Inverse Size Dependent Fano Parameter in Silicon Porous Wires: Consequence of Quasi-Continuum Flattening

Abstract

Quantum confinement typically makes some effects more prominent upon decreasing the size and underlined scientific reasoning must have an explanation if it is otherwise. An anomalous size dependent trend in electron-phonon bound states (interferon) beyond the bulk approximation has been analyzed in silicon (n-type) porous wires. Cross-sectional Raman mapping has been done to assess the longitudinal variation in the size of nanostructures present at the microscopic level. Theoretical analysis of the obtained asymmetric Raman line-shapes yields a decreasing e-phonon coupling strength upon decreasing the size in a weak confinement regime. An in-depth analysis revealed that the combined effect of fabrication technique and associated gradual depletion of dopant atoms (prominent in smaller nanostructures) is found to affect the nature of the electron-phonon coupling. Weaker electron-phonon coupling arising due to flattening of electronic continuum in quantum structures has been validated using appropriate Raman line shape analysis combining the dual effect of quantum confinement and zonal depletion of the dopant atom. A detailed investigation shows that smaller nanostructures present near the tip of the nanowires have less dopant density to provide sufficient electronic continuum to interact with the discrete phonons. Overall, complete ceasation of the Fano interaction at smaller sized nanostructures allows only phonon confinement effects, though weak, to dominate, and are thus seen as an inverse size dependent Fano parameter. © 2021 American Chemical Society.