ABSTRACT
Temperature-dependent Hall-effect measurements under different illumination conditions are performed for AlxGa1-x As:Sn/GaAs modulation doped heterostructures. Two distinct DX center levels are deduced. The two-dimensional electron gas concentration measured under illumination decreases from the value due to persistent photoconductivity to a value less than that measured in the dark when the excitation photon energy is larger than the band gap of the AlGaAs barrier. This decrease occurs only at temperatures below about 90 K. This negative photoconductivity is explained by taking into account the partial freeze-out of electrons into the shallow DX centers and the transfer of holes photo-generated in the barrier into the channel region.
