STM STUDY OF THE CL INDUCED HIGH-TEMPERATURE SI(111) (7X7)[--](1X1) PHASE-TRANSITIONS

The high temperature Si(111) (7 x 7) <-- --> (1 x 1) phase transitions , induced by adsorption and desorption of chlorine at temperatures bet ween 950 and 1030 K, have been followed by scanning tunneling microsco py. Both processes proceed in a nucleation and growth scheme. The diss olution of the (7 x 7) structure during Cl2 adsorption starts at the l ower terrace side of step edges and at domain boundaries. The resultin g (1 x 1) surface is identical to the bulk-like structure with the sur face dangling bonds terminated by adsorbed chlorine in a monochloride configuration and, possibly, a low coverage of adsorbed mobile SiCl(x) species. The regrowth of the (7 x 7) structure during chlorine remova l from the surface is initiated by nucleation and subsequent growth of triangular (7 x 7) domains at upper terrace step edges. This is the t ime reversed process compared to the (7 x 7) dissolution and similar t o the high temperature phase transition at 1100 K on the clean surface . Only on very extended terraces (7 x 7) domains are formed also by ho mogeneous nucleation amid (1 x 1) areas. The size distribution of the (7 X 7) domains in that case reflects random nucleation and uniform gr owth of these domains.