Optical Absorption and Photoluminescence Studies of Free-Standing Porous Silicon Films with High Porosities

We have prepared noncollapsed free-standing porous silicon (PS) films with various porosities even higher than 90% using the electrochemical etching−electropolishing, chemical dissolving, and supercritical drying methods. The result of a study combining optical absorption and photoluminescence (PL) from free-standing PS films with porosities in the range 41−94% is presented. A blue shift of the transmission curves and an increase of the PL intensity with enhancing porosity have been observed. However, no notable blue shift of the PL peak position with increasing porosity has been observed. Furthermore, the square root of the absorption coefficient (α) of the PS samples with high porosities times photon energy (hν) vs photon energy can no longer be fitted by a linear function. Even for PS films with porosity of 94%, the absorption data do not show that the nanometer Si particles in the PS have a direct energy gap.