Piezoelectric models for semiconductor quantum dots

The importance of fully coupled and semi-coupled piezoelectric models for quantum dots are compared. Differences in the strain of around 30% and in the electron energies of up to 30 meV were found possible for GaN/AlN dots.     

Electrical properties of semiconductor quantum dots

A method, which makes it possible to obtain semiconductor particles V ≈ 10^?20 cm³ in volume (quantum dots) with a concentration of up to 10¹¹ cm^?2 and electrical contacts to each of them, is suggested. High variability in the electrical properties of such particles from a metal oxide (CuO or NiO) after the chemisorption of gas molecules is found.     

Exciton photoluminescence and energy in a percolation cluster of ZnSe quantum dots as a fractal object

The results of studies of samples containing ZnSe quantum dots with a density corresponding to or considerably higher than the exciton percolation threshold, at which quantum dots form conglomerates, are reported. Excitonic emission from a percolation cluster of bound quantum dots as a fractal object is observed for the first time. Analysis of the structure of the photoluminescence spectra shows that the spectra are determined by the contribution of exciton states that belong to different structural elements of the percolation cluster, specifically, to the skeleton (backbone), dangling (dead) ends, and internal hollow spaces. A qualitative model is proposed to interpret the dependence of the exciton energy in these structural elements on the concentration of quantum dots in the material.     

Quantitative HRTEM analysis of semiconductor quantum dots

Elastic strains and layer compositions of semiconductor quantum dots are quantified by the measurement of lattice fringe spacings from high-resolution micrographs. Analyses of simulated images, taking thin-specimen relaxation into account by application of finite element simulations, demonstrate that the local slopes of these strain profiles may contain severe artefacts mainly caused by local crystal tilts. Nevertheless, average strain values may be measured with sufficient accuracy and can be used to obtain an estimate on average layer compositions by application of the continuum theory of elasticity when analysing experimental micrographs. Focusing on In(x)Ga1-xAs/GaAs and Ge(x)Si1-x/Si heterostructures, it is demonstrated that elastic strains of nanoscale coherent islands are severely decreased due to an elastic relaxation mechanism compared to fully strained two-dimensional layers. The analysis of self-assembled quantum dots and two-dimensional wetting layers buried by capping layers gives clear evidence for a substantial reduction of the lattice strains compared to the values expected for the nominal layer stoichiometries. This observation originates most presumably from a compositional intermixing during epitaxial growth.     

杂质对方形量子点基态能和束缚能的影响

用有限差分法求解了二维方形量子点中有 杂质时的量子体系,得到了离散薛定谔方程。对体系中电子处于基态时的能量和杂质的束缚能进行了数值计算,讨论了不同间距的杂质离子对不同尺寸量子点中电子的基态能量和束缚能的影响。计算结果表明：量子点中电子基态能量是杂质位置和量子点尺度的函数;基态能量随着量子点尺度的增加先急剧减小后缓慢增大,最后趋于定值;杂质对电子的束缚能随着量子点尺度的增加而减小;杂质间距越小对量子点基态能影响越大。     

An efficient entangled-photon source from semiconductor quantum dots

Photon entanglement,also known as“Spooky Action at a Distance”,is a promising solution to quantum cryptography and quantum computing.The former will construct a cryptosystem that is impossible to break,and the latter will be capable of solving specific problems much more quickly than any classical computer.An ideal entangled-photon source meeting the following criteria is needed for eventually the practical implementation of quantum information processing:on-demand generation[1],high-fidelity[1],ultrabright[2],high extraction efficiency[3],and high-temperature operation[4].For practical applications,it is preferred to have a simple approach that is compatible with current solid-state technologies.Self-organized semiconductor quantum dots(QDs)represent a promising option as an on-demand source of a triggered single-photon and entangled-photon pairs,through the radiative recombination of excitons and biexcitons[5,6].     

Exciton states in wurtzite and zinc-blende InGaN/GaN coupled quantum dots

Based on the effective-mass approximation, exciton states in wurtzite (WZ) and zinc-blende (ZB) InGaN/GaN coupled quantum dots (QDs) are studied by means of a variational method. Numerical results show clearly that both the sizes and In content of QDs have a significant influence on exciton states in WZ and ZB InGaN/GaN coupled QDs. Moreover, the ground-state exciton binding energy decreases when the interdot barrier layer thickness increases in the WZ InGaN/GaN coupled QDs. However, the ground-state exciton binding energy has a minimum if the interdot barrier layer thickness increases in the ZB InGaN/GaN coupled QDs.     

Exciton spin dynamics in zinc-blende GaN/AlN quantum dots: Temperature dependence

The exciton spin alignment is measured in an ensemble of self-organized cubic GaN/AlN quantum dots. By picosecond time-resolved photoluminescence experiments, we show that the exciton linear polarization does not decay with time from 20 to 300 K.     

On the physics of semiconductor quantum dots for applications in lasers and quantum optics

The progression of carrier confinement from quantum wells to quantum dots has received considerable interests because of the potential to improve the semiconductor laser performance at the underlying physics level and to explore quantum optical phenomena in semiconductors. Associated with the transition from quantum wells to quantum dots is a switch from a solid-state-like quasi-continuous density of states to an atom-like system with discrete states. As discussed in this paper, the transition changes the role of the carrier interaction processes that directly influence optical properties. Our goals in this review are two-fold. One is to identify and describe the physics that allows new applications and determines intrinsic limitations for applications in light emitters. We will analyze the use of quantum dots in conventional laser devices and in microcavity emitters, where cavity quantum electrodynamics can alter spontaneous emission and generate nonclassical light for applications in quantum information technologies. A second goal is to promote a new connection between physics and technology. This paper demonstrates how a first-principles theory may be applied to guide important technological decisions by predicting the performances of various active materials under a broad set of experimental conditions.     

Quantum calculations on quantum dots in semiconductor microcavities. Part II

In this work we continue considering solid hybrid systems formed from semiconductor quantum dots and microcavities integrated into a single optical scheme. In the second part, the main emphasis is on the methods of investigation and on the schemes for recording of the spectra and the time dependences of the population energy level, which illustrate the modern control level of the quantum state of these systems.     

Light emission by semiconductor structure with quantum well and array of quantum dots

A new type of composite active region of a laser, which contains an In_0.2Ga_0.8As quantum well (QW) and an array of InAs quantum dots (QDs) embedded in GaAs is studied. The QW acts as accumulator of injected carriers, and the QD array is the emitting system located in tunneling proximity to the QW. A theory for the calculation of electron and hole energy levels in the QD is developed. Occupation of the QDs due to the resonance tunneling of electrons and holes from the QW to the QD is considered; the conclusions are compared with the results obtained in studying an experimental laser with a combined active region.     

Electron transfer between semiconductor quantum dots via laser-induced resonance transitions

The effect of a resonant laser pulse on the quantum dynamics of an electron in a system of two semiconductor quantum dots (QDs) is studied theoretically. The pulse characteristics (frequency, amplitude, and duration) corresponding to the highest probability of electron transition from the ground state of one QD to the ground state of the other QD are determined with regard to possible difference in the QD sizes. A specific model with QDs of near cubic shape is considered.     

Microcavity light-emitting devices based on colloidal semiconductor nanocrystal quantum dots

This letter describes the design, fabrication, and characterization of a microcavity-electroluminescence (EL) device based on colloidal semiconductor nanocrystal quantum dots (NQDs). The device was fabricated by sandwiching a solution-cast film of light-emitting CdSe-CdS core-shell NQDs between two metal mirrors to form a resonant microcavity structure. We have observed a significant reduction in EL emission bandwidth from the fabricated device. Further improvement of the emission efficiency of the NQD-microcavity-EL devices can be achieved upon the minimization of the losses that are pertinent to metal mirrors.     

Excition states in semiconductor quantum dots in the modified effective mass approximation

A new modified effective mass approximation is suggested to describe the excitonic energy spectrum of quantum dots of radii a comparable to the exciton Bohr radius a _ex ⁰ . It is shown that, for quantum dots simulated by infinitely deep potential wells, the effective mass approximation is appropriate for describing excitons in quantum dots of radii a ≈ a _ex ⁰ , if the reduced effective mass of the excitons, μ, is considered as a function of the radius of the quantum dot a, μ = μ(a).     

Exciton states in quasi-zero-dimensional semiconductor nanosystems

The variational method in the context of the modified effective mass approximation is used to calculate the dependence of exciton ground-state energy for a quantum dot embedded in a borosilicate glassy matrix on the quantum dot radius. It is shown that the peaks in the absorption and low-temperature luminescence spectra of such a nanosystem are shifted to shorter wavelengths due to size quantization of the exciton ground-state energy in the quantum dot.     

Exciton states in semiconductor spherical nanostructures

The results of theoretical studies of the energy spectra of excitons moving in semiconductor spherical quantum dots are described. The contributions of the kinetic electron and hole energies, the energy of the Coulomb interaction between an electron and hole, and the energy of the polarization interaction between them to the energy spectrum of an exciton in a quantum dot with a spherical (quantum dot)-(insulator medium) interface is analyzed.     

纤锌矿GaN/AlxGa1-xN量子阱中激子能量

考虑了纤锌矿GaN/Al<,x>Ga<,1-x>N量子阱(QW)材料中空穴带质量和光学声子模的各向异性以及声子频率随波矢变化的效应,采用改进的LLP变分法计算了纤锌矿氮化物QW中激子的基态能量和结合能.给出了激子的基态能量和结合能随着QW宽度和Al组分变化的函数关系,并对闪锌矿和纤锌矿GaN/Al<0.3>Ga<,0....     

Functionalization of nc-Si/SiO2 semiconductor quantum dots by oligonucleotides

nc-Si/SiO₂ crystalline semiconductor quantum dots are very attractive as fluorescent labels for developing biosensors integrated with biomedical materials due to their unique physical properties in the visible region of the spectrum. We report on the functionalization of such nanostructures by single-strand short oligonucleotides using the d(20G,20T) system (d is deoxyribonucleotide, G is guanine, and T is thymine) as an example. Oligonucleotides are obtained by chemical synthesis using the solid-phase phosphoramidite technique. Studies using developed methods of Raman spectroscopy of high spectral and spatial resolution are performed on such complexes. The previously unpredicted phenomenon of multiband selective resonant light scattering by isolated molecular groups, caused by the nonradiative transfer of photoexcited electrons, is observed using a system of nc-Si/SiO₂ quantum dots functionalized by d(20G,20T) oligonucleotides as an example. The results obtained suggest that the developed approach can be used to study the molecular structure of semiconductor quantum-dot and DNA complexes.     

Mechanism of electronic-excitation transfer in organic light-emitting devices based on semiconductor quantum dots

The results of an experimental study of organic light-emitting diodes (LEDs) with luminescent layers based on two types of CdSe/CdS semiconductor quantum dots (QDs) with an average CdSe core diameter of 3 and 5 nm and a characteristic CdS shell thickness of 0.5 nm are presented. The dependences of the LED efficiency on the QD concentration are determined. The experimental data are used to determine the mechanism of electronic-excitation transfer from the organic matrix to the semiconductor QDs. Ways of optimizing the design of the LEDs in order to improve their efficiency are suggested on this basis.