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We develop a low-temperature growth technique of epitaxial Ge layers on a body-centered-cubic ferromagnetic metal (bcc-FM), Fe 3 Si, by combining solid phase epitaxy (SPE) and molecular beam epitaxy (MBE). The insertion of an SPE-grown Ge layer enables two-dimensional epitaxial growth of Ge layers by MBE even at a growth temperature of ∼175 • C. Thanks to the relatively flat surface of the Ge epilayers, we can obtain an epitaxial CoFe (bcc-FM) layer on top of the Ge layers, leading to the all-epitaxial CoFe/Ge/Fe 3 Si trilayer with a reasonable magnetization reversal process. We believe that the all-epitaxial CoFe/Ge/Fe 3 Si trilayer has great potential to be utilized as novel vertical-type Ge-channel spintronic devices.

We experimentally show spin transport up to 125 K in a ∼40-nm-thick p-Ge(111) layer, epitaxially grown on a ferromagnetic Fe3Si. From the magnitude of the spin signals, the spin diffusion length of the p-Ge(111) layer at 10 K can be estimated to be approximately 50 nm. To understand the detectable spin transport in the p-Ge(111), we should consider the energy splitting between heavy-hole and light-hole bands at the L point.

We demonstrate low-temperature growth of all-epitaxial Co2FeSi/Ge/Co2FeSi trilayer structures by developing Sn-induced surfactant-mediated molecular beam epitaxy (SMBE) of Ge on Co2FeSi. Despite the growth of a semiconductor on a metal, we verify that the inserted Sn monolayers between Ge and Co2FeSi enable to promote the 2D epitaxial growth of Ge up to 5 nm at a TG of 250 °C. An understanding of the mechanism of the Sn-induced SMBE leads to the achievement of all-epitaxial Co2FeSi/Ge/Co2FeSi trilayer structures with spin-valve-like magnetization reversals. This study will open a way for vertical-type and high-performance Ge-based spintronics devices.

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