Phase boundaries between the 7 × 7 and the 5 × 2 Au structures on a Si(111) surface, which were located along the two-fold direction of the 5 × 2 structure, were dynamically studied using high-temperature scanning tunneling microscopy (STM) during Au deposition at 500°C. Two types of stable phase boundaries were observed depending on the growth direction of the 5 × 2 domain. The stable phase boundaries had adatoms of the unfaulted half at the edge of the 7 × 7 side. The formation of an unstable phase boundary was quickly followed by the growth of new rows of the 5 × 2 structure next to the boundary which formed a stable phase boundary. These results suggest that the stacking fault of the 7 × 7 structure at a phase boundary is unstable.
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