Investigating surface effects of GaN nanowires using confocal microscopy at below-band gap excitation

We analyze the microscopic origins of subgap photoexcitations of individual gallium nitride (GaN) triangular cross-section nanowires (NWs), which are highly photoactive over a broadband spectral range. Using confocal hyperspectral photoluminescence (PL) microscopy, mid-gap states on the NWs were excited using subgap illumination, resulting in two distinct PL spectra corresponding to the polar (0001) and the semipolar surfaces. Emission spectra are well represented by Gaussian functions with fitted centers of 1.99 +/- 0.01 eV and 2.26 +/- 0.01 eV, respectively. PL collected from the end facets exhibits interference fringes and a relative blue shift. Furthermore, the PL spectrum shifts strongly to the blue when the excitation intensity is increased. These observations are consistent with a qualitative model in which the PL results from excitation into a broad manifold of surface-associated states which are rapidly populated at a high excitation intensity and can couple to etalon modes via longitudinal photon emission.