Light-induced mobility enhancement in delta-doped GaAs/In0.26Ga0.74As/GaAs quantum wells grown by molecular beam epitaxy on GaAs(001)

We have studied persistent photoconductivity in 80 Angstrom strained GaAs/In0.26Ga0.74As/GaAs quantum wells grown by molecular beam epitaxy at 480 degrees C by measuring the Shubnikov de Haas and Hall effects at 4.2 K. Each quantum well was delta doped with Si to 2 X 10(12) cm(-2) either (i) at the center of the quantum well, (ii) at the lower interface, or (iii) the bottom barrier. The free-carrier density after exposure to 654 nm red light with above-band-gap energy did not exceed significantly the intended delta-doping level, but the Hall mobility increased dramatically and was accompanied by stronger Shubnikov de Haas oscillations. We attribute this to electron-hole pair generation and separation and/or the photoionization of deep states, which create a positive space charge in the surface depletion region. This leads to electrostatic bandbending, which converts the quantum well from being asymmetric in the dark to square-like after illumination and shifts the charge distribution away from the delta layer, improving the mobility. This hypothesis is corroborated by the light insensitivity of both an identical heterostructure delta doped to 2 X 10(12) cm(-2) in the top barrier and an equivalent set of heterostructures delta doped with Si to 4.5 X 10(12) cm(-2). (C) 1999 American Institute of Physics. [S0003-6951(99)03814-0].