Double-barrier Tunneling Structures Research Articles

Hot electron photoemission in metal–semiconductor structures aided by resonance tunneling

Enhancement of the surface photoemission from metal into semiconductor by resonance tunneling of photoexcited electrons through (quasi-) discrete level in quantum well, located within Schottky barrier of the metal–semiconductor interface, is studied theoretically taking into account the difference between the electron masses in metal and semiconductor. It is shown, in particular, that resonance tunneling through the discrete level can lead to the redshift of the threshold wavelength of surface photoeffect, higher slope linear growth in photocurrent near the threshold (in contrast to quadratic growth, i.e., Fowler's law), and the possibility to increase substantially the photoemission efficiency similarly to recent experimental results on hot carrier generation in plasmonic structures with a discrete energy level at metal interface. The difference in the effective masses is shown to significantly affect the results. Double-barrier tunneling structures with resonant tunneling may become attractive for applications in photochemistry and in plasmonic photodetectors in near IR and middle IR regions of the spectrum.

Design of Quantum Double-Barrier Tunnel Structures for THz Detection

In this paper, an approach to the design of terahertz tunable detector based on the resonance double-barrier structure is proposed. The approach incorporates the inverse scattering prob- lem method and nonequilibrium Green's function technique. The results of the numerical simulation of the device operation are also presented.

Coulomb-driven terahertz-frequency intrinsic current oscillations in a double-barrier tunneling structure

We investigate time-dependent, room-temperature quantum electronic transport in GaAs/AlGaAs double-barrier tunneling structures (DBTSs). The open-boundary Wigner-Boltzmann transport equation is solved by the stochastic ensemble Monte Carlo technique, coupled with Poisson's equation and including electron scattering with phonons and ionized dopants. We observe well-resolved and persistent terahertz-frequency current-density oscillations in uniformly doped, dc-biased DBTSs at room temperature. We show that the origin of these intrinsic current oscillations is not consistent with previously proposed models, which predicted an oscillation frequency given by the average energy difference between the quasibound states localized in the emitter and main quantum wells. Instead, the current oscillations are driven by the long-range Coulomb interactions, with the oscillation frequency determined by the ratio of the charges stored in the emitter and main quantum wells. We discuss the tunability of the frequency by varying the doping density and profile.

Computation of quantum electron transport with local current conservation using quantumtrajectories

A recent proposal for modeling time-dependent quantum electron transport withCoulomb and exchange correlations using quantum (Bohm) trajectories (Oriols 2007Phys. Rev. Lett. 98 066803) is extended towards the computation of the total (particleplus displacement) current in mesoscopic devices. In particular, two differentmethods for the practical computation of the total current are compared. The firstmethod computes the particle and the displacement currents from the rate ofBohm particles crossing a particular surface and the time-dependent variations ofthe electric field there. The second method uses the Ramo–Shockley theoremto compute the total current on that surface from the knowledge of the Bohmparticle dynamics in a 3D volume and the time-dependent variations of the electricfield on the boundaries of that volume. From a computational point of view, it isshown that both methods achieve local current conservation, but the second ispreferred because it is free from ‘spurious’ peaks. A numerical example, a Bohmtrajectory crossing a double-barrier tunneling structure, is presented, supporting theconclusions.

Decoherence due to contacts in ballistic nanostructures

memoryless on time scales coarsened over their energy-relaxation time, and the evolution of the current- limiting active region can be considered Markovian. Therefore, we first derive a general Markovian map in the presence of a memoryless environment by coarse graining the exact short-time non-Markovian dynamics of an abstract open system over the environment memory-loss time, and we give the requirements for the validity of this map. We then introduce a model contact-active region interaction that describes carrier injection from the contacts for a generic two-terminal ballistic nanostructure. Starting from this model interaction and using the Markovian dynamics derived by coarse graining over the effective memory-loss time of the contacts, we derive the formulas for the nonequilibrium steady-state distribution functions of the forward- and backward- propagating states in the nanostructure's active region. On the example of a double-barrier tunneling structure, the present approach yields an I-V curve that shows all the prominent resonant features. We address the relationship between the present approach and the Landauer-Buttiker formalism and also briefly discuss the inclusion of scattering.

Dynamically modulated tunneling for multipurpose electron devices: Application to THz frequency multiplication

The development of THz electron devices by coupling time-dependent electron quantum transport and electromagnetism is studied. A novel proposal for a frequency multiplier that generates a 1 THz harmonic from a 200 GHz input signal is described in detail with numerical simulations. The proposed electron device is a nanoscale double-gate field-effect transistor with a double barrier tunneling structure. Its functionality is based on dynamically modulated tunneling: a gate voltage forces the oscillation of the double barrier at frequencies comparable to the inverse of the electron transit time. This example points out the viability of using driven tunneling phenomena to develop room-temperature electron device for THz applications that can reduce the cost, the size and the complexity of present prototypes.

Verification of Au Nanodot Size Dependence on Coulomb Step Width by Non-contact Atomic-force Spectroscopy

We demonstrate single electron counting on an alkanethiol-protected Au nanodot in a double-barrier tunneling structure by noncontact atomic-force spectroscopy (nc-AFS). The Coulomb step width dependence on the Au nanodot diameter is observed. Evaluation of fractional charge Q 0 and contact potential difference by nc-AFS reveals a V d -independent voltage shift due to Q 0 .

Single Electron on a Nanodot in a Double-Barrier Tunneling Structure Observed by Noncontact Atomic-Force Spectroscopy

A single electron has been observed on a nanodot in a double-barrier tunneling structure by noncontact atomic-force microscopy at fixed separation. Frequency shift-voltage dependence of an Au-coated cantilever/vacuum/1-decanethiol protected Au nanodot/1-octanethiol self-assembled monolayer/Au substrate structure deviates from the theoretical parabolic curve, which is attributed to the change in the number of quantized electrons on the Au nanodot caused by the Coulomb blockade phenomena.

Tunneling resistance of double-barrier tunneling structures with an alkanethiol-protected Au nanoparticle

Coulomb staircases in double-barrier tunneling junctions consisting of a scanning-probe--vacuum-gap--alkanethiol-protected Au nanoparticle/Au (111) electrode have been measured as a function of the set point current of scanning tunneling spectroscopy. The tunneling resistances of the scanning probe-Au core of a nanoparticle $({R}_{1})$ and the Au core-Au (111) electrode $({R}_{2})$ are evaluated by fitting a theoretical Coulomb staircase into the experimental tunneling current-voltage characteristics measured by scanning tunneling spectroscopy. When a vacuum gap exists between the scanning probe and alkanethiol Au nanoparticles, ${R}_{1}$ is inversely proportional to the set point current. On the contrary, in the case of ${R}_{1}<{R}_{2}$, the top of the tip of the scanning probe tends to penetrate the octanethiol-protecting molecule of an Au nanoparticle. ${R}_{2}$ is found to be independent of the set point current, and ${R}_{2}$ of octanethiol- and hexanethiol-protected Au nanoparticles are evaluated as $7.6\phantom{\rule{0.3em}{0ex}}\mathrm{G}\ensuremath{\Omega}\ifmmode\pm\else\textpm\fi{}10%$ and $460\phantom{\rule{0.3em}{0ex}}\mathrm{M}\ensuremath{\Omega}\ifmmode\pm\else\textpm\fi{}10%$, respectively.

Anomalous negative differential conductance in nanomechanical double barrier tunneling structures

Anomalous negative differential conductance (NDC) is observed along with a Coulomb staircase in the tunneling current-voltage curves of a nanomechanical double barrier tunneling structure consisting of a scanning vibrating probe/vacuum/colloidal Au nanodot/1,6-hexanedithiol/Au substrate. The peak voltages in the NDC phenomena when the sample voltage is applied correspond well with that of the probe voltage. We discuss a candidate mechanism of the NDC phenomena by taking into account the modulation of the tunneling rate between the scanning probe and an Au nanodot due to the nanometer-scale probe vibration.

Spin-dependent resonant tunneling in symmetrical double-barrier structures

A theory of resonant spin-dependent tunneling has been developed for symmetrical double-barrier structures grown of non-centrosymmetrical semiconductors. The dependence of the tunneling transparency on the spin orientation and the wave vector of electrons leads to (i) spin polarization of the transmitted carriers in an in-plane electric field, (ii) generation of an in-plane electric current under tunneling of spin-polarized carriers. These effects originated from spin-orbit coupling-induced splitting of the resonant level have been considered for double-barrier tunneling structures.

Calculation of current-voltage characteristics of gallium arsenide symmetric double-barrier resonance tunneling structures with allowance for the destruction of electron-wave coherence in quantum wells

An approach that describes the effect of electron-wave coherence destruction in quantum wells of GaAs-based symmetric double-barrier resonance tunneling structures on their current-voltage characteristics is suggested. Electron scattering by polar optical phonons, ionized impurities, and surface roughness in quantum wells, as well as thermal fluctuations of particle energies, are analyzed as the major factors responsible for coherence destruction. Current-voltage characteristics of GaAs-based symmetric double-barrier resonance tunneling structures are calculated for three-, two-, and one-dimensional electron gas in the emitter and collector of the structure at different temperatures.

Electron spin filtering in all-semiconductor tunneling structures

In this work we briefly review the present day perspectives for exploiting conventional non-magnetic semiconductor nano-technology to design high speed spin-filter devices. In recent theoretical investigations a high spin polarization has been predicted for the ballistic tunneling current in semiconductor single- and double-barrier asymmetric tunnel structures of III–V semiconductors with strong Rashba spin–orbit coupling. We show in this paper that the polarization in the tunneling can probability be sufficiently increased for producing realistic single-barrier structures by including of the Dresselhaus term into consideration.

Creation and quenching of interference-induced emitter-quantum wells within double-barrier tunneling structures

The initial creation and subsequent quenching of the emitter quantum well within double-barrier resonant tunneling structures (RTSs) is the key process that explains the origin of the hysteresis and plateau-like structure of the I–V characteristics. This fundamental process, which evolves out of quantum-mechanical interference, defines the basic mechanism that can lead to intrinsic high-frequency oscillations. This article presents numerical results, derived from a coupled Wigner–Poisson model, that illustrate the underlying mechanisms responsible for the creation and disappearance of the emitter-quantum well. Additional theoretical results are also given that demonstrates how subband state coupling, between the emitter-quantum well (EQW) and the main-quantum well (MQW) defined by the double-barrier heterostructure, leads to the hysteresis and instability behavior. This article will reveal how the quantum interference that develops between the incident and reflected electron wave function (i.e., from the first barrier) leads to the formation of an emitter-quantum well. An analysis is also performed to define the effects of EQW–MQW subband coupling on the current–density verses voltage (I–V) characteristics and the overall I–V dependence on the initial charging states of the individual wells. In particular, this analysis is used to show how the EQW is formed and quenched and how it influences the time-dependent behavior of the structure when it is subject to forward- and backward-bias sweeps of the applied bias voltages. This article provides fundamental quantum-mechanical explanations for the complicated time-dependent processes within double-barrier RTSs and provides insight into the hysteresis and intrinsic oscillation behavior.

Observation of Displacement Current Staircase and Negative Differential Resistance in Nanomechanical Double-Barrier Tunneling Structures with Scanning Vibrating Probe

Simultaneous measurements of tunneling current and displacement current spectroscopy are demonstrated in nanomechanical double-barrier tunneling structures which consist of scanning vibrating probe/vacuum/colloidal Au dots/self-assembled monolayer (SAM)/Au substrate. The displacement current staircase due to Coulomb blockade is observed together with the tunneling current staircase in the nanomechanical double-barrier tunneling structures. In the case of tunneling current spectroscopy, negative differential resistances (NDRs) are also observed. NDRs are concerned with the change in the number of electrons on the Au dot and the tunneling resistance.

Observation of Coulomb staircases of both tunneling current and displacement current in nanomechanical double barrier tunneling structures

Displacement current staircase due to Coulomb blockade is observed together with a tunneling current staircase in nanomechanical double barrier tunneling structures that consist of scanning vibrating probe/colloidal Au particles/vacuum/PtPd substrate. We discuss the motion of single electrons on and through the Au particles from both the tunneling and displacement current staircases, and demonstrate the electron shuttle phenomenon due to nanomechanical probe vibration.

Shot noise in spin-polarized currents

We investigate spin-dependent fluctuations in spin-polarized electronic currents in a semimagnetic double-barrier tunneling structure. We use both quantum-coherent and semi-classical approaches to study the effects of spin-flip scattering on shot noise. In a limited parameter range, we find that for a fully spin-polarized incoming beam, both descriptions yield shot-noise suppression.

Spectroscopy of growth islands inGaAs/In0.1Ga0.9As/AlAsdouble-barrier structures from photoluminescence and resonant tunneling studies

Photoluminescence spectroscopy is used to investigate interface roughness effects on electron and hole tunneling through ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{I}\mathrm{n}}_{0.1}{\mathrm{Ga}}_{0.9}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}\mathrm{A}\mathrm{s}$ double-barrier tunneling structures. Typical quantum-well photoluminescence spectra present a splitting of 8 meV due to interface roughness and island formation in the quantum well. The states of these islands are selectively populated by electrons and holes by applying bias and changing the photon excitation intensity. Carrier transfer mechanisms between islands are clearly identified as well as intersubband scattering in a sequential tunneling picture. The measurement of activation energies as a function of bias provides an estimate for the electron density in resonant tunneling condition for diodes showing up islands at the interfaces. The possibility of observing growth island effects in $I(V)$ characteristics is also carefully discussed.

Recent Results on Beryllium Chalcogenides

Beryllium chalcogenides are semiconductors with a large bandgap. They have a higher degree of covalent bonding than all of the other II–VI compounds. The bond energies of BeSe and BeS are comparable to those of GaN. Due to their bandgaps and their lattice constants they can be incorporated in quaternary mixed crystals which are lattice matched to GaAs. The hardness of II–VI materials containing beryllium offers new possibilities to enlarge the lifetime of laser diodes, emitting in the green and blue spectral range. First double heterostructure laser diodes with beryllium as a constituent have been produced on p-type GaAs substrates by molecular beam epitaxy. Double barrier tunneling structures of the composition ZnSe–BeTe were realized and investigated in high magnetic fields. Shubnikov-de Haas oscillations were observed in the ZnSe contacts of the devices.

Optical probing of interface roughness in resonant tunneling structures

We report on photoluminescence (PL) and photoluminescence-excitation measurements in GaAs/In0.1Ga0.9As/AlAs double-barrier tunneling structures as a function of bias voltage and temperature. We have observed a splitting in the quantum well photoluminescence due to island formation in the quantum well. We have a good correlation between the variation of integrated PL intensity, linewidth, and tunnel current bias for both lines. The temperature dependence of photoluminescence spectra shows that transfer of carrier between islands can be tuned by the applied bias and that states in different islands are populated by electrons in the resonant tunneling process.