ATOMIC-STRUCTURE AND THERMAL-STABILITY OF 2-DIMENSIONAL ER SILICIDE ON SI(111)

The atomic structure of Er silicides on Si(111) in the submonolayer ra nge has been studied by means of medium-energy ion scattering and surf ace x-ray diffraction. We find good agreement between the two techniqu es. The ion-scattering measurements indicate that the Er adsorbs in a single two-dimensional layer on top of which a completely relaxed (bul klike) Si bilayer is located. This bilayer is exclusively of B type, i .e., rotated by 180 degrees C with respect to the substrate bilayers. The x-ray diffraction results show that Er is positioned on T-4 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 approxima tely 0.4 Angstrom. We attribute this to the fact that the Er does not form a closed layer, leading to the formation of less ordered Er-Si co mplexes in which extra Si adatoms provide the back-bonds for the Si he xagons 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 temperatures above 800 degrees C with concomitant strain relaxation.