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Colleges of Nanoscale Science and Engineering Doctoral Dissertations
Doctoral Dissertations for the Colleges of Nanoscale Science and Engineering at SUNY Polytechnic Institute
Recent Submissions
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Characterization of Regulators of PTP1B OxidationIt is well established that the catalytic activity of Protein tyrosine phosphatases (PTPs) is directly inhibited by cellular oxidants. In this process, reversible oxidation of their catalytic Cys residue can lead to the formation of a sulfenic acid, intra- or intermolecular disulfides, or to the formation of a cyclic sulfenamide form. Reversible oxidation of Cys215 in PTP1B (PTP1B-OX) leads to the formation of a cyclic sulfenamide and to other important changes in its structure, including the solvent exposure of the phosphotyrosine binding loop. We have shown that binding of 14-3-3ζ to phosphoserine50 in the newly exposed phosphotyrosine binding loop was essential to stabilize PTP1B in its oxidized form, an important mechanistic insight into approaches to activate PTPs. In this process, we successfully developed an in vitro activity assay to measure the activity of PTP1B, enriched from cell lysate. We also showed that PTP1B was reversibly oxidized in cardiac hypertrophy and generated a PTP1B cardiomyocyte-specific knockout mouse model to study the inactivation of PTP1B in the heart. Importantly, the cardiac knockout of PTP1B caused a mild hypertrophic phenotype that was exacerbated by pressure overload. Inactivation of PTP1B lead to increased phosphorylation of Argonaute 2, which in turn prevented the association between Argonaute 2 and miR-208b and Argonaute 2-mediated silencing of Thrap1. In the process of these studies, we also updated a protocol for the modified cysteinyl labeling assay. Finally, we describe how cholesterol binds to PTP1B in cells, and how this interaction protects the PTP from reversible oxidation in cells and in mice treated with a high-cholesterol diet. Interestingly, our cell and animal work shows that cholesterol mediated PTP1B reduction prevents proper phosphorylation of the insulin receptor and cause insulin resistance. Overall, our studies bring new insight into the role of underlying mechanisms that regulate the reversible oxidation and reduction of PTP1B and shed light on the function of PTP1B in cardiac hypertrophy and insulin resistance.
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Optimization of 1.2 kV 4H-Silicon Carbide (SiC) Power Devices: High Performance, Reliability, and RuggednessThis research primarily focuses on the design, fabrication, and characterization of 1.2 kV 4H-SiC devices. Power devices play a critical role in numerous high-power applications, including automotive, industrial and energy applications. The development of energy-efficient power devices is essential for reducing power loss during operation. While silicon-based power devices are widely used in high-power applications, they have reached their limit in minimizing power loss. As a result, wide-bandgap materials, particularly 4H-Silicon Carbide (SiC), have gained traction as replacements for their Silicon counterparts due to their superior material properties, enabling further reduction of power dissipation beyond Si. The demand for 1.2 kV 4H-SiC MOSFETs has significantly increased, particularly in the electric vehicle (EV) market where high performance, reliability, and ruggedness are critical to compete with Si counterparts. Hence, the optimization of 1.2 kV 4H-SiC devices is necessary. The most distinctive feature of a power device is its ability to withstand high voltages within the drift region. The breakdown voltage of the power device is determined by the specifications of the drift region. The optimization of the drift region must be performed to enhance power efficiency for each specific application due to the trade-off relationship between on-resistance and breakdown voltage. 4H-SiC enables a thin, heavily doped drift region to support a given breakdown voltage, resulting in a substantial reduction in the on-resistance of the device compared to Si. Moreover, Hybrid Junction Termination Extension (Hybrid-JTE) technique was employed to achieve a near-ideal breakdown voltage and experimentally verified. The influence of deep JFET and P-well implants in 1.2 kV MOSFETs has been examined in terms of their impact on static characteristics and short-circuit ruggedness. To assess the impact on output characteristics and short-circuit ruggedness, the depths of JFET and P-well implants were compared by varying channel lengths and JFET widths. Furthermore, the significance of high channel mobility has been investigated not only for static characteristics but also for short-circuit characteristics. The optimization of static characteristics of 1.2 kV 4H-SiC MOSFETs have been investigated through the analysis of the cell structure. A comprehensive analysis has been conducted to examine the trade-off relationship between specific on-resistance and breakdown voltage, as well as yield, by considering various dimensions within the cell structure. The dimensions explored in this analysis include the channel, JFET, contact opening, ILD (Inter-Layer Dielectric) width, and gate-to-source overlap within the cell structure. A novel approach has been proposed to enhance the trade-off relationship between short-circuit withstand time and specific on-resistance by employing MOSFETs with a deep P-well structure through channeling implantation. For the channeling implantation, a tilt angle of 4 degrees was adjusted to <0001> direction of 4H-SiC in 4H-SiC (0001) substrates with 4 ° off-cut towered <11-20> direction. The utilization of channeling implantation has been employed to overcome the limitations associated with previous random implantation energy. The successful fabrication and demonstration of MOSFETs with deep P-well structures using channeling implantation have been achieved. The MOSFETs with a deep P-well structure enable the short channel lengths, which improve the trade-off relationship between specific on-resistance and breakdown voltage. The implementation of a deep P-well structure effectively suppresses the leakage current originating from the channel during the blocking-mode of operation, thereby enhancing the trade-off relationship. Additionally, the deep P-well structure has significantly reduced the maximum electric field in the gate oxide, leading to improved high temperature reverse bias (HTRB) characteristics. A novel layout approach has been proposed and successfully demonstrated for the monolithic integration of a Schottky diode with 1.2 kV SiC MOSFETs (JBSFETs) to achieve an identical cell pitch compared to the pure MOSFET design. To further reduce cell density, highly doped JFET implantation with narrow widths of JFET/Schottky regions has been conducted. Consequently, the proposed JBSFET demonstrates comparable static performance to the pure MOSFET while exhibiting 3rd quadrant current-voltage characteristics similar to JBS diodes. A thorough comparison of the short-circuit failure mechanisms between 1.2 kV 4H-SiC MOSFETs and Ti JBSFETs, both having identical cell pitch and specific on-resistance, has been successfully accomplished. However, despite the same channel density, different short-circuit characteristics have been observed due to the presence of leakage current from the Schottky contact in the JBSFETs. In order to comprehend the short-circuit failure mechanisms, non-isothermal mixed-mode 2D TCAD device simulations have been employed. Moreover, based on the experimental results and analyses, potential solutions to further enhance the short-circuit characteristics of JBSFETs have been proposed. A 1.2 kV 4H-SiC planar Junction Barrier Schottky (JBS) diode with a deep P+ grid structure, implemented through channeling implantation, has been successfully designed and fabricated. Without the use of a trench structure, a planar JBS diode with a junction depth of 2.2 μm has been successfully fabricated using an implantation energy of 350 keV. The formation of the deep junction significantly suppressed the leakage current originating from the Schottky contact. In summary, extensive examinations have been conducted on 1.2 kV rated 4H-SiC power devices, including MOSFETs, JBSFETs, and JBS diodes, to optimize and enhance their static characteristics, dynamic characteristics, reliability, and ruggedness.
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Development of 4H-SiC SMART (Scalable, Manufacturable, And Robust Technology) Power ICsThe research primarily focuses on the design and development of SMART Power Integrated Circuits (ICs) in silicon carbide (4H-SiC). Over the past decades, power conversion has become more prevalent within the US as technological innovation has enabled the electrification of industrial systems, from energy to aerospace. Silicon (Si)-based power and CMOS devices have been the amicable semiconductor technology for power conversion. However, with the ever-evolving application space, the inherent material properties of Si hamper the capabilities in terms of power processing and high-temperature (HT) operation. The current generation power IC (multiple power integrated functions onto a single chip) technologies, predominantly Bulk-Silicon and Silicon-On-Insulator (SOI) technologies have limitations in their operational temperatures and power handling capability. Based on the theoretical limits, Si-based ICs are rated at 150 oC and are not operational beyond 200 oC due to leakage and reliability issues. Although SOI technology offers relief up to 300 oC, with the insulated region, it also fails beyond 300 oC. In recent decades, 4H-SiC has emerged as a reliable material for the development of high-voltage (HV) and high-temperature power devices. Due v to its superior material properties, 4H-SiC power devices can operate at high power and high temperatures when compared to their Si counterparts. In the current day scenario, Si-based power and control ICs drive the HV discrete 4H-SiC Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs), and this technology appends to an increase in system footprint, and also in the parasitic effects from the interconnects hampering the reliability. Also, the high-temperature operation, one of the significant assets of 4H-SiC cannot be exploited. Hence, a single-chip 4H-SiC-based IC solution by monolithically integrating the HV Power MOSFET with low-voltage (LV) complementary metal-oxide-semiconductor (CMOS) can be a considerable solution to address the high power and extreme temperature challenges of the Si power ICs. The development of 4H-SiC-based power ICs is now seamlessly possible, thanks to considerable progress made over the last decade in material development and device fabrication. The exceptional advancements and the significant progress that was made in developing the technology platform for the demonstration of 4H-SiC Power ICs are reported in this dissertation. The fundamental step in designing the technology roadmap of a semiconductor material is understanding the trade-off performances of that particular semiconductor. Hence a detailed trade-off analysis was reported that was conducted on 4H-SiC and other wide bandgap semiconductors (GaN, Ga2O3, and diamond). This analysis concluded by letting the designers know the criticality of meticulous scrutiny and cautious selection of impact ionization coefficients from the existing literature to ensure accurate assessment and optimization of trade-off performance parameters. Additionally, simplified generalized equations for both non- vi punch-through (NPT) and punch-through (PT) design configurations to effectively design the drift layers in unipolar 4H-SiC power devices are documented. 2D-device, process simulations, and experimental demonstration of the HV lateral MOSFETs and diodes in 4H-SiC, specifically tailored for integration within power ICs are discussed. The cell designs, field management techniques, peripheral designs, and BV tailoring techniques of the HV lateral devices are reported in detail. The HV lateral devices are designed to operate at (400V-600V) and to be integrated with the Power ICs. The experimental results of the HV lateral devices demonstrate that the devices not only have the best-in-class Ron,sp - BV trade-off performance but are also capable of handling large currents validating efficient cell and peripheral design techniques. The design and analysis of critical module processes for CMOS development are also detailed. Channel engineering techniques (accumulation mode vs Inversion mode) are applied to match the threshold voltages (Vth) of the LV NMOS and PMOS. Multiple gate oxide recipes are developed to maximize the channel mobilities of electrons and holes. The results of the efforts dedicated to optimizing CMOS performance through improved ohmic contacts, including the investigation of metal contacts for simultaneous formation of n-type and p-type ohmic contacts were reported. The critical need for high-voltage isolation in power IC technology to ensure safety, reliability, and proper functioning was addressed. The utilization of junction isolation through the P+ Isolation junction implemented via Aluminum channeling implantation has been experimentally verified to yield promising blocking voltages required for the reliable operation vii of the ICs. Another HV isolation requirement which is the interlayer dielectric (ILD) voltage blocking between the adjacent metal layers carrying different voltage potentials is also addressed. Building upon the developed CMOS technology, the performance of digital CMOS ICs at extreme temperatures up to 400 °C has been demonstrated, covering packaging flow, assembly process, employed materials, and encountered challenges during HT measurements. This successful performance of the CMOS ICs at extreme temperatures (400 oC) further confirmed the potential of 4H-SiC as a promising material for the development of high-temperature electronics.
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Development of Electrohydrodynamic Printing Technology with Einzel Lens FocusingAdditive manufacturing has several advantages over conventional subtractive fabrication techniques such as CNC machining, including limited waste and the ability to build complicated structures using fewer processing steps. Recently, a cost-effective, versatile method of high-resolution printing called electrohydrodynamic printing has been developed. With this technique, droplets are deposited from a tip by applying an external electric field. This method allows for spatial resolution in the hundreds of nanometers when used in a drop-on-demand mode of operation. However, this mode has the drawback of relatively slow deposition rates. To increase the rate of deposition, it is desired to deposit the droplets using a continuous spray, which is called the cone jet mode. However, this mode of operation has a much lower resolution because of the space charge effect (lateral spread of the stream of charged droplets from Coulomb repulsion). The primary goal of this research project was to develop an electrohydrodynamic printing system with a focusing element to allow printing at much higher deposition rates than the drop-on-demand mode while maintaining reasonably high lateral resolution. After modeling different focusing systems, an Einzel lens was determined to be the optimal focusing element. A custom electrohydrodynamic printing system with a three-element Einzel lens was fabricated and tested with 10% polyethylene glycol solution. Droplets with a lateral diameter of ~1 μm after drying were successfully deposited. The incorporation of the Einzel lens resulted in an order of magnitude improvement in lateral resolution of the spray.
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Impact of Surface Polarity on the Air Stability and Quantum Efficiency of Cs-Free III-Nitride PhotocathodesPhotocathodes are employed as photodetectors for astronomy and defense applications, as well as electron sources in high energy physics technologies. Photocathodes absorb incident photons, resulting in the emission of electrons. The III-nitride material system is promising for photocathodes due to the wide and tunable band-gap energy spanning infrared to ultraviolet wavelengths. III-nitrides are air-stable, radiation hard, and possess internal polarization charge. Nitrogen polar III-nitrides photocathodes have been predicted to achieve high device quantum efficiency (QE) and effective negative electron affinity due to alignment of the polarization and depletion charges. However, fundamental challenges of p-type doping of III-nitrides, and experimental considerations of unintentional dopant incorporation at surfaces and interfaces initially inhibited repeatable high QE N-polar photocathodes. To address these challenges, a comprehensive approach was used for the development of air-stable, high QE III-nitride photocathodes, investigating the impact of polarity on Mg-dopant incorporation efficiency, distribution of unintentional impurities due to diffusion at interfaces, and on surface passivation. The impact of hillock structures commonly present on the N-polar surface on device QE was investigated, finding a 2x increase in efficiency in photocathodes grown on high hillock density templates. Atom probe tomography (APT) measurements reveal a decrease in Mg-dopant clustering and improved incorporation efficiency in the semi-polar facets of the hillocks, leading to improved optical and electrical characteristics. Building upon this finding, a selective area growth technique is used to create hexagonal pyramid structure in both the N- and Ga-polar orientations as a model to more controllably study the mechanism of Mgincorporation within the semi-polar planes of N-polar hillocks. III-nitride hetero/homo-interfaces are commonly implemented in photocathode device design. Growth of such structures may involve growth interruption and/or exposure of the interface to ambient conditions. Incorporation and diffusion of unintentional impurities including oxygen and carbon from these interfaces has been studied here by SIMS depth profiling and modeling of defect mediated diffusion mechanisms, and their impact on photocathode performance was studied. Through consideration of interface proximity to the surface, and the temperature of processes occurring post-regrowth interface, the unintentional impurity and electrostatic profile is controlled. Emission of photoexcited electrons and hence QE depends greatly on the condition of the surface/states and its impact on surface band bending. It is well known that a non-negligible surface oxide is present on the N-polar III-nitride surface. We have observed removal of the surface oxide and deposition of positive surface charge by HCl cleaning prior to measurement, leading to an order of magnitude increase in QE. However, oxide regrowth occurs following airexposure. We have studied 2D materials including graphene and h-BN as passivation layers to prevent contamination and fully stabilize the surface charge. Sustained decrease in oxygen and carbon coverage have been measured by XPS in h-BN/GaN photocathodes after air exposure on the order of days. The combined findings of the impact of material polarity on Mg-dopant incorporation efficiency, control of the unintentional impurity profile and surface passivation was utilized to optimize the photocathode electrostatic profile for optimal device characteristics, the result of which is a maximum QE of 26.6% at 6 eV photon energy was achieved for an HCl cleaned Npolar p-GaN/u-GaN cap structure grown on high hillock density GaN template without a regrowth interface between active layers. This represents the highest reported QE for a Cs-free GaN photocathode to date [1], [2]. [1] J. Marini, I. Mahaboob, E. Rocco, L. D. Bell, and F. Shahedipour-Sandvik, “Polarization engineered N-polar Cs-free GaN photocathodes,” J. Appl. Phys., vol. 124, no. 11, p. 113101, Sep. 2018, doi: 10.1063/1.5029975. [2] E. Rocco et al., “Overview and Progress Toward High-Efficiency, Air Stable, Cs-free IIINitride Photocathode Detectors,” IEEE Photonics J., vol. 14, no. 2, p. 6818312, 2022.
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RRAM Device Optimization and Circuit Design for Low Power Low Latency Domain Specific Edge ApplicationsConventional von Neumann computing architecture has little parallelism and physically divides memory and logic units. In-memory computation offers further opportunity for power and efficiency improvements as data transport between memory and logic units introduces substantial power consumption and latency. RRAM memory devices are a non-volatile, highly scalable alternative that is compatible with advanced logic CMOS processes. Because of its in-memory computing capabilities, it is a more appealing solution for data-intensive applications. Depending on the needs of the application, RRAM's different switching types such as binary switching, multilayer switching, and analog switching can be employed. These devices can be utilized in applications where they must continuously update their condition (e.g neural network training). The RRAM memory system can be of great use in applications where the device must continuously maintain the resistance state as well, either with or without in-memory processing. Moreover, RRAM devices are currently being researched for in-sensor or near-sensor applications to speed up AI inference. The diversity of RRAM switching schemes and application range creates vast research opportunities to examine the viability of these devices. Hence, leveraging RRAM’s unique switching properties analog (continuous) or binary/multilevel switching, in-memory computation scheme (MAC or bit-wise) as well as integration location (in-pixel, near pixel or memory) there is still a wide range of different applications yet to be explored. In this work, RRAM analog switching properties of based on different materials stacks, inference-like applications such as error correcting code implementation, genome alignment, and in-sensor AI inference acceleration have been investigated.
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Predicting the Effects of Capacity Fade and Thermal Behavior of Silicon Anodes in Lithium-Ion BatteriesSilicon is one of the most promising anode materials for Li-ion batteries, but large volume expansion, pulverization, and strains accelerate electrode disintegration and lead to capacity fade. Our research addressed the problems in silicon anodes in lithium-ion batteries and through modeling proposed solutions. Previous work has used additives and novel electrolytes to create a stable SEI (solid electrolyte interface) to suit silicon surface interaction. Hence, we used a separator which is a porous matrix filled with electrolyte made up of lithium hexafluorophosphate (LiPF6) dissolved in a 3:7 liquid mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). We concentrated the work on the growth and kinetics of SEI layer. Since capacity loss is in part dependent on the cell materials, two different electrodes, LiNi0.8Co0.15Al0.05O2 (NCA) and LiNi1/3Mn1/3Co1/3O2 (NMC 111), were used in combination with silicon to study capacity changes using simulations in COMSOL version 5.5. The results of these studies provide insight into the effects of anode particle size and electrolyte volume fraction on the behavior of silicon anode-based batteries with different positive electrodes. It was observed that the performance of a porous matrix of solid active particles of silicon anode could be improved when the active particles were 150 nm or smaller. The range of optimized values of volume fraction of the electrolyte in the silicon anode were determined to be between 0.55 and 0.40. The silicon anode behaved differently over the cycle time with NCA and NMC cathodes. However, NMC111 gave a high relative capacity in comparison to NCA and proved to be a better working electrode for the proposed silicon anode structure. We also discuss the role of electrode structural characteristics on the thermal behavior of lithium-ion batteries. Preliminary modeling runs have employed a 1D lithium-ion battery coupled to a two-dimensional axisymmetric model using silicon as the battery anode material. The two models are coupled by the heat generated and the average temperature. Our study is focused on the silicon anode particle sizes, and it is observed that silicon anodes with nano-sized particles reduced the temperature of the battery in comparison to anodes with larger particles. These results are discussed in the context of the relationship between particle size and thermal transport properties in the electrode.
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Biomimetic Scaffolds Targeting Remediation of Fibrosis and Regeneration of the Salivary GlandFibrosis, characterized by aberrant deposition of extracellular matrix (ECM) is a contributor to about 45% of deaths worldwide. Fibrosis in the salivary gland, caused by Sjogren’s syndrome, diabetes mellitus, or radiation therapy for head and neck cancers, results in salivary hypofunction characterized by reduced saliva output or changes in its composition, leading to poor oral and digestive health. Current therapies for salivary hypofunction are palliative and inefficient, and regenerative strategies are an appealing therapeutic alternative. Mesenchymal stem/stromal cell (MSC) therapy can limit fibrosis but faces translational challenges due to transient therapeutic effects. Scaffold-based approaches can improve the efficacy of MSC delivery by localizing MSCs near the tissue, improving MSC engraftment and persistence, potentially modulating the in vivo tissue-resident cells, and promoting tissue regeneration. Ideally, scaffolds should emulate native soft tissue ECM to provide key physical, biochemical and mechanical cues that maintain the regenerative potential of MSCs. In this work, we address the limitations of current scaffolding technologies, by developing a novel cryoelectrospinning process, and exploring scaffold chemistry to fabricate scaffolds that mimic the minimal fibrous backbone, porous morphology, and viscoelasticity of decellularized salivary glands (DSG). We used elastin and alginate as natural, compliant biomaterials and water as the solvent for cryoelectrospinning biocompatible scaffolds. We optimized process parameters to produce a unique honeycomb topography, similar to DSG, and optimized collector plate geometries to produce a high throughput yield of >100 scaffolds/run. We demonstrated 3D stromal and epithelial growth on our cryoelectrospun scaffolds (CES) and showed that their coculture facilitated cell-cell interactions resembling normal tissue structure. We demonstrated the feasibility of maintaining MSC-like primary embryonic day 16 (E16) mesenchyme on CES and the ability of CES to repress fibrotic activity, similar to DSG. We also determined that FGF2 supplementation improved stromal health of primary embryonic mesenchyme on CES. Finally, we demonstrated the antifibrotic properties of CES, primary E16 mesenchyme, and FGF2 by the repression of fibrotic activity of myofibroblasts. Overall, in this work, we have developed novel scaffolds that mimic soft tissue ECM and show great potential for use in in vitro organ models and stromal cell delivery for in vivo regenerative therapy.
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The Transport Properties of High-Temperature Superconductor/Graphene JunctionsCharge transport in graphene (Gr) has been studied using conventional superconductors (SCs) in different types of devices. Graphene Josephson junctions (Gr-JJ), using s-wave superconductors, have shown supercurrents that are gate tunable. Andreev reflection (AR), which occurs at both Gr-SC interfaces in a Gr-JJ is responsible for the supercurrent. A prelude to realizing Gr-JJ using unconventional SCs, such as Bi2Sr2CaCu2O8+x (BSCCO), is to understand transport in a single BSCCO/Gr junction which can reveal unique aspects of AR. We have found that charge transfer from BSCCO into graphene creates a hole-doped region at the boundary of the BSCCO/Gr junction. This doped region renders two different interfaces, one with a superconducting graphene region and the other with a bare graphene region. Together they contribute to two scattering mechanisms – AR and Klein tunneling (KT), respectively, that create resonance states. We use a global gate to simultaneously gate the doped and bare graphene regions and observe an asymmetry in conductance as doping configuration changes from p+-p to p-n, a hallmark of KT. A resonant interference condition occurs for every two round trips of carriers within the doped region, which produces oscillations in conductance while sweeping the gate voltage. We observe the suppression of the oscillation amplitude above a junction bias ranging between 10 to 15 mV in over half-a-dozen BSCCO/Gr devices. This energy is consistent with the expected size of the induced gap of Δi ~15 meV measured by others. Furthermore, we have studied the transport properties in BSCCO-Gr-BSCCO junctions that were fabricated by mechanically cracking a single BSCCO flake, to control the separation length from 20 nm to 3 μm between the SCs, while setting it on graphene. We observe that the oscillation period increases as junction length decreases. In long junctions, the period tends to that of the single junction. The oscillations suppress for bias between 20 to 30 meV in the BSCCO-Gr-BSCCO junction which corresponds to 2Δi, as expected for transport between two SCs. These results show the rich transport phenomena in junctions of BSCCO and graphene that may lead to the realization of gate-tunable high-temperature graphene Josephson junctions for quantum computing applications. Superconducting d-wave proximity in graphene could create an effective p-wave pairing, which is one of the ingredients to create Majorana excitations.
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Reliability and Performance Optimization of Resistive Random-Access Memory Devices for Advanced Computing and Memory ApplicationsCurrent trends towards more efficient computing include alternatives to the standard von Neumann architecture and a trend towards in-memory computing with non-volatile memory (NVM) arrays. Resistive Random-Access Memory (ReRAM) is a strong NVM candidate for such applications. In this work, performance optimization and reliability studies were performed using hafnium oxide and tantalum oxide-based ReRAM devices integrated into 65nm CMOS. A thorough analysis of the effects of switching parameters such as maximum current, voltage, pulse width, and temperature-based stress culminated in improved device performance, yielding a memory window (MW) > 30, excellent endurance >2.1x1010, and retention at multiple resistance levels for 104 seconds without degradation. When the operational temperature was ramped from 25-125 °C oxygen vacancy mobility and generation rates shifted in these devices, directly affecting MW by up to 2X. To demonstrate the potential of ReRAM for neuromorphic applications, multilevel (analog) switching was implemented, achieving a total of >10 statistically distinct resistance levels when using large (>50 ns) pulses. When using ultra-short pulses (300 ps) the number of resistance states was limited to < 15 and resulted in a narrow conduction window (CW) of ~2X. Thus, an optimized pulsing scheme, incremental pulsing (ISPP), was utilized in which successive switching pulses increase in voltage amplitude. When used in conjunction with a read-verify scheme, the total number of resistance states increased to >20 and the CW increased to ~31X respectively, while also maintaining the linearity and symmetry of potentiation or depression. Based on these empirical data, the Neural Network (NN) learning algorithm “Cross-Sim” simulator was trained on the MNIST dataset, yielding 96.55% accuracy, on a 96.7% baseline, when using the ISPP algorithm. Taken together, these results demonstrate the potential of ReRAM 24 for non-von Neumann computing applications once proper optimization of electrical switching parameters and operational temperature is achieved.
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Advancement of Gallium Nitride PIN Diode and Development of Novel 3D+planar Core-shell Microstructures for Betavoltaic Device TechnologyA betavoltaic (BV) device converts the kinetic energy from β-electrons emitted by radioactive decay into electricity, with output power in the <1 mW regime. AlxGa1-xN is promising for use as the converter material in a BV device due to its wide bandgap, superior radiation tolerance, chemical inertness, and physical hardness. With the emergence of micro- and nanotechnology, many low power systems require self-sustained power sources where replacement is difficult or impossible in harsh environments, for which BV batteries are a prime candidate. BV batteries have been investigated for several decades; however, the technology still lacks in efficient energy conversion and limited power output. The work reported here has addressed some of the challenges associated with, and improved upon, the GaN-based beta-energy converter, along with engineering new methods for device qualification under high energy irradiation and radioisotope sources. Uniquely, a converter structure has been designed, simulated, and implemented with the growth of novel GaN 3D+planar core-shell microstructures. High performing GaN PIN planar diodes were fabricated by tailoring the fabrication process for BV specific needs such as a “beta-transparent” p-contact, low forward leakage by a KOH wetetch passivation treatment and implementing a large area mesa for greater electron absorption. Electron energy irradiation was performed to mimic radioisotope exposure using an electron flood gun (4-16 keV) and a custom in-operando TEM setup (62 – 200 keV). Beam voltage and beam current dependence is reported for several devices. For the electron flood gun irradiation, the highest efficiency of energy conversion at 7% is reported for GaN PIN exposed to 16 keV electron irradiation. Direct measurement of these GaN PIN diodes under exposure to solid metal radioisotope and liquid radioisotope solution is also reported, for both 63Ni and 147Pm isotopes. Moreover, methodology for device testing in custom enclosures was developed and executed. A combined 3D (planar+core-shell) PIN device has been proposed as the optimal design for GaN application as a BV battery. This optimized structure layout leads to enhanced conversion (depletion) region volume compared to that in a planar device under equilibrium conditions. Monte Carlo simulation indicates there is a 3.75x increase in the amount of power absorbed in the GaN layers (PGaN/cm2) at approximately half the activity density for a 3D structure with 4 μm mesa height compared to planar designs with 10 μm 63Ni thickness with a 5.8x improvement in energy transfer efficiency (ηsrc). Metalorganic chemical vapor deposition (MOCVD) growth of the proposed combined 3D+planar GaN PIN was achieved in both a fin and pillar geometry. High aspect ratio n-GaN seeds with controlled facet stabilization were obtained by optimization of the MOCVD growth conditions. An optimized growth condition is achieved where GaN semipolar sidewalls are replaced by m-plane nonpolar sidewalls characterized by their 90° angle with respect to the c-plane (substrate), which was selected as the seed condition for the combined 3D+planar core-shell structure. The fin combined 3D+planar PIN showed bipolar diode behavior with a threshold voltage of ~3 V.
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Development and Fabrication of Low Voltage (600 V) to High Voltage (15 kV) 4H-Silicon Carbide (SiC) Power DevicesThe research primarily focuses on the development and fabrication of 4H-Silicon Carbide (SiC) power devices. As of today, power devices play a substantial role in high power applications such as fast-charging stations for electric vehicles, inverters for solar power, and energy storage equipment, to name a few. To minimize power loss during the operation, one of the key elements is to develop an energy-efficient power device. Although silicon (Si)-based power devices are currently being used in various high power applications, Si reached its physical limit in power loss reduction. In this aspect, wide-bandgap material, especially 4H-Silicon Carbide (SiC), became an excellent candidate to replace Si to fabricate power semiconductor devices that enable further minimization of power dissipation beyond Si. To advance the present and future low voltage (600 V) and high voltage (15 kV) power applications, the development of both low voltage and high voltage power devices are imperative. The most unique feature of a power device is the ability to withstand high voltages (> 600 V) with a voltage supporting layer, called the “drift region”. The breakdown voltage of the power device depends on the thickness and doping concentration of the drift region, as most of the voltage is supported by the depletion region formed within the drift layer. The optimization of the drift region must be performed to meet the breakdown voltage requirements based on the application while minimizing the on-state voltage drop to reduce power dissipation. When compared to the Si counterparts, SiC allows for the design of a thin, heavily doped drift region to support a specified voltage due to its superior material properties. Additionally, leakage currents generated during the off-state mode are also significantly suppressed due to two orders of magnitude lower intrinsic carrier density than that of Si. These merits of SiC become more substantial when building high voltage power devices (>3.3 kV) where resistance in the drift region dominates the overall on-resistance of the device. The details of optimizing device structures, fabrication details, and electrical characterizations of 600 V to 15 kV 4H-SiC power devices are discussed in this dissertation. The fundamental of the power device including the design of the drift layer and edge termination techniques for the power device will be discussed. To improve the low voltage application (i.e. electrical vehicles and photovoltaic converters), 600 V-rated lateral and vertical MOSFETs were developed and fabricated. From this work, the world's first high current (10 A) and high voltage (600 V) SiC lateral MOSFET was demonstrated. The fabricated lateral MOSFET was compared with the state-of-the-art vertical power MOSFET to identify the performance gaps to further enhance the electrical performances of the lateral MOSFETs. 600 V vertical MOSFETs and JBSFET (Junction-Barrier-Schottky (JBS) diode integrated MOSFET) were also developed to reduce the power loss in the system by replacing the Si-IGBTs (insulated-gate-bipolar-transistor) in the circuitry. The utilization of unipolar devices (i.e. MOSFET) is often more favorable than the bipolar devices (i.e. IGBT) due to faster switching speed and lower switching loss. On the other hand, the development of high voltage (> 6.5kV) devices are essential for high power applications such as power grids, military vehicles, to name a few. The fabrication and application of single-chip, high voltage devices are advantageous in terms of replacing many series-connected devices used to withstand high voltage in power circuits. However, research on ≥ 6.5kV-rated 4H-SiC power devices are very limited. With this motivation, 6.5 kV to 15 kV SiC JBS diodes, MOSFETs, and JBSFETs were designed and fabricated. From this study, we identified that device optimization for high voltage (> 6.5 kV) devices are different from the low voltage (< 1700V) devices due low background doping concentration of high voltage devices. Critical design considerations for fabricating 6.5 kV to 15 kV devices will be discussed. Both static and dynamic characteristics were also evaluated and compared, respectively.
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The Photovoltaic Properties of Carbon Nanotube Network p-n DiodesSingle Walled Carbon nanotubes (SWCNTs) are quasi one dimensional rolled up sheets of graphene with amazing optical and electronic properties. Depending on their diameter and roll up angle, SWCNTs come in varying chiralities with multiple bandgaps giving them exceptional properties that make them attractive for photovoltaic applications. One of such properties is the absorption of light across the broad solar spectrum, a highly desirable property in semiconducting solar cell absorbers. In this dissertation, we will be exploring our attempt to fabricate a fundamental device that enables us harness the full sunlight potential of semiconducting SWCNT (s-SWCNT) networks and have a better understanding of its photovoltaic properties. To fabricate this fundamental device, we look to nature for inspiration on solar energy conversion. We use the process of photosynthesis as a model for building our solar energy conversion device. Nature, through centuries of evolution, has perfected the harvesting of light for energy conversion through the process of photosynthesis by employing two main mechanisms carried out by distinct proteins: excitation energy transfer, where light harvesting complexes capture light from multiple regions of the solar spectrum and funnel photoexcitations to a reaction center, and charge separation, where the photoexcitations become free charges in the reaction center. As we will see in this dissertation, SWCNTs have similar properties to that of photosynthetic systems, one of which is the varying chiralities of SWCNTs with different diameters, analogous to the distinct proteins in photosynthetic systems absorbing light at different wavelengths. We fabricate p-n diodes on various networks of s-SWCNTs, we study the intrinsic electronic and optical properties of nearly monochiral and polychiral s-SWCNT networks and form a fundamental understanding of the best s-SWCNT films required to make more ideal diodes. We examine the current-voltage characteristics of these diodes in the dark and find correlations between the key figure of merits, including the diode leakage current and the ideality factor, to different s-SWCNT networks. We also examine their optical properties by measuring wavelength-dependent photocurrent spectroscopy to gain insights into the dynamics of excitons in a network of s-SWCNTs. We achieve ideal diodes, for the first time in a homogenous network of s-SWCNTs. We discuss the limitations of using ideal diodes in the measurement of the electronic bandgap of s-SWCNT networks and then use non – ideal diodes to measure the electronic bandgaps of the s-SWCNT networks for the first time. After a more in-depth understanding of the dark diode characteristics of the s-SWCNT networks, we progress to fabricating a fundamental solar energy conversion device, modelled after photosynthesis. We fabricate photovoltaic diodes mimicking photosynthetic systems. Using different s-SWCNT chiralities, we create an energy funnel in our diodes by layering different s-SWCNT networks according to their bandgaps. The photo excitations in the larger bandgap s-SWCNTs are funneled down to the smallest bandgap s-SWCNT, allowing us to increase the spectral response of our diodes. We show that the photocurrent generation in our energy funnel is more efficient than in diodes formed using single chirality s-SWCNT networks. Finally, we show that our device architecture increases the photocurrent without increasing the highly undesirable dark leakage current. Using the analogy to photosynthetic systems, we use the smallest bandgap s-SWCNT network to create the diode (Reaction Center). The larger bandgap s-SWCNT networks act as light harvesters. We demonstrate an increase in short circuit current and the open circuit voltage as we add these nanotubes sequentially. We use this device to implement the mechanisms of exciton energy transfer in our p-n diodes and study its properties as it applies to s-SWCNT networks. We see some new and exciting physics which we will cover in this dissertation.
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Regulation of PTP1B Activity and Insulin Resistance by Cellular CholesterolProtein Tyrosine phosphatase 1B (PTP1B), is an endoplasmic resident protein and a well-known negative regulator of the insulin receptor, dephosphorylating Tyr-1162, and Tyr-1163, two residues located in the activation loop of the insulin receptor. Mice lacking the PTPN1 gene encoding for PTP1B exhibit increased insulin sensitivity and improved glucose tolerance. Apart from its role in insulin signaling, mice lacking PTP1B show resistance to weight gain on a highfat diet, increased basal metabolic rate, and decreased cholesterol levels. In addition, PTP1B was previously identified in a proteome-wide mapping of cholesterol-interacting proteins in mammalian cells. However, the relationship between PTP1B and cholesterol is still unclear. To better understand the role of cholesterol on PTP1B function and on insulin signaling, we first used an in silico approach to predict cholesterol-binding sites in the 3D structure of the phosphatase and confirmed the binding sites through fluorescence binding studies and mass fingerprinting. We confirmed that the association between PTP1B and cholesterol occurred in both in vitro and in mammalian cells. In an attempt to understand whether cholesterol affects the ability of PTP1B to dephosphorylate substrates, we performed activity assays in various conditions. We observed that cholesterol could reduce and reactivate the reversibly oxidized form of PTP1B in vitro. Treatment of mammalian cells with cholesterol confirmed that excess cholesterol kept PTP1B reduced, and decreased Insulin Receptor phosphorylation and downstream signaling. In vivo results obtained by exposing mice to a high cholesterol diet support a role in the cholesterol-mediated reduction of PTP1B and decreased insulin sensitivity in the liver. We have established an electron tunneling path between the allosteric site and the catalytic cysteine residue and used a redox-sensitive fluorophore to measure electron tunneling in vitro. Hence, our results demonstrate for the first time that cholesterol binds to PTP1B at an allosteric site and reduces the phosphatase to regulate its activity and insulin signaling. Based on these results we propose a novel role for cholesterol in activating enzymes and in the context of insulin resistance.
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Resist and Process Pattern Variations in Advanced Node Semiconductor Device FabricationPattern variations can cause challenges in device scaling. Since the last few decades, the semiconductor industry has successfully utilized the device scaling technique by reducing the transistor area to meet the requirements needed for optimum device performance and fabrication cost during each generation of development. The main challenges in the development of this technique are imaging resolution and pattern variations. Extreme ultraviolet (EUV) lithography and the multiple-patterning method can be used to push the imaging resolution to sub-30 nm. This thesis investigates the mechanism of pattern variations and proposes methods for pattern improvement. The thesis begins by investigating the origin of pattern variations in an EUV–chemically amplified photoresist system. The experimental results show that the chemical composition and inhomogeneity of the material contribute to pattern variations in EUV lithography. A difference in the localized-material-removal rate indicates the contribution of stochastics chemical kinetics in the photoresist during the development process. The study then investigates the effects of the plasma etching process on the pattern variations. The plasma etching process can alter the pattern variations by modifying the etching behavior and the etching selectivity. The thesis also discusses the system-level or integrated process-induced pattern variations. The method proposed herein involves surface modification and tone inversion technique and reduces the line edge roughness by 26% on a 20-nm pitch line pattern. Using a multicolor line-cut process, the thesis experimentally demonstrated the control of the edge-placement error from system-level pattern variations.
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Microfluidic Imaging Windows for Study of the Tumor MicroenvironmentDespite decades of research and billions of dollars in funding, cancer has maintained its epidemiological prominence as the second leading cause of death in the US for nearly 90 years. Currently, the clinical trial success rates for oncologic drugs is ~3%, and approved drugs often have a modest impact on overall survival. This is due in part to the tumor microenvironment (TME) which promotes cancer development and mitigates therapeutic response. Study of this biological system, however, is limited by conventional in vitro and in vivo techniques, which compromise either physiological relevance or experimental control. To better understand the role of the TME, we have utilized microfabrication techniques to develop the microfluidic imaging window (MFIW), an implantable platform for the observation and manipulation of in vivo TMEs. This technology provides unique opportunities for assessing the pharmacologic effects of therapeutics within intact, living tissue. Among the applications explored, a novel photolithographic technique, termed post exposure lamination, was developed to integrate tapered SU-8 micro-nozzle structures and enhance fluid conduction into porous matrices. Using these features, it was found that micro-nozzles improved axial penetration of fluorescent dextran into agarose tissue mimics and reduced the radial dispersion of Trypan Blue dye. Applications of localized reagent delivery for enhanced assay control were also investigated using small molecule nuclear stains and cell-based reporter systems. Here, significant cell staining occurred rapidly using small volumes of reagent (100 nL), substrate delivery for enzymatic processing was detected using a bioluminescent readout, and induction of cell gene expression was used to upregulate the production of fluorescent protein. Collectively, these capabilities showcase applications of the MFIW for enhanced monitoring and modulation of the TME that are well suited for translation into in vivo animal studies.
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Therapeutic Targeting of Oncogenic Gain-of-Function Mutant p53 by Proteasome InhibitionNon-small cell lung cancer (NSCLC) is a molecularly complex and heterogenous disease. Recent advances in genomic profiling have changed the therapeutic landscape of NSCLC to incorporate targeted and immunotherapeutic approaches. Despite these advances, lung cancer remains the leading cause of cancer mortality in the United States and worldwide. This is partly because these novel treatments are not applicable to all patients and are often associated with primary or secondary resistance. This highlights the need for continued search for new therapeutic agents and strategies for NSCLC patients. However, the drug discovery and development pipeline is protracted and inherently expensive for new drugs. The projected timeline from identification of a new drug candidate from preclinical research to clinical trials and approval is estimated at about 12-15 years with an average cost of $1.3 billion [1]. Moreover, the failure rate for new drugs during the clinical development stage is high, reaching up to 96% by some estimates [2] and is partly due to adverse risk profiles of candidate molecules. Given the ongoing need for continued drug development in lung cancer, repurposing previously approved drugs for new indications when possible is advantageous. Such strategies decrease the cost and timeframe of drug development and pose a lower safety risk to patients since the toxicity profiles of the repurposed drugs are already well established. Drug repurposing has had success in cancer therapy. Some of these include the repositioning of thalidomide for use in multiple myeloma and the repurposing of rituximab from lymphoma to incorporate its use in rheumatoid arthritis [3]. Interestingly, the observations that led to many drug repurposing efforts were serendipitous by nature. However, recently more systematic approaches to repurposing drugs are being employed and include retrospective clinical analysis, genetic associations and pathway matching, binding assays to identify relevant target interactions, and large-scale in vitro drug screens with paired genomic data [3]. In this thesis compilation, I first and foremost lay the groundwork for repurposing proteasome inhibitors for therapeutic targeting of gain-of-function (GOF) oncogenic mutant p53 using lung cancer as a model disease. This has a potential for generalizability across cancers that bear GOF p53 mutations since alterations in TP53 are central to carcinogenesis and prevalent across tumor types. As the ‘guardian of the genome’, p53 maintains the genome integrity by inducing DNA damage repair or forcing aberrant cells into apoptosis or senescence. Failure of this function results in propagation of abnormal cells and the progression from normal to precancerous and malignant cells. Moreover, gain-of-function (GOF) activities of mutated TP53 related the acquisition of novel oncogenic properties are well described in the literature and are related to excess accumulation of the mutant protein. This work describes the mechanism of paradoxical destabilization of GOF p53 by proteasome inhibition in lung cancer and identifies ‘hyperactive’ proteasome genes in mutant p53 as targetable vulnerabilities in this subset of NSCLC. Since proteasome inhibitors are FDA approved drugs and prior drug candidates targeting p53 have not had success in clinical development, the final goal is to repurpose proteasome inhibitors to target GOF p53 mutant NSCLC.
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Stock Price Prediction Using Sentiment Analysis and LSTMThis work presents multiple Long Short-Term Memory neural networks used in con- junction with sentiment analysis to predict stock prices over time. Multiple datasets and input features are used on a LSTM model to decipher which features produce the best output predictions and if there is correlation to the sentiment of posts and the rising of a stock. This project uses embedding based sentiment analysis on a dataset collected from Kaggle which includes over one million posts made on the subreddit r/wallstreetbets. This subreddit recently came under fire by the media with the shorting of Gamestop in the stock market. It was theorized that this subreddit was working as a collective to drive up the price of multiple stock, therefore hurting large corporations such as hedge funds that had large short positions on multiple stocks.
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Application of Resistive Random Access Memory (RRAM) For Non-Von Neumann ComputingThe movement of data between physically separated memory and processing units in conventional computing systems (the so-called von Neumann architecture) incurs significant costs in energy and latency. This is known as the von Neumann bottleneck. With the advent of the Internet of Things (IoT) and edge computing, computing systems are also becoming significantly power limited. In this work, hafnium oxide resistive random access memory (ReRAM) integrated with 65nm CMOS technology on a 300 mm wafer platform was assessed to carry out two novel non-von Neumann computing applications that processes data within memory and avoid excessive data movement. These computing applications are based on regulating the flow of sneak path currents in memory arrays to perform computation, called flow-based computing, and detecting degree of association (correlation) between binary processes in an unsupervised manner using the ReRAM non-volatile accumulative behavior, termed as temporal correlation detection. Electrical characterization of hafnium oxide ReRAM arrays was conducted for multi-level resistance states for flow-based computing, which was then investigated for two functions, approximate edge detection and XOR Boolean logic, through both experiments and simulation. The effect of device non-idealities was also evaluated. A trade-off between the flow-based output resistance ratio and the variability of flow-based outputs was found for different patterned binary resistance Roff/Ron ratios. For the second non-von Neumann application, the feasibility of ReRAM as a non-volatile candidate device was investigated with an empirical ReRAM model through simulation. Experimental ReRAM analog incremental switching data, from both SET and RESET regimes, was also evaluated on the modified temporal correlation detection algorithm, where the RESET regime resulted in better performance. The ReRAM based implementation yielded 36,000-53,000 vi times lower energy consumption than similar implementation with phase change memory for 25 binary processes, and a speed-up of computation time by 1,600-2,100 times than that of a CPU-based implementation using 1xPOWER8 CPU. 1xPOWER8 CPU is a CPU available on the IBM* Power* System S822LC system, the POWER8 system series, where the CPU was run for 1 thread. In summary, hafnium oxide ReRAM based on 65nm CMOS technology has been evaluated for two non-von Neumann computing applications, and the effect of device non-idealities has also been assessed. These ReRAM in-memory computing applications show the promising potential of ReRAM in overcoming the von-Neumann bottleneck.
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sxRNA Switches: Hypothesis Through Automated Design Via a Genetic Algorithm ApproachThe following document is meant to represent an overview of my work on structurally interacting RNA (sxRNA), which has already resulted in three publications with another two in preparation. Where appropriate, some text and data from these publications have been reproduced here. Ribonucleic Acid (RNA) is one of the fundamental macromolecules present in living systems. It can be found in all cells as varying length polymer chains composed of four primary bases (adenine, cytosine, guanine, uracil) capable of numerous modifications. Though generally characterized as an information carrier, RNA is a versatile molecule that exhibits both intra and inter-strand base pairing to form complex structures. Similar to protein, the particular shape of an RNA structure in combination with some degree of sequence specificity, can dictate its function (RNA binding protein recognition sites, ribozyme activity, aptameric affinity, etc.). Structurally interacting RNA (sxRNA) is a molecular switch technology that exploits predictable intermolecular RNA base pairing to form an otherwise absent functional structure in one RNA strand when it interacts with a specific, targeted second strand. Originally proposed as a potential regulatory mechanism in natural systems, we used characteristics of predicted pairings in that context to engineer purely synthetic sxRNA switches that have been successfully tested. There are many non-coding RNAs associated with pathological conditions, the ability to use these as triggers for sxRNA opens the door to potential applications ranging from diagnostics to therapeutics. Furthermore, other prospective triggers (including those synthetically designed) may allow use of the technology as a molecular tool for a variety of purposes including as an alternative to antibiotic selection in cell line development. The typical trigger sequences targeted by sxRNA switches are at least 20 bases in length. Combinatorial options with regard to structure positioning and base composition produce an enormous number of potential sxRNA sequences for any given target. Exhaustively examining these for feasible candidates (i.e., analyzing predicted interactions with unintended targets) is computationally impossible with current systems. Evolutionary computing is a subfield of artificial intelligence (AI) that has been inspired by biology. Genetic algorithms are a type of evolutionary algorithm and apply operators (such as recombination and mutation) to find candidate solutions to an optimization problem. The presented dissertation will describe the original sxRNA research as well as the development and testing of a genetic algorithm that automates the production of new sxRNA switch candidates. This algorithm takes into consideration factors that were previously impossible to account for in manual designs.
