MICROSCOPE OBSERVATION OF A SELF-STANDING FILM OF POROUS SILICON

A self-standing film (SSF) of porous silicon electrochemically isolated from the substrate has been observed under optical and electron microscopes. The SSF consists of two layers: the top layer, on the side from which anodization starts, cracks into cells exhibiting a fine porous structure: the bottom layer, on the side of isolation, shows a uniform and coarse porous structure. Under the optical microscope the film is found to expand upon wetting with methanol and to shrink back when it is dried. The change in the surface area amounts to about 10%, while any change in the thickness is undetectable. This expansion-shrinkage process can be repeated, but the film is occasionally cracked when it shrinks.