Spin transport and relaxation in germanium

Abstract

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.

Spin injection through energy-band symmetry matching with high spin polarization in atomically controlled ferromagnet/ferromagnet/semiconductor structures

“…The detailed fabrication procedure of the LSV devices is described in the Materials and methods. Additionally, similar LSV structures were presented in our previous works [15][16][17]20,21 . We note that the highly spin-polarized material CFAS is utilized as a spin injector and detector for Ge, where CFAS is one of the ferromagnetic Heusler alloys 15,16,22 .…”

“…Additionally, similar LSV structures were presented in our previous works [15][16][17]20,21 . We note that the highly spin-polarized material CFAS is utilized as a spin injector and detector for Ge, where CFAS is one of the ferromagnetic Heusler alloys 15,16,22 . To verify the effect of Fe atomic layer insertion on spin injection and detection, nonlocal and local voltage measurements were carried out on all LSV devices with insertion of various Fe atomic layers in the four-and two-terminal configurations [23][24][25][26] , respectively.…”

“…In this article, to simultaneously achieve highly efficient spin injection at room temperature and low contact resistance in SC devices, we present an efficient spin injection technique using an FM-FM-SC heterostructure, where the top FM electrode is a ferromagnetic Heusler alloy, Co 2 FeAl 0.5 Si 0.5 (CFAS), the middle FM is atomically controlled iron (Fe) layers, and the SC is germanium (Ge), which not only is a next-generation channel material for CMOS but also can be applied to photonics 13 and quantum computing 14 . Although we have thus far developed growth techniques for singlecrystalline CFAS layers on Ge 15 and Schottky-tunnel barrier electrodes with low values of the resistance area product (RA) for Ge spintronic devices 16 , the twoterminal MR ratio at room temperature has been quite low (≤0.001%) because of the low spin accumulation in Ge-based lateral spin-valve (LSV) devices 17 . When five-six atomic layers of Fe (Fe [5][6] ) are inserted between CFAS and Ge, we experimentally demonstrate salient enhancements in spin signals by more than one order of magnitude compared to those in the previous report on Ge 17 , in the LSV devices.…”

Study of Spin Transport and Magnetoresistance Effect in Silicon-based Lateral Spin Devices for Spin-MOSFET Applications

“…In this study, we used small sized LSV devices and obtained values of P < 0.2, estimated from NL measurements. With respect to this, we will use highly ordered ferromagnetic Heusler alloys as ferromagnetic electrodes to obtain large spin polarization, as reported in previous works [55][56] . Although we have so far fabricated Co2FeSi/MgO/Si LSV devices and observed the spin signals even at room temperature 19) , the MR ratio has been still small less than 0.01%.…”

“…Recently, Co-based Heusler alloy/Ge Schottky tunnel contacts with a low contact resistance of less than 0.5 kΩ µm 2 were simultaneously demonstrated for Ge-based LSV devices. 56) In future, we should also explore Co-based Heusler alloy/Si Schottky tunnel contacts for high MR ratios in Si-based LSV devices.…”