Resonant Tunneling Nanostructure Research Articles

Characteristics of Resonant Tunneling in Nanostructures with Spacer Layers

The effect of spacer layers on electron transport through two-barrier nanostructures was studied using the numerical solution of the time-dependent Schrodinger–Poisson equations with exact discrete open boundary conditions. The formulation of the problem took into account both the active region consisting of a quantum well and barriers, as well as the presence of highly doped contact layers and spacer layers. The use of the time formulation of the problem avoids the divergence of the numerical solution, which is usually observed when solving a stationary system of the Schrodinger–Poisson equations at small sizes of spacer layers. It is shown that an increase in the thickness of the emitter spacer leads to a decrease in the peak current through the resonant tunneling nanostructures. This is due to the charge accumulation effects, which, in particular, lead to a change in the potential in an additional quantum well formed in the emitter spacer region when a constant electric field is applied. The valley current also decreases as the thickness of the emitter spacer increases. The peak current and valley current are weakly dependent on the thickness of the collector spacer. The collector spacer thickness has a strong effect on the applied peak and valley voltages. The above features are valid for all three different resonant tunneling nanostructures considered in this study. For the RTD structures based on Al0.3Ga0.7As/GaAs, the optimized peak current value Ipmax = 5.6 × 109 A/m2 and the corresponding applied voltage Vp = 0.44 V. For the RTD structures based on AlAs/In0.8Ga0.2As, Ipmax = 14.5 × 109 A/m2 (Vp = 0.54 V); for RTD structures based on AlAs/In0.53Ga0.47As, Ipmax = 45.5 × 109 A/m2 (Vp = 1.75 V).

Tunneling transport in open nitride resonant tunneling structures taking into account the acoustic phonons: An variational approach

Quantum-mechanical theory of tunneling transport through open nanostructures based on nitride semiconductors, taking into account the contribution of acoustic phonons, is proposed. A variational method taking into account the component of Hamiltonian due to the deformation potential of acoustic phonons at different temperatures is developed. On the basis of the developed theory the transmission coefficient of a two-barrier AlN/GaN resonant tunneling nanostructure, its electronic conductivity and tunneling current are calculated taking into account the impact of acoustic phonons. It was found that with the increase of temperature values the frequencies of quantum transitions between electronic quasistationary states increase, an increase in temperature from cryogenic to room values results in a shift of the tunneling current peak and a decrease in the maximum value of dynamic conductivity in almost seven times.

Investigation of the electron-acoustic phonon interaction via the deformation and piezoelectric potentials in AlN/GaN resonant tunneling nanostructures

The analytical theory of acoustic phonons arising in the layers of a nitride-based nanostructure, taking into account the presence of the piezoelectric effect, was developed. Proposed theory is based on the first obtained exact solutions of the system consisting of equation of motion for a semiconductor medium and Maxwell equation. The theory of the electronic spectrum renormalized by their interaction with acoustic phonons via deformation and piezoelectric potentials was developed using the method of the temperature Green's function and the Dyson equation. Direct calculations of the electron level shifts and their decay rates are performed for geometric and physical parameters of the new experimentally created nanostructure for various temperature values.

NanoLEDs for energy-efficient and gigahertz-speed spike-based sub-λ neuromorphic nanophotonic computing

AbstractEvent-activated biological-inspired subwavelength (sub-λ) photonic neural networks are of key importance for future energy-efficient and high-bandwidth artificial intelligence systems. However, a miniaturized light-emitting nanosource for spike-based operation of interest for neuromorphic optical computing is still lacking. In this work, we propose and theoretically analyze a novel nanoscale nanophotonic neuron circuit. It is formed by a quantum resonant tunneling (QRT) nanostructure monolithic integrated into a sub-λ metal-cavity nanolight-emitting diode (nanoLED). The resulting optical nanosource displays a negative differential conductance which controls the all-or-nothing optical spiking response of the nanoLED. Here we demonstrate efficient activation of the spiking response via high-speed nonlinear electrical modulation of the nanoLED. A model that combines the dynamical equations of the circuit which considers the nonlinear voltage-controlled current characteristic, and rate equations that takes into account the Purcell enhancement of the spontaneous emission, is used to provide a theoretical framework to investigate the optical spiking dynamic properties of the neuromorphic nanoLED. We show inhibitory- and excitatory-like optical spikes at multi-gigahertz speeds can be achieved upon receiving exceptionally low (sub-10 mV) synaptic-like electrical activation signals, lower than biological voltages of 100 mV, and with remarkably low energy consumption, in the range of 10–100 fJ per emitted spike. Importantly, the energy per spike is roughly constant and almost independent of the incoming modulating frequency signal, which is markedly different from conventional current modulation schemes. This method of spike generation in neuromorphic nanoLED devices paves the way for sub-λ incoherent neural elements for fast and efficient asynchronous neural computation in photonic spiking neural networks.

Single-Electron Transistor Based on a Linear Structure of Three Electrically and Optically Controlled Tunnel-Coupled Quantum Dots

A schematic diagram of a single-electron transistor with a sensitive element based on a resonant tunneling nanostructure consisting of three semiconductor quantum dots is discussed. The electron density in the steady (current) mode at the structure’s output is numerically calculated using a model of the incoherent electron transport between the extreme points and metallic reservoir contacts. The dependences of the electron density on time and system parameters are obtained. It is shown that there are sets of parameters that can provide high levels of sensitivity and reliability of the external electric field measurements. An alternative optically controlled transistor circuit is proposed, in which the electron transport through the structure is supported by a resonant laser field.

Electron-phonon interaction in three-barrier nanosystems as active elements of quantum cascade detectors

The theory of electron tunneling through an open nanostructure as an active element of a quantum cascade detector is developed, which takes into account the interaction of electrons with confined and interface phonons. Using the method of finite-temperature Green’s functions and the electron-phonon Hamiltonian in the representation of second quantization over all system variables, the temperature shifts and electron-level widths are calculated and the contributions of different electron-phonon-interaction mechanisms to renormalization of the spectral parameters are analyzed depending on the geometrical configuration of the nanosystem. Due to weak electron-phonon coupling in a GaAs/Al0.34Ga0.66As-based resonant tunneling nanostructure, the temperature shift and rf field absorption peak width are not very sensitive to the electron-phonon interaction and result from a decrease in potential barrier heights caused by a difference in the temperature dependences of the well and barrier band gaps.

High-frequency response and the possibilities of frequency-tunable narrow-band terahertz amplification in resonant tunneling nanostructures

The characteristics of the high-frequency response of single- and double-well resonant tunneling structures in a dc electric field are investigated on the basis of the numerical solution of a time-dependent Schrodinger equation with open boundary conditions. The frequency dependence of the real part of high frequency conductivity (high-frequency response) in In0.53Ga0.47As/AlAs/InP structures is analyzed in detail for various values of the dc voltage V dc in the negative differential resistance (NDR) region. It is shown that double-well three-barrier structures are promising for the design of terahertz-band oscillators. The presence of two resonant states with close energies in such structures leads to a resonant (in frequency) response whose frequency is determined by the energy difference between these levels and can be controlled by varying the parameters of the structure. It is shown that, in principle, such structures admit narrow-band amplification, tuning of the amplification frequency, and a fine control of the amplification (oscillation) frequency in a wide range of terahertz frequencies by varying a dc electric voltage applied to the structure. Starting from a certain width of the central intermediate barrier in double-well structures, one can observe a collapse of resonances, where the structure behaves like a single-well system. This phenomenon imposes a lower limit on the oscillation frequency in three-barrier resonant tunneling structures.

Teoriya vlastyvostey rezonansno-tunel'nykh nanostruktur, yak aktyvnykh elementiv kvantovykh kaskadnykh lazeriv i detektoriv

Teoriya vlastyvostey rezonansno-tunel'nykh nanostruktur, yak aktyvnykh elementiv kvantovykh kaskadnykh lazeriv i detektoriv

Continuous narrow-band amplification tuning effect at terahertz frequencies in double-quantum-well resonant tunneling nanostructures

High-frequency response properties of single and double-quantum-well resonant tunneling diodes (RTD) are examined in a wide frequency range, up to terahertz (THz) frequencies, on the base of proposed quantum theory. Numerical solutions of time-dependent Schrödinger equations with open-system boundary conditions in an external electromagnetic field are performed. The numerical solutions take into account the influence of bias DC voltage on electronic states in RTD with finite height and width of barriers both for monoenergetic and for Fermi-distributed electrons in emitter and collector parts of structures. We show that the presence of an additional level in double-quantum-well structures breaks the response symmetry and leads to selective narrow-band frequency amplification, as well as to the effect of amplification frequency tuning at THz frequencies by variation of applied bias voltage. These phenomena predict an increase of gain coefficient and open new perspectives for engineering of novel types of THz oscillators and other high-frequency units.

Quasi-stationary states of electrons interacting with strong electromagnetic field in two-barrier resonance tunnel nano-structure

An exact solution of non-stationary Schrodinger equation is obtained for a one-dimensional movement of electrons in an electromagnetic field with arbitrary intensity and frequency. Using it, the permeability coefficient is calculated for a two-barrier resonance tunnel nano-structure placed into a high-frequency electromagnetic field. It is shown that a nano-structure contains quasi-stationary states the spectrum of which consists of the main and satellite energies. The properties of resonance and non-resonance channels of permeability are displayed.

Nonresonant transparency channels of a two-barrier nanosystem in an electromagnetic field with an arbitrary strength

The theory of the quasistationary spectrum of the system of electrons interacting with a high-frequency electromagnetic field with an arbitrary strength in a two-barrier resonant tunneling nanostructure has been proposed on the basis of the exact solution of the total one-dimensional Schrodinger equation. It has been shown that nonresonant transparency channels in the planar two-barrier resonant tunneling structure appear owing to the superposition of a certain quasistationary state with first field resonance harmonics of both nearest states; this superposition is due to the interaction of electrons with the electromagnetic field.

Non-perturbation theory of electronic dynamic conductivity for two-barrier resonance tunnel nano-structure

The non-perturbation theory of electronic dynamic conductivity for open two-barrier resonance tunnel structure is established for the first time within the model of rectangular potentials and different effective masses of electrons in the elements of nano-structure and the wave function linear over the intensity of electromagnetic field. It is proven that the results of the theory of dynamic conductivity, developed earlier in weak signal approximation within the perturbation method, qualitatively and quantitatively correlate with the obtained results. The advantage of non-perturbation theory is that it can be extended to the case of electronic currents interacting with strong electromagnetic fields in open multi-shell resonance tunnel nano-structures, as active elements of quantum cascade lasers and detectors.

Evolution of the spectral parameters of quasiparticles in an open symmetrical three-barrier resonant tunneling nanostructure

The spectral parameters (resonance energies and resonance widths) of quasi-stationary states in an open symmetrical three-barrier resonant tunneling nanostructure have been theoretically calculated in terms of the model of spatially dependent effective masses and rectangular potentials of quasiparticles (electrons, holes, excitons) by using the methods of transmission coefficient, scattering matrix, and probability distribution function. The evolution of the spectral parameters of quasi-stationary states of quasiparticles as a function of the variation in the geometric sizes of the nanosystem has been calculated and analyzed for the open three-barrier resonant tunneling structure consisting of three barriers (GaAs) and two wells (In0.25Ga0.75As) as an example. It has been established that the experimental and theoretical results are in satisfactory agreement for the model without fitting parameters in the case of a heavy exciton.

Admittance of a one-dimensional double-barrier resonant tunneling nanostructure

We study the dynamic response of a one-dimensional double-barrier nanostructure to an ac bias. Combining the Schr\odinger equation, Poisson equation and the scattering theory, we calculate the internal potential, charge density, and the ac conductance as well. The results show that the charge distribution is antisymmetric with respect to the center of the double barrier, and depends crucially on the relative position of the Fermi level to the resonant energies of the well. The diagonal emittance is found to have a similar dependence. It is negative (inductive behavior) when the Fermi energy is very close to the resonant energies, and it reaches the negative maximum at resonant energies, while it is always positive (capacitive behavior) when the Fermi energy is within the barrier depth and far from resonance, and develops two peaks closely on both sides of the inductive peak. This result is in agreement with that obtained from discrete model. In addition, we find that the capacitive peaks correspond to the maxima of charge-density fluctuation, and inductive peaks to zero charge-density distribution. Therefore, the sign and magnitude of emittance reflect how the charge piles up inside the device.

Resonant tunneling in time-periodically modulated semiconductor nanostructures

We consider a semiconductor (AlGaAs/GaAs/AlGaAs) double-barrier resonant tunneling nanostructure (DBRTS), in which the bottom of the potential well is modulated by an external periodic potential V(t) = V mod cos Ωt . We examine the resonance response to both mono-energetic and spatially localized incident electrons, extending previous results in three ways. First, we study numerically the resonant characteristics in the transmission probabilities as functions of both the electron's energy and the frequency of the external field. We find that new low-frequency resonant peaks appear in the frequency dependence of the transmission probabilities and discuss the origin and possible experimental observation of these peaks. Second, we develop an analytic perturbation theory (valid for small values of the parameter λ = V mod / h ̷ Ω ) describing the processes of absorption and emission of single quanta of the modulation field and show that this perturbative treatment agrees well with the results of numerical calculations in the appropriate regions, up to and including the regime in which the one-quantum processes saturate. Third, for localized (Gaussian) incident wave packets, we show numerically that temporal modulations of the potential well in the DBRTS can induce spatial modulations in the transmitted and reflected wave packets. We develop a simple qualitative picture that explains our results. Finally, we estimate characteristic values of the parameters relevant for possible observation of these effects in laboratory experiments.

Electrooptical Properties of the Si δ-doped GaAs/AlGaAs Triple-Barrier Resonant Tunneling Structure

The electrooptical properties of a Si δ-doped GaAs/Al0.3Ga0.7As triple-barrier resonant tunneling (TRT) nanostructure have been studied by photoreflectance spectroscopy from 20 K to room temperature. The TRT structure forms two coupled quantum wells. The first and the second electronic subbands in the wide well are partially filled, and the corresponding optical transitions are suppressed. The strongest spectral feature is due to the enhancement of the subband resonance between the two coupled wells. The ionized δ–doping centers in the Al0.3Ga0.7As side barriers can induce internal electric fields, and cause oscillatory spectral features when the photon energy is larger than the gap of Al0.3Ga0.7As. Analyzing the oscillatory part of the spectrum gives an internal electric field of 32 kV/cm, and an estimation of the electron subband energy in the ionized δ-doping center induced potential wells.

Electronic properties of semiconductor nanostructures probed by scanning tunneling microscopy

We demonstrate the novel application of the scanning tunneling microscope to find and probe the electronic transport properties of single semiconductor nanostructures at temperatures from room temperature to as low as 1.5 K. The current-voltage characteristics of InGaAs/InAlAs resonant tunneling nanostructures clearly show negative differential resistance. From the dependence of the current on cross-sectional area the lateral depletion is estimated to be 300 Å.