Thermal Stability at the Interface between Ferromagnetic Alloys and Germanium for Semiconductor Spintronics Devices
Authors: Yamada Michihiro , Matsuoka Teppei , Ueno Tokio , Naito Takahiro , Yamada Shinya , Sawano Kentarou , Hamaya Kohei 
Published: December 2020

Spin MOSFET has drawn much attention as an attractive candidate for future semiconductor devices because of its nonvolatility and reconfigurability [1]. For realization of the spin MOSFET, highly efficient spin injection from ferromagnets to semiconductors with low parasitic resistance is one of the most important key technologies. Recently, we demonstrated room-temperature spin transport in germanium (Ge) based lateral spin valve devices with Schottky-tunnel contact, where the interface resistance at the ferromagnet/Ge is much smaller than that with conventional insulating tunnel barriers [2, 3]. However, when the thermal annealing was conducted to fabricate gate-stack structures for the spin MOSFET, the spin injection/detection efficiency was degraded due to the formation of reaction layers at the ferromagnetic alloys/Ge interface [4]. In this study, we explore the enhancement in the thermal stability of the ferromagnet/Ge interface and demonstrate room-temperature spin signals even for the lateral spin device annealed at 250 oC by an insertion of five Fe atomic layers between the ferromagnetic alloys and Ge.
We fabricated lateral spin valve devices with Co2FeAl0.5Si0.5(CFAS)/Fe (5 atomic-layers)/Ge Schottky tunnel junction [Fig. 1(a)]. A 70-nm-thick phosphorus (P) doped Ge layer was grown on Ge/Si(111) virtual substrate at 350 oC as a spin transport layer by molecular beam epitaxy (MBE). On the P doped Ge layer, five Fe atomic layers and an epitaxial CFAS layer were formed using low temperature MBE. Beneath the ferromagnetic contacts, the P δ-doping with a thin Si layer was inserted to promote the tunnel conduction. The CFAS/Fe and δ-doped Ge layers were patterned into the contacts with 0.4 × 5.0 μm2 and 1.0 × 5.0 μm2, where the edge-to-edge distances between the CFAS/n+-Ge contacts are designed to be 0.45 μm. As a reference sample, a device without the Fe atomic layers was also fabricated. The devices with and without the Fe atomic layers were annealed at 250 oC in N2 atmosphere.
Figures 1(b) and (c) show the nonlocal spin signals at 77 K for devices annealed at 250 oC without and with an Fe insertion, respectively, together with schematics of the CFAS/Ge and CFAS/Fe/Ge interfaces after the annealing. For the device without an Fe insertion [Fig. 1(b)], the spin signal is significantly degraded (~97 %) compared with the as prepared device, where the Ge out-diffusion causes the degradation of the spin injection/detection efficiency [4]. On the other hand, even after the annealing, clear hysteretic curves related to the magnetization states of the ferromagnetic injector and detector contacts can be observed for the device with an insertion of Fe atomic layers [Fig. 1(c)]. Although there is ~50 % decrease of the spin signal by the annealing, the magnitude of nonlocal spin signals is ~ 140 mΩ due to the significant improvement of the spin injection efficiency [5] and of the thermal stability of the CFAS/Fe/Ge interface. As shown in schematics of Figs 1(b) and 1(c), the inserted Fe atomic layers can suppress the Ge out-diffusion into CFAS and maintain the highly efficient spin injection. For the device with an Fe atomic insertion annealed at 250 oC, a spin signal of ~7 mΩ can be obtained even at room temperature.
This work was partly supported by Grants-in-Aid for Scientific Research (A) (No. 16H02333) and (S) (No. 17H06120 and 19H05616) from the Japan Society for the Promotion of Science (JSPS). M.Y. acknowledges JSPS Research Fellowships for Young Scientists (No.18J00502).
[1] S. Sugahara and M. Tanaka, Appl. Phys. Lett. 84, 2307 (2004).
[2] M. Yamada et al., Appl. Phys. Express 10, 093001 (2017).
[3] M. Tsukahara et al., Appl. Phys. Express 12, 033002 (2019).
[4] B. Kuerbanjiang et al., Phys. Rev. B 98, 115304 (2018).
[5] M. Yamada et al., (submitted).

Figure 1