Core-level shifts of the Ge(100)-(2×1) surface and their origins

The surface shifts of the Ge 3d core level from the clean Ge(100)-(2×{}1) surface have been investigated using Si epitaxial layers grown with Sb as a surfactant. We are able to obtain a line shape close to that of the nautral line shape of the Ge 3d and, in turn, we have resolved two surface-related components in the core-level spectrum of the clean surface, in contrast to previously observed single component. The origin of these shifts is addressed: one comes from the top layer and the other from the layer below the surface. Comparison with the Si(100)-(2×{}1) surface is also presented.