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

Main objective of the present work is to develop wide-band gap InGaN solar cells in the 2.4 - 2.9 eV range that can be an integral component of photovoltaic devices to achieve efficiencies greater than 50%. In the present work, various challenges in the novel III-nitride technology are identified and overcome individually to build basic design blocks, and later, optimized comprehensively to develop high-performance InGaN solar cells. Due to the unavailability of a suitable modeling program for InGaN solar cells, PC1D is modified up to a source-code level to incorporate spontaneous and piezoelectric polarization in order to accurately model III-nitride solar cells. On the technological front, InGaN with indium compositions up to 30% (2.5 eV band gap) are developed for photovoltaic applications by controlling defects and phase separation using metal-organic chemical vapor deposition. InGaN with band gap of 2.5 eV is also successfully doped to achieve acceptor carrier concentration of 1e18 cm-3. A robust fabrication scheme for III-nitride solar cells is established to increase reliability and yield; various schemes including interdigitated grid contact and current spreading contacts are developed to yield low-resistance Ohmic contacts for InGaN solar cells. Preliminary solar cells are developed using a standard design to optimize the InGaN material, where the band gap of InGaN is progressively lowered. Subsequent generations of solar cell designs involve an evolutionary approach to enhance the open-circuit voltage and internal quantum efficiency of the solar cell. The suitability of p-type InGaN with band gaps as low as 2.5 eV is established by incorporating in a solar cell and measuring an open-circuit voltage of 2.1 V. Second generation InGaN solar cell design involving a 2.9 eV InGaN p-n junction sandwiched between p- and n-GaN layers yields internal quantum efficiencies as high as 50%; while sixth generation devices utilizing the novel n-GaN strained window-layer enhance the open circuit voltage of a 2.9 eV InGaN solar cell to 2 V. Finally, key aspects to further InGaN solar cell research, including integration of various designs, are recommended to improve the efficiency of InGaN solar cells. These results establish the potential of III-nitrides in ultra-high efficiency photovoltaics.

Transport Properties of Wide Band Gap Semiconductors Louis Tirino III 155 pages Directed by Dr. Kevin F. Brennan The objective of this research has been the study of the transport properties and breakdown characteristics of wide band gap semiconductor materials and their implications on device performance. Though the wide band gap semiconductors have great potential for a host of device applications, many gaps remain in the collective understanding about their properties, frustrating the evaluation of devices made from these materials. The model chosen for this study is based on semiclassical transport theory as described by the Boltzmann Transport Equation. The calculations are performed using an ensemble Monte Carlo simulation method. The simulator includes realistic, numerical energy band structures derived from an empirical pseudo-potential method. The carrier-phonon scattering rates and impact ionization transition rates are numerically evaluated from the electronic band structure. Several materials systems are discussed and compared. The temperature-dependent, high-field transport properties of electrons in gallium arsenide, zincblende gallium nitride, and cubic-phase silicon carbide are compared. Since hole transport is important in certain devices, the simulator is designed to simulate electrons and holes simultaneously. The bipolar simulator is demonstrated in the study of the multiplication region of gallium nitride avalanche photodiodes.