These intensity contour maps were used to identify the interface between Ge and GaInP. From the 1 st to the 12 th position, Ge has been detected while Ga, In and P are detected between the 6 th and the 20 th position. The difference in chemical composition between the two layers provides a clear demarcation: the 1 st to the 5 th positions correspond to pure Ge while the 13 th to the 20 th positions correspond to pure GaInP. Between the 6 th and the 12 th positions, contributions from both layers are detected. This can be due to fringes as well as intermixing of the layers but also to the depth of analysis of the XPS. Photoelectrons will be detected from up to a thickness equal to three times their inelastic mean free path (3λ). According to equation (1), the IMFPs of Ga2p and Ge3d core levels are estimated to respectively 1.0 and 2.1 nm: