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The mechanism of formation of porous silicon layer (PSL) has been studied according to the following experimental results. PSL is formed by the local dissolution of silicon which occurs only at the base of the pores. The HF concentration of the electrolyte in the pores of PSL is constant during anodization and the anodic reaction in the pores proceeds uniformly in the thickness direction. The dissolution of silicon in the pores is the results of the divalent and the tetravalent reactions of silicon with HF , without the disproportionation reaction. The insoluble surface porous film (SPF) exists at the surface of PSL and the silicic acid is formed in PSL. A model of forming PSL is proposed, with emphasis being placed on the anodic reaction and the local dissolution of silicon which is initiated by SPF and is promoted by the hindrance layers composed of the silicic acid.

The mechanism of oxidation of a porous silicon layer (PSL) was studied by examining the oxidation progress, the temperature dependence of the oxidation and the infrared absorption spectra. It is found that the oxidation of PSL is controlled by a surface reaction at both the surface of PSL and the wall of pores in PSL. The progress of the oxidation in PSL is altered at nearly 900°C. Si2O3 as well as SiO2 are produced by low temperature oxidation below 400°C and physically-absorbed water exists in an oxidized porous silicon (OPS) film formed by wet O2 oxidation at temperatures below 700°C. Further, the properties of the OPS film are reported. Etching rate, dielectric constant, tan δ and breakdown strength of the OPS film are strongly dependent on the oxidation temperature, oxidation atmosphere and substrate resistivity. Thick OPS film, which has the same properties as thermal SiO2 of bulk silicon, is formed in a short time by high temperature oxidation above 1000°C in wet O2.

The structure of an as-grown and heat-treated porous silicon layer (PSL) and silicon epitaxial growth on PSL are investigated. Many micropores are formed inside of PSL and zig-zag in the thickness direction. The crystalline structure of PSL is single crystal, but there is a polycrystal silicon on the surface and lattice strain exists. The structure of PSL is changed by high temperature heat-treatment. At 1000~C, PSL is a mosaic crystal. Above 1070~ PSL is a single crystal. After heat-treatment, PSL remains porous and both the pore distribution and the pore size are changed. The surface roughness and the pore size become large with rising heat-treatment temperature. From the experimental results of silicon epitaxial growth on PSL it is found that good epitaxial layers grow on PSL.

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