Germanium is one of the most appealing candidate for spintronic applications, thanks to its compatibility with the Si platform, the long electron spin lifetime and the optical properties matching the conventional telecommunication window. Electrical spin injection schemes have always been exploited to generate spin accumulations and pure spin currents in bulk Ge. Here, we use the spin-Hall effect, which is based on the spin-orbit coupling, to generate a uniform spin current into an epitaxial n-doped Ge channel. The electrically-induced spin accumulation, transverse to the injected charge current density, is equivalent to an effective magnetization that we detect using polar magneto-optical Kerr microscopy at low temperature.
We show that a large spin density up to 400 µm3 can be achieved at the edges of the 100-µm-wide Ge channel for an applied electric field lower than 5 mV/µm. We find that the spin density linearly decreases toward the center of the Ge bar, due to the large spin diffusion length, and such a decay is much slower than the exponential one observed in III-V semiconductors like GaAs. It allows a very large spin accumulations over a length scale of tens of micrometers. We also characterize the electrically-induced spin voltage as a function of the applied bias and temperature, revealing that the spin-to-charge conversion in bulk Ge is preserved and measurable up to 120 K.
These results lay the foundation for multi-terminal spintronic devices, where different spin voltages can be exploited as inputs for magneto-logic gates on the same Ge platform.
Collaborators : C. Zucchetti, F. Bottegoni, S. Dal Conte, J. Frigerio, E. Carpene, G. Isella, F. Ciccacci, G. Cerullo, and M. Finazzi, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy
Paper : F. Bottegoni et al., “Spin-Hall Voltage over a Large Length Scale in Bulk Germanium”, Phys. Rev. Lett. 118, 167402 (2017).