Neste trabalho fazemos um estudo dos campos elétricos existentes em super-redes δ-Si:GaAs As técnicas de espectroscopia Raman e fotorefletancia foram utilizadas para fins de comparação do campo elétrico medido indiretamente (Raman) e diretamente (fotorefletancia). Com o objetivo de eliminar os campos elétricos através do efeito fotovoltaico, foram feitas medidas de fotorefletancia para altas intensidades do feixe de prova, porem apenas reduções de ate 30% no valor destes campos foram obtidas. Os tempos de resposta dos portadores responsáveis pela modulação dos campos elétricos foram inferidos através de medidas de fotorefletancia sensível a fase e estão em bom acordo com os tempos medidos através de uma técnica de fotorefletancia desenvolvida em nosso laboratório / In the present work we report a study of surface and interface electric fields in δ-Si:GaAs superlattices. Techniques such as photoreflectance (PR) and Raman spectroscopy were used to determine surface electric fields in a direct and indirect way, respectively. The results agree with those expected by Fermi level pinning at the surface. In order to eliminate electric fields via photovoltaic effect, photoreflectance spectra were obtained at high probe intensities, but reductions of just 30% were achieved. In time domain, we show that carriers response time responsible for modulation of electric field can be obtained by means of phase sensitive PR, and are in good agreement with those measured in a time resolved PR technique, developed in our laboratory

We are living in an era of digital electronics. The number of robots have already exceeded the human population of the entire earth. An article in the Guardian newspaper dated 30th March 2018 suggests that 10 million UK workers will be jobless within 15 years as they will be replaced by robots. These astonishing facts shed light on the importance of knowledge and how important it is to use it wisely for our benefit without ultimately destroying us. Knowledge in all forms is accessible without going to a library or buying a newspaper. Furthermore to access information, we often use sleek devices such as smart phones, using highly developed multimedia platforms which consume large amounts of power. In 2016, IBM found that humans create 2.5 quintillion bytes of data daily. Since high computing usage is related to large power consumption, the basic building block of electronics i.e. the transistor is required to be more power efficient. This is now possible through spintronics, where the spin of an electron is exploited instead of the charge. A new class of exotic materials called topological insulators are predicted to exhibit efficient spintronic applications. These materials can conduct spin polarised current on their surface while remaining completely insulting from the inside. Moreover, doping topological insulators with magnetic impurities unlocks new avenues for spin memory devices in the form of a single spin polarized dissipationless conduction channel. In topological insulators, there is always a contribution from the inside (bulk) in addition to surface conduction, thereby yielding charge transport rather than spin transport. On this basis, the aim of my PhD was to explore techniques to grow, characterize, fabricate and measure devices on topological Dirac materials, with the hope to experimentally distinguish the bulk from the surface states and also exploit their exotic properties arising from opening of the bulk band gap by intentional magnetic doping. Samples consisted of thin films of Bi2Se3, Sb2Te3, Cr doped Sb2Te3, bilayers of Dy doped Bi2Te3/Cr doped Sb2Te3 and Cd3As2 nanowires. It was found that a seed layer of an undoped topological insulator was a crucial first step to ensure high quality growth by molecular beam epitaxy, followed by the desired stoichiometry. By physically doping Sb2Te3 with Cr, a successful control of the magnetic and electrical properties such as coercivity, anomalous Hall resistance RA xy, Curie temperature Tc, carrier density and mobility were achieved. A substitutional Cr doping ranging from 7.5% to 38% was attained revealing a Tc reaching up to 186 K. Gated electrical measurements displayed a change in RA xy and carrier density by ~ 50% on applicating of just -3 V gate bias in a sample with 29% doping. A comparison between electrical transport, Magneto-optical Kerr effect and terahertz time domain spectroscopy measurements revealed that the mechanism of magnetization was RKKY mediated. Furthermore, the bilayer structure displays a clear exchange bias coupling arising from the proximity of the antiferromagnetic Dy doped Bi2Te3 layer with the ferromagnetic Cr doped Sb2Te3 layer. Electrical transport measurements on Bi2Se3 Hall bars fabricated using Ar+ milling and wet chemical etching were compared. The results showed a more bulk type response in the chemical etched sample even though Ar+ milling was responsible for creating more disorder in the system leading to a higher carrier density and lower mobility. A thickness dependent study on Sb2Te3 thin films revealed a single conducting channel associated with a coupled surface and bulk state for a 12 nm sample, compared to, two conducting channels associated with the top and bottom surfaces for the 25 nm sample. Electrical transport on Dirac semimetal Cd3As2 nanowires reveal an ultra-high mobility of 56884 cm2V-1s-1 at 1.8 K from analysis of Shubnikov-de Haas oscillations. By studying various Dirac materials, new avenues for practical device applications can be explored.

New transparent p- and n-type semiconductors and luminescent materials have been prepared and characterized. Synthesis, structures, optical and electrical properties of new chalcogenide fluoride p-type transparent semiconductors MCuQF (M=Ba, Sr; Q=S, Se, Te) are described. Band-gap tuning and improvement in conductivity through p-type doping are demonstrated in the family. The new Ag sulfide fluoride BaAgSF has been prepared, and its optical and electrical properties have been examined. Phase stabilization as well as optical and electrical properties of the p-type conductors BaCu₂S₂ and BaCu₂Se₂ are considered. New n-type transparent conducting films of W-doped In₂O₃ and W-doped zinc indium oxide (ZIO) have been prepared by pulsed laser deposition, and their electrical properties have been examined. Results on new transparent thin-film transistors containing SnO₂ or ZIO are also presented. Strong near-infrared luminescence of BaSnO3 is described, and the emission brightness is correlated to the crystallite size of assembled nanoparticles. Syntheses, structures, and optical properties of (La,Y)Sc₃(BO₃)₄:Eu³⁺, (Ba,Sr)Sc₂(BO₃)₄:Eu²⁺, and LuAl₃(BO₃)₄:Ln³⁺ (Ln=Eu, Tb, Ce) have been considered with emphasis on relations between structures and optical properties. Finally, the synthesis and luminescence properties of new potential X-ray phosphors Lu₂O₂S:Ln³⁺ (Ln=Eu, Tb) are summarized. / Graduation date: 2005

The neutron and electron radiation effects in Ill-V compound semiconductor heterostructure devices are studied in this thesis. Three types of devices investigated are AlGaAs/GaAs high electron mobility transistors (HEMTs), AlGaAs/InGaAs/GaAs heterostructure insulated gate field effect transistors (HIGFETs), and InP/InCaAs/InGaAs single heterojunction bipolar transistors (SHBTs). HEMTs and HIGFETs are primarily investigated for neutron irradiation effects. Detailed optimized processing of HEMT devices is introduced. Numerical as well as analytical models that incorporate radiation induced degradation effects in HEMTs and HIGFETs are developed. The most prominent radiation effects appearing on both HEMT and HIGFET devices are increase of threshold voltage (V[subscript T]) and decrease of transconductance (g[subscript m]) as radiation dose increases. These effects are responsible for drain current degradation under given bias conditions after irradiation. From our experimental neutron irradiation study and our theoretical models, we concluded that threshold voltage increase is due to the radiation-induced acceptor-like (negatively charged) traps in the GaAs channel region removing carriers. The mobility degradation in the channel is responsible for g[subscript m] decrease. Series resistance increase is also related to carrier removal and mobility degradation. Traps introduced in the GaAs region affect the device performance more than the traps in the AlGaAs doped region. V[subscript T] and g[subscript m] of HIGFET devices are less affected by neutron radiation than they are in HEMTs. This difference is attributed to different shapes of the quantum well in the two devices. The main effects of electron and neutron irradiation of SHBTs are decrease of collector current (I[subscript c]), decrease of common-emitter DC gain, increase of the collector output conductance (��I[subscript c]/��V[subscript CE]), and increase of collector-collector offset voltage. The decrease of breakdown voltage of reverse biased base-emitter junction diode is responsible for increasing the output conductance after irradiation. Base-collector junction degradation also induces collector-emitter offset voltage increase. / Graduation date: 2003

Graduation date: 1996

Thesis (M.E.E.)--University of Delaware, 2006. / Principal faculty advisor: James Kolodzey, Dept. of Electrical and Computer Engineering. Includes bibliographical references.

This thesis reports on the growth and characterization of p-type pseudomorphic A1GaAs /InGaAs /GaAs modulation doped field effect transistor (MODFET) structures. A series of different p-type MODFET structures were grown with a systematic variation of the indium mole fraction and quantum well width of the InGaAs channel region. Extensive characterization of these samples using van der Pauw Hall and photoluminescence measurements showed clear trends in carrier mobility and quantum well quality with respect to the structure of the InGaAs region. From this an optimal indium mole fraction and quantum well width were obtained. Subsequent to material characterization, MODFET devices were fabricated and characterized. The measured DC device performance was reasonable and suggests that high quality p-type MODFETS should be obtainable with a properly optimized device structure and fabrication process. / Graduation date: 1993

In the rst part of this thesis we discuss some details of properties of graphene and we explain the tight-binding approach to nd the energy spectrum in graphene. In the second part of the thesis, we solve a nonlinear integral equation for the electrostatic potential in doped graphene due to an external charge, arising from a Thomas-Fermi (TF) model for screening by graphene's electron bands. In particular, we study the e ects of a nite equilibrium charge carrier density in graphene, non-zero temperature, non-zero gap between graphene and a dielectric substrate, as well as the nonlinearity in the band density of states. E ects of the exchange and correlation interactions are also brie y discussed for undoped graphene at zero temperature. Results from the nonlinear model are compared with results from both the linearized TF model and the dielectric screening model within the random phase approximation (RPA). In addition, the image potential of the external charge is evaluated from the solution of the nonlinear integral equation and compared to the results of linear models. We have found generally good agreement between the results of the nonlinear TF model and the RPA model in doped graphene, apart from Friedel oscillations in the latter model. However, relatively strong nonlinear e ects in the TF model are found to persist even at high doping densities and large distances of the external charge.

In the rst part of this thesis we discuss some details of properties of graphene and we explain the tight-binding approach to nd the energy spectrum in graphene. In the second part of the thesis, we solve a nonlinear integral equation for the electrostatic potential in doped graphene due to an external charge, arising from a Thomas-Fermi (TF) model for screening by graphene's electron bands. In particular, we study the e ects of a nite equilibrium charge carrier density in graphene, non-zero temperature, non-zero gap between graphene and a dielectric substrate, as well as the nonlinearity in the band density of states. E ects of the exchange and correlation interactions are also brie y discussed for undoped graphene at zero temperature. Results from the nonlinear model are compared with results from both the linearized TF model and the dielectric screening model within the random phase approximation (RPA). In addition, the image potential of the external charge is evaluated from the solution of the nonlinear integral equation and compared to the results of linear models. We have found generally good agreement between the results of the nonlinear TF model and the RPA model in doped graphene, apart from Friedel oscillations in the latter model. However, relatively strong nonlinear e ects in the TF model are found to persist even at high doping densities and large distances of the external charge.

The fabrication of broadband single-mode Cr-doped silica fibers (SMCDSFs) using the fiber drawing-tower method with the modified rod-in-tube technique is demonstrated for the first time. A preform was assembled by using the grown Cr:YAG rod as core and the silica tube as cladding. The outer and inner diameters of the silica tube are 20 and 7 mm, respectively. The initial dimension of the Cr:YAG crystal rod had a length of 0.03 m and a diameter of 500 £gm. The Cr:YAG crystal was grown into a diameter of a 290 £gm with a length of 0.12 m by the LHPG method. The SMCDSFs had a 6 £gm core and a 125 £gm cladding. The transmission loss was 0.08 dB/cm at 1550 nm. The far-field pattern measurements indicated the single-mode characteristic when the propagation wavelength was longer than 1310 nm. In order to solve the interface of core and cladding, a novel rod-in-tube(RIT) perform was employed by inserting the Cr:YAG crystal rod of 0.03m length and 500 £gm diameter into the silica capillary tube, which had the same diameter with the drilled silica rod. The single-mode Cr-doped fibers had successfully been fabricated and the loss had been reduced to 0.03 dB/cm at 1550 nm with a 5 £gm core and a 125 £gm cladding. Furthermore, the SMCDSFs also had the single-mode characteristic when they operated in the optical communication window. The successful fabrication of SMCDSFs may be one step forward towards the achievement of utilizing the SMCDSFs as ultra-broadband fiber optical amplifiers to cover the bandwidths in the whole 1300 to 1600 nm range of low-loss and low-dispersion windows of silica fibers and a broadband source for enabling high resolution in optical coherence tomography (OCT).