Spectroscopic investigations of dark Si nanocrystals in SiO2 and their role in external quantum efficiency quenching

Abstract

The percentage of dark silicon nanocrystals, i.e., the nanocrystals that are not able to radiatively recombine after absorption of a photon, is investigated by combining measurements of external and internal quantum efficiencies. The study is conducted on samples prepared by co-sputtering and subsequent heat treatments. We show that the external quantum efficiency is mainly limited by the presence of dark nanocrystals, which induce losses after direct excitation and also, as we propose, by indirect excitation enabled by energy migration. The percentage of dark nanocrystals can be decreased by high quality surface passivation as a result of low-temperature annealing in ambients of O 2 and H 2 . By using a non-passivated sample as a reference, the relation between the size of a nanocrystal and its probability of being dark is studied. Larger nanocrystals are demonstrated to function more likely as dark centers. The study shows that high external quantum efficiencies of Si nanocrystal ensembles can be realized for small, well passivated Si nanocrystals under suppression of excitation diffusion. V C 2013 AIP Publishing LLC. [http://dx

Near-Unity Internal Quantum Efficiency of Luminescent Silicon Nanocrystals with Ligand Passivation

“…22,37,43 In contrast, here we report IQE = 100% for Si NCs passivated with different ligand molecules in a full emission energy range, in which the nonradiative recombination channels of this type seem to be totally suppressed. A partial coverage of nanocrystal surface by oxygen molecules was confirmed here by FTIR measurements ( Figure S2); hence a thick oxide shell is probably required to facilitate such a transition.…”

Spectral Dependencies of the Stretched Exponential Dispersion Factor and Photoluminescence Quantum Yield as a Common Feature of Nanocrystalline Si

“…Obviously, longer dark intervals will result in lower values of PL QY. Ultimately, some NCs can occur to have infinite dark intervals and in literature they are called “dark” NCs . Such “dark” NCs together with blinking NCs in their OFF‐state behave like “black holes”, i.e., they absorb photons, but do not contribute to the emission and so lower the PL QY.…”

Step-like increase of quantum yield of 1.5 μm Er-related emission in SiO2 doped with Si nanocrystals

“…Therefore, the EQY in range #1 indirectly reflects the level of the "optical activity" of Si NCs. 40 In the investigated material, the optical activity of Si NC is quite low and can be improved. Past investigations have shown that in carefully optimized materials the optical activity of Si NCs can reach 60%.…”

“…To this end, the low temperature post-annealing in H 2 environment could be explored, since it has been shown to enhance the optical activity within ensembles of Si NCs, reducing the number of the so-called "dark" NCs undergoing fast nonradiative quenching of excitons. 40 For improvement of EQY in regions #2-4, the hot-carrier mediated excitation must be optimized. Here, the current study provides clear guidance, suggesting that the highest efficiency can be expected for materials with moderate concentrations of relatively small NCs of superior crystalline quality, and a high Er-to-NC concentration ratio.…”