Ultrafast transient grating spectroscopy in silicon quantum dots.

Transient grating spectroscopy detects directly the relaxation of the excited carriers rather than time-resolved photoluminescence and thus it is particularly desired for the indirect semiconductors such as silicon quantum dots. We investigate ultrafast carrier dynamics in silicon quantum dots embedded in silicon oxide matrix using femtosecond transient grating spectroscopy. Two ultrafast decay components are observed with decay time of 800 fs and 4 ps at various detection wavelengths, which are attributed to the transverse optical and transverse acoustic phonon assisted relaxation. Photoexcited electrons and holes are effectively trapped into the localized states on the surface of the silicon quantum dots where electrons and holes have a slow recombination in the time scale of microseconds.