A Variable Temperature Scanning Tunnelling Microscopy Study of the Electronic Response of the Sn/Si(111) α Surface to Extrinsic Defects

The α-phase of Sn/Si(111) surface has been studied with variable temperature STM. When temperature is reduced this system undergoes a phase transition from â 3 à â 3 R30° to 2â 3 à â 3-like. The low temperature phase is characterised by a one-dimensional electronic perturbation of the â 3 surface accompanied by a modulation of the density of the substitutional Si defects having the same periodicity, namely a Defect Density Wave. At room temperature, STM shows that Si defects are slightly shifted off the T4 site. This local break of the C3Ï symmetry is also characteristic, at larger scale, of the low temperature phase. The two observations must be intimately related.