Resistive random access memory (RRAM), based on a two-terminal resistive switching device with a switching element sandwiched between two electrodes, has been an attractive candidate to replace flash memory owing to its simple structure, excellent scaling potential, low power consumption, high switching speed, and good retention and endurance properties. However, due to the current limited understanding of the device mechanism, RRAMs research are still facing several issues and challenges including instability of operation parameters, the relatively high reset current, the limited retention and the unsatisfactory endurance. In this study, we investigated the switching mechanisms, conditions and applications of oxygen vacancy (Vo) filament formation in resistive memories. By studying the behavior of conductive Vo nanofilaments in several metal/oxide/metal resistive devices of various thicknesses of oxides, a resulting model supported by the data postulates that there are two distinct modes of creating oxygen vacancies: i) a conventional bulk mode creation, and ii) surface mode of creating oxygen vacancies at the active metal-dielectric interface. A further investigation of conduction mechanism for the Vo CF only based memories is conducted through insertion of a thin layer of titanium into a Pt/ Ta2O5/Pt structure to form a Pt/Ti/ Ta2O5/Pt device. A space charge limited (SCL) conduction model is used to explain the experimental data regarding SET process at low voltage ranges. The evidence for existence of composite copper/oxygen vacancy nanofilaments is presented. The innovative use of hybrid Vo/Cu nanofilament will potentially overcome high forming voltage and gas accumulation issues. A resistive floating electrode device (RFED) is designed to allow the generation of current/voltage pulses that can be controlled by three independent technology parameters. Our recent research has demonstrated that in a Cu/TaOx/Pt resistive device multiple Cu conductive nanofilaments can be formed and ruptured successively. Near the end of the study, quantized and partial quantized conductance is observed at room temperature in metal-insulator-metal structures with graphene submicron-sized nanoplatelets embedded in a 3-hexylthiophene (P3HT) polymer layer. As an organic memory, the device exhibits reliable memory operation with an ON/OFF ratio of more than 10. / Ph. D.

Owing to its excellent scaling potential, low power consumption, high switching speed, and good retention, and endurance properties, Resistive Random Access Memory (RRAM) is one of the prime candidates to supplant current Nonvolatile Memory (NVM) based on the floating gate (FG) MOSFET transistor, which is at the end of its scaling capability. The RRAM technology comprises two subcategories: 1) the resistive phase change memory (PCM), which has been very recently deployed commercially, and 2) the filamentary conductive bridge RAM (CBRAM) which holds the promise of even better scaling potential, less power consumption, and faster access times. This thesis focuses on several aspects of the CBRAM technology. CBRAM devices are based on nanoionics transport and chemo-physical reactions to create filamentary conductive paths across a dielectric sandwiched between two metal electrodes. These nano-size filaments can be formed and ruptured reliably and repeatedly by application of appropriate voltages. Although, there exists a large body of literature on this topic, many aspects of the CBRAM mechanisms and are still poorly understood. In the next paragraph, the aspects of CBRAM studied in this thesis are spelled out in more detail. CBRAM cell is not only an attractive candidate for a memory cell but is also a good implementation of a new circuit element, called memristor, as postulated by Leon Chua. Basically, a memristor, is a resistor with a memory. Such an element holds the promise to mimic neurological switching of neuron and synapses in human brain that are much more efficient than the Neuman computer architecture with its current CMOS logic technology. A memristive circuitry can possibly lead to much more powerful neural computers in the future. In the course of the research undertaken in this thesis, many memristive properties of the resistive cells have been found and used in models to describe the behavior of the resistive switching devices. The research performed in this study has also an immediate commercial application. Currently, the semiconductor industry is faced with so-called latency scaling dilemma. In the past, the bottleneck for the signal propagation was the time delay of the transistor. Today, the transistors became so fast that the bottleneck for the signal propagation is now the RC time delay of the interconnecting metal lines. Scaling drives both, resistance and parasitic capacitance of the metal lines to very high values. In this context, one observes that resistive switching memory does not require a Si substrate. It is therefore an excellent candidate for its implementation as an o n-chip memory above the logic circuits in the CMOS back-end, thus making the signal paths between logic and memory extremely short. In the framework of a Semiconductor Research Corporation (SRC) project with Intel Corporation, this thesis investigated the breakdown and resistive switching properties of currently deployed low k interlayer dielectrics to understand the mechanisms and potential of different material choices for a realization of an RRAM memory to be implemented in the back-end of a CMOS process flow. / Master of Science

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Muscle disorders and induced muscle atrophy impose critical risks to the well-being of an individual, limiting normal activities of daily living. Several resistance training methods have effectively reversed the progression of muscle atrophy. Weightlifting and hydrotherapy are the two widely practiced schemes for resistance training; however, there is the potential risk of excessive loads exerted on the muscles during weightlifting, and limited accessibility and cost are barriers to hydrotherapy. An alternative is using a resistance band. Some limitations include engaging multiple muscles/joints while only unidirectional resistance is feasible. To address these limitations, a VAriable Resistance Suit (VARS) was designed to provide speed-dependent, bi-directional, and variable resistance at a single joint. As a proof of concept, an elbow module of VARS was developed and validated experimentally through a pilot study [15]. This thesis aims to investigate the feasibility of modulating speed-dependent and adjustable resistance at the targeted joints using a VAriable Resistance Suit and investigate the efficacy of the Variable Resistance Suit to induce muscle hypertrophy. The pilot study shows the changes in flexor and extensor muscle activations in response to eight different levels of resistance modulated by VARS. Furthermore, the evaluation of training using VARS on muscle hypertrophy with a focus on the increase in muscle size and strength has been conducted through a prolonged study involving 12 participants. Some sections of this thesis are reused from our published conference paper which I co-author [15].

付記する学位プログラム名: 京都大学卓越大学院プログラム「先端光・電子デバイス創成学」 / 京都大学 / 新制・課程博士 / 博士(工学) / 甲第24622号 / 工博第5128号 / 新制||工||1980(附属図書館) / 京都大学大学院工学研究科電子工学専攻 / (主査)教授 木本 恒暢, 教授 白石 誠司, 准教授 小林 圭 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

The Hybrid Electric Vehicle (HEV) is a complex electromechanical system with complex interactions among various components. Due to the large number of design variables involved, the design flexibility in the HEV makes performance studies difficult. As the system complexity and sophistication increases, it becomes much more difficult to predict these interactions and design the system accordingly. Also, different variations in the design and manufacture of various components and systems involve a large amount of work and cost to keep updated of all these variations. While the above issues ask for a flexible design environment suitable for vehicle design, most of the existing powertrain design tools are based on experiential models, such as look-up tables, which use idealized assumptions and limited experimental data. The accuracy of the results produced by these tools is not good enough for designing these new generation vehicles. Also, sometimes the designs may lead to components or systems beyond physical limitations. To make the powertrain design more efficient, the models developed must be closely related to the underlying physics of the components. Only such physics-based models can facilitate high fidelity simulations for dynamics at different time scales. This results in the quest for a design tool that manages the vehicle?s development process while maintaining tight integration between the software and physical artifacts. The thesis addresses the above issues and focuses on the modeling of HEV using model integrated computing and employing physics-based resistive companion form modeling method. For this purpose, Generic Modeling Environment (GME), software developed by Institute of Software and Integrated Systems (ISIS), Vanderbilt University is used as the platform for developing the models. A modeling environment for hybrid vehicle design is prepared and a Battery Electric Vehicle (BEV) is developed as an application of the developed environment. Resistive companion form models of various BEV components are prepared and a model interpreter is prepared for integrating the developed component models and simulating the design.

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