Convergent beam electron diffraction extinction distance measurements for quantitative analysis of Si1−xGex

A new method to determine the concentration of germanium in Si1−xGex single crystals is presented. It is based on extinction distance measurements by means of convergent beam electron diffraction (CBED). The two-beam condition CBED intensity oscillation (the so-called rocking curve) is measured for the 004 diffracted beam and compared with a numerical simulation. Using the two-beam dynamical diffraction approximation theory, this approach yields very precise values for both specimen thickness and effective extinction distance (Ultramicroscopy 87 (2001) 5). First a theoretical extinction distance ξg(x) for strain relaxed Si1−xGex is calculated assuming a solid solution and using tabulated atomic scattering factors of silicon and germanium atoms. It is found that for single crystals ξg(x) decreases from 156 nm in pure silicon to 90 nm in pure germanium. Measurements on calibrated strain relaxed SiGe layers with variable germanium concentrations show an excellent agreement between experimental and calculated extinction distances ξg(x). As a consequence the experimental extinction distance ξg(x) becomes an indirect measure of the germanium concentration with a 1–2 at% sensitivity. The method turns out to be insensitive to strain as experimental ξg(x) values obtained on strained SiGe layers fit the theoretical extinction distance curve calculated for strain relaxed SiGe.