(Invited) Metal Assisted Chemical Etching of Silicon Nanowires: Quantum Confinement for Photon Emission and Phonon Transport
Authors: O'Dwyer Colm 
Published: July 2021

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. Reducing size to the confinement regime can also perturbate phonon transport, and this has recently been show to occur in nanocrystallites on rough surfaces of silicon nanowires.

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), and also demonstrate that nanoscale confinement is a cause for limited photon or heat transport in silicon nanowires formed by MACE. n+-Si NWs are composed of a labyrinthine network of silicon nanocrystals in a random mesoporous structure (10). 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. 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. When quantum confinement effects are introduced in crystalline materials such as the nanocrystalline mesoporous NWs, the electron and phonon transport can be significantly altered in tandem with quantum confinement that causes intense red light emission. The data confirm that phonon confinement and scattering for silicon nanowires is due to surface-bound and internal nanostructure, rather than simply a NW diameter reduction in NW materials.

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