Vibration signatures of the structural phase transition of Sn/Ge(111) compared to Sn/Si(111)

A temperature driven structural phase transition between (root 3x root 3) and (3x3) has been reported for Sn adsorbed on the Ge(111) surface, which does not occur for the analogous Sn/Si(111) system. This phase transition has been correlated to a softening of a low-frequency Sn vibration mode, referred to as dynamical fluctuation mode. We have determined the eigenfrequencies of the vibration modes of Sn/Ge(111) and Sn/Si(111) with high accuracy by in situ surface Raman spectroscopy in the temperature range between 300 and 40 K, and calculated the surface reconstructions and vibration eigenmodes by density functional theory. Our calculated vibration eigenfrequencies are in excellent agreement with the observed Raman peak positions and the calculated displacement patterns allow the assignment of all observed vibration modes. Our results for both adsorbate systems Sn/Ge(111) and Sn/Si(111) show the preservation of the global surface atom configuration over the whole investigated temperature range. The emergence of a backfolded Rayleigh wave at approximate to 50 cm(-1) for Sn/Ge(111) below approximate to 200 K is a clear signature of its transition to a static (3x3) reconstruction. The gradual intensity increase of this mode upon further cooling suggests an order-disorder character of this transition.