Facilitating growth of InAs–InP core–shell nanowires through the introduction of Al

InAs nanowires were grown on GaAs substrates by the Au-assisted vapour–liquid–solid (VLS) method in a gas source molecular beam epitaxy (GSMBE) 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, AlxIn1−xAs or AlxIn1−xP 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. Characterisation 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 dislocations.