High-performance Ge nanowire transistors with single-crystalline germanides as Schottky source/drain contacts were fabricated via the solid-state reaction between a single-crystalline Ge nanowire and two Ni contact pads using rapid thermal annealing. The formed high-quality germanides show atomically clean epitaxial interface with the Ge nanowire. The effect of oxide confinement was also studied to control the growth of nickel germanides, and further to passivate the Ge nanowire surface. In addition, a room-temperature ferromagnetic germanide, Mn5Ge3, was formed in the fabrication of Mn5Ge3/Ge/Mn5Ge3 nanowire transistors using a similar approach. Temperature-dependent I-V measurements were performed to extract a Schottky barrier height of 0.25 eV for Mn 5Ge3 conducting to p-Ge, which suggested promising spin injection from Mn5Ge3 into Ge nanowires. Our results open up exciting opportunities to fabricate high-performance Ge nanowire transistors and further explore spintronics applications in Ge nanowire heterostructures with high-quality epitaxial interfaces.