  Two broad bands are usually found in the Raman spectrum of many Te-based
chalcogenides, which include binary compounds, like ZnTe, CdTe, HgTe, GaTe,
GeTe, SnTe, PbTe, GeTe2, As2Te3, Sb2Te3, Bi2Te3, NiTe2, IrTe2, TiTe2, as well
as ternary compounds, like GaGeTe, SnSb2Te4, SnBi2Te4, and GeSb2Te5. Many
different explanations have been proposed in the literature for the origin of
these two anomalous broad bands in tellurides, usually located between 119 and
145 cm-1. They have been attributed to the own sample, to oxidation, to the
folding of Brillouin-edge modes onto the zone center, to the existence of a
double resonance, like that of graphene, or to the formation of Te
precipitates. In this paper, we provide arguments to demonstrate that such
bands correspond to clusters or precipitates of trigonal Te in form of nanosize
or microsize grains or layers that are segregated either inside or at the
surface of the samples. Several mechanisms for Te segregation are discussed and
sample heating caused by excessive laser power during Raman scattering
measurements is emphasized. Finally, we show that anomalous Raman modes related
to Se precipitates also occur in selenides, thus providing a general vision for
a better characterization of selenides and tellurides by means of Raman
scattering measurements and for a better understanding of chalcogenides in
general.
