Erbium silicide growth on weakly boron-doped Si(111) was studied by scanning tunneling microscopy. The reactivity and the strain of this root 3 x root 3R30 degrees surface are different from those observed on Si(111) 7 X 7. These interesting features allow us to study the erbium silicide growth on a new interface. We have observed, in the submonolayer range, the formation of a metastable 2 root 3 X 2 root 3R30 degrees reconstruction and the nucleation of two kinds of stable two-dimensional ErSi2.