Etching-Induced Longitudinal Phase Inhomogeneity in Fractal Silicon: Identification through Depth-Profiled Raman Mapping

Abstract

The coexistence of intermediate amorphous-like and nanocrystalline phases in an etched silicon wafer fabricated using metal-assisted chemical etching is being reported here. In addition, the long-pending origin of amorphous-like features in the Raman spectrum from porous silicon is also being established using spatial Raman spectroscopy and validated using longitudinal Raman spectro-microscopy. Raman spectro-microscopy data have been used here as a bridge to get the complete picture about the presence of coexisting a-Si/nanocrystalline Si phases, about which SEM, TEM, and XRD provide information at different scales. Raman spectra do not show any variation spatially when taken from different parts on the surface, whereas when recorded from different depths by defocusing the laser, they show amorphous and nanocrystalline phases in different proportions. Quantification of the disorder present in terms of medium- or short-range order and estimation of the nanocrystalline size have been done using a modified phonon confinement model and bond polarizability model. Overall, a consolidated picture using three different extraordinary techniques, XRD, SEM, and TEM, that probe microstructures at different scales has been analyzed in unison with Raman spectro-microscopy to reveal structural variation in fractal Si obtained because of porosification.