New growth modes and surface conductivity during epitaxy of metals on a Si(111) surface

We have constructed a high resolution UHV-SEM (scanning electron microscopy) apparatus which can operate at 1.7 x 10(-10) Torr. The observed resolution is similar to 5-7 Angstrom. By using this SEM, metal (Ag, Au) deposited Si(lll) surfaces were investigated in detail. For the delta-7 x 7 structure which was formed by similar to 0.7 ML of Ag deposition, it was observed that two-dimensional Ag clusters of 36 atoms ale arranged with the same spacing as that of the 7 x 7 structure. The domain contrasts for the 2D surface structures, such as root 3 x root 3-Ag and 5 x 2-Au etc., were also observed in detail. The root 3 x root 3-Ag domains nucleated at three parts: (1) along the steps, (2) along the 7 x 7 phase boundaries, and (3) in the middle of the 7 x 7 domains. Network patterns of 7 x 7 phase boundaries were clearly observed. The 5 x 2-Au structure showed elongated domains in the three [110] directions. When Ag and Au were deposited on a Si(111)-[7 x 7 + 1 x 1] surface made by radiation quenching, three contrasts, B (bright), D (dark) and M (middle), were observed for each case. We have carried out in situ measurements of surface conductance. When Ag was deposited on the 7 x 7 surface at room temperature, the change of conductance was normal. However, when Ag was deposited on the root 3 x root 3-Ag surface, the surface resistance showed an abrupt decrease in the initial stage (delta-7 x 7 structure), followed by a gradual decrease during further deposition. Au deposition onto the 5 x 2-Au surface, on the other hand, showed steep increase of resistance in the initial stage, followed by the steep decrease. This curious behavior of the resistance can be understood by considering the S-K growth mode, Fermi level pinning and band bending.