FIR optical study of CdS nanocrystals embedded in SiO2 films

Abstract

A dielectric response due to polar optical phonons in CdS-doped SiO2 films (semiconductor volume fraction f≈6–15%) was studied experimentally by means of FIR and Raman spectroscopies. FIR reflectivity spectra taken at 80–300 K show a broad bulk CdS-like reflection band inconsistent with the relatively low f(≪1). To explain this, a model of fractal aggregates consisting of small crystalline particles was proposed and an effective dielectric function was calculated which fits well with the experimental FIR spectra. The asymmetric Raman lineshape of the 1LO peak observed experimentally can be fairly well fitted by the usual model considering LO-type phonon modes perfectly confined in nanocrystalline spheres. It is argued, however, that in accordance with the aggregate hypothesis, there is not much reason to consider the nanocrystals as regular spheres. Model lattice dynamics calculations for CdS clusters of irregular shape have been performed, and the results are shown to be able not only to fit equally well the LO Raman lineshape, but also to explain an extra feature appearing in the experimental spectra, interpreted as a disorder-activated acoustic mode.