Scanning tunneling microscopy study of the Eu-induced Ge(111)-(3x2)/(3x4) reconstruction

Eu-induced (3x2) reconstruction of the Ge(111) surface has been investigated by scanning tunneling microscopy (STM). The empty-state STM images show the chainlike atomic structure that is similar to those of the metal-induced Si(111)-(3x2) surfaces with an adsorbate coverage of 1/6 monolayer (ML). The filled-state STM images combined with the empty-state images at the low bias voltage reveal that the Ge arrangement of Eu/Ge(111)-(3x2) can be well interpreted in terms of the honeycomb chain-channel (HCC) model with the characteristic Ge = Ge double bond and slightly modified Ge honeycomb chains which are similar to those of the 1/6-ML HCC structure of Si(111)-(3x2). In addition, the Eu/Ge(111)-(3x2) surface is found to have a local x4 periodicity along Eu chains, which can be explained, based on the analysis of STM line profiles, with two nonequivalent adsorption sites occupied by the Eu atoms in the empty channels of the HCC structure. The structural modifications of the Ge honeycomb chains as well as the origin of the x2 and x4 chains of Eu atoms in the HCC structure on the Eu/Ge(111) surface are discussed.