Role of surface vibration modes in Si nanocrystals within light emitting porous Si at the strong confinement regime

Abstract

In this work, we study theoretically the resonant coupling between longitudinal optical surface vibrations of Si-OH and/or Si-O-Si and electron and hole states in the silicon nanocrystals (Si NCs) within light emitting porous Si (PSi) thin films in the framework of the Fröhlich interaction. The results of this analysis are compared with experimental results, which show considerable enhancement and a redshift of the photoluminescence (PL) spectrum of a fresh as-grown PSi thin film after prolonged laser irradiation or after aging in air. These effects coincide with the formation of Si-OH and Si-O-Si bonds on the surface of PSi. The redshift of the PL spectrum is due to the pinning of the bandgap of the light emitting Si NCs, as both oxidation via laser irradiation in air and aging in air introduce energy states in the Si NC band gaps. According to the theoretical analysis, the PL enhancement is assigned to inhibition of nonradiative channels rather than to an enhancement of radiative channels in the Si NCs within the PSi film, due to a strong coupling of the surface Si-OH and/or Si-O-Si vibrational modes to the electronic sublevels in the Si NCs within the PSi layer.