  Unbiased (biased) illuminations are performed at low temperatures on
dopant-free two-dimensional electron gases (2DEGs) at different depths in
undoped GaAs/AlGaAs, while gates are kept grounded (held at a finite voltage,
either positive or negative). Unbiased illuminations in 2DEGs located more than
70 nm away from the surface result in a gain in mobility (for the same electron
density), driven by the reduction of background impurities. In 2DEGs closer to
the surface, unbiased illuminations result in a mobility loss, driven by an
increase in surface charge density. Biased illuminations performed with
positive applied gate voltages result in a mobility gain, whereas those
performed with negative applied voltages result in a mobility loss. The
magnitude of the mobility gain (loss) weakens with 2DEG depth, and is likely
driven by a reduction (increase) in surface charge density. Experimental
results are modeled with Boltzmann transport theory, and possible mechanisms
are discussed.
