  The present study demonstrates a large enhancement in the Seebeck coefficient
and ultralow thermal conductivity (TE) in Sb$_2$Te$_3$-AgSbTe$_2$ nanocomposite
thin film. The addition of Ag leads to the in-situ formation of AgSbTe$_2$
secondary phase nanoaggregates in the Sb$_2$Te$_3$ matrix during the growth
resulting in a large Seebeck coefficient and reduction of the thermal
conductivity. A series of samples with different amounts of minor AgSbTe$_2$
phases are prepared to optimize the TE performance of Sb$_2$Te$_3$ thin films.
Based on the experimental and theoretical evidence, it is concluded that a
small concentration of Ag promotes the band flattening and induces a sharp
resonate-like state deep inside the valence band of Sb$_2$Te$_3$, concurrently
modifying the density of states (DOS) of the composite sample. In addition, the
electrical potential barrier introduced by the band offset between the host TE
matrix and the secondary phases promotes strong energy-dependent carrier
scattering in the composite sample, which is also responsible for enhanced TE
performance. A contemporary approach based on scanning thermal microscopy is
performed to experimentally obtain thermal conductivity values of both the
in-plane and cross-plane directions, showing a reduced in-plane thermal
conductivity value by ~ 58% upon incorporating the AgSbTe$_2$ phase in the
Sb$_2$Te$_3$ matrix. Benefitting from the synergistic manipulation of
electrical and thermal transport, a large ZT value of 2.2 is achieved at 375 K.
The present study indicates the importance of a combined effect of band
structure modification and energy-dependent charge carrier scattering along
with reduced thermal conductivity for enhancing TE properties.
