Persistent photoconductivity in semimetallic AlxGa1-xSb/InAs quantum wells

"Positive" persistent photoconductivity (PPC) and "negative" persistent pho toconductivity (NPPC) have been studied in AlxGa1-xSb/InAs quantum wells. C omparing with the published reports, several interesting results have been observed, and to our knowledge they cannot be explained by previously propo sed models. Through studies of the NPPC and PPC effects under various condi tions, such as the different photon energies of excitation, different tempe rature, and different Al composition x, we suggest that the NPPC and PPC ef fects are caused by two competing processes. At low temperature electrons i n the InAs layer are photoexcited into the valence band of the AlxGa1-xSb l ayer. The return of photoexcited electrons are prohibited by the barrier du e to interface band bending, and thus the NPPC occurs. At high temperature the photoconduction is dominated by the photogenerated electrons in the InA s 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 i n the InAs well. [S0163-1829(99)02203-1].