Semiconductor Quantum Dot Arrays Research Articles

Electrical and Photo-Induced Effects in Graphene Channels When Interfaced with Quantum Dots

ABSTRACTField effect transistors with graphene channels were interfaced with arrays of semiconductor quantum dots (QD). The electrical characteristics of the elements were assessed. The channel response to white light illumination was also assessed as a function of drain-source and gate-source biases.

In‐plane coupling effect on absorption coefficients of InAs/GaAs quantum dots arrays for intermediate band solar cell

AbstractCoupled semiconductor quantum dot (QD) arrays emerged recently as promising structures for the next generation of high efficiency intermediate band solar cell (IBSC), because of their ability to facilitate the formation of minibands. The quantum coupling effect that exists between states in QDs in an array influences the electronic and optical properties of such structures. So far, great experimental and theoretical efforts have been devoted to study the vertically coupled QD arrays. We present here a method based on multi‐band k ⋅ p Hamiltonian combined with periodic boundary conditions, applied to predict the electronic and optical properties of InAs/GaAs QDs‐based lateral QD arrays. Formation of the intermediate band (IB) in all cases was achieved via delocalisation of the electron ground state (e0). We show that the IB in a laterally coupled QD‐IBSC is more robust against external electric field from the solar cell's pn junction than that in a vertically coupled arrangement. Because of symmetry of the QD array lattice and QD states itself, which are C2v for the zinc blend QDs, the electronic and absorption structures were obtained via sampling throughout the reciprocal space in the first Brillouin zone of QD arrays. Copyright © 2014 John Wiley & Sons, Ltd.

Electron microscopy of GaAs-based structures with InAs and As quantum dots separated by an AlAs barrier

Electron microscopy studies of GaAs-based structures grown by molecular beam epitaxy and containing arrays of semiconductor InAs quantum dots and metal As quantum dots are performed. The array of InAs quantum dots is formed by the Stranski-Krastanov mechanism and consists of vertically coupled pairs of quantum dots separated by a GaAs spacer 10 nm thick. To separate the arrays of semiconductor and metal quantum dots and to prevent diffusion-induced mixing, the array of InAs quantum dots is overgrown with an AlAs barrier layer 5 or 10 nm thick, after which a GaAs layer is grown at a comparatively low temperature (180°C). The array of As quantum dots is formed in an As-enriched layer of the low-temperature GaAs by means of post-growth annealing at 400–760°C for 15 min. It is established that the AlAs barrier layer has a surface profile corresponding to that of a subbarrier layer with InAs quantum dots. The presence of such a profile causes the formation of V-shaped structural defects upon subsequent overgrowth with the GaAs layer. Besides, it was obtained that AlAs layer is thinned over the InAs quantum dots tops. It is shown that the AlAs barrier layer in the regions between the InAs quantum dots effectively prevents the starting diffusion of excess As at annealing temperatures up to 600°C. However, the concentration of mechanical stresses and the reduced thickness of the AlAs barrier layer near the tops of the InAs quantum dots lead to local barrier breakthroughs and the diffusion of As quantum dots into the region of coupled pairs of InAs quantum dots at higher annealing temperatures.

Phase-field simulations of GaN/InGaN quantum dot growth by selective area epitaxy

Arrays of semiconductor quantum dots grown by selective area epitaxy, a process in which the size and position of the dots is determined by a lithographically patterned mask, can have a high degree of uniformity in both size and position. However, non-uniformity in the initial stages of growth causes broadening of the energy states of GaN/InGaN heterostructures grown using this technique, limiting their practical utility for device applications. A phase-field model was developed to simulate selective area epitaxy, accounting for a crystallographic-orientation-dependent deposition rate. Model parameters were varied to optimize the uniformity of the InGaN active layers. Conditions that led to the most uniform active layers included low total deposition rate, high surface diffusivity, low deposition of surface adatoms from the mask, and smaller contact angle at the mask–vapor–quantum dot interface. Other factors that improved uniformity were growth on (0001) substrates, which is the fastest growth direction, and more vertical orientation of the sidewalls of the mask holes.

Photoluminescence enhancement and quenching in metal-semiconductor quantum dot hybrid arrays

Hybrid monolayer arrays of metal and semiconductor quantum dots have been prepared to study the exciton-plasmon interaction. We observed crossover from strong quenching to enhancement in photoluminescence of the quantum dots as a function of the emission wavelength for fixed interparticle spacings. Remarkably, the enhancement is observed even for extremely short separation at which strong quenching has been observed and predicted earlier. A significant redshift in emission maxima is also observed for quantum dots with quenched emission. The possible role of collective phenomena as well as strong interactions in such ordered hybrid arrays in controlling the emission is discussed.

Semiconductor Quantum Dots and Quantum Dot Arrays and Applications of Multiple Exciton Generation to Third-Generation Photovoltaic Solar Cells

ADVERTISEMENT RETURN TO ISSUEPREVReviewNEXTSemiconductor Quantum Dots and Quantum Dot Arrays and Applications of Multiple Exciton Generation to Third-Generation Photovoltaic Solar CellsA. J. Nozik*†‡, M. C. Beard†, J. M. Luther†, M. Law§, R. J. Ellingson∥, and J. C. Johnson†View Author Information The National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado 80401, United States, Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, United States, Department of Chemistry, University of California, Irvine, Irvine, California 92697, United States, and Department of Physics and Astronomy, University of Toledo, Toledo, Ohio 43606, United States* To whom correspondence should be addressed. E-mail: [email protected]†The National Renewable Energy Laboratory.‡University of Colorado.§University of California, Irvine.∥University of Toledo.Cite this: Chem. Rev. 2010, 110, 11, 6873–6890Publication Date (Web):October 14, 2010Publication History Received25 August 2009Published online14 October 2010Published inissue 10 November 2010https://doi.org/10.1021/cr900289fCopyright © 2010 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views21423Altmetric-Citations1012LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit Read OnlinePDF (2 MB) Get e-AlertsSUBJECTS:Excitons,Quantum dots,Semiconductors,Solar cells,Solar energy Get e-Alerts

Photoluminescence spectroscopy and lifetime measurements from self-assembled semiconductor-metal nanoparticle hybrid arrays

We present results of photoluminescence spectroscopy and lifetime measurements on thin film hybrid arrays of semiconductor quantum dots and metal nanoparticles embedded in a block copolymer template. The intensity of emission as well as the measured lifetime would be controlled by varying the volume fraction and location of gold nanoparticles in the matrix. We demonstrate the ability to both enhance and quench the luminescence in the hybrids as compared to the quantum dot array films while simultaneously engineering large reduction in luminescence lifetime with incorporation of gold nanoparticles.

Quantum simulation of Fermi-Hubbard models in semiconductor quantum-dot arrays

We propose a device for studying the Fermi-Hubbard model with long-range Coulomb interactions using an array of coupled quantum dots defined in a semiconductor two-dimensional electron-gas system. Bands above the lowest energy band are used to form the Hubbard model, so that a high average electron density may be used to implement the device. We find that depending on the average electron density, the system is well described by a one- or two-band Hubbard model. Our device design enables the control of the ratio of the Coulomb interaction to the kinetic energy of the electrons independently to the filling of the quantum dots, such that a large portion of the Hubbard phase diagram may be probed. Estimates of the Hubbard parameters suggest that a metal-Mott insulator quantum phase transition and a $d$-wave superconducting phase should be observable using current technologies.

Electron trapping in a one-dimensional semiconductor quantum wire with multiple impurities

We demonstrate the trapping of a conduction electron between two identical adatom impurities in a one-dimensional semiconductor quantum-dot array system (quantum wire). Bound steady states arise even when the energy of the adatom impurity is located in the continuous one-dimensional energy miniband. The steady state is a realization of the bound state in continuum (BIC) phenomenon first proposed by von Neuman and Wigner [Phys. Z. 30, 465 (1929)]. We analytically solve the dispersion equation for this localized state, which enables us to reveal the mechanism of the BIC. The appearance of the BIC state is attributed to the quantum interference between the impurities. The Van Hove singularity causes another type of bound state to form above and below the band edges, which may coexist with the BIC.

Self-assembly and optical properties of patterned ZnO nanodot arrays

Patterned ZnO nanodot (ND) arrays and a ND-cavity microstructure were realized on ananodic alumina membrane (AAM) surface through a spin-coating sol–gel process, whichbenefits from the morphology and localized negative charge surface of AAM as well as theoptimized sol concentration. The growth mechanism is believed to be a self-assemblyprocess. This provides a simple approach to fabricate semiconductor quantumdot (QD) arrays and a QD-cavity system with its advantage in low cost andmass production. Strong ultra-violet emission, a multi-phonon process, and itsspecial structure-related properties were observed in the patterned ZnO ND arrays.

Assembly and Analysis of Ordered Semiconductor Quantum Dot Arrays by Focused Ion Beam Nanofabrication and Tomography

Assembly and Analysis of Ordered Semiconductor Quantum Dot Arrays by Focused Ion Beam Nanofabrication and Tomography

Today's challenges in quantum dot materials research for tomorrow's quantum functional devices

AbstractSize, shape, position control, and self‐organized lateral ordering of epitaxial semiconductor quantum dot (QD) arrays are demonstrated. This constitutes the prerequisite for the ultimate control of the electronic and optical properties of man‐made semiconductor heterostructures at the single and multiple charge, spin, and photon level, including their quantum mechanical and electromagnetic interactions in view of applications such as quantum information processing and computing. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Self-organization of Pb islands on Si(111) caused by quantum size effects

Growth of metallic Pb islands on Si(111) by vacuum deposition was studied in real time using synchrotron x-ray diffraction. The islands coarsen and order, maintaining a nearly uniform interisland distance but without angular correlation. The resulting interisland structure is akin to a two-dimensional liquid. Over a wide temperature range, the interisland ordering is well correlated with the development of “magic” island heights caused by energy minimization of the Pb electrons. The results demonstrate quantum confinement effects as a driving force for self-organization, as opposed to strain effects that generally govern the formation of semiconductor quantum dot arrays.

Fluctuation X‐ray microscopy: a novel approach for the structural study of disordered materials

Measuring medium-range order is a challenging and important problem in the structural study of disordered materials. We have developed a new technique, fluctuation x-ray microscopy, that offers quantitative insight into medium-range correlations in disordered materials at nanometre and larger length scales. In this technique, which requires a spatially coherent x-ray beam, a series of speckle patterns are measured at a large number of locations in a sample using various illumination sizes. Examination of the speckle variance as a function of the illumination spot size allows the structural correlation length to be measured. To demonstrate this technique we have studied polystyrene latex spheres, which serve as a model for a dense random-packed glass, and for the first time have measured the correlation length in a disordered system by fluctuation X-ray microscopy. We discuss data analysis and procedures to correct for shot noise and detector noise. This approach could be used to explore medium-range order and subtle spatial structural changes in a wide range of disordered materials, from soft matter to nanowire arrays, semiconductor quantum dot arrays and magnetic materials.

Optical transmission and waveguiding by excitonic quantum dot lattices

Metal-dielectric-metal configurations of optical waveguides have a very high laterally packaging density at the cost of high optical loss. Photonic crystals based on refractive-index-modulation materials have been used in optics, e.g., two materials having different refractive indices form a well-defined Bragg refraction mirror. Such a waveguide has lower loss but also lower packaging density. From the outset of these two notions, we propose a photonic-crystal device based on the exciton-polariton effect in a three-dimensional array of semiconductor quantum dots (QDs) for ultradense optical planar circuit applications. Excitons are first photogenerated in the QDs by the incident electromagnetic field, the exciton-polariton effect in the QD photonic crystal then induces an extra optical dispersion in QDs. The high contrast ratio between the optical dispersions of the QDs and the background therefore creates clear photonic bandgaps. By carefully designing the QD size and the QD lattice structure, perfect electromagnetic field reflection can be obtained at a specific wavelength in the lossless case, thus providing the fundamental basis for ultradense optical waveguide applications.

Complete band gaps in three-dimensional quantum dot photonic crystals

Because of the generation of polaritons, which are quasiparticles possessing the characteristics of both photonics and electronics, active photonic materials offer a possible solution to transfer electromagnetic energy below the diffraction limit and further increase the density of photonic integrated circuits. A theoretical investigation of these exciting materials is, therefore, very important for practical applications. Four different kinds of polaritons have been studied in this thesis, (1) surface polaritons of negative-index-material cylindric rods, (2) exciton polaritons of semiconductor quantum dots, (3) localized plasmon polaritons of metallic nanoshells, and (4) surface plasmon polaritons of subwavelength hole arrays in thin metal films. All these types of polaritons were found to strongly affect the optical properties of the studied active photonic materials. More specifically, (1) for two-dimensional photonic crystals composed of negative-index-material cylindric rods, the coupling among surface polaritons localized in the rods results in dispersionless anti-crossing bands; (2) for three-dimensional diamond-lattice quantum-dot photonic crystals, the exciton polariton resonances lead to the formation of complete band gaps in the dispersion relationships; (3) for metallic nanoshells, the thickness of the metal shell strongly modifies the localized plasmon polaritons, and therefore influences the degree of localization of the electromagnetic field inside the metallic nanoshells; (4) for subwavelength hole arrays in thin metal films, high-order surface-polariton Bloch waves contribute significantly to the efficient transmission. To numerically simulate these active photonic materials, we introduced three approaches, (1) an extended plane-wave-based transfer-matrix approach for negative- index-material media, (2) a plane-wave method for semiconductor quantum-dot photonic crystals, and (3) an auxiliary-differential-equation finite-difference time- domain approach for semiconductor quantum-dot arrays. A brief perspective is also given at the end of this thesis.

Generalized pair approximation in the description of the ac conductivity of a dense disordered array of quantum dots

AbstractThe frequency dependence of the hopping conductivity of dense arrays of semiconductor quantum dots of size $ \bar a $ substantially exceeding the edge‐to‐edge separation $ \bar w $ between the neighboring quantum dots is considered. It is shown that at low frequencies in a wide frequency range the conductivity obeys a fractional power law and its magnitude depends on the structural characteristics of the material. The possibility of the deviations from universality related to the material structure is discussed. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Spin and charge polarization in quantum dot arrays

AbstractThe role of spin and charge distribution in a planar semiconductor quantum dot array (square geometry) with two electrons is studied in the presence of a driver cell and magnetic nanoparticles located near two quantum dots providing a local magnetic field. We use an extended Hubbard model to describe the electrons in the cell, taking into account intra and intercell Coulomb interaction, intracell tunneling, Rashba spin‐orbit interaction while hopping, as well as the local magnetic interaction with the magnetic nanoparticles. We find that under physically suitable conditions, controlled by the driver, spin and charge cell polarization (spin‐switching effect) can be attained, which is fully tunable by an electrical mechanism. This effect can be useful for spintronics and/or quantum computation applications. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Monte-Carlo simulations of photoinduced fluorescence enhancement in semiconductor quantum dot arrays

Photoinduced fluorescence enhancement (PFE) in semiconductor quantum dot (QD) arrays is simulated by a Monte-Carlo method based on the distributed tunneling model. PFE, a property of a QD ensemble, is directly related to the blinking behavior of single QDs. The origin of PFE is attributed not to an increase in the emission intensity during the ‘on’ period, but to the prolongation of average ‘on’ time.

Large nonlinear optical properties of semiconductor quantum dot arrays embedded in an organic medium

We show that an array of semiconductor quantum dots in an organic host leads to a large two-photon absorption. The optical nonlinearity depends on the semiconductor, the dot size, and the dot-to-dot spacing. Using numerical simulations, we demonstrate that a large optical limiting is possible using thin films of this hybrid material.