published_or_final_version / abstract / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

Thesis (Ph. D.)--Ohio State University, 2006. / Title from first page of PDF file. Includes bibliographical references (p. 232-237).

Thesis (M.S.)--Mississippi State University. Department of Electrical and Computer Engineering. / Title from title screen. Includes bibliographical references.

Thesis (Ph.D)--Electrical and Computer Engineering, Georgia Institute of Technology, 2008. / Committee Chair: Honsberg, Christiana; Committee Co-Chair: Ferguson, Ian; Committee Member: Citrin, David; Committee Member: Klein, Benjamin; Committee Member: Rohatgi, Ajeet; Committee Member: Snyder, Robert. Part of the SMARTech Electronic Thesis and Dissertation Collection.

Wide band gap based nanostructures have being attracting much research interest because of their promise for application in optoelectronic devices. Among those wide band gap semiconductors, gallium nitride (GaN) and zinc oxide (ZnO) are the most commonly studied and optoelectronic devices based on GaN and ZnO have been widely investigated. This thesis concentrates on the growth, optical and electrical properties of GaN and ZnO nanostructures, plus their application in solar cells and light emitting diodes (LEDs). GaN-nanowire based dye sensitized solar cells were studied. Different post-growth treatments such as annealing and coating with a TiOx shell were applied to enhance dye absorption. It was found that TiOx increased the dye absorption and the performance of the dye sensitized solar cell. ZnO nanorods were synthesized by vapor deposition and electrodeposition. Post-growth treatments such as annealing and hydrothermal processing were used to modify the defect chemistry and optical properties. LEDs based on GaN/ZnO heterojunctions were studied. The influence of ZnO seed layers on GaN/ZnO LEDs was investigated. GaN/ZnO LEDs based on ZnO nanorods with MgO and TiOx shells were also prepared in order to modify the LED performance. The coating condition of the shell was found to influence the current-voltage (I-V) characteristics and device performance. Moreover, high brightness LEDs based on GaN with InGaN multiple quantum wells were also fabricated. The origin of the emission from GaN/ZnO LEDs was studied using different kinds of GaN substrates. Direct metal contacts on bare GaN substrates were also employed to investigate the optical emission and electrical properties. It is found that the emission from the GaN/ZnO LEDs probably originated from the GaN substrate. GaN/ZnO LEDs with MgO as an interlayer were also fabricated. The MgO layer was expected to modify the band alignment between the GaN and the ZnO. It was shown that GaN/MgO/ZnO heterojunctions (using both ZnO nanorods and ZnO films) have quite different emission performance under forward bias compared to those that have no MgO interlayer. An emission peak was around 400 nm could originate from ZnO. Nitrogen doped ZnO nanorods on n-type GaN have been prepared by electrodeposition. Zinc nitrate and zinc acetate were used as ZnO precursors and NH4NO3 was used as a nitrogen precursor. Only the ZnO nanorods made using zinc nitrate showed obvious evidence of doping and coherent I-V characteristics. Cerium doped ZnO based LEDs were fabricated and showed an emission that depended on the cerium precursor that was employed. This indicates that the choice of precursor influences the growth, the materials properties and the optical properties of ZnO. / published_or_final_version / Physics / Doctoral / Doctor of Philosophy

Thesis (Ph. D.)--Ohio University, 2001. / Title from PDF t.p.

Thesis (Ph. D.)--University of Hong Kong, 2007. / Title proper from title frame. Also available in printed format.

Thesis (M.S.)--West Virginia University, 1998. / Title from document title page. "December 1998." Document formatted into pages; contains viii, 125 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 84-89).

Thesis (Ph. D.)--Oregon State University, 2010. / Printout. Includes bibliographical references. Also available on the World Wide Web.

Orientador: Luiz Fernando Zagonel / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-28T03:52:44Z (GMT). No. of bitstreams: 1 Ramos_Raul_M.pdf: 5673835 bytes, checksum: 005e134cbd8cbb241cbdf367a75f35a3 (MD5) Previous issue date: 2015 / Resumo: Eletrodos condutores transparentes (TCE) possuem grande importância para tecnologias de informação e geração de energia. O TCE mais eficiente na atualidade é o ITO (In2O3 dopado com Sn), que pode alcançar resistividades em torno de 2.10-4 ?cm e uma transmitância ótica de 80% a 90% na região do visível. Entretanto, a escassez dos recursos naturais de Índio e sua grande demanda sugerem a necessidade de materiais alternativos. O presente estudo tem por objetivo investigar as propriedades óticas, eletrônicas e estruturais de filmes finos de TiO2 (fase anatase) dopados com Nitrogênio. A deposição dos filmes foi feita por Deposição por Feixe de Íons (IBD) por bombardeamento de um alvo de titânio puro com íons de Ar+ em atmosfera de O2. Os filmes, com uma espessura de ?90 nm, foram depositados em substrato de quartzo amorfo (Herasil-1) a temperaturas de 400 ou 500°C. Depois, os filmes são dopados com implantação iônica, variando o tempo de 10 a 60 minutos, com feixe de íons misto a baixa energia de N2+ e H2+ com 150 eV e sob a mesma temperatura de crescimento. Após a implantação, medidas Hall indicam que a densidade de portadores majoritários nos filmes de anatase dopados com nitrogênio chegam até ?1019 cm?3 (enquanto filmes não dopados tem densidade de cargas de ?1012 cm?3). A resistividade dos filmes dopados chegam até 10?1 ?cm enquanto mantem boa transmissão ótica (>80%). De fato, dependendo do tempo de dopagem e da temperatura do substrato durante o processo, a transmissão de até 85% podem ser obtida em 550 nm com tal resistividade (?10?1 ?cm). Espectroscopia de fotoelétrons emitidos por raio-x (XPS) realizadas in situ mostram que a composição na superfície é compatível com TiO2?xNx com concentração de nitrogênio de até ? 20%. Difração de raio-x com ângulo de incidência rasante (GIXRD) confirmaram a estrutura cristalina anatase dos filmes antes e após a implantação iônica à baixa energia (150 eV). Este estudo indica que é possível dopar a amostra anatase com nitrogênio através do uso de um feixe de íons de baixa energia. Tal abordagem é interessante por permitir um controle da concentração de dopantes (Nitrogênio através de um precursor gasoso) de forma mais controlada do que usualmente obtido por sputtering reativo / Abstract: Transparent conductive electrodes (TCE) have great importance for information and energy technologies. The most efficient TCE is currently the ITO (Sn-doped In2O3), which may have a resistivity lower than 2·10?4 ?cm and an optical transmittance of 80% to 90% in the visible region. However, the scarcity of natural resources of Indium and its great demand suggests the need of alternative materials. The present study aims to investigate the optical, electronic and structural properties of thin films of TiO2 (anatase phase) doped with nitrogen. The films deposition is made by Ion Beam Deposition (IBD) by bombarding a pure titanium target with Ar+ ions in O2 atmosphere to a thickness of about 90 nm. The films are deposited on an amorphous quartz substrate (Herasil-1) at 400 or 500 °C. Afterwards, the films are doped by ion implantation with low-energy ion beam mixed of N2+ and H2+ at 150 eV and under the same temperature of the growth for times ranging from 10 to 60 minutes. After implantation, Hall measurements indicated that the majority carrier density in the nitrogen doped anatase films reaches up to ? 1019 cm?3 (while the undoped films have a carrier density of ? 1012 cm?3). The resistivity of the doped films is as low as 10?1 ? cm while maintaining good optical transmission. Indeed, depending on the doping time and substrate temperature, transmission of up to 90% could be obtained at 550 nm with this resistivity. X-ray photoelectron spectroscopy (XPS) performed in situ shows that the surface composition is compatible with N:TiO2?x with nitrogen concentrations of up to ? 20%. Small angle x-ray diffraction measurements (SAXRD) confirmed the anatase crystal structure of the films before and after the low energy ion implantation. This study indicates that it is indeed possible to dope anatase thin films with nitrogen by low energy ion beam. This approach is interesting for allowing a greater control of doping concentration with respect to what is usually obtained by reactive sputtering / Mestrado / Física / Mestre em Física / 2013/118682-8 / CAPES