Strain relief, close-packed overlayer formation and discommensurate phases: In on Si(111)

The relief of surface stress, the stabilisation by defects and the tendency to maximise the adatom surface coverage as structure-determining effects are illustrated in a most direct way in the In monolayer structures on Si(111). The scanning tunnelling microscopy images reveal the coexistence of a pseudomorphic (1 x 1) In structure bordering surface defects, a strain-relieving (root 7 x root 3) In reconstruction and a second (root 7 x root 3) In structure due to a close-packed In overlayer of different symmetry to the substrate. The delicate energetic balance in the In monolayer is further emphasised by the destabilisation of these structures in the presence of an external stress field, which induces new ordered discommensurate phases.