Negative persistent photoconductivity in II-VI ${\mathrm{ZnS}}_{1-x}{\mathrm{Se}}_x{/\mathrm{Z}\mathrm{n}}_{1-y}{\mathrm{Cd}}_y\mathrm{Se}$ quantum wells

The carrier concentration of a two-dimensional electron gas in a ${\mathrm{Zn}}_{0.2}{\mathrm{Cd}}_{0.8}\mathrm{Se}$ quantum well was persistently reduced by red-light illumination at low temperature. The deep-level donors were “frozen out” at 50 K and the thermal activation energy was about 42.6 meV. We believe that these deep-level donors are unlikely $\mathrm{DX}$ centers, and the observed negative persistent photoconductivity probably arises from the trapping of electron by the empty localized state of random-local-potential fluctuations in the barrier.