I am presently trying to study the crystalline quality of an ultra highly doped (substitutional, 10 at. %) homoepitaxial thin film. I know from cross section STEM and EBSD analysis that the epilayer is in the same exact atomic registry across the substrate/epilayer interface. Due to the substitutional doping, there is an expanse to the perpendicular lattice constant so that the film is pseduomorphically strained.

I've performed coupled 2theta/omega scans and omega scans in an open detector set-up with a Ge( 2x220) multibounce monochromator, but only the coupled scan demonstrates the epilayer peak (shifted some 0.8 deg at lower angle, again, due to lattice parameter change). Why is it that I cannot see the epilayer using the omega scan? Is the coupled scan the best way to beginning quantifying the defect density in my epifilm?