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The work carried out under this subcontract has provided advanced characterization supporting the development of these materials for photovoltaic applications. The studies, using time-resolved optical methods and positron annihila...
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The work carried out under this subcontract has provided advanced characterization supporting the development of these materials for photovoltaic applications. The studies, using time-resolved optical methods and positron annihilation spectroscopy, focus on characterization of carrier processes and defect states that are important to understand, and thereby control, to optimize photovoltaic efficiency. In this work, systematic studies as a function of key material parameters have been carried out to develop a more detailed understanding of conductivity processes. Femtosecond laser spectroscopic techniques are used to probe photoexcited carrier processes, including carrier trapping and recombination, as well as carrier thermalization, providing key parameters for conductivity models. An important part of the work involves the application of recently developed methods for generation and detection of femtosecond pulses in the far-infrared (or THz) spectral range. Time-resolved measurements of photoexcited carrier dynamics using far-infrared probe pulses provide a direct measure of the photoconductivity on fast time scales.
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SPRITE (Simulation of Particle Reemission, Implantation and Transmission--Extensible) is a Fortran computer program designed to model the transport of a stream of energetic particles as they scatter through a solid or multilayer. ...
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SPRITE (Simulation of Particle Reemission, Implantation and Transmission--Extensible) is a Fortran computer program designed to model the transport of a stream of energetic particles as they scatter through a solid or multilayer. SPRITE is intended to be a user-friendly and easily-extensible engine for performing basic transport calculations, and SPRITE incorporates such physics as is required to simulate the transport process, but specifically omits the details of the scattering mechanisms. PSPRITE is a Fortran computer program, built around the SPRITE transport engine and incorporating physical information necessary for modeling the implantation and thermalization of a stream of positrons or electrons with a solid. This document is intended to be the primary source of information and the only operations manual for SPRITE and the POS-SPRITE family of programs. This information includes the mode of operation of SPRITE, the format of the required and optional file types, as well as information about the output and results of the calculation. Information about installing and running these programs on a variety of computer systems will not be addressed in this report. Such information is rapidly expanding as these programs are adapted to run on different platforms, and thus the user can expect such information to be contained with the source code distribution set. Detailed information about the calculations of the structure of the POS-SPRITE programs are provided in this report, but detailed benchmark comparisons between the output of these calculations and experimental data are an active topic of research, and they refer the reader to the published literature for this information.
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This work investigated surface material modifications in high-purity beryllium foils resulting from 1-keV deuterium ion implantation into specimens for which the anneal temperatures and implantation temperatures were varied. Defec...
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This work investigated surface material modifications in high-purity beryllium foils resulting from 1-keV deuterium ion implantation into specimens for which the anneal temperatures and implantation temperatures were varied. Defects in unimplanted and in deuterium-implanted beryllium were characterized principally by positron-beam depth-profile analyses. Depth-profiles of the defect distributions in the specimens were made by stepping the energy of the positron beam from 0.055 keV to 40 keV, accompanied by measurements of the Doppler-broadened annihilation radiation line shape at each positron energy. These analyses identified a varying defect structure in beryllium, dependent on the previous anneal history of the material and on the temperature of the material during implantation with energetic deuterium ions. For specimens implanted at room temperature with 1-keV/D ions, the beam-induced defect structure had a profile that was peaked near the mean. range of the implanting deuterium and that extended beyond the implantation zone. Isochronal step-thermal anneal experiments revealed that deuterium was released from these defects at a temperature of about 400K, indicative of shallow traps, and that the defect structure annealed at temperatures above 623K. The beam-induced vacancy-defect complexes were estimated to be 1-nm voids for 1-keV/D implantation into Be at room temperature. For beryllium implanted at temperatures of 723K with 1-keV/D ions, these measurements revealed that the beam-induced defect structure was much broader and extended far beyond the implantation zone. Isochronal stepthermal anneal experiments for these specimens revealed a more stable defect structure, with the onset of defect annealing at 775K. There was no evidence of deuterium release at lower temperatures, indicating that for beryllium implanted at elevated temperatures, deuterium is retained in deep traps.
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The results of several positron annihilation (Doppler broadening) experiments have been analyzed using the BNL Monte Carlo implantation profiles and the program VEPFIT. The program VEPFIT has been modified so that scaled, paramete...
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The results of several positron annihilation (Doppler broadening) experiments have been analyzed using the BNL Monte Carlo implantation profiles and the program VEPFIT. The program VEPFIT has been modified so that scaled, parameterized multilayer profiles can also be used as the initial condition for the diffusion equation solution. The authors have looked at both elemental (e.g. amorphous silicon) and multilayer (e.g. Pd/Si) system. Strong correlations between the input implantation profile parameters and the fitted values obtained for the diffusion lengths and overlayer thicknesses for the multilayer systems have been found. The effect of uncertainties in the mean depth on the value of the diffusion length and hence the defect concentrations will be discussed. The impact of reimplanting backscattered positrons on both the implantation profiles and the fitted diffusion lengths will also be presented.
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An experimental investigation of positron channeling was made with a high-angular resolution apparatus, employing positrons of kinetic energy 1 MeV, derived from the Brookhaven National Laboratory Dynamitron. The pattern of transm...
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An experimental investigation of positron channeling was made with a high-angular resolution apparatus, employing positrons of kinetic energy 1 MeV, derived from the Brookhaven National Laboratory Dynamitron. The pattern of transmission through a Si (100) single crystal of thickness 0.245 (mu)m was investigated for a number of major planes. The authors have observed for the first time, in excellent detail, the fine structure of the channeling pattern expected to arise from the particle diffraction effects, theoretically explainable in terms of the quantum-mechanical many-beam calculations.
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A summary of recent positron annihilation experiments on molecular beam epitaxy (MBE) grown III-V layers is Presented. Variable energy positron beam measurements on Al(sub 0.32)Ga(sub 0.68)As undoped and Si doped have been complet...
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A summary of recent positron annihilation experiments on molecular beam epitaxy (MBE) grown III-V layers is Presented. Variable energy positron beam measurements on Al(sub 0.32)Ga(sub 0.68)As undoped and Si doped have been completed. Positron trapping at a open volume defect in Al(sub 0.32)Ga(sub 0.68):Si for temperatures from 300 to 25 K in the dark was observed. The positron trap was lost after 1.3 eV illumination at 25K. These results indicate an open volume defect is associated with the local structure of the deep donor state of the DX center. Stability of MBE GaAs to thermal annealing war, investigated over the temperature range of 230 to 700(degrees)C, Proximity wafer furnace anneals in flowing argon were used, Samples grown above 450(degrees)C were shown to be stable but for sample below this temperature an anneal induced vacancy related defect was produced for anneals between 400 and 500(degrees)C. The nature of the defect was shown to be different for material grown at 350 and 230(degrees)C. Activation energies of 2.5 eV to 2.3 eV were obtained from isochronal anneal experiments for samples grown at 350 and 230(degrees)C, respectively.
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Preliminary positron annihilation studies of ceramic and thin film Pb(Zr,Ti)O(sub 3) (PZT) materials have been completed. This paper examines effects of processing conditions on vacancy related defects. Positron lifetime measureme...
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Preliminary positron annihilation studies of ceramic and thin film Pb(Zr,Ti)O(sub 3) (PZT) materials have been completed. This paper examines effects of processing conditions on vacancy related defects. Positron lifetime measurements on bulk PLZT plates showed an increase in positron trapping to a defect state with increasing grain size consistent with trapping to lead vacancy related defects formed through lead oxide loss during processing. Variable energy positron beam measurements were completed on bulk PLZT plates, sol-gel PZT thin films and laser ablated PLZT thin films. Films processed in a reduced oxygen atmosphere were found to give a higher S-parameter, due to an increase in concentration of neutral or negatively charged vacancy type defects, compared with material processed in an oxidizing ambient.
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Zinc Selenide (ZnSe) crystals grown using the High Pressure Bridgman (HPB) technique were used to fabricate solid-state radiation detectors measuring 10 x 10 x 2 mm(sup 3). Sputtered platinum and gold contacts were applied to poli...
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Zinc Selenide (ZnSe) crystals grown using the High Pressure Bridgman (HPB) technique were used to fabricate solid-state radiation detectors measuring 10 x 10 x 2 mm(sup 3). Sputtered platinum and gold contacts were applied to polished detector blanks. Voltage versus current characteristics were determined for the devices at 25 C. Pulse height spectra were obtained using (sup 241)Am and (sup 109)Cd at both 25 C and 150 C with applied bias of 9,000 V/cm. Current versus temperature was measured over the temperature range of 30 C to 150 C. Performance was measured at energies of 22.1 and 59.5 keV over a temperature range of (minus)70 C to 170 C. Current versus dose rate was measured with 662 keV gamma irradiation. A value of the Mobility-Lifetime product ((mu)(tau)) for electrons was estimated. Time and temperature dependence of photo-peak position using Pulse Height Analysis (PHA) was studied.
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The annealing behaviour open-volume defects introduced in Si(l00) crystals during fluorine implantation and their role in the surface-oriented diffusion of F impurities were investigated by variable-energy positron beam depth prof...
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The annealing behaviour open-volume defects introduced in Si(l00) crystals during fluorine implantation and their role in the surface-oriented diffusion of F impurities were investigated by variable-energy positron beam depth profiling. The defects become mobile and undergo recovery at temperatures well below the onset of fluorine diffusion at 550(degree)C as seen by secondary ion mass spectroscopy (SIMS). The results suggests that after irradiation and annealing the F occupies substitutional sites to which positrons are insensitive. The anomalous F diffusion seen in SIMS has been explained through a two-step diffusion mechanism, in which the diffusion kinetics is determined by dissociation of the substitutional F into an interstitial F and a vacancy, followed by their rapid diffusion to the surface. (ERA citation 19:026785)
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