FREE-CARRIER ABSORPTION AND LUMINESCENCE DECAY OF POROUS SILICON

We present time-resolved photoluminescence (PL) and photo-induced free -carrier absorption measurements on free-standing porous silicon flake s at room temperature. The number of free carriers was monitored by ab sorption of infrared beams, at 3.39 mu m as well as at 1.3 mu m wavele ngth. The excellent scaling between these two wavelengths during the c omplete decay yields clear evidence for an absorption related to free carriers as in bulk Si. Furthermore, the total PL yield in the decay, integrated over the PL spectrum, scales with the photo-induced carrier concentration. Using the bulk-Si free-carrier absorption cross-sectio n we find that of the order of 15% of the initially excited carriers r emain after a fast initial transient, whereas only a few per cent reco mbine radiatively. The competition between various recombination chann els is discussed.