The top-down approach, on the other hand, starts directly with a full device stack. It includes PES depth-profiling methods for which ion gun etching cycles and PES measurements are alternated. In these works Romanyuk et al. combined XPS with Ar gas cluster ion beam sputtering, they studied the GaP/Si(001) interface and the addition of As to suppress the formation of antiphase domains. The top-down approach also includes the more recent Tapered Cross-Section Photoelectron Spectroscopy (TCS-PES) method. A TCS is a very lowangle cross-section that is extended enough so that the lateral resolution of the PES setup is not limiting anymore. Until now proof of concepts have been reported on perovskite solar cells but, to the best of our knowledge, it is the first time that TCS-PES is performed on III-V semiconductor devices. In addition, until now, TCSs were prepared by manual polishing procedures, or using a drilling machine, leading to issues with reproducibility of the prepared cross-sections.