Field-effect persistent photoconductivity in AlAs and GaAs quantum wells with ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ barriers

We report a persistent increase or decrease in the two-dimensional electron density of AlAs or GaAs quantum wells flanked by ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ barriers, brought about by illuminating the samples at $T\sim 4\mathrm{K},$ while simultaneously applying a voltage bias between a back gate and the two-dimensional electron gas. Control of the final carrier density is achieved by tuning the back gate bias during illumination. Furthermore, the strength of the persistent photoconductivity depends on the Al mole fraction in the back ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ barrier, and is largest at $x\simeq \mathrm{0.4.}$