Dramatic dependence of the Fermi level pinning strength on crystal orientation at clean surfaces of n-type In_0.53Ga_0.47As grown by molecular beam epitaxy

The electronic density of states at clean surfaces of n-type In_0.53Ga_0.47As, grown by molecular beam epitaxy on lattice-matched (0 0 1)- and (1 1 1)A-oriented InP substrates, was measured by low-temperature scanning tunneling spectroscopy under ultra-high vacuum. It was found that the surface Fermi level (FL) pinning strength dramatically depends on crystal orientation. The FL at the (0 0 1)-(2×4) surface is pinned near midgap, independently of the dopant concentration in the bulk. In contrast, the FL at the (1 1 1)A-(2×2) surface lies in the conduction band, close to the bulk FL, and increases with dopant concentration.