Anisotropy of porous silicon formation rate in p-Si



We studied the anisotropy of meso- and microporous silicon formation rate on curved surfaces and on flat samples. Experiments on anodization of model p++-cylinders demonstrated that faceting appears on their lateral surface. This indicates that the etching rates vary between different crystallographic planes. Quantitative data on the etching-rate anisotropy were obtained by anodization of wafers through a mask in the form of narrow wedges radiating from a center in all directions (wagon wheel mask). We examined the dependence of porous silicon formation rate anisotropy on current and Si doping. It was shown that for p++ substrates, in all anodization modes, the <100> direction corresponds to the maximum growth rate. The current affects the etching-rate distribution among crystallographic directions: under low currents anisotropy of pore-front propagation rate may be as high as 20%. An even stronger influence on the anisotropy characteristics is exerted by the substrate doping level. It was found for the first time that, for silicon with acceptor concentration NB ≈ 1017–1018 cm−3, the <111> axis becomes the fastest growth direction, and the <100> axis, by contrast, is transformed to the direction of the slowest pore-front propagation. This phenomenon correlates with rotation of the optical axis in birefringent mesoporous silicon on passing from p++-Si to p+-Si, as is known from the literature.