Bias-dependent electron spin lifetimes in n-GaAs and the role of donor impact ionization

In bulk n-GaAs epilayers doped near the metal-insulator transition, we study the evolution of electron spin lifetime $\tau_s$ as a function of applied lateral electrical bias $E_x$. $\tau_s$ is measured via the Hanle effect using magneto-optical Kerr rotation. At low temperatures (T<10 K, where electrons are partially localized and $\tau_s > 100$ ns at zero bias), a marked collapse of $\tau_s$ is observed when $E_x$ exceeds the donor impact ionization threshold at $\sim$10 V/cm. A steep increase in the concentration of warm delocalized electrons -- subject to Dyakonov-Perel spin relaxation -- accounts for the rapid collapse of $\tau_s$, and strongly influences electron spin transport in this regime.