The preparation conditions (temperature and partial pressure) described above correspond to a H chemical potential of about -0.6 eV. For this chemical potential, the 2×2-2D-2H surface is by far the most dominant structure in the phase diagram, see. Fig. . Figure shows an XPS survey scan (top) as well as the Al 2p, In 3d 5/2 , and P 2p (bottom, from left to right) core-level photoemission lines of the 32 nm-thick AlInP(001) film. To increase the surface sensitivity of the measurement, the photoelectron take-off angle (with respect to the surface plane) was varied from 90 • to 30 • against normal emission (Fig. ). The survey scan shows no oxygen or carbon on the sample. The fits of the XPS data show that one component is sufficient to fit the Al 2p 1/2 , Al 2p 3/2 , and In 3d 5/2 core level peaks. The binding energy of metallic Al and In is at lower energy, at 72.75 eV and at 443.75 eV, respectively , and not seen in the measured data. For the P 2p core level, the presence of an extra component is obvious. The data are fitted with two spin-orbit pairs, with the same FWHM and peak ratio of 2:1 for each pair. The second, lower intensity component (red line) is shifted toward higher binding energy, and its binding energy can be correlated with P dimers on the surface . The intensity ratio of the peak related to the P dimers is increasing at the more surface sensitive measurement (P 2p core level, middle), which confirms a P-rich surface. Thus, the XPS measurements exclude cation-rich models and suggest P dimer structures such as the 2×2-2D-2H or the β2-2×4 surface.