Investigating surface effects of GaN nanowires using confocal microscopy at below-band gap excitation
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Nicholas J. Borys Department of Physics & Astronomy, University of Utah, Salt Lake City, Utah 84112, USA; and Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
Tevye R. Kuykendall, P. James Schuck, and Shaul Aloni Molecular Foundry, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
Jordan M. Gertona) Department of Physics & Astronomy, University of Utah, Salt Lake City, Utah 84112, USA (Received 2 April 2017; accepted 18 August 2017)
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 ð1101Þ/ð1101Þ surfaces. Emission spectra are well represented by Gaussian functions with fitted centers of 1.99 6 0.01 eV and 2.26 6 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.
I. INTRODUCTION
Gallium nitride (GaN) has garnered significant attention in recent years for its wide use as a semiconductor material and its low density of defects.1 GaN nanowires (NWs),2 nanoparticles3 and nanotubes4 have various applications within optoelectronic devices,5,6 white LEDs,7 and directional white light sources.8 Variations on NW structures such as core/shell structures, p- and n-type doping, and NWs with incorporated quantum dots display a wealth of PL behaviors.9–12 GaN has also been suggested for use as a catalyst in water splitting using bulk, nanomaterials, and even specific GaN surfaces.2,10,13–17 The use of GaN surfaces for photocatalytic processes is especially attractive since photo-excited carriers are created at the surface, making them readily available to interact with the surrounding medium.18 Many studies of GaN PL over the years have utilized excitation at or above the band gap (3.4 eV), producing band-edge luminescence and the widely reported yellow band luminescence.19–23 However, few experiments have excited directly into mid-gap states.24 Some researchers Contributing Editor: Johan Brand Malherbe a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2017.361
have used excitations below 3.4 eV but have accessed the conduction band via multiphoton processes in both bulk GaN25,26 and NWs.8 Despite a bulk band gap in the ultraviolet, GaN thin films and nanostructures can exhibit unusually strong and broad emission across the ent
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