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

Non-resonant Raman spectroscopy studies of a set of CdS films annealed at d ifferent temperatures were performed and showed a direct correlation betwee n the width of the Raman peak produced by CdS-like optical phonons and the crystalline quality of the semiconductor phase probed by x-ray diffraction (XRD) and transmission electron microscopy (TEM). In order to decribe the R aman lineshape a model proposed by Trallero-Giner et al (1998 Phys. Rev. B 57 4664) was used, which considers optical phonons confined in small semico nductor spheres with a size distribution. The model is shown to give a good reproduction of the spectra of samples where the semiconductor phase is mo st crystalline. However, it required too large values of phonon damping to fit the spectra of several other samples, which, according to XRD and TEM d ata, do contain CdS nano-crystallites. This large broadening of the Raman p eak was considered as inhomogeneous, i,e. associated with disorder, Numeric al lattice dynamics calculations were performed for 2D binary clusters of a rbitrary shape and three kinds of disorder were considered, (i) random vari ation of the Cd-S bond frequency from one nano-crystallite to another, (ii) cluster shape irregularities and (iii) fluctuations of the nearest-neighbo ur interaction constant within one cluster. It is shown that 'ensemble diso rder' (ij can be responsible for a shoulder above the bulk CdS phonon frequ ency observed for some of our samples. The effect of shape disorder (ii) is similar to that of the size dispersion producing some inhomogeneous broade ning of the peak. In addition, it gives rise to an extra low-frequency mode originating from the top of the acoustic band. The force constant's disord er (iii) is shown to result in a stronger asymmetric broadening of the Rama n peak.