Formation and device application of Ge nanowire heterostructures via rapid thermal annealing

We reviewed the formation of Ge nanowire heterostructure and its field-effect characteristics by a controlled reaction between a single-crystalline Ge nanowire and Ni contact pads using a facile rapid thermal annealing process. Scanning electron microscopy and transmission electron microscopy demonstrated a wide temperature range of 400∼500C to convert the Ge nanowire to a single-crystalline Ni₂Ge/Ge/Ni₂Ge nanowire heterostructure with atomically sharp interfaces. More importantly, we studied the effect of oxide confinement during the formation of nickel germanides in a Ge nanowire. In contrast to the formation of Ni ₂Ge/Ge/Ni₂Ge nanowire heterostructures, a segment of high-quality epitaxial NiGe was formed between Ni₂Ge with the confinement of Al₂O₃ during annealing. A twisted epitaxial growth mode was observed in both two Ge nanowire heterostructures to accommodate the large lattice mismatch in the NixGe/Ge interface. Moreover, we have demonstrated field-effect transistors using the nickel germanide regions as source/drain contacts to the Ge nanowire channel. Our Ge nanowire transistors have shown a high-performance p-type behavior with a high on/off ratio of 10⁵ and a field-effect hole mobility of 210cm²/Vs, which showed a significant improvement compared with that from unreacted Ge nanowire transistors.