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.     

高精度太赫兹探测器响应度定标系统

为了保障太赫兹探测器测试的准确度和可信度,对太赫兹探测器响应度定标溯源技术进行研究,针对常用太赫兹探测器黑体辐射测试技术存在对环境、设备要求高和搭建难度大等问题,以中国计量科学研究院的热电型太赫兹探测器作为计量标准,提出一种校准方案,设计搭建了一套太赫兹探测器响应度定标系统。为提高定标准确度,实验测试了定标光学系统的光束质量,并合理设置光阑孔径以满足定标要求。在0.1 THz频点处对2个响应度未知的场效应自混频太赫兹探测器进行响应度定标。结果表明,定标的相对扩展不确定度为6.80% (k=2),验证了定标系统的可行性,系统可实现太赫兹探测器功率响应度溯源到国家激光功率标准,保障太赫兹功率测量的准确可靠。     

Single quantum well transistor with modulation doped AlGaAs/GaAs/AlGaAs structures

Self-consistent calculations have been performed to obtain the wave functions and energy subbands of the two-dimensional electrons confined in a single quantum well of a Al_xGa_1?xAs/GaAs/Al_xGa_1?xAs heterostructure. The wave functions of the two-dimensional electron gas are found to be easily controlled by an external gate voltage applied between the AlGaAs-barriers, indicating a capability of fabricating a novel quantum well device, a modulation-doped single quantum well transistor.     

Study of persistent photoconductivity effect in n-type selectively doped AlGaAs/GaAs heterojunction

A detailed study of the persistent photoconductivity effect (PPE) at selectively doped Al_0.3Ga_0.7As/GaAs interface was carried out at low (4.2 and 77 K) temperatures on samples with different original channel concentrations and mobilities. The observed selectiveness of the PPE to the photon energy allowed us to identify two independent mechanisms of the PPE making almost equal contributions to the total effect. These two mechanisms are: (i) electron photoexcitation from DX centers in AlGaAs layer, (ii) electron—hole generation in bulk GaAs with a charge separation at the interface. It has been found that the behavior of the mobility as a function of the channel concentration (altered by light) depends on a setback thickness d. For a sample with small d a marked mobility drop has been found. The well-resolved structure in the dependence of the electron mobility on the channel concentration has been observed. The first peculiarity is explained by free electron population of AlGaAs layer due to the electron photorelease from DX-centers. The second feature, occurring at higher charge densities in the channel is attributed to the effect of intersubband scattering arising due to the electron occupation of an excited subband at the interface.     

Spectroscopy studies of highly acceptor doped GaAs/AlGaAs quantum wells

We present a theoretical and experimental study of optical properties on highly acceptor doped QWs. Steady state photoluminescence (PL) and PL excitation (PLE) results are compared with theoretical calculations involving exchange and correlation effects for the electron-hole system and their interaction with acceptor ions. We have studied the effects of impurity doping at levels varying from 10⁸ up to 10¹³ cm^-2. Excitons can still be detected at high hole concentrations above the degenerate limit. They survive due to the inefficiency of screening in the 2D system.     

AlGaAs/InGaAs/GaAs赝配高电子迁移晶体管的kink效应研究

利用二维器件模拟软件MEDICI对AlGaAs/InGaAs/GaAs赝配高电子迁移晶体管器(PHEMT)件进行了仿真，研究了PHEMT器件的掺杂浓度与电子浓度分布，PHEMT器件内部的电流走向及传输特性，重点研究了不同温度和不同势垒层浓度情况下PHEMT器件的kink效应.研究结果表明：kink效应主要与处于高层深能级中的陷阱俘获/反俘获过程有关，而不是只与碰撞电离有关. 关键词： 高电子迁移率晶体管 kink效应 二维电子气     

Smith Purcell effect in GaAs/AlGaAs heterostructures

We report a new experimental technique to study the form of the hot electron distribution function in GaAs/AlGaAs heterostructures. A weak periodic surface potential induces Smith-Purcell-FIR-radiation of the electric field driven hot electrons in the 2-dimensional electron gas directly reflecting their velocity distribution. The FIR- radiation is detected by a magnetic field tuned InSb-detector. In samples with very low electron concentration and high mobility the emission spectra show a significant shift to higher energies and develop a steep high energy slope with increasing electric field when we use the geometry with grating vector q directed parallel to the electric field (q ∥ E). In the geometry q ⊥ E smooth decays are observed at lower energies. Comparison of the results with theory gives experimental evidence of a non-equilibrium shape of the distribution caused by the onset of LO-Phonon emission. In addition, the hot electron mean free path of the heated distribution is derived by investigating the experimental emission spectra as a function of the grating period length. The influence of a limited hot electron mean free path on the spectral width is described in terms of a Fourier-analysis of the interaction potential. In drift direction a mean free path of λ = 200 nm is obtained, whereas the mean free path is smaller in the direction perpendicular to the drift direction.     

Correlation between Si diffusion and Si-induced disordering in AlGaAs/GaAs superlattices

Compositional disordering of AlGaAs superlattices induced by Si ion implantation and subsequent annealing has been studied by secondary ion mass spectrometry (SIMS). Distinct correlation is found between the induced disordering and the rapid Si diffusion which occurs above the critical concentration of about 3×10¹⁸ cm⁻³. The annealing-condition dependence of the disordering suggests that AlGa intermixing is induced by the vacancy flow enhanced by the Si_IIISi_V pair movement which causes the rapid Si diffusion. SIMS depth profiles of the heat treated superlattices co-doped with Si and Be do not show any appreciable Si diffusion and induced disordering. This is well-explained by the formation of SiBe pairs which prevents that of Si_IIISi_V pairs.     

n-Type GaAs/AlGaAs heterostructure detector with a 3.2 THz threshold frequency

Terahertz detection using the free-carrier absorption requires a small internal work function of the order of a few millielectron volts. A threshold frequency of 3.2 THz (93 microm or approximately 13 meV work function) is demonstrated by using a 1 x 10(18) cm(-3) Si-doped GaAs emitter and an undoped Al(0.04)Ga(0.96)As barrier structure. The peak responsivity of 6.5 A/W, detectivity of 5.5 x 10(8) Jones, and quantum efficiency of 19% were obtained at 7.1 THz under a bias field of 0.7 kV/cm at 6 K, while the detector spectral response range spans from 3.2 to 30 THz.     

Optical phonon dynamics of GaAs studied with time-resolved terahertz spectroscopy

We investigate the reflection near the reststrahlen band of the optical phonon in bulk GaAs in the time domain, using time-resolved terahertz spectroscopy. We find that the dynamics of the reflection measured for GaAs differs strongly from the reflection dynamics that would be expected for a TO phonon with a frequency-independent dephasing time.     

Proton radiation effect on GaAs/AlGaAs core–shell ensemble nanowires photo-detector

We demonstrate that the GaAs/AlGaAs nanowires(NWs) ensemble is fabricated into photo-detectors. Current–voltage(I–V) characteristics are measured on Ga As/Al Ga As core–shell ensemble NW photo-detectors at room-temperature before and after 1-MeV proton irradiation with fluences from 1.0 × 10~(13) cm~(-2) to 5.0 × 10~(14) cm~(-2). The degradation of photocurrent suggests that the point defects induced by proton radiation could cause both carrier lifetime and carrier mobility to decrease synchronously. Comparing with a GaAs quantum well, the degradations of light and dark current for the irradiated NWs photo-detector indicate that NWs material is a preferable potential candidate for space applications.     

Carrier spin polarization near the Fermi level inn-modulation doped AlGaAs/InGaAs/GaAs quantum well

We studied the carrier-spin polarization in ann-type modulation-doped single quantum well. The sample shows a high electron concentration and the second electron subband is marginally occupied. Photoluminescence and photoluminescence excitation were performed in the continuous-wave regime. The results reveal a polarization profile determined by the hole relaxation. A higher degree of polarization was observed for the luminescence related to the recombination of electrons in the second conduction subband. This suggests that the photo-created electrons localized close to the Fermi level maintain their polarization.     

Carrier transport effects on the photocurrent spectra of an intrinsic GaAs/AlGaAs multiple quantum well with highly doped AlGaAs contact layers

In this study, we demonstrate how electroreflectance (ER) measurements as a function of bias, and of angle of incidence (θ₀), together with bias dependent photocurrent (PC) measurements, can be used to provide understanding of the complex electric field profile and carrier transport effects in a GaAs/Al_0.3Ga_0.7As multiple quantum well (MQW), grown inside n⁺ contact layers. The PC measurements exhibit split excitonic features, the components of which change in strength with the applied bias. The effect is explained by absorption in the front of the MQW stack, with the back of the stack acting as detector. We examine the θ₀-dependence of the ER lineshape, to determine the depth of the layers responsible for each feature. The ER and PC lineshapes and their bias dependence are explained by the unusual electric field profile across the stack. The field profile appears to be determined by tunnelling of the dark current.     

Electron transport and terahertz radiation detection in submicrometer-sized GaAs/AlGaAs field-effect transistors with two-dimensional electron gas

The electronic transport and response in the terahertz range are studied in field-effect GaAs/AlGaAs transistors with a two-dimensional high-mobility electron gas. The special interest expressed in such transistors stems from the possibility of developing terahertz-range radiation detectors and generators on the basis of these devices. Measurements of the value and the magnetic-field dependence of the drain-source resistance are used to estimate the electron density and mobility in the transistor channel. Results of magnetotransport measurements are employed to interpret the nonresonant detection observed in transistors with a gate width from 0.8 to 2.5 μm. __________ Translated from Fizika Tverdogo Tela, Vol. 46, No. 1, 2004, pp. 146–149. Original Russian Text Copyright ? 2004 by Antonov, Gavrilenko, Demidov, Morozov, Dubinov, Lusakowski, Knap, Dyakonova, Kaminska, Piotrowska, Golaszewska, Shur.     

Optical bistability and multistability in a defect slab doped by GaAs/AlGaAs multiple quantum wells

We proposed a new model for controlling the optical bistability(OB) and optical multistability(OM) in a defect slab doped with four-level GaAs/AlGaAs multiple quantum wells with 15 periods of 17.5 nm GaAs wells and 15-nm Al_(0.3)Ga_(0.7)As barriers. The effects of biexciton energy renormalization, exciton spin relaxation, and thickness of the slab on the OB and OM properties of the defect slab were theoretically investigated. We found that the transition from OB to OM or vice versa is possible by adjusting the controllable parameters in a lab. Moreover, the transmission, reflection, and absorption properties of the weak probe light through the slab were also discussed in detail.     

Transverse charge-carrier transfer in selectively doped GaAs/AlGaAs semiconductor heterostructures with longitudinal current flow

Photoreflectance spectra of selectively doped GaAs/AlGaAs heterostructures are studied under the conditions of direct current flow along the structure layers. Using a model developed for the spectra, variations of the internal transverse electric fields are calculated for longitudinal current flow. It is proved experimentally that even a weak heating of electrons in such structures leads to a spatial redistribution of electrons in the direction transverse to the heterostructure layers.     

Free standing modulation doped core–shell GaAs/AlGaAs hetero‐nanowires

Modulation doped AlGaAs/GaAs core–shell nanowire structures were grown by molecular beam epitaxy. A Si delta‐doping was introduced in the AlGaAs shell around the {110} facets of the GaAs core. The wires are typically highly resistive at low temperatures. However, they show a pronounced persistent photoconductivity effect indicating activation of free carriers from the delta‐doped shell to the GaAs core. The n‐type character of the channel is demonstrated by applying a back‐gate voltage. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Carrier dynamics and stimulated radiative terahertz transitions between Landau levels in cascade GaAs/AlGaAs quantum well structures

The carrier distribution over Landau levels was studied in resonant tunneling GaAs/AlGaAs quantum well structures under tunneling pumping of the upper subband. The numerical calculations of the Landau level populations for various values of pumping intensity (tunneling time), magnetic field and structure doping were carried out. The population inversion between zeroth Landau level of the upper subband and the first Landau level of the lowest subband was shown to exist in wide range of the magnetic field strength. The effect of various scattering mechanisms, both two-particle (electron-electron scattering) and single-particle (acoustic phonon and interface roughness scattering) ones, on level population was studied. The way of lifting the selection rule forbidding the inter-Landau level terahertz transitions of interest and achieving considerable values of the dipole matrix element is proposed.     

AlGaAs capping effect on InAs quantum dots self-assembled on GaAs

The effects of AlGaAs capping on InAs quantum dots self-assembled on GaAs are investigated. It is observed that, the photoluminescence intensity becomes stronger up to twice when Al is incorporated into the cap layer. In the mean time, the full width at half maximum of the photoluminescence spectrum becomes narrower, the peak splitting between the ground and first excited exciton levels becomes wider, and the photoluminescence peak wavelength becomes longer. With considerations of the increased barrier height and the changed microstructures of the quantum dots induced by AlGaAs capping, the mechanisms of the observed improvements are discussed.