Impact of disorder on optical phonons confined in CdS nano-crystallitesembedded in a SiO2 matrix

Abstract

Non-resonant Raman spectroscopy studies of a set of CdS films annealed atdifferent temperatures were performed and showed a direct correlation betweenthe width of the Raman peak produced by CdS-like optical phonons and thecrystalline quality of the semiconductor phase probed by x-ray diffraction(XRD) and transmission electron microscopy (TEM). In order to decribe theRaman lineshape a model proposed by Trallero-Giner et al (1998 Phys. Rev. B 57 4664)was used, which considers optical phonons confinedin small semiconductor spheres with a size distribution. The model is shown togive a good reproduction of the spectra of samples where the semiconductorphase is most crystalline. However, it required too large values of phonondamping to fit the spectra of several other samples, which, according to XRDand TEM data, do contain CdS nano-crystallites. This large broadening of theRaman peak was considered as inhomogeneous, i.e. associated with disorder.Numerical lattice dynamics calculations were performed for 2D binary clustersof arbitrary shape and three kinds of disorder were considered, (i) randomvariation of the Cd-S bond frequency from one nano-crystallite to another,(ii) cluster shape irregularities and (iii) fluctuations of thenearest-neighbour interaction constant within one cluster. It is shown that`ensemble disorder' (i) can be responsible for a shoulder above the bulk CdSphonon frequency observed for some of our samples. The effect of shapedisorder (ii) is similar to that of the size dispersion producing someinhomogeneous broadening of the peak. In addition, it gives rise to an extralow-frequency mode originating from the top of the acoustic band. The forceconstant's disorder (iii) is shown to result in a stronger asymmetricbroadening of the Raman peak.