Ultrafast creation and annihilation of space-charge domains in a semiconductor superlattice observed by use of Terahertz fields

We report an experimental study indicating ultrafast creation and annihilation of space-charge domains in a semiconductor superlattice under the action of a THz field. Our experiment was performed for an InGaAs/InAlAs superlattice with the conduction electrons undergoing miniband transport. We applied to a superlattice a dc bias that was slightly smaller than a critical bias necessary for the formation of space-charge domains caused by a static negative differential conductivity. Additionally subjecting the superlattice to a strong THz field, resulted in a dc transport governed by the formation of domains if the frequency of the field was smaller than an upper frequency limit (~3 THz). From this frequency limit for the creation and annihilation of domains we determined the characteristic time of the domain buildup. Our analysis shows that the buildup time of domains in a wide miniband and heavily doped superlattice is limited by the relaxation time due to scattering of the miniband electrons at polar optic phonons. Our results are of importance for both an understanding of ultrafast dynamics of pattern formation in nanostructures and the development of THz electronic devices.Received: 25 March 2004, Published online: 23 July 2004PACS: 72.20.Ht High-field and nonlinear effects - 72.30. + q High-frequency effects; plasma effects - 73.21.Cd SuperlatticesK.N. Alekseev: Permanent address: Department of Physical Sciences, P.O. Box 3000, University of Oulu FIN-90014, Finland.     

Proposal of a Microwave-Driven Semiconductor Superlattice Oscillator for Generation of THz Radiation

We present the proposal of a microwave-driven semiconductor superlattice oscillator. We show that the interplay of a microwave pump field with a synchronous harmonic field can make a semiconductor superlattice to a gain medium for the harmonic field. Placing the superlattice in a resonator for the harmonic field allows the operation of an oscillator. The gain mechanism is based on Bloch oscillations of miniband electrons. The gain is mediated either by the interaction of the high-frequency field with the single electrons or with space charge domains or with both. The microwave-driven superlattice oscillator should be suitable for generation of coherent radiation up to several THz.     

Frequency multiplication of microwave radiation in a semiconductor superlattice by electrons capable to perform Bloch oscillations

We report the observation of frequency multiplication of microwave radiation in a GaAs/AlAs semiconductor superlattice at room temperature. We observed, for a fundamental frequency of 9 GHz, second and third harmonic generation. We associate the harmonic generation with a nonlinear current-voltage characteristic that is determined by Bloch oscillations of electrons propagating along the superlattice axis. Our results suggest for the frequency multiplication an upper limit in the tetrahertz frequency range.     

Rectification of terahertz radiation in semiconductor superlattices in the absence of domains

We study theoretically the dynamical rectification of a terahertz AC?electric field, i.e.?the DC?current and voltage response to the incident radiation, in strongly coupled semiconductor superlattices. We address the problem of stability against electric field domains: a spontaneous DC?voltage is known to appear exactly for parameters for which a spatially homogeneous electron distribution is unstable. We show that by applying a weak direct current bias the rectifier can be switched from a state with zero DC?voltage to one with a finite voltage in full absence of domains. The switching occurs near the conditions of dynamical symmetry breaking of an unbiased semiconductor superlattice. Therefore our scheme allows for the generation of DC?voltages that would otherwise be unreachable due to domain instabilities. Furthermore, for realistic, highly doped wide miniband superlattices at room temperature, the generated DC?field can be nearly quantized, that is, be approximately proportional to an integer multiple of ?ω/ea where a is the superlattice period and ω is the AC?field frequency.     

虚阴极振荡激励高功率微波的数值模拟

模拟了轴对称虚阴极振荡器产生微波的过程。根据实验装置的特点,建立了理想的物理模型。从物理学的基本规律出发,用CIC方法编制了二维空间、三个动量分量的2 1/2维、全电磁、相对论性的数值模拟程序。选择柱状电子束进行模拟,获得了正确的计算结果和清晰的物理图象,并对计算结果作了分析。     

Effect of inter-miniband tunneling on current resonances due to the formation of stochastic conduction networks in superlattices

We study the effects of inter-miniband electron tunneling and electric field domains on the current–voltage and conductance–voltage curves of biased semiconductor superlattices under the action of a magnetic field that is tilted relative to the plane of the layers. For this geometry, electrons in the superlattice minibands exhibit a unique type of stochastic semiclassical motion. At certain critical values of the electric field within the superlattice layers, the stochastic trajectories change abruptly from fully localized to completely unbounded, and map out an intricate web-like mesh of conduction channels in phase space. Delocalization of the electron paths produces a series of strong resonant peaks in the electron drift velocity versus electric field curves. We use these drift velocity characteristics to make self-consistent drift-diffusion calculations of the current–voltage and differential conductance–voltage curves of the superlattices, which reveal strong resonant features originating from the sudden delocalization of the stochastic single-electron paths. We show that this delocalization has a pronounced effect on the distribution of space charge and electric field domains within the superlattices. Inter-miniband tunneling greatly reduces the amount of space-charge buildup, thus enhancing the domain structure and both the strength and number of the current resonances.     

超晶格中电场单极畴与偶极畴的形成和输运

采用分立漂移模型,数值模拟和分析直流偏压下超晶格中单极电场畴和偶极电场畴的形成和输运.随着超晶格两端接触层掺杂浓度和阱中掺杂浓度的不同,可以得到单极畴、偶极畴以及单极畴与偶极畴共存 关键词： 超晶格 单极畴 偶极畴 电流自激振荡     

Microwave generation by a self-sustained current oscillation in an InGaAs/InAlAs superlattice

We observed a self-sustained current oscillation in an InGaAs/InAlAs superlattice at room temperature. The oscillation gave rise to microwave generation at a fundamental frequency around 2 GHz and higher harmonics. The power at the first harmonic was 0.1 mW, corresponding to an efficiency (i.e. ratio of microwave power to dc power) of 3.5%. We attribute the current oscillation to travelling dipole domains.     

低引导磁场下相对论亚纳秒电子束毫米波返波振荡器的粒子模拟

提出了一种Ka波段的相对论亚纳秒电子束毫米波慢波结构,在较低引导磁场情况下,运用粒子模拟(PIC)方法成功地模拟出器件中波束互作用的非线性物理演化过程,得到了一种超辐射状态下的微波辐射,它的产生与波相对于电子束滑移引起的电子束内电子间相互作用有关,辐射微波的峰值功率与电子束总电荷的平方成正比.粒子模拟有利于对超辐射这种束波互作用非线性物理现象的理解,并且对器件的设计有一定的参考价值.     

Bifurcation and stability of dynamical structures at a current instability

We investigate bifurcation and stability of nonuniform current states at a voltage-controlled current instability. We consider a model which exhibits bulk negative differential conductivity due to Bragg scattering of hot electrons. The system is described by balance equations for momentum and energy densities of the carriers. These transport fields are coupled to Maxwell's equations. The uniform stationary current state is unstable against long-wavelength dielectric relaxation modes at a critical field. We find that the softening of these modes gives rise to a family of periodic travelling waves and to a solitary solution (dipole domain). We show that the periodic travelling waves are unstable, wheras the dipole domain can be stabilized by coupling the sample to a suitable external circuit, if the static impedance of the sample in the domain state is negative. The model describes therefore a discontinuous nonequilibrium transition to a large amplitude domain state.Work Supported by the Swiss National Science Foundation     

Dynamic chaotization of the electronic subsystem in graphene superlattice

d'Alembert equation written for the electromagnetic waves propagating in the graphene superlattice is discussed. The chaotic behavior of the electrons in graphene superlattice is studied by the Melnikov method. Dynamic chaos of electron in graphene superlattice is shown to appear for certain intervals of amplitudes of preset alternate current. The frequency dependence of the critical amplitude of alternate current above which would be a chaos is investigated.     

Transition to microwave generation in semiconductor superlattice

We investigate excitation of microwave generation in a semiconductor superlattice under the effect of the applied constant voltage at near-zero temperature in the absence of the external magnetic field. It is shown that the generation is caused by the positive feedback arising from the total constant voltage drop across the superlattice.     

Plasma-Resonance Effects in Nonlinear Reflection of an Electromagnetic Wave from a Semiconductor Superlattice

We show that dynamical localization of electrons in a thin semiconductor superlattice irradiated by a TM electromagnetic wave can lead to the formation of multistable states of the high-frequency field, which is accompanied by significant distortions of the reflection characteristics of superlattices. This effect is caused by the phenomenon of nonlinear plasma resonance and the related significant increase in the component of an alternating electric field, parallel to the superlattice axis, in the case where the real part of the corresponding component of the tensor of nonlinear dielectric permittivity becomes zero. It is found that the generation efficiency of odd harmonics of the radiation increases considerably under resonance conditions.     

Terahertz radiation induced by the Wannier-Stark localization of electrons in a natural silicon carbide superlattice

Intense terahertz electroluminescence from SiC structures with a miniband electron spectrum caused by the natural superlattice has been observed. The shape of the terahertz radiation line, the linear dependence of the position of its maximum on the bias voltage, the typical value of the field required to induce the radiation, and the prevailing polarization of the radiation along the superlattice axis indicate that the observed radiation results from to the excitation of stationary Bloch oscillations of electrons in the natural silicon carbide superlattice.     

Microwave oscillator based on Bloch oscillations of electrons in a semiconductor superlattice

We report on a microwave oscillator based on Bloch oscillations of electrons in a semiconductor superlattice. Our GaAs/AlAs superlattice, at room temperature, was coupled electromagnetically by an antenna to a rectangular cavity resonator, and was operated at a current-voltage state of negative differential conductance. We observed generation of microwave radiation at frequencies, depending on the resonator length, between 7 and 30 GHz. Electronic tuning by several percent was possible; the ratio of linewidth to frequency was of the order of 10^?4. A radiation power up to 1 μW (at 10 GHz) was obtained, corresponding to a generator efficiency of the order of 10^?3 for the conversion of electrical power to microwave radiation.     

Loss and gain in Bloch oscillating super-superlattices: THz Stark ladder spectroscopy

Bloch oscillation in electrically biased semiconductor superlattices offer broadband terahertz gain from DC up to the Bloch frequency or Stark splitting. Useful gain up to 2–3 THz can provide a basis for solid-state electronic oscillators operating at 10 times the frequency of existing devices.A major stumbling block is the inherent instability of the electrically biased doped superlattices to the formation of static or dynamic electric field domains. To circumvent this, we have fabricated super-superlattices in which a large superlattice is punctuated with heavily doped regions. The short superlattice sections have subcritical “nL” products.Room temperature, terahertz photon-assisted transport in short InGaAs/InAlAs superlattice cells allows us to determine the Stark ladder splitting as the superlattice is electrically biased and confirms the absence of electric field domains in short structures.Absorption of radiation from 1.5 to 2.5 THz by electrically biased InAs/AlSb super-superlattices exhibit a crossover from loss to gain as the Stark ladder is opened. Measurements are carried out at room temperature in a novel planar terahertz waveguide defined by photonic band gap sidewalls and loaded with an array of electrically biased super-superlattices. The frequency-dependent crossover voltage indicates 80% participation of the super-superlattice.     

脉冲等离子体辐射微波机理的初步研究

 在分析了等离子体辐射微波的电流波形和喇叭天线接收到的微波信号波形的基础上,对脉冲等离子体辐射微波的机理进行了理论分析：阴阳极间存在TEM和TM两种模式的波;流经等离子体的电流产生的磁场使等离子体中的波成为慢波;高频场的激励因素有两个,一是作加速运动的电子,二是阳极斑点溅射时阳极电位突降;同一时刻具有不同速度的电子将能量转换给相应相速的波是其产生超宽带辐射的原因。     

Double-resonance Semiconductor Superlattice Parametric Oscillator for Generation of Subterahertz Radiation

We report on a subterahertz superlattice parametric oscillator that operated simultaneously at two different harmonic frequencies of a microwave pump field. A pump field (frequency near 100 GHz) was coupled to a GaAs/AlAs superlattice in a resonator for the third and the fifth harmonic. The pump field produced a third harmonic field and this together with the pump field created a fifth harmonic field. A theoretical analysis indicates that the nonlinearity, which is based on the dynamics of miniband electrons, should allow for the upconversion of pump radiation of higher frequency into the terahertz frequency range.      

Numerical simulations of the reditron

The reflected-electrons discrimination microwave generator (reditron) is a high-power, narrow-band, and single-mode microwave generator that makes exclusive use of the oscillatory character of the virtual-cathode of a relativistic electron beam. The complex, nonlinear character of the virtual-cathode device necessitates particle-in-cell plasma simulation techniques. Investigations indicate two sources of the radiation: (1) the trapped electrons reflexing between the real and virtual cathodes, and (2) the oscillation of the virtual cathode. In the conventional design, the two mechanisms coexist and interfere with each other destructively, causing degradation of the efficiency of microwave generation. The authors have investigated a configuration with a slotted, thick anode and an external magnetic field, which effectively eliminates the reflexing electrons. Two-dimensional particle-in-cell simulations showed that such a configuration exploits the oscillation of the virtual cathode exclusively, and it generates single-mode, narrow bandwidth, and high-power microwave radiation with a potential efficiency over 10%. It was found that further optimization could be achieved by the use of a density (current) modulated electron beam at appropriate frequencies