Electron Tunneling Transport Research Articles

Structural and optical properties of position-controlled n-ZnO nanowire arrays: Potential applications in optoelectronics

Highly position-controlled ZnO nanowire (NW) arrays and nanowire-cluster (NW–C) arrays are grown on mask-patterned gallium nitride (GaN) substrates by hydrothermal method. Electroluminescence (EL) emissions with blue-violet light at forward-bias(fb) voltage, EL emissions with yellow light at reverse-bias(rb) voltage, and EL emission at both rb voltages and rb voltages were achieved by the adjustment of the size of the heterojunction interface between wire arrays and film. The high rb current is attributed to electron tunneling transport edge effect via a narrow tunnel barrier. The obtained results highlight that the EL characteristics of ZnO NWs can be tuned and controlled through the size of the heterojunction interface between wire arrays and film. Besides, the nonplanar heterogeneous structure of ZnO NW arrays/GaN film also shown a great spectral response characteristic in the ultraviolet(UV) band, which proves that the structure of is worthy of being studied to achieve high performance UV photodetectors.

Application of the Lewis-Riesenfeld quantum mechanical invariant method for description of electron tunneling transport in nitride multilayer quantum well nanostructures

The Lewis-Riesenfeld method has been applied to study an electron tunneling transport in open nitride nanosystems. The solutions of the time-dependent Schrödinger equation was represented as a linear combination of Fresnel integrals. Calculations of field behavior of the transparency coefficient D(E) revealed the main factors affecting the process of electron tunneling. The oscillations in D(E) behavior with significant distortions and destruction of the quasi-stationary states were observed at some values of parameters.

Modeling electronic conduction in quantum dot constituted assemblies coupled to metallic electrodes

It is shown that the electrical transport in quantum dot (QD) constituted films, synthesized through electrochemical doping, could be described through a two-step model considering (i) the tunneling transport of electrons from an electron source, and subsequent (ii) Ohmic transport through the film governed by a temperature (T) dependent mobility (μ), varying as T−α. A transfer matrix algorithm based approach for electrical field induced tunneling and α ∼ 1, was used. The indicated modeling principles could be deployed to predict the current–voltage characteristics of QD films and assemblies coupled to metallic electrodes.

Quantum transport via dot devices with arbitrarily strong interactions

The paper develops a theory of tunneling electron transport through atomic-scale systems (or briefly quantum dots) with arbitrarily strong interaction. The theory is based on a diagram technique for nonequilibrium Green’s functions defined on Hubbard operators. The use of Hubbard operators, describing many-body states of an entire quantum dot, makes it possible to represent the Hamiltonian of the quantum dot in a universal diagonal form and consider its coupling with two leads within the perturbation theory. It is shown that in the case when all Hubbard operators are defined for the same site, some rules of the diagram technique for Hubbard operators, initially developed for lattice models, have to be modified. As an example of the application of the modified theory, the current-voltage characteristics of the single-impurity Anderson model with infinitely large Coulomb repulsion are calculated. It is shown that taking into account the multiple electron tunneling processes with spin flips results in the dip in the center of the Lorentz distribution peak, describing the density of states of the one level Anderson impurity coupled with two leads. The emergence of this dip in the density of states leads to a peculiar feature in the bias voltage dependence of the differential conductivity, which can be detected experimentally.

Microscopic formation mechanism of Si/Tl5Al1/TiN ohmic contact on non-recessed i-InAlN/GaN heterostructures with ultra-low resistance

In this work, we use Si/Tl5Al1/TiN for a source/drain ohmic contact to demonstrate an ultra-low contact resistance of 0.11 Ω mm (ρc = 2.62 × 10−7 Ω cm2) on non-recessed i-InAlN/GaN heterostructures. The Ti5Al1 alloy was used to suppress the out-diffusion of Al and extract N from the InAlN layer, which aided the formation of ohmic contact by improving the tunneling efficiency of electrons, as we have reported in the past work. A thin Si inter-layer combined with the Ti5Al1 alloy is proposed to further reduce contact resistance. A heavy n-type InAlN layer was obtained through doping with Si atoms to improve the tunneling transport of electrons. Furthermore, the TiN inclusions penetrated into the GaN channel because the in-diffused Si promoted the decomposition of GaN at a high annealing temperature and the in-diffused Ti reacted with GaN. These TiN inclusions provided direct contact with two-dimensional electron gas, offering an additional path for the injection of electrons into the channel. The tunneling and spike mechanism worked alternately to lower the contact resistance at different annealing temperatures (dividing at 900 °C), implying that the joint effect of tunneling and the spike mechanism was initially promoted in the formation of ohmic contact. The mechanism of this Si/Ti5Al1/TiN ohmic contact was fully understood through microscopic and thermodynamic analyses. These results shed light on the mechanism for the formation of ohmic contact in a gold-free metal stack for GaN-based HEMTs.

Electron tunneling spectroscopy of an anisotropic Kitaev quantum spin liquid sandwiched between superconductors

We present the electron tunneling transport and spectroscopic characters of a superconducting (SC) Josephson junction with a barrier of a single anisotropic Kitaev quantum spin liquid (QSL) layer. We find that the dynamical spin-correlation features are well reflected in the direct-current differential conductance $d{I}^{c}/dV$ of single-particle tunneling, including the unique spin gap and dressed itinerant Majorana dispersive band, in addition to an energy shift $2\mathrm{\ensuremath{\Delta}}$ from the two-SC-lead gap. From the spectral characters, we identify different topological quantum phases of the anisotropic Kitaev QSL. We also present the zero-voltage Josephson current ${I}^{s}$, which displays residual features of the anisotropic Kitaev QSL. These results pave a new path to measurement of dynamical spinon or Majorana fermion spectroscopy of Kitaev and other spin-liquid materials.

Increased Molecular Conductance in Oligo[n]phenylene Wires by Thermally Enhanced Dihedral Planarization.

Coherent tunneling electron transport through molecular wires has been theoretically established as a temperature-independent process. Although several experimental studies have shown counter examples, robust models to describe this temperature dependence have not been thoroughly developed. Here, we demonstrate that dynamic molecular structures lead to temperature-dependent conductance within coherent tunneling regime. Using a custom-built variable-temperature scanning tunneling microscopy break-junction instrument, we find that oligo[n]phenylenes exhibit clear temperature-dependent conductance. Our calculations reveal that thermally activated dihedral rotations allow these molecular wires to have a higher probability of being in a planar conformation. As the tunneling occurs primarily through π-orbitals, enhanced coplanarization substantially increases the time-averaged tunneling probability. These calculations are consistent with the observation that more rotational pivot points in longer molecular wires leads to larger temperature-dependence on conductance. These findings reveal that molecular conductance within coherent and off-resonant electron transport regimes can be controlled by manipulating dynamic molecular structure.

Transport Modulation Through Electronegativity Gating in Multiple Nitrogenous Circuits.

Investigating the correlations of electron transport between multiple channels shows vital promises for the design of molecule-scale circuits with logic operations. To control the electron transport through multiple channels, the modulation of electronegativity shows an effective frontier orbit control method with high universality to explore the interactions between transport channels. Here, two series of compounds with a single nitrogenous conductive channel (Sg) and dual-channels (Db) are designed to explore the influence of electronegativity on electron tunneling transport. Single-molecule conductance measured via the scanning tunneling microscope break junction technique (STM-BJ) reveals that the conductance of Db series is significantly suppressed as the electronegativity of nitrogen becomes negative, while the suppression on Sg is less obvious. Theoretical calculations confirm that the effect of electronegativity extends to a dispersive range of molecular frameworks owing to the delocalized orbital distribution from the dual-channel structure, resulting in a more significant conductance suppression effect than that on the single-channel. This study provides the experimental and theoretical potentials of electronegativity gating for molecular circuits.

Tunneling Electron Transport in ZnO Nanograins Prepared by Electrospinning Method: An Ethyl Acetate Vapour Sensor by Chemiresistive Method

Tunneling Electron Transport in ZnO Nanograins Prepared by Electrospinning Method: An Ethyl Acetate Vapour Sensor by Chemiresistive Method

Complementary Vacuum Field Emission Transistor

A complementary vacuum field emission device structure is proposed, and its operation is analyzed by multiphysics simulation. A freely moving double-clamped cantilever, which balances the electrostatic attraction force and elastic restoration force, is used as the source electrode while the drain electrode is fixed. The electron-emitting cathode is formed on the source for the n-type device and on the drain for the p-type. Thus, complementary current–voltage characteristics are obtained only with Fowler–Nordheim tunneling electron transport. The impact of various design parameters such as vacuum gap, cantilever beam thickness, gate width, and tip offset on the drain and gate leakage currents is investigated. Inverter logic operation is verified successfully using complementary devices.

Klein tunneling of optically tunable Dirac particles with elliptical dispersions

We have investigated electron tunneling through an atomically smooth square potential barrier for both the dice lattice and graphene under a linearly polarized off-resonant and high-frequency dressing field. We have demonstrated Klein tunneling for a nonzero angle of incidence which is due to a nonalignment of optically controllable elliptical energy dispersions for the dressed states of Dirac particles and the direction of incoming kinetic particles. This finite angle of incidence has been found to depend on the light-induced anisotropy of energy dispersion, which is a function of the electron-light coupling strength, as well as the misalignment between directions of the light polarization and the electron beam incident on the potential barrier. Additionally, we have discovered much larger off-peak transmission amplitudes for dice lattices in contrast to graphene. We anticipate that the theoretical predictions could be applied to a wide range of Dirac materials and exploited for controlling both coherent tunneling and ballistic transport of electrons in the construction of novel electronic, optical, and valleytronic nanoscale switching devices.3 MoreReceived 27 June 2020Accepted 19 October 2020DOI:https://doi.org/10.1103/PhysRevResearch.2.043245Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasBallistic transportCarrier dynamicsElectrical conductivityElectronic structureOptoelectronicsCondensed Matter, Materials & Applied Physics

Interplay between effects of barrier tilting and scatterers within a barrier on tunneling transport of Dirac electrons in graphene

Dirac-electronic tunneling and transport properties with both finite and zero energy band gap are investigated for graphene with an in-plane tilted potential barrier embedded with scatters. For a tilted barrier, by using Wentzel-Kramers-Brillouin approximation, an analytical solution is obtained first for transmission coefficient of Dirac electrons in gapped graphene in the absence of any scatters. In the presence of either a single or a continuous distribution of scatters embedded within a tilted barrier, however, a numerical scheme based on finite-difference approach is developed for accurately calculating both transmission coefficient and tunneling resistance of Dirac electrons. Here, the combination of a tilted barrier and a scatter potential can be viewed as an effective barrier-potential profile facilitated by a proper gate structure. Meanwhile, a full analysis and detailed comparisons are presented for the interplay between effects of both distributed scatters in a barrier and barrier tilting on tunneling transport of Dirac electrons in graphene. The barrier-tilting field and scatter position are found to play a key role in controlling a peak of tunneling resistance as well as in its switching to a cusp by a mid-barrier-embedded scatter as the incident energy reaches the Dirac point in a barrier. Different from a single scatter, a continuous distribution within a barrier can enhance the unimpeded incoherent tunneling for head-on collision while greatly suppressing skew ones with increasing barrier-tilting field. All these predicted attractive transport properties are expected to be extremely useful for designing both novel electronic and optical graphene-based devices and electronic lenses in ballistic-electron optics.

Features of Tunneling Current in Superlattices with Electrical Domains

Abstract—The tunneling electron transport in GaAs/AlAs and InAs/AlSb superlattices with electric domains at room temperature is studied. At voltages above the threshold for the formation of domains, a series of maxima on the current–voltage characteristics, almost equidistant in voltage, and current hysteresis during direct and reverse voltage sweeps were found. An explanation is proposed relating the origin of these maxima to tunnel transitions between quantum wells in a triangular domain, assisted by the emission of optical phonons, and hysteresis with a transition between modes with a moving and static domain. The large asymmetry of the transition times between domain modes was found.

Transport and magneto-transport properties in La0.8K0.2\u2212x\u25a1xMnO3\u2212\u03b4 (x\u2009=\u20090 and 0.1) manganites

We discuss the effect of the nanometric grain size on the behavior of the electrical and magnetoresistive response of La0.8K0.2−x□xMnO3−δ (x = 0 and 0.1) nanocrystalline samples that were prepared by a sol–gel method. The results from transport and magneto-transport measurements evidence a robust dependence on the nanometric grain size. The temperature dependence of the resistivity was evaluated using different transport models. The results reveal a field-dependent minimum of the resistivity in the low-temperature region, which can be described in terms of intergranular spin-polarized tunneling. Remarkably, a considerable increase of the magnetoresistance (MR) with the decrease of nanoparticle size was found, which might open a new way for the search for potential candidates for magnetoresistive devices. Besides, the magnetic field dependence of the MR was also analyzed, and a distinct drop of MR at low fields was noticed. This behavior was primarily explained by the spin-polarized tunneling transport of conduction electrons across grain boundaries.

Pinhole evolution of few-layer graphene during electron tunneling and electron transport

With graphene as a well-known representative, two-dimensional (2D) atomic crystals are single-layer or few-layer crystalline materials. It is significant to investigate the structure evolution of the 2D atomic crystals in electronic movement process, because it relates to the stability of material properties and the feasibility of the device application. Here, the pinhole evolution of few-layer graphene during electron tunneling and electron transport was observed using in situ transmission electron microscopy (TEM). For few-layer graphene with defects, pinholes expanded with an increase on the electron tunneling current and time, respectively. However, during electron transport processes, with increasing current and time, both, expansion and shrinkage behaviors could be observed among the pinholes distributed within the graphene nanosheet. These behaviors are the result of competition between sublimation and self-repair. Both types of pinhole evolution aim to form a smooth surface with lower surface free energy. The critical boundary conditions for the sublimation and self-repair are determined by the temperature, graphene pinhole structure and active graphene fragment. These findings provide significant reference for the stability of graphene structure and the reliability of graphene based electronic device.

Simulating Tunneling Electron Transport in the Semiconductor–Crystalline Insulator–Si(111) System

Tunneling carrier transport through a thin insulator (e.g., CaF2) layer between a Si(111) substrate and a semiconductor gate is theoretically investigated. Along with the conservation of a large transverse wave vector of tunneling particles, the limitation imposed on the availability of states in the gate is taken into account. Due to this limitation, the tunneling currents at low insulator bias are weaker than in an analogous structure with a metal gate electrode. The same feature leads to a change in the shape of the energy distribution of tunneling electrons, both in transport between the substrate and gate conduction bands and during the Si(111) conduction band–gate valence band transfer.

Interlayer interaction in ultrathin nanosheets of graphitic carbon nitride for efficient photocatalytic hydrogen evolution

Two-dimensional graphitic carbon nitride (g-C3N4) nanosheets (CNNS) have attracted intense interest in photocatalysis, given their small thickness and high specific surface area favoring charge transfer and surface reactions. Herein, a facile strategy of breaking and following repolymerizing the heptazine units in bulk g-C3N4 (BCN) is developed to synthesize ultrathin CNNS with thickness of 1nm in relatively high product yield (∼24%). The as-prepared 1nm-thick CNNS show significantly enhanced photocatalytic performance for hydrogen evolution than BCN and even the 3nm-thick CNNS acquired by thermal oxidation etching of BCN. It is evidenced that the disordered layer structure of the obtained ultrathin CNNS causes strong interlayer CN interaction, tunneling electron transport between the CN layers. Meanwhile, the broken in-plane CN bonds create more unsaturated N sites in the 1nm-thick CNNS, facilitating the electron excitation from the occupied states in g-C3N4 to its unoccupied states for water reduction reaction.

Electronic tunneling through a potential barrier on the surface of a topological insulator

We investigate the tunneling transport for electrons on the surface of a topological insulator (TI) through an electrostatic potential barrier. By using the Dirac equation with the continuity conditions for all segments of wave functions at the interfaces between regions inside and outside the barrier, we calculate analytically the transmission probability and conductance for the system. It is demonstrated that, the Klein paradox can also been observed in the system same as in graphene system. Interestingly, the conductance reaches the minimum value when the incident electron energy is equal to the barrier strength. Moreover, with increasing barrier width, the conductance turns up some tunneling oscillation peaks, and larger barrier strength can cause lower conductance, shorter period but larger oscillation amplitude. The oscillation amplitude decreases as the barrier width increases, which is similar as that of the system consisting of the compressive uniaxial strain applied on a TI, but somewhat different from that of graphene system where the oscillation amplitude is a constant. The findings here imply that an electrostatic barrier can greatly influence the electron tunneling transport of the system, and may provide a new way to realize directional filtering of electrons.

Ultrathin and Atomically Flat Transition-Metal Oxide: Promising Building Blocks for Metal-Insulator Electronics.

We present a new and viable template-assisted thermal synthesis method for preparing amorphous ultrathin transition-metal oxides (TMOs) such as TiO2 and Ta2O5, which are converted from crystalline two-dimensional (2D) transition-metal dichalcogenides (TMDs) down to a few atomic layers. X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and scanning transmission electron microscopy (STEM) were used to characterize the chemical composition and bonding, surface morphology, and atomic structure of these ultrathin amorphous materials to validate the effectiveness of our synthesis approach. Furthermore, we have fabricated metal-insulator-metal (MIM) diodes using the TiO2 and Ta2O5 as ultrathin insulating layers with low potential barrier heights. Our MIM diodes show a clear transition from direct tunneling to Fowler-Nordheim tunneling, which was not observed in previously reported MIM diodes with TiO2 or Ta2O5 as the insulating layer. We attribute the improved performance of our MIM diodes to the excellent flatness and low pinhole/defect densities in our TMO insulting layers converted from 2D TMDs, which enable the low-threshold and controllable electron tunneling transport. We envision that it is possible to use the ultrathin TMOs converted from 2D TMDs as the insulating layer of a wide variety of metal-insulator and field-effect electronic devices for various applications ranging from microwave mixing, parametric conversion, infrared photodetection, emissive energy harvesting, to ultrafast electronic switching.

Scanning Tunneling Electron Transport into a Kondo Lattice**Supported by the National Natural Science Foundation of China under Grant No. 11547203, and the Research Project of Education Department in Sichuan Province of China under Grant No. 15ZB0457

We theoretically present the results for a scanning tunneling transport between a metallic tip and a Kondo lattice. We calculate the density of states (DOS) and the tunneling current and differential conductance (DC) under different conduction-fermion band hybridization and temperature in the Kondo lattice. It is found that the hybridization strength and temperature give asymmetric coherent peaks in the DOS separated by the Fermi energy. The corresponding current and DC intensity depend on the temperature and quantum interference effect among the c-electron and f-electron states in the Kondo lattice.