Atomic structure and thermal stability of two-dimensional Er silicide on Si(111)

The atomic structure of Er silicides on Si(111) in the submonolayer range has been studied by means of medium-energy ion scattering and surface x-ray diffraction. We find good agreement between the two techniques. The ion-scattering measurements indicate that the Er adsorbs in a single two-dimensional layer on top of which a completely relaxed (bulklike) Si bilayer is located. This bilayer is exclusively of B type, i.e., rotated by 180° with respect to the substrate bilayers. The x-ray diffraction results show that Er is positioned on T₄ sites only, i.e., above second-layer substrate atoms. For low coverages (0.23 ML), the height of the top bilayer appears to be reduced by approximately 0.4 Å. We attribute this to the fact that the Er does not form a closed layer, leading to the formation of less ordered Er-Si complexes in which extra Si adatoms provide the back bonds for the Si hexagons on top of the Er. Finally, we investigate the thermal stability of the two-dimensional silicide, and find that the two-dimensional Er layer evolves into relaxed three-dimensional islands at tempera tures above 800 °C with concomitant strain relaxation.