Formation of Co/Ge intermixing layers after Co deposition on Ge(111)2 × 1 surfaces

The formation of a novel surface reconstruction upon Co deposition on freshly cleaved Ge(111)2 × 1 surfaces is studied by means of scanning tunneling microscopy (STM) at 4.5 K. Previously we demonstrated that at this low substrate temperature the deposited Co atoms remain immobile after they become embedded underneath the Ge(111)2 × 1 surface. We now demonstrate that at higher substrate temperatures the embedded Co atoms are able to diffuse below the surface in a direction parallel to the upper π-bonded chain rows. This one-dimensional temperature-induced mobility results in subsurface accumulation of Co atoms at atomic steps, at domain boundaries and on atomically flat Ge terraces at, e.g., vacancies or adatoms, where reconstructed Co/Ge intermixing layers are formed. Voltage dependent STM images reveal that the Co related surface reconstruction locally exhibits an ordered atomic structure with the same inter-atomic distance as that of the initial 2 × 1 reconstructed pure Ge(111) surface. On the other hand, the presence of the Co results in a doubling of the periodicity along the [ 2 1 ̄ 1 ̄ ] ?> direction in the STM images, which can be related to the modified electronic properties of the π-bonded chains.