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

Abstract

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 G₀ = 2e²/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.