PHD, Kent State University, 2022, College of Arts and Sciences / Department of Physics

Organic electrochemical transistors (OECTs) operate at very low voltages, transduce ions into electronic signals, and reach extremely large transconductance values, making them ideally suited for bio-sensing applications. However, despite their importance and promising performance, an incomplete understanding of their working mechanism is currently precluding a targeted design of OECTs and it is still challenging to formulate precise design rules guiding materials development in this field. Here, it is argued that the current capacitive model doesn't capture the full working mechanism of an OECT, and in particular fails to describe the transient response, its equilibrium states, and the dependence of transconductance on device geometry and applied voltages correctly. In this dissertation, current scaling laws for transconductance are revised in the light of a 2D device model that adequately accounts for drift and diffusion of ions inside the polymer channel. It is shown that the maximum transconductance of the devices is found at the transition between the depletion and accumulation region of the transistors. Furthermore, the switching is found to be strongly influenced by lateral ion currents. A consistent treatment of ion and hole currents leads to a dependency of time constants on the applied drain potential, and a complex dependency of the response time constants on the detailed device geometry. In addition to improving the understanding of the device physics of OECTs, studies on the influence of their device geometry are presented. Two different device geometries - top contact and bottom contact OECTs - are compared in terms of their contact resistance, reproducibility, and switching speed. It is shown that bottom contact devices have faster switching times, while their top-contact counterparts are superior in terms of contact-resistance and reproducibility. The origin of this trade-off between speed and reproducibility is discussed, which provides op (open full item for complete abstract)

Committee: Björn Lüssem (Advisor); Antal Jákli (Committee Member); Almut Schroeder (Committee Member); Sangeet Lamichhaney (Committee Member); Elda Hegmann (Committee Member) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 2021, Electrical and Computer Engineering

Ultra-wide bandgap III-Nitride materials, such as AlxGa1-xN, possess many desirable material properties and have been shown to be suitable for a wide range of RF and power applications. These materials possess high electron saturation velocity and extremely high breakdown electric fields. The combination of these two properties makes them ideal for power density scaling at microwave and mm-wave frequencies. Due to the high critical breakdown electric fields in these devices, it is possible to fabricate ultra-scaled devices with these materials as well, which is important for both RF and power devices. Scaling for RF devices helps to significantly boost the electron saturation velocity but maintaining a high electric field profile, which can significantly cut down on the carrier transit time. For power devices, scaling is also important, as the miniaturization of the power circuitry is critically dependent of smaller and more efficient devices. In addition, the large critical breakdown field of these materials could enable more favorable device characteristics for normally-off devices used in power switching applications. It is critical to investigate AlGaN-based devices for high frequency and high-power applications as the necessity for long-range communications at higher frequencies are becoming a necessity. This thesis discusses into the fabrication, analysis and characterization of ultra-wide bandgap AlxGa1-xN channel devices using heterostructure engineered contact layers aimed at improving contact resistance to these devices. Two different approaches to contact formation to metal organic chemical vapor deposition (MOCVD) grown high Al-composition AlGaN films are discussed. Using this approach state of the art device results are also reported. Novel approaches to electric field management in ultra-wide bandgap nitrides have also been explored in this work. Integration of extreme dielectric constant materials like BaTiO3 as an effective solution to electr (open full item for complete abstract)

Committee: Siddharth Rajan (Advisor); Wu Lu (Committee Member); Anant Agarwal (Committee Member) Subjects: Electrical Engineering; Physics

Master of Science, The Ohio State University, 2014, Electrical and Computer Engineering

Due to their unique material properties, 2D semiconductors such as MoS2 have recently attracted significant research interests for few applications including next generation nanoelectronics, flexible and transparent electronics, and optoelectronics. In this MS thesis research, we carried out studies on carrier mobility and transport, ohmic contacts, and gate dielectrics on few layer MoS2 for the purposes of developing high performance 2D MoS2 MOSFETs. Specifically, using the space charge model, we investigated the carrier mobility of few-layer MoS2 grown by chemical vapor deposition (CVD) on sapphire substrates. An electron mobility of 118 cm2/Vs has been demonstrated on structures with transmission line model (TLM) patterns. We also developed a new method to measure and calculate the contact resistance for non-uniform few-layer MoS2 grown by CVD. A specific contact resistivity of 8.4×10-5 Ωcm2 has been demonstrated on Ti/Au contacts on essentially intrinsic CVD grown few-layer MoS2 that were processed by plasma ion bombardment before metallization and annealed by rapid thermal annealing (RTA). Furthermore, we performed a temperature dependent conductance study of MoS2 . The preliminary result suggests that the carrier transport in the CVD grown few-layer MoS2 is Efros-Shklovskii variable range hopping. Finally, an atomic layer deposition (ALD) process has been optimized for the deposition of Al2 O3 on MoS2 as the gate dielectric for future MOSFET development.

Committee: Wu Lu (Advisor); George Valco (Committee Member) Subjects: Electrical Engineering

MS, University of Cincinnati, 2008, Engineering : Mechanical Engineering

Two-phase flow plays a major role in number of technologies used in fuel cells, bioreactors, phase and particle separators, thermal management systems, chemical reactors, etc. Some of these applications involve air–water two-phase flow in channels much less than 1 mm in diameter. The fundamental understanding of flow characteristics, such as flow regime, pressure drop, and heat transfer, is essential in the design and control of these devices. In this work, an experimental set-up is developed to investigate the effects of surface energy/surface wettability and geometry on characteristics of two-phase flow in horizontal microchannels at adiabatic conditions. Two-phase (air-water) slug flow is established in microchannel test sections of varying sol-gel dip coated surface wettabilities. Pressure drop measurements and flow pattern detection by high speed visualization are employed to characterize the flow. Results indicate that two-phase flow resistance is a strong function of surface wettability and it increases with static contact angle. Significant change in contact line in advancing and receding interface, and thus increase in flow resistance, is observed with increase of hydrophobicity while geometry of the channels influences the frequency of the slugs. Presence of thin liquid film, whose thickness is a strong function of wettability, is observed experimentally which clearly elucidates the pressure drop variation with surface wettability. Hence for characterizing two-phase flow in microchannels surface wettability parameters should be accounted appropriately.

Committee: Dr. Sang Young Son (Advisor) Subjects: Engineering, Mechanical

Doctor of Philosophy, The Ohio State University, 2004, Materials Science and Engineering

In this work, AC loss in superconducting composites was described using both an anisotropic continuum model and a discrete model. The efforts were concentrated in three main areas. First, the eddy current coupling loss of composites with rectangular cross section was calculated using an anisotropic continuum description based on a block model with different effective resistivities in each block. In this case, a numerical approach was used. This treatment, like the more typical lumped component network model, was able to describe many factors influencing the eddy current loss in the rectangular composites, such as twist pitch, aspect ratio, and core resistivity. However, the influence of core thickness and the presence of an outer sheath were also described with this model. Certain simplifying assumptions were used here to minimize computation time, while allowing the essential information to be extracted. In the second area, the eddy current loss of round composites were calculated from a discrete (network) point of view, and analytic expressions were developed which allow comparison to analytic expressions which were derived from effective medium theory. We need to measure only the contact resistance between the strands. The eddy current coupling loss of seven-strand MgB2 cables were then calculated by this model. With this model, it was possible to use a measured contact resistance between the strands to both predict the loss and compare to effective medium based resistivities. The results from the block model and from the analytical model give results in reasonable agreement. In the third part of the work, we attempt to compare the developed expressions to experiment. In some cases, data extant in the literature were used; in other cases, direct measurements were performed. For the rectangular geometry composites, existing data were sufficient. In the case of round composites, direct experiments were performed. The specific working medium chosen was round, seven- (open full item for complete abstract)

Committee: Suliman Dregia (Advisor) Subjects: Engineering, Materials Science

Doctor of Philosophy, The Ohio State University, 2003, Welding Engineering

This dissertation study has developed a fundamental understanding of the contact resistance behavior using the virtual contact volume concept and the equivalent contact resistivity definition. In this research, an integrated experimental-numerical approach was used to demonstrate the proposed equivalent contact resistivity versus temperature relationship. Such relationship was further used to study the expulsion behavior of the resistance spot welding aluminum alloy. A concept of using the constriction ring was demonstrated to be effective in preventing weld expulsion in resistance spot aluminum welds. The a-spot model simulated a single ball contact to define the equivalent contact resistivity. For a specific loading range, the generic equivalent contact resistivity versus temperature relationship, consisting of a proportional term and a exponential term, was derived. The proportionality term represents the increasing contact resistivity with temperature reflecting the Wiedemann-Franz-Lorentz behavior. The exponential term represents the softening effect of the contact surface as temperature increases. With the generic equivalent contact resistivity versus temperature relationship established, an a-spot welding which incorporated the contact pairs with the resistivity relationship leaving three factors as parametric variables and an experimental a-spot welding model were conducted. Both the numerical and experimental analyses show the same relationship that is defined by the theoretical hypothesis of a-spot model. The parametric study was conducted to quantify the parametric factors based on comparisons on nugget size and electrical potential drops across the electrodes. For an ideal combination of parameters at the Cu/Al interface and the Al/Al faying surface, both the comparisons of weld nugget size and potential drops are satisfactory. The mechanical analysis proposed a concept that expulsion would occur when the crack tip is within the solidus nugget zone at an (open full item for complete abstract)

Committee: Chon Tsai (Advisor) Subjects: Engineering, Mechanical

Master of Science, Miami University, 2011, Physics

We present experimental research on magneto-transport at Copper and GaMnAs interface. We report the values of specific contact resistance (ARc) between GaMnAs/Cu interfaces using Circular Transmission Line Method (CTLM) for a wide range of temperature (15K to 290K) above and below Curie temperature (Tc) which is at 145K. Our values of specific contact resistance are very low and close to the order of ~10-8 Ωcm2 which agrees with literature and changes abruptly to close to double of the value at Curie temperature when GaMnAs has phase transition between nonmagnetic to ferromagnetic. We suggest that this arises due to suppression of one of the two spin conduction channels when the phase transition of GaMnAs takes place. We also found the Specific contact resistance has a peak shifted towards lower temperature which suggests the magnetization in the GaMnAs film is suppressed near the Copper interface.

Committee: Khalid Eid PhD (Advisor); Michael Pechan PhD (Committee Member); Jan Yarrison-Rice PhD (Other); Herbert Jaeger PhD (Other) Subjects: Physics

Master of Science, Miami University, 2009, Physics

We present experimental research on magneto-transport on micro and nanostructures based on the ferromagnetic semiconductor (Ga,Mn)As, as well as macroscopic (un-patterned) samples. We report the values of contact resistance measurements between (Ga,Mn)As with different materials; copper, silver and aluminum. Copper gave relatively high values of contact resistance whereas aluminum gave extremely high values. Our values for specific resistance for silver of ~10-8 Ωcm2 are very low and agree with literature. We also report a process for the fabrication of the non local spin valves (NLSV's) using electron beam lithography. Finally we present our studies of the effect of native and grown oxides on the Curie temperature and resistivity of (Ga,Mn)As. Our results show that etching the oxide and annealing at temperatures close to growth temperatures enhances the Curie temperature and reduces the resistivity of the samples. The repeated anneal etch process improves the Curie temperature fast compared to continuous annealing.

Committee: Khalid F. Eid PhD (Advisor); Jan Yarrison-Rice PhD (Committee Member); Samir Bali PhD (Committee Member) Subjects: Physics