Picosecond and millisecond dynamics of photoexcited carriers in porous silicon

We report on a detailed study of the dynamics of photoexcited carriers in red-emitting porous silicon at room temperature after excitation by 532-nm picosecond laser pulses. Experimental techniques of time-resolved absorption (pump and probe) and photoluminescence are used to cover a very large time interval 10^-11--10^-4 s. The dynamics exhibits fast and slow components. The fast component ({~}10^-10 s) is interpreted as a bimolecular recombination of free carriers in the core of nanometer-sized silicon nanocrystallites, and the slow component ({~}10^-4 s) originates in the recombination of carriers rapidly trapped in the surface localized states. We propose a rate-equation model which enables us to describe well the complete photoexcited-carrier dynamics from picoseconds to hundreds of microseconds. Our results strongly support the key role of localized states on the surface of a Si network in the steady-state red photoluminescence of porous silicon. {} 1996 The American Physical Society.