Surface structure and long-range order of the Ge(111)-c(2 x 8) reconstruction.

Abstract

We have performed an x-ray diffraction study of the Ge(111)-c(2\ifmmode\times\else\texttimes\fi{}8) surface. From the analysis of 43 in-plane, symmetry-inequivalent structure factors, a simple adatom model with subsurface relaxation is deduced. The relaxations around the adatoms are similar to those observed in the adatom complexes on Si(111)-(7\ifmmode\times\else\texttimes\fi{}7) and Sn/Ge(111)-(\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 ), -(5\ifmmode\times\else\texttimes\fi{}5), and -(7\ifmmode\times\else\texttimes\fi{}7), suggesting that the adatoms are located in threefold sites above the second-layer atoms (${T}_{4}$ sites). Significant distortions around the rest atoms are seen due to rehybridization of the dangling bonds. The diffraction peaks from the eighth-order reflections are displaced away from their nominal positions. This is explained by a model where the long-range order of the c(2\ifmmode\times\else\texttimes\fi{}8) structure is disturbed by faults due to rows of 2\ifmmode\times\else\texttimes\fi{}2 unit cells.