Photo-current and photo-voltage oscillations in the two-dimensional   electron system: screening and "anti-screening" of a potential profile
  We observe in state-of-the-art GaAs based 2D electron systems microwave induced photo-current and photo-voltage oscillations around zero as a function of the applied magnetic field. The photo-signals pass zero whenever the microwave frequency is close to a multiple of the cyclotron resonance frequency. They originate from built-in electric fields due to for instance band bending at contacts. The oscillations correspond to a suppression (screening) or an enhancement ("anti-screening") of these fields by the photo-excited electrons. 
S. I. Dorozhkin, I. V. Pechenezhskiy, L. N. Pfeiffer, K. W. West, V.   Umansky, K. von Klitzing, J. H. Smet