Browsing by Author "Thomas Pearl, Committee Member"

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Domain Patterned Ferroelectric Surfaces for Selective Deposition Via Photochemical Reaction
(2007-12-07) Hanson, Jacqueline Nicole; Laura Clarke, Committee Member; Alexei Gruverman, Committee Member; Thomas Pearl, Committee Member; Robert Nemanich, Committee Chair
In this work, the use of domain patterned ferroelectric materials as a template to direct the assembly of nanostructures via photochemical reactions as well as to direct the assembly of polar molecules is presented. A distinct characteristic of ferroelectric materials is existence of a reversible spontaneous polarization. The direction of the spontaneous polarization can be reversed with the application of an electric field; thus, conductive tip atomic force microscopy (AFM) was employed to "write" nanoscale domain patterns in the ferroelectric and subsequently to visualize the configuration with piezoresponse force microscopy (PFM). Domain patterned lithium niobate and lithium tantalate, which are known as oxygen octahedra ferroelectrics, were used as templates. These materials exhibit unique surface electronic properties resulting from screening of the bound polarization charge, which dictate local reactivity on the surface. The effect of composition (stoichiometry and doping) on photochemical reactions at the surface is explored. Depending on the composition of the material, deposition can occur on the surface of domains or domain boundaries leading to a "bottom-up" method of nanowire formation. Nanowires of various shapes and sizes can be fabricated, as these parameters are dependent only upon the underlying domain configuration. Domain specific adsorption of polar molecules is achieved by utilizing the pyroelectric nature of ferroelectric materials. Surfaces may also be passivated with these polar molecules; subsequent UV irradiation induces photodecomposition of the molecules, which results in the formation of trenches at domain boundaries. Additionally, a scanning Kelvin probe microscopy (SKPM) study of the charge distribution on the surface of lithium niobate emphasizes the external screening mechanism and demonstrates the instability and screening of surface charges.
Spectroscopy of Oxide-GaN Interfaces
(2009-03-05) Craft, Hughes Spalding; Zlatko Sitar, Committee Member; Donald Brenner, Committee Member; Thomas Pearl, Committee Member; Jon-Paul Maria, Committee Chair
GaN-based devices are of interest for applications requiring high-frequency, high-power operation at elevated temperatures. As in traditional, silicon-based devices, integration of semiconducting phases with insulators is critical. Additionally, applications involving the integration of GaN with polar oxides such as perovskite ferroelectrics have been proposed, due to the coupling that may be achieved between the respective polar vector. Devices utilizing such a coupling behavior would make possible two-dimensional electron gases of high charge densities that could be modulated by the oxideâ€™s polarization. The current status of oxide-GaN research is far behind that of oxide-Si research, and large-scale realization of GaN devices will require detailed understanding of oxide-GaN interfaces. This thesis focuses on the characterization of several oxide-GaN interfaces using x-ray photoelectron spectroscopy (XPS), as well as the identification of issues relating to the GaN surface. The rocksalt oxides MgO and CaO have been proposed as candidates for GaN MOSFET gate oxides, passivating layers, and buffer layers in GaN-ferroelectric structures. Thus, knowledge of film growth modes and band alignments is critical. Utilizing in-vacuo molecular beam epitaxy (MBE) and XPS, the growth of MgO on GaN was found to occur by the Volmer-Weber mode, with coalescence occurring at ~12 nm. This coalescence behavior was not found to affect the band alignment. As measured by XPS, the valence band offset at the MgO-GaN interface is 1.2 Â± 0.2 eV, leading to a conduction band offset of 3.5 eV. A similar study was undertaken for the CaO-GaN system, in which more rapid coalescence was observed, leading to the conclusion of a Stranski-Krastanov growth mode. The difference in coalescence behavior is attributed to the increased reactivity of the CaO surface. The band offsets at the CaO-GaN interface were found to be 1.0 Â± 0.2 eV at the valence band, and 2.5 eV at the conduction band. The band structures measured in this thesis are considered to be sufficient for limiting leakage current by Schottky emission for high-temperature devices. Surface chemical stability of rocksalt oxides is a known issue with respect to hydroxylation through water adsorption. XPS characterization of water uptake was performed using the O 1s photoelectron line after several in-vacuo exposures, culminating in a one-hour exposure to a water/oxygen mixture at 1 x 10-6 Torr. Characterization of polycrystalline MgO showed a saturating coverage of â€“OH groups at approximately 1 monolayer, regardless of exposure. CaO films exhibited increased reactivity, with hydroxyl coverage increasing to 3 monolayers, in addition to a similar amount of physisorbed water, suggesting the possibility for further reaction. Complete recovery of both oxide surfaces is shown to be achievable using mild vacuum anneals. Finally, the surface of GaN has been characterized with respect to several issues encountered during these investigations. GaN surfaces are found to be significantly Ga-rich, with surface stoichiometries routinely in excess of Ga2N. Several wet chemistries for GaN preparation were evaluated for their ability to modify the electrical behavior of subsequently grown oxide films. XPS could not unambiguously identify any change in surface chemistry that promotes these effects. Finally, p-type GaN films were noted to consistently possess greater oxide contamination in the as-grown state. Typical n-type or undoped GaN were marked by submonolayer quantities of oxide surface coverage, while p-type GaN typically exhibited coverages in the 1-2 nm scale. This difference has been found to be due to the p-type dopant activation anneal, during which GaN oxidation cannot be suppressed
Synthesis and Field Emission Properties of Carbon Nanotube Films
(2005-02-17) Wang, Yunyu; Sanju Gupta, Committee Member; Griff Bilbro, Committee Member; Hans Hallen, Committee Member; Robert Nemanich, Committee Chair; Thomas Pearl, Committee Member
In this study, vertically aligned multiwalled carbon nanotube films were grown by microwave plasma chemical vapor deposition (MWCVD). Through controlling the thickness of the iron thin film, carbon nanotubes with different diameters were obtained. When the thickness of the iron layer was reduced to 0.3-0.5 nm, single-wall and double-wall nanotubes were obtained with a high areal density (~ 1012/cm2) and vertical alignment. Scanning electron microscopy, Raman spectroscopy and high resolution transmission electron microscopy were employed to characterize the as-deposited nanotubes. In addition, a systematic study of the internal structure transition of the carbon nanotubes has been conducted and a growth model was proposed in terms of carbon surface and bulk diffusion. The field emission of the carbon nanotube films has also been explored in this study. Different measurement systems including a variable distance field emission system, a field emission imaging system, and a field electron emission system (FEEM) were employed. The effects of the diameter (multi-wall vs single- and double- wall), the adsorbates, and the temperature on the field emission properties of carbon nanotubes have been exhibited. Finally, two processes including hydrogen plasma etching and re-growth were used to treat the as-deposited film, and an increased emission site density was observed for the re-grown carbon nanotube film.
Systematic study of film growth in short chain alkylsilanes including issues of phase segreagation and islanding
(2008-06-28) Guy, Laura; Laura Clarke, Committee Chair; Gail McLaughlin, Committee Member; Thomas Pearl, Committee Member; Hans Hallen, Committee Member
Thermionic Energy Conversion and Particle Detection Using Diamond and Diamond-Like Carbon Surfaces
(2007-10-02) Smith, Joshua Ryan; Robert J. Nemanich, Committee Co-Chair; Griff L. Bilbro, Committee Co-Chair; David Aspnes, Committee Member; Thomas Pearl, Committee Member