Tunneling Region Research Articles

High operating temperature interband cascade midwave infrared detector based on type-II InAs/GaSb strained layer superlattice

We report on an interband cascade mid-wave infrared (MWIR) detector based on type-II InAs/GaSb/AlSb strained layer superlattices (T2SL). The reported device has a seven-stage cascade region, each segment containing a MWIR absorber region, a graded T2SL transport region, and an interband tunneling region. Above room temperature spectral response was observed, with a cutoff wavelength of 7 μm at 420 K. Detailed radiometric measurements yielded a Johnson noise limited detectivity of 3.0 × 1011 cmHz1/2W−1 (8.9 × 108 cmHz1/2W−1) and a dark current density of 3.6 × 10−7 A/cm−2 (7.3 × 10−3 A/cm−2) near zero bias with a 100% cutoff wavelength of 5.2 μm and 6.2 μm at 77 K (295 K), respectively, with an estimated 36.2% QE.

Design of GNRFET using different doping profiles near the source and drain contacts

The graphene nanoribbon field effect transistors (GNRFET) suffer from band-to-band tunnelling (BTBT), which in turn causes ambipolar conduction. In this simulation study, we propose a step–linear doping profile for the graphene nanoribbon near the source and drain contacts. This type of doping profile suppresses the BTBT and ambipolar conduction of the transistor. The proposed step–linear dopant distribution efficiently modulates the potential barrier at the source–channel interface, increases the width of the tunnelling region, and thus reduces the OFF-state current by decreasing the possibility of BTBT. A pz orbital band model with the nearest neighbour is used to simulate the graphene nanoribbon. The Schrödinger equation is solved by non-equilibrium Green's function method to achieve the charge density. In order to calculate the electrostatic potential, the Poisson's equation is solved by means of the non-linear finite difference method. We have used the uncoupled mode space approach significantly to reduce the computational time. It is shown that the proposed doping profile improves the performance of graphene nanoribbon transistor by decreasing the OFF-state current and increasing the ON-state current. We have also observed a slight improvement in the subthreshold slope of the proposed GNRFET. The proposed GNRFET is more suitable than the conventional GNRFETs for switching applications.

Rectification in substituted atomic wires: a theoretical insight

Recently, there have been discussions that the giant diode property found experimentally in diblock molecular junctions could be enhanced by the many-body electron correlation effect beyond the mean field theory. In addition, the effect of electron–phonon scattering on an electric current through the diode molecule, measured by inelastic tunneling spectroscopy (IETS), was found to be symmetric with respect to the voltage sign change even though the current is asymmetric. The reason for this behavior is a matter of speculation. In order to clarify whether or not this feature is limited to organic molecules in the off-resonant tunneling region, we discuss the current asymmetry effect on IETS in the resonant region. We introduced heterogeneous atoms into an atomic wire and found that IETS becomes asymmetric in this substituted atomic wire case. Our conclusion gives the other example of intrinsic differences between organic molecules and metallic wires. While the contribution of electron–phonon scattering to IETS is not affected by the current asymmetry in the former case, it is affected in the latter case. The importance of the contribution of the electron–hole excitation to phonon damping in bringing about the current asymmetry effect in IETS in the latter case is discussed.

The Josephson effect between triplet superconductors through a finite ferromagnetic barrier

Charge and spin transport in a junction involving two triplet superconductors and a ferromagnetic barrier are studied. We use Bogoliubov–de Gennes wavefunctions to construct the Green’s function, from which we obtain the Josephson currents in terms of the Andreev reflection coefficients. We focus on the consequences of a finite barrier width for the occurrence of 0–π transitions and for the spin currents, and examine the appropriateness of the common δ-function approximation for the tunneling region.

EXTREMELY SCALABLE CROSS-POINT TOP-GATED HETEROJUNCTION TUNNELING-TRANSISTORS

Energy efficient hetero-junction tunneling transistors in a simple "cross-point" configuration that utilizes top gates are analyzed for use at extremely scaled sub-10 nm gate-lengths. The active tunneling region comprises of a vertical p ++/ n + heterojunction (for example formed at the cross point of p ++ SiGe / n + Si ), where modulation of the energy-bands in the gated n + Si region with a top-gate is used to control the degree of band overlap and tunneling distance and hence current. The sub-threshold swing characteristic of these devices is shown to be potentially highly immune to extreme downscaling to 6 nm gate length allowing for an intact and efficient switching behavior to be retained. The extreme scalability and ultra-low voltage operation could make such cross-point devices useful for alternative applications and architectures that require ultimate energy efficiency.

Transport properties through double-magnetic-barrier structures in graphene

We study electrons tunneling through a double-magnetic-barrier structure on the surface of monolayer graphene. The transmission probability and the conductance are calculated by using the transfer matrix method. The results show that the normal incident transmission probability is blocked by the magnetic vector potential and the Klein tunneling region depends strongly on the direction of the incidence electron. The transmission probability and the conductance can be modulated by changing structural parameters of the barrier, such as width and height, offering a possibility to control electron beams on graphene.

Alignment-dependent ionization of hydrogen molecules in intense laser fields

Recent experiments showed that the alignment-dependent ionization probabilities of hydrogen molecules vary as a function of laser intensity, and the anisotropy deviates from the prediction of the molecular tunneling ionization model (MO-ADK). To investigate the physical origins of the deviation, we systematically studied the anisotropy of hydrogen molecules in intense laser fields for three wavelengths, 400 nm (multiphoton ionization), 1850 nm (tunneling ionization), and 800 nm (in between), by solving the time-dependent Schr\"odinger equation with a model potential. The calculated ratio of the ionization probabilities for laser fields parallel and perpendicular to the molecular axis are in reasonable agreement with experiment. Furthermore, by analyzing the molecular wave function and the model potential, we found that the discrepancies between experiment and the MO-ADK prediction originated from the inaccurate coefficients used in the model and that the assumption of isotropy of the effective potential in the tunneling region is invalid.

Optical rectification and field enhancement in a plasmonic nanogap

Metal nanostructures act as powerful optical antennas because collective modes of the electron fluid in the metal are excited when light strikes the surface of the nanostructure. These excitations, known as plasmons, can have evanescent electromagnetic fields that are orders of magnitude larger than the incident electromagnetic field. The largest field enhancements often occur in nanogaps between plasmonically active nanostructures, but it is extremely challenging to measure the fields in such gaps directly. These enhanced fields have applications in surface-enhanced spectroscopies, nonlinear optics and nanophotonics. Here we show that nonlinear tunnelling conduction between gold electrodes separated by a subnanometre gap leads to optical rectification, producing a d.c. photocurrent when the gap is illuminated. Comparing this photocurrent with low-frequency conduction measurements, we determine the optical frequency voltage across the tunnelling region of the nanogap, and also the enhancement of the electric field in the tunnelling region, as a function of gap size. The measured field enhancements exceed 1,000, consistent with estimates from surface-enhanced Raman measurements. Our results highlight the need for more realistic theoretical approaches that are able to model the electromagnetic response of metal nanostructures on scales ranging from the free-space wavelength, λ, down to ∼λ/1,000, and for experiments with new materials, different wavelengths and different incident polarizations.

Effect of Interface Traps and Oxide Charge on Drain Current Degradation in Tunneling Field-Effect Transistors

In this letter, we report for the first time the degradation mechanism of drain current in tunneling field-effect transistors (TFETs). Using positive-bias and hot-carrier (HC) stress experiments and TCAD simulation, we show that the drain current degradation is mainly induced by the interface traps and/or oxide charge located above the tunneling region, causing reduction of tunneling field and tunneling current. The interface traps mainly induce the degradation in transconductance, while the oxide charge essentially causes a threshold-voltage shift in TFETs. The results show that the interface-trap generation is dominant under a positive-bias stress, while the oxide-charge creation is important under an HC stress in n-TFETs.

Overdoped High Current Density Bi2–xPbxSr2CaCu2O8+δ Intrinsic Josephson Junction Mesas and Their Switching Current Distributions

Small and thin mesa structures of intrinsic Josephson junctions 1–2 μm on a side and 1.5–7.5 nm in thickness are fabricated from single crystals of Bi2–xPbxSr2CaCu2O8+δ with a nominal Pb content x from 0.2 to 0.5. The Josephson critical current density jc is found to be pronouncedly large, ranging from 10 to 50 kA/cm2. Switching current probability measurements for these mesa samples show an indication of a crossover to the macroscopic quantum tunneling region.

A Self-Consistent SDD-NEGF Approach for Modelling Magnetic Tunnel Junctions

Most contemporary work on magnetic tunnel junctions either consider ballistic quantum transport across the tunneling barrer, for instance via the Non Equilibrium Green's Function (NEGF) formalism, or treat the tunneling region as a lump resistance connected to bulk metallic leads in the Spin Drift-Diffusion (SDD) approach. The effects of interfacial barriers and central device physics on the spin accumlation in the leads have largely been ignored in most NEGF calculations. In this work we introduce a self consistent approach that combines the NEGF and SDD methods. We calculate the effects of the barrier potential height and spin orbit interaction on the spin injection from a ferromagnet to a semiconductor through the barrier.

Calculation of the characteristic functions of anharmonic oscillators

The energy levels of quantum systems are determined by quantization conditions. For one-dimensional anharmonic oscillators, one can transform the Schrödinger equation into a Riccati form, i.e., in terms of the logarithmic derivative of the wave function. A perturbative expansion of the logarithmic derivative of the wave function can easily be obtained. The Bohr–Sommerfeld quantization condition can be expressed in terms of a contour integral around the poles of the logarithmic derivative. Its functional form is B m ( E , g ) = n + 1 2 , where B is a characteristic function of the anharmonic oscillator of degree m, E is the resonance energy, and g is the coupling constant. A recursive scheme can be devised which facilitates the evaluation of higher-order Wentzel–Kramers–Brioullin (WKB) approximants. The WKB expansion of the logarithmic derivative of the wave function has a cut in the tunneling region. The contour integral about the tunneling region yields the instanton action plus corrections, summarized in a second characteristic function A m ( E , g ) . The evaluation of A m ( E , g ) by the method of asymptotic matching is discussed for the case of the cubic oscillator of degree m = 3 .

Current noise correlations in double quantum dots asymmetrically coupled to external leads

We investigate the current noise autocorrelations and cross correlations through two interacting quantum dots coupled to four external leads. The results indicate sign reversal of the cross correlation in different tunneling regions, respect to competing or supporting processes. By adjusting the dot-lead coupling in one dot, shot noise in another system can be modulated from sub- to super-Poissonian in the Coulomb blockade regime. Furthermore, spin injection can induce super-Poissonian shot noises in both dots due to the corporation of spin blockade and Coulomb blockade.

Intense field ionization of diatomic molecules: Two-center interference and tunneling

We investigate the ionization of model diatomic molecules exposed in intense laser fields both numerically and analytically. Our simulations show that, in the tunneling region, the interplay of the tunneling effect and molecular two-center structure has an important role in ionization. It can lead to the enhancement of ionization in molecules as compared to their reference atoms with similar ionization potential. Furthermore, it also plays a dominating role in the angle dependence of molecular ionization. This effect is different from that of two-center interference in ionization. The latter manifests itself remarkably in the multiphoton region and can cause the ionization suppression of molecules in this region. Our further comparisons suggest the significant influence of the orbital symmetry of the reference atom on molecular ionization comparison experiments.

Transmission characteristics of evanescent Lamb waves through a tunneling region: a chance for backward propagation

Evanescent waves are characterized by the exponential decay of the amplitude along the propagation direction, such that no phase velocity could be properly defined and the concept of propagation itself has to be properly redefined. However, evanescent waves can carry energy beyond a tunneling region where they are produced, and their effect in the forbidden region may be properly inferred by the outgoing wave. In the present paper, evidence of evanescent Lamb waves on a plate is given, as they are produced within a forbidden region where thickness is properly reduced and the acoustic modes are above threshold of propagation. However, the coupling of modes at each line boundary between different regions makes it difficult to single out the tunneling mode alone, since all modes share the same frequency. Therefore, we resort to the propagation of the backward S1 mode, that can be properly isolated from all the others. That makes the problem of refraction/reflection of backward propagating modes at a boundary, a problem by itself to be investigated and makes it worth to perform experiments on it. This is done in the present paper, as well, by detecting the acoustic field of a backward propagating Lamb mode reflected from the end boundary of a steel plate and the focusing effect from such a boundary is put in evidence in the case that a forward propagating mode is reflected as a backward propagating one.

Simple and efficient scanning tunneling luminescence detection at low-temperature

We have designed and built an optical system to collect light that is generated in the tunneling region of a low-temperature scanning tunneling microscope. The optical system consists of an in situ lens placed approximately 1.5 cm from the tunneling region and an ex situ optical lens system to analyze the emitted light, for instance, by directing the light into a spectrometer. As a demonstration, we measured tip induced photoluminescence spectra of a gold surface. Furthermore, we demonstrate that we can simultaneously record scanning tunneling microscope induced luminescence and topography of the surface both with atomic resolution.

Direct imaging of tunneling from a potential well

We experimentally map the wavefunction in the vicinity of a radial potential well. We photograph light intensity near the tunneling region as well as measure the spiraling phase structure via interference with a reference wave. This spiraling phase structure is required for conservation of angular momentum. The experimental image reveals the non-intuitive emission of light from a region in space that is empty of material and relatively far from the device.

Dwell time in graphene-based magnetic barrier nanostructures

The authors have investigated the dwell time of electrons tunneling through nonuniform magnetically modulated graphene monolayer. Two types of models, i.e., the square magnetic barrier and the δ-function magnetic barrier, are introduced to simulate the magnetic modulation realized by depositing nanoscale ferromagnetic stripes on top of the graphene monolayer. It is found that both the dwell time and the transmission probability show remarkable anisotropy that varies in different magnetically modulated configurations. Particularly, when the electrons tunnel through the graphene monolayer modulated by two antiparallelly aligned ferromagnetic stripes, the corresponding transmission probability exhibits angularly symmetric property, whereas the dwell time does not. Moreover, there exists great discrepancy of the dwell time between in the Klein tunneling region and in the resonant tunneling region, where each region corresponds to the perfect transmission peaks.

Radiation of bremsstrahlung accompanying the α-decay of heavy nuclei

This paper presents the spectrum of the bremsstrahlung emission accompanying the α-decay of 226Ra (E α=4.8 MeV) by measuring the α−γ coincidences. We analyze the spectrum by using the model presented in our previous study on the α-decay of 214Po (E α=7.7 MeV). We compare the experimental data with the quantum mechanical calculation and find a good agreement between theory and experiment. We analyze the bremsstrahlung emission contributions from the tunneling and external regions of the nucleus barrier into the total spectrum, and find destructive interference between these contributions.

Electric-current-induced heat generation in a strongly interacting quantum dot in the Coulomb blockade regime

The heat generation by an electric current flowing through a quantum dot with the dot containing both electron-electron interaction and electron-phonon interaction, is studied. Using the non-equilibrium Keldysh Green's function method, the current-induced heat generation is obtained. We find that for a small phonon frequency, heat generation is proportional to the current. However, for a large phonon frequency, heat generation is in general qualitatively different from the current. It is non-monotonic with current and many unique and interesting behaviors emerge. The heat generation could be very large in the Coulomb blockade region, in which the current is very small due to the Coulomb blockade effect. On the other hand, in the resonant tunneling region, the heat generation is very small despite a large current, an ideal condition for device operation. In the curve of heat generation versus the bias, a negative differential of the heat generation is exhibited, although the current is always monotonously increasing with the bias voltage.