Thermal evolution of Co on the coexisting Ag/Ge(111)- $$ \sqrt 3 \times \sqrt 3 $$ and Ag/Ge(111)-4 × 4 phases

Thermal reaction of Co on Ag/Ge(111)-( {{sqrt 3 imes sqrt 3 } mathord{left/ {vphantom {{sqrt 3 imes sqrt 3 } {4 imes 4}}} ight. kern-0pt} {4 imes 4}} ) phases was studied by scanning tunneling microscopy, low energy electron diffraction, and Auger electron spectroscopy. In this paper, we have addressed the controversies over the chemical composition of Co islands by examining the thermal reaction of Co on the coexisting Ag/Ge(111)-( sqrt 3 imes sqrt 3 ) and Ag/Ge(111)-4 × 4 phases. One of the essential findings is that the reaction of Co with the surface proceeds to shift Ag atoms from the beneath of Co–Ge islands into the ( sqrt 3 imes sqrt 3 ) unit cell. As a result, a transformation from the 4 × 4 phase into the Ag-denser phase occurs on the surface. We also provide the evidence that ( sqrt {13} imes sqrt {13} ) periodicity is composed of Co–Ge compound and the 2 × 2 periodicity is composed of pure Co. Ag-( sqrt 3 imes sqrt 3 ) prevented the diffusion of Co into substrate at annealing temperature ranging from 320 to 730 K.