REAL-TIME OBSERVATION OF THE SI(111) - (7 X 7)-(1 X-1) PHASE-TRANSITION BY SCANNING TUNNELING MICROSCOPY

The initial stage of the (1 x 1)-(7 x 7) phase transition of the Si(111) surface was directly observed by scanning tunneling microscopy. On cooling down to a transition temperature T(c), the (7 x 7) domain nucleated from the step edges and expanded towards the inner region of the terraces continuously, and the steps became straight [112BARBAR] steps. The (7 x 7) domains took the shapes of triangles with acute top angles and fluctuated in size around T(c). The sizes of the observed domains were more than 6 units of (7 x 7) on the terraces. Based on free-energy considerations just at T(c) as well as the energy difference between faulted and unfaulted triangles of (7 x 7) units, we have found that the real shapes of the (7 x 7) domains are equilateral triangles which are surrounded by unfaulted triangles and there exists a critical size around 6 units of (7 x 7) below which the domains disappear rapidly.