Convergent beam electron diffraction studies of strain in Si/SiGe superlattices

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Abstract
Convergent beam electron diffraction (CBED) and large angle CBED have been used to analyse strains in Si1-xGex/Si (001) superlattices with 0.17≤x≤0.33 and wavelengths in the range 200-650 Å. Studies were carried out using plan-view samples, thus largely avoiding thin foil strain relaxations. Rocking curves were obtained for low order and higher order Laue zone (HOLZ) reflections which showed superlattice peaks whose intensity envelope function was symmetric for diffracting planes in the [001] zone and asymmetric for planes inclined to [001]. A kinematical approach is used to give a simple explanation of these features and to investigate the relative contribution of layer strains and differences in layer structure factors and thicknesses. It is shown that strain can be measured either from the rocking curve asymmetry or by modelling the complete intensity envelope function. A quantitative analysis of the experimental results using both kinematical and dynamical simulations showed good agreement for the expected strains to within an accuracy of ±20%.