The ability of uniquely functional thermoelectric materials to convert waste heat directly into electricity is critical considering the global energy economy. Profitable, energy-efficient thermoelectrics possess thermoelectric figures of merit ZT ≥ 1. We examined the effect of metal nanoparticle – oxide film interfaces on the thermal conductivity κ and Seebeck coefficient α in bilayer and multilayer thin film oxide thermoelectrics in an effort to improve the dimensionless figure of merit ZT. Since a thermoelectric's figure of merit ZT is inversely proportional to κ and directly proportional to α, reducing κ and increasing α are key strategies to optimize ZT. We aim to reduce κ by phonon scattering due to the inclusion of metal nanoparticles in the bulk of thermoelectric thin films deposited by Pulsed Laser Deposition. XRD, AFM, XPS, and TEM analyses were carried out for structural and compositional characterization. The electrical conductivities of the samples were measured by a four-point probe apparatus. The Seebeck coefficients were measured in-plane, varying the temperature from 100K to 310K. The thermal conductivities were measured at room temperature using Time Domain Thermoreflectance. / Master of Science

Experimental heat transfer data has been collected at engine representative conditions in this work to use in future work to improve computational models. Tests were carried out in a transonic cascade wind tunnel with the data collected using thin film gages. All of the necessary development to use the thin film gages has been completed, including construction of electronics and analysis tools to reduce the data. Gage installation and calibration techniques have been successfully implemented for the current research facility and those procedures have been documented. Heat transfer tests were carried out at engine design speed as well as conditions both above and below design speed. The resulting effect of different Reynolds numbers on heat transfer has been studied and the data collected has been compared with computer predictions, analytical correlations, and data from other published literature to validate the results obtained. Finally, it has been shown that a transient analysis technique can be used to process the data for gages that do not exhibit steady results during the quasi-steady test run. This transient technique resulted in data that agrees well with published literature and analytical correlations. / Master of Science

A new thin film heat flux array (HFA) was designed and constructed using a series of nickel/copper thermocouples deposited onto a thin Kapton® polyimide film. The HFA is capable of withstanding temperatures up to 300 °C and produces signals of 42 μV/(W/cm²). As a result of its thin film construction, the HFA has a first order time constant of 32 ms. Calibrations were completed to determine the gage's output as well as its time response. In order to measure the signal from the HFA amplifiers were designed to increase the magnitude of the voltage output. An example case is given where the HFA is used in an experiment to correlate time-resolved heat flux and velocities. / Master of Science

In this study, a test technique, referred to as the probe test, has been developed as a quantitative tool for measuring the adhesion in thin adhesive films and coatings. The technique was initially developed as a qualitative test by the Hewlett-Packard Company for measuring adhesion of thin film microelectronic coatings. In the probe test method, an inclined needle-like probe with a conical tip is advanced underneath the free edge of a thin polymeric coating bonded to a substrate, causing the edge to lift-up from the surface of the substrate. A debond is thus initiated at the loading point and propagates as a semi-circular crack at the interface as the probe slides under the coating. A standard test procedure has been developed for testing thin adhesive coating/substrate systems. The sample system used is a thin film epoxy polymer coated silicon system. The interfacial fracture energy (Gc) (or critical strain energy release rate) has been used as a quantitative measure of adhesion for the given adhesive coating/substrate system. The probe test experiments were conducted using an optical microscope and a WYKO optical profiler. Using the optical microscope, the debond radius was measured for different debond sizes. Using the WYKO optical profiler, the three-dimensional surface topography of the debonded coating around the crack front was measured for different debond sizes. Using the experimental data from the probe test, analytical and numerical (finite element-based) techniques have been developed to determine the interfacial fracture energy (Gc) for the given adhesive coating/substrate system. The analytical techniques were developed based on different plate theory formulations (thin/thick plate - small/large deflection) of the probe test geometry and local curvature measurement at the crack tip. The finite element based techniques were developed using a hybrid numerical-experimental approach and surface-based contact interaction analysis in ABAQUS. The results obtained using thick plate-large deflection formulation correlated with finite element contact interaction analysis results. The probe test can be used with transparent or opaque coatings and thus offers a promising alternative to indentation and other tests methods for characterizing thin film and coating adhesion. / Master of Science

<p dir="ltr">Over the past few decades, metamaterials have attracted great research interest due to their extraordinary properties which cannot be easily achieved by natural materials. For example, anisotropic metamaterials that exhibit different properties along different directions, are valuable in different fields of optics. To achieve such anisotropic performance, nanocomposite designs by coupling different materials and functionalities have been demonstrated as an effective approach.</p><p dir="ltr">The goal of this dissertation is to design and fabricate anisotropic multiphase nanocomposite thin films with multifunctionality and tunability. Both transition metal oxides and transition metal nitrides are selected to study due to their high thermal stability, good crystallinity, and unique electromagnetic properties. In addition, different metals, especially plasmonic Au and magnetic Co, are selected as the metallic phase to fabricate nanocomposites. The designs also extend beyond the traditional two-phase nanocomposites to multiphase nanocomposites containing metal, oxide, and nitride, with more metamaterial design possibilities and more functionalities.</p><p dir="ltr">The dissertation consists of the introduction of multiphase nanocomposite thin film and experimental techniques, followed by four research chapters. In the first research chapter, hyperbolic HfO₂-Au with tunable optical properties is fabricated and studied. In the second research chapter, the magnetic Co is introduced into the nanocomposite thin film for multifunctionality design, and the obtained ZrO₂-Co thin film exhibits both hyperbolic optical property and magnetic anisotropy. In the third research chapter, vertically aligned nanocomposite (VAN) design and multilayer design are combined to achieve a complete three-phase HfO₂-Au/TiN-Au multilayer nanocomposite. Such a complete structure can exhibit tunable optical response. In the fourth research chapter, the magnetic Co is combined with the superconducting NbN to explore more applications of such VAN design. Overall, the dissertation work demonstrates various approaches of anisotropic metamaterials designs using oxides, nitrides, and metals. Enhanced functionality and multifunctionalities are demonstrated. Future research is needed for incorporating these new metamaterials designs in optical devices and sensors.</p>

Functional crystalline oxides with perovskite structure have a wide range of electrical properties such as ferroelectric, ferromagnetic, and superconductive, as well as unique properties that make them suited for a wide variety of applications including electro-optics, high-k dielectrics, and catalysis. Therefore, in order to realize the potential of perovskite oxides it is desirable to integrate them with semiconductors. Due to the high surface energy of oxides compared to that of semiconductors and the low number of oxides that are thermodynamically stable against SiO₂ formation, it has been extremely difficult to integrate epitaxial oxides with Si directly. However, in 1998, McKee and co-workers finally succeeded in depositing SrTiO₃ on Si directly using a Sr template via molecular beam epitaxy. This breakthrough opened the possibility of integrating the perovskite oxides with Si to realize potential device applications. In this dissertation, alkaline earth metal (Sr and Ba) templates on semiconductors, which enable epitaxial growth of complex oxides on semiconductors, are investigated using molecular beam epitaxy (MBE) for growth and in-situ X-ray/ultraviolet photoemission spectroscopy (XPS/UPS) for the electronic structure analysis. An epitaxial layer of SrTiO₃ on Si using such alkaline earth templates is used as a pseudo-substrate for the integration of perovskite oxides on Si. Through the use of post-deposition annealing as a function of oxygen pressure and annealing time, the strain relaxation behavior of epitaxial SrTiO₃ films grown on Si is also investigated to determine how the SiO₂ interlayer thickness affects the SrTiO₃ lattice constant. This ability to control strain relaxation can be used as a way to manipulate the properties of other perovskite oxides grown on SrTiO₃/Si. Additionally, SrTiO₃ can be made conductive by doping with La. Conductive SrTiO₃ can be used as a thermoelectric, a transparent conductive layer, and a quantum metal layer in a quantum metal field-effect transistor (QMFET). The structural, electrical, and optical properties of strained conductive La-doped SrTiO₃ are studied in order to understand the relation between elastic strain and electrical properties for electronic device applications. Oxide quantum well systems based on LaAlO₃/SrTiO₃ are also investigated using spectroscopic ellipsometry to understand how the quantum well layer structure affects the electronic structure. Such quantum well systems are good candidates for the monolithic integration of functional perovskites on semiconductors. Oxides quantum wells can be used in various device applications such as in quantum well cascade lasers, laser diodes and high performance transistors. As part of the growth optimization for high quality complex oxide heterostructures, the surface preparation of SrTiO₃ substrates using several different methods was also extensively studied using angle-resolved photoemission spectroscopy (ARPES). We found that acid-free water-based surface preparation is actually more effective at removing SrOx̳ crystallites and leaving the surface TiO₂-terminated compared to the more commonly used acid-based methods. / text

Fabrication techniques and process integration considerations for amorphous oxide semiconductor (AOS) thin-film transistors (TFTs) constitute the central theme of this dissertation. Within this theme three primary areas of focus are pursued. The first focus involves formulating a general framework for assessing passivation. Avoiding formation of an undesirable backside accumulation layer in an AOS bottom-gate TFT is accomplished by (i) choosing a passivation layer in which the charge neutrality level is aligned with (ideal case) or higher in energy than that of the semiconductor channel layer charge neutrality level, and (ii) depositing the passivation layer in such a manner that a negligible density of oxygen vacancies are present at the channel-passivation layer interface. Two AOS TFT passivation schemes are explored. Sputter-deposited zinc tin silicon oxide (ZTSO) appears promising for suppressing the effects of negative bias illumination stress (NBIS) with respect to ZTO and IGZO TFTs. Solution-deposited silicon dioxide is used as a barrier layer to subsequent PECVD silicon dioxide deposition, yielding ZTO TFT transfer curves showing that the dual-layer passivation process does not significantly alter ZTO TFT electrical characteristics. The second focus involves creating an adaptable back-end process compatible with flexible substrates. A detailed list of possible via formation techniques is presented with particular focus on non-traditional and adaptable techniques. Two of the discussed methods, "hydrophobic surface treatment" and "printed local insulator," are demonstrated and proven effective. The third focus is printing AOS TFT channel layers in order to create an adaptable and additive front-end integrated circuit fabrication scheme. Printed zinc indium aluminum oxide (ZIAO) and indium gallium zinc oxide (IGZO) channel layers are demonstrated using a SonoPlot piezoelectric printing system. Finally, challenges associated with printing electronic materials are discussed. Organic-based solutions are easier to print due to their ability to "stick" to the substrate and form well-defined patterns, but have poor electrical characteristics due to the weakness of organic bonds. Inorganic aqueous-based solutions demonstrate good electrical performance when deposited by spin coating, but are difficult to print because precise control of a substrate's hydrophillic/hydrophobic nature is required. However, precise control is difficult to achieve, since aqueous-based solutions either spread out or ball up on the substrate surface. Thickness control of any printed solution is always problematic due to surface wetting and the elliptical thickness profile of a dispensed solution. / Graduation date: 2013

The kinetics of interstitial hydrogen are of great interest and importance for metal-hydride storage, purification, fusion and fission reactor technology, material failure processes, optical sensors for hydrogen gas and many other technologies. In particular nano-sized materials motivate fascinating applications and scientific questions. If hydrogen is absorbed in vanadium it alters the band structure around the Fermi energy. These modifications of the band structurelead to a change in the absorptance of vanadium which are in first order approximation proportional to the concentration. We present a methodto quantify chemical diffusion of hydrogen in nano-sized materials.The induced changes in the absorptance of vanadium hydride (VHx) thin-films are observed visually and in real-time as a function of position.Concentration profiles and their evolution in time, during chemicaldiffusion, were measured down to a hydrogen content corresponding tojust a few effective monolayers, randomly distributed within VHx. For concentrations reached via phase transitions distinct diffusional behavior was found, where a diffusion-front, a strong concentration gradient, migrates in the direction of the diffusive hydrogen flux. The results show that decreased size strongly influences the energy landscape and reveal different rate limiting steps for absorption and desorption.

In this dissertation, a carbon nanotube thin-film transistor is fabricated on a flexible substrate. Combined printing and stamping techniques are used for the fabrication. An ink-jet printing technique is used to form the gate, source, and drain electrodes as well as the dielectric layer. A self aligned carbon nanotube (CNT) thin film is formed by using a new modified dip coat technique before being transferred to the device substrate. This novel modified dip-coat technique utilizes the capillary effect of a liquid solution rising between gaps to coat CNT solution on a large area of the substrate while consuming minimal CNT solution. Several key solutions are addressed to solve the fabrication problems. (1) The source/drain contact with the CNT channel is developed by using droplets of silver ink printed on the source/drain areas prior to applying CNT thin. The wet silver ink droplets allow the silver to "wet" the CNT thin-film area and enable good contact with the source and drain contact after annealing. (2) A passivation layer to protect the device channel is developed by bonding a thin Kapton film on top of the device channel. This thin Kapton film is also used as the media for transferring the aligned CNT thin-film on the device substrate. Using this technique, printing the passivation layer can be avoided, and it prevents the inter-diffusion of the liquid dielectric into the CNT porous thin-film. (3) A simple and cost effective technique to form multilayer metal interconnections on flexible substrate is developed and demonstrated. Contact vias are formed on the second substrate prior bonding on the first substrate. Ink-jet printing is used to fill the silver ink into the via structure. The printed silver ink penetrates through the vias to contact with the contact pads on the on the bottom layer, followed by an anneal process. High drain current of 0.476mA was obtained when V[subscript G]= -3V and source-drain voltage (V[subscript DS]) was -1.5V. A bending test was performed on the CNT TFT showing less than a 10% variation in performance. A bending test was also performed on via structures, which yielded less than a 5% change in resistance. The developed CNT TFT is used to form a switch in a phase shifter for a flexible phased-array antenna (PAA). Four element 1-dimensional and 2-dimensional phased-array antennae are fabricated and characterized. Multilayer metal interconnects were used to make a complete PAA system. For a 2-bit 1x4 PAA system, by controlling the ON/OFF states of the transistors, beam steering of a 5.3GHz signal from 0° to -27° has been demonstrated. The antenna system also shows good stability and tolerance under different bending radii of curvature. A 2-bit 2x2 PAA system was also fabricated and demonstrated. Two dimensional beam steering of a 5.2GHz signal at an angle of [theta]=20.7° and [phi]=45° has been demonstrated. The total efficiency of the 1-dimensional and 2-dimensional PAA systems are 42% and 46%, respectively. / text

Nanolaminate dielectrics combine two or more insulating materials in a many-layered film. These structures can be made to significantly outperform films composed of a single one of their constituent materials by adjusting the composition ratio, arrangement, and size of the component layers. In this work, atomic layer deposition (ALD) is used to fabricate pure-oxide and nanolaminate dielectrics based upon Al₂O₃ and ZrO₂. The relative performance of these dielectrics is investigated with respect to application as gate dielectrics for ZnSnO (ZTO) and InGaZnO (IGZO) amorphous-oxide-semiconductor (AOS) thin-film transistors (TFTs). AOS TFTs are promising candidates for commercial use in applications such as active-matrix displays and e-paper. It was found that the layer thickness, relative composition, and interfacial material all had an effect on TFT performance. Several variants of the Al₂O₃/ZrO₂ nanolaminate were found to exhibit superior properties to either Al₂O₃ or ZrO₂ alone. / Graduation date: 2011