Master of Science (MS), Bowling Green State University, 2019, Physics

Zinc oxide (ZnO) is a compound semiconductor with a direct wide band gap of 3.4 eV with a high exciton binding energy of 60 meV at room temperature. It is a widely investigated semiconductor due to its high potential for optoelectronic applications in the UV region, especially, for light-emitting diodes and lasers. In these applications of ZnO, native point defects play key roles. The understanding of these defects will help us to realize and control the performance of ZnO in these applications. Also, it will help us to realize p-type doping in ZnO, which will open a way for oxide semiconductor based bipolar devices. Due to the reproducibility of high-quality ZnO crystals and their interesting properties, it is preferred for extensive research over other wide band gap semiconductors. So far, as research points out, native point defects are not well understood. In our work, we will present electrical and optical characterization studies done on ZnO single crystals as well as on polycrystals, and we will relate these measurements to defect studies using Positron Annihilation Lifetime Spectroscopy (PALS) and Coincident Doppler Broadening Spectroscopy (CDBS). It was found that the increase in well-known green luminescence is associated with a decrease in conductivity and charge carrier concentration. Positron lifetime spectroscopy measurements were carried out to reveal the origin of defects responsible for decreasing the conductivity and enhancing the green luminescence. Lastly, it was interesting to observe the decrease in the ratio between green luminescence to near band emission as the laser power increased.

Committee: Farida Selim Ph.D. (Advisor); Lewis P. Fulcher Ph.D. (Committee Member); Marco Nardone Ph.D. (Committee Member) Subjects: Physics

Master of Science (MS), Bowling Green State University, 2018, Physics

ZnO is an abundant wide band gap semiconductor with promising applications in optoelectronic technologies. Electronic and optical properties of this material depend critically on the physics of various defects which are very common in ZnO. Controlling those defects is the key to the development of ZnO-based applications, which is still a challenging process. This master thesis work is primarily concerned in studying the pristine ZnO and its native oxygen point defects. The objective is to study, investigate, measure and correlate the electronic, vibrational and thermal properties of the pristine ZnO and its native oxygen point defects, along with drawing necessary inferences for creating substantial theories. Further, the mode of study is the first-principles calculations performed with density functional theory, implemented in the VASP code using the GGA-PBE and LDA+U as functionals. A short discussion of these calculations will be given. At last, we perform a comparative study with these functionals in their application to compute the electronic, vibrational and thermal properties of the pristine ZnO and its native oxygen point defects.

Committee: Alexey Zayak (Advisor); Alexey Zayak (Committee Chair); Lewis Fulcher (Committee Member); Marco Nardone (Committee Member) Subjects: 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, 2008, Physics

ZnO has received renewed attention in recent years due its exciting properties as a wide band gap semiconductor. ZnO has several advantages over GaN including the availability of substrates, a room temperature excitonic emission, and an environmentally benign chemistry. ZnO applications include efficient blue light emitters, surface acoustic wave devices, transparent conductors, high power transistors, and solid state white lighting. Despite this versatility, several hurdles remain before device realization. Firstly, ZnO is almost always p-type. Although high quality n-type ZnO is abundant, there is no stable and reliable p-type doping scheme. Secondly, research into high quality Ohmic and Schottky contacts has been limited. Although there is an abundance of literature, there has yet to be an attempt to understand the physical and chemical mechanisms at metal-ZnO interfaces. In this work, plasma processing techniques are adopted to ZnO. These cold plasmas allow for room temperature modification of the subsurface. Implanting hydrogen has identified it as a primary n-type dopant responsible for a large fraction of the n-type conductivity. Oxygen plasma treatment has yielded an Ohmic to Schottky conversion by reducing oxygen defects at the near surface. Deposition of metals on clean and ordered surfaces reveal the importance that defects play at the metal-semiconductor interface. Higher concentrations of defects promote reactions. This increased reaction eutectic forming and oxide forming. Understanding the nature of the metal allows for engineering of high quality blocking contacts. These contacts can provide added thermal stability to devices. Subsurface introduction of hydrogen and nitrogen provide a potential roadmap to p-type doping and high quality Schottky contacts. Overall, control of transport properties and contact integrity is achieved by remote plasma processing.

Committee: Leonard Brillson Prof (Advisor); Jay Gupta Prof (Committee Member); Thomas Humanic Prof (Committee Member); Julia Meyer Prof (Committee Member); Yann guezennec Prof (Committee Member) Subjects: Physics