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.