  InAs nanowires were grown on GaAs substrates by the Au-assisted
vapour-liquid-solid (VLS) method in a gas source molecular beam epitaxy
(GS-MBE) system. Passivation of the InAs nanowires using InP shells proved
difficult due to the tendency for the formation of axial rather than core-shell
structures. To circumvent this issue, Al$_x$In$_{1-x}$As or Al$_x$In$_{1-x}$P
shells with nominal Al composition fraction of x = 0.20, 0.36, or 0.53 were
grown by direct vapour-solid deposition on the sidewalls of the InAs nanowires.
Characterization by transmission electron microscopy revealed that the addition
of Al in the shell resulted in a remarkable transition from the VLS to the
vapour-solid growth mode with uniform shell thickness along the nanowire
length. Possible mechanisms for this transition include reduced adatom
diffusion, a phase change of the Au seed particle and surfactant effects. The
InAs-AlInP core-shell nanowires exhibited misfit dislocations, while the
InAs-AlInAs nanowires with lower strain appeared to be free of defects.
