Preparation and characterization of freestanding porous silicon films withhigh porosities

We have prepared noncollapsed freestanding porous silicon (ps) films with v arious porosities higher than 90% using electrochemical etching-electropoli shing, chemical dissolving, and supercritical drying methods. For the first time, the optical absorption of ps films with porosities higher than 80% h ave been obtained in transmission measurements. A blue shift of the transmi ssion curve and a sharp increase of the photoluminescence (PL) intensity wi th enhanced porosity have been observed. An experimental result for decreas ing the average diameter of the Si crystallites in the ps film with increas ing porosity has been obtained by Raman scattering measurement. No notable dependence of the PL peak energy of the ps film upon decreasing the sizes o f the Si crystallites or upon increasing the porosity is contrary to the pr ediction of the quantum confinement model for the PL of ps. (C) 1998 The El ectrochemical Society. S1099-0062(98)05-092-5. All rights reserved.