Double-barrier Heterostructures Research Articles

Rate equations description of the asymmetric double barrier electronic cooler

Recent experimental results showed that an electron gas in an asymmetrical double barrier heterostructure can be effectively cooled down under resonant tunneling condition, thus leading to the realization of an electronic cooler. The cooling process is a multi-parameters phenomenon and it is desirable to handle this problem through a reasonably simple approach, in order to understand the role of each parameter. To this end, we present a rate equation modeling of the electron cooling. We model the resonant tunnel injection of the electrons in the well and their thermionic emission assisted by Longitudinal Optical (LO) phonons absorption and emission. The influence of several parameters on the electronic temperature is discussed. This simple model compares rather well to the predictions of non-equilibrium Green function approach and to the experiments.

Spin-resonant tunneling in ferromagnetic conductor/ semiconductor heterostructure for spin-filter application

Spin-dependent tunneling in a ferromagnetic conductor/ semiconductor is analyzed with zero external fields. The barrier transparency, dwell time, tunneling time of electrons through heterostructure and the degree of polarization efficiency are calculated in Fe/GaAs double barrier heterostructure. The polarization efficiency of Fe/GaAs and Fe/InAs double-barrier heterostructures are compared. The Fe/GaAs has a high degree spin-polarization than Fe/InAs structure. The barrier transparency peak is sharper at the high width of the barrier. The results show that the polarization efficiency is maximum when the barrier width is maximum. The tunneling lifetime of electrons is evaluated using Heisenberg’s uncertainty principle. The spin components are completely separated at high barrier width and hence can be used effectively as a spin filter.

A study of tunneling in triangular resonant tunnel diodes: Effect of barrier width, barrier height, pressure, temperature, and heterostructure strain

Using the transfer matrix, the transmission coefficient of electrons is examined in the triangular double-barrier semiconductor heterostructure. GaAs-based heterostructure is considered to study the role of barrier width, barrier height, pressure and temperature. The geometrical parameters of the heterostructures (barrier width and barrier height) and the external perturbation (pressure and temperature) influence the tunneling properties. The tunneling time of the electrons is also studied using Heisenberg's uncertainty principle. The account of the heterostructure strain is also discussed.

GeSn Vertical Heterostructure p-i-n Waveguide Light Emitting Diode for 2 μm Band Silicon Photonics

We demonstrate an electrically-injected GeSn <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p-i-n</i> waveguide light emitting diode on silicon, comprising a vertical Ge/GeSn/Ge double-barrier heterostructure for 2 μm wavelength band integrated photonics. The incorporation of 4.2% Sn into the GeSn active layer results in bandgap shrinkage and extends the operating range to 2200 nm. Our design of the double-barrier heterostructure with the waveguide provides good carrier and optical confinement. The electroluminescence spectra at different current injection densities were measured at room temperature, demonstrating the peak wavelength at 1962 nm with the cutoff at 2200 nm, covering the entire 2 μm wavelength band. The results show that the proposed vertical <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p-i-n</i> heterostructure GeSn diode opens a pathway to meeting the demand for group IV-based light sources required for 2 μm wavelength band integrated photonics.

The effects of the electric-magnetic barrier on the spin transmission in the multibarrier semiconductor heterostructures

In the present paper, we study spin transmission in the multibarrier semiconductor heterostructures based on single particle effective mass approximation. These structures are double-barrier and triple-barrier semiconductor hetero-structures that a metallic ferromagnetic is deposited on them. Using Airy function and magnetic barriers approximated by delta function, we calculate transmission coefficient of tunneling electrons and spin polarization. Our results have shown that the parameters as the height and width of the electrical potential barrier, wave vector parallel to the barrier, applied bias voltage and magnetic field are effective parameters in determination of the transmission coefficient.

Spin-Polarized Transport in InAs/GaAs Double-Barrier Heterostructure with Electric and Magnetic Fields

Spin-Polarized Transport in InAs/GaAs Double-Barrier Heterostructure with Electric and Magnetic Fields

Comparative investigation between complex energy and transfer matrix techniques for quasi-energy states in heterostructure materials

A comparative analysis is performed to study the resonant tunneling through symmetric multiple double-barrier resonant tunneling heterostructure materials composed of nanoscale semiconductors ZnSe/BeTe, AlAs/GaAs, InGaAs/AlInAs, and AlGaAs/GaAs. Two techniques are applied in the present investigation for calculating the quasi-resonant energy states and their associated lifetimes in nanoscale double-barrier quantum well heterostructures. The first technique is based on the complex energy solution of the time-dependent Schrödinger equation with the time–energy uncertainty condition. The second one is employing the transfer matrix method. The quasi-resonant energies, lifetimes, and frequencies in nanoscale double-barrier quantum well heterostructures are calculated by both methods and compared. The validity and accuracy of both techniques are tested and compared extensively with various current numerical methods. Excellent agreements are obtained between our results and the available experimental and theoretical values. It is also found that the complex energy technique is recommended when handling the quasi-energy states and quasi-lifetimes of novel nanoscale devices.

The optimum configuration design of a nanostructured thermoelectric device with resonance tunneling

A nanostructured thermoelectric device is designed by connecting a double-barrier resonant tunneling heterostructure to two electron reservoirs. Based on Landauer’s equation and Fermi–Dirac statistics, the exact solution of the heat flow is calculated. The maximum power output and efficiency are calculated through the optimizations of several key parameters. The optimum characteristic curve of the performance is obtained. The thermodynamic performance characteristics of thermodynamic device are analyzed, including output power and efficiency, and the optimal operation region of device is determined by optimize the main parameter. The results obtained show that the heterojunction may become a perfect energy filter by appropriately regulating the chemical potentials of electron reservoirs and optimally choosing the widths of barrier and quantum well and the nanostructured thermoelectric device with resonance tunneling may obtain simultaneously a large power output and a high efficiency.

Comprehensive Analysis of Electron Evaporative Cooling in Double-Barrier Semiconductor Heterostructures

Based on full quantum-transport simulations, we report a comprehensive study of the evaporative cooling process in a double-barrier semiconductor heterostructure thermionic refrigerator. Our model, which self-consistently solves the nonequilibrium Green's function framework and the heat equation, is capable of calculating the electron temperature and electrochemical potential inside the device. By investigating the dependence of those thermodynamic parameters as a function of the barrier thickness and height, we answer open questions on evaporative cooling in solid-state systems, and give a clear recipe to reach high electron refrigeration. In particular, simulation results demonstrate that the best cooling is obtained when (i) the device operates at the maximum resonant condition; (ii) the quantum well state is symmetrically coupled with the contacts. The present results then shed light on physical properties of evaporative cooling in semiconductor heterostructures and will allow the development of thermionic cooling devices towards unprecedented performances to be sped up.

Phonon-Assisted Tunneling Current in a Double-Barrier Heterostructure with a Quantum Well

Phonon-Assisted Tunneling Current in a Double-Barrier Heterostructure with a Quantum Well

Temperature Characterization of Unipolar-Doped Electroluminescence in Vertical GaN/AlN Heterostructures

An electroluminescence (EL) phenomenon in unipolar-doped GaN/AlN/GaN double-barrier heterostructures—without any p-type contacts—was investigated from 4.2 K to 300 K. In the range of 200–300 K, the extracted peak photon energies agree with the Monemar formula. In the range of 30 to 200 K, the photon energies are consistent with A-exciton emission. At 4.2 K, the exciton type likely transforms into B-exciton. These studies confirm that the EL emission comes from a cross-bandgap (or band-to-band) electron-hole radiative recombination and is excitonic. The excitons are formed by the holes generated through interband tunneling and the electrons injected into the GaN emitter region of the GaN/AlN heterostructure devices.

Pressure effect on spin-resonant tunneling of holes in CdTe/Cd1-xMnxTe double-barrier heterostructure

Light Hole (LH) and Heavy Hole (HH) transmission, spin polarization and dwell time are theoretically studied in CdTe/CdMnTe heterostructures for different pressures. The transfer matrix method is used to find the transport properties taking Dresselhaus spin-orbit coupling into account. Sharp resonance peaks with high transmission probability are obtained for HH. A possibility to manipulate the spin-polarized dwell time of holes in this heterostructure using pressure is inferred from the results.

Spin-resonant tunneling in CdTe/Cd1-xMnxTe double-barrier heterostructure with zero external field: Effect of in-plane wave vector

The tunneling of light hole (LH) and heavy hole (HH) in CdTe/Cd1-xMnxTe double-barrier heterostructure with Dresselhaus spin-orbit interaction is studied. The effect of in-plane wave vector on barrier transparency, polarization efficiency and dwell time is theoretically analyzed. The in-plane wave vector increases the polarization efficiency of LH and HH. The enhanced spin-separation is achieved at high value of in-plane wave vector. The dwell time of LH and HH is also analyzed as a function of in-plane wave vectors.

ДОСЛІДЖЕННЯ СЕНСОРА ТЕМПЕРАТУРИ З ЧАСТОТНИМ ВИХОДОМ НА ОСНОВІ КВАНТОВОЇ ГЕТЕРОСТРУКТУРИ З ВІД’ЄМНИМ ДИФЕРЕНЦІЙНИМ ОПОРОМ

Physical processes in a quantum two-barrier heterostructure, which is the basis for the development of tunnel-resonant diodes, are considered. These studies have shown that tunnel resonance diodes can be used as temperature sensors with a frequency output signal. The use of devices with negative differential resistance makes it possible to significantly simplify the design of temperature sensors in the entire radio frequency range, at which, depending on the operating modes of the sensor, an output signal can be obtained both in the form of harmonic oscillations and in the form of impulse oscillations of a special form. The study of the characteristics of the sensor is based on the equivalent circuit of the tunnel-resonant diode, which takes into account its capacitive and inductive properties. The current-voltage characteristic of the sensor has a falling section, which is responsible for the appearance of a negative differential resistance in this section. The descending section arises due to a decrease in the current that flows through the double-barrier quantum heterostructure, with an increase in voltage. A decrease in the current occurs due to a decrease in the transparency coefficient of the potential barriers of the heterostructure. A mathematical model of the temperature sensor has been developed, on the basis of which the analytical dependences of the change in the elements of the equivalent circuit of the sensor on temperature, as well as the transformation function and sensitivity, have been determined. It is shown that the main contribution to changes in the conversion function and sensor sensitivity is made by the change in the negative differential resistance with a change in temperature. This, in turn, results in different readings of the instrument’s output frequency. The sensor sensitivity was varied from 480 kHz/0С to 220 kHz/0С in the temperature range from -150 0С to 50 0С.

Low bias electron transport properties of the graphene-Ge2Sb2Te5 heterostructure device

We explore the performance improvement of ultra-scaled phase-change memory (PCM) device with graphene- Ge2Sb2Te5 (GST) heterostructure by means of first-principle simulations of electron transport. We compare the tunneling probability of single-barrier(SB) and double barrier(DB) heterostructures and show that the use of DB can have a significant impact on the ON/OFF conductance ratio of the device. The reason for this is that the graphene layers in the middle of the single potential barrier, forming the double potential barrier, perform as an operative phase randomizer and destroy the coherence of the electrons that carry out the transport. Hence, the electric current and conductivity decrease in the amorphous phase and the ON/OFF conductance ratio sharply increases. The results of this work offer great potential to obtain a high ON/OFF ratio in ultra-scaled PCM devices.

Voltage-Controlled Magnetic Anisotropy in Heterostructures with Atomically Thin Heavy Metals

Efficient, fast, high-density magnetoelectric random-access memory (MeRAM) requires both large perpendicular magnetic anisotropy and high-efficiency voltage-controlled magnetic anisotropy. Thus the authors propose a double-barrier ferromagnetic heterostructure with an atomically thin layer of a late transition metal $X$ (Rh, Ir, or Pt). First-principles calculations reveal its anticipated performance to be due to the synergy effects of the large strain-induced magnetism in $X$, the large spin-orbit coupling of $X$, and the giant modulation of magnetocrystalline anisotropy at the $X$/MgO interface. This work provides rules for designing the next generation of spintronic memory devices.

Evaporative electron cooling in asymmetric double barrier semiconductor heterostructures

Rapid progress in high-speed, densely packed electronic/photonic devices has brought unprecedented benefits to our society. However, this technology trend has in reverse led to a tremendous increase in heat dissipation, which degrades device performance and lifetimes. The scientific and technological challenge henceforth lies in efficient cooling of such high-performance devices. Here, we report on evaporative electron cooling in asymmetric Aluminum Gallium Arsenide/Gallium Arsenide (AlGaAs/GaAs) double barrier heterostructures. Electron temperature, Te, in the quantum well (QW) and that in the electrodes are determined from photoluminescence measurements. At 300 K, Te in the QW is gradually decreased down to 250 K as the bias voltage is increased up to the maximum resonant tunneling condition, whereas Te in the electrode remains unchanged. This behavior is explained in term of the evaporative cooling process and is quantitatively described by the quantum transport theory.

Experimental and modeling insight for fin-shaped transistors based on AlN/GaN/AlN double barrier heterostructure

The exploitation of the two-dimensional electron gas (2DEG) channel of an AlN/GaN/AlN double barrier heterostructure, for High Electron Mobility Transistors (HEMTs) with metal–oxidesemiconductor (MOS) tri-gate around a fin-shaped channel (MOS-FinHEMTs), has been investigated by combining fabrication, dc I-V measurements and simulations of single-fin MOS-FinHEMT devices. The dependence of the threshold voltage (Vth) and the maximum drain-source current (Ids,max) on the fin width (Wfin), as well as the effects of ohmic contact resistance, gate-drain and source-gate distance and of the Al2O3 gate dielectric thickness (tox), have been addressed. Fabricated single-fin MOS-FinHEMT devices with tox = 20 nm, exhibited a positive shift of Vth, in comparison to a reference planar-gate device, ranging from +0.8 V for Wfin = 650 nm to +3.4 V for Wfin = 200 nm, due to lateral depletion of the channel by the gate contacts on the fin sidewalls. Simulations reproduced the experimental Vth values and also predicted the Vth of devices with narrower fins, down to Wfin = 10 nm. The boundary for normally-off operation (Vth = 0 V) was determined at Wfin = 17 nm that may increase up to 31 nm if the tensile strain of the top AlN barrier in the fin nanostructure is elastically relaxed. A reduction of maximum drain-source current per top gate width (Ids,max/Wg) with decreasing Wfin in the range of 200–650 nm may result from increased ohmic contact resistance. However, for narrower fins, Ids,max/Wg is predicted to decrease significantly with decreasing Wfin, due to the lateral field of the sidewall gates. The Ids,max/Wg will also decrease with increasing distance between the source, gate and drain contacts for any Wfin. Finally, the Vth and Ids,max/Wg values were calculated for Al2O3 thickness in the range of 5 to 40 nm.

Broken Symmetry Effects due to Polarization on Resonant Tunneling Transport in Double-Barrier Nitride Heterostructures

The phenomenon of resonant tunneling transport through polar double-barrier heterostructures is systematically investigated using a combined experimental and theoretical approach. On the experimental side, GaN/AlN RTDs are grown by MBE. In-situ electron diffraction is employed to monitor the number of monolayers incorporated into each tunneling barrier. Using this precise epitaxial control at the monolayer level, we demonstrate exponential modulation of the resonant tunneling current as a function of barrier thickness. Both the peak voltage and characteristic threshold bias exhibit a dependence on barrier thickness as a result of the intense electric fields present in the polar heterostructures. To get further insight into the asymmetric tunneling injection, we present an analytical theory for tunneling transport across polar heterostructures. A general expression for the resonant tunneling current with contributions from coherent and sequential tunneling processes is introduced. After applying this theory to the case of GaN/AlN RTDs, their experimental current-voltage characteristics are reproduced over both bias polarities, with tunneling currents spanning several orders of magnitude. This agreement allows us to elucidate the role played by the internal polarization fields on the magnitude of the tunneling current and broadening of the resonant line shape. Under reverse bias, we identify new tunneling features originating from highly attenuated resonant tunneling phenomena, which are completely captured by our model. Our analytical model, provides a simple expression which reveals the connection between the polar RTD design parameters and its current-voltage characteristics. This new theory paves the way for the design of polar resonant tunneling devices exhibiting efficient resonant current injection and enhanced tunneling dynamics, as required in various practical applications.

Spin-resonant tunneling in CdTe/Cd1-xMnxTe double-barrier heterostructures with zero external field

Spin-dependent resonant tunneling in CdTe/CdMnTe double-barrier heterostructures was theoretically investigated. The transfer-matrix method was employed to study the spin transport properties under zero applied field. The effect of Dresselhaus spin- orbit coupling was considered in the symmetrical heterostructure. The influences of the barrier width and height on the polarization efficiency, barrier transparency and dwell time of light and heavy holes were observed and discussed.