The experimental demonstration of high-performance p-type enhancement-mode field-effect transistors fabricated from undoped and catalyst-free germanium nanowires.

This paper describes the growth of germanium nanowires (Ge NWs) via vapor–liquid–solid (VLS) mechanism by the low-pressure chemical vapor deposition (CVD) technique. A systematic study of the growth conditions of the Ge NWs has been conducted by varying the size of the Au nanoparticles and the substrate temperature. The tapering of the nanowires has been minimised when the growth temperature is lowered from 300 to 280 °C which also contributes to the decrease in the diameter of the Ge NWs. The growth temperature of 280 °C yields Ge NWs with diameters of less than 5 nm, offering an opportunity for the fabrication of high-performance germanium nanowire field-effect transistors.

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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.

We have successfully passivated the surface of germanium nanowires (Ge NWs) using aluminum oxide (Al2O3) and hafnium oxide (HfO2). The atomic layer deposition (ALD) technique was used to deposit Al2O3 and HfO2. We observed an excellent interface between the nanowire surface and Al2O3 and exceptional uniformity of Al2O3 along the length of the nanowire. In the case of nanowires coated with HfO2, we found that a local crystallization of HfO2 may cause defects on the nanowire surface. We have fabricated devices using Ge NWs coated with ultra-thin Al2O3. The current through the nanowire increased after sequential annealing at 400 °C under a forming gas atmosphere. The current increase is ascribed to the diffusion of germanium into Al2O3 that affects the barrier thickness between the metal and the Al2O3/Ge NW. The ultra-thin Al2O3 serves to eliminate charge trapping and protect the germanium surface from ambient molecules and therefore alleviate hysteresis.