Current rectification through a single-barrier resonant tunneling quantum structure

We propose a simple quantum structure which exhibits resonant tunneling under one bias and simple tunneling under the opposite one, thus acting as a rectifier. The diode consists of a single laterally-indented barrier. Due to its particular conduction-band profile, electrons undergo resonant tunneling when the bias creates a band-profile triangular well which can contain a resonant state aligned to the emitter Fermi energy. A diode with an active layer of ≈ 100Å, realized by AlGaAs/GaAs, has a Rectification Ratio, calculated at the current-peak bias at resonance, of ≈ 100. This value can be enhanced by putting in series several elements of this kind.     

Evidence of the enhanced resonant tunneling effect in a triple-barrier heterostructure

The effect of properly lined-up quantum-well (QW) states, under an external bias, on the electron resonant tunneling is investigated in an InAlAs/InGaAs triple-barrier structure. The degree of alignment of two QW confined ground states at a resonant voltage is analyzed with low-temperature measurement. The experimental data shows the enhanced resonant tunneling effects, and proves that the second QW structure added to the InGaAs/ InAlAs double-barrier heterostructure can act as an effective tool for probing and extracting the resonant tunneling properties deep in a QW.     

磁性隧道结自旋极化电子的隧穿特性

铁磁金属间通过中间层的自旋极化电子隧穿产生的磁性耦合,在自旋电子器件中有许多潜在的应用.考虑由一平面磁性势垒层隔开的两铁磁性金属电极构成的磁性隧道结,针对中间层形成的矩形势垒,在近自由电子模型的基础上,计算零偏压下的隧穿电导、自旋极化率和隧穿磁阻比率,分析势垒层特性、分子场强弱、分子场相对取向等对隧道结自旋极化电子隧穿特性的影响.计算结果对自旋电子器件的设计具有一定的指导意义.     

弱耦合GaAs/AlGaAs/InGaAs双势阱隧穿结构的磁隧穿特性研究

研究了低温(15 K)条件下弱耦合GaAs/AlGaAs/InGaAs双势阱结构的纵向磁隧穿特性. 研究表明,器件在零偏压下处于共振状态. 通过分析不同偏压下的磁电导振荡曲线,可以得到双量子阱中的基态束缚能级随偏压的变化规律,从而可以确定隧穿电流峰对应的隧穿机制. 所得结果可为弱耦合双量子点器件的制备提供基础. 关键词： 双量子阱 隧穿结构 磁电导振荡     

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.     

Isochromat spectroscopy of photons emitted from metal surfaces in an STM

The pronounced variation of the intensity of photons emitted from the tunneling gap of an STM with respect to the applied bias voltage V_t is studied experimentally using simultaneous measurements of tunneling characteristics and photon emission. We show that the structure in isochromat photon spectra are determined by the following: bias-dependent changes in tunneling characteristics, density of initial and final states, and modifications of tip-induced plasmon modes. It is demonstrated that isochromat spectra provide a conclusive test for the inelastic tunneling mechanism. Coupling between tunneling electrons and tip-induced plasmon modes which gives rise to the intense photon emission observed is discussed within this model.     

Enhanced magnetotransport at high bias in quasimagnetic tunnel junctions with EuS spin-filter barriers

In quasimagnetic tunnel junctions with a EuS spin-filter tunnel barrier between Al and Co electrodes, we observed large magnetoresistance (MR). The bias dependence shows an abrupt increase of MR ratio in high bias voltage, which is contrary to conventional magnetic tunnel junctions. This behavior can be understood as due to Fowler-Nordheim tunneling through the fully spin-polarized EuS conduction band. The I-V characteristics and bias dependence of MR calculated using tunneling theory show excellent agreement with experiment.     

分子电子器件简化模型的电子透射谱的计算

基于分子线耦合到电极的构成特点,采用简化的非对称多势垒连续隧穿模型模拟复合分子器件偏压下的电子隧穿过程,推导电子透射谱的解析表达式,同时计算垒宽、垒距、垒高、电子有效质量和所加偏压等参数与透射系数的关系,结果发现：当电子的能量为某些值时,出现明显的共振隧穿,且透射系数对这些参数的变化非常敏感,这表明可以通过适当的控制方式（如改变复合分子组成、构型等）来修改分子电子器件的输运性质. 关键词： 分子器件 非对称势垒模型 电子透射谱 共振隧穿     

磁量子结构中二维自旋电子的隧穿输运

研究了零偏压和偏置电压作用下磁量子结构中自旋电子的隧穿输运性质. 结果表明电子自旋 输运的性质不仅取决于磁量子结构的构型、入射电子的能量和波矢, 而且取决于偏置电压. 在零偏压下, 由等同的磁垒磁阱构成的磁量子结构不具有自旋过滤的特点, 而由不等同的磁 垒磁阱构成的磁量子结构却具有较好的自旋过滤特点. 偏置电压极大地改变了磁量子结构中 电子的极化程度, 使得电子隧穿等同的磁垒磁阱构成的磁量子结构的输运性质也显著地依赖 于电子的自旋指向. 关键词： 磁量子结构 自旋电子 隧穿输运 自旋极化     

Enhanced electron tunneling and Γ–X interlayer intervalley electron transfer in a triple-barrier heterostructure

The enhanced electron tunneling effect and the electron Γ–X intervalley interlayer transfer in the AlAs/GaAs (001) triple-barrier heterostructure have been investigated both experimentally and theoretically. The effects of the external bias on the electronic structure, Γ–X state mixing and higher lying excited energy states are studied. The experimental observations show good agreement with the theoretical predictions based on the scattering theoretical approach of Green's function theory, which can handle electron interlayer intervalley propagation through the layered aperiodic heterostructure under the external bias.     

N/I/d波超导体c轴隧道结的微分电导

以方势垒描述绝缘层,对N/I/d波超导体c轴隧道结的微分电导进行了研究.结果表明:在N/I/d波超导体c轴隧道结的隧道谱中存在V型结构、能隙外的凹陷和小的零偏压电导峰.这一结果能很好的解释相关的实验现象.     

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.     

纳米硅结构中分裂能级对光电输运特性的影响

基于电子在分裂能级系统中同时存在的共振隧穿和子带输运过程,结合光生载流子作用提出了纳米硅结构中的光电输运理论模型.利用该模型计算了纳米硅结构在光照条件下的电流密度、电场强度及电子浓度分布.结果表明,光生电子在具有分裂能级的纳米硅中是以共振隧穿为主要输运方式.在此基础上,详细研究了光电流与吸收系数、外加偏压以及纳米硅层层数之间的关系,发现在特定的外界条件下光电流会出现跳变增加的现象,其物理原因是纳米硅结构中电场强度的二次分布.     

Simplified modeling of frequency behavior in photonic crystal vertical cavity surface emitting laser with tunnel injection quantum dot in active region

In this work, the characteristics of the photonic crystal tunneling injection quantum dot vertical cavity surface emitting lasers(Ph C-TIQD-VCSEL) are studied through analyzing a modified modulation transfer function. The function is based on the rate equations describing the carrier dynamics at different energy levels of dot and injector well. Although the frequency modulation response component associated with carrier dynamics in wetting layer(WL) and at excited state(ES) levels of dots limits the total bandwidth in conventional QD-VCSEL, our study shows that it can be compensated for by electron tunneling from the injector well into the dot in TIQD structure. Carrier back tunneling time is one of the most important parameters, and by increment of that, the bias current dependence of the total bandwidth will be insignificant. It is proved that at high bias current, the limitation of the WL-ES level plays an important role in reducing the total bandwidth and results in rollovers on 3-d B bandwidth-I curves. In such a way, for smaller air hole diameter of photonic crystal, the effect of this reduction is stronger.     

Resonant tunneling properties of inverted Morse double quantum barrier

We study resonant tunneling characteristics of inverted Morse double quantum barrier structures. The effect of electric bias and structure parameters is calculated by using non-equilibrium Green's function method. Results for the transmission coefficients are compared with the structure parameters. Our results show that the widths of the wells and heights of barriers have a significant effect on the transmission properties. We found that the resonant peak of the transmission coefficient decreases with increasing electric field bias. Moreover, resonant energy level increases with increasing barrier height and increasing width parameters.     

Reversible local-modification of surface structure on clean Ge(0 0 1) by scanning tunneling microscopy below 80 K

The structure of the clean Ge(0 0 1) surface is locally and reversibly changed between c(4×2) and p(2×2) by controlling the bias voltage of a scanning tunneling microscopy (STM) below 80 K. It shows hysteresis for the direction of the sample bias voltage change. The c(4×2) structure is observed with the sample bias voltage V_b?−0.7 V. This structure is maintained at V_b?0.7 V with increasing the bias voltage from −0.7 V. When V_b is higher than 0.8 V, the structure changes to p(2×2). This structure is then maintained at V_b?−0.6 V with decreasing the bias voltage from +0.8 V. The area of the structure change can be confined in the single dimer row just under the STM tip using a bias voltage pulse. In particular, the minimum transformed length is four dimers along the dimer row in the transformation from p(2×2) to c(4×2). The observed local change of the reconstruction with hysteresis is attributed to the energy transfer process from the tunneling electron to the Ge lattice in the local electric field due to the STM bias voltage. A phenomenological model is proposed for the structure changes. It is based on a cascade inversion of the dimer buckling orientation along the dimer row.     

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.     

Negative differential conductivity of metal-insulator-metal tunneling structures

The tunneling characteristics of a metal-insulator-metal junction are calculated in the framework of the two-band model, which takes into account the presence of the valence band for the insulator layer. It is demonstrated that, in the case where the Fermi level E _F of the structure under investigation lies below the middle of the band gap of the insulator, the dependence of the tunneling current on the bias voltage across the junction contains portions with a negative differential resistance at V > E _F/e. The iron-aluminum oxide-iron magnetoresistive junctions are considered as samples in which the effect under discussion can be observed. It is shown that, in the given case, the appearance of a portion with a negative differential resistance should be expected at voltages exceeding the Fermi energy \(E_{F_1 } \) for the spin-up electron band.     

Theory of resonant tunneling through the insulator conduction band of a thin film metal-insulator-metal (MIM) heterojunction

An “atomic” model of an insulating barrier between two free-electron model metals is used to investigate resonant tunneling across the insulator in the presence of a medium to large, externally applied electric field (bias). The exact numerically calculated tunneling current exhibits a pronounced oscillatory bias dependence superposed on the dominant roughly exponential tunneling characteristic. The interpretation of these results in terms of an internal field emission or Fowler-Nordheim type tunneling subject to “periodic deviations” (or interferences) seems plausible and was suggested by Maserjian. To test this conjecture, a trapezoidal barrier model of our “atomic” model analyzed numerically. As expected, the trapezoidal barrier model could only qualitatively reproduce the oscillatory bias dependence of the barrier transmissivity and of the current. Furthermore this limited agreement depends on allowing the effective mass in the barrier to become a strictly adjustable parameter. This failure of the conventional model of the junction can be interpreted as follows: (i) For moderate external (bias) fields the trapezoidal barrier fails to account for the correct position dependence of the Blochwave vector in the insulator's conduction band, hence the correct interference conditions cannot be reproduced. (ii) For large external fields the band model itself begins to fail. An explanation of oscillatory bias dependence at the tunneling current in terms of splitting of the insulator's conduction band into a set of discrete Stark levels is suggested. It is demonstrated that a fit of the oscillatory tunneling characteristics in the “Fowler-Nordheim regime” is not a reliable technique to determine the effective mass in the thin insulating film of tunneling junctions over the energy interval containing the forbidden gap and the adjoining conduction-band.     

STM-induced photon emission spectroscopy of granular gold surfaces in air

Light emission has been detected under ambient conditions in the tip–sample region of a scanning tunneling microscope (STM) consisting of an etched gold tip and a granular gold film. The photon yield as a function of surface geometry (photon mapping) has been studied. By means of STM, it was possible to measure photon emission spectra locally. We have studied the effect of grain size and applied bias voltage on the spectrum. We found that the peak position in the photon emission spectrum shifts to a shorter wavelength when increasing the bias voltage and shifts to a longer wavelength when tunneling to larger grains. These effects can be understood in a simple model which considers tunneling electrons exciting localized surface plasmons which decay by emitting photons.