TERAHERTZ SPECTRA EMITTED FROM HOT CARRIERS IN TWO-DIMENSIONAL SEMICONDUCTORS

We propose a simple method to calculate the spectrum of spontaneous terahertz (THz) emission from hot carriers in two-dimensional semiconductors by means of transport quantities, which are easily obtained during the process of solving the transport problem using the recently developed balance-equation approach for THz-driven transport. The method has been applied to examine the surface emission from a GaAs/AlGaAs heterojunction and the edge emission from a GaAs-based multiple quantum-well system subjected to strong dc biases. The theoretical results obtained are in reasonably good agreement with measurements.     

Coherent scattering in a small quantum dot

Ballistic transport in an open small (100 nm) three-terminal quantum dot has been analyzed. The dot is based on the high-mobility 2D electron gas of the AlGaAs/GaAs heterojunction. It has been shown that the gate oscillations of the resistance of such a dot arise due to the coherent scattering of electrons on its quasidiscrete levels and these oscillations are suppressed by a weak magnetic field.     

交叉电场和磁场下GaAs/AIGaAs超晶格子带结构

本文用有效质量理论计算了加平行磁场(方向平行于GaAs/AlGaAs界面)和垂直电场(方向垂直于界面)的超晶格子带结构和光跃迁。加平行磁场后,空穴子带的二重简并解除,轻重空穴混合。加电场后,产生Stark位移,电子和空穴能级发生一定位移。最后,讨论了磁光跃迁概率。     

Terahertz conductivity and possible BLOCH gain in semiconductor superlattices

We have investigated terahertz emission due to dynamical electron transport in wide-miniband GaAs/Al(0.3)Ga0.7As superlattices. By noting that the time-domain THz emission spectroscopy inherently measures the step-response of the electron system to the bias electric field, the obtained THz spectra were compared with the high-frequency conductivities predicted for miniband transport. Excellent agreement between theory and experiment strongly supports that the THz gain due to Bloch oscillating electrons persists at least up to 1.7 THz. It was also found that Zener tunneling into the second miniband sets the high-frequency limit to the THz gain for the samples studied here.     

光电导开关产生太赫兹电磁波双极特性分析

利用光电导体产生太赫兹电磁波(THz波)，THz远场辐射波形与光电导体材料的载流子寿命、偏置电场以及触发光有直接关系.用不同方法对低温GaAs(LT-GaAs)和半绝缘GaAs(SI-GaAs)光电导开关辐射的THz电磁波所呈现的双极特性进行了模拟计算.结果表明，LT-GaAs光电导开关辐射THz波呈现双极性的主要原因是光生载流子寿命小于一个THz波产生时间；而光生载流子寿命大于100ps的SI-GaAs光电导开关，在不同的实验条件（不同偏置电场、不同光脉冲能量）下，产生的THz波呈现双极特性的主要原因分别是载流子发生了谷间散射和空间电荷电场屏蔽. 关键词： 光电导开关 THz电磁波 载流子寿命 空间电荷屏蔽     

Resistivity of dilute 2D electrons in an undoped GaAs heterostructure

We report resistivity measurements from 0.03 to 10 K in a dilute high mobility 2D electron system. Using an undoped GaAs/AlGaAs heterojunction in a gated field-effect transistor geometry, a wide range of densities, 0.16 x 10(10) to 7.5 x 10(10) cm(-2), are explored. For high densities, the results are quantitatively shown to be due to scattering by acoustic phonons and impurities. In an intermediate range of densities, a peak in the resistivity is observed for temperatures below 1 K. This nonmonotonic resistivity can be understood by considering the known scattering mechanisms of phonons, bulk, and interface ionized impurities. Still lower densities appear insulating to the lowest temperature measured.     

Magnetotransport characterization of THz detectors based on plasma oscillations in submicron field-effect transistors

Magnetotransport characterization of field-effect transistors in view of their application as resonant detectors of THz radiation is presented. Three groups of different transistors based on GaAs/GaAlAs or GaInAs/AlGaAs heterostructures are investigated at liquid-helium temperatures and for magnetic fields of up to 14 T. The magnetic-field dependence of the transistor resistance is used for evaluation of the electron density and mobility in the transistor channel. The electron mobility and concentration determined from magnetotransport measurements are used for the interpretation of recently observed resonant detection of terahertz radiation in 0.15 μm gate length GaAs transistors and for the determination of the parameters of other field-effect transistors processed for resonant and voltage tunable detection of THz radiation. From Fizika Tverdogo Tela, Vol. 46, No. 1, 2004, pp. 138–145. Original English Text Copyright ? 2004 by Lusakowski, Knap, Dyakonova, Kaminska, Piotrowska, Golaszewska, Shur, Smirnov, Gavrilenko, Antonov, Morozov. This article was submitted by the authors in English.     

Resonant tunneling transport through GaAs/AlGaAs superlattices in strong tilted magnetic field

An analysis is presented of the transverse resonant tunneling transport through GaAs/AlGaAs superlattices due to tunneling between Landau levels in quantum wells in a strong tilted magnetic field. A high tunneling rate is demonstrated between Landau levels with Δn ≠ 0 in a magnetic field with a nonzero in-plane component. This leads to substantial broadening and shift of the tunneling resonance and significant changes in the current-voltage characteristics of superlattices. The predicted behavior of the current-voltage characteristics of superlattices in tilted magnetic fields is demonstrated experimentally.     

GaAs/AlGaAs异质结的界面束缚二维激子效应

本文讨论了GaAs/AlGaAs异质结界面的H线发光及其性能。用双晶X射线衍射及皮秒光致发光证明了H线与界面质量密切相关,并且有相似于激子跃迁的寿命行为。用限制于异质结界面势阱的二维电子(或空穴)与分布于GaAs边的三维空穴(或电子)组成的二维激子效应,解释了H线的实验结果。并讨论了不同外延生长的异质结与界面有关的发光行为。     

Application of superlattice gate and modulation-doped buffer for GaAs power MESFET grown by MBE

A new power GaAs MESFET (SGMBT), using the undoped superlattice gate and modulation-doped (MD) buffer, has been fabricated successfully by MBE. A much higher gate-drain breakdown voltage (30 V) and lower gate reverse leakage current have been obtained due to the existence of the undoped AlGaAs/GaAs superlattice gate insulator. The use of MD buffer structure introduces a high output resistance and low trap concentration at AlGaAs/GaAs interface. The degradation region at channel-buffer interface is estimated to be smaller than 40 Å. Thus the sharpness and smoothness between active channel and buffer is truly improved by the insertion of MD structure. The maximum output saturation current and output power of SGMBT are 300 mA/mm and 0.67 W/mm, respectively. By optimizing the device geometry and gate dimension, the output performance of SGMBT can be improved further.     

Electrical spin transport in a GaAs (110) channel

A homoepitaxial GaAs (110) channel gives a great interest in the field of semiconductor spintronics due to the longer spin diffusion. By utilizing optimal temperature process and V/III flux ratio control, the GaAs layer is grown without a serious defect. In a ferromagnet/semiconductor hybrid device, Tb₂₀Fe₆₂Co₁₈/Ru/Co₄₀Fe₄₀B₂₀ films are deposited on the GaAs (110) channel as a spin source to investigate the spin transport in (110)-oriented channel. To measure the Hanle signal, an in-plane magnetic field is applied to the perpendicularly polarized spins which are injected from the Tb₂₀Fe₆₂Co₁₈ layer. From the experimental results, the spin diffusion length in a GaAs (110) is longer than that in a GaAs (100) by up to 25%. The proper selection of crystalline growth direction for the spin transport channel is a viable solution for an efficient spin transport.     

Capacitance of a double-heterojunction GaAs/AlGaAs structure subjected to in-plane magnetic fields: results of self-consistent calculations

The capacitance of a double-heterojunction structure with a wide GaAs undoped layer embedded between two selectively doped AlGaAs barriers is calculated self-consistently as a function of intensity of the in-plane magnetic field. With increasing field intensity the capacitance initially increases and after reaching a maximum decreases toward a high field limit which is less than its zero field value. This behaviour is attributed to 'breathing', or charge redistribution, of the 2D electron gas at individual heterojunctions due to a combination of the confining potential and the magnetic field.     

Mean free path of ballistic electrons in GaAs/AlGaAs superlattices

The mean free path of ballistic electrons in GaAs/AlGaAs superlattices was measured using the technique of hot electron spectroscopy in magnetic fields perpendicular to the growth direction. We utilize the fact that the total effective path of an injected hot electron is a function of the applied magnetic field. For a superlattice with 6.5 nm GaAs wells and 2.5 nm GaAlAs barriers we measure a mean free path of 80 nm. The experimental results of a ten-period SL sample are compared to a fully three-dimensional calculation of the transmission including interface roughness with island sizes of 10 nm. We demonstrate that the observed mfp is limited due to interface roughness scattering for temperatures up to 50 K.     

The electron ionization rate in multilayered semiconductor structures

We present theoretical results of the electron impact ionization rate in GaAs/AlGaAs multiquantum well structures as a function of applied electric field for various geometries, i.e., well and barrier widths. In addition, we present preliminary measurements of the current-voltage characteristics of MBE grown devices which demonstrate very low leakage current as well as sharp breakdwon behavior. It is found that the net ionization rate, determined by averaging over the constitutent GaAs and AlGaAs layers, approaches the weighted average of the constituent bulk rates at high electric field strengths; the potential discontinuity is relatively unimportant. The electron ionization rate within the well regions alone is still higher than that in bulk GaAs, but is insufficiently enhanced to compensate for the much lower rate in the AlGaAs layers. As the field is lowered to 250.0 kV/cm, the average ionization rate in the multiquantum well structure becomes larger than in the bulk.     

Ultrafast opto-terahertz photonic crystal modulator

We present an agile optically controlled switch or modulator of terahertz (THz) radiation. The element is based on a one-dimensional photonic crystal with a GaAs wafer inserted in the middle as a defect layer. The THz electric field is enhanced in the photonic structure at the surfaces of the GaAs wafer. Excitation of the front GaAs surface by ultrashort 810 nm laser pulses then leads to an efficient modulation of the THz beam even at low photocarrier concentrations (approximately 10(16) cm(-3)). The response time of the element to pulsed photoexcitation is about 130 ps.     

Optical-Electrical Characteristics and Carrier Dynamics of Semi-Insulation GaAs by Terahertz Spectroscopic Technique

GaAs has been widely used to fabricate a variety of optoelectronic devices by virtue of its superior performance,and it is very important to accurately measure its electrical and optical properties.In this study,a semiinsulation(SI) GaAs wafer is investigated by the terahertz(THz) non-destructive testing technology.Using an air biased coherent generation and detection THz time domain spectroscopy system,the THz time domain waveform and spectrum of SI-GaAs are obtained by the time domain spectroscopy module,and its opticalelectrical characteristics including complex refractive index,permittivity and dielectric loss angle are calculated.Its carrier lifetime is measured by the optical-pump THz-probe module,and the THz pulse induced intervalley scattering in photo-excited SI-GaAs is discussed.     

Coupling of exciton states in superlattice and bulk GaAs optoelectronic modulators,studied by density matrix models and optical modulation spectroscopy

The dielectric function of GaAs/AlGaAs superlattices and bulk GaAs material are studied with density matrix models and modulation spectroscopy on optoelectronic modulator devices, showing the Coulomb enhanced Wannier–Stark effect and the Coulomb enhanced Franz–Keldysh effect, respectively. The models include the Coulomb interaction for the coupled excitonic states and an applied electric field. Transfer matrix models are used to calculate the reflection or transmission spectra of the devices. The dielectric function is modulated by the electric field over the active layer, by optical pumping of the p–i–n–i structure of the superlattice device and by electrical contacts for the bulk GaAs modulator device. In both material structures the exciton dissociates or polarizes due to the applied electric field, modulating the dielectric function and reflected or transmitted light.     

Intense terahertz emission from undoped GaAs/n-type GaAs and?InAs/AlSb structures grown on Si substrates in the transmission-geometry excitation

Intense terahertz radiation was generated from femtosecond laser-irradiated InAs and GaAs layers on Si substrates. Results show that InAs/Si and GaAs/Si films can be excited in reflection and transmission geometries. The InAs/Si film exhibited weaker emission for both excitation cases but it will be more feasible as a spectroscopic THz source due to the absence of complex spectral features in its emission spectrum. The GaAs/Si emission is characterized by Fabry?CPerot oscillations but it is 90% of that of p-InAs bulk crystal emission intensity in the reflection geometry. Excitation fluence measurements showed that the InAs/Si film saturates easily due to the laser??s shallow penetration depth in InAs.     

Absorption and emission of terahertz radiation in doped GaAs/AlGaAs quantum wells

Spontaneous emission of terahertz radiation from structures with GaAs/AlGaAs quantum wells in a longitudinal magnetic field has been studied. It is shown that some bands in the emission spectrum can be related to radiative electron transitions between resonant and localized impurity states, as well as to the transitions with participation of subband states. The temperature dependence of the equilibrium intraband absorption of terahertz radiation and its modulation in a longitudinal electric field in GaAs/AlGaAs quantum wells has been investigated.