SM-INDUCED AND YB-INDUCED RECONSTRUCTIONS OF THE SI(111) SURFACE

Low-energy electron diffraction, scanning tunneling microscopy, and ph otoelectron spectroscopy results from the submonolayer Sm- and Yb-indu ced surface structures are presented. Several similar metal-induced su rface reconstructions are found to exist for Yb and Sm on Si(111) for low submonolayer coverages: 3 x 2, 5 x 1, and 7 x 1. At higher submono layer coverage, Yb induces a 2 x 1 reconstruction while Sm induces a ( square-root 3 x square-root 3)R 30-degrees-like reconstruction. Yb is found to be divalent in all structures, whereas the Sm valence increas es with increasing coverage. In the 3 x 2 structure only divalent Sm i s present, in the 5 x 1 and 7 x 1 structures a small amount of trivale nt Sm appears, and, finally, in the (square-root 3 x square-root 3)R 3 0-degrees structure approximately half of the Sm atoms are trivalent. The surface Fermi-level position in the band gap for the different Sm and Yb reconstructions has been measured. The difference in valence st ability between Sm and Yb is suggested to be the cause of the differen ce in the high-coverage structures found and the differences in pinnin g level for the two elements observed for the 5 x 1 and 7 x 1 structur es.