An alternative approach to energy integrating systems is photon counting which provides higher dose efficiency through efficient noise rejection and optimal energy weighting, and, moreover, is not susceptible to memory artifacts such as image lag and ghosting. The first large-area photon counting imager was Charpak's Nobel Prize winning invention of the gas-filled multiwire proportional chamber (MWPC), which revolutionized the field of radiation detection in 1968. In most applications, however, the use of a solid detection medium is preferable because solid densities are about three orders-of-magnitude greater than gas, and thus, they can yield much smaller detector dimensions with unsurpassed spatial and temporal resolution. Thus far, crystalline Cadmium Zinc Telluride is the only room-temperature solid-state detector that meets the requirements for photon counting imaging. However, the material is grown in small ingots and production costs are high for large-area imaging applications. The problem is that disordered (or non-crystalline) solids, which are easier and less expensive to develop over large-area than single crystalline solids, have been ruled out as viable photon counting detectors because of their poor temporal resolution, or more specifically, extremely low carrier mobilities and transit-time-limited photoresponse. To circumvent the problem of poor charge transport in disordered solids with a conventional planar detector structure, we propose unipolar charge sensing by establishing a strong near-field effect using an electrostatic shield within the material. We introduce the concept of time-differential photoresponse in unipolar solids and show that their temporal resolution can be improved substantially to reach the intrinsic physical limit set by spatial dispersion. Inspired by Charpak's MWPC and its variants, and for the first time, we have implemented an electrostatic shield inside evaporated amorphous selenium (a-Se) using the proposed lithography-based microstrip solid-state detector (MSSD). The fabricated devices are characterized with optical, x-ray, and gamma-ray impulse-like excitations. Using optical time-of-flight (TOF) measurements, we show for the first time a unipolar Gaussian TOF transient from the new MSSD structure, instead of a rectangular response with a Gaussian-integral at the tail which is a typical response of a conventional planar device. The measured optical and x-ray TOF results verify the time-differential property of the electrostatic shield and the practicality of the dispersion-limited photoresponse. Furthermore, we use single gamma-ray photon excitations to probe detector's temporal resolution in pulse mode for photon counting. For the MSSD, we show a depth-independent signal for photon absorption across the bulk and a reduction in signal risetime by a factor of 350, comparing performance limiting factors being hole-dispersion for the MSSD and electron-transit-time for the conventional planar device. The time-differential response obtained from the proposed unipolar detector structure enables disordered photoconductive films to become viable candidates for large-area photon counting applications.

An alternative approach to energy integrating systems is photon counting which provides higher dose efficiency through efficient noise rejection and optimal energy weighting, and, moreover, is not susceptible to memory artifacts such as image lag and ghosting. The first large-area photon counting imager was Charpak's Nobel Prize winning invention of the gas-filled multiwire proportional chamber (MWPC), which revolutionized the field of radiation detection in 1968. In most applications, however, the use of a solid detection medium is preferable because solid densities are about three orders-of-magnitude greater than gas, and thus, they can yield much smaller detector dimensions with unsurpassed spatial and temporal resolution. Thus far, crystalline Cadmium Zinc Telluride is the only room-temperature solid-state detector that meets the requirements for photon counting imaging. However, the material is grown in small ingots and production costs are high for large-area imaging applications. The problem is that disordered (or non-crystalline) solids, which are easier and less expensive to develop over large-area than single crystalline solids, have been ruled out as viable photon counting detectors because of their poor temporal resolution, or more specifically, extremely low carrier mobilities and transit-time-limited photoresponse. To circumvent the problem of poor charge transport in disordered solids with a conventional planar detector structure, we propose unipolar charge sensing by establishing a strong near-field effect using an electrostatic shield within the material. We introduce the concept of time-differential photoresponse in unipolar solids and show that their temporal resolution can be improved substantially to reach the intrinsic physical limit set by spatial dispersion. Inspired by Charpak's MWPC and its variants, and for the first time, we have implemented an electrostatic shield inside evaporated amorphous selenium (a-Se) using the proposed lithography-based microstrip solid-state detector (MSSD). The fabricated devices are characterized with optical, x-ray, and gamma-ray impulse-like excitations. Using optical time-of-flight (TOF) measurements, we show for the first time a unipolar Gaussian TOF transient from the new MSSD structure, instead of a rectangular response with a Gaussian-integral at the tail which is a typical response of a conventional planar device. The measured optical and x-ray TOF results verify the time-differential property of the electrostatic shield and the practicality of the dispersion-limited photoresponse. Furthermore, we use single gamma-ray photon excitations to probe detector's temporal resolution in pulse mode for photon counting. For the MSSD, we show a depth-independent signal for photon absorption across the bulk and a reduction in signal risetime by a factor of 350, comparing performance limiting factors being hole-dispersion for the MSSD and electron-transit-time for the conventional planar device. The time-differential response obtained from the proposed unipolar detector structure enables disordered photoconductive films to become viable candidates for large-area photon counting applications.

Nanolaminate dielectrics combine two or more insulating materials in a many-layered film. These structures can be made to significantly outperform films composed of a single one of their constituent materials by adjusting the composition ratio, arrangement, and size of the component layers. In this work, atomic layer deposition (ALD) is used to fabricate pure-oxide and nanolaminate dielectrics based upon Al₂O₃ and ZrO₂. The relative performance of these dielectrics is investigated with respect to application as gate dielectrics for ZnSnO (ZTO) and InGaZnO (IGZO) amorphous-oxide-semiconductor (AOS) thin-film transistors (TFTs). AOS TFTs are promising candidates for commercial use in applications such as active-matrix displays and e-paper. It was found that the layer thickness, relative composition, and interfacial material all had an effect on TFT performance. Several variants of the Al₂O₃/ZrO₂ nanolaminate were found to exhibit superior properties to either Al₂O₃ or ZrO₂ alone. / Graduation date: 2011

Thesis (M.S.)--Oregon State University, 2010. / Printout. Includes bibliographical references (leaves 139-143). Also available on the World Wide Web.

Thin film transistors (TFTs) can be used to determine the bulk-like mobilities of amorphous semiconductors. Different organic hole transporters (HTs) are under investigation including spiro-TPD, 2TNATA, NPB and TPD which are commonly used in organic light-emitting diodes (OLEDs). In addition, we also measure the TFT hole mobilities of two iridium phosphors: Ir(ppy)3 and Ir(piq)3. These materials were grown on two different gate dielectric surfaces which were SiO2 and polystyrene (PS). On SiO2, the TFT mobilities are found to be 1-2 orders smaller than the bulk hole mobilities as evaluated independently by time-of-flight (TOF) technique. On the other hand, on PS gate dielectric layer, the TFT mobilities of these hole transporters are found to be in good agreement with TOF data. A thickness dependence measurement was carried out on TFT with PS. We found that only 10nm of organic semiconductor is sufficient for TFTs to achieve TOF mobilities. We further investigate why organic semiconductors on SiO2 have such huge reduction of mobilities. Temperature dependent mobility measurements were carried out and the data were analyzed by the Gaussian Disorder Model (GDM). We found that on SiO2 surface, when compared to the bulk values, the energetic disorders (σ) of the HTs increase and simultaneously, the high temperature limits (∞) of the carrier mobilities decrease. Both σ and ∞ contribute to the reduction of the carrier mobility. The increase in σ is related to the presence of randomly oriented polar Si-O bonds. The reduction of ∞ is topological in origin and is related to the orientations of the more planar molecules on SiO2. The more planar molecules tend to lie horizontally on the surface and such orientation is unfavorable for charge transport in TFT configuration. Hybrid organic/inorganic perovskites have emerged as an outstanding material for photovoltaic cells. In the second part of this work, we setup a repeatable perovskite recipe and optimized the system under different conditions. Under certain circumstances, a perovskite solar cell with power conversion efficiency ~9% can be achieved with PEDOT:PSS as hole transporting layer with the conventional structure.

Orientador: Francisco das Chagas Marques / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-08-05T00:47:21Z (GMT). No. of bitstreams: 1 ChampiFarfan_AnaMelva_D.pdf: 65229004 bytes, checksum: 98d3b8d6ecf9c395993d20730f0013f7 (MD5) Previous issue date: 2004 / Resumo: Nesta tese estudamos a influência da incorporação de nitrogênio nas propriedades termomecânicas e estruturais em matrizes de carbono amorfo hidrogenado (a-C:H) e não hidrogenado (a-C). Duas técnicas de deposição foram utilizadas para o crescimento dos filmes. Os filmes hidrogenados foram preparados utilizando um sistema de glow discharge e as não hidrogenadas foram preparadas por IBAD. Um estudo preliminar foi feito em função do bias e a pressão do metano CH4, com o objetivo de escolher duas condições para a incorporação de nitrogênio. Deste primeiro estudo foi observado um aumento do coeficiente de dilatação térnica dos filmes de a-C:H com o aumento das ligações sp2. A partir desta série, uma segunda série de amostras com matriz tipo diamond-like e graphite-like de a-C:H foram estudadas, nas quais observamos que independente da matriz, a incorporação de nitrogênio produz um aumento no coeficiente de dilatação térnica (CTE) de até 9x10-6 C-1 para cerca de 6% de nitrogênio, próximo do valor do CTE do grafite (8x10-6C-1). A influência do nitrogênio no carbono amorfo aumenta a geração das hibridizações sp e sp2, o qual facilita a fornação de clusters grafíticos dentro dos filmes de a-C:H:Nx. O empilhamento dos clusters e a influência destes no CTE dos filmes são amplamente discutidos, para o qual técnicas espectroscópicas como Raman, infravermelho e perda de energia dos elétrons (EELS) são estudadas nesta tese. Por outro lado, para ter uma visão mais clara da influência do N, amostras com matrizes não hidrogenadas de a-C foram preparadas pela técnica de IBAD. Com nesta técnica temos filmes de a-C:Nx com altas concentrações de nitrogênio (até ~30%). Foi observado que mesmo com altas concentrações de N, o CTE dos filmes de a-C:Nx não consegue atingir valores acima de 5xl0-6 C-1. Este fato, faz pensar que o hidrogênio cumpre um papel muito importante no ernpilhamento dos clusters grafíticos. A partir das medidas feitas pela técnica de TIB (o qual foi desenvolvido em nosso laboratório e utilizado para medir o stress, CTE e módulo biaxial dos filmes) e a Nanoindentação obtivemos separadamente o módulo de Young (E) e a razão de Poisson (v), nas três séries de amostras estudadas. Os resultados são discutidos como uma função dos tipos de hibridizações dos filmes de a-C:H, a-C:H:Nx e a-C:Nx / Abstract: In this work, we studied thenno mechanical and structural properties of hydrogenated amorphous carbon (a-C: H), hydrogenated carbon nitride (a-C:H:Nx) and carbon nitride (a-C:Nx) thin films. Two techniques were used to prepare the samples: glow discharge for depositing hydrogenated amorphous carbon (a-C:H) and carbon nitride (a-C:H:Nx) and ion beam assisting deposition (IBAD) for unhydrogenated amorphous carbon nitride (a-C:Nx) films. The curvature of the film/substrate composites was measured using the thermally induced bending (TIB) technique in order to determine the stress of the films. By varying the temperature and using films deposited in several different substrates one was able to obtain the coefficient of thermal expansion (CTE) and the biaxial modulus (E/(l-v)) of the films. Complementarily to this technique, we perfonned nanohardness measurements, which also allows to obtaining the elastic constant (E/(1-v2)) as well. Using both techniques, TIB and nanohardness, we determined, for the first time, separately the Young's modulus (E) and the Poisson's ratio (v) of a-C:H, a-C:H:Nx and a-C:Nx. These thermomechanical properties were investigated as a function of sp2-bond, or graphite's cluster, concentrations, using the following techniques: EELS, infrared spectroscopy, visible and ultraviolet Raman spectroscopy. The influence of the incorporation of nitrogen in the hydrogenated and unhydrogenated matrixes is discussed. We observed that the CTE of a-C:H depends strongly on the concentration of Sp2 sites approaching the value for graphite as the concentration tends to 100 %. The nitrogen incorporation increases the cluster formation in matrixes, inducing a substantial increase of the CTE of the fi1ms. On the other hand, in the matrix free of hydrogen it was observed that the CTE is not strongly influenced by the nitrogen concentration. These observations indicate that hydrogen plays an important role on the value of the CTE in amorphous carbon nitrogen films / Doutorado / Física da Matéria Condensada / Doutor em Ciências

The focus of this thesis is the investigation of thin-film transistors (TFTs) based on amorphous oxide semiconductors (AOSs) in two circuit applications. To date, circuits implemented with AOS-based TFTs have been primarily enhancement-enhancement inverters, ring oscillators based on these inverters operating at peak frequencies up to ~400 kHz, and two-transistor one-capacitor pixel driving circuits for use with organic light-emitting diodes (OLEDS). The first application investigated herein is AC/DC rectification using two circuit configurations based on staggered bottom-gate TFTs employing indium gallium oxide (IGO) as the active channel layer; a traditional full bridge rectifier with diode-tied transistors and a cross-tied full-wave rectifier are demonstrated, which is analogous to what has been reported previously using p-type organic TFTs. Both circuit configurations are found to operate successfully up to at least 20 MHz; this is believed to be the highest reported operating frequency to date for circuits based on amorphous oxide semiconductors. Output voltages at one megahertz are 9 V and ~10.5 V, respectively, when driven with a differential 7.07 Vrms sine wave. This performance is superior to that of previously reported organic-based rectifiers. The second AOS-based TFT circuit application investigated is an enhancement-depletion (E-D) inverter based on heterogeneous channel materials. Simulation results using models based on a depletion-mode indium zinc oxide (IZO) TFT and an enhancement-mode IGO TFT result in a gain of ~15. Gains of other oxide-based inverters have been limited to less than 2; the large gain of the E-D inverter makes it well suited for digital logic applications. Deposition parameters for the IGO and IZO active layers are optimized to match the models used in simulation by fabricating TFTs on thermally oxidized silicon and patterned via shadow masks. Integrated IGO-based TFTs exhibit a similar turn-on voltage and decreased mobility compared to the shadow masked TFTs. However, the integrated IZO-based TFTs fabricated to date are found to be conductive and exhibit no gate modulation. Due to the conductive nature of the load, the fabricated E-D inverter shows no significant output voltage variation. This discrepancy in performance between the integrated and shadow-masked IZO devices is attributed to processing complications. / Graduation date: 2009

The primary objective of this thesis is to develop a process for fabricating integrated circuits based on thin-film transistors (TFTs) using zinc tin oxide (ZTO) as the channel layer. ZTO, in contrast to indium- or gallium-based amorphous oxide semiconductors (AOS), is perceived to be a more commercially viable AOS choice due to its low cost and ability to be deposited via DC reactive sputtering. In the absence of an acceptable ZTO wet etch process, a plasma-etching process using Ar/CH₄ is developed for both 1:1 and 2:1 ZTO compositions. An Ar/CH₄ plasma etch process is also designed for indium gallium oxide (IGO), indium gallium zinc oxide (IGZO), and indium tin oxide (ITO). Ar/CH₄ dry etches have excellent selectivity with respect to SiO₂, providing a route for obtaining patterned ZTO channels. A critical asset of ZTO process integration involves removing polymer deposits after ZTO etching without active layer damage. A ZTO process is developed for the fabrication of integrated circuits which use ZTO channel enhancement-mode TFTs. Such ZTO TFTs exhibit incremental and average mobilities of 23 and 18 cm²V⁻¹s⁻¹, respectively, turn-on voltages approximately 0 to 1.5 V and subthreshold swings below 0.5 V/dec when annealed in air at 400 °C for 1 hour. Several types of ZTO TFT circuits are realized for the first time. Despite large parasitic capacitances due to large gate-source and gate-drain overlaps, AC/DC rectifiers are fabricated and found to operate in the MHz range. Thus, they are usable for RFID and other equivalent-speed applications. Finally, a ZTO process for simultaneously fabricating both enhancement-mode and depletion-mode TFTs on a single substrate using a single target and anneal step is developed. This dual-channel process is used to build a high-gain two-transistor enhancement/depletion inverter. At a rail voltage of 10 V, this inverter has a gain of 10.6 V/V, the highest yet reported for an AOS-based inverter. This E/D inverter is an important new functional block which will enable the realization of more complex digital logic circuits. / Graduation date: 2009

Recentemente, a dopagem de semicondutores (envolvendo compostos II-VI, IV-VI, III-V, e do grupo-IV) com espécies magnéticas tem sido extensivamente investigada em função do seu potencial em spintrônica. Neste contexto, semicondutores magnéticos baseados no Si e no Ge são atraentes devido à sua compatibilidade com a indústria de semicondutores existente. Entretanto, a solubilidade das espécies magnéticas nestes materiais em forma cristalina é muito baixa e, consequentemente, sua atividade magnética é limitada. Este não é o caso para o silício amorfo (a-Si) e o germânio amorfo (a-Ge), que podem conter elementos magnéticos além do limite de solubilidade de seus análogos cristalinos, e apresentar propriedades magnéticas notáveis. Motivado por estes fatos, este trabalho apresenta uma investigação abrangente de filmes finos de Si e Ge contendo diferentes quantidades de Mn e Co, trazendo informações úteis no entendimento das propriedades desta classe de materiais. As amostras foram preparadas por co-sputtering, e possuíram concentrações de Mn na faixa de ~ 0.1-24 at.%, e de Co na faixa de ~ 1-10 at.%. Após a deposição, os filmes foram submetidos a tratamentos térmicos cumulativos até 900 oC, e foram investigados por: espectroscopia de energia dispersiva de raios-x (EDS); espalhamento Raman; difração de raios-x; transmissão óptica; microscopias eletrônica de varredura (SEM), de força atômica (AFM) e de força magnética (MFM); magnetometria SQUID; método de van der Pauw; etc. Para fins comparativos, amostras puras também foram preparadas, tratadas e caracterizadas de forma similar. Os presentes resultados indicam que os átomos de Mn e Co foram incorporados de forma efetiva e homogênea nas matrizes amorfas. Além disso, os filmes sem tratamento (puros ou contendo impurezas) são essencialmente amorfos. Ao contrário, tratamentos em altas temperaturas induzem a cristalização das amostras, e alterações em suas demais características, dependentes da introdução de dopantes. Desta forma: suas propriedades estruturais, ópticas, morfológicas, elétricas, e magnéticas, são notadamente afetadas pela inserção de Mn e Co, e pela temperatura de tratamento térmico. Estas observações foram sistematicamente investigadas e serão apresentadas e discutidas em detalhe. / Along the last few years, the doping of semiconductors (either II-VI, IV-VI, III-V, and group-IV compounds) with magnetic species have been extensively studied due to their potential applications in spintronics. Among them, Si- and Ge-based magnetic semiconductors are very attractive because of their total compatibility with the well-established current semiconductor technology. In the crystalline form, however, these materials exhibit a low solubility limit to magnetic species and, consequently, limited magnetic activity. This is not the case for amorphous (a-)Si and a-Ge, which can contain magnetic elements beyond the solubility limit of their crystalline counterparts, and present improved magnetic properties. Motivated by these facts, this work contains a comprehensive investigation of Si and Ge thin films containing different amounts of Mn and Co, providing useful information concerning the properties of this class of materials. The samples were prepared by co-sputtering, rendering Mn concentrations in the ~ 0.1-24 at.% range, and Co contents in the ~ 1-10 at.% range. After deposition, the films were submitted to isochronal thermal annealing treatments up to 900 oC and investigated by: energy dispersive x-ray spectrometry (EDS); Raman scattering spectroscopy; x-ray diffraction; optical transmission measurements; scanning electron (SEM), atomic force (AFM) and magnetic force (MFM) microscopy techniques; SQUID magnetometry; van der Pauw technique; etc. For comparison purposes, pure samples were also prepared, annealed and characterized in a similar way. The present experimental results indicate that the Mn and Co atoms were effectively and homogenously incorporated into the amorphous hosts. Moreover, the as-deposited films (either pure or doped) are essentially amorphous. On the contrary, thermal annealing at increasing temperatures induces the crystallization of the samples, and changes in their further characteristics, that are dependent of the doping. In this way: their structural, optical, morphological, electrical, and magnetic properties, etc., are notably affected by the insertion of Mn and Co, and by the temperature of thermal annealing. These experimental observations were systematically studied and will be presented and discussed in detail.

Neste trabalho são apresentados resultados da caracterização estrutural e morfológica de filmes de oxinitreto de silício (SiOxNy) depositados pela técnica de deposição química a vapor assistida por plasma (PECVD) a baixa temperatura (320°C). O objetivo deste trabalho é relacionar a composição química de ligas amorfas de SiOxNy com suas propriedades ópticas, estruturais, morfológicas e mecânicas visando sua aplicação em dispositivos elétricos, optoeletrônica e microestruturas. A proposta é dar continuidade a trabalhos prévios desenvolvidos no grupo, que demonstraram a viabilidade de controlar a composição química e, como conseqüência, controlar as propriedades como o índice de refração, constante dielétrica e fotoluminescência de filmes de SiOxNy. As condições de deposição foram ajustadas de forma a obter dois tipos de material: filmes de SiOxNy de composição química controlável entre a do SiO2 e a do de Si3N4 e filmes de SiOxNy com composição rica em Si. O material foi caracterizado pelas técnicas de elipsometria, índice de refração por prisma acoplado, RBS (Rutherford Backscattering Spectroscopy), FTIR (Fourier Transform Infrared Spectroscopy), XANES (X-Ray Absorption Near Edge Spectroscopy) na borda K do Si, O e N, medida de stress residual e microscopia eletrônica de varredura (Scanning Electron Microscopy) e de transmissão (Transmission Electron Microscopy). Os resultados mostraram que os filmes com composição química intermediária entre a do SiO2 e a do Si3N4 apresentam arranjo estrutural estável com a temperatura, mantendo as ligações e a estrutura amorfa mesmo após tratamentos térmicos a 1000°C. Também fora demonstrada a possibilidade de obter um material com baixo stress residual e índice de refração ajustável entre 1,46 e 2, resultados ótimos para aplicações em MOEMS (micro-opto-electro- mechanical systems). Já nas amostras ricas em Si foi observada a formação de diferentes fases, sendo uma delas formada por aglomerados de Si e a outra por material constituído por uma mistura de ligações Si-O e Si-N. Este material apresenta a formação de nanocristais de Si, dependendo do conteúdo de Si e das condições do tratamento térmico, permitindo assim, sua aplicação em dispositivos emissores de luz. / In this work results on the morphological and structural characterization of silicon oxynitride (SiOxNy) films deposited by plasma enhanced chemical vapor deposition technique (PECVD) at low temperature (320°C) are presented. The main goal is to correlate the chemical composition of amorphous SiOxNy alloys to their optical, structural, morphological and mechanical properties intending applications on electrical, optoelectronic and micromechanical devices. The proposal is to continue previous research developed in this group, which demonstrated the possibility of tuning the chemical composition and, consequently, the SiOxNy films properties such as refractive index, dielectric constant and photoluminescence by the precise control of the deposition parameters. The deposition conditions were adjusted in order to obtain to material types, SiOxNy films with tunable chemical composition between SiO2 and Si3N4 and silicon-rich SiOxNy. The characterization was performed by elipsometry, refractive index by coupled prism, RBS (Rutherford Backscattering Spectroscopy), FTIR (Fourier Transform Infrared Spectroscopy), XANES (X-Ray Absorption Near Edge Spectroscopy) on K edge of Si, O and N, residual stress measurement and Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The films with chemical composition between SiO2 and Si3N4 presented stable structural arrangement with temperature, maintaining the chemical bonds and the amorphous structure after high temperature annealing. Also the results demonstrated the possibility of producing a low residual stress material and an adjustable refractive index since in the 1.46 to 2 range, excellent result for MOEMS devices (micro-opto-electro- mechanical systems applications. For silicon rich-samples the formation of different phases was observed, one formed by Si clusters and other one by a mixture of Si-O and Si-N bonds. Depending on the Si content and on the annealing conditions this material can present nanocristals, results which allowed us to understand and to optimize this material for light emitting devices applications.