Single-Crystalline Germanium Nanowire Heterostructure for High-Performance Transistors and Spintronics*

The formation of single-crystalline Ni$_{2}$Ge/Ge/Ni$_{2}$Ge nanowire heterostructure and its field effect characteristics by controlled reaction between a Ge nanowire and Ni contacts were studied. Transmission electron microscopy (TEM) studies reveal a wide temperature range to convert the Ge nanowire to single-crystalline Ni$_{2}$Ge by a thermal diffusion process. The \textit{in-situ} reaction examined by TEM shows atomically sharp interfaces for the Ni$_{2}$Ge/Ge/Ni$_{2}$Ge heterostructure with good epitaxial matches of Ge[-110]//Ni$_{2}$Ge[0-11] and Ge(111)//Ni$_{2}$Ge(100). Field effect transistors (FETs) built on this nanowire heterostructure show a high-performance $p$-type FET behavior with an on/off ratio over 10$^{5}$ and a field-effect hole mobility of 210 cm$^{2}$/Vs. This nanowire heterostructure with atomically sharp interfaces opens an opportunity to achieve high-performance nanowire transistors and explore promising application in spintronics.