The electron cut-off wavelength transistor

We propose a transistor based on the transverse quantization of electron waves. The spread of non-degenerate electrons is some 20 nm at room temperature. In a wire-like structure having a diameter of this or smaller size, the electron transport will be cut-off like that of electromagnetic modes in a wave guide. Accelerated in electric fields, hot electrons have smaller spread and may now propagate through the narrow electron waveguide. This principle presents the basis for novel minuscule and versatile simple devices.     

Theoretical assessment of electronic transport in InN

Among the group-III nitrides, InN displays markedly unusual electronic transport characteristics due to its smaller effective mass, high peak velocity and high background electron concentration. First, a non-local empirical pseudopotential band structure of InN is obtained in the light of recent experimental and first-principles results. This is utilized within an ensemble Monte Carlo framework to illuminate the interesting transport properties. It is observed that InN has a peak velocity which is about 75% higher than that of GaN while at higher fields its saturation velocity is lower than that of GaN. Because of the strongly degenerate regime brought about by the high background electron concentration, the electron–electron interaction is also investigated, but its effect on the steady-state and transient velocity–field characteristics is shown to be negligible. Finally, hot phonon generation due to excessive polar optical phonon production in the electron scattering and relaxation processes is accounted for. The main findings are the appreciable reduction in the saturation drift velocity and the slower recovery from the velocity overshoot regime. The time evolution of the hot phonon distribution is analysed in detail and it is observed to be extremely anisotropic, predominantly along the electric force direction.     

磁场中准二维强耦合磁极化子的有效质量

采用改进的线性组合算符和LLP变分法,研究了外磁场对量子阱中电子与界面光学声子强耦合、与体纵光学声子弱耦合磁极化子的有效质量的影响。对AgCl/KI量子阱进行了数值计算,结果表明,磁极化子的有效质量随阱宽的增加而减小,随磁场的增强而增大,但不同支声子与电子和磁场相互作用对磁极化子有效质量的贡献大不相同。     

Field-induced energy band and effective mass anisotropy in coupled double quantum wells

The effects of the field-induced conduction band anisotropy on the electron effective mass and electrical transport properties of coupled double quantum well structures are discussed. This anisotropy is due to the formation of a partial energy gap in the dispersion curves of coupled double quantum wells by an in-plane magnetic field. The reported data show that the magnitude of this gap is mainly dependent on the interwell coupling, although it also depends on the magnitude of the applied magnetic field. A field-induced anisotropy of the electron effective mass and the in-plane electrical transport properties, due to the formation of a saddle point at the lower edge of this gap, is predicted. Further, a simple model to estimate the critical field at which a saddle point appears is also discussed.     

Electromagnetic-field-induced resonant structures for an open rectangular quantum dot

We study theoretically the electron transport properties for an open rectangular quantum dot under an external electromagnetic field illumination in the ballistic regime. Using the effective mass free-electron approximation, the scattering matrix for the system has been formulated by the time-dependent mode-match method. Some interesting properties of the electron transmission have been demonstrated through serval numerical examples. The dependence of electron transmission on the electron incident energy is found to exhibit Fano dip structures due to the field-induced intersubband scatterings into quasibound states in the dot. Moreover, with an appropriate incident energy the electron transmission as a function of the field frequency and/or amplitude shows a rich structure. Our results suggest that the electron transport properties of an open rectangular quantum dot are affected by the interplay effects between the nonadiabatic dot-lead connection and the applied field.     

量子局域效应和应力对GaSb纳米线电子结构影响的第一性原理研究

采用基于密度泛函理论的第一性原理计算方法, 研究了不同晶体结构和尺寸的GaSb纳米线能带结构特性和载流子的有效质量, 以及单轴应力对GaSb纳米线能带结构的调控. 研究结果表明: 闪锌矿结构[111]方向和纤锌矿结构[0001]方向的小尺寸GaSb纳米线均出现间接带隙的能带结构, 并可通过单轴应力来实现纳米线能带结构由间接带隙到直接带隙的转变, 其中, 闪锌矿结构[111]方向GaSb纳米线仅在受到单轴拉伸应力时才发生能带由间接带隙到直接带隙的转变, 而纤锌矿结构[0001]方向GaSb纳米线无论受单轴拉伸还是压缩应力的作用均可实现能带由间接带隙到直接带隙的转变; [111]和[0001]方向GaSb纳米线的带隙和载流子有效质量与纳米线直径呈非线性关系, 并随纳米线直径的减小而增大; 同一方向和尺寸的GaSb纳米线, 其空穴有效质量要小于电子有效质量, 这表明小尺寸GaSb纳米线有利于空穴载流子输运.     

First-principles Band Structures Calculation of Tin-phthalocyanine

SnPc(Tin-phthalocyanine)因在无机/有机二极管等光电结构器件中表现出了很多有趣的特性而备受关注.为了更深地理解载流子的传输特性,利用密度泛函理论,采用广义梯度近似(DFT-GGA),关联函数选择BLYP计算了SnPc的能带结构.从点波函数、能带带宽以及带隙分析了载流子的传输行为. 从前线轨道的带宽以及电子和空穴的有效质量,可以看到电子的传输要比空穴的传输容易两倍左右.而且,当研究费米能级附近的能带时,发现未占有带的带隙总体上要小于占有带的带隙,这表明在考虑声子参与的情况下,电子在带间的跳跃要比空穴容易得多.以上的事实说明SnPc是一种电子传输占主导的材料.     

Holstein极化子能带和有效质量的温度依赖性

使用正则变换方法,考察了一维Holstein极化子能带和有效质量的温度依赖性。结果表明,对于一定的电子声子耦合强度,Holstein极化子能带宽度随温度升高而变窄,有效质量随温度升高而增大。特别是当电子声子耦合强度足够大时,极化子能带宽度在很小的温度范围内会迅速地变为零,我们认为这种情况实际上是极化子从能带状态向自陷局域态的迅速转变,这与通常的相变现象有点相类似。当电子声子耦合常数越大时,极化子有效质量随温度的升高而增加得越快。很显然,研究电子声子相互作用,对理解固体的光学和输运等性质将有重要的意义。     

Spin polarization and temperature dependence of electron effective mass in quantum wires

The electron effective mass in GaAs quantum wires has been estimated by using a full dynamical random-phase approximation to examine its properties versus spin polarization, temperature, and carrier density. A decrease of mass with spin polarization is seen. The minority mass increases with the polarization while the majority mass decreases and this behaviour is seen for all densities. A maximal enhancement of mass at moderate temperature around 25 K is also presented. These calculations show a qualitative consistence with results in two-dimensional systems and help to control the electronic transport in quantum wires.     

Vertical charge transport in a superlattice

It is demonstrated that the vertical charge transport in a superlattice is influenced uniquely by the difference in the effective mass of the electron between its adjacent layers. The mass difference produces a coupling between the parallel and vertical motions of the electron. As a consequence, the vertical velocity of an electron depends not only on its wavevector in the vertical direction but also on the wavevector in the direction parallel to the layers. As an illustration of the effect, a qualitative analysis is made to show that the vertical motion of the electron wavepacket can be altered by the application of a magnetic field whose direction is normal to the layers.     

无限深势阱下杂质量子点的能级计算

在有效质量近似下,垂直方向采用无限深势阱限制势,在x-y平面上,量子点内采用抛物势近似,在量子点边界处采用与实际情况更接近的无限深势阱.在中心杂质电荷为ηe时,利用波函数近似,得到基态和低激发态的能级,与x-y平面均采用抛物势时得到的能级进行了比较.计算发现在量子点真实半径比较小时,电子的基态和低激发态受其影响很大,而相应的能级随量子点的半径逐步增大.在量子点半径大于5倍有效玻尔半径时,能级受其影响已经变得很弱.并且,随着磁场的变化,量子点半径对基态和第一激发态的能级差的影响也很大.最后我们计算了杂质电子的基态束缚能并讨论了声子对其影响.     

GaSe: A layer compound with anomalous valence band anisotropy

Measurement of the magneto-Stark effect and of electronic transport properties on GaSe show that, although it is a layer structure, this semiconductor has nearly isotropic electronic states at the forbidden gap. In fact, the valence band anisotropy is anomalous: the effective mass normal to the layers is smaller than that in the plane of the layers.     

磁场和耦合强度对极化子有效质量和平均声子数的影响

应用线性组合算符和幺正变换方法,研究磁场和耦合强度对极化子有效质量和平均声子数的影响.数值计算表明:极化子的有效质量随耦合强度的增加而增加,这是由于耦合强度增加时,电子与晶格振动之间的相互作用增加所致;而磁场强度增加时,有效质量是先增加,达到一个极大值后,再逐渐减少,出现共振现象.平均声子数随耦合强度的增加而增加,当磁场强度大于共振时的磁场强度时,随磁场强度的增加而减少,反之,结论相反.     

考虑界面散射的金属纳米线热导率修正

理论分析了声子和电子输运对Cu, Ag金属纳米线热导率的贡献. 采用镶嵌原子作用势模型描述纳米尺寸下金属原子间的相互作用, 应用平衡分子动力学方法和Green-Kubo函数模拟了金属纳米线的声子热导率; 采用玻尔兹曼输运理论和Wiedemann-Franz定律计算电子热导率; 并通过散射失配模型和Mayadas-Shatzkes模型引入晶界散射的影响. 在此基础上, 考察分析了纳米线尺度和温度的影响. 研究结果表明: Cu, Ag纳米线热导率的变化规律相似; 电子输运对金属纳米线的导热占主导地位, 而声子热导率的贡献也不容忽视; 晶界散射导致热导率减小, 尤其对电子热导率作用显著; 纳米线总热导率随着温度的升高而降低; 随着截面尺寸减小而减小, 但声子热导率所占份额有所增加. 关键词： 纳米线 热导率 表面散射 晶界散射     

Dependence of spin susceptibility of a two-dimensional electron system on the valley degree of freedom

We report measurements of the spin susceptibility, chi proportional, variant g(v)g*m*, in an AlAs two-dimensional electron system where, via the application of in-plane stress, we transfer electrons from one ellipsoidal conduction-band valley to another (g(v) is the valley degeneracy, and m* and g* are the electron effective mass and g factor). At a given density, when the two valleys are equally populated (g(v)=2), the measured g*m* is smaller than when only one valley is occupied (g(v)=1). This observation counters the common assumption that a two-valley two-dimensional system is effectively more dilute than a single-valley system because of its smaller Fermi energy.     

Quantum oscillations in a hexagonal boron nitride-supported single crystalline InSb nanosheet

A gated Hall-bar device is made from an epitaxially grown, free-standing InSb nanosheet on a hexagonal boron nitride (hBN) dielectric/graphite gate structure and the electron transport properties in the InSb nanosheet are studied by gate-transfer characteristic and magnetotransport measurements at low temperatures. The measurements show that the carriers in the InSb nanosheet are of electrons and the carrier density in the nanosheet can be highly efficiently tuned by the graphite gate. The mobility of the electrons in the InSb nanosheet is extracted from low-field magneotransport measurements and a value of the mobility exceeding $\sim 1.8\times10^4$ cm$^{2}\cdot$V$^{-1}\cdot$s$^{-1}$ is found. High-field magentotransport measurements show well-defined Shubnikov-de Haas (SdH) oscillations in the longitudinal resistance of the InSb nanosheet. Temperature-dependent measurements of the SdH oscillations are carried out and key transport parameters, including the electron effective mass $m^{\ast }\sim 0.028 m_{0}$ and the quantum lifetime $\tau \sim 0.046 $ ps, in the InSb nanosheet are extracted. It is for the first time that such experimental measurements have been reported for a free-standing InSb nanosheet and the results obtained indicate that InSb nanosheet/hBN/graphite gate structures can be used to develop advanced quantum devices for novel physics studies and for quantum technology applications.     

Jacobian transformed and detailed balance approximations for photon induced scattering

Photon emission and scattering are enhanced by the number of photons in the final state, and the photon transport equation reflects this in scattering–emission kernels and source terms. This is often a complication in both theoretical and numerical analyzes, requiring approximations and assumptions about background and material temperatures, incident and exiting photon energies, local thermodynamic equilibrium, plus other related aspects of photon scattering and emission. We review earlier schemes parameterizing photon scattering–emission processes, and suggest two alternative schemes. One links the product of photon and electron distributions in the final state to the product in the initial state by Jacobian transformation of kinematical variables (energy and angle), and the other links integrands of scattering kernels in a detailed balance requirement for overall (integrated) induced effects. Compton and inverse Compton differential scattering cross sections are detailed in appropriate limits, numerical integrations are performed over the induced scattering kernel, and for tabulation induced scattering terms are incorporated into effective cross sections for comparisons and numerical estimates. Relativistic electron distributions are assumed for calculations. Both Wien and Planckian distributions are contrasted for impact on induced scattering as LTE limit points. We find that both transformed and balanced approximations suggest larger induced scattering effects at high photon energies and low electron temperatures, and smaller effects in the opposite limits, compared to previous analyzes, with 10–20% increases in effective cross sections. We also note that both approximations can be simply implemented within existing transport modules or opacity processors as an additional term in the effective scattering cross section. Applications and comparisons include effective cross sections, kernel approximations, and impacts on radiative transport solutions in 1D geometry. The additional computing time for processing opacities (cross sections) within these approximations is negligible as induced terms are merely added (multipliers) to cross sections at the end of the processing cycle.     

Pressure dependence study of the effective mass in Ga0.47In0.53As/InP heterojunctions

A study of the effective mass in GaInAs/InP heterojunctions under hydrostatic pressure up to 15 kbars is presented. Earlier results have shown the importance of hydrostatic pressure effects on the band parameters of the heterojunction to explain the experimental decrease of the carrier concentration with pressure at the interface (1). Here magnetophonon resonance experiments are performed to work out the increase of mass with pressure in our samples. The effective mass at atmospheric pressure is deduced from high temperature cyclotron resonance experiments and then used to calculate the frequency of the phonon interacting with the 2D electron gas (w_o). The value of w_o is found to be dependent on the carrier concentration of the measured samples. The lowest value is found for the highest carrier concentration sample.A band edge effective mass increase of 1 ± .1% kbar is found in the highest carrier concentration sample. This is two times smaller than the rate found experimentally in GaInAs bulk material and slightly smaller than in an AlInAs/GaInAs heterojunction. The experimental increase could be fitted with multiband k.p theory assuming no pressure dependence for the conduction band-valence band matrix element E_p. However at lower concentration a variation of the matrix element E_p with pressure has to be considered.     

Improved double-gate armchair silicene nanoribbon field-effect-transistor at large transport bandgap

The electrical characteristics of a double-gate armchair silicene nanoribbon field-effect-transistor(DG ASi NR FET)are thoroughly investigated by using a ballistic quantum transport model based on non-equilibrium Green's function(NEGF) approach self-consistently coupled with a three-dimensional(3D) Poisson equation. We evaluate the influence of variation in uniaxial tensile strain, ribbon temperature and oxide thickness on the on-off current ratio, subthreshold swing, transconductance and the delay time of a 12-nm-length ultranarrow ASi NR FET. A novel two-parameter strain magnitude and temperature-dependent model is presented for designing an optimized device possessing balanced amelioration of all the electrical parameters. We demonstrate that employing Hf O2 as the gate insulator can be a favorable choice and simultaneous use of it with proper combination of temperature and strain magnitude can achieve better device performance.Furthermore, a general model power(GMP) is derived which explicitly provides the electron effective mass as a function of the bandgap of a hydrogen passivated ASi NR under strain.     

Small mass enhancement near the metal–insulator transition in gated silicon inversion layers

The strong resistivity changes in the metallic state of two-dimensional electron systems have recently been assigned to quantum interaction corrections in the ballistic regime. We have performed analysis of Shubnikov–de Haas oscillations on high-mobility silicon inversion layers where we have explicitly taken into account that the back scattering angle has different influence on momentum relaxation and quantum life time. The consistent analysis under the assumption of the ballistic interaction corrections leads to smaller increase of the effective mass with decreasing electron density as usually reported.