Photoexcited charge carrier dynamics in silicon nanocrystal/SiO2 superlattices

We report in detail on the dynamics of photoexcited charge carriers in size-controlled silicon nanocrystals in silicon nanocrystal/SiO2 superlattices. The samples were prepared using plasma enhanced chemical vapor deposition and subsequent thermally induced phase separation. This unique approach allows preparation of well-defined Si nanocrystals. Experimental techniques of time-resolved absorption and photoluminescence were used to monitor the carrier dynamics on a wide time scale from picoseconds to milliseconds for a set of samples with different parameters (nanocrystal size, hydrogen annealing). The initial fast decay (tens of picoseconds) dependent on pump intensity for excitation levels exceeding one electron-hole pair per nanocrystal can be interpreted in terms of the bimolecular recombination with constant B=(2-3)×10−10 cm3 s−1. The slow pump intensity independent decay (microseconds) can be reproduced well by a stretched-exponential function. The dependence of stretched-exponential parameters on photoluminescence photon energy and sample properties agrees well with the picture of trapped carriers.