Molecular Mechanism of Etching-Induced Faceting on Si(100): Micromasking Is Not a Prerequisite for Pyramidal Texturing

Chemical control of crystalline surfaces during etching or growth finds application in a wide variety of areas, including the low-cost texturing of silicon solar cells to reduce reflectivity losses. Nevertheless, the kinetic processes that govern these morphological transformations are poorly understood. Here, we study the spontaneous nanoscale faceting of Si(100) surfaces during reaction with deoxygenated H₂O using a combination of scanning tunneling microscopy, infrared spectroscopy, and kinetic Monte Carlo simulation. We show that this reaction is inherently unstable to kinetic faceting and that the flat-to-faceted transition is driven by the reactivity of a single chemical site present in a concentration of less than 0.4% of a monolayer. In contrast to previously postulated mechanisms, pyramidal faceting does not require “micromasks”: chemical heterogeneities, such as impurities, insoluble etch products, or H₂ (g), that collect on the surface during etching and act as transient nanoscale etch masks. Instead, the etching reaction drives the formation of self-propagating, {111}-faced nanoscale hillocks. This study shows that the chemical control of surface morphology can be driven by minority species at concentrations far below the detection limit of surface spectroscopy