Triple Barrier Research Articles

Mid-infrared intersubband electroluminescence from a single-period GaAs/AlGaAs triple barrier structure

This letter reports the observation of intersubband electroluminescence from a single-period resonant tunneling structure. Intersubband emission (λ≈8.4 μm), with a full width at half maximum of 7 meV, was observed from a GaAs/AlGaAs triple barrier structure with quantum well widths of 66 and 33 Å. The emission was coupled out of the sample by a metallic grating with a period of 5 μm deposited on the surface. The intensity of emission follows the resonance behavior in the I–V characteristics. As the temperature increases from 10 to 200 K, the emission intensity decreases monotonically by a factor of ∼2 and the emission energy shifts down slightly (ΔE≈2.7 meV). The temperature dependence of the emission energy is explained by a combination of thermal broadening of the electron distribution and the nonparabolicity of the conduction bands.

Photoluminescence spectroscopy of intersubband population inversion in aGaAs/AlxGa1−xAstriple-barrier tunneling structure

We have used interband photoluminescence (PL) spectroscopy to demonstrate the occurrence of an intersubband population inversion in a ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ triple barrier structure containing asymmetric coupled quantum wells. The relative populations of the $n=1$ $(E1)$ and $n=2$ $(E2)$ electron subbands of the wider quantum well (QW1) are deduced from the relative intensities of the PL peaks arising from recombination of the $E1$ and $E2$ electrons. A significant population inversion is obtained between $E2$ and $E1$ when the structure is biased so that $E1$ is in resonance with the $n=1$ ${(E1}^{*})$ level of the narrower quantum well (QW2). The key importance of the interwell resonance in achieving population inversion is confirmed by comparison with PL results from structures in which ${E1\ensuremath{-}E1}^{*}$ alignment does not occur.

Charge accumulation in GaAs/AlGaAs triple barrier resonant tunneling structures

In this article we present photoluminescence and photoluminescence excitation spectroscopy data from three triple barrier resonant tunneling structures. The spectroscopic techniques are used to estimate the charge accumulation in both tunneling quantum wells of the devices as a function of bias. The charging behavior is extremely asymmetrical, with significant charge accumulation only in the quantum well adjacent to the emitter region of the device and not in the quantum well adjacent to the collector region, irrespective of the direction of bias. This asymmetry in the charging behavior is analogous to highly asymmetrical double barrier resonant tunneling structures. However, due to the two quantum wells present in the triple barrier design it provides a more flexible system to study charge density dependent effects. We also present evidence for negatively charged exciton formation in the first quantum well for both directions of applied bias.

Intersubband Electroluminescence from GaAs/AlGaAs Triple Barrier Resonant Tunnelling Structures

This paper reports the first observation of intersubband electroluminescence from a single period resonant tunnelling structure. The emission (λ = 8.4 μm) with a full width at half maximum of 7 meV, was observed from a GaAs/AlGaAs triple barrier structure with quantum well widths of 66 and 33 Å. The emission was coupled out of the sample by a metallic grating deposited on the surface.

Observation of new type resonances in triple barrier resonant tunneling diodes

Current–voltage characteristics measured in triple barrier resonant tunneling diodes reveal new features about energy level alignment in a superlattice. Our data indicate that energy levels in the two quantum wells are not aligned at current peaks. Current peaks are observed when one of the energy levels in the two wells becomes resonant with the emitter level. One of the current peak was thermally activated, and showed inverted bistability at 77 K temperature. We explain that this is due to X state assisted tunneling.

Phonon bottleneck effects in an InGaAs/InAlAs triple barrier tunnelling diode at high magnetic fields

We investigated non-resonant phonon-assisted tunnelling in an InGaAs/InAlAs triple barrier tunnelling diode in the presence of high magnetic fields up to 40 T in the current direction. We observe quenching of the non-resonant valley current and narrow band GaAs-like and AlAs-like longitudinal optical (LO) phonon peaks related to the phonon bottleneck effect. The peak-to-valley ratio of the device increases significantly up to magnetic fields of 12 T. LO-phonon peaks give information about the electron - LO-phonon coupling strength in the well and the barrier layers.

A dynamic 15N NMR study of kinetic hydrogen/deuterium isotope and tunnel effects on the triple proton transfer in crystalline 3,5‐dimethylpyrazole

AbstractUsing dynamic solid state 15N CPMAS NMR spectroscopy (CP≡cross polarization, MAS≡magic angle spinning) the kinetics of the degenerate intermolecular triple proton and deuteron transfer in the cyclic trimers of 15N‐labeled polycrystalline 3,5‐dimethylpyrazole (DMP) have been studied in a wide temperature range. At high temperatures, rate constants of the various isotopic HHH, HHD, HDD, and DDD transfer reactions are obtained in the millisecond timescale by lineshape analysis of partially deuterated doubly 15N‐labeled DMP. At low temperatures, the kinetics were followed by magnetization transfer methods in the laboratory frame. In order to suppress artifacts arising from 15N‐spin diffusion these experiments were performed on singly 15N labeled DMP, partially diluted in the non‐labeled material. As multiple kinetic hydrogen/deuterium isotope effects on triple proton transfer reactions have not yet been studied these effects are modeled theoretically for the single barrier case involving a concerted proton motion and for the triple barrier case where the three protons are transferred stepwise. The experimental kinetic isotope effects obey the rule of the geometric mean, i.e. kHHH/kHHD ≈︁ kHHD/kHDD ≈︁ kHDD/kDDD ≈︁ 3.6 i.e. ≈︁ kHHH/kDDD ≈︁ 47 at 300 K, which is indicative of a single barrier where all hydrons loose zero‐point energy in the transition state. At low temperatures, strong deviations from an Arrhenius behavior are observed for all isotopic reactions, indicating incoherent triple hydron tunneling processes, which are described in terms of a modified Bell tunneling model.

Enhancement of resonant tunneling current at room temperature

Enhancement of the resonant peak current through the ground quantum-well state at room temperature is observed at a low voltage by adding the second quantum-well structure to the AlAs/GaAs (001) double-barrier single quantum-well heterostructure. The peak to valley current ratio increases slightly with the increasing temperature up to room temperature, and shows stable characteristics up to high temperature in this AlAs/GaAs (001) triple barrier heterostructure. The results indicate that the optimum alignment of the Fermi level with the lined-up quantum-well states can greatly improve the resonant peak current in the low voltage range, and therefore device characteristics.

Full wafer optical characterisation of resonant tunnelling structures using photoluminescence excitation spectroscopy

We report on a novel optical technique for accurately characterising unprocessed GaAs AlGaAs tunnelling structures having two or more thin barriers and thick, heavily doped, contact layers. The results from photoluminescence excitation spectroscopy measurements are presented in which the intensity of the photoluminescence from the heavily doped contact layers rather than photoluminescence from the quantum wells is monitored. This method gives essential information for growth and quality control of “as-grown” layers prior to device fabrication. The usefulness and precision of this technique is illustrated by the measurement of a 0.3% variation in layer thickness from the centre to the edge of an as-grown wafer, and a 1% wafer to wafer variation in layer thickness in a series of triple barrier resonant tunnelling structures that have been designed for use at THz frequencies.

Phonon Assisted Tunneling and Peak-to-Valley Ratio in a Magnetically Confined Quasi Zero Dimensional InGaAs/InAlAs Resonant Tunneling Diode

We studied phonon-assisted tunneling and the peak-to-valley ratio ( P/V ratio) of magnetically confined “dot-like” electron systems in an InGaAs/InAlAs triple barrier resonant tunneling diode. We observe phonon bottleneck, i.e., suppression of the nonresonant valley current and two mode narrow band GaAs- and AlAs-like longitudinal optical (LO)-phonon-assisted tunneling peaks. The P/V ratio increases remarkably with the magnetic confinement and has a maximum each time the energy separation of the confined states matches the GaAs-LO-phonon energy.

GaAs/AlGaAs/InGaAs Vertical Triple Barrier Single Electron Transistors

A gated vertical sub-micron triple barrier structure with undoped Al0.22Ga0.78As barriers and In0.05Ga0.95As wells is used to study the properties of two weakly coupled quantum dots containing just a few electrons. We find that the conductance peaks become sparse as the ac excitation voltage is decreased when there is considerable mismatch between the ladders of energy levels in the two dot. Concurrently, the diamond shaped regions of Coulomb blockade are strongly disrupted. On the other hand, when the mismatch between energy levels is “small”–comparable to or less than the level width–the evolution of conductance peaks in pairs in the presence of a magnetic field applied parallel to the tunneling current is clearly seen and this can be related to spin-degeneracy.

Longitudinal optical phonon relaxation in a GaAs/AlGaAs triple barrier tunnel diode at strong perpendicular magnetic fields

Longitudinal optical (LO) phonon relaxation is investigated in a GaAs/AlGaAs triple barrier tunnel diode at strong perpendicular magnetic fields. We observe LO-phonon assisted tunneling to the ground state as well as to Landau level states. Maxima in the LO-phonon assisted tunnel peak are found each time the LO-phonon energy matches multiple of the Landau level spacing. Corresponding maxima as well as parity effects are found in the integral tunnel conductance. The results indicate enhancement of LO-phonon assisted inter Landau level transitions with increasing magnetic field.

Photon assisted transport through semiconductor quantum structures in intense terahertz electric fields

Quantum transport in resonant tunneling diodes, sequential resonant tunneling superlattices and miniband superlattices in the presence of intense terahertz electric fields is marked by new channels opened by the absorption or emission of one or more terahertz photons. In triple barrier resonant tunneling diodes, new transport channels supported by the absorption or stimulated emission of up to three terahertz photons are observed. In sequential resonant tunneling superlattices, dynamic localization is accompanied by absolute negative resistance. Transport in miniband resonances with controlled by coherent tunneling through several barriers and quantum wells, reveals multi-photon resonances with Bloch oscillation. Photon-assisted transport in these semiconductor quantum structures bears a strong analogy to quasi-particle tunneling and the AC Josephson effect in superconducting junctions and opens the arena of superconducting electronics to semiconductor systems. These experiments are made possible by the UCSB free-electron lasers that deliver kilowatts of tunable radiation from 120 GHz to 4.8 THz.

Resonant tunneling in double-quantum-well triple-barrier heterostructures.

We present a low-temperature (mK) magnetotransport study, using intense pulsed magnetic fields to 50 T, of two double GaAs quantum well, triple AlAs barrier resonant tunneling structures, which demonstrates the critical influence of the second quantum well on the tunneling behavior. We show that charge accumulation in the first well, and thus the overall tunneling characteristic, is controlled by the position of the lowest bound state in the second well, and identify tunneling transitions that are unique to triple-barrier structures. We also demonstrate that the tunneling current is sensitive to integer and fractional quantum Hall effect states, and find that the voltage-tunable accumulation of charge in one of the samples provides an unusual environment for the study of ground states of a two-dimensional electron system. \textcopyright{} 1996 The American Physical Society.

Realizing a completed‐resonant level by using an asymmetrical triple‐barrier structure

AbstractBecause of the rapid development of quantum effect devices, it is necessary to obtain a synthesis method which can give the expected quantum effect symptoms. Using a triple‐barrier structure, quantum effects which cannot be obtained with a double‐bather structure can be realized. However, only a few studies have been made on the triple‐bather structure, in particular, on the asymmetric triple‐bather structure.In this paper, based on the circuit theoiy, the syn thesis method of an asymmetrical triple‐barrier structure where a single perfect resonant level is formed in its quantum well is proposed. It is assumed that an original symmetrical double‐barrier structure has a first resonant level E1 and a second resonant level E2 whose resonant energy is four times higher than that of E1. One of the single barriers is replaced by another symmetrical double barrier with the first resonant level equal to the second resonant level E2 of the original one.In the nonsymmetrical triple‐barrier structure formed by the forementioned method, the transmission probability at E1 is 1 and that at higher‐energy levels is sufficiently low. This synthesis can be achieved by using the power reflection coefficient and the Smith chart of a complex equivalent circuit. the validity of this method can be established by means of the Smith chart. By carrying out a numerical calculation on the electron transmission characteristic of the synthesized nensymmetrical triple barrier, it was confirmed that the transmission probability at the energy level that is four times higher than E1 can be around 0.1.

Luminescence studies of resonant tunneling in a triple barrier structure with strongly coupled quantum wells

The electrical and optical properties of an n-oped triple barrier resonant tunneling diode are investigated. A thin central AlAs barrier provides strong coupling between the two GaAs quantum wells which have equal widths. Photoluminescence (PL) arises from both spatially direct and spatially indirect transitions in the quantum wells. The intense spatially indirect transition shows a pronounced red shift with increasing bias. A strong intrinsic bistability effect is observed in one of the three resonant peaks in the current-voltage curve. This effect is also seen in the photon energy and intensity of the PL from the wells. Photoluminescence excitation spectroscopy is used to identify the optical transitions and the strength of the tunneling interaction.

Electron transport via resonantly coupled energy levels in triple barrier structure investigated by magnetotunneling spectroscopy

Magnetotunneling spectroscopy in a series of GaAs/AlAs triple barrier diodes was performed to clarify the effect of scatterings and dimensionality of electronic states on the resonant tunneling transport. We found a variety of I-V characteristics in such diodes and differences in the behavior of the observed peaks in a magnetic field. By analyzing I-V characteristics in a magnetic field and magneto-oscillations of the tunnel current, we revealed that three different tunneling processes are responsible for the peaks.

Resonant Tunneling in Triple Barrier Diode under Pressure

Pressure-induced changes in the resonant voltages(quantum bound state energies) and the resonant currents(wave functions) in Al0.4Ga0.6As/GaAs triple barrier diode reveal that both of the ΓbXb contact (the subscript b denotes the barrier potential) and the ΓbXw alignment (the w denotes the well potential) in energy induce new boundary conditions for the Kronig-Penny potential.

Circuit theoretical study on the lifetime of the resonant state

AbstractWith a view to investigating the operating speed of a quantum effect device, considerable work has been carried out on the lifetime of the resonant state. The lifetime of the resonant state in a multiple barrier structure is defined as the time constant with which the existence probability of the electron is attenuated. This is a quantum theoretical transient phenomenon.In this paper, by means of a complex‐valued equivalent circuit proposed previously by the authors, this transient problem is discussed from a circuit theoretical viewpoint. First, the complex frequency variable (Laplace variable) is related to the complex energy which permits a circuit theoretical investigation on the lifetime. It was found that the real part of the complex frequency, which makes zero the sum of the admittances looking to the left and the right, corresponds to the lifetime. By means of a numerical experiment, a double‐barrier structure is studied and compared with the research results reported previously. The error of the lifetime estimated from the half‐value width also is studied. In addition, a symmetric triple barrier structure is studied. It is found that a lifetime exists that cannot be estimated from the half‐value width.

Engineering sub-50 nm quantum effect devices and single-electron transistors using electron-beam lithography

Fabrication and characteristics of various nanoscale quantum and single-electron devices are described. Using high-resolution electron beam lithography and double layer PMMA resist, we have successfully fabricated a 20 nm split-gate device, a novel 50 nm single barrier single-electron transistor, and a 75 nm triple barrier tunneling device. Effects of beam dose and the number of writing passes on the actual lithography pattern were investigated for various device gate geometries. Pronounced quantum and single-electron effects have been observed in these devices.