For CMOS-compatible spintronic devices, electrical spin injection and detection in silicon (Si) [1] and germanium (Ge) [2] have been explored. Due to the conductivity mismatch problem [3], the insulator tunnel barriers such as MgO [1,2] were generally used for electrical spin injection from ferromagnets (FM) into Si and Ge, where the resistance area products (RA) values of the tunnel contacts were approximately 10⁴ Ωμm² (10^-4 Ωcm²) [1,2]. However, these contacts with large RA values cannot be utilized as a source and drain for spin-based MOS transistors [4]. If spintronic technologies are integrated with Ge-CMOS and/or Ge-based photonics, novel spin-injection techniques with low- RA contacts should be developed.

In this paper, we have developed high-quality FM/n-Ge heterointerfaces with a delta-doped Ge interlayer for low-RA Schottky-tunnel contacts [5,6]. Because of the tunable doping concentration of the delta-doped layer, we can obtain the contacts with relatively low-RA values of ~10² Ωμm² (~10^-6 Ωcm²) for n-Ge. Using the Schottky-tunnel contacts, we can observe lateral transport of pure spin currents in n-Ge up to room temperature [7]. In addition, by using the low-RA contacts, the two-terminal magnetoresistance effect can be observed even at room temperature [8]. Although we should further reduce the RA value as low as possible, our results with the low-RA Schottky-tunnel contacts will open a road to integrate the spintronic technologies into next generation Ge-based devices on the Si platform.

[1] T. Suzuki et al., Appl. Phys. Exp. 4, 023003 (2011).

[2] Y. Zhou et al., Phys. Rev. B 84, 125323 (2011).

[3] G. Schmidt et al., Phys. Rev. B 62, R4790 (2000).

[4] M.Tanaka and S. Sugahara, IEEE Trans. Electron. Devices 54, 961 (2007).

[5] M. Yamada, KH et al., Mat. Sci. Semicon. Proc. 70, 83 (2017).

[6] Y. Fujita, KH et al., Phys. Rev. Appl. 8, 014007 (2017).

[7] M. Yamada, KH et al., Appl. Phys. Exp. 10, 093001 (2017).

[8] Y. Fujita, KH et al., (submitted).