Undoped and catalyst-free germanium nanowires for high-performance p-type enhancement-mode field-effect transistors
Nominally undoped Ge nanowires were synthesized by a two-step growth method in a low-pressure chemical vapour deposition reactor using a gold nanoparticle-mediated vapour–liquid–solid growth mechanism. The as-grown nanowires were treated by dipping them in an HCl-containing aqueous triiodide solution to remove the gold from the tips and sidewalls of the nanowires. Top-gated field-effect transistors (FETs) fabricated from catalyst-free Ge nanowires (diameters of 20–70 nm and channel lengths of 300–1110 nm) exhibited p-type characteristics as a result of hole accumulation. The gate-modulated hole concentration was calculated to be on the order of 1018 cm−3. On-currents, on/off ratios, transconductances, and field-effect hole mobilities up to 0.29 mA μm−1, 520, 0.44 mS μm−1, and 478 cm2 V−1, respectively, as well as subthreshold slopes of 385 to 580 mV per decade were extracted from these Ge nanowire FETs. These Ge nanowire FETs are expected to exhibit faster intrinsic device speed or gate delay than planar Si p-MOSFETs as the gate length is scaled down to nanometer dimensions. This work provides the first experimental demonstration of p-type enhancement-mode FETs fabricated from undoped and catalyst-free Ge nanowires.