Defect passivation by proton irradiation in ferromagnet-oxide-silicon junctions

Several recent measurements of magnetoresistance in ferromagnet/insulator/semiconductor tunnel junctions have led to controversial claims of spin accumulation, where the signal amplitude and derived spin relaxation time often deviates from theoretically calculated and experimentally confirmed values by several orders of magnitude. These discrepancies cast doubt on the physical origin of the measured magnetoresistance, which was initially attributed to spin precession and dephasing in the semiconductor. More recently, models incorporating transport through localized defects have shown that they can account for device behavior, without any spin accumulation. To directly investigate the role of localized states in this signal, we subject CoFe/SiO2/n-Si junctions to varying doses of proton irradiation. Weak radiation doses not only have little effect on the electrical current-voltage relationship of the junction but also modify the magnetoresistance substantially. Our interpretation of this phenomenon involves the hydrogen passivation of defects within the tunnel barrier, and is consistent with the emerging consensus that defects within the barrier play a crucial role in the physical mechanism behind junction magnetoresistance in this class of devices.