CRITICAL ADATOM DENSITY IN NUCLEATION OF 2-DIMENSIONAL P(1X1) ERSI2 ON SI(111) - IDENTIFICATION OF SUBMONOLAYER ER PHASES

We observe a critical Er adatom density in the nucleation of two-dimen sional (2D) p(1x1)ErSi2 upon Er reaction with Si(lll) (7x7) by means o f scanning tunneling microscopy (STM) and angle-resolved photoemission . When the adatom mobility is sufficient, essentially all the Er is ac commodated into 2D ErSi2 islands for Er densities d greater than or eq ual to 0.5 ML. Silicide phases are formed for d<0.5 ML that differ fro m the well-documented 2D p(1x1) ErSi2 and 3D (root 3x root 3) R30 degr ees defected ErSi1.7 silicides. STM images reveal that at low coverage s the Er corrodes the Si(lll) (7x7) surface and besides 2D ErSi2 forms three specific phases. The most stable one exhibits a p(2x1) surface periodicity and generally grows in the low submonolayer range in the f orm of rod-shaped islands in a highly anisotropic way along [1 (1) ove r bar 0] directions. This kind of silicide may also be formed at large r coverages (>0.5 ML) and adopts then a more isotropic island shape. T he data suggest that it corresponds to a particular form of defected E rSi2-x silicide with a (2x1) arrangement of Si vacancies epitaxially s tabilized on Si(lll). Two metastable phases are also identified that a re 2D in character and can be viewed as submonolayer Er-induced supers tructures or reconstructions of Si(lll) with 2 root 3x2 root 3)R30 deg rees and (5x2) surface cells. Both surface structures convert eventual ly into the (2x1) phase upon further annealing.