  At low temperatures, electrons in semiconductors are bound to shallow donor
impurity ions, neutralizing their charge in equilibrium. Inelastic scattering
of other externally-injected conduction electrons accelerated by electric
fields can excite transitions within the manifold of these localized states.
Promotion of the bound electron into highly spin-orbit-mixed excited states
drives a strong spin relaxation of the conduction electrons via exchange
interactions, reminiscent of the Bir-Aronov-Pikus process where exchange occurs
with valence band hole states. Through low-temperature experiments with silicon
spin transport devices and complementary theory, we reveal the consequences of
this previously unknown spin depolarization mechanism both below and above the
impact ionization threshold.
