| Description |
Gallium nitride (GaN) is an increasingly important semiconductor material showing promising properties for nanomaterial applications. Here the microscopic origins of highly photoactive visible spectra resulting from individual gallium nitride (GaN) triangular crosssection nanowires (NWs) are analyzed with sub- and above-gap photoexcitation using time-resolved measurements, photoluminescence excitation spectroscopy, hyperspectral overlapping and nonoverlapping confocal microscopy, and atomic force microscope (AFM) measurements. Time-dependent measurements show longer lifetimes for electrons producing redder emitted wavelengths and for higher laser excitation energies. Additionally, the spectrum shifts strongly to the blue when the excitation intensity is increased. These observations are consistent with a qualitative model in which the photoluminescence results from excitation into a broad manifold of surface or defect-associated states which are rapidly populated at high excitation intensity and strongly coupled to each other via nonradiative relaxation. This manifold of defect states may contain the oft-cited GaN green and yellow luminescence bands as wavelength dependent data suggest that the fitted amplitudes of these bands decay over time. Confocal measurements reveal delocalized emission along the entire NW with spectral fringes often observed at the NW ends, corresponding to etalon modes. Nonoverlapping confocal measurements and Lumerical simulations suggest that laser excitation photons couple into the NW cavity at the NW ends, producing strong localized fluorescence along the entire length. Fluorescence is also coupled into the cavity where the geometry of the tapered NW effectively filters the emission spectra. These observations suggest that GaN NWs could be used as a nanodirectional spectral filter. |
