UNIDIRECTIONAL AND ISOTROPIC STRAIN RELIEF IN STRIPED AND HEXAGONAL PHASES OF GE(111)IN

We investigated the adsorbate system Ge(lll):In with low energy electron diffraction and scanning tunneling microscopy. Several coverage-driven phase transitions accompanied by a distinct change in symmetry of the surface reconstruction were observed. At coverages around 0.05 monolayer (ML) the surface reconstruction changes from a disturbed c(2 x 8) to an almost hexagonal phase with (2 x 2) domains. This transition is defect induced. Between 0.1 and 0.3 ML In coverage a discommensurate phase with striped domains along (11 ($) over bar 2) is observed. With increasing In coverage the average width of the stripes decreases to a minimum at similar to 0.3 ML (''commensurate limit''). Further increase of coverage yields a superlattice with hexagonal symmetry and In atoms within domains in local (1 x 1)geometry but with a 25% mismatch. The striped discommensurate phase is the result of the counteraction of tensile stress caused by Ge adatoms within the domains and compressive stress by substitutional In in the domain walls. For Theta(In) > 0.3 ML, severe stress by global In substitution leads to strong local strain and the appearance of a domain superstructure. The transition between these two phases reflects the change from unidirectional to isotropic relief of surface stress.