Quantification of the composition of silicon germanium / silicon structures by high-angle annular dark field imaging

The intensity in high-angle annular dark field images of SiGe/Si hetero-structures is studied as a function of the specimen thickness and orientation, the inner detection angle and the energy loss. The approximation by Rutherford scattering where the intensity is proportional to the thickness and the square of the average atomic number is only applicable within a narrow thickness range (50 to 100 nm). At small thicknesses oxide layers on the crystal surface and at large thicknesses plasmon and Si K core loss scattering reduce the image contrast and hence the apparent Ge composition. Also, strain related contrast is visible, even in off-axis conditions and for large inner detection angles.