X-ray-standing-wave analysis of Pb on epitaxial Ge(111) on Si(111).

In the present study we deposited Pb on a Si(111)/Ge-(5\ifmmode\times\else\texttimes\fi{}5) surface. We analyzed the structure of Pb and Ge as a function of Pb coverage with x-ray standing waves. In the monolayer range, Pb forms an incommensurate structure distributed laterally by \ifmmode\pm\else\textpm\fi{}0.6 \AA{} around atop sites on a (1\ifmmode\times\else\texttimes\fi{}1) Ge double layer. At low coverages (0.2 ML) Pb is adsorbed in adatom sites on a (5\ifmmode\times\else\texttimes\fi{}5) reconstructed Ge double layer. The phase diagram of Pb on Ge(111)/Si(111) is strikingly different from both Pb on Si(111) and Pb on Ge(111).