  Hybrid semiconductor-superconductor InAs-Al nanowires with uniform and
defect-free crystal interfaces are one of the most promising candidates used in
the quest for Majorana zero modes (MZMs). However, InAs nanowires often exhibit
a high density of randomly distributed twin defects and stacking faults, which
result in an uncontrolled and non-uniform InAs-Al interface. Furthermore, this
type of disorder can create potential inhomogeneity in the wire, destroy the
topological gap, and form trivial sub-gap states mimicking MZM in transport
experiments. Further study shows that reducing the InAs nanowire diameter from
growth can significantly suppress the formation of these defects and stacking
faults. Here, we demonstrate the in situ growth of ultra-thin InAs nanowires
with epitaxial Al film by molecular-beam epitaxy. Our InAs diameter (~ 30 nm)
is only one-third of the diameters (~ 100 nm) commonly used in literatures. The
ultra-thin InAs nanowires are pure phase crystals for various different growth
directions, suggesting a low level of disorder. Transmission electron
microscopy confirms an atomically sharp and uniform interface between the Al
shell and the InAs wire. Quantum transport study on these devices resolves a
hard induced superconducting gap and $2e^-$ periodic Coulomb blockade at zero
magnetic field, a necessary step for future MZM experiments. A large zero bias
conductance peak with a peak height reaching 80% of $2e^2/h$ is observed.
