  Ultrathin InAs nanowires (NW) with one-dimensional (1D) sub-band structure
are promising materials for advanced quantum-electronic devices, where
dimensions in the sub-30 nm diameter limit together with post-CMOS integration
scenarios on Si are much desired. Here, we demonstrate two site-selective
synthesis methods that achieve epitaxial, high aspect ratio InAs NWs on Si with
ultrathin diameters below 20 nm. The first approach exploits direct vapor-solid
growth to tune the NW diameter by interwire spacing, mask opening size and
growth time. The second scheme explores a unique reverse-reaction growth by
which the sidewalls of InAs NWs are thermally decomposed under controlled
arsenic flux and annealing time. Interesting kinetically limited dependencies
between interwire spacing and thinning dynamics are found, yielding diameters
as low as 12 nm for sparse NW arrays. We clearly verify the 1D sub-band
structure in ultrathin NWs by pronounced conductance steps in low-temperature
transport measurements using back-gated NW-field effect transistors. Correlated
simulations reveal single- and double degenerate conductance steps, which
highlight the rotational hexagonal symmetry and reproduce the experimental
traces in the diffusive 1D transport limit. Modelling under the realistic
back-gate configuration further evidences regimes that lead to asymmetric
carrier distribution and lifts of the degeneracy in dependence of gate bias.
