STUDY OF PHOTOCONDUCTIVITY IN ALXGA1-XAS GAAS MODULATION-DOPED HETEROSTRUCTURES/

Temperature-dependent Hall effect measurements under different illumin ation conditions are performed on AlxGa1-xAs/GaAs heterostructures dop ed with both Si and Sn. For a Sn-doped AlxGa1-xAs/GaAs heterostructure with x=0.35, two distinct DX center levels are observed directly with out exposure to light. 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 AlGaA s barrier. This decrease occurs only at temperatures below 90 K. This negative photoconductivity is explained by taking into account the par tial freeze-out of electrons into the shallow DX centers and the trans fer of holes photogenerated in the barrier into the channel region. No such phenomena are observed in Sn-doped heterostructures with x<0.35 or in the Si-doped heterostructures. The reasons for this are also inv estigated. (C) 1996 American Institute of Physics.