Doctor of Philosophy, University of Toledo, 2017, Electrical Engineering

Electrically conductive hair-like structures, referred to as whiskers, can bridge the gap between densely spaced electronic components. This can cause current leakage and short circuits resulting in significant losses and, in some cases, catastrophic failures in the automotive, aerospace, electronics and other industries since 1946. Detecting a metal whiskers (MWs) is often a challenging task because of their random growth nature and very small size (diameters can be less than 1 µm, lengths vary from 1µm to several millimeters). Many decades ago the industry introduced whisker mitigating Pb in the solders used to fabricate electric and electronic parts. In recent years, this changed because the European Union (EU) passed a legislation in 2006, called “Restriction of the use of Certain Hazardous Substances (RoHS) in Electrical and Electronic Equipment”, which requires a reduction and elimination of the use of Pb in technology. Thus, the issue of undesirable and unpredictable whiskers growth has returned and there is a renewed interest in the mechanisms of formation of these structures. None of the whisker growth models proposed to date are capable of answering consistently and universally why whisker grow in the first place and why Pb addition suppresses their growth. Understanding MW nucleation and growth mechanism are of significant interest to this project, since this would potentially allow the development of new accelerated-failure testing methods of electronic components to replace existing testing methods which are generally found to be unreliable. In particular, this research is intended to study the effects of electric fields on the whisker growth, which according to the recently developed electrostatic theory[1] of whisker growth, are of crucial importance. This theory proposes that the imperfections on metal surfaces can form small patches of net positive or negative electric charge leading to the formation of the anomalous electric field (E), which go (open full item for complete abstract)

Committee: Daniel Georgiev Dr. (Committee Chair); Vijay Devabhaktuni Dr. (Committee Member); Victor Karpov Dr. (Committee Member); Devinder Kaur Dr. (Committee Member); Anthony Johnson Dr. (Committee Member) Subjects: Aerospace Materials; Chemical Engineering; Condensed Matter Physics; Electrical Engineering; Engineering; Experiments; Materials Science; Metallurgy; Nanoscience; Nanotechnology; Physics; Plasma Physics; Solid State Physics; Theoretical Physics

Master of Science, The Ohio State University, 2017, Electrical and Computer Engineering

Zinc oxide (ZnO) has emerged as a promising wide bandgap material (3.35eV at 300K) for use in next-generation nanoelectronics and photonics, with important piezoelectric, pyroelectric, sensing, and optoelectronic properties. ZnO has seen specific application in ultraviolet (UV) photodetectors, UV lasers [1], hydrogen gas sensors [2, 3], surface acoustic wave devices, piezoelectric generators [4], and transparent thin-film transistors for displays [5]. Various forms of ZnO nanostructures, such as nanobelts, nanobows, and nanowires, and have all attracted significant attention due to their ease of fabrication, remarkable relative surface area, and low-dimensional nature [6, 7]. Nanowires of ZnO in particular can exhibit pinch-off of electrical current with surface charge-sensitive depletion depths that are on the order of the wire radius [8, 9]. In bulk ZnO, defects have been shown to strongly affect the behavior of metal contacts, by modifying band bending and allowing trap-assisted tunneling transport through the metal-ZnO Schottky barrier [10]. The electronic impact of native point defects becomes critical at the nanoscale, since their physical properties can dominate charge carrier transport and especially electronic contact behavior. In order to control the distribution of defects at the metal-nanowire interface, various forms of surface modification were investigated. We report the in-situ fabrication of both Ohmic and Schottky platinum (Pt) metal contacts to single ZnO nanowires prepared by pulsed laser deposition (PLD) and carbothermal vapor phase transport, using Ga-ion surface modification and both furnace and electron beam annealing. A Ga focused ion beam (FIB) was operated at 30 keV to implant nanowire surfaces before metallization for production of Ohmic contacts, and at 5 keV to gently mill the defect-rich outer annulus, promoting formation of Schottky contacts. Electron beam induced deposition (EBID) was used to pattern Pt metal contacts to the wire (open full item for complete abstract)

Committee: Leonard Brillson (Advisor); Betty Lise Anderson (Committee Member) Subjects: Electrical Engineering

Doctor of Philosophy, The Ohio State University, 2016, Materials Science and Engineering

Gallium ion irradiation in dual-beam FIB microscopes is well known to cause some degree of damage during the milling process. Although it has been established that cleaning passes with low energy ions can mitigate the extent of this damage, the mechanisms, extent, and type of damage caused have not been well studied due to geometrical limitations inherent to thin foils. By adapting the needle geometry used for atom probe and tomographic work, we can directly measure the extent of damage layers created during milling. Needles were made of multiple semiconductor, intermetallic, and metal systems, confirming previous estimates of damage thickness in Si and GaAs. Materials tested fell into two distinct classes, amorphous dominated and crystalline defect dominated. Amorphous dominated materials consisted of semiconductors and narrow phase field intermetallics, fitting previous radiation work. Crystalline defect dominated materials had semi-crystalline damage layers under 5 nm at all accelerating voltages, and residual defects were shown to have significant effects on lattice clarity in HAADF-STEM. Contrast between amorphous layers in HAADF-STEM was found to be minimal even under ideal conditions, and HRTEM was necessary to accurately confirm and measure damage layer thickness. The causes and extent of damage layer minimization during low keV milling steps were shown to be consistent across all materials.

Committee: Hamish Fraser (Advisor); Wolfgang Windl (Committee Member); Jinwoo Hwang (Committee Member) Subjects: Materials Science; Metallurgy; Radiation

Doctor of Philosophy, Case Western Reserve University, 2015, EECS - Electrical Engineering

These days, sensor chips in very small form factors are ubiquitous. In this dissertation, I explain my efforts and achievements in proposing the first center-clamped 3C-SiC RF microdisk resonators with out-of-plane vibration as a candidate for future resonant sensing applications. First, nanomachining of 3C-SiC material using Focused Ion Beam (FIB) to find sputtering yield and lateral deformations for many different FIB conditions as well different patterns (and scales), is investigated accompanied by SRIM simulations. Then, I report theoretical analysis, fabrication and measurement of (center-clamped) RF multi-mode micromechanical resonators based upon vibrating circular disks made of a ~500nm thin SiC epilayer grown on single crystal Si. The fabrication method - composed of FIB sputtering and HNA etching - has been used for the first time and the vibration resonance peaks (in frequency spectrum) are detected through laser-interferometry measurements. A ~40µm-diameter SiC disk with a slender (~800nm) anchor exhibits more than a dozen flexural modes between ~2MHz¿20MHz with quality factors (Q's) of ~1000¿4000. Two other disks with diameters of ~40µm and ~30µm, and wide anchors (~20µm and ~10.3µm, respectively) have their set of major flexural peaks (associated with modes of zero-circular nodes) between 15¿19MHz with Q's of ~500¿2500. This dissertation then describes the efforts to achieve an improved self-sustained oscillator based on such resonators. First, a theory-inspired method is developed for design of electrostatically-transduced MEMS/NEMS referenced Pierce oscillators to improve phase noise (by increasing motional inductance and thus motional resistance) while also addressing oscillation start-up and other specifications e.g. frequency detuning and power consumption. Finally, the laser-based optical transduction scheme is used to make self-sustained oscillators from microdisk resonators. Laser actuation is very compatible with 3C-SiC material and the (open full item for complete abstract)

Committee: Philip X.-L. Feng (Advisor); Philip X.-L. Feng (Committee Chair); Christian Zorman (Committee Member); Pirouz Pirouz (Committee Member); Francis L. Merat (Committee Member); Soumyajit Mandal (Committee Member) Subjects: Electrical Engineering; Materials Science; Mechanical Engineering; Optics; Physics