Fig. 1: Structural and optical properties of nanowire InGaN/GaN heterostructures.

a Schematic of the InGaN/GaN nanowire array arranged in a photonic crystal structure. The nanowires are enclosed by a thin layer of Al₂O₃ for surface passivation. The top of the nanowire array is covered by a thick SiO₂ layer for insulation during the device fabrication. b Top view SEM image of the as-grown InGaN/GaN PhC structure with lattice constant a of 360 nm and diameter d of 220 nm. Inset: the corresponding schematic for the reciprocal lattice of the nanowire PhC structure. c Cross-sectional STEM image of the active region of the InGaN/GaN heterostructure. The red-emitting InGaN SQW and the InGaN/GaN SPSL are depicted by red and green false color, respectively. d Photoluminescence spectra measured from the as-grown sample (orange curve), after Al₂O₃ passivation (green curve) and SiO₂ deposition (red curve). The evolution of the photonic crystal mode is indicated by the dashed rectangles. e Bandstructure of the nanowire photonic crystal structure without Al₂O₃ passivation (dash-dotted curves), with Al₂O₃ passivation (dashed curves) and with SiO₂ insulation (solid curve). The light lines of GaN and air are shown as magenta and blue lines. f Power-dependent PL measurements of the structure with SiO₂ and Al₂O₃ passivation over an excitation range of over two orders of magnitude