Impact ionization of excitons and donors in ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/(n-type GaAs):Si quantum wells

We have investigated the effect of a parallel electric field on the low-temperature photoluminescence from ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/(n-type GaAs):Si quantum wells. We have studied the electric-field effect in a number of molecular-beam-epitaxy-grown ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/GaAs quantum-well structures doped with Si in the center of the well with well widths from 40 to 190 Å. We observe sharp thresholds in the quenching of luminescence from the free and bound excitons at fields near a few tens of V/cm, accompanied by a sharp increase in the current. The actual threshold is critically dependent on the well width of the sample, which is qualitatively explained by the differences in carrier mobility. The mechanism responsible for the observed effects will be shown to be due to impact ionization of the excitons by the free carriers heated in the electric field.