  The Boson peak is a universal phenomenon in amorphous solids. It can be
observed as an anomalous contribution to the low-temperature heat capacity over
the Debye model. Amorphous phase-change materials (PCMs) such as Ge-Sb-Te are a
family of poor glass formers with fast crystallization kinetics, being of
interest for phase-change memory applications. So far, whether Boson peaks
exist in PCMs is unknown and, if they do, their relevance to PCM properties is
unclear. Here, we investigate the thermodynamic properties of the pseudo-binary
compositions on the tie-line between Ge15Te85 and Ge15Sb85 from a few Kelvins
to the liquidus temperatures. Our results demonstrate the evidence of the
pronounced Boson peaks in heat capacity below 10 K in the amorphous phase of
all compositions. By fitting the data using the Debye model combined with the
Einstein model, we can extract the characteristic parameters of the Boson peaks
and attribute their origin to the excess vibrational modes of dynamic defects
in the amorphous solids. We find that these parameters correlate almost
linearly with the Sb-content of the alloys, despite the nonmonotonic behaviors
in glass forming abilities. A larger contribution of excess vibrational modes
correlates with a larger width of enthalpy relaxation below the glass
transition temperature Tg. In a broader context, we show that the correlations
of the characteristic parameters of the Boson peaks with Tg and kinetic
fragility, vary according to the type of bonding. Specifically, metallic
glasses and conventional covalent glasses exhibit distinct patterns of
dependence, whereas PCMs manifest characteristics that lie in between. A deeper
understanding of the Boson peaks in PCMs holds the promise to enable
predictions of material properties at higher temperatures based on features
observed in low-temperature heat capacity.
