Positive and negative persistent photoconductivities in semimetallic AlxGa1-xSb/InAs quantum wells

We report a detailed investigation of the "positive" persistent photoconduc tivity (PPC) and "negative" persistent photoconductivity (NPPC) in semimeta llic AlxGa1-xSb/InAs quantum wells. The studies of the NPPC and PPC effects have been performed under various conditions, such as different photon ene rgy of excitation, different temperature, different Al composition x, and d ifferent well width. It is found that all the previously proposed mechanism s fail to explain several of our observations. We suggest that the NPPC and PPC effects are produced principally by two competing processes. At a high temperature, the photoconduction is dominated by the photogenerated electr ons in the InAs well, in which the relaxation of the excess electrons is pr ohibited by an energy barrier due to the trapping of photoexcited holes by deep defects in the InAs well. As a result, the PPC is observed. At low tem perature, electrons in the InAs layer are photoexcited into the local poten tial minima induced by compositional fluctuations at the AlGaSb and InAs in terface, the number of electrons in the InAs well decreases, thus the NPPC occurs. (C) 1999 American Institute of Physics. [S0021-8979(99)00818-X].