  Low-temperature illumination of a two-dimensional electron gas in GaAs
quantum wells is known to greatly improve the quality of high-field
magnetotransport. The improvement is known to occur even when the carrier
density and mobility remain unchanged, but what exactly causes it remains
unclear. Here, we investigate the effect of illumination on microwave
photoresistance in low magnetic fields. We find that the amplitude of
microwave-induced resistance oscillations grows dramatically after
illumination. Dingle analysis reveals that this growth reflects a substantial
increase in the single-particle (quantum) lifetime, which likely originates
from the light-induced redistribution of charge enhancing the screening
capability of the doping layers.
