Resonant Tunneling Features Research Articles

Features of vacuum resonant tunneling at one-and two-well barrier potentials

The features of one-dimensional vacuum tunneling and calculation of the tunneling current in barrier quantum structures with one and two wells are considered. The structures are formed by several electrodes: the cathode, the anode and two grids. The potential profiles are constructed by the method of multiple images. Equations for eigenvalues and metastable levels of a structure with an arbitrary potential profile are found. For full resonant tunneling, the metastable levels must fall into the region of the electron kinetic energy distribution at the cathode, which occurs in single-well structures in the absence of an anode voltage or at a low anode voltage compared to the voltage on the grids. A significant voltage at the anode leads to an asymmetric structure and incomplete resonant tunneling. In this case, a two-well structure with a double grid allows obtaining full resonant tunneling for a number of energies and increasing the tunneling current by orders of magnitude. Keywords: resonant tunneling, field emission, vacuum nanotriode, potential barrier, quantum well.

Resonant tunneling and multiple negative differential conductance features in long wavelength interband cascade infrared photodetectors

We report on an investigation of multiple negative differential conductance (NDC) features in long wavelength interband cascade infrared photodetectors (ICIPs) at and above 300 K. Using ICIPs with various structures and carrier concentrations, we employ several approaches to demonstrate that the observed multiple NDC features and their unusual temperature dependence are related to the sequential turn off of resonant tunneling of minority carriers through the electron barriers at high temperatures.

Resonant tunnelling features in a suspended silicon nanowire single-hole transistor

Suspended silicon nanowires have significant potential for a broad spectrum of device applications. A suspended p-type Si nanowire incorporating Si nanocrystal quantum dots has been used to form a single-hole transistor. Transistor fabrication uses a novel and rapid process, based on focused gallium ion beam exposure and anisotropic wet etching, generating <10 nm nanocrystals inside suspended Si nanowires. Electrical characteristics at 10 K show Coulomb diamonds with charging energy ∼27 meV, associated with a single dominant nanocrystal. Resonant tunnelling features with energy spacing ∼10 meV are observed, parallel to both diamond edges. These may be associated either with excited states or hole–acoustic phonon interactions, in the nanocrystal. In the latter case, the energy spacing corresponds well with reported Raman spectroscopy results and phonon spectra calculations.

Understanding resonant tunnel transport in non-identical and non-aligned clusters as applied to disordered carbon systems

We study the conductance spectra and the corresponding current-voltage characteristics of a set of three impurity clusters of different sizes arranged in the form of a scalene triangle and compare with the transport of their horizontal and vertical configurations. The tuning capability of resonant tunnelling features in a quantum dot device made of these non-aligned impurity clusters is demonstrated by re-distributing their diameters and inter-cluster distances in a systematic manner. By manipulating the inter-cluster coupling for a scalene triangular configuration, the transition of current-voltage curves from a step-like feature to a negative differential resistance can be produced. This process also yields conductance features for triangular configurations, which can be compared to the quantum dot structures perfectly aligned to the direction of the propagating wavevector. The strength of inter-cluster coupling or order parameter for these configurations is analysed from the relative variation of the width and the energy difference of the sharp and broad peaks observed in the density of states spectra. Based on the relative change of the inter-cluster coupling with the cluster configurations, a transport model applicable to structurally inhomogeneous systems is proposed in order to explain the experimentally observed variation of the energy band gap with the disorder parameters.

Subband structure of two-dimensional electron gases in SrTiO3

Tunneling between two parallel, two-dimensional electron gases (2DEGs) in a complex oxide heterostructure containing a large, mobile electron density of ∼3 × 1014 cm−2 is used to probe the subband structure of the 2DEGs. Temperature-dependent current-voltage measurements are performed on SrTiO3/GdTiO3/SrTiO3 junctions, where GdTiO3 serves as the tunnel barrier, and each interface contains a high-density 2DEG. Resonant tunneling features in the conductance and its derivative occur when subbands on either side of the barrier align in energy as the applied bias is changed, and are used to analyze subband energy spacings in the two 2DEGs. We show that the results agree substantially with recent theoretical predictions for such interfaces.

Tuning resonant transmission through geometrical configurations of impurity clusters

Resonant tunneling features through impurity clusters embedded in an insulating matrix have been examined through the inter-play between the size of the clusters and the inter-cluster distance. Constructive interference phenomena were tuned through a systematic study of different geometrical configurations, thereby controlling confinement in quasi-bound states. Gaussian trap potentials have been used to simulate the imperfect barrier-well interface associated with disordered materials. Strongly localized states can be formed successfully despite weak disorder as illustrated by breaking the symmetry in the horizontal configuration. To this end, triangular cluster configurations were investigated under a variety of conditions including various shapes and orientations. The effects of disorder created effectively by the arbitary configurations destroy the Fano resonance, which is previlent in conductance spectra and consequently reduce the peak to valley ratio of the resonant peak in current vs. voltage curves. However the formation of two quasi-bound states is demonstrated, suggesting possible applications for disordered naturally grown systems of impurity clusters. This work addresses the controlled lifetime of quasi-bound states and can inform the design of fast switching devices based on high band gap materials by the astute incorporation of impurity clusters with specific geometrical configurations.

Disorder effects on resonant tunneling transport in GaAs/(Ga,Mn)As heterostructures

Recent experiments on resonant tunneling structures comprising (Ga,Mn)As quantum wells [Ohya et al., Nature Physics 7, 342 (2011)] have evoked a strong debate regarding their interpretation as resonant tunneling features and the near absences of ferromagnetic order observed in these structures. Here, we present a related theoretical study of a GaAs/(Ga,Mn)As double barrier structure based on a Green's-function approach, studying the self-consistent interplay between ferromagnetic order, structural defects (disorder), and the hole tunnel current under conditions similar to those in experiment. We show that disorder has a strong influence on the current-voltage characteristics in efficiently reducing or even washing out negative differential conductance, offering an explanation for the experimental results. We find that for the Be lead doping levels used in experiment the resulting spin-density polarization in the quantum well is too small to produce a sizable exchange splitting.

Feasibility, accuracy, and performance of contact block reduction method for multi-band simulations of ballistic quantum transport

Numerical utilities of the contact block reduction (CBR) method in evaluating the retarded Green’s function are discussed for 3D multi-band open systems that are represented by the atomic tight-binding (TB) and continuum k·p (KP) band model. It is shown that the methodology to approximate solutions of open systems, which has been already reported for the single-band effective mass model, cannot be directly used for atomic TB systems, since the use of a set of zinc blende crystal grids makes the inter-coupling matrix non-invertible. We derive and test an alternative with which the CBR method can be still practical in solving TB systems. This multi-band CBR method is validated by a proof of principles on small systems and also shown to work excellent with the KP approach. Further detailed analysis on the accuracy, speed, and scalability on high performance computing clusters is performed with respect to the reference results obtained by the state-of-the-art recursive Green’s function and wavefunction algorithm. This work shows that the CBR method could be particularly useful in calculating resonant tunneling features, but shows a limited practicality in simulating field effect transistors (FETs) when the system is described with the atomic TB model. Coupled to the KP model, however, the utility of the CBR method can be extended to simulations of nanowire FETs.

Resonant tunnelling features in quantum dots

We present a systematic review of features due to resonant electron tunnelling, observablein transport spectroscopy experiments on quantum dots and single donors. The reviewcovers features attributable to intrinsic properties of the dot (orbital, spin and valleystates) as well as extrinsic effects (phonon/photon emission/absorption, features in thecharge reservoirs, coupling to nearby charge centres). We focus on the most commonoperating conditions, neglecting effects due to strong coupling to the leads. By discussingthe experimental signatures of each type of feature, we aim at providing practical methodsto distinguish between their different physical origins. The correct classification of theresonant tunnelling features is an essential requirement to understand the details of theconfining potential or to predict the performance of the dot for quantum informationprocessing.

Tunneling spectroscopy of Stark-shifted image potential states on Cu and Au surfaces

Using low-temperature scanning tunneling microscopy (STM) distance-voltage characteristics at constant current are measured on Cu(111), Cu(110), Au(111), and Au(100) surfaces. Resonant tunneling features are observed that arise from image potential states distorted by the strong electric field of the STM tip. Distance-voltage characteristics are measured over a wide range of current setpoints and the energetic position and width of the resonant features show a significant dependence on this parameter. The lowest energy feature is found to be nearly independent of STM tip condition, while higher energy resonances show substantial tip dependence. The results are discussed and compared with recent calculations.

Structural Contributions to Charge Transport across Ni-Octanedithiol Multilayer Junctions

We report the fabrication and characterization of multilayer thin films incorporating 1,8-octanedithiols and Ni atoms. Low-temperature charge transport measurements exhibit inelastic co-tunneling and resonant tunneling features that correspond energetically to vibrational excitations of the molecular multilayer. Several junctions exhibit changes in conductance features characteristic of charge defect-gating. Transport through our junctions is shown to be dominated by the intrinsic properties of the multilayer.

HIGH FIELD MAGNETOTRANSPORT IN STRONGLY COUPLED InAs/GaSb SUPERLATTICES

Vertical magnetotransport measurements are used to investigate resonant transport processes in strongly coupled InAs / GaSb superlattices. Sample I-V curves display Stark-Cyclotron resonance and longitudinal-optic (LO) phonon mediated transport features. Resonant tunnelling features change dramatically above the magnetophonon condition suggesting that the Landau levels are renormalized due to interaction between LO phonon's and electrons.

Magnetoresistance studies of strongly coupled superlattices

We present magnetotransport results for strongly coupled InAs/GaSb superlattices. The low effective mass of carriers in the InAs/GaSb superlattice allows the study of the transport processes at fields above the magnetophonon condition, where the cyclotron spacing ℏ ω c is greater than the LO phonon energy ℏ ω LO. Magnetotransport results display Stark Cyclotron resonance and the Magnetophonon effect. Resonant tunnelling features change dramatically above the magnetophonon condition and suggest the superlattice undergoes formation of ordered electric field domain states linked to the observation of LO phonon mediated tunnelling.

Transport properties of quasiparticles with fractional exclusion statistics

We consider the ballistic transport of quasiparticles with exclusion statistics through a one-dimensional wire within the Landauer-B\"uttiker approach. We demonstrate that quasiparticle transport coefficients (electrical and heat conductance, as well as thermopower) are determined by the same general formulas as for particles with normal statistics. By applying the developed formalism to the ballistic transport of fractional charge it is shown that for a wire in contact with quasiparticle reservoirs the transport coefficients depend on the fractional charge. Specific features of resonant tunneling of quasiparticles are discussed.

Resonant tunneling in high- Tc superconductors (review)

Papers are reviewed on the resonant tunneling mechanism of the c-axis transport in high-temperature superconducting layered cuprates, proposed by the present author. After an introduction, where the problem is stated and the qualitative principles of the mechanism are described, real physical properties are calculated and compared with experimental data. These include the temperature dependence of the static c-axis conductivity in the normal state, frequency dependence of the optical conductivity and stationary supercurrent along the c-axis. It is demonstrated that for the latter the coherence of resonant tunneling through different centers is of primary importance. In the last section an explanation is proposed of the drastic decrease of T c and increase of the BCS ratio 2 Δ(0)/ T c with underdoping, based on the disruption of interlayer connection and appearance of vortex fluctuations. Two appendices are added illustrating some important features of resonant tunneling on simple models.

Planar quantum transistor based on 2D–2D tunneling in double quantum well heterostructures

We report on our work on the double electron layer tunneling transistor (DELTT), based on the gate control of two-dimensional–two-dimensional (2D–2D) tunneling in a double quantum well heterostructure. While previous quantum transistors have typically required tiny laterally defined features, by contrast the DELTT is entirely planar and can be reliably fabricated in large numbers. We use a novel epoxy-bond-and-stop-etch flip-chip process, whereby submicron gating on opposite sides of semiconductor epitaxial layers as thin as 0.24 μm can be achieved. Because both electron layers in the DELTT are 2D, the resonant tunneling features are unusually sharp, and can be easily modulated with one or more surface gates. We demonstrate DELTTs with peak-to-valley ratios in the source-drain I–V curve of order 20:1 below 1 K. Both the height and position of the resonant current peak can be controlled by gate voltage over a wide range. DELTTs with larger subband energy offsets (∼21 meV) exhibit characteristics that are nearly as good at 77 K, in good agreement with our theoretical calculations. Using these devices, we also demonstrate bistable memories operating at 77 K. Finally, we briefly discuss the prospects for room temperature operation, increases in gain and high speed.

Spin- and wave-vector dependent resonant tunneling through magnetic barriers

Magnetic barriers are relevant to the new class of quantum structures which are realised by the deposition of a two-dimensional electron gas (2DEG) in an inhomogeneous magnetic field. We consider the salient features of resonant tunneling of electrons through double-barrier structures accounting for the dependence of tunneling on the electron wave vector and its spin direction. The scope for forming effective filters which let through electrons with a certain direction of motion and spin polarisation is shown.

Effect of wire width variation on the conductance of a silicon quantum wire

The effect of wire width variation on the conductance of a 1 μm×30 nm silicon quantum wire has been studied. Both resonant tunneling features and conductance fluctuations superimposed on the quantized steps have been observed and explained by the existence of a step-wise constriction in the wire due to the nonideality in the fabrication process.

Excess tunnel currents in AlGaAs/GaAs multiple quantum well infrared detectors

We present experimental evidence of excess tunnel current at low temperatures in AlGaAs/GaAs multiple quantum well infrared detectors that is not accounted for by existing theory. Prior discussions of current mechanisms in these detectors only take into account ideal device properties. For quantum well detectors with thick barrier layers, which are useful for infrared detection, the excess tunnel current component possesses the features of sequential resonant tunneling, i.e., temperature independence of the current, and saturation of the current at intermediate bias voltages with negative conductance oscillations. However, the current is orders of magnitude larger than theory predicts. Comparison with data reported by other groups shows the magnitude of the discrepancy is sample dependent. These results suggest that defects play an important role in determining the tunnel current magnitude. This current component is significant because it limits attainable detector performance at low temperatures for applications requiring operation in reduced infrared backgrounds.

Modeling of cross-well carrier transport in a multiple quantum well modulator

A self-consistent time and spatially resolved model is developed to explain the nature of cross-well transport in multiple quantum well devices. It is this cross-well transport that sets fundamental limits on the speed of such devices. This model is solved numerically to obtain a fit to experimental excite-probe data. A good fit to rise times is obtained with resonant tunneling features predicted and observed. It is also predicted that as the transient features in the transmission data relax, many carriers are still bound in the quantum wells.