Monte Carlo simulation of charge transport in amorphous selenium photoconductors

The electronic properties of amorphous materials are greatly affected by the density of localized states in the mobility gap of these materials. The exact shape of the density of states (DOS) distribution in amorphous selenium (a-Se) is still unresolved despite decades of research. One of the most commonly employed methods to investigate charge transport properties in high resistivity materials is time-of-flight (TOF) transient photoconductivity experiment. The TOF transient photoconductivity technique is used to measure the induced photocurrent in the external circuit when the sample is photoexcited. Information pertaining to carrier mobility and other carrier parameters are deduced from the shape of the photocurrent. The investigation of the charge transport phenomenon is well known to be a complicated task. Monte Carlo (MC) simulation method has become a standard method for carrier transport studies in amorphous materials. The purpose of this research work is to develop a Monte Carlo simulation model for charge transport in typical TOF transient photoconductivity experiment to investigate the DOS distribution in a-Se. The MC simulations were first performed for relatively simpler models for which theoretical and analytical solutions were available. The MC model developed here is based on simulating the drift of carriers resulting from photogeneration, subject to the influence of an applied electric field and multiple trapping events. The free drift time of photocarriers and their dwell time in the traps are stochastic in nature, in accordance with the probabilities of these events. Electron time-of-flight transient photocurrents were calculated in amorphous selenium as a function of the electric field. The distribution of localized states (DOS) in a-Se has been investigated by comparing the experimentally measured and calculated transient photocurrents. The analysis of multiple-trapping transport has been done by the discretization of a continuous DOS. The DOS distribution has been optimized to produce the best agreement between the calculated and measured transient photocurrents. The resulting DOS has distinct features: A first peak at ~0.30 eV below Ec with an amplitude ~1017 eV–1 cm–3, a second small peak (or shoulder) at 0.45–0.50 eV below Ec with an amplitude 1014–1015 eV–1 cm–3, and deep states with an integral concentration 1011–1014 cm–3 lying below 0.65 eV, whose exact distribution could not be resolved because of the limitations of the available experimental data. The density of states (DOS) distribution in the vicinity of the valence band mobility edge in vacuum coated a-Se films has been investigated by calculating the MC hole transient photocurrents at different temperatures, and also the dependence of the drift mobility on the temperature and field. The calculated TOF transient photocurrents were compared with experimental data published elsewhere. It is shown that, analogous to electron transport in a-Si:H, the DOS near Ev is a featureless, monotonically decreasing distribution in energy up to Ev + 0.4 eV, without the 0.28 eV peak near the valence band which was thought to control the hole drift mobility. Such a DOS was able to account for hole TOF data reported previously by several authors to date

Optical properties of amorphous selenium films

Recently there has been a substantial renewed interest in the electrical and optical properties of amorphous selenium (a-Se) films due to its use as an ultra-sensitive photoconductor in the newly developed flat panel x-ray image detector and high definition digital and video camera. This project has examined the optical properties of a range of a-Se films fabricated by conventional vacuum deposition technique. The films were deposited at two substrate temperatures called hot and cold deposition: (i) at 51C, which is above the glass transition temperature (Tg ≈ 40C), and (ii) at 3C, well below the glass transition temperature. The transmission spectrum T() of all the a-Se films were measured over a wide range of wavelengths from 500 nm to 2500 nm, and analyzed using the Swanepoel technique. The thickness, absorption coefficient (), refractive index n(), and optical bandgap, EgT and EgU, in different absorption regions of a-Se were all extracted using the Swanepoel technique. A systematic way to improve the accuracy of the calculations was also developed and the n vs.  data was fitted to the Sellmeier dispersion equation. The optical bandgap EgT was obtained from the h vs h plot of the absorption spectrum and the Urbach tail was obtained from a semi-logarithmic plot of  vs h. With all essential optical properties collected, the influence of thermal annealing, the deposition temperature, doping, thickness and aging time on the optical properties of a-Se films was studied. Amorphous selenium is essentially a glass, and all glasses exhibit some degree of structural relaxation effects during which the physical properties such as the refractive index and the bandgap change with time, called "aging". First of all, thermal annealing an a-Se film at T = 51C for an hour reduces the thickness by ~15% and increases the refractive index by ~0.5%. The optical bandgap does not change by more than 0.2%. While the decrease in the thickness is thought to be due to the rearrangement of the molecular clusters and the amorphous network, the increase in the refractive index is believed to be linked to the densification and the polarizability of the material. Although the influence of the thickness of the film, the temperature of the glass substrate, and the doping of 67ppm of chlorine on the optical properties of the a-Se film was found to be negligible, there are noticeable changes during aging. It was found that an aged a-Se film that was cold deposited has an increased refractive index that is believed to be caused by densification alone. In a period of 2 months, the thickness of a cold deposited a-Se:67ppm-Cl film shrinks by 3.5% and the refractive index increases by 3.7%. The optical bandgap of the film decreases by 0.6%. Similar aging trends in the thickness, the refractive index, and the optical bandgap were also observed in a cold deposited a-Se film but no significant changes, at least not more than 0.5% deviation, were noted in a period of about 1-3 weeks in any aging a-Se films that were hot deposited or annealed

Technology 2001: The Second National Technology Transfer Conference and Exposition, volume 1

Papers from the technical sessions of the Technology 2001 Conference and Exposition are presented. The technical sessions featured discussions of advanced manufacturing, artificial intelligence, biotechnology, computer graphics and simulation, communications, data and information management, electronics, electro-optics, environmental technology, life sciences, materials science, medical advances, robotics, software engineering, and test and measurement

Modeling and Performance Evaluation of Avalanche Detector Structures for Low Dose Medical X-ray Imaging

The flat-panel x-ray detectors based on large area integrated circuit called active matrix array ensure excellent image quality and provide wide dynamic range. As a result, active matrix flat panel imagers (AMFPIs) are now commercially available for chest radiography and mammography. However, at low level exposures, most of the AMFPIs are not quantum noise limited due to the electronic noise of the readout circuitry. Therefore, AMFPIs are not fully commercialized for x-ray fluoroscopy (used for interventional procedures and deployment of endovascular devices) which requires maintaining a very low x-ray exposure. An active research is underway to make AMFPIs quantum noise limited at fluoroscopic exposure level. This work investigates the feasibility of avalanche gain as a solution. At high electric field, avalanche multiplication of charge carrier improves the signal strength to overcome the effects of electronic noise in both direct and indirect conversion x-ray detectors. Indirect conversion detectors are suitable for the avalanche multiplication. It is because the gain fluctuation is minimum since the x-ray absorption and electric charge collection occur in two separate layers. However, in indirect conversion detectors, the image resolution in terms of modulation transfer function (MTF) deteriorates due to depth dependent x-ray absorption (Lubberts effect) and omni-directional propagation of light photons in the phosphor. In this research work, a cascaded linear-system model is proposed to calculate the image quality of CsI-based indirect conversion a-Se avalanche x-ray detectors in terms of spatial frequency dependent detective quantum efficiency (DQE). The depth dependent MTF and noise power spectrum (NPS) are modeled by incorporating the Lubberts effect. The theoretical model also considers MTF due to K-fluorescence reabsorption. The model is then compared with experimentally determined DQE(f) and shows a better fit than previously published models. On the other hand, direct conversion directors show a better performance in terms of image resolution as the x-ray photons are directly converted into electron and hole pairs. However, the direct conversion detectors are still vulnerable to the electronic noise at low exposures. Utilization of mesh electrode in order to separate the x-ray absorption and gain region has been proposed in the literature to reduce the avalanche gain fluctuation in direct conversion detectors. This work includes a cascaded linear-system model to calculate the DQE(f) of an a-Se based direct conversion avalanche x-ray detector. The proposed model evaluates charge collection efficiency using the Ramo-Shockley theorem and the actual weighting potential of an individual pixel. A 2-Dimensional simulation is performed to calculate the actual weighting potential in the presence of a mesh electrode. The optimal design parameters and operational condition for a-Se based direct conversion multilayer avalanche x-ray detectors are described in this work. In order to ease the fabrication process and eliminate the need of applying two different voltages to the aforementioned mesh electrode based direct conversion avalanche detector, a novel structure for direct conversion avalanche detector is proposed. The proposed structure contains a hole trapping layer instead of a mesh electrode to separate the absorption layer from the gain region. A numerical model is developed using Semiconductor Module of COMSOL Multiphysics to analyze the device performance. The electric field profile as a function of various device parameters is calculated. A detailed analysis on the transient behavior of the dark current in presence of blocking and trapping layers is performed. A modified cascaded linear-system model that considers the effect of reabsorption of K-fluorescent x-rays, carrier trapping in different layers and avalanche multiplication of charge carrier is used to calculate the DQE(f) and the MTF of the proposed structure. The DQE(f) of the proposed structure is then compared with published experimental results of a commercially available detector at low x-ray exposures (e.g., exposures used in tomosynthesis). The relative performance of these detector structures influences their clinical effectiveness. Therefore, a comparison of the performance of these detectors for different x-ray imaging modalities is also presented in this research work

Abstracts on Radio Direction Finding (1899 - 1995)

The files on this record represent the various databases that originally composed the CD-ROM issue of "Abstracts on Radio Direction Finding" database, which is now part of the Dudley Knox Library's Abstracts and Selected Full Text Documents on Radio Direction Finding (1899 - 1995) Collection. (See Calhoun record https://calhoun.nps.edu/handle/10945/57364 for further information on this collection and the bibliography). Due to issues of technological obsolescence preventing current and future audiences from accessing the bibliography, DKL exported and converted into the three files on this record the various databases contained in the CD-ROM. The contents of these files are: 1) RDFA_CompleteBibliography_xls.zip [RDFA_CompleteBibliography.xls: Metadata for the complete bibliography, in Excel 97-2003 Workbook format; RDFA_Glossary.xls: Glossary of terms, in Excel 97-2003 Workbookformat; RDFA_Biographies.xls: Biographies of leading figures, in Excel 97-2003 Workbook format]; 2) RDFA_CompleteBibliography_csv.zip [RDFA_CompleteBibliography.TXT: Metadata for the complete bibliography, in CSV format; RDFA_Glossary.TXT: Glossary of terms, in CSV format; RDFA_Biographies.TXT: Biographies of leading figures, in CSV format]; 3) RDFA_CompleteBibliography.pdf: A human readable display of the bibliographic data, as a means of double-checking any possible deviations due to conversion

Space transportation system and associated payloads: Glossary, acronyms, and abbreviations

A collection of some of the acronyms and abbreviations now in everyday use in the shuttle world is presented. It is a combination of lists that were prepared at Marshall Space Flight Center and Kennedy and Johnson Space Centers, places where intensive shuttle activities are being carried out. This list is intended as a guide or reference and should not be considered to have the status and sanction of a dictionary

Probing the Electronic Structure and Dynamics of Anions in the Gas Phase

The electronic structure and dynamics of a number of anions in the gas phase have been investigated through joint experimental and computational methods. Photoelectron imaging was used to obtain both frequency-, time- and angle-resolved photoelectron spectra. Quantum chemistry calculations modelled different experimental parameters and provided insight into the complex photoelectron signatures observed. A particular emphasis on advancing the understanding and interpretation of the photoelectron angular distributions in both the frequency- and time-domain was made. The need for caution and understanding of the considerations one needs to make, and the requirement to be flexible with the theoretical model used to interpret a specific problem are presented. This was emphasized throughout the results section, where the complexity of models required ranged from a simple Hückel model to a multi-configurational multi-reference model. An analysis of the sensitivity of the parameters used in the calculation of photoelectron angular distributions with the Dyson orbital approach was presented. The results were presented within three broad themes. Section A presented results in which the photoelectron angular distributions were modelled and interpreted to give novel insight into the character and structure of the molecular anions investigated. Analysis of the photoelectron angular distributions was presented both in the frequency- and time-domain. Section B presented studies in which the ‘bottom-up’ approach was used to elucidate information on the structure-function dynamics of small components of more complex systems. Finally, Section C presented results in which the prevalence and importance of non-valence states were probed

Interactions of spacecraft and other moving bodies with natural plasmas

Abstract compilation on spacecraft interactions with ionosphere, magnetosphere, and interplanetary plasm