Optically induced long-lifetime photoconductivity in semi-insulating bulk GaAs

Persistent illumination with photons of energy ranging from 1 to 1.25 eV is known to produce metastable transformations in the photoelectronic properties of GaAs. In this work the generation of a high-photosensitivity state is reported. This state is shown to be associated with a center, labeled C*A, which is optically created by those photons. These centers are found to have a high hole photoionization, the dominant conduction associated with them upon illumination is p type, and the photoholes remain long-time delocalized in the valence band before their capture by the ionized C*A centers (persistent photoconductivity effect). This effect is associated with a screening of C*A centers by ionized shallow donors located nearby. The formation of the C*A centers is found to occur in two or more stages and is the result of a reordering inside native defect complexes called C, more likely AsGa aggregates, followed by the association with shallow acceptor levels due to a carbon impurity (CAs). The driven force leading to such a defect configuration is assumed to be associated with Coulombic attractive forces appearing upon photoionization of the As antisite defects. This leads to a strong lattice relaxation and hence the formation of the new C*A centers accounting for the observed photocurrent properties of the samples. The destruction of these centers is accomplished by heating above 135 K; this thermal cycle converts the sample to its normal photosensitivity state.