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.     

Temperature dependence of InAs/GaAs quantum dots solar photovoltaic devices

This paper presents the temperature dependence measurements characterisation of several InAs/GaAs quantum dots （QDs） solar cell devices. The devices with cylindrical geometry were fabricated and characterised on-wafer under 20 suns in a temperature range from 300°K to 430°K. The temperature dependence parameters such as open circuit voltage, short circuit density current, fill factor and efficiency are studied in detail. The increase of temperature produces an enhancement of the short circuit current. However, the open circuit voltage is degraded because the temperature increases the recombination phenomena involved, as well as reducing the effective band gap of the semiconductor.     

Spin-flip relaxation due to phonon macroscopic deformation potential in quantum dots

We have calculated the spin-flip relaxation rates of electrons in a single CdTe spherical quantum dot due to the interaction with acoustical phonons via macroscopic deformation potential, also called the ripple mechanism. The relaxation of spin states is possible considering that the applied magnetic field and the coupling between valence and conduction bands produce admixture, both in the electrons as well as in the hole spin states. We have demonstrated that the inclusion of spin states mixture produces an appreciable probability of spin-preservingand spin-flip transitions. The electronic states are calculated within the multiband k·p approximation and the phonon modes are described as plane-waves. We have studied the dependence of the transition rates with the magnetic field and with the dot size. Our results show a fair compatibility with available experimental data.     

Size dependence saturation and absorption of PbS quantum dots

The saturation intensity for lead sulfide quantum dots in a titanium dioxide-glycerol matrix (PbS/TiO₂-glycerol) on a glass substrate has been studied as a function of quantum cluster concentration and cluster size. The saturation intensity in these materials is strongly dependent on the size of the semiconductor nanocrystals, their concentration, or the sample thickness. The samples are reflective at a certain range of the incoming intensity of the optical field and become transparent over a threshold intensity beyond which the output and input intensities are linearly related. The system studied involves very dilute distribution of PbS QD of dimension∼10 nm embedded in a matrix of TiO₂-glycerol. Since the distribution is relatively constant in each deposited layer, the total number of QDs in a given area is proportional to the thickness. We found that the threshold of power separating absorption-bleaching, Pth is linearly related to the thickness, with values of Pth ∼few mW/cm², more than 2-3 orders of magnitude below that of QDs with dimension∼1 μm, representing a typical solid.     

Quantum size effect on excitons in zinc-blende GaN/AlN quantum dot

Within the framework of effective-mass approximation, exciton states confined in zinc-blende GaN/AlN quantum dot (QD) are investigated by means of a variational approach, including three-dimensional confinement of the electron and hole in the QD and finite band offsets. Numerical results show that the exciton binding energy and the interband emission energy are both decreased when QD height (or radius) is increased. Our theoretical results are in agreement with the experimental measurements.     

Property comparison of polarons in zinc-blende and wurtzite GaN/AlN quantum wells

The properties of polarons in zinc-blende and wurtzite GaN/AlN quantum wells with Fr(o|¨)hlich interaction Hamiltonians are compared in detail.The energy shifts of polarons at ground state due to the interface(IF), confined(CO) and half-space phonon modes are calculated by a finite-difference computation combined with a modified LLP variational method.It is found that the two Fr(o|¨)hlich interaction Hamiltonians are consistent with each other when the anisotropic effect from the z-direction and the x-y plane is neglected.The influence of the anisotropy on the polaron energy shifts due to the IF phonon modes for a smaller well width or due to the CO phonon modes for a moderate well width is obvious.In addition,the built-in electric field has a remarkable effect on the polaron energy shifts contributed by the various phonon modes.     

Property comparison of polarons in zinc-blende and wurtzite GaN/AlN quantum wells

本文详细比较闪锌矿和纤锌矿GaN/AlN量子阱中Fr?hlich电声子相互作用哈密顿量及极化子性质。采用LLP变分方法计算由界面、局域和半空间声子模导致的基态极化子能移。结果表明,若忽略z方向和x-y平面之间异性的影响,闪锌矿和纤锌矿哈密顿量是一致的。各向异性在窄阱情形对界面声子能移的影响较为明显,而在略宽阱时对局域声子能移影响明显。此外,内建电场对各支光学声子引起的能移的影响较大。     

CdTe/CdS core shell quantum dots for photonic applications

In this work, we show a simple experimental procedure to obtain CdTe/CdS (diameter=3-7 nm) core shell nanocrystals in colloidal suspension. The nanocrystals were characterized by transmission electronic microscopy and powder X-ray diffraction analysis. The optical properties were determined by electronic absorption, excitation and emission spectroscopies and are discussed in terms of chemical changes that occur at the surface of the particles.     

Coherence and localization in AC-driven quantum dots

We investigate the effects of an AC field on the dynamics of interacting electrons in quantum dots (QDs). We first consider a double QD containing two electrons. By mapping the system to an effective lattice model and employing Floquet theory we analyze the dynamics of the two-electron wavefunctions in the singlet space. This system presents a richer phenomenology than the case of a single electron in a double QD, displaying two distinct regimes of behaviour depending on whether the applied field strength or the inter-electron Coulomb repulsion is dominant. We then study the dynamics of two electrons in a 2D square QD in the limit of very low density, where the Coulomb interaction causes the electrons to become highly localized, forming a ‘Wigner molecule’. Using the same techniques we investigate how the AC field drives the dynamics of the Wigner states, and show how the parameters of the effective model may be measured in experiment.     

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.     

Pulsed laser annealing of self-organized InAs/GaAs quantum dots

The effect of pulsed laser annealing (PLA), using an excimer laser, on the luminescence efficiency of self-organized InAs/GaAs and In_0.4Ga_0.6As/GaAs quantum dots has been investigated. It is found that such annealing can enhance both the peak and integrated photoluminescence (PL) efficiency of the dots, up to a factor of 5–10 compared to as-grown samples, without any spectral shift of the luminescence spectrum. The improved luminescence is attributed to the annealing of nonradiative point and extended defects in and around the dots.     

Self-assembled InAs/InP quantum dots and quantum dashes: Material structures and devices

The advances in lasers, electronic and photonic integrated circuits (EPIC), optical interconnects as well as the modulation techniques allow the present day society to embrace the convenience of broadband, high speed internet and mobile network connectivity. However, the steep increase in energy demand and bandwidth requirement calls for further innovation in ultra-compact EPIC technologies. In the optical domain, advancement in the laser technologies beyond the current quantum well (Qwell) based laser technologies are already taking place and presenting very promising results. Homogeneously grown quantum dot (Qdot) lasers and optical amplifiers, can serve in the future energy saving information and communication technologies (ICT) as the work-horse for transmitting and amplifying information through optical fiber. The encouraging results in the zero-dimensional (0D) structures emitting at 980 nm, in the form of vertical cavity surface emitting laser (VCSEL), are already operational at low threshold current density and capable of 40 Gbps error-free transmission at 108 fJ/bit. Subsequent achievements for lasers and amplifiers operating in the O-, C-, L-, U-bands, and beyond will eventually lay the foundation for green ICT. On the hand, the inhomogeneously grown quasi 0D quantum dash (Qdash) lasers are brilliant solutions for potential broadband connectivity in server farms or access network. A single broadband Qdash laser operating in the stimulated emission mode can replace tens of discrete narrow-band lasers in dense wavelength division multiplexing (DWDM) transmission thereby further saving energy, cost and footprint. We herein reviewed the1 progress of both Qdots and Qdash devices, based on the InAs/InGaAlAs/InP and InAs/InGaAsP/InP material systems, from the angles of growth and device performance. In particular, we discussed the progress in lasers, semiconductor optical amplifiers (SOA), mode locked lasers, and superluminescent diodes, which are the building blocks of EPIC and ICT. Alternatively, these optical sources are potential candidates for other multi-disciplinary field applications.     

Inelastic light scattering from electronic excitations in quantum dots

A review is given of progress in our understanding of the electronic excitations in semiconductor quantum dots as studied by inelastic light scattering spectroscopy. Such experiments have revealed the characteristics of single particle, charge density, and spin density excitations of many-electron dots in zero and applied magnetic fields. Theoretical calculations, which reveal an electronic shell structure, are in general accord with the experimental results. Although the behaviour in a magnetic field is extremely complex, it is now feasible to study collective excitations from a range of strongly correlated ground states using this technique.     

Quadratic electro-optic effects and electro-absorption process in InGaN/GaN cylinder quantum dots

We carried a detailed calculation of quadratic electro-optic effects (QEOE) and electro-absorption (EA) process as a function of pump photon energy ?ω in InGaN/GaN cylinder quantum dots. The third-order susceptibility dispersion behaviors of direct current are obtained. It is found that with the increase of the quantum dot (QD) height and radius, the magnitudes of the real part of the quadratic electro-optic susceptibility and the imaginary part of the EA susceptibility increase at the resonant frequency, and its resonant position shifts to the lower energy region. In addition, lower In content induces larger χ⁽³⁾.     

Optical properties of wurtzite GaN/AlN quantum dots grown on non-polar planes: The effect of stacking faults in the reduction of the internal electric field

The optical emission of non-polar GaN/AlN quantum dots has been investigated. The presence of stacking faults inside these quantum dots is evidenced in the dependence of the photoluminescence with temperature and excitation power. A theoretical model for the electronic structure and optical properties of non-polar quantum dots, taking into account their realistic shapes, is presented which predicts a substantial reduction of the internal electric field but a persisting quantum confined Stark effect, comparable to that of polar GaN/AlN quantum dots. Modeling the effect of a 3 monolayer stacking fault inside the quantum dot, which acts as zinc-blende inclusion into the wurtzite matrix, results in an additional 30% reduction of the internal electric field and gives a better account of the observed optical features.     

Optical characterisation of MOVPE grown vertically correlated InAs/GaAs quantum dots

Structures with vertically correlated self-organised InAs quantum dots (QDs) in a GaAs matrix were grown by the low-pressure metal-organic vapour phase epitaxy (MOVPE) and characterised by different microscopic techniques. Photoluminescence in combination with photomodulated reflectance spectroscopy were applied for characterisation of QDs structures. We show that combination of both methods allows detecting optical transitions originating both from QDs and wetting (separation) layers, which can be than compared with those obtained from numerical simulations. On the basis of obtained results, we demonstrate that photoreflectance spectroscopy is an excellent tool for characterisation of QDs structures wetting layers and for identification of spacer thicknesses in vertically stacked QDs structures.     

Self-organized InAs/GaAs quantum dots multilayers with growth interruption emitting at 1.3 μm

We have grown single, 10 and 20 InAs/GaAs quantum dots (QDs) multilayers by molecular beam epitaxy in Stranski-Krastanov growth mode with and without growth interruption. Multilayer structures of InAs QDs have been studied by photoluminescence (PL) and atomic force microscopy (AFM) techniques. Between 1 and 10 layers of QDs, 10 K PL shows a shift energy, and a PL linewidth reduction. Moreover, AFM image of the 10 layers sample shows that the InAs QDs size remains constant and almost uniform when the growth is without interruption. These effects are attributed to electronic coupling between QDs in the the columns. However, we show the possibility of extending the spectral range of luminescence due to InAs QDs up to 1.3 μm. Realisation of such a wavelength emission is related to formation of lateral associations or coupling of QDs (LAQDs or LCQDs) during InAs deposition when growth interruption (20 s) is used after each InAs QDs layer deposition. The growth interruption applied after the deposition of the InAs layer allows the formation of well-developed InAs dots (large dot size).     

PAMBE growth of (1 1 2¯ 2)-oriented GaN/AlN nanostructures on m-sapphire

We report on the plasma-assisted molecular-beam epitaxial growth of (1 1 2¯ 2)-oriented GaN/AlN nanostructures on (1 1¯ 0 0) m-plane sapphire. Moderate N-rich conditions enable to synthesize AlN(1 1  2) directly on m-sapphire, with in-plane epitaxial relationships [1 1 2¯ 3¯]_AlN∥[0 0 0 1]_sapphire and [1  0 0]_AlN∥[1 1 2¯ 0]_sapphire. In the case of GaN, a Ga-excess of one monolayer is necessary to achieve two-dimensional growth of GaN(1 1 2¯ 2). Applying these growth conditions, we demonstrate the synthesis of (1 1 2¯ 2)-oriented GaN/AlN quantum well structures, showing a strong reduction of the internal electric field. By interrupting the growth under vacuum after the deposition of few monolayers of GaN under slightly Ga-rich conditions, we also demonstrate the feasibility of quantum dot structures with this orientation.     

Green synthesis of yellow emitting PMMA–CdSe/ZnS quantum dots nanophosphors

We herein report the fabrication of highly fluorescent yellow emitting nanophosphors using CdSe/ZnS quantum dots (QDs) dispersed in polymethyl methacrylate (PMMA). The QDs were synthesised via a simple, non-phosphine and one pot synthetic method in the absence of an inert atmosphere. The as-prepared nanocrystallites were characterised by Fourier transform infrared spectroscopy (FTIR), ultraviolet–visible (UV–vis) and photoluminescence spectroscopy, energy-dispersive spectroscopy (EDS), Raman spectroscopy, transmission electron microscopy (TEM) and high resolution TEM (HRTEM) microscopy. Optical analysis confirmed that the as-synthesised CdSe/ZnS QDs were of high quality with sharp absorption peaks, bright luminescence, narrow emission width and high PL quantum yield (up to 74%). The electron microscope images showed that the QDs are small and spherical in shape with narrow size distributions while the HRTEM micrograph confirmed the high crystallinity of the material. The Raman analysis of the QDs revealed the formation of core–shell structure and the energy dispersive spectroscopy confirmed the presence of the corresponding elements (i.e., Cd, Se, Zn and S). The dispersion of the core–shell QDs in PMMA matrix led to the red-shifting of the emission position from 393 nm in the neat PMMA to 592 nm in the nanocomposite. The fabricated highly fluorescent yellow emitting PMMA–CdSe/ZnS core–shell QDs polymer nanocomposite film display excellent optical properties without loss of luminescence. Furthermore, the as-synthesised organic soluble CdSe/ZnS QDs were successfully converted into highly water soluble QDs after ligand exchange with mercaptoundecanoic acid (MUA) without the loss of their emission properties. The simplicity of the method and the quality of the as-synthesised nanocomposite make it a promising material for the large scale fabrication of diverse optical devices.