(Invited) Intense Red Light Emitting Mesoporous Silicon Nanowires for Luminescent Devices
Authors: O'Dwyer Colm , McSweeney William , Collins Gillian 
Published: February 2020

Recent advancements in the growth and formation of semiconducting nanostructures, particularly group 14 metalloid semiconductors such as silicon and germanium have shown how quantum effects can be engineered and controlled (1,2) to modify thermal and optical properties (3). In particular, the nanostructuring of silicon materials mediates phonon scattering and confinement, including reduction of thermal conductivity (4,5) and Si-based nanowire heterostructures have been employed as solar cell materials and nanoelectronic power sources. Crystal sizes below the Bohr radius also results in light emission, and effective bandgap modification can influence absorbed thermopower in nanoscale silicon, and for photovoltaics.
We report intense red luminescence from mesoporous n+-Si(100) nanowires (NWs) and nanocrystal-decorated p-Si NWs fabricated using electroless metal assisted chemical (MAC) etching (6-9). n+-Si NWs are composed of a labyrinthine network of silicon nanocrystals in a random mesoporous structure. p-type Si(100) NWs exhibit solid core structure, with a surface roughness that contains surface-bound nanocrystals. Both mesoporous n+-Si NWs and rough, solid p-Si NWs exhibit red luminescence at ~1.7 and ~1.8 eV, respectively. Time-resolved photoluminescence (PL) measurements indicated long (tens of µs) radiative recombination lifetimes. The red luminescence is visible with the naked eye and the red light is most intense from mesoporous n+-Si NWs, which exhibit a red-shift in the emission maximum to 1.76 eV at 100 K. The red PL from monolithic arrays of p-type NWs with nanocrystal-decorated rough surfaces is comparatively weak, but originates from the surface bound nanocrystals. Significant PL intensity increase is found during excitation for mesoporous NWs. X-ray photoelectron spectroscopy identifies a stoichiometric SiO2 on the rough p-Si NWs with a SiOx species at the NW surface. No distinct oxide is found on the mesoporous NWs. The analysis confirms that long life-time PL emission arises from quantum confinement from internal nanoscale crystallites, and oxidized surface-bound crystallites, on n+- and p-Si NWs respectively.

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