Experimental Demonstration of Room-Temperature Spin Transport in $n$-Type Germanium Epilayers

We report an experimental demonstration of room-temperature spin transport in $n$-type Ge epilayers grown on a Si(001) substrate. By utilizing spin pumping under ferromagnetic resonance, which inherently endows a spin battery function for semiconductors connected with a ferromagnet, a pure spin current is generated in the $n\text{−}\mathrm{Ge}$ at room temperature. The pure spin current is detected by using the inverse spin-Hall effect of either a Pt or Pd electrode on $n\text{−}\mathrm{Ge}$. From a theoretical model that includes a geometrical contribution, the spin diffusion length in $n\text{−}\mathrm{Ge}$ at room temperature is estimated to be 660 nm. Moreover, the spin relaxation time decreases with increasing temperature, in agreement with a recently proposed theory of donor-driven spin relaxation in multivalley semiconductors.

Figure 1

A schematic illustration of (a) the spin transport experiment in the $\mathrm{Py}/n\text{−}\mathrm{Ge}/{\mathrm{Me}}^{\mathrm{SOI}}$ device, the (b) $\mathrm{Py}/n\text{−}\mathrm{Ge}/\mathrm{Cu}$ and (c) $\mathrm{Cu}/n\text{−}\mathrm{Ge}/\mathrm{Py}/n\text{−}\mathrm{Ge}/\mathrm{Pd}$ samples used in the control experiments, and the (d) $\mathrm{Py}/{\mathrm{SiO}}_2$ samples used for the spin current density estimation.

Figure 2

(a) FMR signal $dI/dH$ dependence on the in-plane external magnetic field $\mathbf{H}$ for the $\mathrm{Py}/n\text{−}\mathrm{Ge}/\mathrm{Pd}$ (black line) and $\mathrm{Py}/{\mathrm{SiO}}_2$ (red line) samples at ${\theta }_H=0°$. $H_{\mathrm{FMR}}$ and $I$ denote the resonance field and the microwave absorption intensity, respectively. (b) The electromotive force $V$ detected from the Pd strip dependence on the in-plane external magnetic field $\mathbf{H}$ for the $\mathrm{Py}/n\text{−}\mathrm{Ge}/\mathrm{Pd}$ sample at ${\theta }_H=0°$. Open circles denote the experimental data, and the colored lines show the fitting result.

Figure 3

The electromotive force $V$ detected at different microwave powers from the Pt strip of the $\mathrm{Py}/n\text{−}\mathrm{Ge}/\mathrm{Pt}$ sample under FMR for the two external magnetic field $\mathbf{H}$ orientations (a) ${\theta }_H=0°$ and (b) ${\theta }_H=180°$. (c),(d) Microwave power dependence of $V_{\mathrm{ISHE}}$ (filled circles) and $V_{\text{asym}}$ (empty circles) contributions to the electromotive force $V$ detected from the ${\mathrm{Me}}^{\mathrm{SOI}}$ strip of the $\mathrm{Py}/n\text{−}\mathrm{Ge}/{\mathrm{Me}}^{\mathrm{SOI}}$ device, where the solid lines denote the linear fit. (c) ${\mathrm{Me}}^{\mathrm{SOI}}=\mathrm{Pt}$. (d) ${\mathrm{Me}}^{\mathrm{SOI}}=\mathrm{Pd}$.

Figure 4

The FMR signal $dI/dH$ dependence on the in-plane external magnetic field $\mathbf{H}$ for $\mathrm{Py}/n\text{−}\mathrm{Ge}/\mathrm{Cu}$ for (a) ${\theta }_H=0°$ and (b) ${\theta }_H=180°$, and for $\mathrm{Cu}/n\text{−}\mathrm{Ge}/\mathrm{Py}/n\text{−}\mathrm{Ge}/\mathrm{Pd}$ for (c),(e) ${\theta }_H=0°$ and (d),(f) ${\theta }_H=180°$. The electromotive force $V$ detected under FMR from the Cu strip of the $\mathrm{Py}/n\text{−}\mathrm{Ge}/\mathrm{Cu}$ sample (g) for ${\theta }_H=0°$ and (h) ${\theta }_H=180°$, from the Cu strip of the $\mathrm{Cu}/n\text{−}\mathrm{Ge}/\mathrm{Py}/n\text{−}\mathrm{Ge}/\mathrm{Pd}$ sample (i) for ${\theta }_H=0°$ and (j) ${\theta }_H=180°$, from the Pd strip of the $\mathrm{Cu}/n\text{−}\mathrm{Ge}/\mathrm{Py}/n\text{−}\mathrm{Ge}/\mathrm{Pd}$ sample (k) for ${\theta }_H=0°$ and (l) ${\theta }_H=180°$.

Figure 5

Temperature dependence of the (a) spin diffusion length and (b) spin relaxation time of electrons in the $n\text{−}\mathrm{Ge}$ channel normalized by RT values. The red line is an inverse square root fitting.