ATOM-SCALE MANIPULATION OF SI(111)7X7 BY STM - DEPENDENCE ON POLARITYAND AMPLITUDE OF VOLTAGE PULSES

We monitor the vertical motion of a scanning tunnelling microscope tip above a Si(111)7 X 7 surface during 30 ms voltage pulses. Sudden jump s of the tip towards or away from the sample are observed, due to atom ic-scale modifications occurring on either the surface or the tip. The temporal distribution of these modification events is strongly depend ent on pulse amplitude, more weakly dependent on pulse polarity. In ad dition, the modification is more probable if the constant current feed back is cut during the pulse. These observations are explained in term s of variations of the field between tip and sample, as well as field- assisted migration occurring near the tunnel junction.