Quantum box resonant tunneling spectroscopy: Experiments and modelisation

We have calculated the potential profile and the electronic levels in resonant tunneling double barrier structures with nanometric lateral dimensions (≤ 500 nm) for various contact doping. At biases for which the box states (laterally confined quantum well) are resonant with the emitter Fermi level, fine structures are expected in the resonant tunneling current. Comparison with I(V) characteristics measured on nanometric GaAs/GaAlAs and GaAs/GaAlAs/InGaAs resonant tunneling diodes shows that our model accounts for the resonance bias voltage and explains the shape of the current peak. The fine structure observed in the current peak provides a spectroscopy of the confined states in the quantum box.     

The variably spaced superlattice energy filter

A new superlattice device concept which provides for high energy injection of electrons into a semiconductor layer is presented. The device is based on resonant tunneling of electrons between adjacent aligned quantum well levels in a variably spaced superlattice structure. By a judicial choice of well and barrier widths the energy levels under reverse bias become aligned such that resonant tunneling of electrons through the structure can occur. Thus, electrons are injected into a semiconductor layer at an energy corresponding to the energy of the first subband in the last quantum well. This structure has significant advantages over the conventional method of producing hot electrons in that a nearly monoenergetic high-energy electron distribution is created at low reverse bias and with high efficiency, since energy loss to phonons is inhibited as a consequence of the channeling of electrons through a narrow band of quantum states. Applications of the VSSEF structure to avalanche photodiodes, IMPATT diodes and electroluminescent devices are discussed.     

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.     

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.     

Quantum oscillation of the tunneling conductance in fully epitaxial double barrier magnetic tunnel junctions

We investigated spin-dependent tunneling conductance properties in fully epitaxial double MgO barrier magnetic tunnel junctions with layered nanoscale Fe islands as a middle layer. Clear oscillations of the tunneling conductance were observed as a function of the bias voltage. The oscillation, which depends on the middle layer thickness and the magnetization configuration, is interpreted by the modulation of tunneling conductance due to the spin-polarized quantum well states created in the middle Fe layer. This first observation of the quantum size effect in the fully epitaxial double barrier magnetic tunnel junction indicates great potential for the development of the spin-dependent resonant tunneling effect in coherent tunneling regime.     

Resonant-tunneling structure of quantum wells in the p-i-n photovoltaic element

The method for efficient separation of photoexcited carriers, based on the resonant tunneling phenomenon in the quantum well structure placed into the i-region of the p-i-n photovoltaic element, is proposed. The parameters of quantum well structures based on the GaAs/Ga_1?x In _x As system, implementing the mode of sequential resonant tunneling in the electric field of the GaAs p-i-n junction, is calculated. A microscopic model of resonant-tunneling transport in such structures is constructed, and the kinetic tunneling times are calculated depending on well and barrier parameters. The possibility of achieving sufficiently short (<～10 ps) tunneling times and, hence, quite efficient removal of photoelectrons and photoholes from quantum wells at a proper choice of barrier powers is shown.     

Tunneling in double-barrier ZnSe/ZnTe structures — time-dependent analysis

Wave-packet time-dependent quantum mechanics is used to calculate the tunneling probability through a double-barrier ZnSe/ZnTe structure. The time-dependent transmission characteristics are obtained for several structures, and detailed electron dynamics is presented. The resonant peaks due to the presence of the discrete energy levels in the quantum well as well as in the barrier region are observed.     

Theoretical aspects of electron transport in modulated structures

In the theory of transport in modulated structures we have studied both transport perpendicular and parallel to the heterojunction interfaces. In perpendicular transport we have investigated models for tunneling through double barriers and find that resonant tunneling and sequential tunneling lead to the same expression for the current as long as the width of the energy distribution of the injected electrons is larger than the width of the resonant level in the diode. We present results for phonon assisted tunneling between two wells in a model which remains valid even when the barrier shrinks and the tunneling probability becomes very high. In parallel transport we show that very satisfactory agreement with extensive measurements of the mobility in modulation doped structures in the whole temperature range from 4 K to 300 K can be obtained if one takes into account the complete quasi-two-dimensional subband structure and all the relevant scattering mechanisms. Having established this we apply this program to systems with more complicated double channel structures, and show how one can tailor the conductivity of a channel in which perpendicular resonant tunneling affects parallel transport.     

The hole transport characteristics in SiGe/Si double and triple barrier resonant tunneling structures

Experimental measurements and theoretical calculations have been used to study the hole transport characteristics in SiGe/Si double and triple barrier resonant tunneling structures. The main emphasis is put on discussing the symmetry of I–V characteristics with forward and reverse bias, their temperature dependences and relations to quantum well designs. The calculations show that at current resonance, the sub-level can be much lower (e.g, for heavy hole resonance) or much higher (e.g, for light hole resonance) than the quasi-Fermi-level in the spacer. The distinctly different features of the measured first and second resonances for SiGe/Si double and triple barrier resonant tunneling, can be understood, by considering the different population of the heavy hole and light hole bands in the spacer region and the temperature dependences of Fermi-level, carrier mobility and effective masses. The analysis of dependences of the transmission and I–V curve with quantum well designs presents the possibility of using an asymmetric triple barrier structure to improve the resonant tunneling performance.     

Experimental evidence of resonant tunneling via localized DQW states in an asymmetric triple barrier structure

In this work we report on field-induced features appearing in the tunneling current traces of a biased asymmetric triple barrier resonant tunneling device in the presence of an in-plane magnetic field. A theoretical model that satisfactorily explains the origin of these features is discussed. The reported data evidences the localized nature of the quantum states in thin layer asymmetric double-quantum-well structures.     

Resonant tunnelling in O-D systems

O-D energy spectroscopy by means of transport and magneto-transport measurements has been carried out in double barrier resonant tunnelling heterostructures. Two models for the fabrication imposed lateral confining potential have been considered to account for the resonant lines associated with tunnelling of electrons through the O-D states of the quantum well. Preliminary measurements in a magnetic field are consistent with the values of magnetic length and undepleted conducting core radius in our structure, and indicate that up to 6 T the energy spectrum is dominated by spatial quantization. Single-charging effects in our structures are discussed.     

Controllable spin transport in dual-gated silicene

Based on the dual-gated silicene, we have evaluated theoretically the spin-dependent transport in lateral resonant tunneling structure. By aligning the completely valley-polarized beam with spin-resolved well state in concerned structure, large spin polarization can be expected owing to spin-dependent resonant tunneling mechanism. Under the gate electric field modulation, the forming quantum well state can be externally manipulated, triggering further the emergence of externally-controllable spin polarization. Importantly, integrating the considered structure with a proper valley-filter, which might be constructed from valley-contrasting physics as that in graphene valleytronics, completely-polarized spin beam can also be attained without the assistance of ferromagnetic component, providing thus some profitable strategies to develop nonmagnetic spintronic devices residing on silicene.     

Macroscopic resonant tunneling in an rf-SQUID flux qubit

We have observed the macroscopic resonant tunneling of magnetic flux between macroscopically distinct quantum states in a superconducting flux qubit.The dependences of the macroscopic resonant tunneling on the barrier height of the potential well,the flux bias and the initial state are investigated.Detailed measurements of the tunneling rate as a function of the flux bias reveal the feature of the quantum noise in the superconducting flux qubit.     

Effects of intersubband interaction on multisubband electron transport in single and double quantum wells

The multisubband electron transport properties are studied for doped single quantum well and gated double asymmetric quantum well structures. The effects due to intersubband interaction and screening of the ionized impurity scattering are also investigated. We show that intersubband coupling plays an essential role in describing the screening properties as well as the effect of ionized impurity scattering on the mobility in a doped single quantum well. For coupled double quantum well structures, negative transconductance is found theoretically which is due to resonant tunneling between the two quantum wells.     

Growth and time-resolved photoluminescence study of self-organized CdSe quantum dots in ZnSe

Employing two different growth methods: standard molecular beam epitaxy (MBE) and low-temperature atomic layer epitaxy (ALE) with subsequent annealing, we have obtained high-quality quantum dot structures consisting of CdSe embedded in ZnSe. Single dot emission lines are observed in micro-luminescence. The samples have been investigated by further optical methods including time-resolved photoluminescence under resonant excitation at 4.2 K. Distinct properties of systems with three-dimensional confinement are observed such as the suppression of the interaction between isolated quantum dots (QDs). In standard quantum wells tunneling/hopping processes generally lead to a pronounced red shift of the luminescence over time due to a lateral localization of excitons in potential fluctuations. A much less pronounced red shift is observed for the QDs reflecting only the different lifetimes of single dots and higher excited states. The red shift completely vanishes under resonant excitation that selectively excites only a few QDs of the ensemble in the layer. Typical behaviour is also observed from the halfwidth of the quantum dot emission.     

Level width of a quasibound state in a double-barrier parabolic-well resonant tunneling structure

Taking exact Airy functions and Hermitian functions as envelope functions, we investigate in detail the level width of a quasibound state for electrons coherent resonant tunneling through symmetric and asymmetric double-barrier parabolic-well resonant tunneling structures (DBRT) with the transfer-matrix formalism. It is found that for the symmetric structure and the asymmetric structure with left barrier thicker than the right one, both the level width and the peak value vary monotonously with increasing applied bias, but for the asymmetric DBRT structure with left barrier thinner than the right one, they change nonmonotonously. The nonmonotonous variations of the level width and the peak value reflect the transition of tunneling type (i.e. first from incompletely resonant tunneling to completely resonant tunneling, and then from completely resonant tunneling back to incompletely resonant tunneling). The effects of well width, barrier thickness and barrier height on the level width and the peak value are also inspected.     

铁磁金属量子阱态的共振隧穿

非铁磁金属层中的量子阱态在磁输运过程中的重要性已被广泛认识.铁磁金属层中自旋极化的量子阱态以前并没有详尽的理论研究;实验上也没有清晰地观测到自旋极化量子阱态的隧穿.文章介绍了最近由卢仲毅、张晓光和Pantelides［1］预言的Fe/MgO/FeO/Fe/Cr和其他铁磁量子阱隧道结中的共振隧穿,并解释铁、钴、铬的Δ1能带的对称性在这种共振隧穿中的作用.     

Velocity fluctuations of a column of water streaming down a wire: the possibility of one-dimensional turbulence

In this paper we study the influence of the magneto-coupling effect between the longitudinal motion component and the transverse Landau orbits of an electron on transmission features in single barrier structures. Within the parabolic conduction-band approach, a modified one-dimensional effective-mass Schr?dinger equation, including the magneto-coupling effect generated from the position-dependent effective mass of the electron, is strictly derived. Numerical calculations for single barrier structures show that the magneto-coupling effect brings about a series of the important changes for the transmission probability, the above-barrier quasi-bound states, and the tunneling time. Through examining the variation of the above-barrier resonant-transmission spectrum with the barrier width and observing the well-defined Lorentzian line-shape of the above-barrier resonant peaks, we convincingly show that the above-barrier resonant transmission in single barrier structures is delivered by the above-barrier quasibound states in the barrier region, just as the below-barrier resonant tunneling in double barrier structures is mediated by the below-barrier quasi-bound states in the well. Furthermore, we come to the conclusion that the magneto-coupling effect brings about not only the splitting of the above-barrier quasi-bound levels but also the striking reduction of the level-width of the quasi-bound states, correspondingly, the substantial increase of the density of the quasi-bound states. We suggest that magneto-coupling effects may be observed by the measurements of the optical absorption spectrum associated with the above-barrier quasi-bound states in the single barrier structures. Received: 26 September 1997 / Revised: 26 November 1997 / Accepted: 15 December 1997     

Magnetotunneling spectroscopy of a biased triple-barrier semiconductor heterostructure

In this work we describe a magnetotunneling spectroscopy technique for probing the localization degree of two-dimensional states and mapping the subbands in the active region of a resonant-tunneling semiconductor heterostructure. The reported experimental data consist of the low-temperature tunneling current traces of an asymmetric triple-barrier structure measured by sweeping an in-plane magnetic field up to 10 T. According to our interpretation model, the main features observed in the tunnel current traces are due to the field-induced resonant transitions between two-dimensional states at the crossing region between dispersion curves. The data reveals the highly localized nature of the quantum states in an asymmetric double-quantum-well structure even for those with very narrow middle barriers.     

限域量子态调控研究

电子、激子和声子等量子态在固体中的行为早已被人们所熟知. 然而,当体系的尺寸只有纳米量级的时候,已有的固体理论常常不能适用,需要新的低维物理理论的建立. 我们系统研究了低维体系限域量子态（包括电子、激子和声子）的行为对环境、应力、压力及光的响应和性质的调控. 较早认识到低维体系之显著的表面-体积比对量子态性质调控之有效性,系统地揭示了低维体系的一系列由表面和应力决定的新颖性质,证明了低维体系的表面和应力效应同量子限域效应同等重要. 本文概况了如下五个方面的结果：（1）一种使用应力效应调控电子能带结构的方法和（2）一种使用表面效应调控电子能带结构的方法（这两个方法都可将低维体系能带从间接能隙调控至直接能隙能带结构）;（3）一种低维体系表面掺杂方法,该方法将在低维体系掺杂中取代传统方法;（4）量子点表面诱导的光致异构现象;（5）基于表面自催化半导体低维结构的形成机理. 希望我们的研究工作有助于促进低维体系在光电子、纳电子、环境、能源、生物和医学等领域的应用.