Resonant magnetotunneling through individual self-assembled InAs quantum dots

A single-barrier GaAs/AlAs/GaAs heterostructure, with self-assembled In-based quantum dots incorporated in the AlAs tunnel barrier, exhibits a series of resonant peaks in the low temperature current–voltage characteristics. We argue that each peak arises fromsingle-electrontunneling through thediscrete zero-dimensionalstate of anindividualInAs dot. We use the tunneling for fine probing of the local density of states in the emitter-accumulation layer. Landau-quantized states are resolved at magnetic field B∥ as low as 0.2 T. Spin-splitting of the dot electron states has been observed forB⊥I.     

Resonant tunneling of electrons through single self-assembled InAs quantum dot studied by conductive atomic force microscopy

Resonant tunneling of electrons through a quantum level in single self-assembled InAs quantum dot (QD) embedded in thin AlAs barriers has been studied. The embedded InAs QDs are sandwiched by 1.7-nm-thick AlAs barriers, and surface InAs QDs, which are deposited on 8.3 nm-thick GaAs cap layer, are used as nano-scale electrodes. Since the surface InAs QD should be vertically aligned with a buried one, a current flowing via the buried QD can be measured with a conductive tip of an atomic force microscope (AFM) brought in contact with the surface QD-electrode. Negative differential resistance attributed to electron resonant tunneling through a quantized energy level in the buried QD is observed in the current–voltage characteristics at room temperature. The effect of Fermi level pinning around nano-scale QD-electrode on resonance voltage and the dependence of resonance voltage on the size of QD-electrodes are investigated, and it has been demonstrated that the distribution of the resonance voltages reflects the size variation of the embedded QDs.     

Detection of single photons by a resonant tunneling heterostructure with a quantum dot layer

Light absorption by GaAs/AlAs heterostructures with a layer of self-assembled InAs quantum dots (QDs) at resonant tunneling through an energy-selected QD has been investigated. A high sensitivity of the current through this selected tunneling channel to the absorption of single photons with a wavelength λ ≲ 860 nm up to a temperature of 50 K is demonstrated; this sensitivity is caused by the Coulomb effect of the photoexcited holes captured by surrounding QDs on the resonance conditions. It is shown that single-photon absorption can discretely change the current through the system under study by a factor of more than 50. The captured-hole lifetimes have been measured, and a model has been developed to qualitatively describe the experimental data. It is also demonstrated that the InAs monolayer can effectively absorb photons. The properties of the heterostructure studied can be used not only to detect photons but also to design logical valves and optical memory devices.     

A Small Signal Equivalent Circuit Model for Resonant Tunnelling Diode

We report a resonant tunnelling diode （RTD） small signal equivalent circuit model consisting of quantum capacitance and quantum inductance. The model is verified through the actual InAs/In0.53Ga0.47As/AlAs RTD fabricated on an InP substrate. Model parameters are extracted by fitting the equivalent circuit model with ac measurement data in three different regions of RTD current-voltage （I-V） characteristics. The electron lifetime, representing the average time that the carriers remain in the quasibound states during the tunnelling process, is also calculated to be 2.09ps.     

Fine structure splitting and biexciton binding energy in single self-assembled InAs/AlAs quantum dots

We report on photoluminescence investigations of individual InAs quantum dots embedded in an AlAs matrix which emit in the visible region, in contrast to the more traditional InAs/GaAs system. Biexciton binding energies, considerably larger than for InAs/GaAs dots, up to 9 meV are observed. The biexciton binding energy decreases with decreasing dot size, reflecting a possible crossover to an antibinding regime. Exciton and biexciton emission consists of linearly cross polarized doublets due to a large fine structure splitting up to 0.3 meV of the bright exciton state. With increasing exciton transition energy the fine structure splitting decreases down to zero at about 1.63 eV. Differences with InAs/GaAs QDs may be attributed to major dot shape anisotropy and/or larger confinement due to higher AlAs barriers.     

Dependence of current-voltage characteristics of pseudomorphic AlAs/In0.53a0.47s/InAs resonant tunneling diodes on quantum well widths

This paper studies the dependence of I-V characteristics on quantum well widths in AlAs/In_0.53Ga_0.47As and AlAs/In_0.53Ga_0.47As/InAs resonant tunneling structures grown on InP substrates. It shows that the peak and the valley current density in the negative differential resistance region are closely related with quantum well width. The measured peak current density, valley current densities and peak-to-valley current ratio of resonant tunneling diodes are continually decreasing with increasing well width.     

Simulation of resonant tunneling devices: origin of the I–V multi-peak behavior

Multi-peak behavior of the $I$ – $V$ curves of an AlAs/GaAs/AlAs double-barrier resonant tunneling diode combined with a layer of InAs quantum dots (QDs) is simulated. Our simulation results show that the coupling between the energy level in the emitter quantum well (quantum dot) layer and that in the central quantum well is the key point in understanding the origin of the $I$ – $V$ multi-peak behavior. The embedded designed QD layer at the emitter spacer can enhance this effect. The effects of device temperature on the $I$ – $V$ characteristics are also obtained. Our results provide the physical basis for understanding and utilizing the multi-peak behavior of $I$ – $V$ curves in designing resonant tunneling devices.     

Experimental analysis of resonant tunneling transit time using high-frequency characteristics of resonant tunneling transistors

We systematically estimate the resonant tunneling transit time from the high-frequency characteristics of resonant tunneling transistors. It is found that the transit time across an InGaAs/InAlAs resonant tunneling structure is more than one order of magnitude shorter than that for GaAs/AlAs with the same barrier layer thickness. In addition, the obtained times agree reasonably well with calculated phase time. It is probably the elastic resonance width and not the inelastic scattering effect that mainly determines the resonant tunneling transit time.     

Emission of photoexcited charge carriers from InAs/GaAs quantum dots grown by gas-phase epitaxy

A model describing the emission of photoexcited electrons and holes from an array of InAs quantum dots into the GaAs matrix is suggested. The analytical expression obtained for the emission efficiency takes into account the thermal emission of charge carriers into the GaAs matrix and two-dimensional states of the InAs wetting layer, tunneling and thermally activated tunneling escape, and electron transitions between the quantum-confinement levels in the conduction band of InAs. The temperature dependences of the photosensitivity in the regions of the ground-state and first excited-state optical transitions in InAs/GaAs quantum dots grown by gas-phase epitaxy are investigated experimentally. A number of quantum dot parameters are determined by fitting the results of a theoretical calculation to the experimental data. Good agreement between the theoretical and experimental results is obtained in this way.     

Resonant tunneling in magnetic semiconductor tunnel junctions with arbitrary magnetic alignments

Combining an extended Julliere model with transfer matrix method, we study the spin-polarized resonant tunneling in GaMnAs/AlAs/GaAs/AlAs/GaMnAs double barrier ferromagnetic semiconductor (FS) tunnel junctions with the arbitrary angle θ between the magnetic directions of two FS's. It is shown that tunneling magnetoresistance (TMR) ratio linearly varies with sin²(θ/2). We also demonstrate that for the heavy and light holes, the properties of the spin-polarized resonant tunneling are obviously different. The present results are expected to be instructive for manufacturing the relevant semiconductor spintronic devices.     

A bistable, self-latching inverter by the monolithic integration of resonant tunnelling diode and high electron mobility transistor

This paper reports that the structures of AlGaAs/InGaAs high electron mobility transistor (HEMT) and AlAs/GaAs resonant tunnelling diode (RTD) are epitaxially grown by molecular beam epitaxy (MBE) in turn on a GaAs substrate. An Al_0.24Ga_0.76As chair barrier layer, which is grown adjacent to the top AlAs barrier, helps to reduce the valley current of RTD. The peak-to-valley current ratio of fabricated RTD is 4.8 and the transconductance for the 1-μm gate HEMT is 125mS/mm. A static inverter which consists of two RTDs and a HEMT is designed and fabricated. Unlike a conventional CMOS inverter, the novel inverter exhibits self-latching property.     

Resonant enhancement of tunneling magnetoresistance in double-barrier magnetic heterostructures

We show that spin-dependent resonant tunneling can dramatically enhance tunneling magnetoresistance. We consider double-barrier structures comprising a semiconductor quantum well between two insulating barriers and two ferromagnetic electrodes. By tuning the width of the quantum well, the lowest resonant level can be moved into the energy interval where the density of states for minority spins is zero. This leads to a great enhancement of the magnetoresistance, which exhibits a strong maximum as a function of the quantum well width. We demonstrate that magnetoresistance exceeding 800% is achievable in GaMnAs/AlAs/GaAs/AlAs/GaMnAs double-barrier structures.     

InAs/InP柱型量子线中隧穿时间和逃逸问题的研究

在有效质量近似和绝热近似下,利用转移矩阵法研究了电子通过In As/In P/In As/In P/In As柱形量子线共振隧穿二极管的输运问题,分析和讨论了电子居留时间以及电子的逃逸过程.详细研究了外加电场、结构尺寸效应对居留时间和电子逃逸的影响.居留时间随电子纵向能量的演化呈现出共振现象;同时,结构的非对称性对电子居留时间有很大的影响,随着结构非对称性的增加,居留时间表现出不同的变化.利用有限差分方法研究了非对称耦合量子盘中电子的相干隧穿逃逸过程.     

Bandgap engineering of self-assembled InAs quantum dots with a thin AlAs barrier

We investigate the effects of a thin AlAs layer with different position and thickness on the optical properties of InAs quantum dots (QDs) by using transmission electron microscopy and photoluminescence (PL). The energy level shift of InAs QD samples is observed by introducing the thin AlAs layer without any significant loss of the QD qualities. The emission peak from InAs QDs directly grown on the 4 monolayer (ML) AlAs layer is blueshifted from that of reference sample by 219 meV with a little increase in FWHM from 42–47 meV for ground state. In contrast, InAs QDs grown under the 4 ML AlAs layer have PL peak a little redshifted to lower energy by 17 meV. This result is related to the interdiffusion of Al atom at the InAs QDs caused by the annealing effect during growing of InAs QDs on AlAs layer.     

Structure of InAs/GaAs quantum dots grown with Sb surfactant

InAs quantum dots in GaAs, grown under the presence of Sb by metalorganic chemical vapor deposition, were studied with cross-sectional scanning tunneling microscopy. Large flat quantum dots with a truncated pyramidal shape, base lengths between 15 and 30 nm, heights of 1–3 nm, and a rather pure InAs stoichiometry were found for the case of an Sb supply during the InAs deposition. If Sb is already supplied during GaAs stabilization prior to InAs deposition, the dots become even larger and tend to get intermixed with Ga, but remain coherently strained with a reversed cone-like In distribution. Regarding the quantum dot growth Sb acts as surfactant, whereas an incorporation of individual Sb atoms was observed in the wetting layer.     

MBE自组织生长多层竖直自对准InAs量子点结构的研究

利用MBE方法在(001)GaAs衬底上生长了多层竖直自对准InAs量子点结构。透射电子显微镜的观察表明,多层量子点成一系列柱状分布。同单层量子点相比,多层量子点的光荧光谱线发生红移。这表明由于量子点中载流子波函数的扩展和交迭,柱中量子点之间有耦合现象发生。光荧光谱线半高宽随温度的反常变化说明载流子还会在邻近柱中隧穿.     

Structural and optical properties of high-density (>10/cm) InAs QDs with varying Al(Ga)As matrix layer thickness

We have obtained high-density (>10¹¹/cm²) InAs quantum dot (QD) structures by using an Al(Ga)As matrix layer. With increase of the AlAs matrix layer thickness, the density of QDs increases a little and the luminescence intensity emitted from InAs QDs decreases. We have used a thin GaAs insertion layer (IL) for the reason of keeping InAs QDs from an aluminum intermixing towards QDs. As the thickness of GaAs IL increases, the density of QDs decreases slightly due to the reduction of the roughness of an AlAs matrix layer. However, the luminescence intensity increases with increase in the thickness of GaAs IL resulting from the efficient blocking of an aluminum intermixing towards QDs.     

Intersubband photocurrent from double barrier resonant tunneling structures

Simple models of semiconductor-based double barrier resonant tunneling structures predict a large accumulation of charge carriers in the structure. These carriers can be excited optically from one subband to another generating photocurrent. In this work we have investigated the photo-induced current due to intersubband excitation in double barrier structures. We have found that the origin of the photocurrent is accumulation of quantized carriers in the emitter-barrier junction of the structure, rather than accumulation of carriers in the double barrier quantum well. This photon assisted tunneling process in double barrier structures may be used for infrared detection.     

Negative differential conductance in InAs wire based double quantum dot induced by a charged AFM tip

We investigate the conductance of an InAs nanowire in the nonlinear regime in the case of low electron density where the wire is split into quantum dots connected in series. The negative differential conductance in the wire is initiated by means of a charged atomic force microscope tip adjusting the transparency of the tunneling barrier between two adjoining quantum dots. We confirm that the negative differential conductance arises due to the resonant tunneling between these two adjoining quantum dots. The influence of the transparency of the blocking barriers and the relative position of energy states in the adjoining dots on a decrease of the negative differential conductance is investigated in detail.     

Photoluminescence dynamics due to exciton and free carrier transport in GaAs/AlAs superlattices

Photogenerated carrier transfer is investigated in a set of three GaAs/AlAs short-period superlattices (SPSs) with different barrier thicknesses by steady-state and time-resolved photoluminescence (PL) spectroscopy at 15–20 K as a function of excitation power. The tunneling transport of carriers is evaluated by detecting excitonic PL signals from an embedded GaAs single quantum well (SQW) in the middle of the SPS layer. We find that, as the barrier thickness is decreased, the PL intensity ratio of SQW/SPS increases systematically due to enhanced tunneling efficiencies of both electrons and holes. However, the PL intensity ratio significantly increases with decreases in the excitation power by more than two orders of the magnitude. We attribute the enhanced PL intensity of SQW relative to the SPS to the faster transport of electrons that can recombine with residual holes to form excitons in SQW. The PL dynamics of SQW and SPS thus shows unique density-dependent PL intensity and time behaviors due to variations in relative amounts of excitons and free carriers to be transported into the SQW layer.