ROLE OF 2X1 DOMAIN BOUNDARIES ON THE TRANSITION FROM 2X1 TO C(2X8) ATGE(111) SURFACES

We studied the role of 2X1 domain boundaries and steps on the irrevers ible transition from 2X1 to c(2 x8) reconstruction at the Ge(lll) surf ace. The as cleaved Ge(lll) surface reconstructs to a 2X1 structure wi th two types of domain boundaries and steps. The pi-bonded chains rota te 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 whic h 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 thi s 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 deta iled analysis of scanning tunneling microscopy images, we proposed an atomic structure model of a type-B boundary in which staggered rr-bond ed chains were smoothly bridged with no additional dangling bonds. How ever, smooth bridging was impossible at the typed boundary, and the ed ge 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 stru cture.