Anomalous scanning tunneling microscopy images of GaAs(110) surfaces due to tip-induced band bending

Scanning tunneling microscopy (STM) images of clean nondegenerate GaAs(110) surfaces, which lack surface states in the band gap, often show the As sublattice at low positive sample bias (empty state image) for p-type material, or conversely, the Ga sublattice at low negative sample bias (filled state image) for n-type material. This happens because as the Fermi level of the tip is positioned inside the GaAs energy gap, no current can flow between the sample and the tip. As a result, the STM feedback brings the tip very close to the surface and the tip-induced electric field is greatly enhanced, creating a subsurface accumulation layer.