Force- and bias-dependent contrast in atomic force microscopy based photocurrent imaging on GaAs-AlAs heterostructures

In this work, photocurrent images of lithographically processed GaAs/AlAs heterostructures are recorded by an atomic force microscope. 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. Due to the complex behavior of the photocurrent as a function of tip-bias and tip-force, the contrast in photocurrent images is non-monotonic and can be reversed under appropriate bias and force conditions.