Theoretical studies of polarization dependent electro-optical modulation in lattice matched and strained multi-quantum well structures

We report on polarization dependent optical absorption for excitonic and interband transitions in lattice matched (GaAs/AlGaAs) and strained (biaxial tensile strain - GaAsP/AlGaAs; biaxial compressive strain - InGaAs/AlGaAs) multiquantum well structures in the presence of transverse electric fields. The hole states are solved by using the Kohn-Luttinger Hamiltonian and using an eigenvalue technique. The effect of heavy-hole and light-hole mixing due to the strain, electric field and quantization is studied. Under biaxial tensile strain the heavy-hole and light-hole transition can coincide, leading to interesting polarization dependent effects. Results are presented for excitonic and interband transitions.     

Efficacy of proposed 2DEG-based photoconductive antenna using magnetic bias-controlled carrier transport

An externally applied magnetic field was used to induce increased photocarrier transport along the high mobility channel in GaAs/AlGaAs modulation-doped heterostructures (MDH). The terahertz (THz) emission from GaAs/AlGaAs MDH increases with increasing magnetic field, applied parallel to the heterojunction. The THz emission enhancement factors due to the magnetic field in MDH are higher than in undoped GaAs/AlGaAs heterojunction and in bulk SI-GaAs. This demonstrates that properly utilizing the high-mobility channel for carrier transport promises to be a viable design consideration for efficient THz photoconductive antenna (PCA) devices. Moreover, it was observed that for MDH, as well as for an undoped GaAs/AlGaAs heterojunction, the enhancement for one magnetic field direction is greater than the enhancement for the opposite direction. This is in contrast to the symmetric enhancement with magnetic field direction observed in a bulk SI-GaAs. An analysis of photocarrier trajectories under an external magnetic field supports the explanation that the enhancement asymmetry with magnetic field direction in MDH is due to the cycloid motion of electrons as affected by the GaAs/AlGaAs interface.     

Photoreflectance study of InAs ultrathin layer embedded in Si-delta-doped GaAs/AlGaAs quantum wells

Photoreflectance and photoluminescence studies were performed to characterize InAs ultrathin layer embedded in Si-delta-doped GaAs/AlGaAs high electron mobility transistors. These structures were grown by Molecular Beam Epitaxy on (1 0 0) oriented GaAs substrates with different silicon-delta-doped layer densities. Interband energy transitions in the InAs ultrathin layer quantum well were observed below the GaAs band gap in the photoreflectance spectra, and assigned to electron-heavy-hole (E_e-hh) and electron-light-hole (E_e-lh) fundamental transitions. These transitions were shifted to lower energy with increasing silicon-δ-doping density. This effect is in good agreement with our theoretical results based on a self-consistent solution of the coupled Schrödinger and Poisson equations and was explained by increased escape of photogenerated carriers and enhanced Quantum Confined Stark Effect in the Si-delta-doped InAs/GaAs QW. In the photoreflectance spectra, not only the channel well interband energy transitions were observed, but also features associated with the GaAs and AlGaAs bulk layers located at about 1.427 and 1.8 eV, respectively. By analyzing the Franz-Keldysh Oscillations observed in the spectral characteristics of Si-δ-doped samples, we have determined the internal electric field introduced by ionized Si-δ-doped centers. We have observed an increase in the electric field in the InAs ultrathin layer with increasing silicon content. The results are explained in terms of doping dependent ionized impurities densities and surface charges.     

Field-induced lifetime enhancement and ionization of excitons in GaAs/AlGaAs quantum wells

Picosecond and nanosecond luminescence studies of GaAs/AlGaAs quantum wells exposed to electric fields perpendicular to the layers are reported. The drastic red-shift of the photoluminescence with the electric field strength is accompanied by a strong increase of the electron-hole recombination lifetime. These features are most dominant for the wider wells due to the stronger polarization of the confined electron-hole pairs. Our observations are consistent with the model of the Quantum-Confined Stark Effect. In contrast, for high fields in the regime of ∼100 kV/cm field ionization limits the confinement of electrons and holes and leads to a strong decrease of the photoluminescence yield and lifetime with increasing field strength.     

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.     

Broad-band microwave detection with a novel 2D hot-electron device

We present the operation of a new AlGaAs–GaAs multiquantum well hot-electron microwave detector. The working principle of this device is based on the interaction of two-dimensional free carriers inside the wells with the in-plane electric field.We shall report room-temperature responsivity of several 10³V W^− 1, comparable to that of conventional solid-state devices. The different physical principle of operation, however, yields for the present detector a broader frequency range, extending up to the submillimetre band, and short response times which can be estimated around 10 ps. Finally, we report a characterization of the device from the point of view of noise.     

High temperature gate control of quantum well spin memory

Time-resolved optical measurements in (110)-oriented GaAs/AlGaAs quantum wells show a tenfold increase of the spin-relaxation rate as a function of applied electric field from 20 to 80 kV cm(-1) at 170 K and indicate a similar variation at 300 K, in agreement with calculations based on the Rashba effect. Spin relaxation is almost field independent below 20 kV cm(-1) reflecting quantum well interface asymmetry. The results indicate the achievability of a voltage-gateable spin-memory time longer than 3 ns simultaneously with a high electron mobility.     

Preparation of curved two-dimensional electron systems in InGaAs/GaAs-microtubes

We present a preparation method to realise transport measurements on evenly curved two-dimensional electron systems (2DESs). By combining the method of self-rolling strained double layers with a special lithographic procedure we are able to roll-in and contact AlGaAs/GaAs/AlGaAs quantum well structures into tubes or curved lamellas. Applying a magnetic field to such structures results in a strong modulation with changing sign of the magnetic field components perpendicular to the curved 2DES plane. Our preparation method allows transport measurements along or perpendicular to this modulation. We present and discuss our first magneto-transport measurements on such rolled 2DESs.     

Origin of current instability in GaAs/AlGaAs heterostructures

We propose a mechanism to explain the electric instability often observed in modulation-doped heterostructures GaAs/AlGaAs when current is passed along the heterostructure layers. The instability is caused by hot electron transport in AlGaAs layer that is not only heavily doped, but also strongly compensated due to the presence of DX-centers. This layer contains a large-scale random potential of significant magnitude, which strongly affects electron transport. The heating of electrons in the percolation cluster net and electron transfer from the cluster into the random potential wells result in the appearance of latent negative differential conductivity causing the current instability. When the instability gives rise to the formation of a high electric field domain, one of the domain walls blocks the current flow through the two-dimensional electron gas. Experimental results supporting this mechanism are given.     

Optically biased photoreflectance spectroscopy of a GaAs epitaxial layer and an AlGaAs/GaAs quantum well

We propose optically biased photoreflectance (OBPR) spectroscopy, which is performed by continuous illumination of a secondary monochromatic light on a sample with conventional photoreflectance (PR), as a useful tool to investigate the internal electric fields dependence of the PR signals associated with band to band and quantum level transitions. Line shape of the PR signal has a strong dependence on the internal electric field. In OBPR, if a secondary incident light is absorbed, the internal electric field is suppressed by the photo-generated electron–hole pairs. On the other hand, if the secondary light is not absorbed, the internal electric field is not affected. Through the OBPR investigation of a GaAs epitaxial layer and an AlGaAs/GaAs quantum well, we are able to obtain an absorption like spectrum by performing a wavelength scan of a secondary monochromatic light. The results of OBPR measurements at each PR peak position show the contribution of the electric fields modification by the photo-generated carriers in each layers to the PR signals that are related to band gap and quantum level transitions.     

半导体异质结构中的谷间电子转移效应

本文使用单带双谷理论研究了GaAs/AlGaAs异质结构中的谷间电子转移效应。计入每一异质结界面上的能带交错,谷间耦合和电场的贡献,导出了计算异质结构隧穿概率和隧道电流的公式。以GaAs/AlGaAs异质结构为例,算出不同结构、不同合金组分比及不同电压下的隧穿概率和隧道电流。讨论了异质结界面、势阱和势垒材料的能带结构以及外加电压对谷间电子转移效应的影响。算得的隧道电流同实验结果相符合,证实了这一理论在研究多能谷系统中的适用性。在此基础上进一步分析了这一谷间电子转移效应与熟知的Gunn效应间的区别,并讨论了它在半导体器件设计中的应用。 关键词：     

Electric-field dependence of the intersubband optical absorption in a semiconductor quantum well

We present theoretical results of intersubband linear optical absorption in the conduction band of a GaAsAlGaAs quantum well with an applied electric field taking into account the field dependent linewidth. Our analysis is based on the one electron density matrix formulation with intrasubband relaxation processes due to polar optical phonon scattering and tunneling of electrons. We show that (a) for an increasing electric field the absorption peak corresponding to the transition of states 1 → 2 is shifted higher in energy and (b) the peak amplitude increases if the Fermi level is fixed and decreases if the electron density in the well is fixed when an increasing electric field is applied. The linewidth broadening also reduces the peak absorption amplitude.     

Full electrical control of the electron spin relaxation in GaAs quantum wells

The electron spin dynamics in (111)-oriented GaAs/AlGaAs quantum wells is studied by time-resolved photoluminescence spectroscopy. By applying an external electric field of 50 kV/cm a two-order of magnitude increase of the spin relaxation time can be observed reaching values larger than 30 ns; this is a consequence of the electric field tuning of the spin-orbit conduction band splitting which can almost vanish when the Rashba term compensates exactly the Dresselhaus one. The measurements under a transverse magnetic field demonstrate that the electron spin relaxation time for the three space directions can be tuned simultaneously with the applied electric field.     

The role of the electron-hole coulomb interaction in quantum confined stark effect

The luminescence peak energy and tunneling lifetime of an exciton in a semiconductor quantum well with a small valence band offset in the presence of a perpendicular electric field is calculated by generalizing the variational approach of quantum confined Stark effect normally used for systems of GaAs/AlGaAs quantum wells. At a finite electric field, the electron-hole Coulomb interaction provides additional confinement to each of the carriers and significantly enhances the Stark shift and the exciton lifetime against field ionization. Numerical results are presented for ZnSe/Zn_1−xMn_xSe heterostructures studies in recent experiments.     

Picosecond dynamics of hot carrier relaxation in highly excited multi-quantum well structures

Dynamics of hot carrier relaxation, exciton screening and subband level renormalization in GaAsAlGaAs multiple quantum well structures are investigated by time resolved measurements of optical absorption and gain following subpicosecond optical excitation. The cooling rate of the electron hole plasma in these two dimensional structures is approximately the same as for bulk GaAs at comparable photoexcitation density.     

The phonon-assisted photon-drag effect in a two-dimensional semiconductor quantum-well structure

The phonon-assisted photon-drag effect in a two-dimensional semiconductor quantum well structure is investigated. By making use of second-order perturbation theory and the classical Boltzmann equation we found, by considering both intrasubband and the intersubband electronic transitions, that the electric field generated by the photon-drag effect in a typical GaAs–AlGaAs two-dimensional system is enhanced by almost one order of magnitude as compared with that of a bulk system. Moreover, the results can qualitatively account for the observed absorption spectra due to intersubband electronic transitions occurring in GaAs quantum wells.     

Barrier lowering effect and dark current characteristics in asymmetric GaAs/AlGaAs multi quantum well structure

In this study, we investigate dark current voltage characteristics of GaAs/AlGaAs staircase-like asymmetric multiquantum well structure at various temperatures experimentally. The activation energy is calculated by using Arrhenius plots at different voltages. It is found that the activation energy decreased with increasing electric field. This result is evaluated using a barrier lowering effect which is a combination of geometrical and Poole–Frenkel effects. Measured dark current density–voltage (J–V) characteristics compared with the Levine model, 3D carrier drift model and the emission capture model. The best agreement with the experimental results of dark current densities is obtained by the Levine model.     

The eigenstates and tunnelling time in an interdiffused AlGaAs/GaAs quantum well by the transfer matrix method

A matrix method has been used for the determination of the confined eigenstates and their corresponding tunnelling times in an interdiffused AlGaAs/GaAs single quantum well structure. The nonlinear confinement potential profile of the disordered quantum well is due to the interdiffusion of the constituent Al and Ga atoms and is modeled here by an error function. The results indicate that the interdiffusion, as well as the applied electric field, affect the tunnelling times and the energy linewidths of the corresponding states. The results are compared with those for an AlGaAs/GaAs as-grown single square quantum well without any interdiffusion.     

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.     

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