Luminescence of Silicon Nanocrystals in SiO_2: Effects of Excitation Spectrum

摘要

Silicon nanocrystals formed by ion implantation and annealing of fused silica wafers show a strong, broad photoluminescence (PL) peak centered at a wavelength between 750 and 900 nm, depending on the processing conditions. This luminescence has been extensively investigated and trial device structures based on these materials have been built. However, relatively few studies also report the optical absorption spectra. In fact, the absorbance of these specimens is quite low (usually < 10%) at wavelengths greater than 450 nm (i.e., at the pump wavelengths typically used for PL studies). This suggests that in numerous studies of Si nanocrystals produced by ion implantation, only a small fraction of the nanocrystals is responsible for the observed PL at the typical pump wavelengths. In this study, we investigated how the PL spectrum and intensity depend on the power and wavelength of the pump laser. We find that the PL intensity approaches saturation at high pump fluences, and that the peak emission wavelength is sensitive to the excitation power. These observations can be attributed to the dynamics of the excitation/recombination processes at different energies, and indicate that considerable care must be taken when comparing the emission spectra of different specimens. Our data are uniformly consistent with a mechanism of light emission involving subgap states (i.e., radiative trap sites) and are not supportive of a "pure" quantum confinement model.