Function Of Barrier Thickness Research Articles

Molecular beam epitaxy of polar III-nitride resonant tunneling diodes

Advances in molecular beam epitaxy (MBE) have been crucial for the engineering of heterostructures in which the wave nature of electrons dictates carrier transport dynamics. These advances led to the first demonstration of negative differential conductance (NDC) in arsenide-based resonant tunneling diodes (RTDs) in 1974. In contrast to the 17 years elapsed between the initial MBE growth of arsenide semiconductors and the first demonstration of room-temperature GaAs/AlAs RTDs, the development of polar III-nitride RTDs has been remarkably different. After pioneering growths of nitride materials by MBE in 1973, it would take 43 years—until 2016—to demonstrate the first GaN/AlN RTD that exhibits repeatable NDC at room temperature. Here, we discuss, from the crystal growth point of view, the key developments in the epitaxy of III-nitride heterostructures that have led us to the demonstration of robust resonant tunneling transport and reliable NDC in III-nitride semiconductors. We show that in situ tracking of the crystal electron diffraction allows us to deterministically control the number of monolayers incorporated into the tunneling barriers of the active region. Employing this technique, we fabricate various GaN/AlN RTD designs showing the exponential enhancement of the resonant tunneling current as a function of barrier thickness. In addition, we experimentally demonstrate that tunneling transport in nitride RTDs is sensitive to epitaxial parameters such as the substrate growth temperature and threading dislocation density. This new insight into the MBE growth of nitride resonant tunneling devices represents a significant step forward in the engineering of new functionalities within the family of III-nitride semiconductors, allowing to harness quantum interference effects for the new generation of electronic and photonic devices.

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.

Magnetic and electronic transport properties of some tunnel junctions with AgBr symmetry-filter barriers

A new prototype tunnel junction with AgBr based barrier is proposed in this study. Electronic and magnetic properties of Fe/AgBr/Fe (001) heterostructures are investigated on the basis of first-principles and ballistic conductance calculations. The interlayer exchange coupling and electronic transport properties are analyzed as functions of the AgBr barrier thickness. The results show that Fe magnetism at Fe/AgBr (001) interfaces is enhanced as compared with the bulk material. A very strong direct tunnelling of Fe Δ1 states is observed. The minority-spin channel is characterized by resonant tunnelling of interfacial states that are forming at Fe/AgBr (001) interfaces. Spin-polarization of the tunnelling current is positive and can exceed 99%. Tunnelling magnetoresistance ratios of about 8000% are evidenced for the junctions with clean interfaces. Fe/AgBr (001) coherent interfaces could act as nearly ideal symmetry-filters. In the presence of interfacial disorder the tunnelling magnetoresistance ratios decrease dramatically. The exchange coupling between Fe electrodes oscillates as a function of the barrier thickness.

Thermal conductance by Dirac fermions in a normal-insulator-superconductor junction of silicene

We theoretically study the properties of thermal conductance in a normal-insulator-superconductor junction of silicene for both thin and thick barrier limit. We show that while thermal conductance displays the conventional exponential dependence on temperature, it manifests a nontrivial oscillatory dependence on the strength of the barrier region. The tunability of the thermal conductance by an external electric field is also investigated. Moreover, we explore the effect of doping concentration on thermal conductance. In the thin barrier limit, the period of oscillations of the thermal conductance as a function of the barrier strength comes out be $\pi/2$ when doping concentration in the normal silicene region is small. On the other hand, the period gradually converts to $\pi$ with the enhancement of the doping concentration. Such change of periodicity of the thermal response with doping can be a possible probe to identify the crossover from specular to retro Andreev reflection in Dirac materials. In the thick barrier limit, thermal conductance exhibits oscillatory behavior as a function of barrier thickness $d$ and barrier height $V_0$ while the period of oscillation becomes $V_0$ dependent. However, amplitude of the oscillations, unlike in tunneling conductance, gradually decays with the increase of barrier thickness for arbitrary height $V_0$ in the highly doped regime. We discuss experimental relevance of our results.

A Low-Frequency Dielectric Barrier Discharge System Design for Textile Treatment

The ultimate goal of the conducted research was to design and test a simplified and energy efficient low-frequency dielectric barrier discharge reactor for the textile processing industry. For this purpose, a 50 Hz AC source and a variac were used to ionize the ambient air column between two asymmetric conductive electrodes separated by a dielectric layer between them. It was observed that the discharge system with such electrode configuration undergoes an electrical breakdown with large number of shortly lived microdischarges indicating the effectiveness of the model. The variational trend in discharge power and transported charge as a function of AC input voltage and dielectric barrier thickness were also studied in this work. In addition, the spectroscopic investigations were carried out on electron number density and electron temperature as a function of barrier thickness, AC input voltage, and interelectrode air gap. A significant influence of the system input parameters on discharge properties was noticed in these investigations.

Observation on array of PbTe nanocrystals embedded in amorphous InSe multiple quantum wells

The array of three dimensionally confined PbTe nanocrystals embedded in the amorphous InSe barrier grown by a thermal evaporation technique has been characterized as a function of barrier thickness. The face centered cubic structure of the PbTe nanocrystals were revealed by Transmission electron microscopy (TEM) images and X-ray diffraction (XRD) pattern. The crystallites size evaluated from the XRD peak width using Scherer formula were consistent with the PbTe nanocrystals size obtained from TEM images. The observed blue shift in the band gap values of the PbTe nanocrystals were correlated with the phenomenon of three dimensional quantum confinement effects.

Quantum-confined Stark effects in coupled InAs/GaAs quantum dots

We report the effects of tunnel coupling on the Quantum-Confined Stark Effect (QCSE) for excitons in InAs/GaAs coupled quantum dots (CQDs). As the barrier separating the dots is reduced, the zero-field dipole moment and the polarizability are both found to increase. This systematic variation as a function of barrier thickness is due to factors including the formation of molecular wavefunctions, the electron/hole effective masses, and the CQD structural properties. The dipole moment for the interdot exciton is found to be up to 100 times larger than that of the intradot exciton resulting in a predominantly linear shift with field. The ability to control the QCSE of the exciton in a single CQD could be useful for a new class of single photon optical switches and tunable emitters.

Spin transport in ferromagnet/semiconductor/ferromagnet structures with cubic Dresselhaus spin-orbit-interaction

We have investigated the spin transport in ferromagnet (FM)/semiconductor (SC)/ferromagnet (FM) structures with a central SC barrier region exhibiting cubic Dresselhaus spin-orbit-interaction (SOI). The energy profile of the barrier is assumed to be a square with height V and thickness d along z-direction. The magnetoresistance (MR) ratio has been calculated for three different barriers, GaAs, GaSb, and GaAs without SOI as a function of barrier thickness. We have found that the MR ratio has a negative value for GaAs barrier with SOI except for very thin barrier thickness. In the case of GaSb barrier, the MR ratio changes sign from negative to positive with increasing the barrier thickness. Also, we have calculated the MR ratio with changing the spin coupling constant.

Giant oscillations in spin-dependent tunneling resistances as a function of barrier thickness in fully epitaxial magnetic tunnel junctions with a MgO barrier

Giant oscillations in spin-dependent tunneling resistances as a function of MgO barrier thickness $({t}_{\text{MgO}})$ were observed for fully epitaxial magnetic tunnel junctions with Heusler alloy ${\text{Co}}_{2}{\text{Cr}}_{0.6}{\text{Fe}}_{0.4}\text{Al}$ electrodes and a MgO barrier. The oscillations in tunneling resistances were well approximated by a superposition of an exponential function of $\text{exp}(a{t}_{\text{MgO}}+b)$ and a periodic function of $1+C\text{ }\text{cos}[(2\ensuremath{\pi}/T){t}_{\text{MgO}}+\ensuremath{\varphi}]$ with significantly large amplitudes $C\ensuremath{\sim}0.16\ifmmode\pm\else\textpm\fi{}0.01$ even at 293 K for both parallel and antiparallel magnetization orientations. The period was found to be almost independent of temperature and bias voltage $(V)$. The amplitudes $C$ showed only weak dependence on $V$ at least up to 0.2 V. These features should be a key to understand the origin of the pronounced oscillations.

DC Characterization of InP/InGaAs Tunneling Emitter Bipolar Transistor

An InP/InGaAs tunneling emitter bipolar transistor (TEBT) is studied and demonstrated. From the simulation results, the band diagram, tunneling transmission, and carrier distribution of the device are analyzed as functions of barrier thickness. The higher the emitter injection efficiency, the higher the current gain and the larger the current drivability for the device studied by employing a tunneling barrier layer of suitable thickness. In addition, experimentally, InP/InGaAs TEBT has been fabricated successfully. Due to its excellent tunneling barrier structure, the studied device can be operated under an extremely wide collector current range. The operation range is larger than 11 decades of collector current (10-12 to 10-1 A). Moreover, the studied device exhibits a very small collector-emitter offset voltage (ΔVCE) of 40 mV and an extremely wide operation range of output current. Thus, the studied device is suitable for low-voltage and low-power circuit applications.

Spin Depolarization via Tunneling Effects in Asymmetric Double Quantum Dot Structure

Time-resolved photoluminescence experiments have been performed to investigate exciton spin relaxation processes at low temperatures in InAlAs-InGaAs asymmetric double quantum dots embedded in AlGaAs layers. By decreasing the thickness of the AlGaAs barrier between the dots, the spin relaxation times decreased from 3 ns to 1 ns. The observed spin relaxation as a function of barrier thickness was reasonably described by two components: one was a constant and the other was exponentially depended on barrier thickness. The origin of the latter component is discussed in connection with spin-flip tunneling.

Optical property of self-assembled InAs quantum dots in Al0.5Ga0.5As barriers grown by molecular beam epitaxy

Photoluminescence (PL) of quantum dots embedded in high potential barriers is studied as functions of barrier thickness and temperature. With the increase of barrier thickness, both the strengthened two-dimensional electron gas (2DEG) structure and strongly localized electron wave functions can increase the carrier recombination. The optical properties of two different-barrier-thickness samples exhibit different characteristics with the decreased measurement temperatures. The PL recombination characteristic of the samples with the barriers adjacent to a Si-doping GaAs layer is different from that of samples with the barrier adjacent to an i-GaAs layer.

Effect of Barrier Thickness and Barrier Doping on the Properties of InGaN/GaN Multiple‐Quantum‐Well Structure Light Emitting Diode

Optical and electrical properties of InGaN/GaN multi-quantum-well (MQW) structure light-emitting diodes (LED) are systematically studied as a function of barrier thickness and Si doping of barriers. The emission energy of photoluminescence (PL) and electroluminescence (EL) of MQW-LEDs decrease with barrier thickness increased, but with Si doping of barriers decreased. The electrical characteristics of MQW-LEDs strongly depend on barrier thickness and Si-doped content in barriers. With increasing barrier thickness, forward voltages of MQW-LEDs dramatically reduce as well as the barriers are increasingly doped with Si. Reverse currents of MQW-LEDs measured at reverse bias 6 V present a similar behavior that Ir increase with increasing barrier thickness as well as increasing Si doped content in barriers. The experimental results imply an optimized quality of MQW-LED fabrication by an appropriate combination of barrier thickness and carrier concentration.

Fabrication and properties of high- Tc ramp junctions with manganite barriers

We report on fabrication and characterization of YBa 2Cu 3O 7− δ (YBCO) ramp junctions with a barrier of doped manganite, La 0.67Sr 0.33MnO 3 (LSMO). Multilayers of YBCO and LSMO are epitaxially grown on (100) SrTiO 3 substrates by on-axis pulsed laser ablation. The resistance and current–voltage characteristics of the ramp junctions have been measured as a function of barrier thickness, in the range of 10–30 nm, and of temperature. Junctions with 20 nm thick barriers had RSJ-like current–voltage characteristics with a small excess current. The critical current density and normal state resistance of the junctions at 4.2 K were 5×10 3 A/cm 2 and 300 Ω, respectively.

Fabrication of high IcRn YBCO ramp junctions using Ga doped Pr-Ba-Cu-O barriers at 65 K

Yttrium Barium Copper Oxide ramp junctions have been fabricated via off-axis rf magnetron sputtering using Praseodymium Barium Copper Gallium Oxide as a barrier material. Optimal wafer processing conditions and junction behavior as a function of barrier thickness are presented. IcRn values of 500 microvolts at 65 K have been achieved with a two step deposition/anneal process using Praseodymium Barium Copper Gallium Oxide as the barrier. This junction process has been demonstrated on 2 inch wafers.

Transition behavior from coupled to uncoupled GaAs/InAs double quantum wells

The electronic structures of the InAs single and double quantum wells buried in bulk GaAs (001) are presented based on the sp3s* empirical tight-binding model. Both electrons and holes are found to be confined in the c-axis direction around the inserted InAs monomolecular plane with a localization length of the order of 110 Å. The inserted InAs monolayer is, therefore, playing the role of a quantum well for all charge carriers and, as a consequence, the formed heterojunction is of type I. The system, composed of two InAs monolayers buried in GaAs and separated by N monolayers of GaAs, is studied versus the barrier thickness (N). Our results of the variation of band gap energy as a function of barrier thickness (N) are in excellent agreement with the available photoluminescence data when a small valence band offset (of order 80 meV including the spin-orbit effects) is employed. A critical barrier thickness of about 220 Å is suggested to decouple the InAs quantum wells.

Ramp-type junction parameter control by Ga doping of PrBa2Cu3O7−δ barriers

We analyzed the transport of charge carriers across PrBa2Cu3−xGaxO7−δ (PBCGO) barriers as a function of barrier thickness, Ga-doping level, temperature, and bias voltage. It was found that by Ga doping, the normal state resistance (Rn) of the junctions was systematically increased, while the critical current (Ic) remained constant. We argue that pair transport takes place by direct tunneling, whereas the quasiparticles have access to channels formed by one or more localized states inside the barrier. By Ga doping the IcRn products were increased, up to 8 mV at 4.2 K for junctions with 8 nm thick PrBa2Cu2.6Ga0.4O7−δ barriers.

Capture time versus barrier thickness in quantum-well structures measured by infrared photoconductive gain

Photoconductive gain measurements in quantum-well (QW) infrared detectors are used to determine the variation of the capture time of electrons in QWs as a function of barrier thickness. The capture time is shown to be proportional to the multi-quantum-well period, which is consistent with a quantum mechanical description of the capture process. The measured values are far higher than the ones measured by time-resolved photoluminescence, ranging from 8 to 150 ps, depending on the applied electric field and barrier thickness. The reasons for this discrepancy are discussed.

Experimental determination of photon energy--angle distributions for cobalt-60 after penetration of thick concrete barriers.

The number photon spectra of transmitted cobalt-60 gamma-rays scattered by thick concrete barriers were determined as functions of the angle of scatter and of the barrier thickness under broad incident beam conditions. The energy region for which the maximum number of photons occurs, shifts toward lower energies with increasing thickness, but the rate of this shift as a function of the thickness of concrete becomes small for thick slabs. When the number of photons is plotted as a function of barrier thickness, it is found that after an initial buildup an “equilibrium” thickness is approached. For greater thicknesses the slope of the curve plotted is a constant. The constants determined for plots corresponding to all angles of scattered radiation greater than 0° with the normal to the surface are equal to one another.

MEASUREMENTS OF SPECTRAL AND ANGULAR DISTRIBUTIONS OF SECONDARY GAMMA-RAYS IN MATTER

The distribution in energy and angle of the secondary gamma radiation emerging from the face of a concrete barrier containing a point source of cobalt-60 has been measured as a function of barrier thickness. Results on energy spectra and angular distributions are presented, and some of their features are compared with theoretical predictions. The operation of the two-crystal spectrometer and the photographic system for recording pulse-height distributions are described in some detail.