KINETICS OF THE SI(111)2 X 1-]5 X 5 AND 7 X 7 TRANSFORMATION STUDIED BY SCANNING TUNNELING MICROSCOPY

The scanning tunneling microscope is used to study the transformation of the Si(111)2 x 1 surface to the 7 x 7 structure, as a function of time and annealing temperature. Two intermediate states are found in the transition: a disordered adatom covered 1 x 1 surface, and a well ordered 5 x 5 structure. The surface transformation is observed to occur in two stages. First, nucleation at steps or 2 x 1 domain boundaries produces disordered arrangements of adatoms on a 1 x 1 surface, which then order, producing small domains of 5 x 5 and 7 x 7 structure. Second, growth from these small domains occurs, with the remaining 2 x 1 surface converting entirely into 5 x 5 structure. It is argued that the 5 x 5 structure is favored over 7 x 7 because it contains the same number of atoms as the 2 x 1 structure. Kinetic measurements of the transformation rate as a function of time and temperature have been performed, for temperatures in the range 280-425-degrees-C. An overall activation energy of 2.1 +/- 0.5 eV is obtained.