New type of persistent photoconductivity related to DX-center: the study of interband PPC in Si-doped AlGaAs

We studied the build-up of the persistent photoconductivity (PPC) in AlxGa1-xAs (x = 0.15) doped with Si. The Hall electron concentration and the conductivity were measured at 77 K as a function of time while the sample was illuminated by the monochromatic light. The case of excitation by photons with energy below the gap was compared to that by photons with energy just above the gap. The first mode leads to optical ionization of DX centers. The new mechanism of persistent emptying of DX centers, specific for inter-band PPC, is related to the capture of photo-created holes by negatively charged DX ions. Our results indicate the importance of effects of correlated space distribution of charges on impurity sites. The evolution of mobility during the build-up indicates that illumination with above-gap photons increases the degree of correlation, in contrast to the lower-energy ones. By analyzing the transients n(t) we conclude that the effective cross section for hole capture decreases dramatically during the build-up process. This indicates that the hole capture by DX- ions is strongly influenced by the Coulomb potential from the nearest impurity ions. (C) 1999 Elsevier Science B.V. All rights reserved.