Spin-photodiodes for SiGe spin-optoelectronics

We report on spin-photodiodes based on fully epitaxial Fe/MgO/Ge(001) heterostructures for room temperature integrated detection of light helicity at 1300 nm and 1550 nm wavelengths. The degree of circular polarization of light determines the spin direction of photo-carriers in Ge that are filtered by the Fe/MgO analyzer. Spin-detection experiments are performed by measuring the photocurrent while illuminating the spin-photodiodes with left or right circularly polarized light, under the application of a magnetic field parallel to the light direction which drives the Fe magnetization out of plane. We found that the spin-photodiodes spin filtering asymmetry is reduced by ∼40% in forward bias and by less than 15% in reverse bias, when increasing the photon wavelength from 1300 nm to 1550 nm. This result, apparently counterintuitive because of the larger spin polarization of the photo-carriers generated at 1550 nm with respect to that at 1300 nm, is explained in terms of the different spatial profile of carrier generation inside Ge. The larger penetration depth of light at 1550 nm leads to a smaller polarization of photocarriers when they reach the MgO tunneling barrier, due to the more efficient spin relaxation during transport.