Investigation of optical properties of free-standing porous silicon films by absorption and mirage effect

• Published in: Journal of luminescence 1993-11, Vol.57 (1), p.217-221
• Main Authors: Vincent, G., Leblanc, F., Sagnes, I., Badoz, P.A., Halimaoui, A.
• Format: Article
• Language: English
• ISSN: 0022-2313; 1872-7883
• DOI: 10.1016/0022-2313(93)90136-B

Absorption coefficients α of free-standing porous silicon (PS) films obtained from p- and p + -Si have been measured between 0.75 and 2.3 eV. Using both transmission and mirage effect, we have measured variations of α over more than four decades. The experimental data confirm the large difference between p- and p + -PS samples and also demonstrate a clear difference with bulk silicon as well as amorphous silicon: the α-PS exhibits tail states below 1.1 eV (the bulk energy gap), whereas it is lower than the α bulk above 1.1 eV and the α a-Si: H above 1.5 eV. It is also shown that a partial oxidation of the samples, which is believed to modify the inner surface coverage of the material and reduce the dimension of the Si skeleton, resulted in a red shift of the transmission curve. Furthermore, after oxide removal this transmission curve is blue-shifted. These striking results are discussed in terms of bulk and surface effects.