Mi. Vasilevskiy et al., Impact of disorder on optical phonons confined in CdS nano-crystallites embedded in a SiO2 matrix, J PHYS-COND, 13(14), 2001, pp. 3491-3509
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.