Photoelectron diffraction study of the (3x3)-Sn/Ge(111) structure

The photoemission spectra of the Sn 4d electrons from the (3 x 3)-Sn/Ge(111 ) surface present two components which are attributed to inequivalent Sn at oms in T4 bonding sites. This structure has been explored by photoelectron diffraction experiments performed at the ALOISA beamline of the Elettra sto rage ring in Trieste (Italy). The modulation of the intensities of the two Sn components, caused by the backscattering of the underneath Ge atoms, has been measured as a function of the emission angle at fixed kinetic energie s and vice versa. The bond angle between Sn and its nearest neighbor atoms in the first Ge layer (Sn-Ge-1) has been measured by taking polar scans alo ng the main symmetry directions and it was found almost equivalent for the two components. The corresponding bond lengths are also quite similar, as o btained by studying the dependence on the photoelectron kinetic energy with the photon polarization and the collection direction parallel to the Sn-Ge -1 bond orientation (bond emission). A clear difference between the two bon ding sites is observed when studying the energy dependence at normal emissi on, where the sensitivity to the Sn height above the Ge atom in the second layer is enhanced. The (3 x 3)-Sn/Ge(111) is thus characterized by a struct ure where the Sn atom and its three nearest neighbor Ge atoms form a rather rigid unit that presents a strong vertical distortion with respect to the underneath atom of the second Ge layer.