Carrier Dynamics in Porous Silicon: from the Femtosecond to the Second

Abstract

ABSTRACTThe luminescence in red-emitting porous silicon exhibits a distribution of lifetimes in the μsec time domain at room temperature and in the msec time domain at cryogenic temperatures. However, the luminescence and carrier dynamics in porous silicon display transients that vary from much less than 1 psec to ∼ 1 sec, depending on the measurement conditions and sample preparation. We have investigated the carrier dynamics in porous silicon by two time-resolved techniques. The blue photoluminescence of oxidized porous silicon has been measured with 100 ps time resolution as a function of the oxidation method, emission wavelength, excitation intensity and measurement temperature. The blue luminescence has a distinct origin from the well-studied red luminescence and we attribute it to defects in the oxide. Femtosecond photoinduced absorption measurements have been performed on thin red-emitting porous silicon films. The wavelength and intensity dependence of the recovery are interpreted in terms of trapping and of Auger recombination at high excitation intensity. Our results also show conclusively that red-emitting porous silicon is not a direct gap semiconductor.