Single-crystalline Ni₂Ge/Ge/Ni₂Ge nanowire heterostructure transistors

In this study, we report on the formation of a single-crystalline Ni ₂Ge/Ge/Ni₂Ge nanowire heterostructure and its field effect characteristics by controlled reaction between a supercritical fluid-liquid-solid (SFLS) synthesized Ge nanowire and Ni metal contacts. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies reveal a wide temperature range to convert the Ge nanowire to single-crystalline Ni₂Ge by a thermal diffusion process. The maximum current density of the fully germanide Ni₂Ge nanowires exceeds 3.5 × 10⁷ A cm^-2, and the resistivity is about 88 μΩ cm. The in situ reaction examined by TEM shows atomically sharp interfaces for the Ni₂Ge/Ge/Ni₂Ge heterostructure. The interface epitaxial relationships are determined to be Ge[011̄] || Ni ₂Ge[01̄1] and Ge(11̄1̄) || Ni₂Ge(100). Back-gate field effect transistors (FETs) were also fabricated using this low resistivity Ni₂Ge as source/drain contacts. Electrical measurements show a good p-type FET behavior with an on/off ratio over 10³ and a one order of magnitude improvement in hole mobility from that of SFLS-synthesized Ge nanowire.