Temperature- and exposure-dependent study of the Ge(001)c(8×2)-Au surface

Abstract

Using scanning tunneling microscopy (STM), Auger electron spectroscopy, and low-energy electron diffraction, we have determined the optimal gold exposure for the Ge(001) $c$(8 \ifmmode\times\else\texttimes\fi{} 2)-Au surface. We find deposition of submonolayer (ML) gold onto a Ge(001) surface held at temperatures between 570 and 870 K produces a $c$(8 \ifmmode\times\else\texttimes\fi{} 2) surface reconstruction. The relative extent of $c$(8 \ifmmode\times\else\texttimes\fi{} 2) domains increases with Au exposure, and at 0.75 \ifmmode\pm\else\textpm\fi{} 0.05 ML the surface is entirely covered by $c$(8 \ifmmode\times\else\texttimes\fi{} 2) chains. The 0.75-ML exposure is equivalent to six gold atoms per unit cell. Beyond 0.75 ML, exposure to additional Au leaves the $c$(8 \ifmmode\times\else\texttimes\fi{} 2) domains intact, and extra Au is accommodated at three-dimensional islands on the surface. STM images of the $c$(8 \ifmmode\times\else\texttimes\fi{} 2) phase are dominated by bright chains running along the Ge{110} directions with an interchain spacing of 1.6 nm. At low coverage the domains are highly asymmetric, and extended along the chain direction. These atomically flat domains routinely span several germanium terraces and indicate that chain formation involves considerable mass transport of gold and germanium atoms.