Photovoltage imaging of a single As-vacancy at a GaAs(110) surface: Evidence for electron trapping by a charged defect?

We employed atomically-resolved surface photovoltage (SPV) imaging with a scanning tunneling microscope, using super- and sub-band gap energy photons, to map the potential distribution around a single As vacancy at a p-GaAs(110) surface. While the super-band gap (532 nm) SPV reflects the band bending (including the tip-induced effect) in the dark, the sub-band gap (1064 nm) SPV shows the change in band bending due to charge injection into the defect. We conclude that in the dark, the As vacancy introduces a surface state, ∼0.63 eV above the valence band maximum, that has to be filled with two electrons to maintain surface neutrality. However, under 1064 nm (sub-band gap) illumination at room temperature, we observed a large negative SPV feature (radius of 70–100 Å), and a zero or low positive SPV zone in its center, with a six-pointed star shape of ∼15 Å radius. We interpret our findings as either negative photo-charging of the defect, or positive charging that leads to a trapped electron state around the defect.