A self-tracking method for local surface-photovoltage measurements on semiconducting surfaces

We present a method for determining the surface photovoltage (SPV) on semiconductor surfaces with non-metallic electronic structure under continuous wave illumination using scanning tunneling microscopy and spectroscopy. Tip-induced band bending effects are largely avoided by keeping the electric field between tip and sample constant during the measurement. This is achieved with an additional feedback loop that tracks the position of characteristic nonlinearities in the I(V) spectrum by applying a suitable compensation voltage. The method is benchmarked for the case of Si( 100)-c(4x2) at 78 K with different bulk doping levels. The results are discussed in view of carrier lifetimes, depletion widths and SPV saturation at different doping levels.