Cadmium Zinc Telluride (CZT) has been one of the most promising semiconductor materials for many years. Due to its high atomic number, suitable band-gap energy and ability to function at room temperature, CdZnTe has become the material of choice to be used as a room temperature radiation detector for many applications in the fields of medical imaging, process monitoring and national security, where demands and specifications set by those applications require that these detectors can operate well at the extreme conditions while maintaining good resolution, high detection efficiency, good reliability and high throughput. In most applications, detectors are exposed to high flux of X-ray radiation. One of the most common issues is the degradation of these detectors due to the presence of extended and point defects, which can act as traps for the charge carriers. This charge trapping causes the build-up of space charge and disturbing the electric field, resulting incomplete charge collection and signal formation of the detectors. This thesis investigates the associated failure modes by identifying the types of defects that exist in the CZT crystal and studies their roles in the performance of X-ray radiation detectors using in-house diagnostic tools. The results from different screening methods are compared and studied in order to find meaningful relationships and correlations that will help researchers to better understand the underlying physics and provide information and means for corrections and improvements of the crystal quality. / Graduate

This thesis reports on experimental research on structural defects and magnetic species on the nanoscale. The latter project involved considerable development work on the production of a spin-polarised mono-energetic positron beam. The construction of the system is described through various trial steps with emphasis on the methods of maximum practical polarization of the positron beam and of electrons in the sample with the smallest possible loss of beam intensity. A new sodium-22 source capsule was purchased, having beryllium foil backing to minimise the depolarisation effects of backscattering, and the source-moderator spacing was increased. Different types of sample were tested, varying in atomic structure, purity, magnetic susceptibility, electronic structure, and electric conductivity - including iron of different purity and structure, mu metal and solid oxygen. After these tests measurements were taken on single crystal iron, and the results suggest that the positron response to magnetic structures is very small, and that prospects for depth profiling of dilute magnetic systems are not favourable at this time. A large number of other investigations have been performed on non-magnetic defect structures in various materials. Variable Energy Positron Annihilation Spectroscopy – here involving beam-based Doppler broadening – was applied to novel materials of relevance to photonic or electronic structures on the nanoscale. These included thin films of technological interest such as AlGaN and Ar plasma-treated TiO2: silicon and silicon-on-insulator samples implanted with He and Si ions to engineer vacancies: Si-rich SiO2 and SiN to form nanocrystals for photonic applications in which new findings on the evolution of the nanocrystals, and the role of the nanocrystal-oxide interface in optical emission, could be very useful in the technological development of such systems: and a study of the structural phase and nano-pore properties of water ice films grown from vapour on a cold copper surface. The variety of these experimental studies serves to underline the wide applicability of positron beam spectroscopy in research on defect and nanostructure structures. A list of papers published to date resulting from this work is given at the end of the thesis; a number of others are planned.

Positron studies of defects in thin films and semiconductors are reviewed. The results obtained from experimental studies of Doppler broadening of annihilation radiation (DBAR) from variable energies are presented. Normalisation methods for the DBAR parameters S and W have been developed, allowing for direct comparisons of the results for different samples taken over long periods of time. The evaluation of the P:V parameter, the peak-to-valley ratio in a full annihilation spectrum, has been improved via a correction method that produced a fourfold increase in sensitivity to o-Ps annihilation. The spectrum ratio curve technique was improved and developed to investigate the chemical composition of the environment surrounding a positron-trapping defect. By fitting to multiple-element and/or defect-type responses the percentage of that particular element or defect contained within the sample could be found. Ratio curves were found to rely on the positron affinity to different vacancy types. Beam-based Doppler broadening spectroscopy, variable-energy positron annihilation spectroscopy (VEPAS), was used as a probe of oxide film and film/substrate interface characteristics. Different film growth methods were found to play a significant role in defining the features of films and their interfaces. Vacancies have a profound effect on the properties of semiconductor-type devices. A range of different structures and the effects of implantation have been investigated. VEPAS has been found to be useful in studying the more exotic types of materials, silica aerogel and niobium.

Digitized by Kansas Correctional Industries

Various methods of estimating the fault probabilities based on defect data of random defects seen in integrated circuit manufacturing are examined. Estimates of fault probabilities based on defect data are less costly than those based on critical area analysis and are potentially more reliable because they are based on actual manufacturing data. Due to limited sample size, means of estimating the confidence interval associated with these estimates are also examined. Because the mathematical expressions associated with defect data-based estimates of the fault probabilities are not amenable to analytical means of obtaining confidence intervals, bootstrapping was employed. The results show that one method of estimating the fault probabilities based on defect data proposed previously is not applicable when using typical in-line data. Furthermore, the results indicate that under typical fab conditions, the assumption of a Poisson random defect distribution gives accurate fault probabilities. The yields as predicted by the fault probabilities estimated from the limited yield concept and kill ratio and those estimated from critical area simulation are shown to be comparable to actual yields observed in the fab. It is also shown that with in-line data, the FP estimated for a given inspection step is a weighted average of the fault probabilities of the defect mechanisms operating at that inspection step. Four bootstrapped based methods of confidence interval estimation for fault probabilities of random defects are examined. The study is based on computer simulation of randomly distributed defects with pre-assigned fault probabilities on dice and the resulting count of different categories of die. The results show that all four methods perform well when the number of fatal defects is reasonably high but deteriorate in performance as the number of fatal defects decrease. The results also show that the BCA (bias-corrected and accelerated) method is more likely to succeed with a smaller number of fatal defects. This success is attributed to its ability to account for change of the standard deviation of the sampling distribution of the FP estimates with the PP of the population, and to account for median bias in the sampling distribution. / Graduation date: 2004

Semi-insulating, powder samples of Cadmium Telluride (CdTe) have been studied using ������In Time Differential Perturbed Angular Correlation (PAC) Spectroscopy. The samples have been lightly doped (~10���� cm�����) with ������In atoms, which occupy well-defined metal (Cd) lattice sites and act as probes of the local environment. These substitutional donors form a single defect complex in CdTe. This complex has been identified and characterized as a function of temperature. Those indium probes that are not complexed occupy metal lattice sites with no defect in the local vicinity. Samples containing metal vacancy concentrations as large as 500 ppm have been prepared by a high temperature anneal and quench. The defect complex involves the trapping of a cadmium metal vacancy bound to the indium probe. The electric field gradient (EFG) experienced by probe atoms has a coupling constant of V[subscript Q]=61.5(5) MHz and is not axially symmetric, with the asymmetry parameter given by ��=0.16(4). It is believed that this asymmetry results from a relaxation of the chalcogen (Te) atoms adjacent to the metal vacancy, with the tellurium atom shared by the probe atom and the vacancy providing the dominant contribution. The fraction of complexed probe atoms increases as the sample temperature is decreased, and is still increasing at room temperature. Complexed fractions are reproducible on cycling within the temperature range 40 to 200��C. The binding energy of the complex has been measured to be 0.15(2) eV and is independent of metal vacancy concentration, which varies and is dependent on the details of the quench. In rapidly cooled samples, a non-equilibrium number of these defect complexes is observed. This state equilibrates with a time constant of 45(5) hours at 15��C, implying that at least one of the two constituents involved in the complex has a significant diffusion rate at this temperature. Under the assumption that vacancy diffusion mechanisms dominate at this temperature, it is found that the cadmium vacancies overcome an energy barrier of 0.9(1) eV with a jump time of 20(2) minutes in CdTe at 15��C. / Graduation date: 2002

The self-interstitial defect is commonly regarded as important in regulating diffusion in silicon. A review of the literature reveals that the scientific bases for invoking the self-interstitial defect are weak, while an alternate defect, the vacancy cluster, has been largely ignored. One argument which has been used to establish dominance of the self-interstitial defect over vacancies is based upon attempts to model gold diffusion. Possible behavior of vacancies are considered, and the past analysis is found to be inconclusive. Another argument which has been presented as evidence for presence of silicon self-interstitials is based on observations of type "A" swirl defects. These defects are amenable to interpretation in terms of solidification theory, without need to invoke the interstitial point defect. Two experiments were designed to demonstrate influence of heat treatments upon gold diffusion in electronic grade silicon crystal when the heat treatment was performed prior to gold deposition. Results are interpreted in terms of Ostwald ripening of vacancy clusters retained in the crystal from high temperature processing. / Graduation date: 1996

No description available.

With today's technology, a variety of styles and a variety of different LED material was produced. The development of LED in the same time, the vendor industry in order to defend their own interests and under the huge business opportunities, and each side of the application of the style-related intellectual property rights in the LED,although the wise limits of property rights, but also contributed to a more diverse The LED manufacturing methods have been developed. Our experiment is the YAG:Ce3+ doped other compounds (for example: copper,aluminum, ...) in a small test tube and in within the near vacuum of a state to do the open end of the seal, and then make a small test tube temperature heating, hopes the doping elements in the impact of high temperatures contributed to YAG:Ce3+ to adjust the lasing wavelength, and in vitro experiments also found with the increase of heating temperature photoluminescence intensity for a gradual downward trend. The YAG:Ce3+ yellow phosphor surface facing the shortcomings of a defect, so we research for YAG:Ce3+ phosphors adsorbed on the surface of anti-reflective film to be improved and modified YAG:Ce3+ has a surface defect observation showed that after modification surface defects of the YAG:Ce3+ emission intensity on increase in the phenomenon. We will also anti-reflective film adsorbed on the YAG:Ce3+ compounds used in fluorescent plastic surface on top, and with the original YAG:Ce3+ phosphors are made of rubber making to compare, comparison of the fluorescent plastic found on YAG:Ce3+ surface adsorption anti-reflective film for luminescence efficiency and the absorption rate for the blue light has a better performance.

In the studies described in this dissertation, we investigated the roles of p53 in normal development, teratogen-induced apoptosis, and birth defects. In the first study, the activation of p53 and its target genes, p21, NOXA, and PUMA, were examined during neural tube closure in mouse embryos exposed to hyperthermia (HS) or 4- peroxycyclophosphamide (4CP), teratogens known to induce neural tube defects (NTDs). In the second study, using p53-deficient mice, we examined the expression of mRNAs and microRNAs (miRNAs) during neural tube closure. In the third study, the incidence of NTDs was investigated in p53- and p21-deficient mouse embryos exposed to HS. Finally, we examined the induction of apoptosis in p53-deficient mouse embryos exposed to HS. HS and 4CP induced the activation of p53 by phosphorylation and accumulation of the protein, leading to an increase in p21 proteins and mRNAs. Although HS and 4CP also induced the expression of Noxa and Puma mRNAs, no significant increases in NOXA and PUMA proteins were observed, suggesting a possible role of transcriptionindependent apoptosis. In the second study, we showed that the expression of 388 genes and 5 miRNAs were significantly altered in p53 -/- compared to p53 +/+ embryos. Finally, we showed that 10% of p53 -/- pups exhibit exencephaly, spina bifida, and/or preaxial polydactyly, whereas no malformations were observed among p21 -/- offspring in the absence of HS. HS resulted in an increased incidence of exencephaly in both p53 and p21 null mice indicating that these two proteins act as teratogen suppressors. Our preliminary data additionally showed that a decreased level of apoptosis was observed in HS-treated embryos lacking a p53 allele, suggesting that too little apoptosis may be causally linked to NTDs observed in embryos exposed to HS. Taken together, these studies suggest that precise control of apoptosis and cell cycle arrest pathways are critical for neural tube development and the prevention of teratogen-induced NTDs.