Tunneling Images of Germanium Surface Reconstructions and Phase Boundaries

We have imaged with the tunneling microscope the microscopic surface topography of germanium layers grown by molecular-beam epitaxy on Si (111) -77. The surface structures corresponding to Ge (111) -c28 and Ge (111) -77 are resolved. The Ge-c28 structure is shown to consist of a coherent combination of two simpler units both of which exist independently on the surface. The associated phase boundaries are illustrated. The Ge 77 structure is shown to be similar to that obtained for Si (111). Defects in the cell are also imaged.