X-ray structure analysis on alkali metals adsorbed on Ge(001)(2 x 1).

Abstract

Surface x-ray diffraction has been used to investigate the geometric structure of Cs, K, and Na adsorbed on Ge(001)(2\ifmmode\times\else\texttimes\fi{}1) at room temperature. At low and saturation coverage corresponding to about 0.3 monolayer (ML) and 0.6--0.7 ML, respectively (1 ML=6.25\ifmmode\times\else\texttimes\fi{}${10}^{14}$ atoms ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$), adsorption takes place in the large grooves between the Ge-dimer atoms. For all adsorption systems investigated, the statistical occupation of two different sites is observed: first, above the third-layer Ge atoms (T3), and second, in an asymmetric site close to the dangling bonds of the Ge-dimer atoms (a-T4). At about half saturation coverage we have evidence for the preferential occupation of the adsorption site T3 indicating a coverage-dependent ordering and the formation of a linear densely packed chain along the [110] direction of the substrate. Whereas the bond lengths of the T3-adsorbed alkali metals to nearest-neighbor Ge atoms are in the regime found for bulk alkali-metal--Ge structures, which can be interpreted as indicative of covalent bonding, they are shorter for alkali metals adsorbed in a-T4 to the Ge-dimer atoms, where average distances in the regime between covalent and ionic bonding are observed. This can be interpreted by partial charge transfer to the empty dangling bond states. The alkali metals Cs and K are generally found to occupy different adsorption heights, which can be related to a strong disorder normal to the surface due to steric reasons. This is not observed for the smaller Na atoms. Generally, the Ge-dimer bond length (2.45 \AA{}) and the dimer asymmetry (inclination angle of the dimer bond to the surface \ifmmode\pm\else\textpm\fi{}17.6\ifmmode^\circ\else\textdegree\fi{}) are not influenced by alkali-metal adsorption within 0.1 \AA{} and about 4\ifmmode^\circ\else\textdegree\fi{}, respectively. Shifts of Ge atoms at least down to the fourth layer below the surface are observed.