  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.
