Critical adatom density in nucleation of two-dimensionalp(1×1) ErSi2on Si(111): Identification of submonolayer Er phases

Abstract

We observe a critical Er adatom density in the nucleation of two-dimensional (2D) $p(1\ifmmode\times\else\texttimes\fi{}1){\mathrm{ErSi}}_{2}$ upon Er reaction with Si(111) (7\ifmmode\times\else\texttimes\fi{}7) by means of scanning tunneling microscopy (STM) and angle-resolved photoemission. When the adatom mobility is sufficient, essentially all the Er is accommodated into 2D ${\mathrm{ErSi}}_{2}$ islands for Er densities $dg~0.5\mathrm{ML}$. Silicide phases are formed for $dl0.5\phantom{\rule{0ex}{0ex}}\mathrm{ML}$ that differ from the well-documented 2D $p(1\ifmmode\times\else\texttimes\fi{}1) {\mathrm{ErSi}}_{2}$ and 3D $(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}) R30\ifmmode^\circ\else\textdegree\fi{}$ defected ${\mathrm{ErSi}}_{1.7}$ silicides. STM images reveal that at low coverages the Er corrodes the Si(111) (7\ifmmode\times\else\texttimes\fi{}7) surface and besides 2D ${\mathrm{ErSi}}_{2}$ forms three specific phases. The most stable one exhibits a $p(2\ifmmode\times\else\texttimes\fi{}1)$ surface periodicity and generally grows in the low submonolayer range in the form of rod-shaped islands in a highly anisotropic way along 〈11\ifmmode\bar\else\textasciimacron\fi{}0〉 directions. This kind of silicide may also be formed at larger coverages (g0.5 ML) and adopts then a more isotropic island shape. The data suggest that it corresponds to a particular form of defected ${\mathrm{ErSi}}_{2\ensuremath{-}x}$ silicide with a (2\ifmmode\times\else\texttimes\fi{}1) arrangement of Si vacancies epitaxially stabilized on Si(111). Two metastable phases are also identified that are 2D in character and can be viewed as submonolayer Er-induced superstructures or reconstructions of Si(111) with $2\sqrt{3}\ifmmode\times\else\texttimes\fi{}2\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}$ and (5\ifmmode\times\else\texttimes\fi{}2) surface cells. Both surface structures convert eventually into the (2\ifmmode\times\else\texttimes\fi{}1) phase upon further annealing.