We observe a critical Er adatom density in the nucleation of two-dimensional ͑2D͒ p(1ϫ1)ErSi 2 upon Er reaction with Si͑111͒ ͑7ϫ7͒ by means of scanning tunneling microscopy ͑STM͒ and angle-resolved photoemission. When the adatom mobility is sufficient, essentially all the Er is accommodated into 2D ErSi 2 islands for Er densities dу0.5 ML. Silicide phases are formed for dϽ0.5 ML that differ from the well-documented 2D p(1ϫ1) ErSi 2 and 3D (ͱ3ϫͱ3) R30°defected ErSi 1.7 silicides. STM images reveal that at low coverages the Er corrodes the Si͑111͒ ͑7ϫ7͒ surface and besides 2D ErSi 2 forms three specific phases. The most stable one exhibits a p(2ϫ1) surface periodicity and generally grows in the low submonolayer range in the form of rod-shaped islands in a highly anisotropic way along ͗110͘ directions. This kind of silicide may also be formed at larger coverages ͑Ͼ0.5 ML͒ and adopts then a more isotropic island shape. The data suggest that it corresponds to a particular form of defected ErSi 2Ϫx silicide with a ͑2ϫ1͒ arrangement of Si vacancies epitaxially stabilized on Si͑111͒. Two metastable phases are also identified that are 2D in character and can be viewed as submonolayer Er-induced superstructures or reconstructions of Si͑111͒ with 2ͱ3ϫ2ͱ3)R30°and ͑5ϫ2͒ surface cells. Both surface structures convert eventually into the ͑2ϫ1͒ phase upon further annealing.

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