Depth-profiling XPS confirmed the tendency of In to segregate at the GaAs|GaInP interface and give rise to mixed phases. Strategies to investigate buried interfaces with PES can be divided into two main groups: bottom-up and top-down approaches. The first strategy mainly consists of so-called interface experiments. Those consist of alternating between the stepwise deposition of a thin layer of a contact material on top of a substrate material and the measurement of the resulting deposit and formed junctions by PES to provide spectroscopic information on the emerging contact.