A direct scanning tunnelling microscopic-imaging study was made of electrically active Si dopants, near to the (110) surface, at 4.2K. In the case of the filled-state images, patterns of rings were observed which were centered around the individual dopant atoms. It was suggested that these ring patterns were produced by individual impurities which, due to their charge, disturbed the local potential and caused (Friedel) oscillations in the charge density. In the case of the empty state images, no Friedel oscillations were observed.

M.C.M.M.Van der Wielen, A.J.A.Van Roij, H.Van Kempen: Physical Review Letters, 1996, 76[7], 1075-8