[diss] Perustieteiden korkeakoulu / SCI

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Browsing [diss] Perustieteiden korkeakoulu / SCI by Degree programme/Major subject "Fysiikan instrumentointi ja puolijohdefysiikka"

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  • Positron lifetime spectroscopy : digital spectrometer and experiments in SiC
    (2006-06-29) Aavikko, Reino
     Doctoral dissertation (article-based)
    A digital positron lifetime spectrometer has been designed, set up and tested comprehensively. The system consists of a fast commercial digitizer connected to a computer, a simple coincidence circuit and software to extract the timing from the collected detector pulses. The system has the same time resolution as a conventional analog apparatus using the same detectors. The pulse processing part of the spectrometer is able to analyze and store in real time several thousands of events per second, which is an order of magnitude more than the count rates in typical positron lifetime experiments. The data acquisition can handle small pulses, down to a few tens of millivolts, and its time scale linearity and stability are very good. Positron spectroscopy has been used to study native vacancy defects in High Temperature Chemical Vapor Deposition grown semi-insulating silicon carbide. The material is shown to contain (i) vacancy clusters consisting of 4-5 missing atoms and (ii) Si vacancy related negatively charged defects. The growth of the clusters due to the annealing of the samples was observed. The total open volume bound to the clusters anticorrelates with the electrical resistivity both in as-grown and annealed material. It is concluded that Si vacancy complexes compensate electrically the as-grown material, but are suggested to migrate to increase the size of the clusters during annealing, leading to loss of resistivity.