STM investigation of the boundary structure of the Na/Ge(111) - 3 x 1 surface

With the use of the scanning tunneling microscope (STM) technique we have investigated the atomic structure of the domain boundary of the Na-driven 3 x 1 reconstruction of the Ge(111) surface. At both the c(2 x 8)/3 x 1-Na and the 3 x 1-Na/3 x 1-Na boundaries, 3 x 1-Na domains are found to be terminated with double rows in the filled-state STM images. This is in contrast to the termination of the 3 x 1-Na islands on the Si(111) surface, which has a single-row feature. The observed double-row feature at the boundary is interpreted in terms of an atomic structure without Ge=Ge double bonds. We argue that the occurrence of this boundary structure supports the structural model proposed to explain the STM images of the Na-induced 3 x 1 reconstruction of the Ge(111) surface, which differs from that of the Si(111) surface.