Quantum confinement and recombination dynamics in silicon nanocrystals embedded in Si/SiO2 superlattices

In this study the structural and optical properties of nanocrystalline Si/SiO2 superlattices have been investigated and discussed. Ordered planar arrays of silicon nanocrystals (Si-nc) have been formed by thermal annealing of ten period amorphous Si/SiO2 superlattices prepared by plasma enhanced chemical vapor deposition. Thermal processing of the superlattices results in well separated (by about 5 nm of SiO2) nanocrystalline Si layers, when the annealing temperature does not exceed 1200 degrees C. The photoluminescence (PL) properties of these layers have been studied in details. The PL peaks wavelength has been found to depend on the laser pump power; this intriguing dependence, consisting in a marked blueshift for increasing power, has been explained in terms of the longer lifetime characterizing larger Si-nc. It is also observed that these decay lifetimes exhibit a single exponential behavior over more than two orders of magnitude, in clear contrast with the typical, nonsingle exponential trends observed for Si-nc uniformly dispersed inside an insulating matrix. We attributed this peculiar behavior to the lack of interaction among nanocrystals, due to their large reciprocal distance. In agreement with the carrier quantum confinement theory, we have found that the wavelength of the PL peak can be properly tuned by changing the annealing temperature and/or the thickness of the Si layers of the superlattices, and, in turn, the Si-nc mean size. Moreover, the observed lifetimes remain very long (about 0.3 ms) even at room temperature, revealing the absence of relevant nonradiative decay processes in these samples. Furthermore, we have used the experimental PL intensities and decay times to evaluate the radiative rate as a function of the temperature; the obtained data are in good agreement with a model proposed by Calcott in the case of porous silicon. All of these data are presented, discussed, and explained within a consistent picture. (C) 2000 American Institute of Physics. [S0021-8979(00)01611-X].