The application of group-IV based metallic-nanodots (NDs) such as silicide- and germanide-NDs to a floating gate in MOS memories has been attracting much attention as charge retention characteristics can be improved and they can be compatible with Si-based semiconductor devices [1]. By replacing nonmagnetic NDs with ferromagnetic NDs, floating gate MOS memories can have the new functionality to control charging and discharging properties in NDs floating gate and to improve retention characteristics by the tunnel magnetoresistance effect between NDs and ferromagnetic gate electorodes [2]. Recently, Fe₃Si-NDs have received much attention as the promising materials for application to the Si-based spintronics and/or magnetic memory devices such as magnetoresistive random access memory [3]. Previously, we demonstrated the self-assembling formation of metal (Ni, Pt, Pd, Co and Fe) NDs on thermally grown SiO₂ with an areal density as high as ~10¹² cm^-2 by simply exposing ultrathin metal films to remote H₂ plasma (H₂-RP) without external heating [4]. In this work, we extended our research to form high density Fe₃Si-NDs and to study their magnetization and local I-V characteristics measured with a magnetic probe under external magnetic field application.

After conventional wet-chemical cleaning steps for p-type Si(100) wafers, a ~3.6nm-thick SiO₂ layer was grown at 850 ˚C. After that, a ~2.0nm-thick Fe layer was first deposited uniformly on the SiO₂ layer by electron beam evaporation, and then uniformly with a ~2.0nm-thick amorphous Si (a-Si) layer was covered without air exposure. Subsequently, a ~1.5nm-thick Fe layer was deposited, where the film thicknesses were designed with stoichiometric ratio of Fe:Si=3:1. The Fe/a-Si/Fe trilayer stack was exposed to a H₂-RP without external heating. By exposing the trilayer stack to the H₂-RP, the formation of NDs with an areal density as high as ~5×10¹¹ cm^-2 and with an average height of ~10 nm was observed. Considering the fact that the temperature was raised up to ~480 ˚C in the case of a Fe foil during H₂-RP exposure, this result suggests that Fe-Si alloying was promoted with agglomeration by cohesive action of Fe and Si atoms on the SiO₂ surface during the H₂-RP exposure. In addition, by X-ray diffraction and magnetization measurements, the formation of DO₃-orderd NDs was confirmed.

To evaluate the local electron transport properties of the DO₃-orderd Fe₃Si-NDs, a CoPtCr-coated Si cantilever was contacted with the sample surface and kept at the same position, and the I-V characteristics were measured at room temperature, where electrons are injected from the tip. Without any external magnetic field application, the current level was slightly increased with tip bias over ~-3.5 V. When applying a magnetic field of 0.5 kOe in the same direction as the tip magnetization, a distinct increase in the current level was detected, namely, the threshold voltage which is defined as a bias voltage at 0.1 nA was decreased from -4.0 to -0.3 V. With an increase in the magnetic field over 0.6 kOe, no further changes in the I-V curve were observable, which means that the resistance was kept at a significantly decreased level under magnetic fields of 0.5k Oe and over. When the CoPtCr-coated tip was replaced by a Rh-coated tip, almost no change was detected in the I-V curve by the application of a magnetic field of 4.5 kOe, irrespective of the field direction. We also confirmed a remarkable decrease in the current level and an almost complete recovery to the initial current level by applying a magnetic field of 0.5 kOe in the opposite direction to the first magnetization at 0.5kOe. Noted that, when the magnetic field of 0.5 kOe was applied in the same direction again, remarkable increase in the current level was observed. Here, the magnetization direction of the CoPtCr-coated tip is not affected by the magnetic field of 0.5 kOe. Therefore, the observed increase and decrease in the current level, namely, the change in the magnetoresistance can be interpreted in terms that the magnetization of the Fe₃Si-NDs was aligned in parallel or antiparallel to that of the CoPtCr-coated tip by the external magnetic field.

References

[1] F. M. Yang et al., Thin Solid Films, 516, 360 (2007).

[2] T. Sakaguchi et al., Jpn. J. Appl. Phys., 43, 2203 (2004).

[3] Y. Nakamura et al., Thin Solid Films, 519, 8512 (2011).

[4] K. Makihara et al., Jpn. J. Appl. Phys., 47, 3099 (2008).