Asymmetric Double Barrier Research Articles

Spin-polarizated transmissivity in an asymmetrical double barrier

The spin-polarized electron resonant tunnelling at zero magnetic field through a double barrier heterostructure like InAs/GaSb/InAs/GaSb/InAs has been calculated as a function of the electron energy. A model is proposed to study the combined effects of Dresselhaus and in-plane Rashba spin-orbit interactions on the spin-dependent tunnelling, taking into account the k3 dependence of the Dresselhaus Hamiltonian. For the directions and the spin mixing produces a 100% efficiency of polarization. Moreover, the effect of the Dresselhaus and Rashba spin-orbit interactions are shown to be quite favorable for the fabrication of spin filters and spintronic devices.

Tunneling processes in asymmetric double barrier magnetic tunnel junctions with a thin top MgO layer

Dynamic conductance dI/dV and inelastic electron tunneling spectroscopy (IETS) d2I/dV2 have been measured at different temperatures for double barrier magnetic tunnel junctions with a thin top MgO layer. The resistance in the antiparallel state exhibits a normal tunnel-like behavior, while the resistance in the parallel state shows metallic-like transport, indicating the presence of pinholes in the thin top MgO layer. Three IETS peaks are the zero-bias anomaly, interface magnons, and barrier phonons in both the parallel and antiparallel states. The zero-bias anomaly is the strongest peak in the parallel state and its intensity decreases with temperature. The magnon has the largest intensity in the antiparallel state and its intensity also decreases with temperature. The origins of the dips and peaks in the dI/dV-V curve are also discussed.

Tunable electron wave filter and Goos–Hänchen shift in asymmetric graphene double magnetic barrier structures

Electronic transmission and Goos–Hänchen shift for Dirac fermions in asymmetric graphene double magnetic barrier structure are investigated theoretically. Numerical results for rectangular barriers demonstrate that a transmission gap appears owing to the magnetic confinement and the width of gap is dependent on the incident angle and changed by the width of barrier and the distance between the double barriers. At the edge of the transmission gap, the Goos–Hänchen shifts of electrons have strong peaks and are tuned by the structure parameters. Furthermore, for the real magnetic structure induced by ferromagnetic strips, the transmission gap and GH shift remain. The tunable transmission gap indicates that the graphene magnetic structure can be use as electron wave filter and the tunable GH shift shows that the structure can be applied to laterally manipulate Dirac fermions in graphene.

Electron Transport through Magnetic Superlattices with Asymmetric Double-Barrier Units in Graphene

We investigate the transport properties through magnetic superlattices with asymmetric double-barrier units in monolayer graphene. In N-periodic asymmetric double-barrier units, there is (N − 1)-fold resonant peak splitting for transmission, but the splitting is (2N − 1)-fold in N-periodic symmetric units. The transmission depends not only on the value of incident wavevectors but also on the value and the direction of transverse wavevectors. This renders the structure's efficient wavevector filters. In addition, the conductance of standard electrons with a parabolic energy spectrum is suppressed more strongly than that of Dirac electrons, whereas the resonances are more pronounced for Dirac electrons than for standard ones.

Line-type resonance peaks and their suppression through graphene-based symmetric and asymmetric double barriers

Resonant tunneling through symmetric and asymmetric double barriers based on monolayer graphene at non-normal incidence is investigated. Due to the evanescent modes inside the barrier, the transmission, as a function of the incident energy, has a gap which can be tuned by the height of the barrier and the incident angle of the electrons. In terms of the coupling between the barriers and the well in the symmetric double barriers, several line-type resonance peaks with a unity value appear in the transmission gap, and the number of the resonance peaks is closely related to the incident angle, the height, and the width of the barrier. The resonant conditions in the transmission gap are derived and discussed. However, the line-type resonance peaks are greatly suppressed through the asymmetric double barriers. The authors demonstrate that line-type peaks possess potential applications in resonant tunneling devices and energy filters.

Fabry-Pérot resonances in graphene microstructures: Influence of a magnetic field

Fabry-P\'erot resonances in the transmission through single and double, graphene-based barriers (of height $V$) and wells are investigated and their dependence on an applied perpendicular magnetic field. For rectangular barriers the conductance decreases with increasing magnetic field while the resonances weaken (become more pronounced) with increasing magnetic field for ${E}_{F}<V$ $({E}_{F}>V)$. The position of the resonances exhibit a linear shift with magnetic field which move to lower (higher) energy for ${E}_{F}<V$ $({E}_{F}>V)$. Compared to semielliptic- or Gaussian-shaped barriers they show a smaller number of resonances in the absence of a magnetic field and an overall lower conductance but the resonant structure is more pronounced. The conductance of asymmetric double barriers show two major regions of resonances while the symmetric ones show one, that of three asymmetric barriers three, and so on.

Coupling Asymmetry Effect on the Coherent Spin Current Through a Ferromagnetic Lead/Quantum Dot/Ferromagnetic Lead System

The spin dependent transport through asymmetric double barrier magnetic tunnel junction system is theoretically studied, based on the non-equilibrium Green's function obtained via the equation of motion method. In the system, a single energy level is considered in the semiconductor quantum dot, which is asymmetrically coupled to the left and the right leads. It is found that due to the asymmetric coupling, the threshold voltages for charge current I - V characteristics are different for positive and negative applied bias, so are the spin current I - V characteristics. Besides, it is also found that the direction of current flow also modifies the electron occupancy within the dot.

Spin-Polarized Transport Phenomena in Double Magnetic Tunnel Junctions Caused by Ferromagnetic CoFe Nanoparticles

We present some unusual spin-polarized transport phenomena in asymmetric double barrier magnetic tunneling junctions (ADBMTJs) with CoFe/AlOx/ferromagnetic nanoparticle (FM-NPs)/AlOx/Ta structures. The conductance curves and the magnetoresistance ratio clearly oscillate with applied bias voltage, indicating the presence of Coulomb blockade effects due to isolated ferromagnetic nanoparticles in the parallel configuration in the ADBMTJ. The oscillation period is about 1.5 mV at 2 K.

Resonant Tunnelling and Storage of Electrons in Si Nanocrystals within a-SiNx/nc-Si/a-SiNx Structures

The a-SiNx/nanocrystalline silicon (nc-Si)/a-SiNx sandwiched structures with asymmetric double-barrier are fabricated in a plasma enhanced chemical vapour deposition (PECVD) system on p-type Si substrates. The nc-Si layer in thickness 5nm is fabricated from a hydrogen-diluted silane gas by the layer-by-layer deposition technique. The thicknesses of tunnel and control SiNx layers are 3nm and 20nm, respectively. Frequency-dependent capacitance spectroscopy is used to study the electron tunnelling and the storage in the sandwiched structures. Distinct frequency-dependent capacitance peaks due to electrons tunnelling into the nc-Si dots and capacitance-voltage (C - V) hysteresis characteristic due to electrons storage in the nc-Si dots are observed with the same sample. Moreover, conductance peaks have also been observed at the same voltage region by conductance-voltage (G - V) measurements. The experimental results demonstrate that electrons can be loaded onto nc-Si dots via resonant tunnelling and can be stored in our a-SiNx/nc-Si/a-SiNx structures.

MoRe-based tunnel junctions and their characteristics

Perspective Josephson Mo-Re alloy-oxide–Pb, Mo-Re alloy-normal metal–oxide–Pb and Mo-Re alloy-normal metal–oxide–normal metal–Mo-Re alloy junctions have been fabricated and investigated. Thin (~50-100 nm) MoRe superconducting films are deposited on Al2O3 substrates by using a dc magnetron sputtering of MoRe target. Normal metal (Sn, Al) thin films are deposited on the MoRe films surfaces by thermal evaporation of metals in vacuum and oxidized to fabricate junctions oxide barriers. Quasiparticle I–V curves of the fabricated junctions were measured in wide range of voltages. To investigate a transparency spread for the fabricated junctions barriers the computer simulation of the measured quasiparticle I–V curves have been done in framework of the model of multiple Andreev reflections in double-barrier junction interfaces. It's demonstrated the investigated junctions can be described as highly asymmetric double–barrier Josephson junctions with great difference between the two barrier transparencies. The result of the comparison of experimental quasiparticle I–V curves and calculated ones is proposed and discussed. Also I–V curves of the fabricated junctions have been measured under microwave irradiation with 60 GHz frequency, clear Shapiro steps in the measured I–V curves were observed and discussed.

Transport Characteristics of Mesoscopic Radio-Frequency Single Electron Transistor

The transport property of a quantum dot under the influence of external time-dependent field is investigated. The mesoscopic device is modelled as semiconductor quantum dot coupled weakly to superconducting leads via asymmetric double tunnel barriers of different heights. An expression for the current is deduced by using the Landauer–Buttiker formula, taking into consideration of both the Coulomb blockade effect and the magnetic field. It is found that the periodic oscillation of the current with the magnetic field is controlled by the ratio of the frequency of the applied ac-field to the electron cyclotron frequency. Our results show that the present device operates as a radio-frequency single electron transistor.

Structural and electrical characterization of gold nanoclusters in thin SiO2 films: Realization of a nanoscale tunnel rectifier

Conductive atomic force microscopy has been used to measure the I-V characteristics of nanometric Au clusters embedded in a SiO2 film prepared by sputter deposition and low temperature annealing. Highly local asymmetric rectifier I-V characteristics were evidenced and modelled in terms of electrical transport through an asymmetric double barrier tunnel junction SiO2/Au cluster/SiO2. The threshold voltage depends strongly on the cluster size and barrier thickness according to the model given.

Spin transport through a ZnSe‐based diluted magnetic semiconductor resonant tunneling structure in the presence of electric and magnetic fields

AbstractSpin‐dependent tunneling of electrons in asymmetric double quantum wells and barriers composed of different sequences of diluted magnetic and nonmagnetic well and barrier materials are investigated theoretically for ZnSe‐based semiconductor heterostructures in the presence of parallel magnetic and electric fields. In the studied systems the transmission of electrons and the degree of spin polarization depends on the strength of the magnetic (due to the s–d exchange‐enhanced spin splitting) and electric fields and on the direction of the applied bias, resulting in a highly spin polarized gas. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Resonant Tunneling Transmission Characteristics and Tunneling Time in Asymmetrical Double-Barrier Structures with Multiple Prewells

Resonant tunneling phenomenon in the asymmetrical double-barrier structures with multiple prewells is studied theoretically. First, analytical expressions for the tunneling transmission coefficient and the resonance condition are derived. Second, analytical expressions for the dwell time and for the shortest dwell time condition are obtained. It is found that the dwell time in the main well region decreases greatly with increasing number of prewells, and that there exists an optimum number of prewells which makes the dwell time in the whole region shortest.

Spin-dependent conductance in nonmagnetic InGaAs asymmetric double barrier devices

The spin dependence of the conductance of an asymmetric double-barrier InGaAs device is studied within the multiband k¢p and envelope function approximations. The spin-dependent transmission probability for electrons across the structure is obtained using transfer matrices and the low bias conductance per unit area is calculated as a function of the Fermi energy (or doping) in the contacts. The possibility to obtain spin polarized currents in such devices is demonstrated, however, the resulting degree of polarization is rather small (a few percent) in the specific InGaAs structures considered here.

Resonant tunneling of polarized electrons through nonmagnetic III-V semiconductor multiple barriers

The quantum transport of spin-polarized electrons across nonmagnetic III-V semiconductor multiple barriers is considered theoretically. We have calculated the spin dependent transmission coeficient, for conducting electrons transversing lattice-matched In0. 53Ga0. 47As/GaAs0. 5Sb0: /In0. 53Ga0. 47As/ InP/In0. 53Ga0. 47As nanostructures with different numbers of asymmetric double barriers, as a function of electron energy and angle of incidence. Spin-orbit split resonances, due to the Rashba term, are observed. The envelope function approximation and the Kane k ×p model for the bulk are used. For an unpolarized incident beam of electrons, we also obtain the spin polarization of the transmitted beam. The formation of spin dependent minibands of energy with nonzero transmission is observed.

Electron spin–orbit split minibands in semiconductor asymmetric superlattices

Semiconductor superlattices can be either symmetric or asymmetric with respect to specular reflection along the growth direction. The electronic miniband structure of asymmetric superlattices is in general spin dependent, due to spin‐orbit interaction. Using Kane’s kip model for the bulk and standard envelope function formalism, we have calculated the spin dependent transmission probability for electrons crossing different III‐V politype multibarrier nanostructures. We have obtained spin dependent intervals of energy with nonzero transmission, corresponding to the minibands of allowed electronic states in the superlattice. Spin‐orbit split minibands for InGaAs superlattices, with asymmetric double barrier unit cells and different pairs of latticematched barrier materials, are obtained from the transmission and reflection coefficients for the unit cell. The miniband structure is well reproduced by the transfer matrix calculation with already three unit cells. The symmetric‐asymmetric crossover as well as the miniband formation from the double barrier spin split resonances were also investigated. The effect of electron spin polarization by resonant tunneling is shown to be enhanced with the use of multibarrier or superlattice structures.

Role of interface optical phonons in magnetotunneling in asymmetric double-barrier structures

The role of interface-optical (IO) phonons in tunneling through an asymmetric double barrier structure in a magnetic field perpendicular to the barriers is studied. The phonon-assisted tunneling current densities are calculated and the numerical results for typical AlxGa1−xAs/GaAs/AlyGa1−yAs structures are given. The theoretical results show that the applied magnetic field sharpens and heightens the phonon-assisted tunneling peaks in agreement with experimental observations. In contrast to previous work, the calculated IO phonon-assisted tunneling current peak occurs at the energy the same as that of the bulk GaAs longitudinal-optical (LO) phonon energy. The IO phonon-assisted tunneling is much more important than the confined LO phonon-assisted tunneling. The phonon-assisted tunneling currents are sensitive to the thickness of the emitter barrier. Only one phonon-assisted tunneling peak can be easily observed for the wider well case.

Terahertz switching action of a double-barrier resonant tunneling device

The response of an asymmetric double barrier resonant device to the passage of terahertz radiation is discussed. Within the bistable region the radiation is able to turn the current flowing through the system on or off, with an onset that depends on the bias, the strength of the incoming radiation, and its frequency.

Scanning tunneling spectroscopy of InAs nanocrystal quantum dots

Scanning tunneling spectroscopy is used to investigate single InAs nanocrystals, 20--70 \AA{} in diameter, in a highly asymmetric double barrier tunnel junction configuration. The $I\ensuremath{-}V$ characteristics reflect contributions of both single-electron charging and the atomiclike level structure of the quantum dots. The spectra are simulated and well described within the framework of the ``orthodox model'' for single-electron tunneling. The peaks in the tunneling spectra display a systematic broadening with the reduction of dot diameter, from 40 to 150 meV over the studied quantum dot size range. This is assigned to a decreased electron dwell time on the dot, due to reduction of the barrier height, induced by the blueshift of the quantum-confined levels. The distribution of peak spacings within charging multiplets in the tunneling spectra is found to be Gaussian, resembling observations on metallic quantum dots.