Growth of GaN free-standing nanowires by plasma-assisted molecular beam epitaxy: structural and optical characterization

This paper reports on the growth, structural and optical properties of GaN free-stranding nanowires synthesized in catalyst-free mode on Si(111) substrate by plasma-assisted molecular beam epitaxy. Cylindrical nanowires with a hexagonal cross-section defined by planes and diameters down to 20 nm were observed. The nanowire length increases as a function of their diameter, following the Gibbs–Thomson expression. The growth rate in the lateral direction was studied using thin AlN marker layers showing that the lateral over axial growth rate ratio can be tuned from ∼1% to ∼10% by changing the III/V flux ratio, with the lateral growth remaining homogeneous along the NW axis. Nanowire ensembles showed a strong near band edge photoluminescence up to room temperature. Low-temperature micro-photoluminescence from a single wire is peaked at 3.478 eV with broadening of 6–10 meV. This emission is similar to the luminescence of nanowire ensembles, which demonstrates strain homogeneity from wire to wire. The optical properties along the wire axis probed by micro-cathodoluminescence were found to be uniform, with no evidence of a higher defect density in the bottom part of the nanowires next to the Si substrate.