Spin accumulation in Fe/MgO/Si heterostructures*

We report on spin injection experiments at Fe/MgO/Si interfaces using all electrical injection and detection. MgO is a promising magnetic tunnel junction material, and its incorporation with Si-based spintronics has only recently been reported in degenerately doped Si (n $\sim 10^{20} cm^{-3})$ [1]. We focus here on spin accumulation under the injecting contact for much lower n-doping levels by measuring the Hanle effect in a standard 3-terminal scheme where injection and detection are done using the same contact. The Fe/MgO spin injector was sputter deposited onto various n-doped Si bulk substrates using a variety of different substrate temperatures. The best tunnel barriers were obtained when the MgO was deposited at 70$^{\circ}$C and annealed $in-situ$ before Fe deposition. Fits to Hanle curves using the drift-diffusion model for Si samples with n=$4x10^{18} cm^{-3}$ yield spin lifetimes $\tau_{s}$ = 0.28 ns up to 30 K and a spin diffusion length L$_{s}$=$\sqrt{D\tau_{s}}$ of 0.65 $\mu$m (the diffusion constant D is obtained from the mobility assuming degenerate statistics). We determine the dependence on n, and comment on the potential differences between SOI and bulk Si wafer transport channels. [1] T. Sasaki, et al., Appl. Phys. Exp. 2 (2009).