Lead-induced reconstructions of the Ge(001) surface

The two-dimensional phases of Pb on Ge(001) have been studied with scanning tunneling microscopy and electron diffraction. The common feature of all the low coverage reconstructions is the coexistence of Pb and Ge dimers either in parallel or in perpendicular orientations depending on the coverage. The tunneling voltage polarity dependence in the STM images of the c(8 x 4) reconstruction and the atomic displacements in the (2 x 2) structure are explained by buckling of the Pb surface dimers. Structural models for (2 x 2), c(8 x 4), (1 x 5), and [GRAPHICS] phases are presented which elucidate the changes taking place with increasing coverage. The high coverage c(8 x 4)i reconstruction is interpreted as an incommensurate hexagonal overlayer on the cubic substrate. Heat treatment of the higher coverage reconstructions at 300 degrees C leads to the formation of multiple steps, which can be interpreted as facets, whereas heating of the low coverage phases induces a roughening of the surface.