Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2007. / Vita. Includes bibliographical references (leaves 134-143). Also available in electronic version.

Thesis (M.S.)--University of Missouri-Columbia, 2006. / The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file (viewed on September 14, 2007) Vita. Includes bibliographical references.

Thesis (Ph. D.)--Lehigh University, 2006. / Includes vita. Includes bibliographical references (leaves 121-133).

No description available.

Thienoacenes represent an intriguing class of organic semiconducting molecules with potential applications in organic electronics. Some of thienoacenes have been reported with high charge carrier mobility in organic field-effect transistors (OFET). OFETs based on naphtho[2,1-b:3,4-b’]dithiophene (NDT) exhibited moderate device performance and low-band gap donor-acceptor copolymers based on NDT showed a promising solar power conversion efficiency. In this thesis, four novel series of thienoacenes based on naphtho[2,1-b:3,4-b’]dithiophene backbone were designed and synthesized for OFET applications. Firstly, a novel series of p-type semiconducting naphthodithieno[3,2-b]thiophene derivatives (NDTT-n) composed of six-fused aromatic rings were designed and synthesized (Figure 1). The OFETs based on NDTT-10, and NDTT-12 fabricated by vacuum deposition showed a hole mobility of 0.22 and 0.13 cm2/(Vs), respectively with Ion/Ioff above 107 after annealing at 80 oC. Secondly, the derivatives of NDT fused with benzene rings at the flanks of thiophene, namely NBBT-n (Figure 2) were also designed and synthesized. OFETs based on NBBTF-10 fabricated by vacuum deposition exhibited a hole mobility of 0.35 cm2/(Vs) with a current on/off ratio of 106 107 after annealing at 160 oC. Further extension of π-conjugation of NDTT by incorporating with fused thiophenes leading to a new NBTBT-n series was also developed (Figure 3). The OFETs fabricated by NBTBT-10 showed the hole mobility up to 0.25 cm2/(Vs) with a current on/off ratio of 105 106 after annealing at 220 oC. Lastly, two dimensionally π-extended, butterfly-shaped thienoacenes (Figure 4) were also synthesized. The OFETs based on SMB-10 fabricated by spin-coating showed the best performance in this series with an average mobility of 0.027 cm2/(Vs) for five devices and the highest mobility of 0.038 cm2/(Vs) with a current on/off ratio of 106 107 by from chloroform. Key words: organic semiconducting molecules, organic field-effect transistor, thienoacene, charge carrier mobility.

The object of the work described in this thesis was to develop GaAs integrated circuit modeling techniques based on a modified version of SPICE 2, then layout, fabricate, model and test ion implanted GaAs MESFET integrated sample and hold circuits. A large signal GaAs MESFET model was used in SPICE to evaluate the relative performance of inverted common drain logic (ICDL) digital integrated circuits compared to other circuit configurations. The integrated sample and hold subsequently referred to as an integrated sampling amplifier block(ISAB), uses a MESFET switch with either one or two guard gates to suppress strobe feedthrough. Performance guidelines suggested by the project sponsor indicate an optimal switch sampling pulse width capability of 25 ps with 5 ps rise and fall time. Guard gates are included in the switch layout to evaluate pulse feedthrough minimization. The project sponsor suggested -20 dB pulse feedthrough isolation and minimum sampling switch off isolation of -20 dB at 10 GHz as project guidelines. Simulations indicate that a 0.5 µm gate length process approaches the suggested performance guidelines. A mask layout was designed and modeled including both selective implant and refractory self aligned gate processes. The refractory self aligned gate process plasma etched t-gate structure produces a sub 0.5 µm gate length. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

For VLSI applications, it is essential to have consistent device characteristics for devices fabricated on different fabrication runs, on different wafers, and especially across a single wafer. MESFETs fabricated on GaAs have been found to have an orientation dependence in their threshold voltage and other characteristics. For MESFETs with gate length less than 2 μm, changing the device orientation can so significantly alter the device characteristics that it must be considered during the transistor design stage. The causes for the orientation dependence in the device characteristics have been suggested to be the piezoelectric property of GaAs and stress in the substrate. Stress produced by the encapsulating dielectric film generates a polarization charge density in the substrate. If the magnitude of the polarization charge density is large enough to alter the channel doping profile, then the device characteristics are changed. In this thesis, the effects of stress on GaAs MESFET device characteristics were studied by modelling and experimental works. In the modelling part, polarization charge densities under the gate of an encapsulated MESFET were calculated by using the so called distributed force model and the edge concentrated model. The distributed force model is a much better model because it describes more realistically the stress distribution in the film and in the substrate. It should provide a much more accurate calculation of the induced polarization charge density. The results show that the polarizarition charge densities calculated by the two models have similar distribution pattern, but the magnitudes are very different. With an identical set of conditions, a much larger polarization charge density is predicted by the edge concentrated model. In addition, the distributed force model distinguishes different films by a "hardness" value, based on their elastic property, whereas the edge concentrated model does not. A film with a larger "hardness" value is predicted to generate a larger polarization charge density. Two types of film were considered, SiO₂ and Si₃N₄. Using bulk film characteristics, the calculations showed that Si0₂ film is "harder" than Si₃N₄ film. If an equal built-in stress value is assumed, then a larger polarization charge density is predicted for Si0₂ than for Si₃N₄ encapsulated substrates. In the experimental part, stress was applied to test devices by bending strips of GaAs wafers in a cantilever configuration. MESFETs tested were oriented in the [011] or the [011̅] direction. Both static stress and time-varying stress were applied. In the statics stress experiment, the changes in the barrier height and the C-V profile were measured. It was found that, with equal stress applied, Schottky barriers with a larger ideality factor showed a larger change in the barrier height. In the time-varying stress experiment, attempts were made to measure the effect of the polarization charge density on device characteristics by measuring changes in the drain-source current. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

Dual gate MOSFET structures such as FinFETs are widely regarded as the most promising option for continued scaling of silicon based transistors after 2010. This work examines key process modules that enable reduction of both device area and fin width beyond requirements for the 16nm node. Because aggressively scaled FinFET structures suffer significantly degraded device performance due to large source/drain series resistance (RS/D), several methods to mitigate RS/D such as maximizing contact area, silicide engineering, and epitaxially raised S/D have been evaluated.

Le contrôle du magnétisme aux interfaces s’est avéré essentiel pour la spintronique et ses applications, en particulier celles basées sur des structures chirales de spin appelées skyrmions magnétiques. Ces skyrmions, décrits comme des solitons magnétiques, sont de potentiels vecteurs d’information. Dans des empilements ultraminces du type métal lourd / ferromagnétique / isolant, les skyrmions sont stabilisés par une interaction d’échange antisymétrique d’interface appelée interaction de Dzyaloshinskii-Moriya (DMI); celle-ci entre en compétition avec d’autres interactions telles que l’interaction d’échange symétrique ou l’anisotropie magnétique. Afin de contrôler ces skyrmions, les propriétés magnétiques aux interfaces doivent être ajustées finement et modulées par une excitation extérieure. Le champ électrique s’est avéré être un outil efficace pour manipuler ces propriétés d’interface. Il a notamment été montré dans un certain nombre d’études depuis 2009 qu’une différence de potentiel permet de modifier localement et de manière dynamique des propriétés telles que l’anisotropie magnéto-cristalline ou l’aimantation à saturation. Cependant, cet effet sur DMI, qu’il est crucial d’intégrer pour les systèmes avec skyrmions, n’avait pas été observé pour les films ultraminces.Cette thèse présente tout d'abord une optimisation des systèmes tricouches de type métal lourd/ferromagnétique/oxide dans lesquels peuvent exister des skyrmions. J’ai plus particulièrement étudié le système Ta/FeCoB/TaOx qui permet de énucléer des skyrmions en présence d’un faible champ magnétique appliqué perpendiculairement au plan des couches. Une étude approfondie en fonction de l’épaisseur de FeCoB et de l’état d’oxydation du TaOx a notamment été menée, permettant ainsi d’identifier les différentes zones présentant des skyrmions. D’autre part, le résultat majeur de cette thèse est la démonstration de la modulation de DMI par un champ électrique dans une tricouche Ta/FeCoB/TaOx. Des mesures de spectroscopie Brillouin sous champ électrique ont montré une très grande variation allant jusqu’à 130%. Puis, des observations complémentaires en microscopie à effet Kerr magnéto-optique ont permis de mesurer simultanément une variation monotone de DMI et de la taille des skyrmions en fonction du champ électrique avec une efficacité sans précédent. Puisque le champ électrique agit principalement sur l’interface entre le matériau ferromagnétique et l’oxyde (FeCoB/TaOx), cette étude indique l’existence d’une DMI de type Rashba, expliquant la forte sensibilité au champ électrique. Ces observations montrent également qu’un renversement du signe de l’IDM est possible, qui conduirait à une inversion de la chiralité des skyrmions. Cette manipulation dynamique de DMI permettrait de conférer un degré de contrôle supplémentaire pour le développement de mémoires ou de dispositifs logiques ou neuromorphiques à base de skyrmions. / Control of interfacial magnetism has emerged to be of paramount importance for spintronics applications specially involving chiral magnetic structures called skyrmions. Skyrmions are envisaged to be the future information carriers owing to their solitonic properties. In heavy metal/ ferromagnet/ insulator heterostructures, skyrmions are stabilized by interfacial Dzyaloshinskii-Moriya interaction which is an antisymmetric exchange and competes with other interactions like symmetric exchange and magnetic anisotropy. In order to tune skyrmions, the interfacial magnetic properties need to be modulated. One of the energy efficient tools to maneuver interfacial magnetism is electric field effect. Voltage gating has been shown, in a number of studies since 2009, to locally and dynamically tune magnetic properties like interface anisotropy and saturation magnetization. However, its effect on interfacial Dzyaloshinskii-Moriya Interaction (DMI), which is crucial for the stability of magnetic skyrmions, has been challenging to achieve and has not been reported yet for ultrathin films.This thesis demonstrates an optimization of trilayer systems consisting of a heavy metal/ ferromagnet/ oxide where skyrmions can be stabilized. In particular, I focussed on the Ta/FeCoB/TaOx system to nucleate skyrmions in the presence of very small out of plane magnetic field. Further, the different skyrmionic zones as a function of the FeCoB thickness and TaOx oxidation state are studied. We then show electric field induced modulation of interfacial DMI which forms the most important result of this thesis. We demonstrate 130% variation of DMI with electric field in Ta/FeCoB/TaOx trilayers through Brillouin Light Spectroscopy (BLS). Using polar Magneto-Optical-Kerr-Effect microscopy, we further show a monotonic variation of DMI and skyrmionic bubble size with electric field, with an unprecedented efficiency. Since the electric field acts mainly on the FeCoB/TaOx interface, this study also points at the existence of the Rashba DMI explaining its high sensitivity to an applied voltage. We anticipate through our observations that a sign reversal of DMI with electric field is possible, leading to a chirality switch. This dynamic manipulation of DMI establishes an additional degree of control to engineer programmable skyrmion based memory, logic or neuromorphic devices.

This thesis concerns itself with the scientific study of the recently discovered organic magnetoresistance (OMAR) whose underlying mechanism is currently not known with certainty. As an introduction, we briefly review the major findings from prior work done by my colleagues. They found that OMAR can be as large as ~10% magnetoresistance at 10 mT magnetic fields at room temperature. Both OMAR and other kinds of magnetic field effect data in organics can be fitted using the empirical laws B^2/(B^2+B_0^2) or B^2/(|B|+B_0)^2, dependent on material. The fitting parameter B_0 is a measure of the characteristic magnetic field strength of OMAR. We explore the dependence of B_0 on material parameters to clarify the origin of OMAR. Various pi-conjugated semiconductor OMAR devices were studied to explore the possibility that hyperfine interaction causes OMAR. For a quantitative analysis of the experiments, we developed a theoretical fitting formula to relate B_0 to the hyperfine coupling strength. In addition, organic materials with different spin-orbit coupling strengths were also measured. Fluorescence and phosphorescence spectroscopies were used to estimate the spin-orbit coupling strength from the measured spectra. For analyzing our measurements, we developed a fitting formula from the time-dependent Schrodinger equation that takes into account the combined effect of hyperfine and spin-orbit coupling on spin-dynamics. We found that in the case of strong spin-orbit coupling, it dominates the behavior, resulting in magnetic field effect traces that are much wider than those in ordinary organics. However, a small cone remains at zero field with a width equal to the hyperfine coupling strength. We find qualitative agreement between the experimental results and the model. We also investigated the question whether OMAR is related to an excitonic effect, or is primarily a transport effect. We measured the magnetic field effects on current, photocurrent and electroluminescence to address this question. By varying the injection efficiency of the minority carriers, we show that OMAR most likely is not an excitonic effect. Our results provide strong evidence in support of the claim that OMAR is caused by spin-dynamics. However, further study is required to study the mechanism connecting spin-dynamics and conductivity.