A new model, based on an extremely strong coupling between the electronic and vibrational systems of certain defect centers, is proposed to explain the phenomenon of persistent photoconductivity observed in some compound semiconductors. The model is supported by data on donor-related defects in $n$-type ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ which exhibit the features characteristic of this effect: a very large Stokes shift (thermal depth, ∼0.1 eV; optical depth, ∼1.2 eV); and a very small (<${10}^{-30\mathrm{}}$ ${\mathrm{cm}}^2$), thermally activated, electron-capture cross section at temperatures below 77 K.

• Received 6 April 1977

DOI:https://doi.org/10.1103/PhysRevLett.39.635