Helium scattering structure analyses of the c(2x8) reconstruction and the high-temperature (1x1) structures of Ge(111)

We present a quantitative analysis of high-resolution helium-atom scattering measurements obtained from the Ge(lll)-c(2 x 8) surface. The presence of quarter-order beams as well as the symmetries observed along <[11(2)over bar]><[2(11)over bar]> confirm that the angular distributions are originated by three equivalent c(2 x 8) domains. The corrugation function derived from the calculations reveals clearly that the two rest atoms within the unit cell are buckled, as was also observed previously with scanning tunneling microscopy. Intensity analyses of diffraction scans measured above and below the order-order transition at T-c = 1050 K suggest that for T>T-c the first-layer atoms are shifted from their regular bulk places to the hexagonal diamond sites.