Pore Formation Mechanisms for the Si-hf System

A model is presented with the potential to account for all processes of the reactive Si – liquid interface including, e.g., current oscillations, and the formation of nano-, meso, and macropores with their specific dependence on crystal orientation. The model assumes that current flow is spatially and temporally inhomogeneous current thus flows in current "lines" occurring in current "bursts". The mean cycle time between correlated current bursts is mostly given by the kinetics of oxide dissolution and hydrogen passivation (which introduces a strong surface orientation dependence). Structure generation at the Si electrode (current oscillations in the time domain or pore formation in the space domain) under these assumptions is a self-organized process resulting from an interplay of synchronizing and desynchronizing mechanisms. Synchronizing mechanisms always couple the nucleation of a new current burst in a specific area to the history of that area, desynchronizing mechanisms may also depend on the interaction of current burst. Examples for synchronizing mechanisms are enhanced nucleation probabilities on (100) surfaces, response to local oxides from another current burst, or coupling of current bursts by space charge region effects. Desynchronisation results, e.g., from quantum wire effects, or local reduction of reactants or potential by a current line. The model accounts qualitatively for most if not all observed phenomena, gives a number of quantitative relations, and makes numerous predictions.