Wide-range optical spin orientation in Ge from near-infrared to visible light

Ge-based spin-photodiodes have been employed to investigate the spectral dependence of optical spin orientation in germanium, in the range 400–1550 nm. We found the expected maximum in the spin polarization of photocarriers for excitation at the direct gap in $\Gamma$ (1550 nm) and a second sizable peak due to photogeneration in the $L$ valleys (530 nm). Data suggest distinct spin depolarization mechanisms for excitation at $\Gamma$ and $L$, with shorter spin relaxation times whether the $X$ point is involved. These devices can be used as integrated photon-helicity detectors over a wide spectral range.

Figure 1

(a) Band structure of Ge from the $\Gamma$ to the $L$ point of the Brillouin zone (along the direction $\Lambda )$. (b) Sketch of a Fe/MgO/Ge spin photodiode.

Figure 2

(a) Photocurrent $I_{\mathrm{photo}}$ photogenerated by linearly polarized light vs the bias voltage $V_{\mathrm{bias}}$, for several values of the photon energy $h\nu$. (b) Helicity-dependent photocurrent $\Delta I$ due to the full reversal of the circular polarization of light. (c) Spin transport asymmetry $\left(A_{\mathrm{SF}}\right)$ after subtraction of the MCD contribution. Data are not shown in the region were $\Delta I$ is almost zero and $A_{\mathrm{SF}}$ diverges [shaded region of (c)].

Figure 3

Spin filtering asymmetry of spin PD's vs photon energy. Continuous and dashed lines represent fits obtained with the diffusive model presented in the text. (b) Calculated degree of optical spin orientation $P_S$ [13]. The light absorption length ${\lambda }_L$ [19] is compared with the spin-diffusion length $l_{\mathrm{SF}}$ values for carriers [9] excited around $\Gamma$ (open circles on the ${\lambda }_L$ curve).