Epitaxial silicon layers fabricated on porous silicon have many applications, and a recent one is the production of silicon-on-insulator substrates. In this work, the microstructure and crystallinity of n(-) (lightly doped, n-type) n(+) (heavily doped, n type), p(-) (lightly doped, p type), and p(+) (heavily doped, p type) porous silicon are systematically, investigated by transmission electron microscopy and high-resolution x-ray diffraction. The results show that p(+) porous silicon has the best quality compared to n(-), n(+), and p(-) porous silicon and is the best substrate to fabricate epitaxial silicon. Non-uniform porosity is detected in n(+) porous silicon formed without exposure to light. Silicon epitaxial layers produced by ultrahigh vacuum electron evaporation on p(+) (100) porous silicon are consequently investigated. Our study, shows that preoxidization of porous silicon before epitaxy is very important because it not only improves the crystal quality of the epitaxial layer but also prevents boron diffusion into the epitaxial layer during growth. (C) 2003 American Vacuum Society.