Force and bias dependent contrast in photocurrent imaging on GaAs–AlAs heterostructures

In this work, an atomic force microscope (AFM) with conductive tip was applied to investigate the local photoconductive behavior of lithographically processed GaAs/AlAs heterostructures. Photocurrent images and current vs. voltage (IV) curves were recorded under different tip-sample voltage and tip force conditions. It is found that the AFM tip-sample contact is strongly dependent on the thickness of the native oxide layer on the sample surface. Therefore, the photocurrents increase if a successively increasing tip-sample force is applied, which leads to a gradual penetration of the surface oxide layer. The force dependence of the Schottky barrier height was applied to build a laser free AFM feedback for photocurrent spectroscopy. This feedback system was used to record photocurrent vs. wavelength spectra at a constant tip-sample force.