This is in contradiction to the EDX measurements, but EDX is much less surface sensitive than XPS. The unexpected stoichiometry is most probably due to the presence of oxidized species that have been created during the polishing procedure. Indeed, O1s emissions are observed all along the TCS (see Figure ) as well as additional Ga-, In-, Ge-, P-based states at higher BE (see Figure ). For the Ge substrate, the Ge 0 state doublet is measured at 29.5 eV and 30.1 eV; the state at 3.2 eV higher BE corresponds to Ge IV (GeO2), and the one at 0.9 eV higher BE corresponds to an intermediate GeOx state (see Figure ). We quantified that almost half of the p-Ge(100) emission is related to oxidic species that presumably have formed during the polishing procedure. For instance, at position 1, the composition of the Ge states was the following: 48 %at. for Ge 0 , 25%at. for GeOx, and 27%at. for GeO2 (see Figure for the detailed fitting procedure of Ge3d). Similar findings were obtained for the GaInP layer. Two states were identified for each of the Ga2p, In3d, and P2p core levels: at 1118.5 eV and 1119.8 eV, at 445.2 and 445.8 eV, as well as at 129.8 eV and 134.4 eV, respectively. The absolute BE values vary from previous reports but the BE difference of 4.5 eV between the two P2p chemical states is the same and it is characteristic of the GaInP and the formation of InPO4