Self-consistent approach for calculations of exciton binding energy in quantum wells

We introduce a computationally efficient approach to calculating characteristics of excitons in quantum wells. In this approach we derive a system of self-consistent equations describing the motion of an electron–hole pair. The motion in the growth direction of the quantum well in this approach is separated from the in-plane motion, but each of them occurs in modified potentials found self-consistently. The approach is applied to shallow quantum wells, for which we obtained an analytical expression for the exciton binding energy and the ground state eigenfunction. Our numerical results yield lower exciton binding energies in comparison to standard variational calculations, while require reduced computational effort.     

载流子复合对时间分辨法拉第旋转光谱的影响

利用二能级体系速率方程,推导了半导体中探测光探测到的法拉第旋转光谱的理论模型,发现电子-空穴对的复合对法拉第旋转信号随时间的衰减有重要影响,并利用该模型对GaAs量子阱中实验测得的法拉第旋转光谱进行拟合,得到GaAs量子阱材料中的电子自旋弛豫时间为73.5 ps,而直接利用单指数进行拟合得到的电子自旋弛豫时间仅为51.3 ps. 因此,直接利用单指数对法拉第旋转光谱进行拟合得到电子自旋弛豫时间的传统做法是不准确的. 关键词： 自旋弛豫时间 时间分辨法拉第旋转光谱 GaAs量子阱     

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.     

Generation of infrared entangled light in asymmetric semiconductor quantum wells

We proposed a scheme to achieve two-mode CV entanglement with the frequencies of entangled modes in the infrared range in an asymmetric semiconductor double-quantum-wells (DQW), where the required quantum coherence is obtained by inducing the corresponding intersubband transitions (ISBTs) with a classical field. By numerically simulating the dynamics of system, we show that the entanglement period can be prolonged via enhancing the intensity of classical field, and the generation of entanglement doesn't depend intensively on the initial condition of system in our scheme. Moreover, we also show that a bipartite entanglement amplifier can be realized in our scheme. The present research provides an efficient approach to achieve infrared entangled light in the semiconductor nanostructure, which may have significant impact on the progress of solid-state quantum information theory.     

Effects of InGaN barriers with low indium content on internal quantum efficiency of blue InGaN multiple quantum wells

Blue In_0.2Ga_0.8N multiple quantum wells (MQWs) with In_xGa_{1 - x}N (x=0.01-0.04) barriers are grown by metal organic vapour phase epitaxy. The internal quantum efficiencies (IQEs) of these MQWs are studied in a way of temperature-dependent photoluminescence spectra. Furthermore, a 2-channel Arrhenius model is used to analyse the nonradiative recombination centres (NRCs). It is found that by adopting the InGaN barrier beneath the lowest well, it is possible to reduce the strain hence the NRCs in InGaN MQWs. By optimizing the thickness and the indium content of the InGaN barriers, the IQEs of InGaN/InGaN MQWs can be increased by about 2.5 times compared with conventional InGaN/GaN MQWs. On the other hand, the incorporation of indium atoms into the intermediate barriers between adjacent wells does not improve IQE obviously. In addition, the indium content of the intermediate barriers should match with that of the lowest barrier to avoid relaxation.     

Spin-polarization dependent carrier recombination dynamics and spin relaxation mechanism in asymmetrically doped (110) n-GaAs quantum wells

Carrier recombination and electron spin relaxation dynamics in asymmetric n-doped (110) GaAs/AlGaAs quantum wells are investigated with time-resolved pump-probe spectroscopy. The experiment results reveal that the measured carrier recombination time depends strongly on the polarization of pump pulse. With the same pump photon flux densities, the recombination time of spin-polarized carriers is always longer than that of the spin-balanced carriers except at low pump photon flux densities, this anomaly originates from the polarization-sensitive nonlinear absorption effect. Differing from the traditional views, in the low carrier density regime, the D'yakonov–Perel' (DP) mechanism can be more important than the Bir–Aronov–Pikus (BAP) mechanism, since the DP mechanism takes effect, the spin relaxation time in (110) GaAs QWs is shortened obviously via asymmetric doping.     

Coherent state polarons in quantum wells

We investigate the polaronic effects of an electron confined in a quantum well, which we describe through its algebraic properties using su(1,1), taking into account the electron-bulk longitudinal-optical phonon interaction. We construct the variational wave function as the direct product of an electronic part and a part describing coherent phonons generated by the Low–Lee–Pines transformation from the vacuum state. We use two explicit forms of coherent states, Perelomov and Barut-Girardello states, to represent the electronic part in the quantum well spectrum. Our results show that in a coherent state basis for electrons the basic polaron parameters such as the energy gap shift and effective mass are further enhanced compared to those obtained with the conventional sinusoidal form of the basis. The difference between the two types of quantum well coherent states appears in polaronic interactions in quantum wells. We extend the calculations in order to estimate polaron lifetimes for a variety of different material systems.     

Phase dependence of cross-phase modulation in asymmetric quantum wells

We study the Kerr nonlinearity associated with cross-phase modulation based on electromagnetically induced transparency in asymmetric double quantum wells. It is shown that, different from atomic system, not only the nonlinear dispersion and absorption but also the linear absorption depends on the relative phase of the laser fields because of the Fano interference. By choosing the parameters appropriately, large cross-phase modulation with nearly vanishing two-photon absorption, even π phase shift with single photon level, could be achieved in the asymmetric quantum wells.     

Phase control of two-dimensional probe gain-absorption spectra in semiconductor quantum wells

We investigate the two-dimensional gain and absorption of a weak probe field via two orthogonal standing-wave lasers in a four-level inverted-Y asymmetric quantum well system. We find that, due to the spatial-dependent quantum interference effect, the spatial distribution of the 2D gain and absorption spectra can be easily controlled by adjusting the system parameters. More importantly, the probe gain-absorption spectrum can be controlled at a particular position and the 2D localization effect is indeed achieved efficiently. Thus, our scheme shows the underlying probability for the formation of the 2D localization effect by using a QW structure.     

GaAs体材料及其量子阱的光学极化退相特性

采用飞秒时间分辨瞬态简并四波混频技术，在室温下测量了GaAs体材料及其量子阱材料GaAs/Al_0.3Ga_0.7As的光学极化超快退相时间，当激光中心波长为785nm，受激载流子浓度为10¹¹cm^-2时，它们的退相时间分别为28fs和46fs.量子阱材料的退相时间比体材料的长，这是由于量子阱中的载流子在垂直于GaAs/AlGaAs界面的运动受到限制，运动呈现二维特性，大大减小了载流子的散射概率.实验中观察到瞬态简并四波混 关键词： 时间分辨简并四波混频 飞秒激光脉冲 退相 密度矩阵     

Features of hopping conductivity in weakly doped and weakly compensated semiconductor quantum wells

Main features of hopping transport in quantum wells in the conditions of low doping and low compensation are considered. Based on the features of the classical law of interaction between the impurities and on the percolation problem in quantized wells, it is shown that the specific resistance and activation energy of donor centers of the well are determined by the doping parameters and sharply depend on the width of the well.     

Magnetoelectronic transport of the two-dimensional electron gas in CdSe single quantum wells

Hall mobility and magnetoresistance coefficient for the two-dimensional (2D) electron transport parallel to the heterojunction interfaces in a single quantum well of CdSe are calculated with a numerical iterative technique in the framework of Fermi-Dirac statistics. Lattice scatterings due to polar-mode longitudinal optic (LO) phonons, and acoustic phonons via deformation potential and piezoelectric couplings, are considered together with background and remote ionized impurity interactions. The parallel mode of piezoelectric scattering is found to contribute more than the perpendicular mode. We observe that the Hall mobility decreases with increasing temperature but increases with increasing channel width. The magnetoresistance coefficient is found to decrease with increasing temperature and increase with increasing magnetic field in the classical region.      

Phonon heating of two-dimensional exciton gases in GaAs/AlGaAs quantum wells

We report the first measurements of the interaction of non-equilibrium phonons with two-dimensional exciton gases (2DExGs). The rise in the effective temperature of the 2DExG produced by the phonons depends on the width of the quantum well and the exciton sheet density and hence on the ratio τ^?1 (ex-ph)/τ^?1 (ex-ex). The dependence of the effective temperature rise on this ratio is attributed to the non-equilibrium frequency distribution of the phonons incident on the 2DExG.     

Enhancement of spin polarization in asymmetrically coupled CdSe and CdZnMnSe quantum dots in ZnSe matrix

An asymmetrically coupled double quantum dot (QD) system consisting of adjacent CdSe and CdZnMnSe QD layers in a ZnSe matrix was investigated using polarization-selective magneto-photoluminescence (PL). Two well-resolved PL peaks are observed corresponding, respectively, to the CdSe and the CdZnMnSe QDs. The peaks exhibit significant change in the intensity and energy position when a magnetic field is applied. The enhancement of the degree of σ⁻ circular polarization emitted by the non-magnetic CdSe QDs is observed in the double layer system, as compared to that observed in CdSe QDs without the influence of neighboring CdZnMnSe QDs. This behavior was discussed in terms of antiferromagnetic interaction between carrier spins localized in pairs of CdSe and CdZnMnSe QDs that are electronically coupled.     

InAs/GaAs透镜形量子点超晶格材料的纵向和横向周期对应变场分布的影响

对量子点超晶格材料中量子点纵向周期和同层量子点的横向周期间距对量子点及其周围应变场分布的影响进行了系统的研究.结果表明，横向和纵向周期通过衬底材料之间的长程相互作用对量子点沿中心轴路径应变分布的影响效果正好相反，在适当条件下，两者对量子点应变场分布的影响可以部分抵消.同时也论证了在单层量子点和超晶格量子点材料中，计算量子点的电子结构时，应综合考虑量子点空间周期分布对载流子限制势的影响，不能简单的利用孤立量子点模型来代替. 关键词： 应变 半导体量子点 自组织     

Phonon satellites and time-resolved studies of carrier recombination dynamics in InGaN quantum wells

We have studied the photoluminescence and time-resolved photoluminescence of a set of InGaN quantum wells with well thickness from 1 to 7.5 nm. An analysis of the phonon satellites at 5 K shows Huang–Rhys factors from 0.32 to 0.44. The increase of this factor is caused by the electron–hole separation induced by the piezoelectric field. The time-resolved photoluminescence at room temperature shows that the decay time of the 1 and 2 nm wells does not depend on the wavelength. The maximum decay time is around 600 ps for the 2, 3 and 4 nm wells. However, for the 3 and 4 nm wells a decrease of the photoluminescence decay time is observed at the highest wavelengths. This suggest the onset of a non-radiative process in these samples. The optimum well width for efficient emission for these single quantum wells was found to be 2 nm.     

Effect of continuum resonances on electronic transport in quantum wells

We investigate the influence of continuum resonances on the release of electrons by a quantum well. We find that the inclusion of resonance effects leads to a decrease in the rate of scattering by non-polar phonons from bound confined states to unbound continuum states.     

Effect of screening by two-dimensional charge carriers on the binding energy of excitonic states in GaAs/AlGaAs quantum wells

The spectrum of excitonic excited states in GaAs/AlGaAs quantum wells of different width is studied together with its change due to the screening of electron-hole interaction by two-dimensional electrons. The exciton binding energy decreases sharply with an increase in the concentration of two-dimensional electrons. The temperature dependence of screening parameters is studied for the ground and excited excitonic states down to ultralow temperatures T=50 mK.     

Highly efficient radiative recombination of electron–hole pairs localized at compound semiconductor quantum dots embedded in Si

We have studied the optical properties of compound semiconductor quantum dots (CSQDs) embedded in Si. Both photoluminescence and electroluminescence spectra were found to be associated with an inhomogeneously broadened band in the near-infrared. A long decay lifetime of luminescence was observed, which is in support of an indirect transition in both k- and real-space. Strong localization of electron–hole pairs was found to occur due to a deep potential well created by the built-in electric dipole at the III–V/Si interface. A Si-based light-emitting diode with GaSb-CSQDs in the active layer showed a high value of quantum efficiency. Light amplification was also observed under pulsed laser excitation.