Double-barrier Tunnel Junctions Research Articles

Metallic nanofilms on Si(100) and SiO2 grown with a ruthenium precursor

Ruthenium (Ru) nanofilms (<3 nm) were prepared using tricarbonyl(trimethylenemethane)ruthenium, Ru(TMM)(CO)3 at 230 °C. We show that the surface morphology and electrical conductance of Ru nanofilms are substantially different on H:Si(100) and SiO2/Si(100) substrates. Two-dimensional (2D) Ru nanofilms (∼1 nm) were formed on H:Si(100), while thick (∼3 nm) granular Ru films were formed on SiO2 substrate under the same growth conditions, as confirmed by cross-sectional transmission electron microscopy and X-ray photoelectron spectroscopy. Using scanning probe microscopy, the metallic conductance of Ru grains on H:Si(100) substrates was recognized. On ultrathin (1 nm) SiO2/Si(100) substrates, the spatial separation of Ru grains facilitates the single electron tunneling (SET) phenomenon in the double barrier tunnel junction structure. The results emphasized the difference in carrier transport in Ru nanofilms on Si and SiO2 substrates.

Tunnel-magneto resistance of double-barrier tunnel junctions in the presence of spin-orbit coupling within spin-filter barriers

The tunnel magneto-resistance ratio is investigated for double-barrier tunnel junctions with spin-filter (magnetic insulators) barriers. The transmission probability is calculated in the tight-binding model by the non-equilibrium Green's function formalism considering spin-orbit coupling within barriers. The formation of resonant states inside a non-magnetic spacer gives rise to an enhancement in tunnel magneto-resistance ratio. Double spin-filter tunnel junctions have also shown a higher tunnel magneto-resistance ratio compared to that in double-barrier magnetic tunnel junctions. These findings will be valuable to improve the performance of spintronics devices.

Strong signature of electron-vibration coupling in molecules on Ag(111) triggered by tip-gated discharging

Electron-vibration coupling is of critical importance for the development of molecular electronics, spintronics, and quantum technologies, as it affects transport properties and spin dynamics. The control over charge-state transitions and subsequent molecular vibrations using scanning tunneling microscopy typically requires the use of a decoupling layer. Here we show the vibronic excitations of tetrabromotetraazapyrene (TBTAP) molecules directly adsorbed on Ag(111) into an orientational glassy phase. The electron-deficient TBTAP is singly-occupied by an electron donated from the substrate, resulting in a spin 1/2 state, which is confirmed by a Kondo resonance. The TBTAP•− discharge is controlled by tip-gating and leads to a series of peaks in scanning tunneling spectroscopy. These occurrences are explained by combining a double-barrier tunneling junction with a Franck-Condon model including molecular vibrational modes. This work demonstrates that suitable precursor design enables gate-dependent vibrational excitations of molecules on a metal, thereby providing a method to investigate electron-vibration coupling in molecular assemblies without a decoupling layer.

Observation of Coulomb blockade and Coulomb staircases in superconducting Pr0.8Sr0.2NiO2 films

Motivated by the discovery of superconductivity in the infinite-layer nickelate family, we report an experimental endeavor to clean the surface of nickelate superconductor Pr0.8Sr0.2NiO2 films by Ar+ ion sputtering and subsequent annealing, and we study their electronic structures by cryogenic scanning tunneling microscopy and spectroscopy. The annealed surfaces are characterized by nano-sized clusters and Coulomb staircases with periodicity inversely proportional to the projected area of the nanoclusters, consistent with a double-barrier tunneling junction model. Moreover, the dynamical Coulomb blockade effects are observed and result in well-defined energy gaps around the Fermi level, which correlate closely with the specific configuration of the junctions. These Coulomb blockade-related phenomena provide an alternative plausible cause of the observed gap structure that should be considered in the spectroscopic understanding of nickelate superconductors with the nano-clustered surface.

Spin torque oscillator and magnetization switching in double-barrier Rashba Zeeman magnetic tunnel junction

In this work, we have studied the spin torque based magnetization oscillations and switching in presence of Rashba–Zeeman (RZ), Ruderman–Kittel–Kasuya–Yosida (RKKY) and Dzyaloshinskii–Moriya (DM) interactions in a double-barrier RZ|Heavy Metal (HM)|RZ magnetic tunnel junction (MTJ). The system has stable magnetization oscillations and can work as an oscillator or a switcher for a significant difference in the strength of RKKY and DM interaction under suitable spin transfer torque (STT). For the proposed system with same order of RKKY and DM interaction, a nonlinear characteristic of the magnetization oscillation is observed. However, this nonlinearity of oscillations can be reduced by an external magnetic field or considering a material with suitable RZ interaction. In addition to this, our study reveals the magnetization switching can be tuned by using suitable STT. A dependence of switching time on layer thickness is also observed. Also, the switching speed increases with the thickness for systems having either same order of RKKY and DM interaction or dominated by RKKY interaction. An opposite characteristic is seen when DM interaction dominates over RKKY interaction.

Direct Fabrication of Vertically Stacked Double Barrier Tunnel Junctions Based on Graphene and h-BN

Direct Fabrication of Vertically Stacked Double Barrier Tunnel Junctions Based on Graphene and h-BN

Thermal Spin Torque in Double-Barrier Tunnel Junctions with Magnetic Insulators

The thermal spin torque induced by the spin-dependent Seebeck effect in double-barrier tunnel junctions is derived considering free-electron and tight-binding calculations. We show that in systems comprising ferromagnetic electrodes and nonmagnetic barriers, the in-plane component of the thermal spin torque is the dominant term, whereas in junctions comprising nonmagnetic electrodes and ferromagnetic barriers, both components, the in-plane and the out-of-plane, are comparable in magnitude. Moreover, larger torque amplitudes up to 3 orders of magnitude are obtained in the second system as a result of the spin-filtering effect; consequently, double-barrier tunnel junctions in the presence of magnetic insulators offer an enhanced thermal spin-torque mechanism for reliable applications. We propose taking advantage of quantum resonant tunneling through resonance states below the Fermi level in these structures that can pave a route toward achieving larger spin-torque efficiencies, even when considering smaller values of the exchange splitting. Furthermore, we identify the parameters needed to tune efficiently these resonant states.

Dielectric Confinement and Excitonic Effects in Two-Dimensional Nanoplatelets.

Quasi-two-dimensional (2D) semiconductor nanoplatelets manifest strong quantum confinement with exceptional optical characteristics of narrow photoluminescence peaks with energies tunable by thickness with monolayer precision. We employed scanning tunneling spectroscopy (STS) in conjunction with optical measurements to probe the thickness-dependent band gap and density of excited states in a series of CdSe nanoplatelets. The tunneling spectra, measured in the double-barrier tunnel junction configuration, reveal the effect of quantum confinement on the band gap taking place mainly through a blue-shift of the conduction band edge, along with a signature of 2D electronic structure intermixed with finite lateral-size and/or defects effects. The STS fundamental band gaps are larger than the optical gaps as expected from the contributions of exciton binding in the absorption, as confirmed by theoretical calculations. The calculations also point to strong valence band mixing between the light- and split-off hole levels. Strikingly, the energy difference between the heavy-hole and light-hole levels in the tunneling spectra are significantly larger than the corresponding values extracted from the absorption spectra. Possible explanations for this, including an interplay of nanoplatelet charging, dielectric confinement, and difference in exciton binding energy for light and heavy holes, are analyzed and discussed.

Tunneling spectroscopy of an indium adatom precisely manipulated on the cross-sectional surface of InAs/GaSb quantum structures

Indium atom manipulation was achieved on the (110) cross-sectional surface of a quantum structure consisting of InAs/GaSb multilayer using a scanning tunneling microscope (STM) at low temperature. Positioning, removing, and lateral displacement by means of tip-induced hopping was performed on both InAs and GaSb layers. Electron tunneling via the electron orbital of the manipulated atom (adatom) was observed not only on the InAs layer but also on the GaSb layer, even when the energy of the orbital was in the GaSb band gap. The tunneling process from the STM tip to the InAs conduction band via the adatom and the GaSb band gap is explained well by the single electron tunneling model through the double-barrier tunneling junction. In the case of the adatom being very close to the InAs/GaSb heterointerface, interference with the electron tunneling to the InAs conduction electronic states through the GaSb barrier with/without passing the adatom was observed. The peak energy in the tunneling spectrum corresponding to the orbital of the adatom depends on the adatom position on the GaSb, demonstrating the fundamental interactions between the surface atomic-scale structures and semiconductor heterostructures.

Directional Excitation of Surface Plasmon Polaritons via Molecular Through-Bond Tunneling across Double-Barrier Tunnel Junctions.

Directional excitation of surface plasmon polaritons (SPPs) by electrical means is important for the integration of plasmonics with molecular electronics or steering signals toward other components. We report electrically driven SPP sources based on quantum mechanical tunneling across molecular double-barrier junctions, where the tunneling pathway is defined by the molecules' chemical structure as well as by their tilt angle with respect to the surface normal. Self-assembled monolayers of S(CH2)nBPh (BPh = biphenyl, n = 1-7) on Au, where the alkyl chain and the BPh units define two distinct tunnel barriers in series, were used to demonstrate and control the geometrical effects. The tilt angle of the BPh unit with respect to the surface normal depends on the value of n, and is 45° when n is even and 23° when n is odd. The tilt angle of the alkyl chain is fixed at 30° and independent of n. For values of n = 1-3, SPPs are directionally launched via directional tunneling through the BPh units. For values of n > 3, tunneling along the alkyl chain dominates the SPP excitation. Molecular level control of directionally launching SPPs is achieved without requiring additional on-chip optical elements, such as antennas, or external elements, such as light sources. Using the molecular tunneling junctions, we provide the first direct experimental demonstration of molecular double-barrier tunneling junctions.

Resonant magnetoresistance in double-barrier antiferromagnetic tunnel junction

Resonant magnetoresistance in double-barrier antiferromagnetic tunnel junction

Damping by sequentially tunneling electrons

Sequential tunneling of electrons through a single chlorine vacancy on a multilayer NaCl film on Cu(100) in the junction of a non-contact atomic force microscope with a conductive tip can lead to a negative damping of the cantilever. The characteristic features in the damping signal as a function of NaCl-layer thickness, tip position and tip-sample bias can be explained in a simple rate-equation model for the sequential tunneling process through the double-barrier tunnel junction. The first barrier results from the vacuum gap between tip and vacancy and its tunneling rate is tuned by the tip position and the applied bias. The second barrier results from the NaCl film and its tunneling rate is governed by the film’s thickness. The current-induced damping is strongest if the two tunneling rates through each of the barriers individually are both comparable to the cantilever’s oscillation frequency. The damping signal can be employed for detecting subfemtoampere tunneling currents and furthermore, maps of the damping signal can be used to inspect the mesoscopic tip shape. The observed and described current-induced damping should be a common phenomenon in non-contact atomic force microscopy for double-barrier tunnel junction geometries.

Single-electron transport through stabilised silicon nanocrystals.

We have fabricated organically capped stable luminescent silicon nanocrystals with narrow size distribution by a novel, high yield and easy to implement technique. We demonstrate transport measurements of individual silicon nanocrystals by scanning tunnelling microscopy at a low temperature in a double-barrier tunnel junction arrangement in which we observed pronounced single electron tunnelling effects. The tunnelling spectroscopy of these nanocrystals with different diameters reveals quantum confinement induced bandgap modifications. Furthermore, from the features in the tunnelling spectra, we differentiate several energy contributions arising from electronic interactions inside the nanocrystal. By applying a magnetic field, we have detected a variation in the differential conductance profile that we attribute to arising from higher order tunnelling processes. We have also systematically simulated our experimental data with the Orthodox theory, and the results show good agreement with the experiment. The study establishes a correlation between the nanocrystal size and quantum confinement induced band-structure modifications which will pave the way to devise tailored nanocrystals.

Influence of a ZnO layer on Seebeck coefficients in asymmetric double-barrier tunnel junctions

The Seebeck coefficients are studied in asymmetric double-barrier magnetic tunnel junctions (DBMTJs) with the structures as CoFeB/MgO/ZnO/MgO/LSMO. In the linear response regime, oscillatory behavior is observed with change in the ZnO layer thickness because of the resonant tunneling in the DBMTJs. Effects of average temperature and magnetizations orientation are also discussed. It is found that the Seebeck coefficients are greater in the DBMTJs than asymmetric single-barrier MTJs (SBMTJs). Therefore, it is to be achieved the greater Seebeck coefficients by choosing a proper thickness for the ZnO layer in the proposed asymmetric DBMTJs.

Enhancement of Thermopower by Structural Asymmetry in Double-Barrier Tunnel Junctions

Thermopower is studied in double-barrier magnetic tunnel junctions (DBMTJs) with a non-magnetic metal (NM) spacer by considering structural asymmetry—related to the different thicknesses of the MgO insulators—in the linear response regime. Using non-equilibrium Green’s function (NEGF) formalism, it is found that the thermopower is larger in the case with thicker left insulator than the other cases which is related to the voltage drop within the insulator layer. The thermopower has shown its maximum value as large as S = 482μ V/K in the anti-parallel configuration of the magnetizations and T = 300K. Therefore, it is possible to achieve the large thermopower in the thermoelectric devices based on MTJs.

Resonant enhancement of thermal spin-transfer torque in asymmetric double-barrier tunnel junctions

Charge current and thermal spin-transfer torque (TSTT) in asymmetric double-barrier magnetic tunnel junctions (DBMTJs) with a non-magnetic metal (NM) spacer as CoFeB/MgO/NM/MgO/LSMO are studied by non-equilibrium Green’s function (NEGF) formalism. It is found that the results show oscillatory behavior with an increase of the NM spacer thickness due to the resonant tunneling effect through the DBMTJs structures. About the dependence of the temperature difference and angle between the magnetizations, the charge current and TSTT are larger at the specific thicknesses (resonant positions) of the NM spacer than the others. The results also illustrate a magnitude enhancement for asymmetric DBMTJs compared with asymmetric single-barrier MTJs (SBMTJs). Therefore, it is possible to achieve a large TSTT for magnetization dynamics by new asymmetric DBMTJs.

Enhanced thermal spin transfer in MgO-based double-barrier tunnel junctions

Based on atomic first principles, we predict enhanced thermal spin transfer (TST) effects and small switching temperature gradient in Fe MgO Fe MgO Fe double-barrier magnetic tunnel junctions (MTJs). At room temperature, temperature gradient ∼10 with ∼10 K nm−1 across barriers would be sufficient to switch the magnetic configurations circularly in a junction with 3 MgO atomic layers (L), which is about one order smaller than that in Fe MgO(3L) Fe MTJs. This temperature gradient is under the current experimental capability. The resonant quantum-well states in companion with resonant interfacial states are responsible for the enhancement. Moreover, a thermal induced ‘off’ state is found in a double-barrier MTJ.

Rhombic Coulomb diamonds in a single-electron transistor based on an Au nanoparticle chemically anchored at both ends.

Rhombic Coulomb diamonds are clearly observed in a chemically anchored Au nanoparticle single-electron transistor. The stability diagrams show stable Coulomb blockade phenomena and agree with the theoretical curve calculated using the orthodox model. The resistances and capacitances of the double-barrier tunneling junctions between the source electrode and the Au core (R1 and C1, respectively), and those between the Au core and the drain electrode (R2 and C2, respectively), are evaluated as 4.5 MΩ, 1.4 aF, 4.8 MΩ, and 1.3 aF, respectively. This is determined by fitting the theoretical curve against the experimental Coulomb staircases. Two-methylene-group short octanedithiols (C8S2) in a C8S2/hexanethiol (C6S) mixed self-assembled monolayer is concluded to chemically anchor the core of the Au nanoparticle at both ends between the electroless-Au-plated nanogap electrodes even when the Au nanoparticle is protected by decanethiol (C10S). This is because the R1 value is identical to that of R2 and corresponds to the tunneling resistances of the octanedithiol chemically bonded with the Au core and the Au electrodes. The dependence of the Coulomb diamond shapes on the tunneling resistance ratio (R1/R2) is also discussed, especially in the case of the rhombic Coulomb diamonds. Rhombic Coulomb diamonds result from chemical anchoring of the core of the Au nanoparticle at both ends between the electroless-Au-plated nanogap electrodes.

Effect of silver doping on the current–voltage characteristic of PbS nanorods

We report on field emission property from a single nanorod measured by using scanning tunnelling spectroscopy. It has been shown that field emission from nanorods of small band gap semiconductor is significantly increasing by doping. The current transport mechanism is explained using double barrier tunnel junction formalism. It is observed experimentally that the Fowler–Nordheim tunnelling mechanism is dominant and governs the transport mechanism. The transport properties of PbS nanostructures in the form of nanorod are investigated in terms of various conduction mechanism. The minimum voltage necessary for triggering Fowler–Nordheim tunnelling under the revised biased for intrinsic sample ~0.95V and decreases to ~0.67V for increase doping concentration up to 1.76wt%.

Sub-picowatt volatile memory cell based on double-barrier tunnel junction

A bistable resistor as a volatile memory is proposed. A silicon island sandwiched with a double barrier oxide tunnel junction is used as storage node. The write operations are conducted by direct tunneling. The charges are localized on one side of the two silicon-oxide interfaces, and the position of the charge is interchanged upon the polarity of the applied voltages. The bistable data states are clearly identified and all operations are performed under a voltage below 1 V and a current below 1 pA. The present device can be a promising candidate for ultra low-power capacitorless dynamic random access memory.