Study of photoconductivity in AlxGa1−xAs/GaAs modulation‐doped heterostructures

Temperature‐dependent Hall effect measurements under different illumination conditions are performed on AlxGa1−xAs/GaAs heterostructures doped 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 without 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 AlGaAs barrier. This decrease occurs only at temperatures below 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 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 investigated.