Fabrication of high-quality Co2FeSi0.5Al0.5/CoFe/MgO/Si spin injectors for Si-channel spin devices

In recent years, spin-MOSFETs [1] have received much attention for low-power and high-performance integrated circuits. Figure 1 shows the schematic device structure of a spin-MOSFET that employs half-metallic ferromagnet tunnel contacts for the source and drain. To realize spin-MOSFETs, highly efficient spin-injector/detector for the Si channel is required. Half-metallic full-Heusler alloy Co₂FeSi_0.5Al_0.5 (CFSA) is promising for a ferromagnetic electrode material owing to its high Curie temperature (>>RT) and extremely high spin polarization. Although CFSA has the completely ordered L2₁, partially ordered B2, and disordered A2 structures, its half-metallicity remains even for the partially disordered B2 structure [2]. This structural robustness is beneficial for device process of spin-MOSFETs. Recently, we demonstrated spin accumulation in a Si channel using a CFSA/MgO/Si tunnel contact (Fig. 2) [3]. The Hanle-effect signal for spin accumulation in the channel was successfully observed. However, its intensity was weak, i.e., the spin injection efficiency was low. The roughening of the CFSA/MgO interface was also observed for the tunnel contact, which could be the one of the origins of the low spin injection efficiency.