Role of 2x1 domain boundaries on the transition from 2x1 to c(2x8) at Ge(111) surfaces

We studied the role of 2X1 domain boundaries and steps on the irreversible transition from 2X1 to c(2 x8) reconstruction at the Ge(lll) surface. The as cleaved Ge(lll) surface reconstructs to a 2X1 structure with two types of domain boundaries and steps. The pi-bonded chains rotate by 120 degrees at the type-A boundary, whereas the pi-bonded chains are parallel but stagger by a half-unit cell at the type-B boundary. The bilayer-height step is also classified into a type-A step, to which rr-bonded chains make an angle of 120 degrees, and a type-B step, to which pi-bonded chains are parallel. In annealing, we found that the 2x1 reconstruction was first disordered, and then changed to a c(2X8) reconstruction. The disordered area included adatoms, arranged with c(2 x 4) and 2 X 2 short-range ordering, and holes. We observed that this disordering began preferentially from the typed boundary and type-ii, step of the 2 x 1 domain above 120 degrees C. In contrast, the type-B boundary and type-B; step were stable against disordering. In a detailed analysis of scanning tunneling microscopy images, we proposed an atomic structure model of a type-B boundary in which staggered rr-bonded chains were smoothly bridged with no additional dangling bonds. However, smooth bridging was impossible at the typed boundary, and the edge atoms at this boundary were unstable. The typed boundary and typed step, therefore, can release additional adatoms. This created holes at the boundary and changed the 2 x 1 area nearby into a disordered structure.