Defect Structures in Neutron Irradiated 6H-SiC Studied by X-Ray Diffraction Line Profile Analysis
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Defect Structures in Neutron Irradiated 6H-SiC Studied by X-Ray Diffraction Line Profile Analysis C. Seitz1, A. Magerl1, R. Hock1, H. Heissenstein2, R. Helbig2 1
Lehrstuhl für Kristallographie und Strukturphysik, Universität Erlangen-Nürnberg, Bismarckstr. 10, D-91054 Erlangen, Germany 2 Lehrstuhl für Angewandte Physik, Universität Erlangen-Nürnberg, Staudtstr. 7/A3, D-91058 Erlangen, Germany ABSTRACT We have investigated by x-ray diffraction defect structures in 6H-SiC after neutron irradiation with different fluences and followed by different annealing procedures. An interpretation along a model of Klimanek [1, 4-6] shows, that higher fluences lead to a stronger than linear reduction of the correlation length, whereas higher annealing temperatures correlate with a better recovery of the correlation length. In addition defects of 1st kind created by irradiation are reduced by annealing. We find that annealing changes the character of the defects and it accentuates a defect structure already present in the original samples. THEORY Crystal defects can be divided into two kinds: 1 kind: defects disturbing dominantly the short range order but not the long range order, such as point defects (e.g. self interstitials or dopands). 2nd kind: defects reducing dominantly the structural correlation length such as dislocations or stacking faults. As defects of 1st kind have no effect on the crystal periodicity they do not lead to a change of the line profile of Bragg reflections. If the point defects are not correlated as can be expected from an irradiation experiment there is no constructive interference of the scattered x-ray radiation. The Fourier transformation of a single point defect (nearly δ-shaped in real space) corresponds to a constant intensity (in reciprocal space). Thus defects of 1st kind lead to a weak, but homogeneous intensity between Bragg reflections due to diffuse scattering. Defects of 2nd kind involve a coherent dislocation pattern in the lattice and therefore they lead to a reduction of the structural correlation length. An infinitely large crystal gives δ-shaped Bragg reflections. Defects of 2nd kind will broaden this line shape. Klimanek [1, 4-6] and Ungár [2] have developed models to extract quantitative information on defect densities from such line broadening. We used the description by Klimanek [4-6], which assumes that only dislocations are present. Under this assumption the evaluation in terms of dislocation densities is quantitative. If, however, other defects like small angle grain boundaries were present, the absolute value in terms of dislocation density must be considered with caution. Nevertheless, the model provides a measure of the structural correlation length relating to defects of the 2nd kind. Furthermore, the character of the dislocation is being considered by the anisotropy of the apparent correlation lengths along crystal axis. The intensity distribution of an x-ray peak for a large crystal containing defects of 2nd kind can be described by the expression [3, 4] st
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