Effect of “buffer layers” on the optical properties of silicon nanocrystal superlattices

Silicon nanocrystal superlattices consisting of alternating layers of silicon-rich oxide (SRO) and silica (SiO2) were grown by electron beam evaporation, followed by thermal processing. All of the SRO depositions and the processing steps were done under conditions that were as similar as possible so as not to vary the average nanocrystal size. Three sets of samples were synthesized. In the first set, the SRO layers were grown with a different thickness in the different samples while the buffer layers were always 20 nm thick, and in the second set, the SRO layers were 20 nm thick while the SiO2 buffer layers were varied. There were pronounced differences in the photoluminescence spectrum and decay dynamics due to the differences in the thickness of the various layers. The results are interpreted in terms of the migration of charge carriers between nanocrystals, and illustrate that the peak of the PL spectrum can vary between 750 and 950 nm without changing the average particle size. Finally, a third set of samples in which the buffer layers were erbium oxide instead of silica were grown, and the results were interpreted in terms of efficient carrier migration through the nanocrystal-containing layers.