Double-barrier Structures Research Articles

Spin and Valley Filter Based on Two-Dimensional WSe2 Heterostructures

In this work, we investigate the spin and valley transport properties of a $\mathrm{W}{\mathrm{Se}}_{2}$ monolayer placed on top of a ferromagnetic insulator. We are interested in controlling the transport properties by applying external potentials to the system. To obtain spin and valley polarizations, we consider a single- and a double-barrier structure with gate potentials. We analyze how the efficiency of these polarized transport properties depends on the gate-potential intensities and geometrical configurations. Additionally, we investigate how the spin and valley transport properties are modified when an ac potential is applied to the system. We obtain a controllable modulation of the spin and valley polarizations as a function of the intensity and frequency of the ac potential, mainly in the terahertz range. These results validate the proposal of double quantum well structures of $\mathrm{W}{\mathrm{Se}}_{2}$ as candidates to provide spin- and valley-dependent transport within an optimal geometrical parameter regime.

Supertunneling effect in graphene

The ballistic transmission of the Dirac ultrarelativistic quasielectrons in graphene structures with the rectangular potential barrier is considered, and both the single and the double-barrier structures are analyzed. Within the framework of the continuum model, the transmission coefficient of quasielectrons T is calculated depending on the parameters of the problem. It is believed that there is an electrostatic barrier, as well as the Fermi velocity barrier, due to the fact that this quantity may acquire different values in the barrier and out-of-barrier regions (υF2 and υF1, respectively) of the considered structures. It is shown that the effect of the supertunneling manifests itself in these structures which consists in the fact that under certain conditions the transmission through the structure is perfect (transmission rates T = 1) for the arbitrary angle of incidence of quasielectrons on the barrier. In the case of different values of the Fermi velocities in the barrier and out-of-barrier regions (the parameter β = υF2/υF1, which characterizes the velocity barrier, is not equal to unity), the supertunneling is observed for a certain ratio between the energy E and the barrier height U and significantly depends on β. The expression is given that determines the specified conditions for the supertunneling. In the case of equal velocities (β = 1), the supertunneling effect is observed for the quasielectron energy value E equal to half the height of the electrostatic barrier U. The analysis of the dependence of the transmission on the problem parameters is also provided.

Controlling Phase‐Coherent Electron Transport in III‐Nitrides: Toward Room Temperature Negative Differential Resistance in AlGaN/GaN Double Barrier Structures

AbstractResonant tunneling of electrons is important for the manufacture of high‐speed electronic oscillators and the electron injection control in quantum cascade lasers. In this work, room temperature negative differential resistance (NDR) in AlGaN/GaN double barrier structure with AlN/GaN digital alloy (DA) barriers is demonstrated. The peak‐to‐valley current ratio (PVCR) ranges from 1.1 to 1.24 at room temperature and becomes 1.5 to 2.96 at low temperatures, whereas no NDR is observed in double barrier structures with conventional ternary AlGaN barriers. The room temperature NDR together with the high PVCR at low temperature is attributed to the suppression of alloy disorder scattering by introducing AlN/GaN DA barriers. This work presents the successful control of phase‐coherent electron transport in III‐nitride heterostructures and is expected to benefit the future design of nitride‐based resonant tunneling structures and high‐speed electronic devices.

Armchair Graphene Nanoribbon Gate-Controllable RTD With Boron Nitride Barriers

Resonant-tunneling diodes (RTDs) based on graphene nanoribbons have attracted the attention of many researchers recently. In this article, we propose a new RTD based on graphene nanoribbon/h-boron nitride. In this structure, two hexagonal boron nitride (h-BN) slices are used as potential barriers to form a double-barrier structure, due to the higher bandgap of h-BN. Two gate electrodes are used in order to control the peak-to-valley current ratio (PVR). In the proposed structure, a PVR of 1.11 at a valley current of $5.56~\mu \text{A}$ and a PVR of 2.41 at a valley current of $0.49~\mu \text{A}$ have been obtained that are better than many reported structures in the literature. The proposed structure is a suitable candidate for use in various high current RTD-based applications. A numerical tight-binding model coupled with nonequilibrium Green’s function formalism is used for simulation and studying electronic properties of this structure.

Exploring bilayer graphene lateral quantum structures for valley filtering

Energy bands in AB-stacked bilayer graphene are featured by the co-presence of trigonal warping and a Mexican hat structure around each Dirac point, and electrically modulatable band edges with the modulation valley-dependent (valley-independent) in the case of in-plane (vertical) fields. This theoretical work takes into account the foregoing realistic features and exploits them for valleytronic applications. Specifically, it investigates in detail electrical valley transport in the armchair direction, in AB-stacked bilayer graphene lateral quantum structures with an interface, a single barrier, and double barriers, respectively, and demonstrates that due to the band structure features as well as an involved interplay among them, such structures harbor an interesting potential for valley filtering. An analysis of transmitted current valley polarization is provided. Optimization of the structures for the polarization is explored. The thermal effect on valley filtering is examined. Sizable polarizations of 88% (97%) and 23% (27%) at low and room temperatures, respectively, are shown to be achievable in the case of one-barrier (double-barrier) structures.

External quantum efficiency of a resonant tunneling diode photo detector: Structural parameters and wavelength dependencies

In this study, a resonant tunneling diode with AlAs/GaAs double barrier structure is simulated by using the non-equilibrium green’s function. A lattice mastched InGaAs absorption layer is added to the device for light sensing at the wavelength λ = 600 nm. The Photo current of the device and source photo current curves versus light intensity are compared. The quantum efficiency of the device and its dependences on structural parameters (absorption layer thickness, spacer thickness, collector and emitter width) and different wavelengths were simulated and their influences on operation of the device were investigated. The external quantum efficiency of 0.95 was obtained for the device at room temperature. The simulation results show that changing of external quantum efficiency upon absorption layer thickness is nonlinear. Electric field through the device and the absorption curve versus wavelength are presented. Also with increasing of spacer layer and collector thicknesses, external quantum efficiency also changes.

A simple analytical model for the resonant tunneling diode based on the transmission peak and scattering effect

A new analytical model for the current–voltage characteristic of a resonant tunneling diode (RTD) is presented herein, being derived from the basic integral of the Tsu–Esaki equation. The model is physics based and developed as a function of material parameters such as the effective mass, barrier heights, and Fermi levels as well as geometrical parameters such as the barrier height and well width. The material chosen for the double-barrier RTD structure is AlGaAs/GaAs. The dependence of the peak transmission on the applied voltage and the scattering effect in the active region, which are neglected in previous models, are also taken into account. The model is implemented and validated using numerical simulations, revealing that the resulting electrical characteristic is in good agreement with numerical simulations using the Green’s function formalism.

Enhancement of transparency in a double-barrier structure by the Fano antiresonance

We show that the presence of a side-attached state strongly modifies the transmission through a one-dimensional double-barrier system in the window of wavevectors around the Fano antiresonance. Specifically, the interplay between the Fano interference and the size quantization inside the structure gives rise to narrow resonant peaks in the transmission coefficient. The height of the peaks may become close to unity (perfect transmission) even for an asymmetric setup with strong barriers, where the transmission coefficient in the absence of the Fano state is strongly suppressed at all other wavevectors. Thus, the two types of the interference phenomena, each by itself leading to the suppression of the transmission, conspire in a peculiar way to produce the transparency enhancement.

THz photodetector using sideband-modulated transport through surface states of a 3D topological insulator

The transport properties of the surface charge carriers of a three dimensional topological insulator under a terahertz (THz) field along with a resonant double barrier structure is theoretically analyzed within the framework of Floquet theory to explore the possibility of using such a device for photodetection purposes. We show that due to the contribution of elastic and inelastic scattering processes in the resulting transmission, side-bands are formed in the conductance spectrum. This side band formation is similar to the side-bands formation in cavity transmission spectra in an optical cavity and this information can be used to detect the frequency of unknown THz radiation. The dependence of the conductance on the bias voltage, the effect of THz radiation on resonances and the influence of zero energy points on the transmission spectrum are also discussed.

Surface state transport in double-gated and magnetized topological insulators with hexagonal warping effects

We explore the scattering of Dirac electrons in a double-gated topological insulator in the presence of magnetic proximity effects and warped surface states. It is found that a magnetic field can shift the Dirac cone in momentum space and deform the constant-energy contour, or opens up a band gap at the Dirac point, depending on the magnetization orientation. The double gate voltage induces quantum wells and/or quantum barriers on the surface of topological insulators, generating surface resonant tunnelling states. It is found that the hexagonal warping effect can increase the electronic transport at high energies when the constant-energy contour exhibits a snowflake shape. The energy-dependent conductances in the parallel and antiparallel magnetic configurations exhibit out-of-phase oscillations due to the quantum interference of propagating waves in the region between the two magnetized segments. Although the conductance spectrum of the double-well structure is higher than that of the double-barrier structure, the magnetoresistance ratio versus the separation distance between the two magnetized barriers exhibits pronounced oscillations due to the resonant tunnelling states. We show that the surface state transport can be controlled by the exchange field and gate voltage without breaking time reversal symmetry, suggesting that the double gated and magnetized topological insulators can be utilized to achieve a large magnetoresistance ratio with a tunable sign.

Four-port SNAP microresonator device.

It is well known from quantum mechanics that the transmission amplitude of a symmetric double-barrier structure can approach unity at the resonance condition. A similar phenomenon is observed in optics for light which propagates between two waveguides weakly coupled through a microresonator. Examples of microresonators used for this purpose include ring, photonic crystal, toroidal, and bottle microresonators. However, ring and photonic crystal photonic circuits, once fabricated, cannot be finely tuned to arrive at the mentioned resonant condition. In turn, it is challenging to predictably adjust coupling to toroidal and bottle microresonators by translating the input-output microfibers, since the modes of these resonators are difficult to separate spatially. Here we experimentally demonstrate a four-port micro-device based on a SNAP microresonator introduced at the surface of an optical fiber. The eigenmodes and corresponding eigenwavelengths of this resonator are clearly identified for both polarization states by the spectrograms measured along the length of the fiber. This allows us to choose the resonant wavelength and simultaneously determine the positions of the input-output microfiber tapers to arrive at the required resonance condition.

Spin-dependent quantum beating of the conductance oscillations in asymmetric velocity modulated ferromagnetic graphene junction

Abstract The mismatch effect of a double Fermi-velocity barrier structures NG/FG 1 /NG/FG 2 /NG junction on the electrically driven SdH-like oscillations of spin-dependent conductance is investigated, where NG is normal graphene and FG 1,2 is the ferromagnetic-gate with Fermi-velocity modulation ξ 1,2 where ξ 1,2 is the Fermi velocity ratio between NG region and FG 1,2 region. The results show that the effect of asymmetry of the double Fermi-velocity barrier structures, the difference value between ξ 1 and ξ 2 cause of the oscillations the spin-dependent quantum beating pattern. Which the beating patterns of the spin transport appeared as a result of the superposition of the two oscillatory components in the double Fermi velocity barriers. In addition, we found that the shifting of the spin-dependent quantum beats of the G ↑ and G ↓ is due to the presence of the exchange field, H, that is, the G ↑ and G ↓ are shifted in the opposite direction. By the difference value of the phase shift can be estimated to be 2H . In case of U + η σ H > > E F which electrons are collimated, the oscillations behaviors show the quantum beating with holding constant amplitude and the period of the beating amplitude depend on the Fermi velocity ratio mismatch in FG regions. These interesting characteristic of spin-dependent quantum transport may identify the effect of Femi-velocity asymmetry on double proximity-induced ferromagnetic graphene structure for making spintronic devices.

Electronic transport on graphene armchair-edge nanoribbons with Fermi velocity and potential barriers

The transport properties of Dirac fermions through armchair-edge graphene nanoribbons (AGNRs) with a single and double rectangular Fermi velocity vF and electrostatic potential U barriers is investigated. We employ a transfer matrix method (TMM) to compute the transmission coefficient of the full set of propagating mode which is used to obtain the conductance and Fano factor spectra for both metallic and semiconducting nanoribbons. We show that a reduced Fermi velocity within the barrier region can partially suppress the backscattering resulting from the electrostatic potential. In a double barrier structure, the emergence of high-order transmitting modes is shown to substantially reduce the Fano factor in the spectral region around U. These results indicate that the simultaneous tuning of vF and U in barrier regions can be explored to control the electronic transport in graphene-based nanoelectronics structures.

Valley aspect of lateral tunneling transport

We theoretically study the valley aspect of lateral tunneling transport in valleytronic materials, using AB-stacked (Bernal-stacked) bilayer graphene as a material example. A lateral structure with multiple well/barrier interfaces is considered. A full-zone tight-binding method is employed, allowing for the study of intervalley mixing in electron states. We study the degree of tunneling current valley contrast under the condition of a source-drain bias or barrier asymmetry. For a single-barrier structure, it is found that the intervalley mixing is negligible, and the contrast is positively correlated with the barrier width and height. For a double-barrier structure, the effect of resonant tunneling is investigated. The intervalley mixing is shown to be significantly enhanced at resonant tunneling. In addition, the resonant levels are found valley split, and a proposal is given which exploits the splitting for the generation of a sizable tunneling current valley polarization. Based on a simple model using momentum-shifted thermal distribution for obliquely incident electrons, the temperature dependence of tunneling current valley polarization is also examined. An interesting crossover from the valley-orbit interaction-dominant polarization to the warping-dominant polarization is found to occur as the temperature is increased.

A high-quality spin and valley beam splitter in WSe2 tunnelling junction through the Goos–Hänchen shift

We have theoretically studied the Goos–Hänchen shift of spin and valley electron in single and double barrier WSe2 tunnelling junctions. For single barrier structure, it has shown that a two-fold degenerate lateral shift induced by the spin-valley locking occurs at Fabry–Perot resonance width. By introducing the ferromagnetic exchange field, one spin and valley dependent lateral shift can be further obtained. Different from single barrier structure, the double barrier structure exhibits two series of Fabry–Perot interferences with lateral shifts exceeding thousands of Fermi wavelengths due to the effect of localized states, and more important, these interferences generated between two potential barriers are even higher than the shift formed inside of two potential barriers. In addition, for a specified Fermi energy, one can modulate the desired spatial separation of spin and valley beam by controlling the incident angle or potential barrier. Our results demonstrate that the proposed WSe2 double barrier structure is a fascinating candidate for designing high-quality spin and valley beam splitter.

Electronic Transport and Resonant Tunneling Properties of Hyperbolic Pöschl-Teller Double-Barrier Structures

The ballistic electron transport characteristics of hyperbolic Pöschl-Teller double-barrier resonant tunneling structures are investigated. The transmission coefficient and the current density-voltage characteristic in electrically biased one dimensional hyperbolic Pöschl-Teller double-barrier structures with different structure parameters have been calculated by using non-equilibrium Green’s function method. Results for the transmission coefficients and the current densities are compared with the barrier shapes. The results show that the widths of the wells and heights of barriers have a significant effect on the transmission properties. The current density-voltage characteristic presents N-type negative differential resistance.

Quantum Pumping with Adiabatically Modulated Barriers in Three-Band Pseudospin-1 Dirac-Weyl Systems.

In this work, pumped currents of the adiabatically-driven double-barrier structure based on the pseudospin-1 Dirac–Weyl fermions are studied. As a result of the three-band dispersion and hence the unique properties of pseudospin-1 Dirac–Weyl quasiparticles, sharp current-direction reversal is found at certain parameter settings especially at the Dirac point of the band structure, where apexes of the two cones touch at the flat band. Such a behavior can be interpreted consistently by the Berry phase of the scattering matrix and the classical turnstile mechanism.

Repeatable asymmetric resonant tunneling in AlGaN/GaN double barrier structures grown on sapphire

We report repeatable AlGaN/GaN resonant tunneling diodes (RTDs) grown on a sapphire substrate by metal organic chemical vapor deposition. The RTDs exhibit clear negative differential resistance at low temperature in both bias directions. Peak-to-valley current ratios of 1.4 and 1.08 and peak current densities of 6 kA/cm2 and 0.65 kA/cm2 are extracted at 6.5 K for forward and reverse bias, respectively. The polarization fields in III-nitrides are found to affect the diode electrical behaviors by modulating the symmetry of the two barriers and altering the thickness of the depletion region, which eventually triggers asymmetric resonant tunneling transport.

InAs/GaSb superlattice resonant tunneling diode photodetector with InAs/AlSb double barrier structure

We report on a resonant tunneling diode (RTD) photodetector using type-II InAs/GaSb superlattices with an InAs/AlSb double barrier structure. At 80 K, the maximum response of the detector is at about 4.0 μm and the 50% cutoff wavelength is 4.8 μm. The resonant tunneling mechanism is confirmed by observing the negative differential resistance (NDR) phenomenon. The detector is also tested under illumination by a laser with a wavelength of 3.3 μm. A significant photocurrent and NDR peak shift are observed when changing the laser illumination power. The internal multiplication factor, which means how many excess electrons can be triggered by one absorbed photon, is estimated to be 1.01 × 105 at 4.9 V and is 1.90 × 103 at 1.4 V.

Direct writing of heterostructures in single atomically precise graphene nanoribbons

Precision control of interfacial structures and electronic properties is the key to the realization of functional heterostructures. Here, utilizing the scanning tunneling microscope (STM) both as a manipulation and characterization tool, we demonstrate the fabrication of a heterostructure in a single atomically precise graphene nanoribbon (GNR) and report its electronic properties. The heterostructure is made of a seven-carbon-wide armchair GNR and a lower band gap intermediate ribbon synthesized bottom-up from a molecular precursor on an Au substrate. The short GNR segments are directly written in the ribbon with a STM tip to form atomic precision intraribbon heterostructures. Based on STM studies combined with density functional theory calculations, we show that the heterostructure has a type-I band alignment, with manifestations of quantum confinement and orbital hybridization. Our finding demonstrates a feasible strategy to create a double-barrier quantum dot structure with atomic precision for functionalities, such as negative differential resistance devices in GNR-based nanoelectronics.