Surface phase transformations in the Ni/Si(111) system observed in real time using low-energy electron microscopy

Submonolayer Ni deposits quenched from above 840 °C produce an ‘‘impurity stabilized’’ 1×1 phase, which scanning tunnel microscope observations reveal to be a 1×1 lattice gas of ring clusters (RCs) together with Si adatoms in local 2×2 domains. We call this phase 1×1-RC. Real time low-energy electron microscopy observations show the following: terraces are covered with the 1×1-RC phase except for narrow strips of 7×7 along the top side of bilayer steps. The strip width decreases with increasing Ni coverage and increasing quench rate. Slow cooling results in clean 7×7 only. Depositing Ni at 700 °C causes the 1×1-RC phase to expand, consuming 7×7. At 720 °C, the 1×1-RC phase mixes with 7×7 to cover the terraces uniformly. Depositing at 500 °C instead causes formation of NiSi2 islands in a matrix of clean 7×7. These reactions are summarized in terms of a temperature versus coverage phase formation diagram.