Optical properties of free-standing ultrahigh porosity silicon films prepared by supercritical drying

Highly luminescent free-standing porous silicon thin films of excellent optical quality have been manufactured by using electrochemical etching and lift-off steps combined with supercritical drying. One to 50 mu m thick free-standing layers made from highly (p(+)) and moderately (p) Boron doped single crystal silicon (c-Si) substrates have been produced with porosities (P) up to 95 %. The Fabry-Perot fringes observed in the transmission and photoluminescence (PL) spectra are used to determine the refractive index. At the highest P the index of refraction is below 1.2 from the IR to 2 eV. The absorption coefficients follow a nearly exponential behavior in the energy range from 1.2 eV and 4 eV. The porosity corrected absorption spectra of free-standing films made from p type c-Si substrates are blue shifted with respect to those prepared from p(+) substrates. For P > 70 % a blue shift is also observed in FL. At equal porosities the luminescence intensities of porous silicon films made from p(+) and p type c-Si are different by one order of magnitude.