Instantaneous growth of single monolayers as the origin of spontaneous core-shell InxGa1-xN nanowires with bright red photoluminescence.

Increasing the InN content in the In_xGa_1-xN compound is paramount for optoelectronic applications. It has been demonstrated in homogeneous nanowires or deliberately grown nanowire heterostructures. Here, we present spontaneous core-shell In_xGa_1-xN nanowires grown by molecular beam epitaxy on Si substrates at 625 °C. These heterostructures have a high InN fraction in the cores around 0.4 and sharp interfaces, and exhibit bright photoluminescence at 650 nm. The surprising effect of material separation is attributed to the periodically changing environment for instantaneous growth of single monolayers on top of nanowires. Due to a smaller collection length of N adatoms, each monolayer nucleates under a balanced V/III ratio, but then continues under highly group III rich conditions. As a result, the miscibility gap is suppressed in the cores but remains in the shells. These results provide a simple method for obtaining high-quality InGaN heterostructures emitting in the extended wavelength range.