PES is one of the few technics that provides spectroscopic (i.e., chemical) and electronic information at the same time. However, for measurements from the top of a layer stack, the inelastic mean free path (IMFP -λ) of photoelectrons limits the thickness that XPS can probe to maximum 10-20 nm using lab-based X-ray sources. For measurements from the side of a layer stack, the dimension of its normal cross-section is usually below 1 µm. It is thus one or two orders of magnitude smaller than the lateral resolution of a lab-based PES setup. Thus, buried interfaces cannot be directly assessed by PES, and appropriate strategies to analyze them are needed.