Reinvestigation of the alkali-metal-induced Ge(1 1 1)3×1 reconstruction on the basis of boundary structure observations

We have investigated the surface atomic structure of boundary area of Li- and Na-induced Ge(1 1 1)3×1 reconstruction using scanning tunneling microscope. On Li/Ge(1 1 1)3×1, the 3×1 phase was found to be terminated with a single row in the filled-state image and with dimer-like features in the empty-state image. The images of both interior and boundary of the Li/Ge(1 1 1)3×1 surface are compatible with the honeycomb-chain-channel (HCC) model, which has substrate atoms with double bonds and is well established as the structure of AM/Si(1 1 1)3×1 surfaces. In contrast, termination with zigzag double rows at the domain boundary edges was observed in the filled-state images of the Na/Ge(1 1 1)3×1 phase, which is not reconcilable with the HCC structure. The filled-state STM feature of the boundary region of the Na/Ge(1 1 1)3×1 phase supports a structural model not having Ge=Ge double bonds, which was proposed to interpret its empty-state images. The trend of bondings between atoms in the surface layer of the AM-induced 3×1 reconstruction of Si and Ge is discussed in terms of electronegativity differences.