Enhancement of Room-Temperature Effective Spin Diffusion Length in a Si-Based Spin MOSFET With an Inversion Channel

We have demonstrated a Si-based bottom-gate-type spin metal-oxide-semiconductor field-effect transistor (spin MOSFET) with an electron inversion channel at room temperature and showed the enhancement of the room-temperature effective spin diffusion length by “spin drift”. Our spin MOSFET was fabricated on a (001)-oriented silicon-on-insulator (SOI) substrate with a p-type ultrathin (8 nm) Si layer, in which the channel length was 1.0 μm, the spin injector/detector electrodes were ferromagnetic multilayer (from top to bottom) Fe(4nm)/Mg(1nm)/MgO(1nm) tunnel junctions, and a 200-nm-thick buried oxide layer was used for the gate dielectric. Using various gate electric fields and source-drain electron currents, two types of spin-dependent transport signals were measured: spin-valve signals with an inplane magnetic field and Hanle signals with an out-of-plane magnetic field. Clear Hanle signals and spin-valve signals were obtained for various bias conditions. We analyzed the Hanle signals and the change of the spin-valve signals based on our original formulas that take into account the distribution of the lateral electric field along the electron transport, and revealed that the spin drift can enhance the effective spin diffusion length by the lateral electric field parallel to the electron transport in the inversion channel. The effective spin diffusion length becomes 3-13 times larger than the intrinsic spin diffusion length λ
_S
= 0.89 μm owing to the increase in the lateral electric field. It was confirmed that the spin drift is very useful to achieve larger spin-valve signals in spin MOSFETs with an electron inversion channel.