Bi isoelectronic impurities in GaAs

GaAs1-xBix is a mixed-anion semiconductor alloy. In the As-rich regime (0.4%<x<4%), isovalent Bi creates a series of bound states but this alloy nonetheless exhibits properties characteristic of regular semiconductors. The dual impurity-alloy character can be tuned by varying the temperature. Below 100 K, multiple Bi bound states appear at low energy in the luminescence spectrum. These states are associated with the pseudodonor potential created by isovalent Bi impurities. Taking into consideration the concentration regime at which these bound states are observed, they likely involve excitons bound to clusters composed of a few substitutional Bi atoms, indirectly implying that the isolated Bi state is resonant with the valence band. At ambient temperature, these localized states are strongly suppressed and luminescence from the band edges is measured. The important Bi-induced atomic disorder creates a strong intraband coupling resulting in an important reduction of the band gap for a relatively small alloy concentration. These results on a pseudodonor isoelectronic alloy are reminiscent of the better known pseudoacceptor GaAsN, but offer a complementary view of this intriguing and yet little understood class of alloys.