Metallicity and gap states in tunneling to Fe clusters in GaAs(110)

We report the characteristics of tunneling to a GaAs(110) substrate with distinct, nanometer-size Fe clusters, as a function of distance from and size of the clusters. We show that Fe clusters of volumes {~}150 A³, corresponding to {}13 atoms, are observed to be nonmetallic with a gap at the Fermi level. Larger clusters with >35 atoms begin to show metallic characteristics. We observe a continuum of cluster-induced gap states in tunneling to the GaAs substrate surrounding the metallic Fe clusters. The decay length of these states has a minimum decay of 3.4 A{} at midgap and diverges at the valence- and conduction-band edges.