1,720,961 research outputs found
Position-sensitive multi-wavelength photon detectors based on epitaxial InGaAs/InAlAs quantum wells
No full textBeam monitoring in synchrotron radiation or free electron laser facilities is extremely important for calibration and diagnostic issues. Here we propose an in-situ detector showing fast response and homogeneity for both diagnostics and calibration purposes. The devices are based on In0.75Ga0.25As/In0.75Al0.25As QWs, which offer several advantages due to their direct, low-energy band gap and high electron mobility at room temperature. A pixelation structure with 4 quadrants was developed on the back surface of the device, in order to fit commercially available readout chips.
The QW devices have been tested with collimated monochromatic X-ray beams from synchrotron radiation. A rise in the current noise with positive bias was observed, which could be due to deep traps for hole carriers. Therefore, an optimized negative bias was chosen to minimize dark currents and noise. A decrease in charge collection efficiency was experienced as the beam penetrates into deeper layers, where a dislocation network is present. The prototype samples showed that individual currents obtained from each quadrant allow the position of the beam to be monitored for all the utilized energies. These detectors have a potential to estimate the position of the beam with a precision of about 10 μm
Fast pixelated quantum-well-based sensor for multi-wavelength photon detection
No full textIn order to cover a wide range of experimental applications, the opportunity to use InGaAs/InAlAs quantum well (QW) devices as fast pixelated photon detectors has been investigated. QW structures are planar objects in which electrons are confined in one dimension. Devices with several combinations of barrier and well materials can be fabricated by using compound semiconductors; in the case of InGaAs/InAlAs QWs this allows to tune the energy band gap down to 0.6 eV. Thanks to their direct, low-energy gap such devices operated at room temperature may be used as detectors for photon energies ranging from visible to hard X-ray. Internal charge amplification mechanism can be applied for very low signal levels, while the high carrier mobility allows the design of very fast photon detectors with sub-nanosecond response times. QW devices grown with molecular beam epitaxy have been pixelated by using standard photolithographic techniques. In order to fit commercially available readout chips, a pixelated sensor with pixel size of 172 × 172 μm2 is currently under development. A small-scale version of the pixelated QW sensor has been preliminarily tested with synchrotron radiation, conventional X-rays and UV laser light. The reported results indicate that these devices show fair charge sharing in the clearances between the pixels and feature very short response times to 100-fs-wide laser pulse
Fast, multi-wavelength, efficiency-enhanced pixelated devices based on InGaAs/InAlAs quantum-well
No full textSeveral applications utilizing either synchrotron or conventional light sources require fast and efficient pixelated detectors. In order to cover a wide range of experiments, this work investigates the possibility to use InGaAs/InAlAs quantum well devices as photon detectors for a broad range of energies.
Owing to their direct, low-energy band gap and high electron mobility, such devices may be used also at room temperature as multi-wavelength sensors from visible light to hard X-rays. Furthermore, internal charge-amplification mechanism can be applied for very low signal levels, while the high carrier mobility allows the design of very fast photon detectors with sub-nanosecond response times.Samples were grown by solid source molecular beam epitaxy on GaAs substrates. Metamorphic In0.75Ga0.25As/ In0.75Al0.25As heterostructures were obtained by relaxing the strain due to the lattice mismatch in the substrate by means of a composition-graded buffer layer. A two-dimensional electron gas forming in an In0.75Ga0.25As quantum well is sandwiched between In0.75Al0.25As barriers and is modulation-doped by a Si δ on its top. The samples have been pixelated by using standard photolithographic techniques. In order to fit commercially available readout chips, a pixelated sensor with pixel size of 172 × 172 μ m2 is currently under development. A small-scale version of the pixelated quantum well sensor has been preliminary tested with 100-fs-wide laser pulses and X-ray synchrotron radiation. The reported results indicate that these sensors respond with 100-ps rise-times to ultra-fast laser pulses. Synchrotron X-ray tests show how these devices exhibit high charge collection efficiencies, which can be imputed to the charge-multiplication effect of the 2D electron gas inside the well
Fast, multi-band photon detectors based on quantum well devices for beam-monitoring in new generation light sources
Full text linkIn order to monitor the photon-beam position for both diagnostics and calibration purposes, we have investigated the possibility to use InGaAs/InAlAs
Quantum Well (QW) devices as position-sensitive photon detectors for Free-Electron Laser (FEL) or Synchrotron Radiation (SR).
Owing to their direct, low-energy band gap and high electron mobility, such QW devices may be used also at Room Temperature (RT) as fast multi-band sensors for photons ranging from visible light to hard X-rays. Moreover, internal charge-amplification mechanism can be applied for very low signal levels, while the high carrier mobility allows the design of very fast photon detectors with sub-nanosecond response times. Segmented QW sensors have been preliminary tested with 100-fs-wide 400 nm laser pulses and X-ray SR. The reported results indicate that these devices respond with 100 ps rise-times to such ultra-fast laser pulses. Besides, linear scan on the back-pixelated device has shown that these detectors are sensitive to the position of each ultrashort beam bunch
Position sensitive photon detectors using epitaxial InGaAs/InAlAs quantum wells
No full textThis work deals with the investigation of novel position-sensitive devices based on InGaAs/InAlAs quantum wells, which could be applied to several applications of either synchrotron or conventional light sources. Such devices may be used as fast and efficient detectors due to the direct, low-energy band gap and high electron mobility at room temperature. Metamorphic In0.75Ga0.25As/In0.75Al0.25As quantum wells containing a two-dimensional electron gas were grown by molecular beam epitaxy. Two devices with size of 5 × 5 mm2 were prepared by using optical lithography. In the first, the active layers were segmented into four electrically insulated quadrants. Indium ohmic contacts were realized on the corner of each quadrant (for readout) and on the back surface (for bias). In the second, the quantum well was left unsegmented and covered by 400 nm of Al providing a single bias electrode, while four readout electrodes were fabricated on the back side by depositing and segmenting a Ni/Ge/Au layer. Photo-generated carriers can be collected at the readout electrodes by biasing from either the QW side or the back side of the devices during beam exposure. Individual currents obtained from each electrode allow monitoring of both the position and the intensity of the impinging beam for photon energies ranging from visible to hard X-ray. Such detector prototypes were tested with synchrotron radiation. Moreover, the position of the beam can be estimated with a precision of 800 nm in the segmented QW. A lower precision of 10 μm was recorded in the unsegmented QW due to the charge diffusion through the 500-μm-thick wafer, with however a lower electronic noise due to the better uniformity of the contacts
Development of quantum well structures for multi band photon detection
Full text link2013/2014La ricerca qui presentata è incentrata sullo sviluppo di tecnologie innovative per la produzione di rivelatori di posizione di fasci fotonici veloci (pBPM) per applicazioni in luce di sincrotrone (SR) e laser a elettroni liberi (FEL). Nel nostro lavoro abbiamo proposto un rilevatore in-situche ha dimostrato velocità di risposta ed omogeneità sia per scopi di diagnostica che di calibrazione. I dispositivi sono basati su pozzi quantici (QW) dimateriali semiconduttori InGaAs / InAlAs,che offrono diversi vantaggi grazie alla loro gap di banda diretta e a bassa energia, e all’alta mobilità elettronica a temperatura ambiente. I QW metamorfici diIn0.75Ga0.25As/In0.75Al0.25As contenenti un gas di elettroni bidimensionali (2DEG) sono staticresciuti tramite epitassia a faci molecolari (MBE). Tali materiali presentano alcune differenze notevoli rispetto al diamante, che è il materiale utilizzato per i rivelatori commerciali allo stato dell’arte. Innanzitutto, i costi di produzione e di fabbricazione sono molto più bassi. Poi, il coefficiente di assorbimento è molto superiore al diamante su una vasta gamma di energie di raggi X, il che li rende ampiamente complementari in possibili applicazioni. Inoltre, utilizzando semiconduttori composti si possono fabbricare dispositivi con diverse combinazioni di materiali per la barriera ed il QW;ciòha permesso di ridurre la gap di energia fino a 0.6 eV. La disponibilità e la ripetibilità di fabbricazione dei dispositivi è migliore rispetto a quelle del diamante. Quattro configurazioni di dispositivi a QW pixelati sono stati testati con diverse fonti di luce, come radiazione di sincrotrone, tubo a raggi X convenzionali e laser ultra veloce nel vicinoUV.
In questa tesi, dopo aver introdotto i dispositivi a QW per utilizzo comepBPM, saranno riportati e discussii risultati più importanti ottenuti. Tali risultati indicano che questi rivelatori rispondono con tempi di 100-ps a impulsi laser ultraveloci, cioè un fattore 6 più velocirispetto a rivelatori a semiconduttori commerciali allo stato dell’arte. La precisione raggiunta nella stima della posizione del fascio fotonico è di 800nm, da confrontare con i 150nm di rivelatori a diamante commerciali. Inoltre, i nostri rivelatori di fotoni a QW lavorano a tensioni molto inferiori rispetto aipBPMs esistenti.Infine, test con raggi X da radiazione di sincrotrone mostrano come questi dispositivi presentano elevate efficienze di raccolta di carica, che possono essere imputabili all'effetto di moltiplicazione di carica del gas di elettroni 2D all'interno del pozzo. Tutti questi vantaggi rispetto ai rivelatori esistenti basati sul diamante, rendono i nostri dispositivi potenzialmente molto attrattivi come alternativa a quelli commerciali.XXVII Ciclo198
Fast Synchrotron and FEL Beam Monitors Based on Single Crystal Diamond Detectors and InGaAs/InAlAs Quantum-Well Devices
No full textSimultaneous photon-beam position and intensity monitoring is becoming of increasing importance for new-generation synchrotron-radiation (SR) sources and free-electron lasers (FEL). Thus, novel concepts of beam diagnostics are required in order to keep such beams under control.
From this perspective diamond is a promising material for the production of semitransparent in situ photon-beam monitors which can withstand the high dose rates occurring in such radiation facilities. Here, we report on the development of freestanding, single-crystal chemical-vapor-deposited diamond detectors with segmented electrodes.
Due to their direct, low-energy band gap, InGaAs quantum-well (QW) devices operated at room temperature may be used as fast detectors for photons ranging from visible to X-ray. These features are valuable in low-energy and time-resolved FEL applications. In particular, a novel segmented InGaAs/InAlAs device has been developed and will be discussed.
Dedicated measurements carried out on both these devices at the Elettra Synchrotron show their capability to monitor the position and the intensity of the photon beam with bunch-by-bunch temporal performances. Furthermore, preliminary tests have been performed on diamond detectors at the Fermi FEL, extracting quantitative intensity and position information for 100-fs-wide FEL pulses with a photon energy of 28.8 eV
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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