Design and implementation of microstrip filters for a radio over fiber network demonstrator

Dissertação de mest., Engenharia Eléctrica e Telecomunicações, Faculdade de Ciências e Tecnologia, Univ. do Algarve, 2010The need for networks able of integrating services such as voice, video, data and mobility is growing. To satisfy such needs wireless networks with a high data transmission capacity are required. An efficient solution for these broadband wireless networks is to transmit radio signals to the Base Stations (BS) via optical fiber using Wavelength Division Multiplexing (WDM). The WDM usage helps this growing, allowing the use of a single optical fiber to feed several BSs using for each one a different wavelength (or WDM channel). Additionally, in the RoFnet project in order to improve radio coverage within a cell, it is considered a sectorized antenna interface. The combination of subcarrier multiplexing (SCM) with WDM, further simplifies the network architecture, by using a specific wavelength channel to feed an individual BS and different subcarriers to drive the individual antenna sectors within the BS. This dissertation reports the design and simulation of the microstrip bandpass filters used at the BS on of the RoFnet demonstrator. These bandpass filters are used for the filtering of fours subcarrier multiplexed channels located at (9, 11, 13, 15 and 17 GHz). The design and simulation of the lowpass root raised cosine filter required for testing is also discussed. Additionally, the design and testing of two power splitter is reported. Finally, all the designed components were brought together and the overall BS performance is assessed. The microstrip components have been designed and simulated using both ADS (Agilent’s Advanced Design System) and Momentum simulators

Electrical characterization of metal-oxide-polymer devices for non-volatile memory applications

The objective of this thesis is to study the properties of resistive switching effect based on bistable resistive memory which is fabricated in the form of Al2O3/polymer diodes and to contribute to the elucidation of resistive switching mechanisms. Resistive memories were characterized using a variety of electrical techniques, including current-voltage measurements, small-signal impedance, and electrical noise based techniques. All the measurements were carried out over a large temperature range. Fast voltage ramps were used to elucidate the dynamic response of the memory to rapid varying electric fields. The temperature dependence of the current provided insight into the role of trapped charges in resistive switching. The analysis of fast current fluctuations using electric noise techniques contributed to the elucidation of the kinetics involved in filament formation/rupture, the filament size and correspondent current capabilities. The results reported in this thesis provide insight into a number of issues namely: (i) The fundamental limitations on the speed of operation of a bi-layer resistive memory are the time and voltage dependences of the switch-on mechanism. (ii) The results explain the wide spread in switching times reported in the literature and the apparently anomalous behaviour of the high conductance state namely the disappearance of the negative differential resistance region at high voltage scan rates which is commonly attributed to a “dead time” phenomenon which had remained elusive since it was first reported in the ‘60s. (iii) Assuming that the current is filamentary, Comsol simulations were performed and used to explain the observed dynamic properties of the current-voltage characteristics. Furthermore, the simulations suggest that filaments can interact with each other. (iv) The current-voltage characteristics have been studied as a function of temperature. The findings indicate that creation and annihilation of filaments is controlled by filling and neutralizing traps localized at the oxide/polymer interface. (v) Resistive switching was also studied in small-molecule OLEDs. It was shown that the degradation that leads to a loss of light output during operation is caused by the presence of a resistive switching layer. A diagnostic tool that predicts premature failure of OLEDs was devised and proposed. Resistive switching is a property of oxides. These layers can grow in a number of devices including, organic light emitting diodes (OLEDs), spin-valve transistors and photovoltaic devices fabricated in different types of material. Under strong electric fields the oxides can undergo dielectric breakdown and become resistive switching layers. Resistive switching strongly modifies the charge injection causing a number of deleterious effects and eventually device failure. In this respect the findings in this thesis are relevant to understand reliability issues in devices across a very broad field.As memórias resistivas baseiam-se na alteração da resistência elétrica de um material ou componente quando submetido a uma tensão elétrica. Este fenómeno deu origem a um novo elemento eletrónico que se passou a designar por “memristor” por sugestão de Leon Chua em 1971. [1] O “memristor” juntou-se assim aos componentes elétricos mais conhecidos, o condensador, a bobine e a resistência. Desde os anos 60 que a comutação resistiva tem sido observada numa variedade de materiais. No contexto desta tese os mais interessantes são por exemplo o SiOx, Al2O3, Ta2O5, ZrO2 e TiO2, onde a comutação resistiva é um processo eletrónico e não envolve uma mudança de fase do material. Os processos físicos envolvidos na comutação de resistência tem permanecido pouco claros. Os vários mecanismos propostos não tem merecido o consenso da comunidade científica. A ausência de um modelo físico tem impedido o desenvolvimento tecnológico destas memórias que têm assim progredido de forma empírica. Apesar da falta de conhecimento sobre os mecanismos físicos, as memórias resistivas oferecem um conjunto de vantagens sobre as tecnologias atuais. Isto despoletou uma intensa atividade de pesquisa quer no meio académico quer pela industria para comercializar este tipo de componente. As memórias resistivas combinam num só componente as vantagens de diversas tecnologias atuais. Podem ter a velocidade de acesso das memórias aleatórias de acesso dinâmico (DRAMs) com um custo muito inferior, com menor consumo de energia e sem necessidade de periodicamente fazer o restauro ou “refeshing”. Oferecem as características não-voláteis de uma memória do tipo “flash”, mas mais robustas, permitindo assim mais ciclos de leitura e escrita. Possibilitam uma elevada densidade e não sofrem dos problemas mecânicos dos discos duros associados com as cabeças de leitura. A comercialização deste tipo de memórias irá revolucionar as tecnologias de informação ao disponibilizar uma elevada capacidade de memória a baixo custo, em dimensões reduzidas e com muito baixo consumo de energia. As memórias resistivas também não precisam de alguma da eletrónica que acompanha os sistemas atuais, nomeadamente os sistemas de “cache”, reduzindo substancialmente os custos e a complexidade dos circuitos. O trabalho desenvolvido nesta tese foi focado nas propriedades elétricas das memórias resistivas com o objetivo de aumentar o nosso conhecimento sobre os mecanismos físicos e elétricos que controlam a comutação resistiva e a velocidade de acesso. As memórias estudadas nesta tese são estruturas do tipo metal-isolador-semicondutor (MIS). Foi usado óxido de alumínio e um polímero conjugado para a camada isolante e semicondutora respetivamente. Estas memórias comutam entre dois estados resistivos diferentes quando submetidas a voltagens definidas durante um certo período de tempo. Paralelamente, foi identificado que o processo físico que conduz a bi-estabilidade elétrica do óxido de alumínio é também responsável pela falha prematura de díodos emissores de luz orgânicos (OLEDs). A presença de óxido de alumínio nativo nos eletrodos dos OLEDs pode dar origem a transições resistivas que alteram o equilíbrio da injeção de portadores de carga e leva a degradação da eletroluminescência. Quer as memórias quer os díodos emissores de luz foram caraterizados usando técnicas elétricas e óticas. Medidas da resposta da corrente a degraus e/ou rampas de tensão permitiram avaliar a velocidade de comutação resistiva. Medidas da impedância no domínio da frequência foram usadas para estudar variações de carga nas interfaces da memória, e por último medidas do ruído elétrico complementadas com medidas óticas permitiram estudar flutuações na corrente causadas pela criação e aniquilação de pequenos caminhos condutores ou filamentos. Todas as medidas foram feitas num grande intervalo de temperatura e frequência. Esta tese contribui para o esclarecimento dos mecanismos físicos que originam comutações entre estados resistivos não-voláteis. As constantes de tempo que controlam o tempo de acesso à memória, isto é, o tempo para ler, escrever ou apagar foram também estudadas. Os resultados obtidos contribuíram para elucidar o mecanismo físico que determina o tempo de acesso. Estratégias para otimizar a rapidez deste tipo de memoria foram propostas. Foi identificado que a condução elétrica é não-homogénea. A corrente é transportada por filamentos. Foi possível quantificar as dimensões físicas e a densidade de corrente transportada por filamentos individuais. O estudo da dinâmica destes filamentos usando técnicas de análise de ruído elétrico permitiu concluir que os filamentos não são criados nem destruídos, mas sim ligados e desligados como interruptores. O mecanismo que liga os filamentos são buracos armadilhados na camada de óxido de alumínio. Quando os buracos são neutralizados por eletrões o filamento é desligado. Este resultado foi um dos contributos mais importantes para a área científica. A condução filamentar dá origem a um conjunto de observações não intuitivas. Concretamente, dá origem a uma dependência anómala da corrente elétrica com a temperatura. A corrente aumenta de forma discreta à medida que a temperatura diminui, isto porque o armazenamento de cargas em armadilhas a baixas temperaturas liga mais filamentos. Adicionalmente, a existência de condução filamentar dá origem a que a corrente elétrica diminua, quando as rampas de tensão rápidas são aplicadas sucessivamente. Os resultados desta tese também sugerem que filamentos de corrente vizinhos podem interatuar e dar origem a fenómenos correlacionados, quer durante o ligar, quer durante o desligar de filamentos. O campo elétrico associado a dois filamentos vizinhos induz um campo elétrico adicional na região intermédia que pode ligar um terceiro filamento. Se um filamento for desligado os filamentos na vizinhança terão mais probabilidade de ser desligados. Simulações usando o “COMSOL Multiphysics” parecem suportar a correlação destes fenómenos. A comutação resistiva é uma propriedade de óxidos binários. Este fenómeno pode ocorrer de forma não intencional, nomeadamente em díodos emissores de luz, células solares, válvulas de spin, transístores de efeito de campo e de uma forma geral, todos os componentes que usam elétrodos que oxidam. O conhecimento adquirido nesta tese é assim relevante para detetar e prevenir problemas de confiabilidade num conjunto vasto de componentes eletrónicos.Dutch Polymer Institute (DPI), Project No. 704, BISTABLE (resistive switching and OLED reliability) and by Fundação para Ciência e Tecnologia (FCT) through the Instituto de Telecomunicacões (IT)

Unipolar resistive switching in metal oxide/organic semiconductor non-volatile memories as a critical phenomenon

Diodes incorporating a bilayer of an organic semiconductor and a wide bandgap metal oxide can show unipolar, non-volatile memory behavior after electroforming. The prolonged bias voltage stress induces defects in the metal oxide with an areal density exceeding 10(17) m(-2). We explain the electrical bistability by the coexistence of two thermodynamically stable phases at the interface between an organic semiconductor and metal oxide. One phase contains mainly ionized defects and has a low work function, while the other phase has mainly neutral defects and a high work function. In the diodes, domains of the phase with a low work function constitute current filaments. The phase composition and critical temperature are derived from a 2D Ising model as a function of chemical potential. The model predicts filamentary conduction exhibiting a negative differential resistance and nonvolatile memory behavior. The model is expected to be generally applicable to any bilayer system that shows unipolar resistive switching. (C) 2015 Author(s).Dutch Polymer Institute (DPI), BISTABLE [704]; Fundacao para Ciencia e Tecnologia (FCT) through the research Instituto de Telecommunicacoes (IT-Lx); project Memristor based Adaptive Neuronal Networks (MemBrAiNN) [PTDC/CTM-NAN/122868/2010]; European Community Seventh Framework Programme FP7', ONE-P [212311]; Dutch Ministry of Education, Culture and Science (Gravity Program) [024.001.035]info:eu-repo/semantics/publishedVersio

Polichromatic image sensor with microlenses for stereoscopic acquisition

This paper presents the fabrication of microlenses for integration on a stereoscopic image sensor in CMOS technology and a low-cost technology for fabricating optical filters arrays tuned for the primary colors. The material selected for fabricating the microlens was the AZ4562 positive photoresist and the fabrication process explained. Moreover, the fabrication process presented in this paper is for directly printing the optical filters into a transparent flexible substrate (acetate).This work was fully supported by the Portuguese Foundation for Science and Technology under the project FCT/PTDC/EEA-ELC/109936/2009 and R. P. Rocha is supported by the Foundation for Science and Technology financial grant SFRH/BD/33733/2009

Photonic sensors based on flexible materials with FBGs for use on biomedical applications

This chapter is intended for presenting biomedical applications of FBGs embedded into flexible carriers for enhancing the sensitivity and to provide interference-free instrumentation.This work was fully supported by the Algoritmi’s Strategic Project UI 319-2011-2012, under the Portuguese Foundation for Science and Technology grant Pest C/EEI/UI0319/2011

Unipolar resistive switching in metal oxide/organic semiconductor non-volatile memories as a critical phenomenon

Diodes incorporating a bilayer of an organic semiconductor and a wide bandgap metal oxide can show unipolar, non-volatile memory behavior after electroforming. The prolonged bias voltage stress induces defects in the metal oxide with an areal density exceeding 1017m-2. We explain the electrical bistability by the coexistence of two thermodynamically stable phases at the interface between an organic semiconductor and metal oxide. One phase contains mainly ionized defects and has a low work function, while the other phase has mainly neutral defects and a high work function. In the diodes, domains of the phase with a low work function constitute current filaments. The phase composition and critical temperature are derived from a 2D Ising model as a function of chemical potential. The model predicts filamentary conduction exhibiting a negative differential resistance and nonvolatile memory behavior. The model is expected to be generally applicable to any bilayer system that shows unipolar resistive switching.</p

Reversible post-breakdown conduction in aluminum oxide-polymer capacitors

Aluminum/Al2O3/polymer/metal capacitors submitted to a low-power constant current stress undergo dielectric breakdown. The post-breakdown conduction is metastable, and over time the capacitors recover their original insulating properties. The decay of the conduction with time follows a power law (1/t)(alpha). The magnitude of the exponent alpha can be raised by application of an electric field and lowered to practically zero by optical excitation of the polyspirofluorene polymer. The metastable conduction is attributed to formation of metastable pairs of oppositely charged defects across the oxide-polymer interface, and the self-healing is related to resistive switching. (C) 2013 AIP Publishing LLC [http://dx.doi.org/10.1063/1.4802485

Lithium fluoride injection layers can form quasi-Ohmic contacts for both holes and electrons

Thin LiF interlayers are typically used in organic light-emitting diodes to enhance the electron injection. Here, we show that the effective work function of a contact with a LiF interlayer can be either raised or lowered depending on the history of the applied bias. Formation of quasi-Ohmic contacts for both electrons and holes is demonstrated by electroluminescence from symmetric LiF/polymer/LiF diodes in both bias polarities. The origin of the dynamic switching is charging of electrically induced Frenkel defects. The current density-electroluminescence-voltage characteristics can qualitatively be explained. The interpretation is corroborated by unipolar memristive switching and by bias dependent reflection measurements. (C) 2014 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License

Opto-electronic characterization of electron traps upon forming polymer oxide memory diodes

Metal-insulator-polymer diodes where the insulator is a thin oxide (Al2O3) layer are electroformed by applying a high bias. The initial stage is reversible and involves trapping of electrons near the oxide/polymer interface. The rate of charge trapping is limited by electron transport through the polymer. Detrapping of charge stored can be accomplished by illuminating with light under short-circuit conditions. The amount of stored charge is determined from the optically induced discharging current transient as a function of applied voltage and oxide thickness. When the charge density exceeds 8 1017/m2, an irreversible soft breakdown transition occurs to a non-volatile memory diode

Lithium fluoride injection layers can form quasi-Ohmic contacts for both holes and electrons

Thin LiF interlayers are typically used in organic light-emitting diodes to enhance the electron injection. Here, we show that the effective work function of a contact with a LiF interlayer can be either raised or lowered depending on the history of the applied bias. Formation of quasi-Ohmic contacts for both electrons and holes is demonstrated by electroluminescence from symmetric LiF/polymer/LiF diodes in both bias polarities. The origin of the dynamic switching is charging of electrically induced Frenkel defects. The current density-electroluminescence-voltage characteristics can qualitatively be explained. The interpretation is corroborated by unipolar memristive switching and by bias dependent reflection measurements.</p