimer

Abstract: High-resolution near-infrared spectra are reported for all of the O-H stretch vibrational bands of the water dimer. The four O-H vibrations are characterized as essentially independent proton-donor or proton-acceptor motions. In addition to the rotational and vibrational information contained in these spectra, details are obtained concerning the internal tunneling dynamics in both the ground and excited vibrational states. These results show that, for tunneling motions which involve the interchange of the proton donor and acceptor molecules, the associated frequencies decrease substantially due to vibrational excitation. The predissociation lifetimes for the various states of the dimer are determined from linewidth measurements. These results clearly show that the predissociation dynamics is strongly dependent on the tunneling states, as well as the Ka quantum number, indicating that the internal tunneling dynamics plays an important role in determining the dissociation rate in this complex.

New negative differential resistance device based on resonant interband tunneling

Abstract: We propose and demonstrate a novel negative differential resistance device based on resonant interband tunneling. Electrons in the InAs/AlSb/GaSb/AlSb/InAs structure tunnel from the InAs conduction band into a quantized state in the GaSb valence band, giving rise to a peak in the current-voltage characteristic. This heterostructure design virtually eliminates many of the competing transport mechanisms which limit the performance of conventional double-barrier structures. Peak-to-valley current ratios as high as 20 and 88 are observed at room temperature and liquid-nitrogen temperature, respectively. These are the highest values reported for any tunnel structure.

Intersubband emission from semiconductor superlattices excited by sequential resonant tunneling

Abstract: We present the first observation of infrared light emission from a semiconductor superlattice in a resonant-tunneling experiment. Radiation from the three lowest intersubband transitions is observed, proving resonant tunneling an effective means of populating high-lying states of the superlattice. The relative strength of the emission lines enables us to estimate the electron temperature below the optical-phonon energy to reach about 140 K, whereas transitions originating above the optical-phonon energy are strongly quenched.

Enhanced Photon Emission in Scanning Tunnelling Microscopy.

Abstract: We have investigated the inverse photoelectric effect stimulated by electron tunnelling in the scanning tunnelling microscope (STM). Light emission spectra are reported for polycrystalline silver films. In accordance with similar observations for other tunnelling devices, the high-energy threshold of the spectral distributions has been found to equal the voltage between tip and sample. Characteristic maxima of the photon emission at ~1.9 eV and ~2.4 eV, independent of tunnel voltage, are attributed to resonant excitation and radiative decay of localized surface plasmon polariton modes at the silver surface. This represents the first observation of plasmon excitations with the STM and provides unambiguous evidence of local inelastic processes occurring on silver surfaces. We propose that our novel results may have particular relevance to the interpretation of optical phenomena on the atomic scale.

Resonant interband tunnel diodes

Abstract: Novel device structures are proposed, incorporating quantum wells and a pn diode structure. Such a device combines the structure and behavior of both resonant tunneling diodes and conventional tunnel diodes, leading to high speed and low excess current. There is interband tunneling between the conduction band and the valence band, as is in the case for a tunnel diode, but carriers are confined within quantum wells. Under small forward bias the diodes are expected to behave in a manner very similar to that of a tunnel diode formed of bulk material. Under large forward bias, however, the devices act much like resonant tunneling diodes, and display additional negative resistance regions.

Realization of a three-terminal resonant tunneling device: The bipolar quantum resonant tunneling transistor

Abstract: A new three‐terminal resonant tunneling structure in which current transport is controlled by directly modulating the potential of the quantum well is proposed and demonstrated Typical current gains of 50 at room temperature are observed

Single-electron tunnel junction array: an electrostatic analog of the Josephson transmission line

Abstract: The authors carried out static and dynamic analyses of uniform one-dimensional arrays of ultrasmall tunnel junctions. The correlated single-electron tunneling in the junctions of the array results in a behavior qualitatively similar to that of the Josephson transmission line. In particular, external electric fields applied to the array edges can inject single-electron-charged solitons into the array interior. The shape of such a soliton and the character of its interactions with other solitons and the array edge are very similar to those of Josephson vortices (sine-Gordon solitons) in the Josephson transmission line. Under certain conditions, a coherent motion of the soliton train along the array is possible, resulting in generation of narrowband SET (single-electron tunneling) oscillations with frequency f=/e, where is the DC current flowing along the array. >

A theoretical study of the dissociation of H2/Cu

Abstract: We present calculations for the dissociative adsorption of hydrogen molecules on a Cu surface as a function of initial translational energy and vibrational quantum state. Classical, semiclassical, and fully quantum calculations are performed and the results compared. The potential energy surface was based upon a total energy calculation for H2 on a small Cu cluster and has been previously employed in dynamical simulations. Our results show that for low primary beam energies, dissociation occurs primarily via tunneling through the activation barrier in the vibrational coordinate. Populating the initial vibrational states is shown to enhance reactivity, but not simply by a total energy shift. By changing the hydrogen isotope it is shown that tunneling effects can persist up to quite high molecular masses. This occurs because the activation barrier lies in the vibrational coordinate, where the reduced mass of the molecule determines the dynamics.

Conduction in granular metals-variable-range hopping in a Coulomb gap?

Abstract: The author sets out the model for conduction in disordered systems by variable-range tunnelling in a Coulomb gap. Detailed analysis of experimental data for three different types of granular metal shows that the model cannot apply to these systems. He concludes that the problem of the mechanism of conduction in granular metals remains unsolved.

Tunneling of molecules in low-temperature media: an elementary description

Abstract: The authors discuss the influence of the environment on the tunneling of molecules at low temperature, using elementary quantum mechanics. We show that even in the gas phase dephasing processes can render the tunneling incoherent and that the tunneling rate due to dephasing is often enormously decreased relative to the unperturbed period. In addition to these effects, in the condensed phase, a renormalization of the effective tunneling matrix element occurs.

Effect of high-frequency electrodynamic environment on the single-electron tunneling in ultrasmall junctions

Abstract: Single-electron tunneling in Al/${\mathrm{AlO}}_{\mathrm{x}}$/Al junctions with areas below 0.01 \ensuremath{\mu}${\mathrm{m}}^{2}$ was studied at temperatures close to 1 K. The junctions, placed in different high-frequency environments but similar in all other aspects, exhibited different dc I-V curves, in accordance with the theory of correlated single-electron tunneling. Our results imply that a tunneling electron can effectively probe its electrodynamic environment at distances much larger than c${\mathrm{\ensuremath{\tau}}}_{\mathrm{t}}$, where ${\mathrm{\ensuremath{\tau}}}_{\mathrm{t}}$ is the ``traversal'' time of its passage through the energy barrier.

Demonstration of the tunnel-diode effect on an atomic scale

Abstract: THE tunnel diode1, which is widely used in high-speed electronics applications2, depends on the property of negative differential conductivity, that is, a negative slope in the current–voltage curve. The mechanism underlying the tunnel diode's behaviour, namely the existence of a range of biases for which tunnelling is forbidden or suppressed following a bias for which tunnelling is strongly favoured, has been employed subsequently in the design of new devices that also display the conductance anomaly, such as the double-barrier resonant-tunnelling device3. It has been predicted4 that the conductance anomaly could result from a similar mechanism at the tunnel junction of the scanning tunnelling microscope (STM), where localized states on adsorbate and tip atoms give rise to allowed and suppressed energies for tunnelling. The STM has the capability to image regions of negative differential conductivity induced by individual atoms on a surface. Here we report the observation of negative differential conductivity on particular binding sites of a Si (111) surface doped with boron. Specific current–voltage characteristics are shown to be related to the presence or absence of the dopant at individual atomic sites, and negative differential conductivity is observed at –1.4 V tip bias at a specific type of site. Tunnelling spectroscopy indicates that the effect results from a tunnel-diode mechanism.

Spectroscopy of semiconductor microstructures

Abstract: MBE Growth of Custom-Designed III-V Semiconductor Microstructures Scaled to the Physical Limit: Ultrathin Layer Superlattices and Monolayer Doping.- MOCVD - Grown Atomic Layer Superlattices.- Aspects of the Growth of InP/InGaAs Multi-Quantum Well Structures by Gas Source Molecular Beam Epitaxy.- Quantum Wires and Related Superstructures.- Spectroscopy of One-Dimensional Electronic Systems.- Electrons in Lateral Microstructures on Indium Antimonide.- Ballistic Transport in Quasi-One-Dimensional Structures.- Phonons in Superlattices.- Resonance Raman Scattering in Short Period GaAs - AlAs Superlattices.- Phonons and Optical Properties of Si / Ge Superlattices.- Characterisation of Ge - Si Interfaces and Ultra-thin Ge layers by Raman Scattering.- Phonons of Ideal (001) Superlattices: Application to Si / Ge.- Calculations of Phonons in Superlattices. The Ge / Si Superlattices along [001].- Structural and Optical Properties of Periodic Fibonacci Superlattices.- Raman Scattering from Phonons in Quasiperiodic Superlattices Based on Generalizations of the Fibonacci Sequence.- Can We Tune the Band Offset at Semiconductor Heterojunctions?.- Optical Properties and Band Alignments of III-V Heterostructures.- Indirect GaAs/AIAs Superlattices.- Optical Testing of Probability Densities in Quantum Well Eigenstates.- Photoemission from A1GaAs/GaAs Heterojunctions and Quantum Wells under Negative Electron Affinity Conditions.- Wannier - Stark Quantization and Bloch Oscillator in Biased Semiconductor Superlattices.- Electronic Raman Scattering in Photoexcited Quantum Wells: Field Effects and Charge-Density Domains.- Linear and Nonlinear Optics of Confined Excitons.- Magneto-Excitons in GaAs/GaAlAs Quantum Wells.- Anisotropy of Magneto-Optical Properties of (Al, Ga)As Quantum Wells.- Magneto-Optical Study of Miniband Dispersion and Excitonic Effects in GaAs/GaAIAs Superlattices.- Magnetotunnelling in Semiconductor Microstructures.- Properties of a Dense Quasi Two Dimensional Electron-Hole Gas at High Magnetic Fields.- Magneto - Absorption in a Hg1?xXMnxTe - CdTe Superlattice: Evidence of the Exchange Interaction in a Semimagnetic Semiconducting Superlattice.- Polaron Coupling in GaAs - GaA1As Heterostructures observed by Cyclotron and Magnetophonon Resonance.- Cyclotron Resonance of 2D Electron Systems in Intentionally Doped A1GaAs/GaAs Quantum Wells and Heterostructures.- Magnetophonon Resonance Condition in Quasi One- and Two-Dimensional Electron Systems.- Interaction of Surface Acoustic Waves with Two-Dimensional Electron Systems in GaAs/AlGaAs Heterojunctions.- Screening of Impurities in the Quantum Hall Regime.- Ultrafast Luminescence Studies of Tunneling in Semiconductor Microstructures.- Monte-Carlo Simulation of Femtosecond Carrier Relaxation in Semiconductor Quantum Wells.- Ultrafast Transport Measurements in Bulk Semiconductors and Tunneling Devices Using Electro-Optic Sampling.- Optical Transport Experiments in Heterostructures.- Optical Properties of GaAs / AlAs Superlattices.- Hot Phonons in Microstructures.- Contributors.- Participants.

On the modelling of tunnelling currents in reverse-biased p-n junctions

Abstract: It is experimentally shown that the conventional expression for Zener tunnelling can be used as a model for tunnelling in reverse-biased p - n junctions, provided that the maximum electric field at the junction is used in the expression and not, as sometimes suggested in the literature, an average field. It is shown that this distinction can only be made by considering different types of p - n junctions. This finding is supported theoretically by taking the actual electric field distribution near the junction into account in the derivation of the dependence of the tunnelling current on the applied junction voltage. The implications for incorporation of this model in a numerical device simulation package are briefly discussed.

Scanning tunnelling microscopy of Z-DNA.

Abstract: SCANNING tunnelling microscopy (STM) has been used to map the surface topography of inorganic materials at the atomic level, and is potentially one of the most powerful techniques for probing biomolecular structure1–3. Recent STM studies of calf thymus DNA4,5and poly(rA) · poly(rU)5 have shown that the helical pitch and periodic alternation of major and minor grooves can be visualized and reliably measured. Here we present the first STM images of poly(dG-me5d) · poly(dG-me5dC) in the Z-form. Both the general appearance of the fibres and measurements of helical parameters are in good agreement with models derived from X-ray diffraction6–4.

Physics and applications of semiconductor microstructures

Abstract: Description of non-interacting electrons in periodic structures - free-electron model quantum states of electrons interacting with a weak periodic potential - nearly-free-electron band structure localized representation of electron states in solids - tight-binding model basic observable properties of semiconductor materials - thermal excitations, lattice vibrations, optical absorption and emission, impurity levels, p-n junctions, diffusion - Einstein's relation, electron motion in electric field electronic structure - optical and transport parameters of technologically important semiconductors physics of semiconductor interfaces quantum wells, superlattices, quantum wires and dots electron transport in microstructures - Poole-Frenkel effect, Josephson tunnelling optical properties of microstructures - propagation of electromagnetic waves in dielectrics, origin of non-linear effects wave mixing, bistability, self-focusing and soliton propagation fabrication and characterization of semiconductor microstructures - methods of crystal growth, lithography, dry etching, direct writing, Langmuir-Blodgett-Roberts films, digital MBE growth, surface spectroscopy, quantum Hall effect, determination of heterojunction band line-ups application of semiconductor microstructures in electronic devices - planar silicon technology, MOSFET and biopolar transistors, high-electron-mobility transistor (HEMT), hot-electron transistors application of semiconductor microstructures in optoelectronics - heterojunction lasers, photodetectors, optical fibres and switches, application of photonics in information technology. Appendices: 1 - hydrogenic levels 2 - particle in a one-dimensional rectangular well 3 - solution of the Schrodinger equation for a periodic one-dimensional (Kronig-Penney) potential.

Self‐consistent model of transport in quantum well tunneling structures

Abstract: We present a fully self‐consistent model for the current‐voltage characteristic of quantum well tunneling structures. Our approach is based on the independent particle approximation and incorporates both screening of the carrier potential and the response from the contacts. It is shown that a self‐consistent treatment of charge supply from the contacts is essential for meaningful description of the system far from equilibrium. Results are presented for GaAs‐AlGaAs double‐barrier structures. Compared to standard calculations, this approach leads to strong quantitative changes in the I‐V characteristic. In particular, peak and valley currents increase and current peaks shift to higher voltages. Doping effects on the I‐V characteristic are discussed.

Tunneling escape time of electrons from a quantum well under the influence of an electric field

Abstract: We have performed time‐resolved photoluminescence on GaAs/AlxGa1−xAs single quantum well structures with an electric field applied perpendicular to the well plane. The quantum wells are coupled to the GaAs continuum through a thin barrier; the escape time of the electrons in the well was measured by time‐resolved photoluminescence. The dependence of the decay time on applied bias was found to agree very well with a simple semiclassical model.