Bias-Dependent Electron Spin Lifetimes in n-Type GaAs and the Role of Donor Impact Ionization

We present a study of electron spin lifetimes $\tau_s$ in n-type GaAs as a function of applied lateral electrical bias [1]. Using the magneto- optical Kerr effect, $\tau_s$ is obtained from Hanle depolarization measurements on n-GaAs epilayers doped near the metal-insulation transition ($n_e\sim 0.4 - 5.0$ x $10^{16} $ cm$^{-3}$) . Below 10 K, we observe that applied electric fields larger than 10 V/cm result in a large and sudden decrease of $\tau_s$. This collapse results from impact ionization of the cold donor-bound electrons into free electron states that are subject to Dyakonov-Perel spin relaxation. The effect is less dramatic at higher temperatures and at higher doping concentrations above the metal- insulator transition, where most electrons are delocalized even in the absence of an applied electrical bias. The collapse of $\tau_s$ is shown to strongly influence lateral spin transport in the impact ionization regime. [1] M. Furis, D. L. Smith, S. A. Crooker, and J. L. Reno, Appl. Phys. Lett. \textbf{89}, 102102 (2006)