LAYER-BY-LAYER ATOMIC MANIPULATION ON SI(111)-7X7 SURFACE - SURFACE-STRUCTURES AND STAIRCASE CONDUCTANCE VARIATION WITH ATOM REMOVAL

Si atom removal from a Si(111)-7 x 7 surface was successfully executed by allowing the biased STM tip to approach the surface to the point c ontact region followed by retraction to the tunnelling region. This te chnique was found to have the capabilities of revealing: (1) the atomi c structures on the newly exposed surfaces; and (2) tip-substrate curr ent variation in relation to the Si atom removal. When the adatoms wer e removed, the second layer pi the dimer-adatom-stacking fault (DAS) m odel structure appeared without reconstruction except for the relaxati on of the dimer rows. When a higher biased tip (>2 V) was used, the to p three layers were removed. Clear metastable structures such as c(2 x 4) and root 3 x root 3 were observed in the holes, and the shape of t hese structures was easily changed with the expansion of the size of t he hole. A staircase-shaped current drop was observed in the tip retra ction process when the adatoms were removed. This is closely related t o the decrease in the discrete number of Si atoms al the tip-substrate junction in an atom-by-atom manner, which shows a clear contrast to t he nanowire formation in the metal surface indentation. The last drop in the current to return to the tunnelling region might correspond to a junction with a single atom between the tip and the substrate, which corresponds to 4 x 10(5) Omega. (C) 1997 Elsevier Science B.V.