Origin of 3.45 eV Emission Line and Yellow Luminescence Band in GaN Nanowires: Surface Microwire and Defect

The physical origin of the strong emission line at 3.45 eV and broadening yellow luminescence (YL) bard centered at 2.2 eV in GaN nanowire (NW) has been debated for many years. Here, we solve these two notable issues by using state-of-the-art first-principles calculations based on many-body perturbation theory combined with polarization-resolved experiments. We demonstrate that the ubiquitous surface microwires with amazing characteristics, i.e., the outgrowth nanocrystal along the NW side wall, are vital and offer a new perspective to provide insight into some puzzles in epitaxy materials. Furthermore, inversion of the top valence bands, in the decreasing order of crystal-field split-off hole (CH) and heavy/light hole, results in the optical transition polarized along the NW axis due to quantum confinement. The optical emission from bound excitons localized around the surface microwire to CH band is responsible for the 3.45 eV line with El c polarization. Both gallium vacancy and carbon-related defects tend to assemble at the NW surface layer, determining the broadening YL band.