A variable temperature scanning tunnelling microscopy study of the electronic response of the Sn/Si(111) alpha surface to extrinsic defects

The alpha -phase of Sn/Si(111) surface has been studied with variable tempe rature STM. When temperature is reduced this system undergoes a phase trans ition from root3 x root3 R30 degrees to 2 root3 x root3-like. The low tempe rature phase is characterised by a one-dimensional electronic perturbation of the root3 surface accompanied by a modulation of the density of the subs titutional Si defects having the same periodicity, namely a Defect Density Wave. At room temperature, STM shows that Si defects are slightly shifted o ff the T-4 site. This local break of the C-3v symmetry is also characterist ic, at larger scale, of the low temperature phase. The two observations mus t be intimately related.