Structural Origin of the Sn $4d$ Core Level Line Shape in $\mathrm{Sn}/\mathrm{Ge}(111)\mathrm{\text{−}}(3\times 3)$

High-resolution photoemission of the Sn $4d$ core level of $\mathrm{Sn}/\mathrm{Ge}(111)\mathrm{\text{−}}(3\times 3)$ resolves three main components in the line shape, which are assigned to each of the three Sn atoms that form the unit cell. The line shape found is in agreement with an initial state picture and supports that the two down atoms are inequivalent. In full agreement with these results, scanning tunnel microscopy images directly show that the two down atoms are at slightly different heights in most of the surface, giving rise to an inequivalent-down-atoms ($3\times 3$) structure. These results solve a long-standing controversy on the interpretation of the Sn $4d$ core-level line shape and the structure of $\mathrm{Sn}/\mathrm{Ge}(111)\mathrm{\text{−}}(3\times 3)$.

Figure 1

Sn $4d$ core-level spectra (normal emission) for Sn coverages below 0.5 ML. All spectra correspond to a ($3\times 3$) LEED pattern, with the exception of spectrum A.

Figure 2

Normal emission Sn $4d$ core-level spectra (circles). Left: solid lines show the four doublets used to deconvolute the spectra and the resulting fit. Right: zooms of the $4d_{5/2}$ area. Vertical lines indicate the BEs of $C_2$ and $C_3$. The continuous line joining the experimental points is a guide to the view.

Figure 3

Filled [(a), $V=-1.0\mathrm{V}$, $I=1.0\mathrm{nA}$] and empty [(b), $V=-1.0\mathrm{V}$, $I=2.0\mathrm{nA}$] states’ STM images for the ($3\times 3$) phase at 77 K. Note the different heights of the down atoms in the zoom from (a) shown on the left. (c) Height profiles corresponding to the highlighted directions from (a) and (b). The height of the down atoms is marked with horizontal lines for the upper (lower) down atom. The directions are selected so that no nearest neighbor is a defect.