Formation and Properties of Porous Silicon Film

Porous silicon film (PSF) on silicon was formed by anodic reaction in 50 weight percent (w/o) hydrofluoric acid. Effects of parameters on the PSF formation rate, density, crystalline structure, and cross section structure have been studied. Current density and acceptor concentration in p‐type silicon PSF formation, as well as current density, illumination light intensity, and donor concentration in n‐type silicon PSF formation, were mainly found to play important roles in determining the PSF crystalline and porous structure. The surface crystalline structure in the p‐type silicon PSF changes from the clean Kikuchi pattern to a broad pattern when acceptor concentration decreases. In the n‐type silicon PSF, the crystalline structure tends to change from single crystal to amorphous as current density becomes smaller, donor concentration decreases, illumination light intensity becomes strong, and reaction time is longer. The cross section, in the p‐ and the n‐type silicon PSF, is virtually dense at a very thin layer close to the surface, becoming more porous at the inner region compared with the surface. The holes injection effect on the PSF formation in n‐type silicon has been investigated. It is found that PSF formation without holes injection occurs uniformly at anodic reaction after the order of 0.1 μm thick PSF is formed with holes injection. It is thought that anodic dissolution occurs at the local silicon surface under anodic conditions by which the single crystal PSF is obtained. It was found that the PSF composition consists mainly of silicon and a small amount of hydrogen, oxygen, and fluorine.