Electrochemical conditioning of fractal topographies at the silicon oxide/silicon interface
H. J. Lewerenz
On n-type silicon photoelectrodes, immersed in concentrated ammonium fluoride, fractal etch patterns are observable at anodic potentials near 6V. The formation of these patterns was investigated in dependence upon light intensity. For low photon flux, and correspondingly low density of electron-hole pairs, the etch structures exhibit random dendritic branching. For medium photon flux, regular patterns are achieved preserving, e.g., the four-fold symmetry of Si(100) surfaces. At increased light intensities, a transition to chaotic corrosion was found. Model considerations relate the rate of oxide formation to the strength and homogeneity of local stress and strain forces exerted onto silicon bonds. Numerical simulations of strain- and stress-induced crack propagation, carried out in dependence upon light intensity and oxidation rate respectively, are reproducing the structures in good agreement with experimental data.