Origin of deep localization in GaAs1-xBix and its consequences for alloy properties

The addition of Bi isoelectronic dopants to GaAs provides an attractive avenue for tailoring its electronic band structure, yet it also introduces less appealing and very strong hole localization. The origin of the localization is still not thoroughly understood, which has in part inhibited the practical use of $\mathrm{GaA}{\mathrm{s}}_{1\ensuremath{-}x}\mathrm{B}{\mathrm{i}}_{x}$ alloys. In this study, the evolution of hole localization was evaluated as a function of composition. We find that spatial overlap of Bi-related bound states at concentrations $g0.6%$ Bi effectively enables holes to be channeled to those at the lowest energies, thereby aiding localization of excitons $\ensuremath{\ge}150\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ below the band gap. The large energy gap between these bound states and the GaAs valence-band edge combined with the slow upward movement of the valence band with composition causes deep localization to persist to high concentrations $g6%$ Bi. The results provide important insight into the optical and transport behavior of $\mathrm{GaA}{\mathrm{s}}_{1\ensuremath{-}x}\mathrm{B}{\mathrm{i}}_{x}$ and its implications for device applications.