Transition from Metal-Rich to N-Rich Growth for Core-Shell InGaN Nanowires on Si (111) at the Onset of In Desorption

The growth of InGaN on Si in the regime slightly above the onset of In desorption naturally leads to the formation of core-shell InGaN nanowires (NWs) by plasma-assisted molecular beam epitaxy. The onset of In desorption is gradual and depends on the growth temperature, N flux, and metal fluxes. The dependence on the growth temperature has been reported before; here, the dependence on the N flux or N/metal flux ratio is the topic. Based on the analysis of InGaN layers directly grown on Si (111) substrates without substrate rotation, we present a detailed correlation of the morphological, structural, and optical properties close to the transition from metal-rich to N-rich growth conditions due to changing N flux across the wafer with very high precision. The different regions are characterized by scanning electron microscopy, energy-dispersive X-ray spectroscopy, atomic force microscopy, X-ray diffraction, and photoluminescence spectroscopy. In desorption for the In-poor InGaN NW shell formation is reduced by increasing N flux, while the In-rich core is generally much less affected by In desorption with a stable, high In content, confirming the previous growth model. Good optical quality requires a N flux a few percent higher than the metal flux, highlighting the importance of the transition from metal-rich to N-rich growth as a crucial reference for realizing high-quality core-shell InGaN NWs.