A study for the cartography of the interface roughness of V-shaped AlGaAs/GaAs quantum wires

We present a theoretical study for the cartography of the interface roughness of AlGaAs/ GaAs V-shaped quantum wires which is reflected on the photoluminescence and micro-photoluminescence spectra of these structures. The model developed is based on the existence of microscopic compositional fluctuations at the interfaces. The fine structure of the micro-photoluminescence spectrum is attributed to localized excitonic states in island-like fluctuations which act as quantum boxes distributed along the free direction of the wire. The fluctuation of the concentration of these boxes together with the estimation of their sizes are used to explain the evolution of the signals along the wire axis and to produce the overall photoluminescence spectrum. The model is applied to a V-shaped Al_0.3Ga_0.7As/GaAs quantum wire and reproduces successfully the observed photoluminescence and micro-photoluminescence characteristics.     

Exchange-induced splitting of radiative exciton levels in a single quantum wire

We report on polarization-resolved micro-photoluminescence experiments performed on a single GaAs/GaAlAs V-shaped quantum wire. At low temperature the micro-photoluminescence spectra are composed of sharp peaks corresponding to excitons localized in naturally formed quantum boxes. We observed splittings of the radiative doublet of these exciton levels into two linearly polarized states due to the exchange interaction. The exchange splittings are of the order of 100 μeV. A theoretical model of the exchange interaction on localized states in quantum wires is developed. It shows that the exchange splitting is characteristic of the uniaxial anisotropy of the localized states and thus related to their extension along the wire axis. The experimental results are discussed within the frame of this model.     

V形GaAs/AlGaAs量子线结构的微区光致发光谱研究

在室温下用显微光致发光的方法对单根V形GaAs/AlGaAs量子线进行了沿垂直于量子线方向的 空间分辨扫描测试,观察到各种量子结构的光致发光谱随空间位置的变化.在量子线区域附 近观察到来自量子线(QWR)、颈部量子阱(NQWL)和垂直量子阱(VQWL)等各种结构的发光,而 在距离量子线约1μm以远的发光光谱表现出侧面量子阱(SQWL)的发光.对全部发光光谱用高 斯线形进行了拟合,发现QWR和SQWL的发光包含了两个荧光峰,将它们分别归诸为电子到轻 、重空穴的跃迁.拟合后发光强度的空间变化直接确定了与量子线 关键词： V形GaAs/AlGaAs量子线 显微光致发光 空间分辨扫描     

Fabrication and microscopic characterization of a single quantum-wire laser with high uniformity

A T-shaped single quantum-wire laser with high spatial uniformity has been fabricated by a cleaved-edge overgrowth method with molecular beam epitaxy and a growth-interrupt annealing technique. Using micro-photoluminescence imaging and spectroscopy, we confirmed the formation of a spatially uniform quantum wire over 20 μm long without hetero-interface roughness. By optical pumping, we achieved single-mode lasing from the ground state of the single quantum wire at a threshold excitation power as low as 5 mW at 5 K.     

Comparison of the optical properties of as-grown ensembles and single ZnO nanowires

By use of micro-photoluminescence spectroscopy we analyze and compare the optical properties of individual zincoxide nanowires, with diameters 90 nm<d<620 nm, and of the as-grown ensemble. After special preparation techniques individual nanowires of different morphologies and crystalline qualities are observed which possess distinctively different near band-edge excitonic features. We show that the spectral shape of these excitonic emission lines correlates with the morphology of the nanowires. Our results clearly show that for decreasing wire diameter, distinct surface-related spectral features strongly contribute to the optical properties of individual ZnO nanowires. Finally, the temperature dependence of the near band-edge emission is analyzed. The results obtained from individual wires provide information about the homogeneity of the optical properties of the wires in the as-grown ensembles, and show that easily performed ensemble measurements indeed reflect the properties of typical individual, single nanowires. PACS 78.55.-m; 78.55.Et; 78.67.-n; 78.67.Bf     

The self-consistent electronic structure of rectangular free-standing quantum wires: Fourier expansion approach

A method for the self-consistent calculation of rectangular free-standing quantum wires is presented. The method is also used for the electronic structure calculation of cladded quantum wires. It relies on the wavefunction's Fourier expansion, and uses the structure symmetry to save computation time. Calculations are performed for these two types of quantum wire at a number of dopant and surface state densities, and the influence of various parameters is analyzed. At low temperatures, the occurrence of Friedel oscillations is noticed, arising from a very limited number of well separated electronic states.     

Single-Photon Emission from a Single InAs Quantum Dot

Excitation power-dependent micro-photoluminescence spectra and photon-correlation measurement are used to study the optical properties and photon statistics of single InAs quantum dots. Exciton and biexciton emissions, whose photoluminescence intensities have linear and quadratic excitation power dependences, respectively, are identified. Under pulsed laser excitation, the zero time delay peak of second order correlation function corresponding to exciton emission is well suppressed, which is a clear evidence of single photon emission.     

Photoluminescence of single quantum wires and quantum dots

The results of a study into the photoluminescence spectra of a set of quantum dots based on GaAs enclosed in AlGaAs nanowires are presented. The steady state and time resolved spectra of photoluminescence under optical excitation both from an array of quantum wires/dots and a single quantum wire/dot have been measured. In the photoluminescence spectra of single quantum dots, emission lines of excitons, biexcitons and tritons have been found. The binding energy of the biexciton in the studied structures was deduced to be 8 meV.     

抛物量子线中弱耦合极化子的有效质量和光学声子平均数

讨论电子与体纵光学(LO)声子弱耦合时对抛物量子线中极化子性质的影响.采用Tokuda改进的线性组合算符法、Lagrange乘子和变分法,导出了抛物量子线中弱耦合极化子的有效质量和光学声子平均数随拉格朗日乘子变化的规律及极化子振动频率随量子线约束强度的变化规律.并以ZnS量子线为例进行了数值计算,结果表明:抛物量子线中弱耦合极化子的有效质量m^*和光学声子平均数N随着拉格朗日乘子u的增加而增大;该结论与体材料中结论基本一致,但量子线中的效应比体材料更明显,表明量子线对电子约束的增强,使极化子效应更明显.同时,极化子振动频率λ随约束强度ω₀的增强而增大.     

Detecting spatially localized excitons in InGaN quantum well structures with a micro-photoluminescence technique

Spatially localized excitons are observed in InGaN quantum well structures at 4 K by using a micro-photoluminescence (PL) technique. By combining PL and nano-lithographic techniques, we are able to detect PL signals with a 0.2 μm spatial resolution. A sharp PL line (linewidth of <0.4 meV) is clearly obtained, which originates from a single localized exciton induced by a quantum dot like a local potential minimum position. Sharp PL spectra detected in three QWs with different indium compositions confirm that there are exciton localization effects in quantum wells in the blue-green (about 2.60 eV, 477 nm) to purple (about 3.05 eV, 406 nm) regions.     

量子线中强耦合极化子的温度效应

采用Tokuda改进的线性组合算符法和有效质量下的变分法,研究在抛物势作用下,同时考虑电子与LO声子相互作用时,温度对量子线中强耦合极化子特性的影响。对RbCl晶体所作的数值计算结果表明,量子线中强耦合极化子的基态能量、平均数和光学声子平均数均随温度的升高而增加。     

Quantum confined carrier transition in a GaN/InGaN/GaN single quantum well bounded by AlGaN barriers

We investigated the carrier transition properties of the GaN/InGaN/GaN single quantum well bounded by AlGaN barriers. In order to confirm the carrier transition coming from the single quantum well, the single quantum well layer was etched by reactive ion etching method. The structural property of the samples was characterized by high resolution X-ray diffraction measurements. In micro-photoluminescence measurements, it is clearly shown that the donor bound exciton transition of the single quantum well sample was redshifted compared to the etched one due to strain. Moreover, a lot of peaks were observed below the GaN band gap energy due to carrier localization in the InGaN/GaN single quantum well, including carrier localization center and quantum confined states. The excitation power dependence and time resolved photoluminescence spectra were investigated to characterize the optical transition of the single quantum well.     

抛物量子线中强耦合极化子的有效质量

采用改进的线性组合算符法、Lagrange乘子和变分法,在考虑电子与LO声子相互作用情况下,研究了抛物量子线中强耦合极化子的有效质量和光学声子平均数。通过数值计算,讨论了约束强度ω₀和拉格朗日乘子u对极化子的有效质量m^*和光学声子平均数N及极化子振动频率λ的影响。计算结果表明:有效质量m^*和光学声子平均数N及极化子振动频率λ都随着约束强度ω₀和拉格朗日乘子u的增加而增大。     

Correlated photons from multi-carrier complexes in GaAs quantum dots

We have studied micro-photoluminescence spectra of a self-assembled single GaAs quantum dot under 8 K. With strong pulsed excitation, the micro-photoluminescence spectrum shows bright emission lines originated from an exciton, a positively charged exciton, and a biexciton, together with weak lower energy emissions reflecting multi-excitonic structures with more carriers. We have identified the origins of these weak emission lines, and showed the existence of charged biexciton states, through single photon correlation measurements and excitation power dependence of the photoluminescence intensity. In addition, investigating the radiative recombination process of the charged biexciton, we have determined the electron–hole exchange energy in the GaAs quantum dot.     

Optical and electronic properties of single modulation doped GaAs/AlGaAs V-grooved quantum wire modified by ion implantation

1 Introduction Recently, there is considerable interest in the fabrication and study of quasi-one di-mensional quantum wires (QWRs) due to their potential application for novel optoelec-tronic devices such as QWR laser array [1,2] etc. Among the various techniques devel-oped for producing quasi-one dimensional (quasi-1D) QWRs, the self-organized growth on patterned substrates has been proven to be one of the most promising methods, due to the simplicity of fabrication[3―5]. The QWR is fa…     

Quantum decay of supercurrent in thin superconducting wires

Quantum fluctuations cause a decay of the supercurrent in thin superconducting wires making them resistive even at very low temperatures. We derive a microscopic effective action formalism that goes beyond the usual TDGL approach and study quantum fluctuations of the superconducting order parameter at all temperatures belowT _C . We calculate the quantum phase slip rate in thin superconducting wires, demonstrate the importance of dissipation in a quantum phase slip process, and evaluate the resistanceR(T) of the wire. In very thin wires the effect is well observable, even at zero temperature.     

Time-resolved cathodoluminescence of InGaAs/AlGaAs tetrahedral pyramidal quantum structures

An original time resolved cathodoluminescence set up has been used to investigate the optical properties and the carrier transport in quantum structures located in InGaAs/AlGaAs tetrahedral pyramids. An InGaAs quantum dot formed just below the top of the pyramid is connected to four types of low-dimensional barriers: InGaAs quantum wires on the edges of the pyramid, InGaAs quantum wells on the (111)A facets and segregated AlGaAs vertical quantum wire and AlGaAs vertical quantum wells formed at the centre and at the pyramid edges. Experiments were performed at a temperature of 92 K, an accelerating voltage of 10 kV and a beam probe current of 10 pA. The cathodoluminescence spectrum shows five luminescence peaks. Rise and decay times for the different emission wavelengths provide a clear confirmation of the peak attribution (previously done with other techniques) to the different nanostructures grown in a pyramid. Moreover, experimental results suggest a scenario where carriers diffuse from the lateral quantum structures towards the central structures (the InGaAs quantum dot and the segregated AlGaAs vertical quantum wire) via the InGaAs quantum wires on the edges of the pyramid. According to this hypothesis, we have modeled the carrier diffusion along these quantum wires. An ambipolar carrier mobility of 1400 cm²/V s allows to obtain a good fit to all temporal dependences. PACS 78.67.-n     

Study on magnetization reversal of cobalt nanowire arrays by magnetic force microscopy

The magnetic properties of self-assembly cobalt nanowire arrays formed in anodic porous alumina template were investigated by nanosize imaging method and macroscopic magnetic measurement. We have successfully made a wire-by-wire observation of magnetization reversal of a cobalt nanowire array using magnetic force microscopy with a home-made FePt tip. The nanowires in this medium have uniaxial anisotropy with easy axis along the wire due to the large aspect ratio of the wires (30 nm in diameter and 300 nm in length). Considering the nanowires as single-domain structures, we can obtain the average DC demagnetization curve from nanosize images by calculating the number of wires in each magnetized direction, and the results agreed well with the DC demagnetization curve measured by macroscopic measurement. The magnetostatic field between wires was evaluated by a new nanosize imaging method. Macroscopic measurement shows that reversible magnetization occurs in this medium. Nanosize images of the remanent and saturated states prove that the reversible magnetization processes mainly take place inside individual wires and reversed wires induced by magnetostatic field just give a little contribution to the reversible magnetization.     

Si quantum dot structures and their applications

This paper presents briefly the history of emission study in Si quantum dots (QDs) in the last two decades. Stable light emission of Si QDs and NCs was observed in the spectral ranges: blue, green, orange, red and infrared. These PL bands were attributed to the exciton recombination in Si QDs, to the carrier recombination through defects inside of Si NCs or via oxide related defects at the Si/SiOx interface. The analysis of recombination transitions and the different ways of the emission stimulation in Si QD structures, related to the element variation for the passivation of surface dangling bonds, as well as the plasmon induced emission and rare earth impurity activation, have been presented.The different applications of Si QD structures in quantum electronics, such as: Si QD light emitting diodes, Si QD single union and tandem solar cells, Si QD memory structures, Si QD based one electron devices and double QD structures for spintronics, have been discussed as well. Note the significant worldwide interest directed toward the silicon-based light emission for integrated optoelectronics is related to the complementary metal-oxide semiconductor compatibility and the possibility to be monolithically integrated with very large scale integrated (VLSI) circuits. The different features of poly-, micro- and nanocrystalline silicon for solar cells, that is a mixture of both amorphous and crystalline phases, such as the silicon NCs or QDs embedded in a α-Si:H matrix, as well as the thin film 2-cell or 3-cell tandem solar cells based on Si QD structures have been discussed as well. Silicon NC based structures for non-volatile memory purposes, the recent studies of Si QD base single electron devices and the single electron occupation of QDs as an important component to the measurement and manipulation of spins in quantum information processing have been analyzed as well.     

Negative Coulomb drag in coupled wire and quantum dot system

We measure the Coulomb drag between parallel split-gate quantum wires with a quantum dot embedded in one of the two wires (drive wire). We observe negative Coulomb drag when a Coulomb oscillation peak appears in the drive wire and the conductance of the other wire (drag wire) is slightly below the first plateau. This indicates that correlation holes are dragged in the drag wire by single electron tunneling through the quantum dot in the drive wire. The drag is only promoted in the drag wire near the barrier regions of the dot, and low compressibility of the drag wire is necessary for the negative drag to occur.