The first studied interface is the simplest one because it contains distinguishable elements: Ge on one side and Ga, In, and P on the other side. The core level spectra of the stepwise XPS line scans are depicted in Figure as intensity contour maps. On each position, from 1 to 20 XPS core level spectra of Ga2p, O1s, In3d, P2p, Ge3p, and Ge3d were acquired. A color code is used, from dark to light colors, to show the intensity of each spectrum. White corresponds to the most intense peak of the series.