Papers by Author: Pavel Tománek

Abstract: This study describes the principles of synthesis and technological features of composition ceramics formation on the basis of silicon carbide and aluminum nitride by hot-pressing. The structural properties and composition of the ceramics were investigated by scanning electron microscope and the formation of the solid solution is confirmed. The elements distribution on the surface of failure pattern is shown. The results of the study are useful for optimization of manufacturing process of structural and functional high-density ceramics.

Abstract: Local defects, as micro-fractures, precipitates and other material inhomogeneities in solar cell structure, evidently modify electrical and photoelectrical behavior of the latter. To improve the efficiency and lifetime of existing solar cells, it is important to localize these defects which influence the p-n properties, and assign them corresponding electrical characteristics. Although the electric breakdown can be evident in current-voltage plot, the localization of local defects in the sample, that generate this breakdown, is not so easy task. It has to be done by microscopic investigations and measurement of light emission from defects under electrical bias conditions. Thus to contribute to this end, the structure of defects is microscopically investigated and consequently, the defects can be removed by focused ion beam milling. The experimental results obtained from samples before and after milling are also discussed.

Abstract: Solar cells, or photovoltaic cells, are used to convert sunlight into electrical power. The defects or imperfections in silicon solar cells lower the light-current conversion and consequently also an efficiency of the device. These defects in the semiconductor structure are normally detected by electric measurements. The thermal dependency of breakdown voltage is positive and the defects can be revealed by surface inhomogenity. To ensure a higher quality of the solar cells, advanced local quality assessment is provided and experimental results of solar cell defect measurement in microscale region are presented. Using Near-field optical beam induced current and voltage method, both current and voltage in defect area were detected and individual defects were localized with higher spatial resolution. This measurement also verifies that in reverse biased electroluminescence spots the quantum efficiency is lower and so these spots affect overall quality of the cell.

Abstract: The local spatial distribution of photoluminescence due to the creation of hot luminescence centers was measured in the optical near-field by Scanning near-field optical microscope at emission peaks of materials (λ =595nm), which is due to the luminescence of Mn2+ in ZnS. The excitation bandgap of ZnS forms exitons, and these excitons get the center of Mn2+ through nonradiation dominates, by means of transition of 4T1 – 6A1 luminescence. This spectrum is evidence that Mn2+ has been incorporated into the ZnS nanoparticles. In comparison with the bulk ZnS:Mn phosphors these nanoparticles have clearly higher luminescent efficiency with its luminescent decay time at least 4 orders of magnitude slower. It means that the oscillator intensity of luminescent centers in ZnS:Mn nanocrystal enhances at least 4 orders of magnitude than that in corresponding bulk ZnS:Mn.

Abstract: InAs/GaAs quantum dots (QDs) with ordered structure, due to their peculiar properties, open new way to design novel semiconductor devices such as single-electron transistors or highly parallel computing architectures. The lateral quantum dot alignment achieved during the selfassembly process is not well understood heretofore. The reason is, that quantum structures areusually small and studied at low temperatures. Conversely, the Scanning near-field optical microscopy (SNOM) allows study nanometer details in the non-offensive manner, in the room temperature with high spatial and temporal resolution. The first results of near-field optical study on aligned QDs are presented.