Electronic coupling and structural ordering of quantum dots using InAs quantum dot columns

In this article, recent investigations of vertically aligned quantum dot columns conducted at Stanford University are reviewed. The quantum dots are InAs in a matrix of GaAs. Both the quantum dots and quantum dot columns are formed through strain-induced islanding, without lithography. Two aspects of these columns are discussed. First, the electronic coupling of quantum dots within columns of up to ten quantum dots is demonstrated. The coupling is adjusted and improvements to a simple light-emitting diode are shown. Second, increased uniformity of a surface quantum dot layer is shown when a subsurface layer of these columns are used. The most impressive results occur when the columns contain a large number of islands. Reduced variations in average ensemble height and diameter, called size uniformity, and average nearest neighbor distances, called structural uniformity, are shown. A surface unit cell of islands is demonstrated and the lack of a surface lattice is discussed.     

The influences of an anisotropic parabolic potential on the quantum dot qubit* Zhao Cui-Lan(赵翠兰), Cai Chun-Yu(蔡春雨), Xiao Jing-Lin(肖景林)?

To study the influence of an anisotropic parabolic potential(APP)on the properties of a quantum dot(QD)qubit,we obtain the eigenenergies and eigenfunctions of the ground and first excited state of an electron,which is strongly coupled to the bulk longitudinal optical(LO)phonons,in a QD under the influence of an APP by the celebrated Lee–Low–Pines(LLP)unitary transformation and the Pekar type variational(PTV)methods.Then,this kind of two-level quantum system can be excogitated to constitute a single qubit.When the electron locates at the superposition state of its related eigenfunctions,we get the time evolution of the electron’s probability density.Finally,the influence of an APP on the QD qubit is investigated.The numerical calculations indicate that the probability density will oscillate periodically and it is a decreasing function of the effective confinement lengths of theAPPindifferentdirections.Whereasitsoscillatoryperiodisanincreasingoneandwilldiminishwithenhancing the electron–phonon(EP)coupling strength.     

The engineering and properties of InAs quantum dot molecules in a GaAs matrix

Arrays of InAs quantum dot (QD) molecules in the GaAs matrix, which consist of pairs of vertically aligned InAs QDs, have been synthesized by molecular beam epitaxy. A study of the resulting structures by transmission electron microscopy demonstrated that the vertically aligned QDs are equal in size. Photoluminescence measurements revealed that the spectra of the samples under study contain bands corresponding to electronic states in QD molecules.     

Issues of practical realization of a quantum dot register for quantum computing

We present a quantum dot structure fabricated by the lithographic positioning, which can be used as a prototype of the quantum dot register for quantum computing. Using simple model calculations we show that parameters of our quantum dot structure are very close to the ones required for two possible embodiments of a quantum computer. Results of numerical simulation of the quantum dot register, as well as discussion of materials and technological issues of fabrication of quantum logic gates are also presented.     

The quest for the quantum dot

Semiconductor devices that confine electrons within two-dimensional quantum wells are already in production. The zero-dimensional quantum dot promises super-efficient lasers with the last word in spectral purity. The author briefly discusses the definition of the electron and then goes on to discuss in more detail the quantum well obtained in the case of a GaAlAs-GaAs heterojunction. Further quantisation of the electron energy produces one dimensional quantum wires. By removing the one degree of freedom of the quantum wire leaves the quantum dot, zero degrees of freedom, the ultimate in energy quantisation     

Enhanced optical polarization anisotropy in quantum wells under anisotropic tensile strain

Anisotropic in-plane strain in quantum wells leads to an optical polarization anisotropy that can be exploited in optoelectronic devices such as modulators. A theoretical model shows that the behavior of the polarization anisotropy with increasing strain anisotropy is radically different for quantum wells under anisotropic tensile and compressive strains of equal magnitude. This strikingly different behavior arises from the different valence-subband mixing that occurs in the cases of anisotropic tensile and compressive strain. Specifically, the mixing of the first heavy- and light-hole subbands that occurs only under anisotropic tensile strain is central to the polarization anisotropy.     

Identification of candidate material systems for quantum dot solar cells including the effect of strain

Heterostructures that include self‐assembled quantum dots (SAQDs) have been suggested as model systems for the realization of novel high efficiency solar cells such as those based on intermediate bands (IBs). The lattice mismatch in the epitaxial growth of these structures, necessary for the formation of SAQDs, introduces strain throughout the structure, making the selection of materials systems with appropriate physical parameters problematic. The model solid theory is used to calculate the energy band edge alignment at Γ point of such quantum dot (QD) heterostructures including the effects of strain. With the modified band gaps due to strain, a materials search was performed for high efficiency QD solar cells among III‐V binaries and ternaries with negligible valence band offsets. This requirement of the valence band offset along with the limited band gap ranges for optimum efficiency results in only a few feasible materials systems being identified. The optimum barrier/dot material system found was Al_0.57In_0.43As/InP_0.87Sb_0.13 grown on lattice matched metamorphic buffer layer, but due to miscibility gap concerns it is suggested that the Al_0.50In_0.50As/InAs_0.41P_0.59 fully strained system may be preferred. Copyright © 2010 John Wiley & Sons, Ltd.     

抛物量子点中的二能级体系

采用Pekar类型的变分方法,在抛物量子点中电子与体纵光学声子（LO）强耦合的条件下,得出了电子的基态,第一激发态的能量及相应的波函数。讨论了电子-声子耦合强度,量子点受限长度对电子在基态∣0〉, 激发态∣1〉,叠加态时的概率密度分布的影响。结果显示叠加态时的概率密度随着电子-声子耦合强度的增加而增大,随着量子点受限长度的减小而增大。该结果对以量子点制备量子比特有重要的指导意义。     

Heteroepitaxial growth of InAs on Si: A new type of quantum dot

The mechanism for heteroepitaxial growth in the InAs/Si system is studied by reflection highenergy electron diffraction, scanning tunnelling microscopy, and photoluminescence. For certain growth conditions, InAs nanostructures are found to develop on the Si surface immediately during the growth process in the course of molecular beam epitaxy. The range of substrate temperatures that lead to formation of nanosized islands is determined. InAs quantum dots grown on a buffer Si layer with a silicon layer of thickness 50 nm grown on the top produced photoluminescence lines at a wavelength of 1.3 μm at 77K and 1.6 μm at 300 K. Fiz. Tekh. Poluprovodn. 33, 1066–1069 (September 1999)     

Influence of strain on hydrogenic impurity states in a GaN/AlxGa1?xN quantum dot

Within the effective-mass approximation, we calculated the influence of strain on the binding energy of a hydrogenic donor impurity by a variational approach in a cylindrical wurtzite GaN/Al _x Ga_1−x N strained quantum dot, including the strong builtin electric field effect due to the spontaneous and piezoelectric polarization. The results show that the binding energy of impurity decreases when the strain is considered. Then the built-in electric field becomes bigger with the Al content increasing and the binding energy of hydrogenic donor impurity decreases when the Al content is increasing. For dot height L < 2 nm, the change of the binding energy is very small with the Al content variety. This work has been supported by the National Natural Science Foundation of China (No. 10564003) and the Key Project of the Science and Technology Research of the Educational Ministry of China (No. 208025)     

Effects of GaAs-spacer strain on vertical ordering of stacked InAs quantum dots in a GaAs matrix

Vertical ordering in stacked layers of InAs/GaAs quantum dots is currently the focus of scientific research because of its potential for optoelectronics applications. Transmission electron microscopy was applied to study InAs/GaAs stacked layers grown by molecular-beam-epitaxy with various thicknesses of GaAs spacer. Thickness dependencies of quantum dot size and their ordering were observed experimentally and, then, compared with the results of strain calculations based on the finite element method. The vertical ordering did occur when the thickness of the GaAs spacer was comparable with the dot height. The ordering was found to be associated with relatively large InAs dots on the first layer. Quantum dots tend to become larger in size and more regular in plane with increasing numbers of stacks. Our results suggest that the vertical ordering is not only affected by strain from the InAs dots on the lower layer, but by total strain configuration in the multi-stacked structure.     

Impedance spectroscopy for quantum dot light-emitting diodes

Impedance spectroscopy has been increasingly employed in quantum dot light-emitting diodes(QLEDs) to investigate the charge dynamics and device physics. In this review, we introduce the mathematical basics of impedance spectroscopy that applied to QLEDs. In particular, we focus on the Nyquist plot, Mott-Schottky analysis, capacitance-frequency and capacitance-voltage characteristics, and the d C/d V measurement of the QLEDs. These impedance measurements can provide critical information on electr...     

量子点电视技术浅析

本文分析了WLED电视的技术缺点和量子点电视的技术优越性。传统的WLED电视大多采用“蓝光LED＋黄色荧光粉”的发光模式,虽可让人感觉到白光,不过缺乏红光和绿光使得混色较为困难,是导致色域覆盖率偏低和背光转换效率不高的原因。近年来,随着量子点发光技术的成熟及其在液晶背光中的应用,量子点电视应运而生,性能十分优越。与传统WLED电视比较,量子点电视的色域和能效更高,所以其画面色彩更自然亮丽,并且兼顾节能环保。     

Single electron transport in a free-standing quantum dot

We have fabricated a single quantum dot in the two-dimensional electron gas of a free-standing GaAs/AlGaAs heterostructure. Standard processing techniques are used to define the dot on a narrow mesa and selective etching undercuts the device to form a 120 nm thick free-standing beam connected to the substrate by source and drain regions. Work by Weig et al. [Phys. Rev. Lett. 92(4) (2004) 046804] on similar devices has provided evidence for phonon blockade which suppresses conduction at low temperatures and bias. Further studies are presented here that can only partially be explained by this theory. A similar gap in conductance has been seen, but evolution of the structure in a magnetic field suggests an alternative explanation of multiple dots in series, may better explain the results.     

柱形量子点的电子拉曼散射研究

本文对CdS柱形量子点的电子拉曼散射进行了研究;推导了拉曼散射截面的表达式,给出了有关的选择定则,获得了丰富的拉曼散射谱线并对谱线特点进行了分析和讨论,讨论结果直接给出了电子的有关信息。     

Properties of a polaron in a quantum dot:a squeezed-state variational approach

The ground-state energy and the average number of virtual phonons around the electron in a parabolic quantum dot for the entire range of the electron-phonon coupling constant are obtained using the single-mode squeezed-state variational approach.The variational approach we applied is based on two successive canonical transformations and using a displaced-oscillator type unitary transformation to deal with the bilinear terms which are usually neglected.In order to study the relationship between the ground-state energy and the average number of virtual phonons around the electron of a polaron in a parabolic quantum dot with the electron-LO-phonon coupling constant and the confinement length,numerical calculations are carried out in the electron-LO-phonon strong-and weak-coupling regions.     

量子点中极化子性质:压缩态变分法

利用单模压缩态变分法得到了在电子-体纵光学声子全耦合条件下抛物量子点极化子的基态能量及电子周围平均声子数的表达式。所采用的变分方法是基于逐次正则变换的变分方法,变换处理了包含经常被忽略的声子产生湮灭算符的双线性项。在电子-体纵光学声子全耦合条件下,讨论了受限长度与抛物量子点极化子的基态能量及电子周围平均声子数之间的依赖关系。     

Characteristic temperature of a tunneling-injection quantum dot laser: Effect of out-tunneling from quantum dots

A laser structure is studied, which exploits tunneling-injection of electrons and holes into quantum dots (QDs) from two separate quantum wells (QWs). An extended theoretical model is developed allowing for out-tunneling leakage of carriers from QDs into the opposite-to-injection-side QWs (electrons into the p-side QW and holes into the n-side QW). Due to out-tunneling leakage, parasitic recombination of electron-hole pairs occurs outside QDs – in the QWs and optical confinement layer. The threshold current density j_th and the characteristic temperature T₀ are shown to be mainly controlled by the recombination in the QWs. Even in the presence of out-tunneling from QDs and recombination outside QDs, a tunneling-injection laser shows potential for significant improvement of temperature stability of j_th – the characteristic temperature T₀ remains very high (above 300 K at room temperature) and not significantly affected by the QD size fluctuations.     

High-performance 980 nm quantum dot lasers for high-powerapplications

High-performance quantum dot lasers emitting at 980 nm with output powers of up to 4 W CW from a single facet (AR/HR coating, 100 μm stripe width) have been fabricated. Wall-plug efficiencies >50%, were achieved at room temperature. Owing to an improved carrier confinement output powers as high as 1 W CW can be obtained from the fundamental dot transition even at temperatures as high as 110°C     

Coherence and phase sensitive measurements in a quantum dot

Via a novel interference experiment, which measures magnitude and phase of the transmission coefficient through a quantum dot in the Coulomb regime, we prove directly, for the first time, that transport through the dot has a coherent component. We find the same phase of the transmission coefficient at successive Coulomb peaks, each representing a different number of electrons in the dot, however, as we scan through a single Coulomb peak we find an abrupt phase change of π. The abrupt phase change can be explained in the framework of a one-dimensional model, which we develop.