Initial stages of porous Si formation on Si surfaces investigated by infrared spectroscopy

We used infrared absorption spectroscopy in the multiple internal reflection geometry (MIR-IRAS) to investigate the mechanism of formation of porous silicon (por-Si) on Si(1 0 0), (1 1 0) and (1 1 1) electrode surfaces. We analyze IRAS spectra in the SiH stretching vibration region of the Si electrode surface during anodization (electrochemical etching) in dilute hydrofluoric acid (HF) solution. We observe that during anodization, hydride species on the topmost layer of the surface are etched away and instead, another types of hydride species are generated on the etched portions. The evolution of each hydride species can be interpreted in terms of an etching model in which the atomic bonding configuration of surface Si atoms that depends on the crystallographic orientation is considered. On the basis of Monte Carlo (MC) simulations, we suggest that the etching rate for monohydride species (SiH∗) is higher than that for dihydride species, which could be the driving force behind pore formation.