Silicon nanowires are a nanostructure consisting of elongated crystals of silicon. Like many nanostructures, silicon nanowires have properties that change with size. In particular, silicon nanowires have a band-gap that is tuneable with the diameter of the nanowire. They tend to absorb a large portion of the light incident upon them and they form a highly textured surface when grown on an otherwise flat substrate. These properties indicate silicon nanowires are good candidates for use in solar cells. Nanostructured silicon, in the form of nanocrystalline silicon, has been used to produce thin film solar cells. Solar cells produced using silicon nanowires could combine the properties of the silicon nanowires with the low material costs and good stability of nanocrystalline based solar cells. This thesis describes the process of optimisation of silicon nanowire growth on a plasma enhanced chemical vapour deposition system. This optimised growth of silicon nanowires is then used to demonstrate a prototype solar cell using silicon nanowires and amorphous silicon. Several steps had to be accomplished to reach this goal. The growth of silicon nanowires was optimised through a number of steps to produce a high density film covering a substrate. Developments were made to the standard deposition technique and it was found that by using pulsed plasma enhanced chemical vapour deposition the density of nanowire growth could be improved. Of a range of catalysts trialled, gold and tin were found to be the most effective catalysts for the growth of silicon nanowires. A range of substrates was investigated and the nanowires were found to grow with high density on transparent conductive oxide coated glass substrates, which would allow light to reach the nanowires when they were used as part of a solar cell. The silicon nanowires were combined with doped and intrinsic amorphous silicon layers with the aim to create thin film photovoltaic devices. Several device designs using silicon nanowires were investigated. The variant that showed the highest efficiency used doped silicon nanowires as a p-layer which was coated with intrinsic and n-type amorphous silicon. By the characterisation and optimisation of the silicon nanowires, a prototype silicon nanowire solar cell was produced. The analysis of these prototype thin film devices, and the nanowires themselves, indicated that silicon nanowires are a promising material for photovoltaic applications.

Orientador: Mario Antonio Bica de Moraes / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-09T02:50:52Z (GMT). No. of bitstreams: 1 Rouxinol_FranciscoPauloMarques_M.pdf: 1569781 bytes, checksum: dbadfea1682b172f533c81260f079ca8 (MD5) Previous issue date: 2003 / Resumo: Este trabalho descreve uma nova técnica de deposição de filmes finos de óxido de tungstênio e apresenta uma investigação sobre o processo de deposição e propriedades dos filmes. Os filmes foram obtidos em uma câmara de deposição a vácuo contendo um filamento de tungstênio aquecido por uma fonte de corrente alternada. O oxigênio era admitido na câmara usando um fluxômetro de massa. Os filmes eram formados em um substrato posicionado próximo ao filamento pelos precursores voláteis WxOy, formados na superfície aquecida do tungstênio pela reação entre o oxigênio e o tungstênio. A espessura dos filmes era medida com um perfilômetro e a taxa de deposição, R, foi determinada em função da temperatura do filamento, TF, da pressão de oxigênio na câmara, PO2 e da distância substrato-filamento, D. Um aumento no valor de R foi observado com o aumento de TF de 1500 a 1950 K, mantendo-se PO2fixo. A taxa de deposição também mostrou um aumento quando PO2 era aumentada de 0,8 para 2,1 Pa, para TF fixo. Foi alcançada uma taxa de deposição de 93 nm/min para TF= 1950 K e PO2 = 2.1 Pa. Através do espectro de transmissão ultravioleta-visível dos filmes, o coeficiente de absorção e o gap óptico, EG, foram determinados. Foi encontrado um valor médio de EG de 3,56 ± 0,08 eV. As ligações químicas nos filmes foram investigadas usando-se espectroscopia infravermelha por transformada de Fourier, tanto com luz não polarizada como polarizada, e espectroscopia de fotoelétrons por raios-X. Os dados de espectroscopia infravermelha revelaram a presença de água nos filmes. A análise do pico de XPS W4f mostrou que os átomos de W estão no estado de valência W +6 , típica do tungstênio na estequiometria WO3. Através da análise dos filmes por espectroscopia de espalhamento Rutherford, foi determinada uma razão atômica média entre o oxigênio e tungstênio igual a 3,5 ± 0,3. A difração de raio-X mostrou que os filmes são amorfos. A análise por microscopia de varredura por elétrons mostrou que a superfície dos filmes era uniforme, enquanto as micrografias da seção retas dos mesmos revelaram um material sem estrutura e compacto. As propriedades eletrocrômicas dos filmes foram estudas pela intercalação de Li + em uma cela eletrolítica. Foram encontradas eficiências ópticas de 78 e 125 cm 2 C -1 para 623,8 e 950 nm, respectivamente / Abstract: This work describes a novel technique for the deposition of tungsten oxide thin films and reports an investigation on their deposition process and properties. The films were obtained in a vacuum deposition chamber fitted with a tungsten filament heated by an ac power supply. Oxygen was admitted into the chamber using mass flow meters. The films were formed on a substrate positioned near the filament from the volatile Wx Oy precursors generated on the heated filament surface from the reactions between oxygen and tungsten. Film thicknesses were measured using a perfilometer and the deposition rate, R, was determined as a function of the filament temperature, TF, pressure of oxygen in the chamber, PO2, and substrate-filament distance, D. An increase in the R-value was observed for TF increasing from 1500 to 1950 K, for a fixed PO2. The deposition rate also increased for a PO2 increase from 0.8 to 2.1 Pa, for a fixed TF. Deposition rates as high as 93 nm/min was observed for TF= 1950 K and PO2= 2.1 Pa. From the transmission ultraviolet-visible spectra of the films, the absorption coefficient and the optical gap, EG, were determined. An average EG-value of 3.56 ± 0.08 eV was found. The chemical bonds in the films were investigated using Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. In the former, unpolarized and polarized infrared beams were employed. The infrared spectroscopy data revealed the presence of water in the films. Lineshape analyses of the W4f XPS peak showed that the W-atoms were in the W+6 valence state, typical of tungsten in the WO3 stoichiometry. From Rutherford backscattering spectroscopy analysis of the films, an average O/W atomic ratio of 3.5 ± 0.3 was determined. X-ray diffraction revealed that the films were amorphous. Scanning electron microscopy analysis showed that the film surface was smooth and uniform, while the fracture micrographs of the film cross-section revealed a structureless and compact material. The electrochromic properties of the films were studied for Li + intercalation using an electrochemical cell. Coloration efficiencies of 78 and 125 cm 2C -1 at wavelengths of 632,8 and 950 nm, respectively, were measured. / Mestrado / Física da Matéria Condensada / Mestre em Física

Cubic boron nitride (cBN) synthesis has gained lot of interest during the past decade as it offers outstanding physical and chemical properties like high hardness, high wear resistance, and chemical inertness. Despite of their excellent properties, every application of cBN is hindered by high compressive stresses and poor adhesion. The cost of equipment is also high in almost all the techniques used so far. This thesis deals with the synthesis of cubic phase of boron nitride on Si (100) wafers using electron beam evaporator, a low cost equipment that is capable of depositing films with reduced stresses. Using this process, need of ion beam employed in ion beam assisted processes can be eliminated thus reducing the surface damage and enhancing the film adhesion. Four sets of samples have been deposited by varying substrate temperature and the deposition time. scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR) techniques have been used to determine the structure and composition of the films deposited. X-ray diffraction (XRD) was performed on one of the samples to determine the thickness of the film deposited for the given deposition rate. Several samples showed dendrites being formed as a stage of film formation. It was found that deposition at substrate temperature of 400oC and for a period of one hour yielded high quality cubic boron nitride films.

Near infrared region (NIR) of the electromagnetic spectrum (EM) is defined as 700nm to 1400nm wavelength interval by International Commission on Illumination(CIE). This wavelength interval is extensively used for target acquisition, night vision, wireless communication etc. Therefore, filtering the desired portion of EM spectra becomes a need for that kind of applications. Interference filters are multilayer optical devices which can be designed and produced for the desired wavelength intervals. The production of near infrared interference filters is a process of depositing thin material layers on the suitable substrates. In this thesis, a multilayer NIR filter will be designed for a selected wavelength interval by the use of dierent materials. Then, transmission quality, thermal stability, dependence of the transmission values on the incoming beam angle, performance and durability of the filter will be studied.

O presente trabalho apresenta o desenvolvimento de um método de obtenção e a caracterização de filtros ópticos de interferência de banda passante variável que, constituídos por refletores dielétricos multicamadas de filmes finos intercalados por cavidades de Fabry- Perot não planares com espessuras linearmente variáveis, apresentam a propriedade do deslocamento linear da transmitância máxima espectral em função da posição, isto é, um Filtro de Interferência Variável (FIV). Este método apresenta novas e abrangentes possibilidades de confecção de filtros ópticos de interferência variável: lineares ou em outras formas desejadas, de comprimento de onda de corte variável (passa baixa ou alta) e filtros de densidade neutra variável, através da deposição de metais. A primeira etapa do trabalho foi o projeto e a construção de um filtro óptico convencional, de um comprimento de onda central, com camadas homogêneas. A etapa seguinte foi, com base na teoria espectrométrica, promover a variação das espessuras das camadas em um perfil inclinado e linear. Para tanto, de acordo com os requisitos de projeto, foram feitas adaptações em uma evaporadora por e-Beam (elétron-Beam), acrescentando um obliturador mecânico especialmente projetado, ajustando parâmetros de técnicas de deposição e caracterizando o perfil inclinado de um filme depositado para ajustar o obliturador. E, ao final destas etapas, foi projetado e construído o FIV especificado. Também é descrita uma possível aplicação do FIV: um dispositivo multicanal espectral para análise ambiental, e, dentre muitas, outras podem ser mencionadas: sistemas Lab-on-Chip, biosensores, detectores chip-sized, espectrometria de fluorescência on-chip, detectores de deslocamento de comprimento de onda, sistemas de interrogação e etc. / This work presents the development of a method to obtain and characterize a variable interference optical bandpass filter, made up of a number of thin films forming dielectrical reflectors intercalated by non flat Fabry-Perot cavities whose thickness variates linearly. These filters present the propriety of a linear variation in the maximum spectral transmittance as a function of the position in the filter, for this reason this is called Variable Interference Filter (VIF). This method allows of manufacturing linear interference filters or any other function disered, with variable cut wavelength (low or high pass) and variable neutral density filters by means of metallic depositions. The first step in this work was to design and built a conventional filter, with homogeneous layers and a fixed central wavelength. The following step was, using spectrometric theory basis, introduce the variation in the thickness of the layers in a linear inclined outline. Accordingly with the design requirements, it was made some adaptations in an e-beam evaporator (electron-beam), adding a mechanical obliterator adjusted with series of depositions and characterizations of a single layers in order to find a linearly inclined outline. In the end of this step it was designed and built the specified VIF. It is also described a possible application of this VIF: a multichannel spectral device for environmental analysis. Between many applications, others can be cited, such as: Lab-on-Chip systems, biosensors chip-sized detectors, on-chip fluorescence spectrometry, shift wavelength detectors, interrogation systems, etc.

O presente trabalho apresenta o desenvolvimento de um método de obtenção e a caracterização de filtros ópticos de interferência de banda passante variável que, constituídos por refletores dielétricos multicamadas de filmes finos intercalados por cavidades de Fabry- Perot não planares com espessuras linearmente variáveis, apresentam a propriedade do deslocamento linear da transmitância máxima espectral em função da posição, isto é, um Filtro de Interferência Variável (FIV). Este método apresenta novas e abrangentes possibilidades de confecção de filtros ópticos de interferência variável: lineares ou em outras formas desejadas, de comprimento de onda de corte variável (passa baixa ou alta) e filtros de densidade neutra variável, através da deposição de metais. A primeira etapa do trabalho foi o projeto e a construção de um filtro óptico convencional, de um comprimento de onda central, com camadas homogêneas. A etapa seguinte foi, com base na teoria espectrométrica, promover a variação das espessuras das camadas em um perfil inclinado e linear. Para tanto, de acordo com os requisitos de projeto, foram feitas adaptações em uma evaporadora por e-Beam (elétron-Beam), acrescentando um obliturador mecânico especialmente projetado, ajustando parâmetros de técnicas de deposição e caracterizando o perfil inclinado de um filme depositado para ajustar o obliturador. E, ao final destas etapas, foi projetado e construído o FIV especificado. Também é descrita uma possível aplicação do FIV: um dispositivo multicanal espectral para análise ambiental, e, dentre muitas, outras podem ser mencionadas: sistemas Lab-on-Chip, biosensores, detectores chip-sized, espectrometria de fluorescência on-chip, detectores de deslocamento de comprimento de onda, sistemas de interrogação e etc. / This work presents the development of a method to obtain and characterize a variable interference optical bandpass filter, made up of a number of thin films forming dielectrical reflectors intercalated by non flat Fabry-Perot cavities whose thickness variates linearly. These filters present the propriety of a linear variation in the maximum spectral transmittance as a function of the position in the filter, for this reason this is called Variable Interference Filter (VIF). This method allows of manufacturing linear interference filters or any other function disered, with variable cut wavelength (low or high pass) and variable neutral density filters by means of metallic depositions. The first step in this work was to design and built a conventional filter, with homogeneous layers and a fixed central wavelength. The following step was, using spectrometric theory basis, introduce the variation in the thickness of the layers in a linear inclined outline. Accordingly with the design requirements, it was made some adaptations in an e-beam evaporator (electron-beam), adding a mechanical obliterator adjusted with series of depositions and characterizations of a single layers in order to find a linearly inclined outline. In the end of this step it was designed and built the specified VIF. It is also described a possible application of this VIF: a multichannel spectral device for environmental analysis. Between many applications, others can be cited, such as: Lab-on-Chip systems, biosensors chip-sized detectors, on-chip fluorescence spectrometry, shift wavelength detectors, interrogation systems, etc.

Interference filters are multilayer thin film devices. They use interference effects between the incident and reflected radiation waves at each layer interface to select wavelengths. The production of interference filters depend on the precise deposition of thin material layers on substrates which have suitable optical properties. In this thesis, the main target is to design and produce two optical filters (short-pass filter and long-pass filter) for the CCDs that will be used in the electronics of a space camera. By means of these filters, it is possible to take image in different bands (RGB and NIR) by identical two CCDs. The filters will be fabricated by plasma ion-assisted deposition technique.

The dissertation is focused on the study of electric and especially dielectric properties of thin film organic materials with their huge potential for optoelectronics and other industrial sectors. The theoretical part deals with the use of organic materials in organic photovoltaic cells, the methods of deposition techniques and characterization. The theoretical knowledge of the dielectric spectroscopy, including methods for determining the physical properties and evaluation of experimental data are also described. The experimental part is devoted to the study of small molecule organic solar cells with bulk heterojunction composed of electron donor molecule of DPP(TBFu)2 and electron acceptor fullerene derivatives, such as PC60BM, PC70BM and TC60BM. The experimental part is divided into two main parts. The first part deals with the study of processes occurring at the interface between the active layer and the contact; the second part is focused on transport processes inside the structure of photovoltaic cells and also contains a study of perovskite solar cells.