The role of composition and diameter in the crystal purity of InAs_xP_1-x nanowires

Ternary InAs_xP_1-x 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_xP_1-x nanowires with different compositions x, ranging from 0 to 0.54, grown via 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 x for larger diameters of Au colloids (in particular, x = 0.36 for 20 nm diameter and x = 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 x_c corresponding to the transition from a polytypic structure at x < x_c to a pure wurtzite structure at x > x_c. For the InAs_xP_1-x 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_xP_1-x and other III-V ternary nanowires.