Persistent photoconductivity in semimetallic ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{S}\mathrm{b}/\mathrm{I}\mathrm{n}\mathrm{A}\mathrm{s}$ quantum wells

“Positive” persistent photoconductivity (PPC) and “negative” persistent photoconductivity (NPPC) have been studied in ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{S}\mathrm{b}/\mathrm{I}\mathrm{n}\mathrm{A}\mathrm{s}$ quantum wells. Comparing with the published reports, several interesting results have been observed, and to our knowledge they cannot be explained by previously proposed models. Through studies of the NPPC and PPC effects under various conditions, such as the different photon energies of excitation, different temperature, and different Al composition x, we suggest that the NPPC and PPC effects are caused by two competing processes. At low temperature electrons in the InAs layer are photoexcited into the valence band of the ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{Sb}$ layer. The return of photoexcited electrons are prohibited by the barrier due to interface band bending, and thus the NPPC occurs. At high temperature the photoconduction is dominated by the photogenerated electrons in the InAs well, in which the relaxation of the excess electrons is prohibited by an energy barrier due to the trapping of photoexcited holes by deep defects in the InAs well.