Bulk and surface contributions to second-order susceptibility in crystalline and porous silicon by second-harmonic generation

Accurate second-harmonic generation (SHG) measurements in reflection on native-oxide-covered porous silicon samples and on (1 0 0) p-type doped crystalline silicon wafers have been performed. Non-linear reflection coefficients of both materials have been measured as a function of the azimuthal angle around the direction normal to the surface. An absolute calibration has been performed to allow a quantitative analysis of second-harmonic efficiencies. Following the theory of SHG in reflection in centrosymmetric media, surface and bulk terms of χ(2) have been estimated for the crystalline and nano-crystalline materials; linear optical constants of porous Si have been independently determined by spectroscopic ellipsometry. χ(2) of porous Si is found to be more than two orders of magnitude smaller than in crystalline Si.