Oxide-confined formation of germanium nanowire heterostructures for high-performance transistors

Over the past several years, the formation of nanowire heterostructures via a solid-state reaction between a semiconductor nanowire and metal contact pads has attracted great interest. This is owing to its ready application in nanowire field-effect transistors (FETs) with a well-controlled channel length using a facile rapid thermal annealing process. We report the effect of oxide confinement on the formation of Ge nanowire heterostructures via a controlled reaction between a vapor-liquid-solid-grown, single-crystalline Ge nanowire and Ni pads. In contrast to the previous formation of Ni₂Ge/Ge/Ni ₂Ge nanowire heterostructures, a segment of high-quality epitaxial NiGe was formed between Ni₂Ge and Ge with the confinement of Al ₂O₃ during annealing. Significantly, back-gate FETs based on this Ni₂Ge/NiGe/Ge/NiGe/Ni₂Ge heterostructure demonstrated a high-performance p-type transistor behavior, showing a large on/off ratio of more than 10⁵ and a high normalized transconductance of 2.4 μS/μm. The field-effect hole mobility was extracted to be 210 cm²/(V s). Temperature-dependent I-V measurements further confirmed that NiGe has an ideal ohmic contact to p-type Ge with a small Schottky barrier height of 0.11 eV. Moreover, the hysteresis during gate bias sweeping was significantly reduced after Al₂O₃ passivation, and our Ω-gate Ge nanowire FETs using Al₂O₃ as the top-gate dielectric showed an enhanced subthreshold swing and transconductance. Therefore, we conclude that the Al₂O₃ layer can effectively passivate the Ge surface and also serve as a good gate dielectric in Ge top-gate FETs. Our innovative approach provides another freedom to control the growth of nanowire heterostructure and to further achieve high-performance nanowire transistors.