With the use of photoluminescence (PL) spectroscopy one can able to get a great deal of information about electronic structure and optical processes in semiconductors by the aid of optical characterization. Among various compound semiconductors, Cadmium Sulfide (CdS) and Gallium Selenide (GaSe) are interesting materials for their PL emissions. Particularly, due to its strong anisotropy, investigation of GaSe necessitates new experimental approaches to the PL technique. We have designed, fabricated and used new experimental set-up for this purpose. In this thesis, we have investigated the PL spectra of both CdS and GaSe as a function of temperature. We observed interesting features in these samples. These features were analyzed experimentally and described by taking the band structure of the crystals into account. From the excitonic emissions, we determined the bandgap energy of both materials. We studied various peaks that appear in the PL spectra and their origin in the material. We have found that donor acceptor transitions are effective in CdS at low temperatures. A transition giving rise to a red emission was observed and attributed to a donor level which is likely to result form an S vacancy in CdS crystal. The PL peaks with energy close to the bandgap were observed in GaSe. These peak were attributed to the bound excitons connected either to the direct or indirect band edge of GaSe. The striking experimental finding in this work was the PL spectra of GaSe measured in different angular position with respect to the crystal axis. We observed that PL spectra exhibit substantial differences when the angular position of the laser beam and the detector is changed. The optical anisotropy which is responsible for these differences was measured experimentally and discussed by considering the selection rules of the band states of GaSe.

In this report a novel low temperature synthesis approach of CdS nanocrystals is described starting from well known precursors, Cd(SA) and TOP-S, in a ligand system of aliphatic long chain alcohols. A one-pot synthesis approach is applied using a laboratory microwave heating source. The resulting CdS nanocrystals exhibit an absorbance with a pronounced fine-structure, a photoluminescence with a very high ratio between the band gap peak and the defect peak and a fluorescence quantum yield of 33%. Different synthesis approaches have been investigated by changing heating rate, temperature, precursor concentration and chain length of the aliphatic alcohol ligand as well as chain length of the Cadmium precursor. It was found that small changes in the heating rate do not affect the reaction. Changing the reaction temperature between 200°C and 160°C has no visible effects on the quality of the resulting CdS nanocrystals. At 140°C the nanoparticles experience a significant drop in quality, probably because there is a major change in the growth mechanism of the nanocrystals at that low temperature. At 100°C and 120°C the creation of so-called CdS nanoclusters is observed, and a growth mechanism towards nanocrystals based on cluster aggregation is suggested. For the synthesis of high quality nanoparticles it was found that a ratio of 1:25 between precursor and aliphatic alcohol is preferable as well as a ratio of 1:1 between the two precursors. If the chain length of both the precursors and the alcohol is short, the reaction rate is enhanced. If the chain length is too short the nanocrystals grow very fast and the size distribution gets broad, the photoluminescence intensity decreases and the ratio between band gap luminescence and defect luminescence decreases. The best Cd-precursor was found to be Cd-Laurate and the most suitable ligand evaluated was Tetradecanol.

X-ray medical imaging provides invaluable medical information, while subjecting patients to hazardous ionizing radiation. The dosage that the patient is exposed to may be reduced, at the cost of image resolution. A technology that promises lower dosage for a given resolution is direct conversion digital imaging, typically based on amorphous Selenium semiconductor. Sufficient exposure should be used for the first exposure to avoid subsequent exposures; a challenge is then to reduce the necessary exposure for a suitable image. To quantify how little radiation the detector can reliably discriminate, one needs an analysis of the variance that 1/f and white noise contribute to the signal of such detectors. An important consideration is that the dark current, which varies with time, is subtracted from the photo-current, to reduce the spurious spatial variance in the image. In this thesis, the variance that 1/f noise contributes to integrating detectors is analysed, for a very general integrating detector. Experiments were performed to verify the theoretical results obtained for the 1/f noise variance contribution.

Ponencias y comunicaciones del Primer Encuentro Internacional de Informática y Ciencias Humanas realizado en Lima del 24 al 26 de Agosto de 1995

Ponencias y comunicaciones del Primer Encuentro Internacional de Informática y Ciencias Humanas realizado en Lima del 24 al 26 de Agosto de 1995

An increase in conversion efficiency of CIGS thin film solar cells has been reached at Ångström Solar Center (ÅSC) due to an introduction of a post-deposition-treatment (PDT) with potassium fluoride (KF) on the CIGS absorber layer. The PDT will however affect the growth of the cadmium sulfide (CdS) buffer layer normally deposited on the CIGS layer. The purpose of this study has therefore been to increase the conversion efficiency of the CIGS solar cells by optimizing the process parameters when growing CdS with a chemical bath deposition process (CBD) on CIGS with KF-PDT. The purpose has also been to understand how CdS grows on CIGS with KF-PDT. CdS has been deposited with CBD on both CIGS with KF-PDT and on soda lime glass samples by varying process parameters such as time, bath temperature and concentrations. The solar cells were characterized using current-voltage measurements as well as external quantum efficiency measurements. Further methods of analysis were profilometry, x-ray fluorescence spectroscopy and scanning electron microscopy. It was found that the solar cells with the best performance were obtained at a bath temperature of 70 °C, process time of 3 min after preheating the cadmium acetate and ammonia solutions for 6 min and using the concentrations of the baseline process for CBD of CdS at ÅSC. The best cell of this sample resulted in a conversion efficiency of 19.1 % without antireflective coating.

No description available.

<p> This report presents a study of the sputtering of vacuum deposited thin films of cadmium sulphide on a (111) face of single crystal silicon by Rutherford backscattering (RBS) technique. Cadmium was found to be preferentially sputtered when bombarded to high fluences of 80 kV Bi+ while no significant preferential sputtering was observed in the case of 40 kV Ar+ bombardment. </p> <p> The structural study by reflection high energy electron diffraction (RHEED) revealed that the films grew epitaxially in the wurtzite structure. The epitaxial relations are (00.1) Cds || (111) Si with [10.0] II [110] Si. </p> <p> Scanning electron microscope (SID4) microphotographs showed smooth surface features with a large grain size (surface grain size was ~ 83 nm) for a film of about 60 nm thickness. </p> <p> The basic structure did not change with highest fluences of Bi+ (Sxlo16 ions/cm2 ) and Ar+ (6.7xlo16 ions/cm2). He+ beam channeling was done for unbombarded and bombarded CdS films. It was found that the critical angle of channeling for cadmium increased for bombarded samples while for sulfur the statistics were too poor for any conclusion. </p> <p> Saturation fluences for bismuth and argon retention were observed and are compared with calculated values. </p> / Thesis / Master of Engineering (MEngr)

<p> As part of a CdS-cu2s thin film solar cell research project, a CdS evaporation system was designed and built using an Edwards 19E6 coating unit. With the overall aims of the project in mind, the apparatus was designed as part of a CdS-Cu2s dual, in situ, evaporation system. CdS films, ranging in thickness from lμ to 25μ, produced by this system, were characterized optically, electrically and crystallographically as functions of the various evaporation parameters. </p> / Thesis / Master of Engineering (MEngr)

Experiments were conducted to characterize and compare the localized corrosion susceptibility of the granular microstructure of aluminum (Al) alloy AA7050 in the peak aged T6 condition cast by the novel controlled diffusion solidification (CDS) process against the conventional wrought plate counterpart. CDS is a casting technique that involves mixing two precursor melts at specific temperatures and compositions before solidification. This process avoids the problem of hot tearing by causing copious nucleation of the solid phase within the melt before solidification, reducing the amount of solute segregation and, thus dendritic growth creating a consistently equiaxed microstructure. The effect of the CDS processing route on its relative localized corrosion susceptibility was elucidated by making links between the microstructure heterogeneities and the localized corrosion susceptibility as evaluated in aqueous saline solutions. Microstructures were characterized and compared with the use of the following techniques: 1. Scanning electron microscopy (SEM) to characterize grain size, shape and distribution. 2. Electron backscattered diffraction (EBSD) in SEM mode to characterize grain misorientation and the associated distribution. 3. Auger electron spectroscopy (AES) to characterize the composition of the grain boundary region including the precipitate free zone (PFZ) and the grain boundary precipitates. 4. Atom probe tomography (APT) to define the size, distribution, and composition of the strengthening matrix precipitates along with the grain boundary region (PFZ and grain boundary precipitates). Electrochemical experiments were conducted to characterize and compare the localized corrosion susceptibility of the two materials (CDS and conventional wrought) exhibited in aqueous saline solutions. Specific techniques include the following: i. Potentiodynamic polarization measurements of mechanically-abraded surfaces to determine the corrosion potential (Ecorr) and breakdown potential (Eb). ii. Potentiostatic anodic polarization of mechanically-abraded surfaces to observe the mode and extent of localized corrosion. iii. Open-circuit potential (OCP) measurements of fracture surfaces to determine the OCP of a surface with a significantly higher grain boundary area fraction relative to the bulk material. iv. Cyclic acidified salt (sodium chloride (NaCl)) fog testing (ASTM-G85-Annex 2) to validate the relative localized corrosion susceptibility under more realistic atmospheric corrosion exposure conductions. The CDS casting technique resulted in an entirely equiaxed microstructure. The microstructure was isotropic with an average grain size of 25±1 µm and an aspect ratio of around 1. This grain structure was in stark contrast with the wrought material, which exhibited a granular structure elongated along the rolling direction. The wrought material had a cord length of 56±3.2 µm in the rolling direction, 51±3 µm in the traverse direction and 13.3±1.6 µm in the short traverse direction. The wrought material had an aspect ratio of around 4 in the longitudinal plane (LS), 2.6 in the short transverse plane (ST) and 1.2 in the rolling plane (LT). AES and APT revealed that the CDS material had a higher amount of copper (Cu) segregation into the grain boundary precipitates. Electrochemical testing showed that the wrought material had a Eb of −750 ± 3 mV while the CDS had a higher Eb of −697 ± 4 mV. The Cu segregation into the grain boundary precipitates yielded more electrochemically active grain boundaries, as revealed by the OCP measurements. Despite this fact, localized corrosion of the CDS material initiated as pitting and propagated as a mixed mode involving intergranular corrosion (IGC) and pitting. The localized corrosion mode exhibited by the wrought material was purely IGC: both in initiation and propagation. The difference in corrosion mode was found to be due to the differences in the size of the Fe-based IMPs and the distribution of the Cu secondary phase precipitates: The CDS had large Fe IMP trapped at the grain boundary triple points and clustering of Cu secondary phase precipitates. Conversely, the wrought material had finely dispersed Fe IMP of significantly smaller size than those found in the CDS, and its Cu secondary phase precipitates are evenly distributed along the grain boundaries. These differences in precipitate distribution enhanced susceptibility for pitting in the CDS and reduced the driving force for IGC. The propagation of localized corrosion was markedly reduced in the CDS material: about half of that exhibited by the wrought material (under identical exposure conditions). Cyclic acidified salt fog testing revealed industry acceptable levels of localized corrosion susceptibility in-line with the benchmark alloys that are currently used in automotive applications. / Thesis / Master of Applied Science (MASc)