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Ko, D. Y. K. "Quantum tunnelling in heterostructures." Thesis, University of Exeter, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.384673.
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Martin, Robert W. "Quantum magnetotransport in strained layer heterostructures." Thesis, University of Oxford, 1991. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.315751.
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Dhillon, S. S. "Terahertz intersubband electroluminescence from quantum cascade heterostructures." Thesis, University of Cambridge, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.598519.
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Mid-infrared quantum cascade lasers (QCLs) have been extensively developed since their realisation in 1994, with a spectral range covered from 3.4μm (88THz) to 24μm (12.5THz). This is a direct result of advances in molecular beam epitaxy and band-structure engineering. QCLs are fabricated from multi-quantum well semiconductor heterostructures in which an appropriate engineering of the thickness and composition of the semiconductor layers adjusts the intersubband transition energies, offering considerable design flexibility of the band profile. By application of a suitable electric field and stacking together successive active regions, each injected electron cascades through the device, generating a number of photons. QCLs have shown considerable advances in performance with high powers and room temperature operation demonstrated. Extension of this quantum cascade scheme to the far-infrared, or terahertz (THz) range, is now being investigated, where the lack of sources remains acute. Specifically, operation is sought at energies smaller than the characteristic LO phonon energy of the semiconductor material, where currently no lasing has been shown (<36meV, 9 THz). The dynamics of this spectral range, however, are considerably different to those in the mid-infrared. LO phonon emission is effectively forbidden for subband spacings less than the phonon energy but increases in electron-electron scattering are expected to dominate. Although THz electroluminescence has been shown from cascade structures, systematic investigations into key parameters have not been reported. This dissertation reports a comprehensive study of THz electroluminescence from n-type A1GaAs/GaAs quantum cascade emitters as a basis for understanding the radiative and scattering mechanisms that occur in this spectral range, forming the foundations of the development of a THz semiconductor laser. The electroluminescence was correlated thoroughly with band structure calculations, along with the structural and electrical properties of the samples. The many features observed in the far-infrared were characterised, with the intersubband peaks investigated extensively to confirm their origin.
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Smith, Ainsley H. "Quantum confined states in cylindrical nanowire heterostructures." DigitalCommons@Robert W. Woodruff Library, Atlanta University Center, 2007. http://digitalcommons.auctr.edu/dissertations/2364.
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We present an investigation of quantum confinement effects in nanowire heterostructures through the use of an effective-mass model with a band-offset induced potential barrier.
The characteristic size of microelectronics is rapidly approaching the nanometer scale and because of this, nanostructure based devices in the field of nanomaterial research is continually being emphasized. The quantum confinement effect exhibited by the nanowire is the most interesting in one-dimensional nanostructures. Potential applications for the nanowire include its use in the fabrication of high performance devices such as the p — n junction diode, the p-channel or n-channel coaxial gated field effect transistor, and the complimentary field effect transistors, to name a few.
In the fabrication of such devices, a doping process is used in order to supply free carriers. This process involves introducing doped impurities which unfortunately causes difficulties. These difficulties are characterized by a marked decrease in the mobility of the aforementioned carriers and include the scattering of the free carriers. To remedy these problems a novel doping mechanism has been proposed. It involves the use of a radial heterojunction in a core-shell nanowire where it has been suggested that one can dope impurities in the shell and inject free carriers to the core or vice versa. This separation of free carriers reduces their scattering rate and improves their mobility, both preferred properties for high-speed devices.
A better understanding of the heterojunction under strong cylindrical confinement is important to guide the future fabrication of nanowire-based high-speed devices. In order to achieve this, the question as to whether the band offset evolves with the size of the nanowire needs to be addressed.
The inquiry into the relationship between band offset evolution and nanowire size led us to employ an effective-mass model with a band-offset induced potential barrier to study the band structure of carriers in cylindrical core-shell and core-multishell nanowires. Quantum confined states and band alignment effects are found to be dependent upon the height of the potential barrier, the core-shell radius ratio, and the diameter of the quantum wire.
The subband charge densities are studied for clarifying the quantum confinement. By numerically solving the effective-mass model we were able to provide an interpretation of experimental observations on carrier accumulation and one-dimensional ballistic transport in Ge-Si core-shell nanowire heterostructures. The model serves as the continuum limit to the first-principles simulation approach.
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Torresani, Patrick. "Hole quantum spintronics in strained germanium heterostructures." Thesis, Université Grenoble Alpes (ComUE), 2017. http://www.theses.fr/2017GREAY040/document.
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Le travail exposé dans cette thèse de doctorat présente des expériences à basse température dans le domaine de la spintronique quantique sur des hétérostructures à base de germanium. Tout d’abord, les avantages attendus du germaniumpour la spintronique quantique sont exposés, en particulier la faible interaction hyperfine et le fort couplage spin-orbite théoriquement prédits dans le Ge. Dans un second chapitre, la théorie des boites quantiques et systèmes à double boite sont détaillés, en se focalisant sur les concepts nécessaires à la compréhension des expériences décrites plus tard, c’est-à-dire les effets de charge dans les boites quantiques et double boites, ainsi que le blocage de spin de Pauli. Le troisième chapitre s’intéresse à l’interaction spin-orbite. Son origine ainsi que ses effets sur les diagrammes d’énergie de bande sont discutés. Ce chapitre se concentre ensuite sur les conséquences de l’interaction spin-orbite spécifiques aux gaz bidimensionnels de trous dans des hétérostructures de germanium, c’est-à-dire l’interaction spin-orbite Rashba, le mécanisme de relaxation de spin D’Yakonov-Perel ainsi que l’antilocalisation faible.Le chapitre quatre présente des mesures effectuées sur des nanofils coeur coquillede Ge/Si. Dans ces nanofils une boite quantique se forme naturellement et celui-ci est étudié. Un système à double boite quantiques est ensuite formé par utilisation de grilles électrostatiques, révélant ainsi du blocage de spin de Pauli.Dans le cinquième chapitre sont détaillés des mesures demagneto-conductance de gas de trous bidimensionnels dans des hétérostructures de Ge/SiGe contraints dont le puit quantique se situe à la surface. Ces mesuresmontrent de l’antilocalisation faible. Les temps de transport caractéristiques sont extraits ainsi que l’énergie de séparation des trous 2D par ajustement de courbe de la correction à la conductivité due à l’antilocalisation. De plus, les mesures montrent une suppression de l’antilocalisation par un champ magnétique parallèle au puit quantique. Cet effet est attribué à la rugosité de surface ainsi qu’à l’occupation virtuelle de sous-bandes inoccupées.Finalement, le chapitre six présente des mesures de quantisation de la conductancedans des hétérostructures de Ge/SiGe contraints dont le puit quantique est enterré. Tout d’abord, l’hétérostructure est caractérisée grâce à des mesures de magneto-conductance dans une barre de Hall. Ensuite, un second échantillon dessiné spécialement pour la réalisation de points de contact quantiques est mesuré. Celui-ci montre des marches de conductance. La dépendance en champ magnétique de ces marches est mesurée, permettant ainsi une extraction du facteur gyromagnétique de trous lourds dans du germaniumThis thesis focuses on low temperature experiments in germaniumbased heterostructure in the scope of quantumspintronic. First, theoretical advantages of Ge for quantum spintronic are detailed, specifically the low hyperfine interaction and strong spin orbit coupling expected in Ge. In a second chapter, the theory behind quantum dots and double dots systems is explained, focusing on the aspects necessary to understand the experiments described thereafter, that is to say charging effects in quantum dots and double dots and Pauli spin blockade. The third chapter focuses on spin orbit interaction. Its origin and its effect on energy band diagrams are detailed. This chapter then focuses on consequences of the spin orbit interaction specific to two dimensional germaniumheterostructure, that is to say Rashba spin orbit interaction, D’Yakonov Perel spin relaxation mechanism and weak antilocalization.In the fourth chapter are depicted experiments in Ge/Si core shell nanowires. In these nanowire, a quantumdot formnaturally due to contact Schottky barriers and is studied. By the use of electrostatic gates, a double dot system is formed and Pauli spin blockade is revealed.The fifth chapter reports magneto-transport measurements of a two-dimensional holegas in a strained Ge/SiGe heterostructure with the quantum well laying at the surface, revealing weak antilocalization. By fitting quantumcorrection to magneto-conductivity characteristic transport times and spin splitting energy of 2D holes are extracted. Additionally, suppression of weak antilocalization by amagnetic field parallel to the quantum well is reported and this effect is attributed to surface roughness and virtual occupation of unoccupied subbands.Finally, chapter number six reportsmeasurements of quantization of conductance in strained Ge/SiGe heterostructure with a buried quantumwell. First the heterostructure is characterized by means ofmagneto-conductance measurements in a Hall bar device. Then another device engineered specifically as a quantum point contact is measured and displays steps of conductance. Magnetic field dependance of these steps is measured and an estimation of the g-factor for heavy holes in germanium is extracted
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Harrison, Paul Anthony. "Resonant tunnelling and luminescence in coupled quantum wells." Thesis, University of Nottingham, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.363933.
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Wee, Siew Fong. "Interdiffusion of semiconductor alloy heterostructures." Thesis, University of Surrey, 1998. http://epubs.surrey.ac.uk/844156/.
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This thesis is concerned with a quantitative study of intermixing in GaAs/AlGaAs and ZnSe/ZnCdSe single quantum well semiconductor structures. In this study, a method of iterative isothermal anneals and photoluminescence used to characterize this phenomenon has enabled the evolution of the diffusion coefficients for the interdiffusion process with anneal time to be followed. The blue-shift emissions arising from this method are predicted by a model based on Fick's law of diffusion. This model is developed in an attempt to relate the energy shift that is observed experimentally to the diffusion length. The mixing is modelled using an error function expression to solve the diffusion equation so as to describe the variation in well shape which is attributed to compositional disordering induced during thermal processing. Using this approach, where evidence of intermixing was monitored, the emission would be expected to shift measurably. Data has been taken to cover a wide temperature range to establish values for the activation energy EA. From this data, it has been found that the diffusion coefficients at various temperatures are thermally activated with an energy of 3.6 +/- 0.2 eV in GaAs/AlGaAs. The data is compared with the available literature data taken under a wide range of experimental conditions. We show that despite the range of activation energies quoted in the literature all the data appears to be consistent with a single activation energy. Departures from the 'mean' value are ascribed to experimental uncertainties in determining the diffusion coefficients for example, to fluctuations in the composition of the material, to techniques used, or to a wide range of perturbations. Photoluminescence observations on ZnSe/ZnCdSe show that an improvement in the optical quality of these quantum well structures was found for anneals at temperatures (~500°C). A value of EA = 2.9 +/- 0.3 eV was derived from the experiments for the interdiffusion process over a 250 K temperature range and four decades of interdiffusion coefficient. The interdiffusion process of both these systems was inferred to be Fickian with no dependence on alloy composition or strain.
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Chung, Sung-Yong. "Si/SiGe heterostructures materials, physics, quantum functional devices and their integration with heterostructure bipolar transistors /." Columbus, Ohio : Ohio State University, 2005. http://rave.ohiolink.edu/etdc/view?acc%5Fnum=osu1132244278.
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Malins, David Brendan. "Ultrafast dynamics in InAs quantum dot and GaInNAs quantum well semiconductor heterostructures /." St Andrews, 2007. http://hdl.handle.net/10023/404.
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Malins, David B. "Ultrafast dynamics in InAs quantum dot and GaInNAs quantum well semiconductor heterostructures." Thesis, University of St Andrews, 2008. http://hdl.handle.net/10023/404.
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The quantum confined Stark effect (QCSE) and ultrafast absorption dynamics near the bandedge have been investigated in p-i-n waveguides comprising quantum confined heterostructures grown on GaAs substrates, for emission at 1.3um. The materials are; isolated InAs/InGaAs dot-in-a-well (DWELL) quantum dots (QD), bilayer InAs quantum dots and GaInNAs multiple quantum wells (MQW). The focus was to investigate these dynamics in a planar waveguide geometry, for the purpose of large scale integration in optical systems. Initial measurements of the QCSE using photocurrent measurements showed a small shift for isolated QDs whilst a significant shift of 40nm (at 1340nm) was demonstrated for bilayer dots, comparable to that of GaInNAs MWQ (30nm at 1300nm). These are comparable to InP based quaternary multiple quantum wells used in modulator devices. With the use of a broadband continuum source the isolated quantum dots exhibit both a small QCSE (15nm at 1280nm) and minimal broadening which is desirable for saturable absorbers used in monolithic modelocked semiconductor lasers (MMSL). A robust experimental set-up was developed for characterising waveguide modulators whilst the electroabsorption and electro-refraction was calculated (dn=1.5x10⠻³) using the Kramers-Kronig dispersion relation. Pump probe measurements were performed at room temperature using 250fs pulses from an optical parametric oscillator (OPO) on the three waveguide samples. For the isolated QDs ultrafast absorption recovery was recorded from 62ps (0V) to 700fs (-10V and the shortest times shown to be due to tunneling. Additionally we have shown good agreement of the temperature dependence of these dots and the pulse width durations from a modelocked semiconductor laser using the same material. Bilayer QDs are shown to exhibit ultrafast absorption recovery from 119ps (0V) to 5ps (-10V) offering potential for applications as modelocking elements. The GaInNAs multiple quantum wells show absorption recovery of 55ps (0V), however under applied reverse bias they exhibit long lived field screening transients. These results are explained qualitatively by the spatial separation of electrons and holes at heterobarrier interfaces.
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Denton, Graham John. "Coherent optical transient effects in semiconductor quantum wells." Thesis, University of Cambridge, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.320979.
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Haysom, Joan E. "Quantum well intermixing of InGaAs(P)/InP heterostructures." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 2001. http://www.collectionscanada.ca/obj/s4/f2/dsk3/ftp04/NQ66152.pdf.
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Suckling, Andrew. "The fractional quantum hall effect in semiconductor heterostructures." Thesis, University of Oxford, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.258168.
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Freyland, Jan Moritz. "Optical studies of V-groove quantum wires." Thesis, University of Oxford, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.360256.
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Jaszek, Ryszard. "Electrical properties of modulation-doped InAs quantum-well heterostructures." Thesis, Imperial College London, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.286504.
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Lutti, Julie. "Optical properties of InP/AlGaInP quantum dot laser heterostructures." Thesis, Cardiff University, 2005. http://orca.cf.ac.uk/56016/.
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Optical characterisation of InP/AlGaInP quantum dot laser structures, involving laser threshold and wavelength, optical modal gain and absorption spectra, and radiative efficiency are presented. The samples were grown by MOVPE in Sheffield University on (100) 10 off and (211)B GaAs substrates, and consist of 5 layers of self-assembled InP dots, with each layer grown on Alo.3Gao.7InP and placed in a GaInP quantum well. A record low threshold current density of 290A/cm2 at a wavelength of 740nm for a 1.6 mm-long device with uncoated facets is obtained from one of the sample grown on (100) 10 off substrate at 690 C. This sample has an internal optical mode loss of 4 1 cm"1 and an internal quantum spontaneous emission efficiency of 30% for current densities corresponding to the threshold. The ground state modal gain is shown to saturate at 17cm"1 at room temperature, which is about lA of the full population inversion limit, and the saturation level increases with decreasing temperature. A sample grown at a lower temperature of 650 C has higher optical mode loss (7.5 1 cm"1) and quantum efficiency under 15%. A sample grown on (211)B substrate exhibits optical transitions at higher photon energies, consistent with smaller dot sizes. The thesis also presents an analysis of the segmented contact technique used to measure gain and absorption spectra, determining criteria for the excitation and detection geometries required to ensure accurate measurement. It is shown that the collection angle and device nearfield should be limited so that rays that intercept the stripe edges are not collected. If this is not satisfied, the measurement underestimates the modal gain. An exponential variation of the measured ASE upon stripe length cannot be taken as evidence for a correct collection geometry.
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Smith, Colin. "The theory of Auger recombination in quantum well heterostructures." Thesis, Durham University, 1985. http://etheses.dur.ac.uk/6805/.
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This thesis is concerned with calculations of the Auger recombination rate in direct gap semiconductors. It is composed of two parts: in the first and major part, the calculation of the CHCC Auger recombination process in a model of a quantum well heterostructure is considered; and in the second part, the overlap integrals between the cell periodic parts of the conduction band and heavy hole band Bloch functions are calculated using a 15-band full zone empirical K.p method. These overlap integrals are important factors in determining the Auger rate involving the recombination of electrons with heavy holes. The calculation of the quantum well CHCC Auger recombination rate differs from the bulk CHCC Auger calculations because carriers trapped in quantum wells reside within sub-bands associated with different bound states of the wells. The quantum well CHCC Auger recombination rate is thus calculated by considering all the possible intra and inter-sub-band carrier transitions (Hereafter referred to as bound-bound transitions). Processes in which the excited electron starts in a bound state of the well but makes a transition to an unbound state are also considered, and it is shown that although these 'bound-unbound' transitions have customarily been ignored, they can make a significant contribution to the Auger rate. Simple physical descriptions are then used to explain the relative importance of the processes, and numerical results are presented for the Auger rate in 1.3 um and 1.55 urn In GaAsP/InP quantum well systems. In these alloys it is found that the quantum well and bulk Auger rates are - very similar for the same carrier concentrations, and similar approximations. In the second part of this thesis conventional approximations for estimating conduction band - heavy hole with the wave vectors in the (001) direction, where the discrepancy is much larger, showing that the usual assumptions as to the dominant terms that appear ineffective mass rules, are incorrect. Also shown is the underestimation of the overlap ntegrals by the 4 band k.p method. Finally the significance of the results is discussed
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Borsosföldi, Zoltan. "Surface gated quantum dots in shallow GaAs-AlGaAs heterostructures." Thesis, University of Glasgow, 1998. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.264107.
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Sakai, Joao Wesley Lopes. "Donor-assisted resonant tunnelling in semiconductor heterostructures." Thesis, University of Nottingham, 1997. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.387861.
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Gopir, Geri Kibe Ak. "Electronic and optical properties of III-V heterostructures." Thesis, University of Newcastle Upon Tyne, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.336297.
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Stavrinou, Paul Nicholas. "A study of InP-based strained layer heterostructures." Thesis, University College London (University of London), 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.261711.
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Guthrie, Daniel K. "Analysis of quantum semiconductor heterostructures by ballistic electron emission spectroscopy." Diss., Georgia Institute of Technology, 1998. http://hdl.handle.net/1853/14915.
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Hoare, David. "Monte Carlo simulations of electron transport in quantum well heterostructures." Thesis, Durham University, 1993. http://etheses.dur.ac.uk/5557/.
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The parallel transport of electrons in pseudomorphic In(_z)Ga(_1-x)As/GaAs quantum wells is influenced by the degree of spacial confinement and by the effect of the indium concentration which determines, the amount of alloy scattering, the subband structure, and material parameters. The indium content changes the bandstructure and material parameters through both direct compositional and strain effects. We use the single particle and ensemble methods of Monte-Carlo simulation to investigate how the above phenomena influence the transport properties of electrons in In(_x)Ga(_1-x)As/GaAs quantum wells. To understand the effects of alloying and strain on the electron transport properties we first consider electrons in bulk In(_x)Ga(_1-x)As/GaAs. Alloying and strain are considered in artificial systems were the effect(s) of these factors on electron transport may be isolated. For a range of indium compositions, we consider independently the effects of alloying (with and without alloy scattering) and strain on the bandstructure and material parameters and, in turn, their effects on the electron transport properties. We show that increasing the indium concentration generally improves the carrier low field mobility and peak velocity of unstrained materials but has a detrimental effect on the saturation velocity. Strain reduces the low field mobility and peak velocity but gives a slightly higher saturation velocity when compared to GaAs, and the unstrained system. Comparison of transient and steady state transport phenomena is made for strained In(_0.15)Ga(_0.085)As/GaAs quantum well structures, at fields high enough for real- and reciprocal-space transfer to occur. An artificial case, called the unstrained system, where the strain effects on the bandstructure and material parameters are neglected is also considered. Differences between the strained and unstrained well results are small and mainly transient. At steady state, most of the electrons for almost all fields reside in unbound states. The strained and unstrained systems show higher low field mobilities when compared to bulk GaAs. Lattice vibrations are also affected by heterostructures and we have made a study of the effects on the low field transport of electrons in a 70Å Al(_0.3)Ga(_0.7)As/GaAs quantum well when the polar optical phonon modes which interact with the electrons are described by three phonon models which describe the lattice vibrations of the heterostructure; the Hydrodynamic Model (EDM), the Dielectric Continuum Model (DCM), and the Bulk Phonon Approximation (BPA). We show that the BPA and EDM predict similar transport effects and are in good agreement with experimental results. We conclude that, at present, the BPA is an adequate model to describe the phonon modes in heterostructure quantum wells for use in transport calculations.
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Smith, George St J. V. "The characterisation of InSb quantum well heterostructures by electrical measurement." Thesis, Cardiff University, 2019. http://orca.cf.ac.uk/119553/.
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This thesis describes both experimental and theoretical work on the electronic transport properties of 30 nm InSb/AlInSb quantum well 2DEG heterostructures. Advances in the epitaxial growth of large lattice constant III-V materials using mismatched substrates like GaAs or Si has generated renewed interest in developing high mobility devices. Similarly, narrow gap semiconductors are promising candidates for the advancement of spintronic devices taking advantage of their extreme material parameters, such as the small effective mass and large effective Landé g-factor. An investigation of the low temperature Hall effect and Shubnikov-de Haas oscillations of asymmetrically doped InSb quantum well heterostructures has been made to determine the scattering mechanisms present for carriers in the 2D system. Modelling these oscillations by calculation of the density of states at the Fermi energy as a function of magnetic field was performed to analyse the effects of parameter variation on the observed oscillation. Application of a dielectric layer and gate electrode to the material surface has allowed for a carrier density dependent investigation of the transport properties to be performed. These investigations have provided a detailed understanding of the transport limiting scattering mechanisms over a range of carrier densities and temperatures. A novel study of the current-voltage characteristics of high resistance contacts has been performed to investigate the energetic distribution of electron states in the quantum well under the application of large magnetic fields. Clear Landau level quantisation of the 2D density of states for the first subband of the quantum well has been observed. Analysis of the high field asymmetry of the fundamental Landau level has revealed the presence of significant spin dependent broadening within the heterostructure, which has previously been suggested to exist from an asymmetry of the Fourier transform of Shubnikov-de Haas oscillation.
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Lai, Andrew P. (Andrew Pan). "Investigation of lateral gated quantum devices in Si/SiGe heterostructures." Thesis, Massachusetts Institute of Technology, 2013. http://hdl.handle.net/1721.1/83775.
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Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Physics, 2013.Cataloged from PDF version of thesis.
Includes bibliographical references (pages 73-75).
Quantum dots in Si/SiGe have long spin decoherence times, due to the low density of nuclear spins and weak coupling between nuclear and electronic spins. Because of this, they are excellent candidates for use as solid state qubits. The initial approach towards creating controllable Si/SiGe quantum dots was to fabricate them in delta doped heterostructures. We provide evidence that the delta doping layer in these heterostructures provides a parallel conduction path, which prevents one from creating controllable quantum dots. Instead, it may be more favorable to supply electrons in the 2DEG through capactive gating, instead of a delta doping layer. We therefore discuss efforts to fabricate Si/SiGe quantum dots from undoped heterostructures and the difficulties encountered. A new method for fabricating ohmics in undoped heterostructures is discussed. We also discuss parallel conduction which occurs in the Si cap layer of these undoped heterostructures, which appears to be a major obstacle towards achieving workable devices in undoped Si/SiGe heterostructures.
by Andrew P. Lai.
S.M.
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Krishna, Kumar Roshan. "High temperature quantum transport in graphene/hexagonal-boron nitride heterostructures." Thesis, Lancaster University, 2017. http://eprints.lancs.ac.uk/88867/.
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The past decade has seen a new paradigm in solid state physics, where a new class of layered crystals can be thinned down to a monolayer and exhibit drastic changes in their electronic and optical properties in comparison to their bulk counterpart. Graphene was the first, and certainly most outstanding, of this set of so called two-dimensional (2D) materials. Aside from its obvious appeal which earnt its discovery the 2010 Nobel Prize, the electronic properties of graphene are truly unique. Perhaps the most familiar is its linear electron dispersion which hosts quasi-particles that obey the Dirac equation. This has enabled the study of a plethora of transport phenomena, as well as the realisation of novel device architectures that will be used in the next generation electronics. In general, experimental signatures of electron transport are most prominent at liquid helium temperatures when lattice vibrations are weak, for example in quantum hall physics. In this Thesis, we explore the regime of intermediate temperatures where the physics of interest is strongest between 100 and 300 K. Equipped with the state of the art high quality graphene samples, we demonstrate novel electron transport unique to graphene. The experimental work consists of two themes. In the first work, we study hydrodynamic electron flow in graphene encapsulated with hexagonal boron nitride devices. At elevated temperatures, electron-electron collisions become significant, and the electron viscosity starts to influence the steady state current distribution in a variety of surprising ways. In the first work, we perform transport experiments on standard graphene hall bars in a unique measurement geometry which allows the detection of negative non-local voltages intrinsic to viscous flow. In another experiment, we study viscous electron flow through graphene nano-constrictions/classical point contacts. Here, we observed anomalous temperature dependence in the conductance measured across the constriction. Specifically, the conductance increases with increasing temperature and even exceeded the semi-classical limit which is expected for single-particle ballistic transport. The underlying mechanism originates from electron-electron collisions, which, counter-intuitively, act to enhance current flow. In the second work, we slightly change our experimental system by studying magneto transport in a graphene/hexagonal boron nitride superlattice. Owed to the large periodicity of the superlattice unit cell, these devices have allowed experimental observation of the long sought Hofstadter butterfly, which addresses the electronic dispersion of electrons in a periodic potential and magnetic field. Here, we again go to elevated temperatures, where all the spectral gaps related to Hofstadter butterflies are completely smeared, and instead find a new type of quantum oscillation. These new oscillations are periodic in 1/B with a frequency corresponding to one flux quantum piercing the superlattice unit cell. Whilst these oscillations are related to Hofstadter physics, they are in fact more primal in origin. The most fascinating feature is their robustness with respect to increasing temperature. The oscillations are easily observable at room temperature in fields as low as 3 T and still remained prominent at 373 K, the boiling point of water.
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Corbin, Elizabeth Ann. "Infra-red optical properties of SiGe/Si heterostructures." Thesis, University of Newcastle Upon Tyne, 1995. http://hdl.handle.net/10443/810.
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We present full-scale relativistic pseudopotential calculations of the first-order susceptibility in p-type SiGe/Si multiple quantum well structures with a view to exploring the suitability of such systems for infrared applications in the 3-5yrn and 8-15itm ranges. A derivation of an expression for the linear susceptibility, or absorption, is given and the frequency dependence of the linear response due to transitions between the valence minibands is determined. The microscopic origin of the absorption is demonstrated for both parallel and normal incident light. Comparisons between calculated and experimental results are presented and shown to be in good agreement. The effects of changing well width, temperature, doping concentration and germanium concentration in the well are considered. We also consider Auger recombination and discuss the possibility of engineering the miniband structure in order to prevent certain Auger processes occuring, Preliminary results from full scale Auger calculations are also presented.
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Schill, Alexander Wilhem. "Interesting Electronic and Dynamic Properties of Quantum Dot Quantum Wells and other Semiconductor Nanocrystal Heterostructures." Diss., Georgia Institute of Technology, 2006. http://hdl.handle.net/1853/11514.
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Some interesting electronic and dynamic properties of semiconductor nanocrystal heterostructures have been investigated using various spectroscopic methods. Semiconductor nanocrystal heterostructures were prepared using colloidal synthesis techniques. Ultrafast transient absorption spectroscopy was used to monitor the relaxation of hot electrons in CdS/HgS/CdS quantum dot quantum wells. Careful analysis of the hot electron relaxation in CdS/HgS/CdS quantum dot quantum wells reveals an energy dependent relaxation mechanism involving electronic states of varying CdS and HgS composition. The composition of the electronic states, combined with the layered structure of the nanocrystal permits the assignment of CdS localized and HgS localized excited states. The dynamic effect of surface passivation is then shown to have the strongest influence on excited states that are localized in the HgS layer.
New quantum dot quantum well heterostructures of different sizes and compositions were also prepared and studied. The dynamic properties of CdS/CdSe/CdS colloidal quantum wells suggest simultaneous relaxation of excited electrons within the CdS core and CdSe shell on the sub-picosecond time scale. Despite the very different electronic structure of CdS/CdSe/CdS compared to CdS/HgS/CdS, the time scales of the relaxation and electron localization were very similar.
Enhancement of trap luminescence was observed when CdS quantum dots were coated with silver. The mechanism of the enhancement was investigated using time-resolved spectroscopic techniques.
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Wang, Yongqian. "Nanometer characterization of quantum compound semiconductor heterostructures grown by molecular beam epitaxy." Diss., Georgia Institute of Technology, 2002. http://hdl.handle.net/1853/20192.
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Weston, Steven John. "The optical spectroscopy of novel cadmium telluride/cadmium manganese telluride heterostructures." Thesis, University of Hull, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.321049.
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Howe, H. H. T. "Amplitude, temperature, and frequency dependence of quantum pumps in semiconductor heterostructures." Thesis, University College London (University of London), 2016. http://discovery.ucl.ac.uk/1474205/.
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In the rapidly growing field of integrated quantum devices, two particular areas of interest are the development of an on-chip cryogenic current comparator (CCC) for completing the metrological triangle and the development of integrated de- vices for fast qubit operations. This thesis aims to significantly further our understanding of a quantum pump, a device integral to the CCC and potentially critical for realising fast qubit operations. A quantum pump is a device that transfers a discrete number of electrons between two electrically isolated regions when a potential barrier is cyclically oscillated. Initially, quantum pumps were single electron turnstile devices, which were limited in operational frequency by the Coulomb potential of the turnstile. Modern quantum pumps, utilising a dynamic quantum dot in a 2-dimensional electron gas (2DEG), are not limited by frequency. The fast operation of these modern pumps makes them very promising devices for accurately measuring the electron charge and performing fast qubit operations. In this study, we address the technical challenges of measuring a Al- GaAs/GaAs quantum pump and detail the processing and measurement setup. One of the challenges is rectified current swamping pump current. We develop a model for the rectified current and investigate ways to suppress it. We then show how the accuracy of a quantum pump changes as a function of amplitude, temperature, and frequency, and develop a model towards explaining the changes.
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Yang, Qi-Zhong. "Yield analysis of split-gate quantum wires in high mobility heterostructures." Thesis, University of Cambridge, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.612057.
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Murillo, Carlos Alberto Parra. "Study of semiconductor heterostructures with embedded quantum dots:: micropillars and photodetectors." Universidade Federal de Minas Gerais, 2009. http://hdl.handle.net/1843/ESCZ-7YSHXD.
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In this work, we study some of the optical processes that take place into semiconductor systems, specially heterostructures of two types with embedded quantum dots: infrared photodetectors and microcavity pillars. Quantum dots are the source of electrons and/or quase-particles such as excitons and bi-excitons, which are fundamental in the operation of devices based on pillar microcavities and photodetectors. The importance of infrared detectors is enormous, with a huge variety of applications, and the relevance of microcavities have increased due to its promising technological applications. We present here a theoretical and experimental study of these two heterostructures in specific cases of our interest. In order to investigate the coupling between the photonic modes and the emission of quantum dots embedded in microcavity pillars we implemented a code using the free software CAMFR [Peter Bienstmann. Cavity modelling framework, http://camfr.sourceforge.net], which allows to model photonic devices such as VCSELs and microcavities. From the analysis of the intensity of excitation of the modes in the pillars, we showed that it is possible to infer on polarization of the emission of the embedded quantum dots. Furthermore, to help in the interpretation of the response of quantum dot infrared photodetectors, we developed a code on the C-language which is based in a numerical diagonalization of Schr¨odinger equation for the effective mass aproximation, in order to obtain the energy levels and wavefunctions of the system. The oscillator strengths are computed to quantify which are the most probable optical transitions, and to understand some interesting phenomena that appear in the study of infrared photodetectors. We conclude that Auger scattering has a significant role in the response of these devices.Neste trabalho estudamos alguns processos ópticos em sistemas semicondutores, em especial, heteroestruturas de dois tipos que contêm pontos quânticos: fotodetectores de infravermelho e pilares de microcavidades. Os pontos quânticos têm a função de fornecer elétrons e/ou quasi-partículas como éxcitons e bi-éxcitons, fundamentais para a operação de dispositivos baseados em pilares de microcavidades e fotodetectores. A importância dos detectores de infravermelho é enorme, com uma imensa variedade de aplicações, e a relevância das microcavidades têm crescido devido às suas promissoras aplicações tecnológicas. Apresentamos aqui o estudo teórico e experimental destas duas heterostruturas em casos específicos de nosso interesse. Para investigar o acoplamento entre os modos fotônicos e a emissão de pontos quânticos inseridos em pilares de microcavidades, foi implementado um código baseado no software livre CAMFR [Peter Bienstmann. Cavity modelling framework, http://camfr.sourceforge.net], que permitenos modelar dispositivos fotônicos como VCSELs e microcavidades. Mostramos que a partir da análise da intensidade de excitação dos vários modos dos pilares, é possível inferir sobre a polarização dos pontos quânticos neles inseridos. Para auxiliar na interpretação da resposta de fotodetectores de infravermelho baseados em pontos quânticos semicondutores, foi desenvolvido um código na linguagem de programação C, o qual é baseado na diagonalização numérica da equação de Schrödinger na aproximação de massa efetiva, obtendo assim a estrutura de níveis de energia e funções de onda do sistema. As magnitudes de oscilador são calculadas para quantificar quais são as transições ópticas mais prováveis, e entender alguns fenômenos interessantes que aparecem no estudo dos detectores de infravermelho. Concluimos que o espalhamento Auger é um processo determinante na resposta desses dispositivos.
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Krishtopenko, Sergey. "Spin splitting and collective effects in InAs/AlSb quantum well heterostructures." Toulouse 3, 2011. http://thesesups.ups-tlse.fr/1459/.
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Les travaux de cette thèse, essentiellement théorique, concernent l'étude des phénomènes dépendants du spin, à un électron et à n-corps, dans les puits quantiques (PQ) de semiconducteurs (SC) à faible gap InAs/AlSb. Les résultats obtenus permettent de prévoir de nouveaux effets physiques, ils sont comparés aux résultats expérimentaux existants. L'asymétrie du champ électrique aux interfaces InAs/AlSb est étudiée expérimentalement et théoriquement, son effet sur le spectre d'énergie des sous bandes électriques est mis en évidence. La possibilité de contrôler optiquement ce champ électrique, et par là le clivage de spin par effet Rashba sous champ magnétique nul, est démontrée. La prise en compte des interactions e-e sur les niveaux de Landau des quasi-particules ainsi que sur la densité d'états au niveau de Fermi est réalisée dans ce système pour la première fois. Le calcul théorique de l'exaltation du facteur g par échange dans les puits quantiques à faible gap est développé. Le calcul permet de prédire l'évolution du facteur g "magnéto-optique" dans les hétérostructures InAs/AlSb déterminé par résonance de spin, il met en évidence la violation du théorème de Larmor dans les hétérostructures à base de SC à faible gap. L'étude théorique de la résonance cyclotron, en régime quantique, d'un gaz bidimensionnel d'électrons de haute mobilité démontre aussi la violation du théorème de Kohn dans les hétérostructures InAs/AlSb. Les résultats obtenus dans ce travail de thèse apportent des informations utiles pour le "design" et la mise au point de nouveaux dispositifs électroniques ou optoélectroniques basés sur des hétérostructures InAs/AlSbThe Thesis is devoted to the study of "single-particle" and "many-body" spin-related phenomena in narrow-gap InAs/AlSb quantum well (QW) heterostructures. The scientific significance of the results obtained consists in the discovering and prediction of new physical effects. The asymmetry of the built-in electric field in InAs/AlSb QW heterostructures has been probed both experimentally and theoretically and its effect on the electron energy spectrum splitting in electric subbands is demonstrated. A principle possibility to control by optical means the "built-in" electric field and the Rashba spin splitting in zero magnetic field is exhibited. The theoretical investigation into e-e interaction effect on quasiparticle Landau levels and density-of-states at the Fermi level is undertaken for the first time. Theory of the exchange enhancement of quasiparticle g-factor in narrow gap QW heterostructures is developed in the Thesis. Calculation results on the "magnetooptical" g-factor in InAs/AlSb heterostructure measured in electron spin resonance are the first demonstration of Larmor theorem violation in narrow gap QW heterostructures. Cyclotron resonance study in the samples with high mobility 2D electron gas in quantizing magnetic fields provides evidences of Kohn theorem violation in InAs/AlSb heterostructures. The results obtained in the Thesis can be utilized at the designing new electronic and optoelectronic units as well the spintronic devices based on InAs/AlSb heterostructures
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Haysom, Joan E. "Quantum well intermixing of indium gallium arsenide(phosphorus)/indium phosphorus heterostructures." Thesis, University of Ottawa (Canada), 2001. http://hdl.handle.net/10393/9400.
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This thesis studies several aspects of the interdiffusion of InGaAs(P)/InP quantum well (QW) heterostructures, from the fundamental defect mechanisms, through optimization of processing parameters, to novel device applications. Conclusions from each of these areas have been drawn which further the scientific understanding and the manufacturability of the technique. The thermal stability of a series of different wafers is studied to highlight how poor quality of growth can cause increased interdiffusion, and to review the requirements for achieving repeatable annealing. Purposeful and controlled interdiffusion is accomplished through the introduction of excess defects into layers above the QWs, which during a subsequent anneal, diffuse through the QWs and enhance interdiffusion of atoms of the QWs with atoms of the barriers. These excess defects are introduced using two different techniques, via growth at low temperatures (LT) using chemical beam epitaxy (CBE), and via implantation of phosphorus ions. The CBE LT growth technique is new, and reported for the first time in this thesis. Characterization of the as-grown layers leads us to believe that they have an excess of phosphorus. The diffusion rate of the mobile defects which cause the intermixing is also measured, and the interdiffusion is shown to occur predominantly on the group-V sublattice. Due to many similarities between this and the results of the implantation technique, it is proposed that these mobile defects are the same for both intermixing approaches, and that the behaviour can be explained by a phosphorus interstitial mechanism. Annealing recipes for the implantation-induced technique are optimized, and the sample-to-sample reproducibility of the blueshift for this method was found to be quite good (standard deviations of ∼6 meV on blueshifts of ∼70 meV). The lateral selectivity and refractive index changes are characterized, and used in combination to create novel buried waveguide devices.
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McBride, Patrick M. "The Effect of Polarization and InGaN Quantum Well Shape in Multiple Quantum Well Light Emitting Diode Heterostructures." DigitalCommons@CalPoly, 2012. https://digitalcommons.calpoly.edu/theses/822.
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Previous research in InGaN/GaN light emitting diodes (LEDs) employing semi-classical drift-diffusion models has used reduced polarization constants without much physical explanantion. This paper investigates possible physical explanations for this effective polarization reduction in InGaN LEDs through the use of the simulation software SiLENSe. One major problem of current LED simulations is the assumption of perfectly discrete transitions between the quantum well (QW) and blocking layers when experiments have shown this to not be the case. The In concentration profile within InGaN multiple quantum well (MQW) devices shows much smoother and delayed transitions indicative of indium diffusion and drift during common atomic deposition techniques (e.g. molecular beam epitaxy, chemical vapor deposition). In this case the InGaN square QW approximation may not be valid in modeling the devices' true electronic behavior. A simulation of a 3QW InGaN/GaN LED heterostructure with an AlGaN electron blocking layer is discussed in this paper. Polarization coefficients were reduced to 70% and 40% empirical values to simulate polarization shielding effects. QW shapes of square (3 nm), trapezoidal, and triangular profiles were used to simulate realistic QW shapes. The J-V characteristic and electron-hole wavefunctions of each device were monitored. Polarization reduction decreased the onset voltage from 4.0 V to 3.0 V while QW size reduction decreased the onset voltage from 4.0 V to 3.5 V. The increased current density in both cases can be attributed to increased wavefunction overlap in the QWs.
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Farrokhi, M. Javad. "ELECTRONIC PROPERTIES OF ATOMICALLY THIN MATERIAL HETEROSTRUCTURES." UKnowledge, 2019. https://uknowledge.uky.edu/physastron_etds/67.
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There is a movement in the electronic industry toward building electronic devices with dimensions smaller than is currently possible. Atomically thin 2D material, such as graphene, bilayer graphene, hBN and MoS2 are great candidate for this goal and they have a potential set of novel electronic properties compare to their bulk counterparts due to the exhibition of quantum confinement effects. To this goal, we have investigated the electric field screening of multilayer 2D materials due to the presence of impurity charge in the interface and vertical electric fifield from back gate. Our result shows a dramatic difference of screening behavior in high and low charging limit, which depends on the number of layers as well. We also have an extensive study on quantum tunneling effect in graphene and bilayer graphene heterojunctions. The peculiar electronic properties of graphene lead to an unusual scattering effect of electron in graphene n-p junction. We implement the cohesive tunneling effect to explain the nonlinear electron transport in ultrashort channel graphene devices. This nonlinear behavior could make them tremendously useful for ultra-fast electronic applications.
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Godfrey, M. D. "Transport properties of GaAs/InGaAs double quantum wells and graded InGaAs heterostructures." Thesis, University of Cambridge, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.599457.
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In the first structure presented a single two-dimensional electron gas is positioned in a region of graded (0 ≤ x ≤ 0.1) InGaAs composition. Through a series of MBE grown wafers the technique of successfully growing InGaAs as an InAs/GaAs superlattice was demonstrated. Varying the period of the superlattice was used to achieve the graded InGaAs region in the final device design. The exchange-enhanced g-factor was measured via thermal excitation seen to increase with the application of positive back-gate voltages is. One-dimensional conductance is observed in a graded alloy system for the first time, a stepping-stone to the implementation of single electron devices. Through analysis of the low-field resistivity, the appearance of a second subband in the two-dimensional electron gas, attributed to the zero-field spin-splitting from the Rashba interaction, was seen, and a dependence on back-gate voltage observed. Gate-voltage control of the spin-orbit interaction has only previously been observed in much higher indium concentration samples. A second structure investigated consists of two two-dimensional electron gases and forms a new kind of double quantum well device. Two-dimensional electron gases are located in separate GaAs and InGaAs quantum wells, separated by an AlGaAs barrier. Devices presented in this thesis allow two different gating schemes to be investigated. Firstly large-area front- and back-gates allow the isolation of a two dimensional electron gas in either well. This means two-dimensional conduction can be limited to either the GaAs or InGaAs layer. Secondly through use of a split-gate midline device it is possible to select the conduction pathway through the device with quasi-one-dimensional channels. This technique uses surface gates only, and again it gives the ability to select the material composition in which the electron wavefunction is situated. Such a double quantum well system gives the possibility of investigating the effect of the local g-factor and spin-orbit coupling on various low-dimensional spin-related phenomena.
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Smet, Jurgen Hubert. "Intrawell and interwell intersubband transitions in single and multiple quantum well heterostructures." Thesis, Massachusetts Institute of Technology, 1995. http://hdl.handle.net/1721.1/11853.
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Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1995.Includes bibliographical references (p. 228-251).
by Jurgen Hubert Smet.
Ph.D.
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Walsh, T. M. "Theoretical characterisation of spheroidal PbSe/PbS core/shell colloidal quantum dot heterostructures." Thesis, University of Salford, 2016. http://usir.salford.ac.uk/41075/.
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Nanocrystal quantum dots (NQDs) show great promise in the advancement of the field of photovoltaics. While the maximum efficiency of conventional solar cell (SC) devices is limited to ∼ 31% (Shockley-Queisser limit), devices based on NQDs may attain a maximal thermodynamic efficiency of 42% through the exploitation of multiple exciton generation (MEG). In this process, several electron- hole pairs are created by the absorption of a single high energy photon, as opposed to the single excitons created in conventional solar cell devices. IV-VI semiconductor nanocrystals (PbS, PbSe) are of particular interest as candidates for the exploitation of MEG due to the narrow band gap, high confinement energies, and long radiative carrier lifetimes observed in these systems. In order to realise the full potential of MEG devices, full characterisation of the optoelectronic properties of the underlying nanoparticles is desirable. While the size-dependent properties of NQDs are well understood, the effects of NQD shape are less so. This thesis investigates the effect of ellipticity on the optoelectronic properties associated with spheroidal NQDs. To this end, a four-band, anisotropic, and radially variant k · p system Hamiltonian is expanded in a planewave basis in order to calculate single-particle eigenenergies and eigenfunctions of colloidal PbSe/PbS core/shell heterogeneous NQDs of varying ellipticity. Many-body effects are accounted for via a full configuration interaction (CI) Hamiltonian, the basis of which is comprised of the single-particle states. Exci- tonic and bi-excitonic corrections are then found by mixing of the basis states. In this manner, such diverse electronic and optical properties as quasi-particle binding energies, momentum matrix elements, and charge carrier lifetimes, both radiative and non-radiative, may be predicted.
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Lillianfeld, Robert Brian. "Experimental Observation of Geometric Phases in Narrow-Gap Semiconductor Heterostructures." Diss., Virginia Tech, 2011. http://hdl.handle.net/10919/26889.
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We have studied the electron quantum phase by fabricating low dimensional (d â ¤ 2) mesoscopic interferometers in high-quality narrow-gap semiconductor (NGS) heterostructures. The low effective-mass electrons in NGS heterostructures enable observation of delicate quantum phases; and the strong spin-orbit interaction (SOI) in the systems gives us means by which we can manipulate the quantum-mechanical spin of these electrons through the orbital properties of the electrons. This enables the observation of spin-dependent phenomena otherwise inaccessible in non-magnetic systems. We have performed low temperature (0.4 K â ¤ T â ¤ 8 K), low noise (â V ~ 1μV ) transport measurements, and observed evidence of Aharonov-Bohm (AB) and Alâ tshuler-Aronov-Spivak (AAS) quantum oscillations in meso- scopic devices that we fabricated on these NGSs. Our measurements are unique in that we observe both AB and AAS in comparable magnitude in ballistic networks with strong SOI. We show that, with appropriate considerations, diffusive formalisms can be used to describe ballistic transport through rings, even in the presence of SOI. This work also contains an introduction to the physics of geometric phases in mesoscopic systems, and the experimental and analytic processes through which these phases are probed. A discussion of the results of our measurements presents the case that quantum interferometric measurements of geometric phases can be understood quite thoroughly, and that these measurements may have deeper utility in discovery than has yet been recognized.Ph. D.
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Shi, Teng. "Confined States in GaAs-based Semiconducting Nanowires." University of Cincinnati / OhioLINK, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1460447182.
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Anufriev, Roman. "Optical properties of InAs/InP nanowire heterostructures." Thesis, Lyon, INSA, 2013. http://www.theses.fr/2013ISAL0133/document.
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Ce travail de thèse porte sur l’étude des propriétés optiques de nanofils InP et d’hétérostructures nanofils InAs/InP épitaxiés sur substrat silicium. Ce travail de thèse a été réalisé principalement dans le cadre du projet ANR «INSCOOP»This thesis is focused upon the experimental investigation of optical properties of InAs/InP NW heterostructures by means of photoluminescence (PL) spectroscopy. First, it was demonstrated that the host-substrate may have significant impacts on the optical properties of pure InP NWs, as due to the strain, created by the difference in the LTECs of the NWs and the host-substrate, as due to some other surface effects. Next, the optical properties of such nanowire heterostructures as quantum rod (QRod) and radial quantum well (QWell) NWs were investigated. The features of obtained spectra were explained using theoretical simulation of similar NW heterostructures. The polarization properties of single InP NWs, InAs/InP QWell-NWs, InAs/InP QRod-NWs and ensemble of the InAs well ordered NWs were studied at different temperatures. Further, we report on the evidences of the strain-induced piezoelectric field in WZ InAs/InP QRod-NWs. Finally, PL QE of NW heterostructures and their planar analogues are measured by means of a PL setup coupled to an integrating sphere. In general, the obtained knowledge of the optical and mechanical properties of pure InP NWs and InAs/InP NW heterostructures will improve understanding of the electrical and mechanical processes taking place in semiconductor NW heterostructures and will serve for the fabrication of future nanodevice applications
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Nemitz, Ian R. "Synthesis of Nanoscale Semiconductor Heterostructures for Photovoltaic Applications." Bowling Green State University / OhioLINK, 2010. http://rave.ohiolink.edu/etdc/view?acc_num=bgsu1277087935.
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Cheriton, Ross. "Design and Characterization of InGaN/GaN Dot-in-Nanowire Heterostructures for High Efficiency Solar Cells." Thesis, Université d'Ottawa / University of Ottawa, 2018. http://hdl.handle.net/10393/37905.
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Light from the sun is an attractive source of energy for its renewability, supply, scalability, and cost. Silicon solar cells are the dominant technology of choice for harnessing solar energy in the form of electricity, but the designs are approaching their practical efficiency limits. New multijunction designs which use the tunable properties of the more expensive III-V semiconductors have historically been relegated to space applications where absolute power conversion efficiency, resilience to radiation, and weight are more important considerations than cost. Some of the more recent developments in the field of semiconductor materials are the so-called III-nitride materials which mainly use either indium, aluminum or gallium in combination with nitrogen. Indium gallium nitride (InGaN) is one of these III-nitride semiconductor alloys that can be tailored to span the vast majority of the solar spectrum. While InGaN growth traditionally requires expensive substrate materials such as sapphire, three-dimensional nanowire growth modes enable high quality lattice mismatched growth of InGaN directly on silicon without a metamorphic buffer layer. The absorption and electronic properties of InGaN can also be tuned by incorporating it into quantum confined regions in a GaN host material. This opens up a route towards cost-effective, high efficiency devices such as light emitted diodes and solar cells which can operate over a large range of wavelengths. The combination of the two material systems of InGaN/GaN and silicon can marry the low cost of silicon wafers with the desirable optoelectronic properties of III-nitride semiconductors. This thesis investigates the potential for highly nanostructured InGaN/GaN based devices using quantum-dot-in-nanowire designs as novel solar cells which can enable intermediate band absorption effects and multiple junctions within a single nanowire to absorb more of the solar spectrum and operating more efficiently. Such semiconductor nanostructures can in principle reach power conversion efficiencies of over 40\% on silicon, with a cost closer to conventional silicon solar cells as opposed to methods which use non-silicon substrates.
In the primary strategy, the nanowires contain InGaN quantum dots which act as photon absorption/carrier generation centres to sequentially excite photons within the large band gap semiconductor. By using this intermediate band of states, large operating voltages between contacts can be maintained without sacrificing the collection of long wavelength solar photons. In this work, we characterize the properties of such nanowires and experimentally demonstrate sub-bandgap current generation in a large area InGaN/GaN dot-in-nanowire solar cell.
Experimental characterization of InGaN / GaN quantum dots in nanowires as both LEDs and solar cells is performed to determine the nanowire material parameters to understand how they relate to the nanowire device performance. Multiple microscopy techniques are performed to determine the nanowire morphology and contact effectiveness. Optical characterization of bare and fabricated nanowires is used to determine the anti-reflection properties of nanowire arrays. Photoluminescence and electroluminescence spectroscopy are performed. Illuminated current-voltage characteristics and quantum efficiencies are determined. Specular and diffuse reflectivities are measured as a function of wavelength.
Technology computer-aided design (TCAD) software is used to simulate the performance of the overall nanowire device. The contribution from quantum dots or quantum wells is simulated by solving for the carrier wavefunctions and density of states with the quantum structures. The discretized density of states from the quantum dots is modelled and used in a complete drift-diffusion device simulation to reproduce electroluminescence results. The carrier transport properties are modified to demonstrate effects on the overall device performance.
An alternate design is also proposed which uses an InGaN nanowire subcell on top of a silicon bottom subcell. The dual-junction design allows a broader absorption of the solar spectrum, increasing the operating voltage through monolithically grown series-connected, current-matched subcells. The performance of such a cell is simulated through drift-diffusion simulations of a dual-junction InGaN/Si solar cell. The effects of switching to a nanowire subcell based on the nanowires studied in this thesis is discussed.
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Yildirim, Hasan. "Nonlinear Optical Properties Of Semiconductor Heterostructures." Phd thesis, METU, 2006. http://etd.lib.metu.edu.tr/upload/12607438/index.pdf.
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The nonlinear optical properties of semiconductor heterostructures, such as GaAsAl/GaAs alloys, are studied with analytic and numerical methods on the basis of quantum mechanics. Particularly, second and third-order nonlinear optical properties of quantum wells described by the various types of confining potentials are considered within the density matrix formalism. We consider a Pöschl-Teller type potential which has been rarely considered in this area. It has a tunable asymmetry parameter, making it a good candidate to investigate the effect of the asymmetry on the nonlinear optical properties. The calculated nonlinear quantities include nonlinear absorption coefficient, second-harmonic generation, optical rectification, third-harmonic generation and the intensity-dependent refractive index. The effects of the DC electric field on the corresponding nonlinearities are also studied. The results are in good agreement with the results obtained in other types of quantum wells, such as square and parabolic quantum wells. The effects of the Coulomb interaction among the electrons on the nonlinear intersubband absorption are considered within the rotating wave approximation. The result is applied to a Si-delta-doped, square quantum well in which the Coulomb interaction among the electrons are relatively important, since there has been no work on the nonlinear absorption spectrum of the Si-delta-doped quantum well. The results are found to be new and interesting, especially when a DC electric field is included in the calculations.
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Zieliński, Marcin. "Nanoscale engineering of semiconductor heterostructures for quadratic nonlinear optics and multiphoton imaging." Phd thesis, École normale supérieure de Cachan - ENS Cachan, 2011. http://tel.archives-ouvertes.fr/tel-00585601.
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Nonlinear coherent scattering phenomena from single nanoparticles have been recently proposed as alternative processes for fluorescence in multiphoton microscopy staining. Commonly applied nanoscale materials, however, have reached a certain limit in size dependent detection efficiency of weak nonlinear optical signals. None of the recent efforts in detection of second-harmonic generation (SHG), the lowest order nonlinear process, have been able to cross a ~40 nm size barrier for nanoparticles (NPs), thus remaining at the level of "large" nanoscatterers, even when resorting to the most sensitive detection techniques such as single-photon counting technology. As we realize now, this size limitation can be significantly lowered when replacing dielectric insulators or wide gap semiconductors by direct-gap semiconducting quantum dots (QDs). Herein, a new type of highly nonlinear nanoprobes is engineered in order to surpass above mentioned size barrier at the single nanoparticle scale. We consider two-photon resonant excitation in individual zinc-blende CdTe QDs of about 12.5 nm diameter, which provide efficient coherent SHG radiation, as high as 105 Hz, furthermore exhibiting remarkable sensitivity to spatial orientation of their octupolar crystalline lattice. Moreover, quantum confinement effects have been found to strongly contribute to the second-order nonlinear optical susceptibility χ(2) features. Quantitative characterization of the χ(2) of QDs by way of their spectral dispersion and size dependence is therefore undertaken by single particle spectroscopy and ensemble Hyper-Rayleigh Scattering (HRS) studies. We prove that under appropriate conditions, χ(2) of quantum confined semiconducting structures can significantly exceed that of bulk. Furthermore, a novel type of semiconducting hybrid rod-on-dot (RD) QDs is developed by building up on crystalline moieties of different symmetries, in order to increase their effective quadratic nonlinearity while maintaining their size close to a strong quantum confinement regime. The new complex hybrid χ(2) tensor is analyzed by interfering the susceptibilities from each component, considering different shape and point group symmetries associated to octupolar and dipolar crystalline structures. Significant SHG enhancement is consequently observed, exceeding that of mono-compound QDs, due to a coupling between two nonlinear materials and slower decoherence, which we attribute to the induced spatial charge separation upon photoexcitation.
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Klar, Peter Jens. "Magneto-optical studies of wide-gap dilute magnetic semiconductor heterostructures and quantum dots." Thesis, University of East Anglia, 1996. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.338223.
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Sanchez-Yamagishi, Javier Daniel. "Superlattices and quantum spin Hall states in graphene and hexagonal boron nitride heterostructures." Thesis, Massachusetts Institute of Technology, 2015. http://hdl.handle.net/1721.1/99289.
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Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2015.Cataloged from PDF version of thesis.
Includes bibliographical references (pages 159-178).
Two-dimensional (2d) layered materials, such as graphene and hexagonal boron nitride (hBN), can be isolated separately and then stacked together to form heterostructures with crystalline interfaces between the layers. In this thesis, I present a series of experiments which explore the quantum transport of electrons in heterostructures made from graphene and hBN. Depending on the relative alignment, or "twist", between the layers, a crystal of hBN can be either a non-perturbing substrate for the graphene, or a method to induce a band gap and superlattice potential for the graphene electrons. In the case of two stacked graphene layers, a relative twist can electronically decouple the layers from each other, despite a tiny 0.34nm interlayer spacing. This twist-dependent physics can be used to realize new electronic states in graphene, especially in the presence of strong magnetic fields and electron-electron interactions. By applying a strong tilted magnetic field to graphene which is decoupled from its hBN substrate, we are able to realize a quantum spin Hall state and measure its electronic properties. An analogous bilayer quantum spin Hall state is also realized in twisted bilayer graphene, by taking advantage of the twist decoupling between the layers and the effects of electron-electron interactions. A different set of experiments explores the competition of a magnetic field with the effects of the superlattice potential which arises when a graphene sheet is nearly aligned to its hBN substrates. The large superlattice potential allows us to study graphene transport in Hofstadter's butterfly-the fractal spectrum for electrons under the simultaneous influence of a lattice and a magnetic field.
by Javier Daniel Sanchez-Yamagishi.
Ph. D.
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House, Jody L. (Jody Lee) 1970. "The growth and microstructural characterization of ZnSe/GaAs quantum wells and double heterostructures." Thesis, Massachusetts Institute of Technology, 1997. http://hdl.handle.net/1721.1/46114.
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Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 1998.Includes bibliographical references (leaves 189-198).
by Jody Lee House.
Sc.D.