Real topography, atomic relaxations, and short-range chemical interactions in atomic force microscopy:: The case of the α-Sn/Si(111)-(√3x√3)R30° surface

We have investigated the phases of the Sn/Si(111)-(root 3 x root 3)R30 degrees surface below 1/3 ML coverage at room temperature by means of atomic force microscopy (AFM) and density functional theory based first-principles calculations. By tuning the Sn concentration at the surface we have been able to discriminate between Sn and Si adatoms, and to assure that the AFM topography for the different phases resembles the one reported using scanning tunneling microscopy. In the mosaic and the intermediate phases, a dependence of the topographic height of the Si adatoms on the number of surrounding Sn adatoms has been identified. In the pure phase, however, variations in the measured height difference between the Sn adatoms and the substitutional Si defects, which are intrinsic to the AFM observation, are reported. Reliable room-temperature force spectroscopic measurements using the atom-tracking technique and first-principles calculations provide an explanation for these striking induced height variations on the pure phase in terms of both the different strength of the short-range chemical interaction and tip-induced atomic relaxations. Our results suggest that the corrugation measured with true atomic resolution AFM operated at low interaction forces and close to the onset of significant short-range chemical interactions provides direct access to the real structure of heterogeneous semiconductor surfaces.