Vidros a base de óxido de telúrio para dispositivos fotônicos

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Conversão descendente de energia envolvendo íons lantanídeos. Vidros emissores de luz na região de 1000 nm

Pós-graduação em Química - IQ

Formação e crescimento do óxido de zinco e óxido de zinco dopado com cobre

Pós-graduação em Química - IQ

Frequency upconversion involving triads and quartets of ions in a Pr3+/Nd3+ codoped fluoroindate glass

Frequency upconversion (UC) processes involving energy transfer (ET) among Nd 3+ and Pr 3+ ions in a fluoroindate glass are reported. In a first experiment, the excitation of Pr 3+ [transition 3H 4→ 1D 2] and of Nd 3+ [transition 4I 9/2→( 2G 7/2+ 4G 5/2)] was achieved with a dye laser operating in the 575-590 nm range. In a second experiment, the Nd 3+ ions were excited with the second harmonic of a Nd: YAG laser at 532 nm. The ET processes leading to UC in both experiments were studied by monitoring the blue fluorescence decay at 480 nm due to the transition 3P 0→ 3H 4 in Pr 3+. In the more relevant UC process, quartets of ions (Nd-Nd-Pr-Pr) are excited due to absorption of three laser photons by two Nd 3+ ions which transfer their energy to two Pr 3+ ions. Each Pr 3+ ion promoted to the 3P 0 level decays to the ground state emitting one photon in the blue region. This conclusion was achieved investigating the dependence of the UC fluorescence intensity as a function of laser intensity, samples concentrations, and temporal behavior of the UC signal. Other UC processes involving nonisoionic groups of three ions are also reported. © 2002 American Institute of Physics.

Formation of an extended CoSi2 thin nanohexagons array coherently buried in silicon single crystal

A Co-doped silica film was deposited on the surface of a Si(100) wafer and isothermally annealed at 750 degrees C to form spherical Co nanoparticles embedded in the silica film and a few atomic layer thick CoSi2 nanoplatelets within the wafer. The structure, morphology, and spatial orientation of the nanoplatelets were characterized. The experimental results indicate that the nanoplatelets exhibit hexagonal shape and a uniform thickness. The CoSi2 nanostructures lattice is coherent with the Si lattice, and each of them is parallel to one of the four planes belonging to the {111} crystallographic form of the host lattice. (C) 2012 American Institute of Physics. [doi:10.1063/1.3683493]

Untersuchungen zur Löslichkeit von Kobalt und Eisen in TiO 2-Bulk sowie in TiO 2-Nanoproben und deren Einfluss auf die magnetischen Eigenschaften

Co- und Fe-dotierte Rutil- und Anatas-Bulkproben wurden über einen Sol-Gel Prozess und anschließende thermische Behandlung dargestellt und auf ihre Zugehörigkeit zu der Gruppe der verdünnten magnetischen Oxide untersucht. Die Untersuchungen der dotierten Rutil-Proben mittels Röntgenbeugung, Elektronenmikroskopie und magnetischen Methoden zeigen, dass die Löslichkeit von Co und Fe in der TiO2-Modifikation Rutil sehr gering ist. Oberhalb von 1at% Co bzw. Fe wird neben Rutil die Bildung der Oxide CoTiO3 bzw. Fe2TiO5 beobachtet. Weitere thermische Behandlung im Argon-H2-Strom führte aufgrund der Bildung von metallischem Co bzw. Fe zu einem ferromagnetischen Verhalten. Die TiO2-Modifikation Anatas besitzt eine höhere Löslichkeit, so dass erst oberhalb von 4at% Co bzw. 10at% Fe die Phasen Co3O4 bzw. FeTiO3 neben Anatas auftreten. Entsprechende Proben zeigen ein paramagnetisches Verhalten. Oberhalb der Löslichkeitsgrenze führt die Reduktion im Argon-H2-Strom zu einem ferromagnetischen Verhalten, welches auf metallisches Co bzw. Fe zurückzuführen ist. Analog zu den Bulkproben wurden Co- und Fe-dotierte TiO2-Nanodrähte hergestellt. Das magnetische Verhalten der Fe-dotierten TiO2-Nanodrähte entspricht dem der Fe-dotierten Anatas-Bulkproben. Dagegen führt die Co-Dotierung nicht zu einem Einbau in die TiO2-Nanodrähte, sondern zur Bildung von CoOx-Nanopartikeln. Die entsprechenden Proben zeigen ein schwach ferromagnetisches Verhalten. Dies ist jedoch nicht auf eine ferromagnetische Dotierung der TiO2-Nanodrähte zurückzuführen, sondern auf nicht kompensierte Momente an den Oberflächen der als Verunreinigungen auftretenden CoOx-Nanopartikel. Zusammenfassend wird festgestellt, dass die Löslichkeit von Co und Fe in TiO2 für die Ausbildung eines ferromagnetischen Verhaltens zu gering ist. Der beobachtete Ferromagnetismus lässt sich eindeutig auf magnetische Verunreinigungen zurückführen. Somit können die dotierten TiO2 Proben nicht den verdünnten magnetischen Oxiden zugeordnet werden.

Open-atmosphere structural depth profiling of multilayer samples of photovoltaic interest using laser-induced plasma spectrometry

The present work aims to assess Laser-Induced Plasma Spectrometry (LIPS) as a tool for the characterization of photovoltaic materials. Despite being a well-established technique with applications to many scientific and industrial fields, so far LIPS is little known to the photovoltaic scientific community. The technique allows the rapid characterization of layered samples without sample preparation, in open atmosphere and in real time. In this paper, we assess LIPS ability for the determination of elements that are difficult to analyze by other broadly used techniques, or for producing analytical information from very low-concentration elements. The results of the LIPS characterization of two different samples are presented: 1) a 90 nm, Al-doped ZnO layer deposited on a Si substrate by RF sputtering and 2) a Te-doped GaInP layer grown on GaAs by Metalorganic Vapor Phase Epitaxy. For both cases, the depth profile of the constituent and dopant elements is reported along with details of the experimental setup and the optimization of key parameters. It is remarkable that the longest time of analysis was ∼10 s, what, in conjunction with the other characteristics mentioned, makes of LIPS an appealing technique for rapid screening or quality control whether at the lab or at the production line.

Desenvolvimento de materiais nanoestruturados à base de SiO2-Nb2O5 preparados por uma nova rota sol-gel com aplicações em fotônica

Este trabalho concentra-se na preparação e caracterizações estrutural e espectroscópica de materiais nanoestruturados à base de SiO2-Nb2O5 dopados e codopados com íons Er3+, Yb3+ e Eu3+ na forma de pós e guias de onda planares. Os nanocompósitos foram preparados através de uma nova rota sol-gel utilizando óxido de nióbio como precursor em substituição ao alcóxido de nióbio. A correlação estrutura propriedades luminescentes foi estudada por difração de raios X, microscopia eletrônica de transmissão, espectroscopia vibracional de absorção no infravermelho, espectroscopia vibracional de espalhamento Raman, análise térmica, reflectância difusa e especular, espectroscopia de fotoluminescência e acoplamento M-line. Inicialmente foi avaliado a influência da concentração de nióbio nas propriedades estruturais e luminescentes de nanocompósitos (100-x)Si-xNb dopados e codopados com íons Er3+, Yb3+ e Eu3+ tratados termicamente a 900 °C por 3h. A cristalização do Nb2O5 foi dependente da concentração de Nb na matriz, com a distribuição dos íons lantanídeos preferencialmente no Nb2O5, afetando as propriedades luminescentes. Para os nanocompósitos codopados com íons Er3+ e Yb3+ foram obtidos valores de largura de banda a meia altura (FWHM) da ordem de 70 nm na região de 1550 nm e tempos de vida de até 5,2 ms. A emissão na região do visível, decorrente de processos de conversão ascendente, revelou-se dependente da concentração de nióbio. Foi verificada emissão preferencial na região do verde para menores concentrações de Nb. Enquanto que, para as maiores concentrações, processos de relaxação cruzada levaram a um aumento relativo na intensidade de emissão na região do vermelho. A eficiência quântica de emissão dos nanocompósitos (100-x)Si-xNb dopados com Eu3+ variou com o comprimento de onda de excitação, refletindo os diferentes sítios de simetria ocupados por este íons nesta estrutura complexa. A influência da temperatura de tratamento térmico no processo de cristalização do Nb2O5 em nanocompósitos 70Si:30Nb codopados com íons Er3+ e Yb3+ foi avaliada. Material amorfo foi obtido a 700 °C enquanto que a 900 e 1100 °C foram identificas as fases ortorrômbica (fase T) e monoclínica (fase M) do Nb2O5. Intensa emissão na região de 1550 nm com valores de FWHM de 52 e 67 nm e tempos de vida de 5,6 e 5,4 ms foram verificados a 700 e 900 °C sob excitação em 977 nm, respectivamente. Por fim, foram obtidos guias de onda planares com excelentes propriedades ópticas e com grande potencial de aplicação em dispositivos de amplificação óptica. Especificamente, materiais fotônicos com banda larga de emissão na região do infravermelho foram preparados, indicando fortemente a potencialidade para a aplicação em telecomunicações envolvendo não somente a banda C como também as bandas L e S em materiais contendo somente íons Er3+ como centros emissores.

Oxide Semiconductors For Organic Opto-electronic Devices

Development of transparent oxide semiconductors (TOS) from Earth-abundant materials is of great interest for cost-effective thin film device applications, such as solar cells, light emitting diodes (LEDs), touch-sensitive displays, electronic paper, and transparent thin film transistors. The need of inexpensive or high performance electrode might be even greater for organic photovoltaic (OPV), with the goal to harvest renewable energy with inexpensive, lightweight, and cost competitive materials. The natural abundance of zinc and the wide bandgap ($sim$3.3 eV) of its oxide make it an ideal candidate. In this dissertation, I have introduced various concepts on the modulations of various surface, interface and bulk opto-electronic properties of ZnO based semiconductor for charge transport, charge selectivity and optimal device performance. I have categorized transparent semiconductors into two sub groups depending upon their role in a device. Electrodes, usually 200 to 500 nm thick, optimized for good transparency and transporting the charges to the external circuit. Here, the electrical conductivity in parallel direction to thin film, i.e bulk conductivity is important. And contacts, usually 5 to 50 nm thick, are optimized in case of solar cells for providing charge selectivity and asymmetry to manipulate the built in field inside the device for charge separation and collection. Whereas in Organic LEDs (OLEDs), contacts provide optimum energy level alignment at organic oxide interface for improved charge injections. For an optimal solar cell performance, transparent electrodes are designed with maximum transparency in the region of interest to maximize the light to pass through to the absorber layer for photo-generation, plus they are designed for minimum sheet resistance for efficient charge collection and transport. As such there is need for material with high conductivity and transparency. Doping ZnO with some common elements such as B, Al, Ga, In, Ge, Si, and F result in n-type doping with increase in carriers resulting in high conductivity electrode, with better or comparable opto-electronic properties compared to current industry-standard indium tin oxide (ITO). Furthermore, improvement in mobility due to improvement on crystallographic structure also provide alternative path for high conductivity ZnO TCOs. Implementing these two aspects, various studies were done on gallium doped zinc oxide (GZO) transparent electrode, a very promising indium free electrode. The dynamics of the superimposed RF and DC power sputtering was utilized to improve the microstructure during the thin films growth, resulting in GZO electrode with conductivity greater than 4000 S/cm and transparency greater than 90 %. Similarly, various studies on research and development of Indium Zinc Tin Oxide and Indium Zinc Oxide thin films which can be applied to flexible substrates for next generation solar cells application is presented. In these new TCO systems, understanding the role of crystallographic structure ranging from poly-crystalline to amorphous phase and the influence on the charge transport and optical transparency as well as important surface passivation and surface charge transport properties. Implementation of these electrode based on ZnO on opto-electronics devices such as OLED and OPV is complicated due to chemical interaction over time with the organic layer or with ambient. The problem of inefficient charge collection/injection due to poor understanding of interface and/or bulk property of oxide electrode exists at several oxide-organic interfaces. The surface conductivity, the work function, the formation of dipoles and the band-bending at the interfacial sites can positively or negatively impact the device performance. Detailed characterization of the surface composition both before and after various chemicals treatment of various oxide electrode can therefore provide insight into optimization of device performance. Some of the work related to controlling the interfacial chemistry associated with charge transport of transparent electrodes are discussed. Thus, the role of various pre-treatment on poly-crystalline GZO electrode and amorphous indium zinc oxide (IZO) electrode is compared and contrasted. From the study, we have found that removal of defects and self passivating defects caused by accumulation of hydroxides in the surface of both poly-crystalline GZO and amorphous IZO, are critical for improving the surface conductivity and charge transport. Further insight on how these insulating and self-passivating defects cause charge accumulation and recombination in an device is discussed. With recent rapid development of bulk-heterojunction organic photovoltaics active materials, devices employing ZnO and ZnO based electrode provide air stable and cost-competitive alternatives to traditional inorganic photovoltaics. The organic light emitting diodes (OLEDs) have already been commercialized, thus to follow in the footsteps of this technology, OPV devices need further improvement in power conversion efficiency and stable materials resulting in long device lifetimes. Use of low work function metals such as Ca/Al in standard geometry do provide good electrode for electron collection, but serious problems using low work-function metal electrodes originates from the formation of non-conductive metal oxide due to oxidation resulting in rapid device failure. Hence, using low work-function, air stable, conductive metal oxides such as ZnO as electrons collecting electrode and high work-function, air stable metals such as silver for harvesting holes, has been on the rise. Devices with degenerately doped ZnO functioning as transparent conductive electrode, or as charge selective layer in a polymer/fullerene based heterojunction, present useful device structures for investigating the functional mechanisms within OPV devices and a possible pathway towards improved air-stable high efficiency devices. Furthermore, analysis of the physical properties of the ZnO layers with varying thickness, crystallographic structure, surface chemistry and grain size deposited via various techniques such as atomic layer deposition, sputtering and solution-processed ZnO with their respective OPV device performance is discussed. We find similarity and differences in electrode property for good charge injection in OLEDs and good charge collection in OPV devices very insightful in understanding physics behind device failures and successes. In general, self-passivating surface of amorphous TCOs IZO, ZTO and IZTO forms insulating layer that hinders the charge collection. Similarly, we find modulation of the carrier concentration and the mobility in electron transport layer, namely zinc oxide thin films, very important for optimizing device performance.

Filmes de óxido de zinco e nitreto de zinco depositados por magnetron sputtering com diferentes pressões de argônio, oxigênio e nitrogênio.

O óxido de zinco é um material semicondutor que apresenta alta transparência óptica no espectro visível, alta energia de ligação de éxcitons e piezoeletricidade. Por suas propriedades, ele é utilizado na área de sensores, eletrodos transparentes e dispositivos optoeletrônicos. No entanto, sua utilização ainda é limitada pela dificuldade de obtenção de condutividade tipo p, cujo principal dopante é o nitrogênio, devido à assimetria de dopagem ocasionada por defeitos intrínsecos do material, dopagem em valências diferentes das esperadas e formação de níveis de aceitadores profundos na banda proibida. A aplicação em dispositivos piezoelétricos também exige alta resistividade e ótimas propriedades cristalinas. Muitos processos de deposição estabelecidos hoje ainda utilizam altas temperaturas, o que impede sua deposição sobre superfícies ou substratos sensíveis a altas temperaturas. O objetivo deste trabalho é desenvolver técnicas de deposição de filmes de ZnO, principalmente em baixas temperaturas ( 100°C), pelo método de magnetron sputtering de rádio frequência, para avaliar a influência dos gases de processo nas características estruturais, estequiométricas, elétricas e ópticas dos filmes. Para isso, foram obtidos filmes utilizando pressão total de argônio, e pressões parciais de argônio e oxigênio e argônio e nitrogênio, utilizando alvo cerâmico de óxido de zinco ou alvo metálico de zinco. Para alvo de ZnO, filmes com condutividade tipo n foram obtidos em ambiente de argônio, em condições que geraram deficiências de oxigênio. Filmes altamente resistivos foram obtidos com a utilização de pressão parcial de oxigênio no gás de processo, em condições que resultaram em filmes estequiométricos, inclusive com condutividade tipo p. Condutividade tipo p mais alta foi observada, apenas por ponta quente, para uma amostra obtida em argônio logo após a utilização de nitrogênio na câmara de processo, que provavelmente sofreu influência da dopagem não intencional do cobre, que foi identificado como um contaminante do processo devido à estrutura da câmara. Para alvo de Zn, observou-se a formação de nitreto de zinco, que demonstrou alta capacidade de oxidação em ambiente atmosférico, e portanto, transforma-se naturalmente ao longo do tempo ou por processos de oxidação térmica em ZnO dopado com nitrogênio. Filmes de ZnO produzidos a partir de nitreto de zinco foram os únicos dos testados que apresentaram fotoluminescência característica do ZnO, mesmo para processos onde não houve aquecimento intencional.

The relationships between microstructure and crystal structure in zincite solid solutions

Single phase (Zn,Fe)(1-x) O zincite solid solution samples have been prepared by high temperature equilibration in air and in reducing atmospheres, followed by quenching to room temperature. The Fe2+/Fe3+ concentrations in the samples have been determined using wet chemical and XPS techniques. Iron is found to be present in zincite predominantly in the form of Fe3+ ions. The transition from an equiaxed grain morphology to plate-like zincite crystals is shown to be associated with increasing Fe3+ concentration, increasing elongation in < 001 > of the hexagonal crystals and increasing anisotropic strain along the c-axis. The plate-like crystals are shown to contain planar defects and zincite polytypes at high iron concentrations.

Fabrication and application of short and novel structure in-fibre Bragg gratings

This thesis presents details on progress made in the fabrication and application of short and novel structure fibre Bragg gratings. The basic theoretical concepts of in-fibre Bragg gratings and photosensitive mechanisms are introduced together with an overview of fabrication methods and applications presented to date. The fabrication of fibre Bragg gratings using a quadrupled Nd:YAG laser is presented and some of the issues of grating fabrication using a fabrication using a phasemask are investigated, including the variation of the separation of the fibre and phasemask, and other alignment issues. A new apodisation technique is presented, enabling the production of gratings with a wide range of spectral profiles. The technique is used to investigate the design and fabrication of length limited fibre Bragg gratings for use in telecommunication systems as filters. Application to devices designed for use in WDM systems is presented. The use of fibre Bragg gratings as high spatial resolution distributed sensors is investigated. Grating sensing arrays comprising very short apodised gratings are demonstrated and Chirped Moiré gratings are implemented as distributed sensors achieving high spatial resolution with miniature point sensing sub-elements. A novel grating sensing element designed to imitate an interferometer is also presented. Finally, the behaviour of gratings fabricated in Boron-Germania-co-doped fibre is investigated, revealing atypical behaviour of the Bragg wavelength during ageing.

Writing and characterisation methods for UV-photosensitivity based devices and their applications

The underlying work to this thesis focused on the exploitation and investigation of photosensitivity mechanisms in optical fibres and planar waveguides for the fabrication of advanced integrated optical devices for telecoms and sensing applications. One major scope is the improvement of grating fabrication specifications by introducing new writing techniques and the use of advanced characterisation methods for grating testing. For the first time the polarisation control method for advanced grating fabrication has successfully been converted to apodised planar waveguide fabrication and the development of a holographic method for the inscription of chirped gratings at arbitrary wavelength is presented. The latter resulted in the fabrication of gratings for pulse-width suppression and wavelength selection in diode lasers. In co-operation with research partners a number of samples were tested using optical frequency domain and optical low coherence reflectometry for a better insight into the limitations of grating writing techniques. Using a variety of different fabrication methods, custom apodised and chirped fibre Bragg gratings were written for the use as filter elements for multiplexer-demultiplexer devices, as well as for short pulse generation and wavelength selection in telecommunication transmission systems. Long period grating based devices in standard, speciality and tapered fibres are presented, showing great potential for multi-parameter sensing. One particular scope is the development of vectorial curvature and refractive index sensors with potential for medical, chemical and biological sensing. In addition the design of an optically tunable Mach-Zehnder based multiwavelength filter is introduced. The discovery of a Type IA grating type through overexposure of hydrogen loaded standard and Boron-Germanium co-doped fibres strengthened the assumption of UV-photosensitivity being a highly non-linear process. Gratings of this type show a significantly lower thermal sensitivity compared to standard gratings, which makes them useful for sensing applications. An Oxford Lasers copper-vapour laser operating at 255 nm in pulsed mode was used for their inscription, in contrast to previous work using CW-Argon-Ion lasers and contributing to differences in the processes of the photorefractive index change

Advanced fibre gratings and their applications

This thesis describes a detailed study of advanced fibre grating devices using Bragg (FBG) and long-period (LPG) structures and their applications in optical communications and sensing. The major contributions presented in this thesis are summarised below. One of the most important contributions from the research work presented in this thesis is a systematic theoretical study of many distinguishing structures of fibre gratings. Starting from the Maxwell equations, the coupled-mode equations for both FBG and LPG were derived and the mode-overlap factor was analytically discussed. Computing simulation programmes utilising matrix transform method based on the models built upon the coupled-mode equations were developed, enabling simulations of spectral response in terms of reflectivity, bandwidth, sidelobes and dispersion of gratings of different structures including uniform and chirped, phase-shifted, Moiré, sampled Bragg gratings, phase-shifted and cascaded long-period gratings. Although the majority of these structures were modelled numerically, analytical expressions for some complex structures were developed with a clear physical picture. Several apodisation functions were proposed to improve sidelobe suppression, which guided effective production of practical devices for demanding applications. Fibre grating fabrication is the other major part involved in the Ph.D. programme. Both the holographic and scan-phase-mask methods were employed to fabricate Bragg and long-period gratings of standard and novel structures. Significant improvements were particularly made in the scan-phase-mask method to enable the arbitrarily tailoring of the spectral response of grating devices. Two specific techniques - slow-shifting and fast-dithering the phase-mask implemented by a computer controlled piezo - were developed to write high quality phase-shifted, sampled and apodised gratings. A large number of LabVIEW programmes were constructed to implement standard and novel fabrication techniques. In addition, some fundamental studies of grating growth in relating to the UV exposure and hydrogenation induced index were carried out. In particular, Type IIa gratings in non-hydrogenated B/Ge co-doped fibres and a re-generated grating in hydrogenated B/Ge fibre were investigated, showing a significant observation of thermal coefficient reduction. Optical sensing applications utilising fibre grating devices form the third major part of the research work presented in this thesis. Several experiments of novel sensing and sensing-demodulating were implemented. For the first time, an intensity and wavelength dual-coding interrogation technique was demonstrated showing significantly enhanced capacity of grating sensor multiplexing. Based on the mode-splitting measurement, instead of using conventional wavelength-shifting detection technique, successful demonstrations were also made for optical load and bend sensing of ultra-high sensitivity employing LPG structures. In addition, edge-filters and low-loss high-rejection bandpass filters of 50nm stop-band were fabricated for application in optical sensing and high-speed telecommunication systems

Optically tuning of chromatic dispersion compensators

We present a new tuning method for chromatic dispersion compensators, which can be optically tunable. The dispersion compensators were made in Er/Yb co-doped fiber and were pumped with 980nm laser diodes. The tunable dispersion for a chirped grating and also a uniform-period grating was successfully demonstrated in the experiment.