Spin transport and relaxation in germanium

This paper reviews the recent progress in germanium (Ge) spintronics on the basis of the electrical spin injection from ferromagnets (FM), where Ge is a next generation semiconductor for applications such as CMOS and optical communication on the silicon platform. In general, four-terminal nonlocal voltage measurements in FM–Ge lateral spin-valve devices are important to discuss the spin transport and spin relaxation in n-Ge. First, to obtain relatively low contact resistance compared to the FM/MgO/Ge contacts, we introduce the formation of high-quality FM/Ge heterointerfaces with a phosphorus δ-doped Ge layer, where the atomic arrangement matching at the interface between the (1 1 1) surface of body-centered cubic FMs and Ge(1 1 1) is important. Next, we explain electrical detections of the spin transport in degenerate n-Ge. Owing to the Heusler alloy/Ge Schottky-tunnel barrier contacts, we obtain relatively large spin signals compared to those detected by using conventional CoFe contacts. Furthermore, we can experimentally determine the spin diffusion length and the spin lifetime in degenerate n-Ge by quantitatively analyzing the contact-distance dependence of the spin signals and the Hanle-effect curves. Since we can clarify the temperature dependence of the spin lifetime from 8 to 296 K, the spin relaxation mechanism in n-Ge can be understood as a consequence of the intervalley spin-flip scattering in the conduction band. We propose an advantage over GaAs systems by comparing the spin lifetimes between Ge and GaAs at around room temperature. Finally, we describe future prospects of Ge spintronics, including vertically fabricated device structures such as vertical spin MOSFETs and spin LEDs. (topical review)