Synthesis and characterization of Ge-core/a-Si-shell nanowires with conformal shell thickness deposited after gold removal for high-mobility p-channel field-effect transistors.

Ge-core/a-Si-shell nanowires were synthesized in three consecutive steps. Nominally undoped crystalline Ge nanowires were first grown using a vapor-liquid-solid growth mechanism, followed by gold catalyst removal in an etching solution and deposition of a thin layer of amorphous silicon on the nanowire surface using a chemical vapor deposition method. Catalyst removal is necessary to avoid catalyst melting during temperature increase prior to a-Si shell deposition. Field effect transistors based on Ge-core/a-Si-shell nanowires exhibited p-channel depletion-mode characteristics as a result of free hole accumulation in the Ge channel. Scaled on-currents and transconductances up to 3.1 mA µm-1 and 4.3 mS µm-1, respectively, as well as on/off ratios and field-effect hole mobilities up to 102 and 664 cm2 V-1 s-1, respectively, were obtained for these Ge-core/a-Si-shell nanowire FETs. The minimum subthreshold slope was measured to be 300 mV dec-1. The present work also demonstrates for the first time the conductance quantization in one-dimensional Ge-core/a-Si-shell nanowires at low temperatures. The quantization of conductances at discrete values of G 0 = 2e 2/h at low temperatures suggests that our Ge-core/a-Si-shell nanowires are multi-mode ballistic conductors with a mean-free-path up to 500 nm. The results provided here are relevant for the synthesis of high-quality Ge-core/Si-shell nanowires for high-mobility devices with transparent contacts to hole carriers.

Germanium nanowires with 3-nm-diameter prepared by low temperature vapour-liquid-solid chemical vapour deposition.

We report the growth of germanium nanowires (Ge NWs) with single-step temperature method via vapour-liquid-solid (VLS) mechanism in the low pressure chemical vapour deposition (CVD) reactor at 300 degrees C, 280 degrees C, and 260 degrees C. The catalyst used in our experiment was Au nanoparticles with equivalent thicknesses of 0.1 nm (average diameter approximately 3 nm), 0.3 nm (average diameter approximately 4 nm), 1 nm (average diameter approximately 6 nm), and 3 nm (average diameter approximately 14 nm). The Gibbs-Thomson effect was used to explain our experimental results. The Ge NWs grown at 300 degrees C tend to have tapered structure while the Ge NWs grown at 280 degrees C and 260 degrees C tend to have straight structure. Tapering was caused by the uncatalysed deposition of Ge atoms via CVD mechanism on the sidewalls of nanowire and significantly minimised at lower temperature. We observed that the growth at lower temperature yielded Ge NWs with smaller diameter and also observed that the diameter and length of Ge NWs increases with the size of Au nanoparticles for all growth temperatures. For the same size of Au nanoparticles, Ge NWs tend to be longer with a decrease in temperature. The Ge NWs grown at 260 degrees C from 0.1-nm-thick Au had diameter as small as approximately 3 nm, offering an opportunity to fabricate high-performance p-type ballistic Ge NW transistor, to realise nanowire solar cell with higher efficiency, and also to observe the quantum confinement effect.