Structural Investigation of InAs-AlInAs and InAs-AlInP Core-Shell Nanowires

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.