Silicon nanocrystals as light sources: stable, efficient and fast photoluminescence with suitable passivation

Abstract

Silicon optoelectronics is an emerging technological platform, which holds promise for providing better performance, mostly in terms of lower dissipation losses, than the traditional electronics. However, a monolithic integrated light source is still missing since bulk silicon is a very poor light emitter due to its indirect bandgap. The situation dramatically changes when the size of the crystal is reduced down into nanometric dimension (<5 nm), when efficient room–temperature luminescence of these tiny silicon nanocrystals sets in. In this contribution, we present a study of silicon nanocrystals as light sources. We compare the photoluminescence properties of silicon nanocrystals with three different types of surface passivation (hydrogen, silicon oxide and methyl–based capping), which has substantial impact. We show that with sufficiently small sizes and suitable surface passivation, the photoluminescence properties of silicon nanocrystals can reach a level comparable with direct–bandgap semiconductor nanocrystals (radiative lifetime of 10 ns, stable macroscopic quantum yield of 20%). Apart from studying photoluminescence properties on a macroscopic level, we also carried out microscopical room–temperature single–nanocrystal photoluminescence spectroscopy experiments. These spectra revealed the occurrence of a fine structure (peaks 150 meV apart), practically identical with a structure already observed in single–nanocrystal spectra of silicon by other groups and very similar to a structure observed in a fundamentally different type of semiconductor nanocrystals (IIVI material). We propose that all these observations are linked with the same process, most probably the emission of trions in nanocrystals, although further measurements are necessary to support this claim.