Substrate-structure dependence of Ag electromigration on Au-precovered Si(111) surfaces

Electromigration of Ag on Au-precovered Si(111) surfaces was investigated b y in-situ ultrahigh vacuum scanning electron microscopy and mu-probe reflec tion-high-energy electron diffraction (RHEED). Migration behaviors of a Ag- film patch strongly depended on Au coverage theta(Au) and corresponding sur face structures. When theta(Au) < 0.7 monolayer (ML), the patch expanded pr eferentially towards the cathode to attain a maximum area in which the sum of Ag and Au coverages were always about 1 ML irrespective of theta(Au), re sulting in two-dimensional (2D) alloy phases (showing root 3 x root 3 RHEED patterns) with different Au/Ag concentration ratios. The largest expansion of the patch area was achieved on a (5 x 2 + alpha-root 3 x root 3)-Au mix ed phase structure (theta(Au) similar to 0.7 ML). However, when theta(Au) > 0.7 ML, the patch expansion was greatly reduced. Especially on the beta-ro ot 3 x root 3-Au surface (theta(Au) similar to 1.0 ML), the patch showed no directional expansion towards the cathode. But Ag atoms were observed to m igrate inside the patches on all substrates (including the beta-root 3 x ro ot 3-Au surface) to form 3D islands near terrace edges.