Columnar structure in anodized porous silicon

Porous silicon (PS) anodized below a constant temperature, for example 12 degrees C which depends on anodization parameters, is composed of an assemble of columnar shaped sponge like Si. The width of a column is several micrometers, and columns stand almost perpendicular to Si substrate, and the laterals of columns are in contact with each other directly. The average cross-sectional area of columns is related to both the boron density in Si substrate and HF concentration in anodization solution, but is not related to both anodization current density and anodization temperature. The average area of a column is determined such like that only one boron atom exists in an atomic layer in a column. In samples anodized at the temperature around and above 12 degrees C, needle and wall-shaped unanodized crystal Si are observed at the interface between PS columns, respectively.