OPTIMIZATION OF MULTI-JUNCTION SOLAR CELLS FOR SPACE APPLICATIONS MODELED WITH SILVACO ATLAS

Dual junction solar cells are used in space applications for their high efficiency. In this thesis, we model an indium gallium phosphide/gallium arsenide dual-junction solar cell. The solar cell is modeled using Silvaco ATLAS software. Solar cell layer thicknesses and doping concentrations were varied to find optimum efficiency parameters for the solar cell under a variety of radiation conditions. These radiation conditions mimic the damage done at various orbits around Earth for an arbitrary mission length of 12 years. The optimization process resulted in an improved efficiency of 15.1% to 22.4%.http://archive.org/details/optimizationofmu1094559615Lieutenant, United States NavyApproved for public release; distribution is unlimited

Optimization of a Quantum Cascade Laser Operating in the Terahertz Frequency Range Using a Multiobjective Evolutionary Algorithm

A quantum cascade (QC) laser is a specific type of semiconductor laser that operates through principles of quantum mechanics. In less than a decade QC lasers are already able to outperform previously designed double heterostructure semiconductor lasers. Because there is a genuine lack of compact and coherent devices which can operate in the far-infrared region the motivation exists for designing a terahertz QC laser. A device operating at this frequency is expected to be more efficient and cost effective than currently existing devices. It has potential applications in the fields of spectroscopy, astronomy, medicine and free-space communication as well as applications to near-space radar and chemical/biological detection. The overarching goal of this research was to find QC laser parameter combinations which can be used to fabricate viable structures. To ensure operation in the THz region the device must conform to the extremely small energy level spacing range from ~10-15 meV. The time and expense of the design and production process is prohibitive, so an alternative to fabrication was necessary. To accomplish this goal a model of a QC laser, developed at Worchester Polytechnic Institute with sponsorship from the Air Force Research Laboratory Sensors Directorate, and the General Multiobjective Parallel Genetic Algorithm (GenMOP), developed at the Air Force Institute of Technology, were integrated to form a computer simulation which stochastically searches for feasible solutions

DC and Microwave Analysis of Gallium Arsenide Field-Effect Transistor-Based Nucleic Acid Biosensors

Sensitive high-frequency microwave devices hold great promise for biosensor design. These devices include GaAs field effect transistors (FETs), which can serve as transducers for biochemical reactions, providing a platform for label-free biosensing. In this study, a two-dimensional numerical model of a GaAs FET-based nucleic acid biosensor is proposed and simulated. The electronic band structure, space charge density, and current-voltage relationships of the biosensor device are calculated. The intrinsic small signal parameters for the device are derived from simulated DC characteristics and used to predict AC behavior at high frequencies. The biosensor model is based on GaAs field-effect device physics, semiconductor transport equations, and a DNA charge model. Immobilization of DNA molecules onto the GaAs sensor surface results in an increase in charge density at the gate region, resulting from negatively-charged DNA molecules. In modeling this charge effect on device electrical characteristics, we take into account the pre-existing surface charge, the orientation of DNA molecules on the sensor surface, and the distance of the negative molecular charges from the sensor surface. Hybridization with complementary molecules results in a further increase in charge density, which further impacts the electrical behavior of the device. This behavior is studied through simulation of the device current transport equations. In the simulations, numerical methods are used to calculate the band structure and self-consistent solutions for the coupled Schrodinger, Poisson, and current equations. The results suggest that immobilization and hybridization of DNA biomolecules at the biosensor device can lead to measurable changes in electronic band structure and current-voltage relationships. The high-frequency response of the biosensor device shows that GaAs FET devices can be fabricated as sensitive detectors of oligonucleotide binding, facilitating the development of inexpensive semiconductor-based molecular diagnostics suitable for rapid diagnosis of various disease states

Readout and Control Beyond a Few Qubits: Scaling-up Solid State Quantum Systems

Quantum entanglement and superposition, in addition to revealing interesting physics in their own right, can be harnessed as computational resources in a machine, enabling a range of algorithms for classically intractable problems. In recent years, experiments with small numbers of qubits have been demonstrated in a range of solid-state systems, but this is far from the numbers required to realise a useful quantum computer. In addition to the qubits themselves, quantum operation requires a host of classical electronics for control and readout, and current techniques used in few-qubit systems are not scalable. This thesis presents a series of techniques for control and readout of solid-state qubits, working towards scalability by integrating classical control with the quantum technology. Two techniques for reducing the footprint associated with readout of gallium arsenide spin qubits are demonstrated. Gate electrodes, used to define the quantum dot, are also shown to be sensitive state detectors. These gate-sensors, and the more conventional Quantum Point Contacts, are then multiplexed in the frequency domain, where three-channel qubit readout and ten-channel QPC readout are demonstrated. Two types of superconducting devices are also explored. The loss in superconducting coplanar waveguide resonators is measured, and a suppression of coupling to the parasitic electromagnetic environment is demonstrated. The thesis also details software for the simulation of Josephson-junction based circuits including features beyond what is available in commercial products. Finally, an architecture for managing control of a scalable machine is proposed where classical components are distributed throughout a cryostat and cryogenic switches route control pulses to the appropriate qubits. A simple implementation of the architecture is demonstrated that incorporates a double quantum dot, a gallium arsenide switch matrix, frequency multiplexed readout, and cryogenic classical computation

Photoluminescence blueshift mechanisms in molecular beam epitaxy grown dilute nitride hetrostructures

Ph.DDOCTOR OF PHILOSOPH

Elasticity, lattice dynamics and parameterisation techniques for the Tersoff potential applied to elemental and type III-V semiconductors.

The focus of this thesis is the techniques used in constructing a library of improved parameters for the Tersoff bond-order potential energy model which is used in atomistic modelling applications. The parameters presented here are for the elemental type-IV diamond structure semiconductors and the binary III-As, III-P, III-Sb and the cubic III-N compound semiconductors. The parameters are fitted to a number of experimental and DFT predicted properties of the materials including the lattice parameter, the cohesive energy, the elastic constants and the lattice dynamical properties, including phonon frequency and mode-Griineisen parameters, for three pertinent locations in the Brillouin zone. The conclusions of this work demonstrate that the elastic and dynamical properties of a material cannot be simultaneously predicted with the Tersoff potential due to a lack of flexibility in the current functional form. The balance between the radial and angular force contributions available in the bond-order term cannot replicate the delicate nature of the equilibrium in a real system and so two modifications to the Tersoff potential energy model have been proposed. The modifications include the addition of a second parameter and a linear contribution to the crystal anti-symmetry modelling term and the addition of a fourth parameter to the angular bonding term, which has been re-designed to be a more"" flexible summation of cosine terms. Also included in this work is: 1) a re-modelling of Keyes' relation which relates the dimensionless elastic properties of the cubic III-V semiconductors to the lattice parameter of the material to include a second-order term for the modelling of the III-N materials, 2) a simple method for the prediction of the effective ionic charge q* of the cubic III-V semiconductor materials based upon the X-point phonon energies and 3) the first Tersoff parameterisation of the materials GaP, InP, GaSb and InSb available in the literature

Determination of the interface structures of the multilayer system MgO/Fe/GaAs(001)

Die vorliegende Arbeit befasst sich mit der Untersuchung der Grenzflächen des Spintronik Mehrschichtsystems MgO/Fe/GaAs(001). Der magnetische Tunnelwiderstand (TMR) und der Riesenmagnetowiderstand (GMR), die in Spintronikbauelementen Anwendung finden, treten an den Grenzflächen auf und werden durch die chemischen und strukturellen Eigenschaften beeinflusst. Die Photoelektronenspektroskopie eignet sich besonders zur Analyse von dünnen Filmen und Grenzstrukturen von Mehrschichtsystemen. Sie ermöglicht eine genaue chemische Untersuchung über hochaufgelöste Spektren der Rumpfniveaus einzelner Elemente. Die spektralen Komponenten in einem Signal beinhalten Informationen über die lokalen Bindungen, z.B. ob der Emitter in einen Oberflächen-Dimer oder in einer tiefer liegende Schicht gebunden ist. Die Signalintensität variiert als Funktion von Polar- und Azimutwinkel. Ursächlich hierfür sind Beugungs- und Streuungseffekte der emittierten Elektronenwelle an benachbarten Atomen. Die Kombination aus hochaufgelösten Spektren und Beugungsbildern erlaubt eine detaillierte Analyse jeder einzelnen Schicht. Das MgO/Fe/GaAs(001)-System wurde in-situ präpariert und sukzessive unter Verwendung von Synchrotronstrahlung der Strahllinie 11 des Speicherings DELTA untersucht. Dabei wurde die Notwendigkeit einer GaAs Oberflächenrekonstruktion für epitaktisches Wachstum des Fe-Films nachgewiesen. Die Spektren der Fe/GaAs(4x2) und Fe/GaAs(001) Systeme weisen eindeutige chemische Bindungen auf. Die Beugungsbilder zeigen ein epitaktisches Wachstum von Fe auf GaAs(4x2) in einer pyramidalen Struktur, die durch das gleichzeitige Insel- und Lagenwachstum verursacht wird. Das Eisen reichert sich auf einer gereinigten GaAs(001) Oberfläche an, allerdings ist die Austausch-Diffusion so stark, dass der Fe-Film komplett amorph ist. Die Ga-reiche (4x2)-Rekonstruktion verhindert eine Diffusion und gewährleistet ein epitaktisches Wachstum der Fe-Schicht. Das MgO wurde auf der wohlgeordneten Fe(001)-Oberfläche aufgebracht und ist dort epitaktisch aufgewachsen. Die Fe-Oberfläche ist aufgrund der MgO-Anlagerung oxidiert, was sich in einer zwei Lagen dünnen FeO-Schicht zeigt. Der MgO-Film liegt in einer Steinsalzstruktur vor, bildet aber einen Gitterfehler in Form von leicht verschobenen Mg-Atomen aus. Diese Verschiebung ist sowohl für den dünnen als auch für einen dickeren Film vorhanden. Dies könnte durch das Substrat induziert sein, da die MgO/Fe Grenzfläche deutlich durch die Fe/GaAs Grenzstruktur beeinflusst wird. Chemische und strukturelle Veränderungen beeinflussen die magnetischen Eigenschaften des Mehrlagensystems. Daher wurden diese durch Magneto-optische Messung mittels T-MOKE untersucht. Die Spektren zeigen eine starke Reaktion des Eisens auf das externe magnetische Feld. Die Analyse der T-MOKE Daten ergab weder magnetischen Eigenschaften des GaAs-Substrats noch der dünne MgO-Schicht. Eine zusätzliche Hysterese Messung belegt die exzellenten ferromagnetischen Eigenschaften der Fe Zwischenschicht trotz der chemischen und strukturellen Veränderungen.In this thesis, the interfaces of the spintronics multilayer system MgO/Fe/GaAs(001) are determined. The Tunnel Magneto Resistance (TMR) and Giant Magneto Resistance (GMR) effects used for spintronics devices arise at the interfaces of the junctions and are influenced by the chemical and structural properties. A very suitable tool for investigating thin-films and interfaces of multilayers is photoelectron spectroscopy. It allows a detailed chemical investigation by high-resolution core level spectra of each element. The spectral components contained in the signals provide information of the local bonding, e.g. whether the emitter is located as a part of a dimer at the surface or located within a bonding beneath the surface. The intensity of the signals varies as a function of polar- and azimuth-angle. This is a result of diffraction and scattering events of the out-going electron wave at neighboring atoms around the emitter atom. Therefore, photoelectron diffraction allows a structure analysis of the surface. The combination of high-resolution spectra and diffraction patterns enable a detailed analysis of each individual layer. The MgO/Fe/GaAs(001) system was prepared in-situ and investigated successively by using synchrotron light from beamline 11 of the electron storage ring DELTA. Thereby, the necessity of a reconstructed GaAs-surface to ensure an epitaxial growth of the Fe-film is verified by XPD patterns of the Fe/GaAs(001) and Fe/GaAs(4x2)-system. The spectra clearly show a chemical bonding between the Fe-film and the GaAs-substrate. The diffraction patterns reveal an epitaxial growth of Fe on GaAs(4x2), but in a pyramid-like structure due to simultaneous layer and island growth. Iron is deposited on a cleaned GaAs(001)-surface, but the inter-diffusion is strong as the Fe-film is completely amorphous. The Ga-rich (4x2)-reconstruction prevents a diffusion and ensures an epitaxial growth of the Fe-layer. The topmost MgO-layers were prepared on the well-ordered Fe(001)-surface and grow epitaxial. The Fe was oxidized at the surface and was found in a two layer thick FeO-film due to MgO-deposition. The MgO-film is halite structured and shows a lattice misfit by slightly shifted Mg-atoms. This shift was confirmed at thin and thick MgO-films and may be induced by the substrate, because the Fe/GaAs-interface clearly influences the MgO/Fe-interface structure. Furthermore, the chemical and structural changes may influence the magnetic properties of the multilayer system. Hence, they are verified by magneto-optical measurements using T-MOKE. The spectra reveal a strong reaction of the Fe-interlayer on the external magnetic field. Further, the T-MOKE analysis reveals non-magnetic properties of the GaAs-substrate nor the thin MgO-film. An additional hysteresis measurement showed excellent ferromagnetic properties of the remaining Fe despite all chemical and structural changes

Design of Hybrid Nanoantenna-Dielectric-Cavity with Strong Near-Field Intensity Enhancement

This dissertation investigates the Near-Field Intensity Enhancement (NFIE) of plasmonic nano-antennas in the visible and near-infrared frequency region and proposes a gold nanoantenna on a dielectric nanocavity for use in Surface Enhanced Raman Spectroscopy (SERS). Plasmonic nanoantennas allow direct coupling of light to metal and have since gained traction as fabrication methods have become more precise, allowing designs that were previously limited to theories. Attempts have been made to improve the NFIE but most of these efforts have been in improving the nano-antenna design as well as in the use of Fabry-Perot cavities. This dissertation therefore proposes a “hybrid antenna” in the form of a dielectric resonant cavity (DRC) coupled to a plasmonic dipole as a receiver. The interaction that occurs during the coupling is investigated in detail and a relation is made between the overall NFIE and the NFIE introduced by the dielectric resonator modes. A simple way to design dielectric resonator cavities for use at optical frequencies is introduced and data from two commercial electromagnetic field solvers and self-coded FDTD numerical analysis are compared against results found in literature. It is found that the addition of the proposed dielectric resonator nanocavity increases the NFIE of a plasmonic gold dipole by 21 times, from 1750 to 37k. It was further found that the NFIE can be increased further to 68k when mirrored with a gold slab, where other published results not employing optical cavities have a maximum NFIE of 29k. This dissertation contributes by proposing an effective and simple DRC that can significantly improve the near-field properties of surface plasmon resonance to be used in SERS measurements

Heuristic modelling of multijunction solar cells using a parallel genetic algorithm

In order to fabricate solar cells with the highest possible values of efficiencies, material type, layer thickness and doping have to be properly selected. It can be achieved if light absorption and carrier generation are maximized and losses minimized. The same parameters that increase carrier generation, can increase certain types of losses. Parameters which reduce one type of losses, tend to increase the other types. Structural complexity of these devices combined with already mentioned conflicting requirements create vast parameter space which is highly uneven and contains a huge number of local minima and maxima. It makes it difficult for the most of search methods to locate the global maximum. Therefore, heuristic optimization is crucial for solving problems as complex as this one. To find the optimal combination of these parameters, genetic algorithm is used with drift-diffusion model and all material parameters calculated as a function of energy gap. This way we have very realistic material parameter set which, together with detailed losses modeling, provide reliable results. To test the model, findings were compared with the record setting devices. Results were in agreement, which makes the model trustworthy. Two types of devices were optimized: multi-junction solar cells (MJSC) and photon energy converters (PEC). In case of MJSCs, parameters which were optimized are thicknesses, impurity concentrations, energy gaps and optimal current. And in case of PECs, thicknesses, impurity concentrations and optimal current were optimized. The optimization was repeated with different types of losses accounted in order to see how each one of them affects the overall efficiency. From the results of optimization it was possible to see what are the main drawbacks in the device efficiency and how to overcome them. Calculations were carried out with ASTM G173−03 Global tilted solar spectrum in case of MJSCs and laser with intensity of 5W/cm2 and wavelength of 855nm in case of PECs. The absorption was calculated from kppw code. The maximum efficiencies achieved for the unconstrained device are 30.158%,41.479%, 45.669%, 50.775% and 53.653% for MJSC devices with one, two, three, four and five subcells, respectively, when all types of losses are taken into account. In case of the series constrained device, the results are 31.080%, 42.467%, 48.276%, 50.777%, 53.653%, 54.917% and 55.317% for devices with up to seven subcells, respectively, when all types of losses are taken into account, as well. The values for the PECs are 69.431%, 68.838%, 66.676% and 65.698% for one, five, ten and fifteen subcells, respectively. This time all losses are accounted as well. If the model is applied to the record setting devices, the results are 32.34% for 2JSC and 38.1%, while actual, measured, values are 31.6±1.5% and 37.9±1.2%, respectively, which is an outstanding match. Detailed device parameters obtained through the optimization process are presented. Examination of those results leads to possible recipe how to fabricate the highest possible efficiency devices. It was concluded that the radiative recombination is the most dominant type of losses in III-V semiconductors and can be suppressed by increasing the material’s energy gap. Diffusion dark current can be suppressed by increasing the energy gap as well, while the doping levels shoud be increased. On the other hand, Auger recombination can be reduced by decreasing the doping, while the increase of energy gap reduces Auger much more than the other two. This leads to significant drop in efficiency when the algorythm tries to suppress the Auger, in comparison when only the two other types of losses are accounted. Nevertheless, the suppression of all losses leads to more efficient devices. This analysis can be a guide the future experiments and indicate how much more efficiency can be achieved with these devices, which materials to target and how to correctly balance between various contradicting requirements imposed by the nature of semiconductor materials