  The growing market for sensors, internet of things, and wearable devices is
fueling the development of low-cost energy-harvesting materials and systems.
Film based thermoelectric (TE) devices offer the ability to address the energy
requirements by using ubiquitously available waste-heat. This review narrates
recent advancements in fabricating high-performance TE films and superlattice
structures, from the aspects of microstructure control, doping, defects,
composition, surface roughness, substrate effect, interface control,
nanocompositing, and crystal preferred orientation realized by regulating
various deposition parameters and subsequent heat treatment. The review begins
with a brief account of heat conduction mechanism, quantum confinement effect
in periodic layers, film deposition processes, thin film configurations and
design consideration for TE in-plane devices, and characterization techniques.
It then proceeds to alayzing the latest findingd on the TE properties of
Bi2(Te,Se)3 and (Bi,Sb)2Te3, PbTe, GeTe, SnSe, SnTe, Cu2-xSe, and skutterudite
films, including superlattices and the performance of TE generators, sensors,
and cooling devices. Thickness dependent microstructure evolution and TE
characteristics of films in relation to temperature are also analyzed. In the
context of spin Seebeck effect (SSE) based systems, SSE mechanism analysis,
developments in enhancing the spin Seebeck signal since its first observation,
and recent developments are covered from the facets of new system design,
signal collection, magnetic manipulation, interface conditions,
thickness-dependent longitudinal spin Seebeck signal, and length scale of
phonon and magnon transport in longitudinal SSE (LSSE) in different bi-layer
systems. At the end, possible strategies for further enhancing zT of TE films
and spin Seebeck signals of many systems are addressed.
