The role of composition and diameter in the crystal purity of InAs_<i>x</i>P_1−<i>x</i> nanowires

Ternary InAs_<i>x</i>P_1-<i>x</i> nanowires are widely considered promising building blocks for fundamental studies and applications in nano- and opto-electronics. However, it is admittedly challenging to maintain the necessary control over the crystal purity of nanowires. Furthermore, the crystal phase trends in ternary III-V nanowires remain generally unknown. In this work, we present wurtzite InAs_<i>x</i>P_1-<i>x</i> nanowires with different compositions <i>x</i>, ranging from 0 to 0.54, grown <i>via</i> chemical beam epitaxy on InP(111)B substrates using Au colloidal catalysts of different diameters. Transmission electron microscopy studies reveal that a pure wurtzite phase of the nanowires requires larger InAs fractions <i>x</i> for larger diameters of Au colloids (in particular, <i>x</i> = 0.36 for 20 nm diameter and <i>x</i> = 0.54 for 30 nm diameter Au colloids). We developed a model for the critical composition of thin vapor-liquid-solid III-V ternary nanowires <i>x</i>_c corresponding to the transition from a polytypic structure at <i>x</i> < <i>x</i>_c to a pure wurtzite structure at <i>x</i> > <i>x</i>_c. For the InAs_<i>x</i>P_1-<i>x</i> material, the critical composition increases with the nanowire diameter, which explains our experimental findings. These results shed more light on the composition- and diameter-dependent polytypism, offering valuable insights for the design and crystal phase control of InAs_<i>x</i>P_1-<i>x</i> and other III-V ternary nanowires.