Thesis (MTech(Electrical Engineering))--Cape Technikon, Cape Town,1999 / This thesis describes the design and implementation of a new feedback controller for a scanning tunneling microscope or STM. The previous controller had several shortcomings when it came to the data throughput rate of the data acquisition system, the scan rate, and the way the data was stored and displayed. The initial investigation was done to determine the most cost effective way to implement the data acquisition system. Various approaches such as DSP systems, analogue systems and microcontroller systems were looked at. The investigation also looked at the best way to get the data from the Z directional control loop to the PC for displaying the data. The final choice was to use an ultra fast microcontroller for the control loop implementation and to change the DOS based software for Windows based software. The embedded system was divided into two parts. The first was the controller for the X and Y scan directions, and the second was for the Z scan direction. A digital PI control loop was implemented on the Z controller to control the height of the scan tip above the specimen surface. The microcontroller that was chosen for this was the Microchip PIC17c43. The data transfer to the PC was done with a PC-14 programmable digital input/output card. Two options for the implementation of the PC-14 software were considered. The first option was the software that was bundled with the card. This software proved to be very slow, so special device-driver-based software was developed to control the PC-14 card and the data transfer to and from the Pc. The PC software was implemented using Visual C++. Both the XY and the Z controllers proved to be working satisfactorily in the existing STM arrangement. It was discovered that the XY controller was overloaded with the many tasks that it has to perform, and a suitable alternative system to replace the XY controller is proposed. The selection of the PC that will be used for the data acquisition system is also discussed. It was found that this choice had a very big influence on the design of the final system because of the difference in PC bus design. Several proposals to increase the functionality of the PC software are also made.

Since the single-molecule diode was first envisioned by Aviram and Ratner in 1974, researchers have investigated how the electronic properties of molecules might be designed to achieve a variety of device functionality. However, although electron-phonon and electron-photon interactions have been studied in systems where the molecule is poorly electronically coupled to the environment, only a few experimental modalities exist for studying inelastic transport in two-terminal single-molecule junctions. Furthermore, each phenomena typically has a few possible mechanisms which must be distinguished. The objective of this dissertation is to expand the experimental tools available for probing inelastic transport in single-molecule junctions, with special attention to electron-photon interactions. Throughout the dissertation, we utilize the scanning tunneling microscope break-junction technique to form either tunnel junctions or single-molecule junctions. By repeatedly pushing and pulling a Au STM tip into a Au-coated mica substrate, a variety of junction geometries are sampled to give a distribution of device performances. Transport and optical measurements are made while controlling the electrode displacement and junction bias independently, which permits flexible experimental design. The body of the dissertation is divided into three chapters, each chapter exploring a different phenomenon. In the first chapter, I study light emission from tunnel junctions driven at high bias. It was shown previously that electroluminescence from tunnel junctions can have photon energies exceeding the classical limit, so-called overbias emission. Multi-electron processes and blackbody radiation have been proposed as possible explanations for this extraordinary result. We demonstrate that the intensity of the overbias emission depends superlinearly on the junction conductance even at room temperature, which strongly supports the theory from multi-electron processes. Additionally, we show that blackbody radiation-like effects can be produced by multi-electron processes. In the second chapter, I demonstrate experimentally the enhanced conductance of single-molecule junctions under illumination. Again, we consider two mechanisms for enhancement, namely photon-assisted tunneling and hot-electron distributions. By carefully comparing the two theories, we find that their steady-state signatures are nearly identical, but that the contribution from hot-electron distributions is larger in our system. This is confirmed by measuring a conductance enhancement at a polarization where photon-assisted tunneling is negligible. In the third chapter, I explore both junction rupture and nonlinear transport phenomena in single-molecule junctions around the resonant tunneling regime. Importantly, we develop nonlinear regression curve-fitting to allow straightforward extraction of key transport parameters on individual single-molecule junctions. We observe a strong correlation between the bias at which the junction ruptures and the level alignment of the dominant transport orbital, which suggests that, in the resonant tunneling regime, the tunneling electrons interact strongly with the nuclear degrees of freedom. However, we also find that not all junctions rupture and those that sustain display negative differential resistance and hysteresis. We hypothesize that this nonlinear behavior is due to a change in the charge state of the molecule. We study the stability of this charge state and find that the dynamics of charging and discharging occur on millsecond timescales. Although the blocking-state and polaron models each predict parts of our data, neither are fully consistent with the experiments in their entirety. This reveals opportunities for further experimental and theoretical investigations into transport in the resonant tunneling regime.

Nous avons étudié par des méthodes numériques et en théorie k.p avancée les propriétés tunnel d’électrons et de trous dans des systèmes modèles et hétérostructures composés de semi-conducteurs impliquant des interactions spin-orbite de volume. Nous démontrons que le couplage entre les interactions spin-orbite et d’échange à l’interface de jonctions tunnel résulte en un fort contraste de transmission de porteurs selon le signe de la composante de leur vecteur d’onde dans le plan de la jonction. Cet effet conduit à un effet tunnel anormal d’interface que nous appelons « Effet Hall Tunnel Anormal » (ATHE). De façon similaire, des processus tunnel non-conventionnels se manifestant sur des isolants topologiques ont été prédits par d’autres auteurs. Alors que l’ensemble de ces effets Hall anormaux sont liés aux interactions spin-orbite, les effets tunnel anormaux diffèrent des effets Hall tunnel, des effets Hall et des effets Hall de spin par la forte amplitude prédite ainsi que par des phénomènes de chiralité. Ces propriétés possèdent un lien nontrivial avec la symétrie du système. L’ensemble de ces résultats démontre l’existence d’une nouvelle classe d’effets tunnel qui devaient être étudiés expérimentalement dans un futur proche. En ce qui concerne la bande de valence, nous démontrons, en utilisant un Hamiltonien 14x14 prolongeant un modèle 2x2, que le calcul décrivant l’ATHE repose sur un traitement subtil des états dits « spurious » (états non-physiques) et nous donnons quelques éléments d’amélioration et de compréhension. Dans ce mémoire de thèse, nous développons deux méthodes numériques pour résoudre le problème des états spurious en développant en parallèle des méthodes k.p respectivement à 14 bandes et 30 bandes afin de décrire des matériaux semiconducteurs à gap indirect. Les calculs menés dans la bande de valence d’hétérostructures semiconductrice incluant interfaces et barrières tunnel (approches 6x6 et 14x14) sans centre de symétrie d’inversion mettent en évidence des propriétés d’asymétrie équivalente à celles obtenues dans la bande de conduction. De tels effets sont interprétés, dans le cadre de calculs de perturbation en transport basés sur des techniques de fonctions de Green, par des effets chiraux orbitaux lors du branchement tunnel des fonctions évanescentes dans la barrière. / We report on theoretical, analytical and computational investigations and k.p calculations of electron and hole tunneling, in model systems and heterostructures composed of exchange-split III-V semiconductors involving spin-orbit interaction (SOI). We show that the interplay of SOI and exchange interactions at interfaces and tunnel junctions results in a large difference of transmission for carriers, depending on the sign of their incident in-plane wave vector (k//): this leads to interfacial skew-tunneling effects that we refer to as Anomalous Tunnel Hall Effect (ATHE). In a 2x2 exchange-split band model, the transmission asymmetry (A) between incidence angles related to +k// and -k// wave vector components, is shown to be maximal at peculiar points of the Brillouin zone corresponding to a totally quenched transmission (A = 100%). More generally, we demonstrate the universal character of the transmission asymmetry A vs. in-plane wavevector component, for given reduced kinetic energy and exchange parameter, A being universally scaled by a unique function, independent of the spin-orbit strength and of the material parameters. Similarly, striking tunneling phenomena arising in topological insulators have just been predicted. While they all are related to the spin-orbit directional anisotropy, ATHE differs from the tunneling Hall effect, spontaneous anomalous, and spin Hall effects, or spin-galvanic effect, previously reported for electron transport, by its giant forward asymmetry and chiral nature. These features have non-trivial connection with the symmetry properties of the system. All these results show that a new class of tunneling phenomena can now be investigated and experimentally probed.When valence bands are involved, we show (using 14x14 Hamiltonian and within a 2x2 toy model) that ATHE accurate calculations rely on a subtle treatment of the spurious (unphysical) states and we give an insight into the topology of the complex band structure. We introduce two numerical methods to remove spurious states and successfully, include them in 30-band codes able to describe indirect bandgap group-IV semiconductors. Calculations performed in the valence bands of model heterostructures including tunnel barriers, in both 6x6 and 14x14 k.p Hamiltonians without inversion asymmetry, more astonishingly highlight the same trends in the transmission asymmetry which appears to be related to the difference of orbital chirality and to the related branching (overlap) of the corresponding evanescent wave functions responsible for the tunneling current. Besides, we built an analytical model and developed scattering perturbative techniques based on Green’s function method to analytically deal with electrons and holes and to compare these results with numerical calculations. The agreement between the different approaches is very good. In the case of holes, the asymmetry appears to be robust and persists even when a single electrode is magnetic.

No description available.

PhD Thesis / Recently, gallium nitride high electron mobility transistor [GaN HEMT] has evolved as a promising device in the field of power electronics. It has excellent material qualities such as high bandgap, high saturation velocity, and good thermal stability which is expected to give superior device performances compared to its Si counterparts. One of the major challenges in GaN technology is to achieve enhancement operation (or normally off mode) due to the presence of its inherent two-dimensional electron gas[2DEG]. Among many methods developed to realize this, pGaN HEMT has emerged as the most encouraging technique for power GaN technology due to its high threshold voltage and good reliability. However, one of the major issues in pGaN HEMTs is that it suffers from high gate leakage current which limits their device performance. In this thesis, we have made a detailed study of the gate leakage process in pGaN HEMTs in terms of modeling, TCAD simulations, and alternative methods being used to reduce gate leakage in pGaN devices. A numerical model has been developed to model the gate leakage in pGaN HEMTs as a function of gate bias and temperature. This model is validated against 5 devices with different contact metals, geometries, and process conditions. A single model with a consistent set of parameters can fit the experimental data for all these 5 devices without the need to invoke multiple mechanisms to explain the gate leakage process. The numerical model relied on some simplifications, such as ignoring series resistance, using the compact diode model, and using a simplified expression to describe trap-assisted tunneling. Using commercial TCAD simulations, can address these limitations since the simulator computes the electric field distribution throughout the structure. Furthermore, using TCAD some of the trap levels have been identified which accounts for leakage at low bias. We were able to calibrate our TCAD simulations against published data for the drain current and then used the calibrated simulation environment to accurately simulate gate leakage using parameters that closely correspond to the physical phenomena described, including interface trap parameters, which we identify with known trap levels in GaN. Finally, we have examined different strategies that have been implemented so far to reduce leakage current. The pGaN layer is important in the whole device operation. Its doping concentration and thickness affect the leakage characteristics. Three modified structures have been studied through TCAD simulations which decrease gate leakage current. In each case, we used our calibrated TCAD model to study the impact on the drain current as well as the leakage current. Our results closely fit published experimental results and therefore provide confidence on the simulated dependence of leakage and drive current behavior on process modifications. The specific results, and our model overall, are expected to be of benefit to device designers in optimizing device structures for leakage while maintaining the required drive current. / Thesis / Doctor of Philosophy (PhD)

<p> The present knowledge of lattice dynamics in particular solids is applied to the theory of strongly coupled superconductors. From existing phonon data, the product function α²(ω)F(ω) is determined in various materials, where α²(ω) is the electron-phonon coupling term, and F(ω) is the phonon density of states of that material. The Eliashberg gap equations are solved for these particular materials using this product function and predictions of the superconducting energy gap ∆(∆₀) and tunneling electron density of states N_T(ω) are made. </p> <p> Tunneling experiments are performed on selected Tℓ-Pb-Bi alloys where this phonon information is available and comparisons are made both of the predicted and obtained ∆(∆₀) and the tunneling density of states N_T(ω). </p> / Thesis / Doctor of Philosophy (PhD)

Master of Science

We imitated the sandwich structure of TMR(Tunneling Magnetoresistance) to apply to CMR(Colossal Magnetoresistance) material . We choose one of the Colossal Magnetoresistance material La0.67Sr0.33MnO3(113) to be the Ferromagnetic(FM) layers as top and bottom layer in sandwich structure and use La0.67Sr0.33MnO3 ¡P SrO(214) to be middle layer which have Antiferromagnetic(AFM) property to form FM-AFM-FM structure. The FM and AFM layer can match their lattice in interface joint. What its purpose is to use this structure to enhance SPT (Spin Polarization Tunneling) effect and let spintronics can periodical spin-flip in supper lattice structure of antiferromagnetic. Upon this compose we try to show increase the LFMR (Low Field Magnetoresistance) by use CMR. The experiment result shows maybe the film structure damage occurred in our made TMR tunneling device process (ex. Ion etching process), so we should improvement the process to get the exactly experiment data. Additional, due to the alignment of the moment of La0.67Sr0.33MnO3 is unstable, the back and forth hysteresis loop can¡¦t overlap, So that afterwards we can use the more stable material La0.67Ca0.33MnO3 which have more stable moment.

A study of the nucleation, growth and final microstructure of vacuum deposited nickel films has been performed using scanning tunneling microscopy (STM) as the primary research instrument. Typical nucleation conditions are reported for nickel films grown on partially shadowed highly-oriented pyrolytic graphite (HOPG), and techniques are developed for using the STM to catalog film islands instead of more conventional electron microscopes. Values for the activation energy of surface diffusion, critical nucleus size, changes in the saturation nucleation density with temperature, and spatial variations in the nucleation rate are included. Roughening and microstructure changes observed with STM are reported as functions of substrate temperature and deposition angle for nickel films grown on highly-oriented pyrolytic graphite and fused silica. Conventional film RMS roughness values are compared to microRMS values derived from STM data and STM images of film microstructure are compared with SEM and optical microscope photographs.

published_or_final_version / abstract / toc / Physics / Master / Master of Philosophy