Kohei Okumura

Step structures and dislocations of SiC single crystals grown by modified Lely method

Step structures on the {0 0 0 1} facet of SiC single crystals grown by the modified Lely method were examined through an optical microscopy and an atomic force microscopy to discuss the dependence on the polytype and the polarity. The results were compared with those of CVD method. Step structures around micropipes were quite different for the different polarity of a growing surface. No micropipes were detected at the center of spirals with a step height of 1.5 nm for 6H-SiC. Quantitative morphological features could be corresponded to the variety of etch pits by chemical etching on the growing surface. The dislocations and their motions during the growth were also characterized through etching and polishing the grown crystal from the surface to the inside successively. The screw dislocations shifted outwards from the center of a giant spiral as the growth proceeded. In contrast, the edge dislocations glided in any directions. Discussion was carried out on these results.

Quantitative analysis of transient surface reactions on planar catalyst with time-resolved time-of-flight mass spectrometry.

A procedure for the quantitative analysis of transient surface catalytic reactions in millisecond time resolution has been studied constructing a specially designed apparatus employing (1) pulsed-gas valves for the injection of reactant molecules onto catalysts and (2) a time-of-flight mass spectrometer (TOF-MS) to detect every reaction product simultaneously. For a better understanding of the catalytic activity and selectivity for products quantitatively, a procedure for measuring an amount of reactant molecules injected onto catalyst surface and calibrating the intensity of mass signal were proposed and implemented. We tested the applicability of this procedure for the quantitative analysis of products of NO+H(2) reaction on Pt-Al(2)O(3) catalysts (a planar catalyst: Pt-Al(2)O(3)Si substrates inserted into a micro-tube-reactor with SiC balls). Although the surface area of the planar catalyst was very small, the mass signal intensities of the reaction products were found to be sufficient for the above procedure. We measured the fragmentation patterns and the inherent sensitivity factors in the TOF-MS using the mixture of the internal standard gas Ar and the N-containing gases. The relative sensitivity factors for NH(3), N(2), NO, and N(2)O and the relative intensities of fragment peaks to the molecular ion peak of H(2)O and N(2)O were estimated. The procedure constructed here has enabled us to analyze the transient consecutive secondary catalytic reactions as well as primary reactions based on the formation rate of product molecules per millisecond instead of the mass signal intensities of the reaction products.