Quantum corrections to the conductivity of a two-dimensional system with antidots

The quantum corrections to the conductivity of a two-dimensional electron system with antidots in the limit of a small antidot density are investigated. The corrections to the conductivity and magnetoconductivity due to the presence of antidots in a magnetic field perpendicular to the plane of the system are considered. It is assumed that the mean free path l of electrons on the impurities is far smaller than the antidot radius. Fiz. Tekh. Poluprovodn. 32, 1461–1466 (December 1998)     

Reflection of electrons during tunneling and an intersubband polaron in the 2D electron system of a delta-layer in GaAs

Al-δ-GaAs structures are studied where tunneling to one or more subbands of the 2D electron system of a near-surface delta-doped layer is observed. Reflection of electrons at the threshold of the emission of an LO-phonon is observed. This phenomenon occurs when a new subband is involved in the tunneling process and when intersubband transitions of the electrons in a 2D-system with the emission of an LO-phonon are added to the inelastic tunneling within a single subband. It is shown that, under the conditions of an intersubband polaron resonance, the reflection processes dominate during tunneling into the delta-layer. If, however, tunneling from the delta-layer occurs, the reflection processes are observed when two subbands are occupied.     

Electron scattering in AlGaN/GaN heterostructures with a two-dimensional electron gas

The temperature and concentration dependences of electron mobility in AlGaN/GaN hetero-structures are studied. The mobility for the samples under study at T = 300 K lies in the range of 450–1740 cm²/(V s). It is established that scattering at charged centers is dominant for samples with low mobility (lower than 1000 cm²/(V s) right up to room temperature. These centers are associated with a disordered piezoelectric charge at the heterointerface because of its roughness or with a piezoelectric charge similarly to the Al-GaN barrier because of alloy disorder, as well as with the deformation field around dislocations. Scattering at optical phonons is dominant for samples with mobility exceeding 1000 cm²/(V s) at T = 300 K. Scattering at alloy disorders, heterointerface roughness, and dislocations are dominant at temperatures lower than 200 K. A decrease in the influence of scattering at roughness with improvement of the heterointerface morphology increases room-temperature mobility from 1400 cm²/(V s) to 1700 cm²/(V s).     

Spin properties of trions in a dense quasi-2D electron gas

The reflection and photoluminescence spectra of modulation-doped CdTe/CdMgTe quantum-well structures have been studied. It was found that the magnitude and sign of the Zeeman splitting of the trion reflection line depend on the electron concentration in the quantum well, whereas the magnitude and sign of the splitting of the exciton line are absolutely the same for all the electron concentrations under study. In the photoluminescence spectra, the magnitude and sign of the Zeeman splittings for the exciton and trion were the same. This ??renormalization?? of the trion g factor is explained in terms of the model of combined exciton-electron processes.     

电子显微镜中的康普顿散射

在电子显微镜中,通过记录大角非弹性散射电子能量损失谱来研究固体材料基态电子动量密度分布的方法,称为固体的电子康普顿散射技术(electron Compton scattering from solids,ECOSS)。利用该技术得到的康普顿轮廓的动量分辨率可以达到从同步辐射光源上得到的康普顿轮廓的动量分辨率水平;而其非常短的收集时间,使利用康普顿散射对固体材料电子动量密度的系统研究成为可能。本文简要介绍了该技术的背景、实验方法、理论基础,研究进展和应用。     

Picosecond optical studies of 2D electron - 2D phonon dynamics

We describe recent developments in time-resolved optical measurements of electron-phonon interactions in quantum wells which reveal the fundamental two-dimensional properties of the process. Picosecond photoluminescence experiments show enhanced energy relaxation in narrow quantum wells. Time-resolved Raman measurements of inter- and intra-subband relaxation of electrons reveal the participation of confined optical phonons; in narrow quantum wells there is strong coupling to interface phonons.     

弹性散射截面对计算电子束投影成像衬度的影响

我们采用蒙特卡洛方法模拟了100keV电子通过W／Cr／Si3N4掩膜的透射率。发现在此高能量下,电子弹性散射的Mott截面与经典的Rutherford截面仍有较大差异。由于这两种总截面和小角散射的微分截面不同,因此采用何种截面将直接影响限角散射电子束投影成像的模拟结果。本文通过比较电子透射的实验测量和模拟计算结果,确认了使用Mott截面的必要性。     

Contact-free determination of the parameters of a 2D electron gas in GaAs/AlGaAs heterostructures

The parameters of a 2D electron gas in a GaAs/AlGaAs heterostructure — density, conductivity, and mobility in zero magnetic field, transport and quantum (one-particle) relaxation times, Dingle temperature, and spacer width — have been determined by a contact-free (acoustic) method. Fiz. Tekh. Poluprovodn. 31, 1092–1094 (September 1997)     

Estimate of the electron density in a plasma after-glow from a study of electromagnetic scattering

The decay of electron density in an argon-plasma afterglow has been determined by analysis of the scattering of microwave radiation. The results are applicable to overdense plasmas and are in good agreement with a short-wavelength cutoff measurement; they lead to a reasonable value of the ambipolar diffusion coefficient.     

Analysis of time domain scattering by a flat plate

A numerical modeling technique is developed to calculate time-domain responses for a range which radiates short pulse signals. The theoretical time domain response for the case of bistatic scattering by a finite flat plate is calculated by combining frequency data from uniform geometrical theory of diffraction (UTD) and moment method (MM) solutions. The theoretical responses are compared to measured time waveforms showing good agreement.     

基于GPU的并行FDTD方法在二维粗糙面散射中的应用

利用显卡(Graphics Processing Unit, GPU)加速时域有限差分(Finite-Difference Time Domain, FDTD)法计算二维粗糙面的双站散射系数, 介绍了FDTD的理论公式以及计算模型.采用各向异性完全匹配层(Uniaxial Perfectly Matched Layer, UPML)截断FDTD计算区域.重点讨论了基于GPU的并行FDTD计算粗糙面双站散射系数的并行设计方案计算流程.在NVIDIA GeForce GTX 570显卡上获得了50.7×的加速比.结果表明:通过对FDTD计算粗糙面散射问题的加速, 极大地提高了计算效率.     

Testing of geometrized models of 2D electron beams

Geometrized models of a dense electron beam are tested using two sets of references: solutions with multiplicative separation of variables and analytic solutions with additive separation of variables. The effectiveness of the geometrized approach for describing nonparaxial flows with the nonuniform distribution of parameters over the cross section is demonstrated. Models are considered in which the length of the base current tube or the potential plays the role of the longitudinal coordinate. The effect of remarking the transverse coordinate, which determines the geometry of the current tube, is studied. The asymptotic theory is compared with the first-approximation geometrized theory. It is shown that each exact axially symmetric solution to the equations of a beam in elementary functions corresponds to the exact solution to the equation for a tubular beam in the asymptotic theory. For optimal utilization of possibilities of the geometrized approach, it is proposed to use the value of discrepancy as a criterion oor accuracy.     

Analysis of scattering from 2D penetrable object using iterativeFEM with novel mesh truncation

An iterative finite element method is proposed to analyse scattering from a two dimensional penetrable object. Conventionally, to solve this kind of problem, the whole penetrable object is discretised, which is unduly costly if the object contains a large homogeneous region. By introducing a fictitious boundary inside in addition to outside the scatterer, the problem region can be greatly reduced. Also, the typical finite element method is applied with iteratively improved radiation-type boundary conditions. To validate the proposed method, a numerical example is presented     

Properties of 2D electron gas in AlGaAs/GaAs heterojunctions with thin AlGaAs layers

Transport properties of a 2D electron gas located at a short distance from the surface (15–32.5 nm) in an AlGaAs/GaAs heterojunction were studied. A pronounced effect of the surface on the behavior of the conductivity of the 2D electron gas, as well as metallic gate screening of the scattering of two dimensional electrons by charged centers, were observed.     

Electric-field control of electron interference effects in 2D semiconductor nanostructures with parabolic quantum wells

The possibility of the influence of a constant electric field on spatial reproduction effects for quantum-mechanical current density, which arise during interference of electron waves in 2D semiconductor nanostructures, is studied theoretically. It is found that, in structures comprising a narrow rectangular quantum well and a wide parabolic quantum well placed successively in the direction of propagation of an electron wave, the transverse current density distribution existing at the input of the wide quantum well is reproduced in this well with a certain accuracy in periodically repeated cross sections. The inhomogeneous current density distribution arises because of the interference of electron waves propagating in the wide quantum well simultaneously in different quantum-dimensional subbands. It is shown that it is possible to control these effects via the use of a constant electric field perpendicular to the axis of the structure in the region of the wide quantum well.     

Compton scattering in a large-aperture positron imaging system

Compton scattering of gamma rays within the image volume has been assessed for a large-aperture positron-emission-tomography imaging system. The Compton scattered attenuation and the Compton scattered background were both modeled and measured for point sources centered in scattering spheres up to 10 cm in diameter. Good agreement was obtained between simulations and measurements. The attenuation problem is independent of the detector system, but its correction is more difficult in a large-aperture system. The scattered coincidence background is large in this system (43% for a 10-cm-diameter scattering sphere), but the background overlap is reduced with 3D imaging.     

Effect of ionised impurity scattering on the electron transit time in GaAs and InP f.e.t.s

The effect of ionised impurity scattering on the transient drift velocity of electrons in short-gate GaAs and InP field-effect transistors is investigated by Monte Carlo techniques.     

静电场中高斯激光束与电子作用产生单个阿秒脉冲

提出了利用线极化的高斯光束穿过固体表面与其电 离的一个电子在Thomson散射作用下产生单个阿秒脉冲辐射的方法。发现了上述环境中有合 适大小均匀静电场的情况下可以产生单个阿秒 脉冲。分别研究了4种不同强度的静电场大小对处在光场半腰束位置的电子产生脉冲的影响 。结果显 示在相同参数,束腰半径r₀=5,脉冲宽度w₀=3,持续周期时间为200ms,P=1的情况下,四种不 同大小静电场环境中均可以产生阿秒脉冲。但是只有当高斯光速强度为5,静电场大小为归 一化的 E_s=5×10－4时,可以产生单个阿秒脉冲,脉冲宽度为16as。并且比无其他静电场产生脉冲的强度更 强,方向单一化。因此得出高斯强度E_s=5×10 －4的静电场与静止的单个电子作用可以得出单个阿秒脉冲辐射。     

Nd:YAG四倍频激光抽运D2和D2/He受激拉曼散射的实验研究

使用脉冲Nd:YAG四倍频激光抽运充有纯D2气体和D2/He混合气体的拉曼池.实验研究了受激拉曼散射的能量转换效率和能量稳定性与系统主要参量,包括抽运光能量、D2气体压强和加入惰性气体He的关系.实验表明,适量惰性气体He的加入在没有降低一阶斯托克斯散射光(S1,波长:289.04 nm)能量稳定性的前提下,有利于提高其能量转换效率,最大能量转换效率达到22.1%.通过实验分析,得到了受激拉曼散射一阶斯托克斯散射光的能量转化效率和能量稳定性的优化条件.     

Determination of the LO-phonon and Γ→L intervalley scattering time in GaAs from hot electron luminescence spectroscopy

Both the LO-phonon scattering time and the Γ→L intervalley scattering time for electrons in the conduction band of GaAs are of fundamental importance, and they are needed for the modelling of devices. We measure the steady state distribution of hot electrons in lightly p-doped bulk GaAs under carrier densities of 10¹³–10¹⁴cm⁻³, which is orders of magnitude lower than in pulsed laser experiments. Using a 16×16 k.p Hamiltonian and taking into account the transition matrix elements in a dipole model, we determine the hot electron lifetime from comparison with the experimentally found lifetime broadening. For electrons with a kinetic energy of 100meV to 300meV we obtain τ_LO=(132±10)fs. We also determine the Γ→L intervalley separation as E_ΓL=(300±10)meV. We find Γ→L scattering times around 150fs to 200fs, corresponding to a value for the associated deformation potential of D_ΓL=(9.4±1.5)×10⁸eV/cm.