Thermoelectric
Power Factor Enhancement by Pulsed Plasma Engineering in Magnetron
Sputtering Induced Ge<sub>2</sub>Sb<sub>2</sub>Te<sub>5</sub> Thin
Films
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Abstract
Precise
control over microstructure and composition is desired prerequisite
for the performance enhancement of thermoelectric materials. In conventional
magnetron plasma sputtering synthesis, composition control is challenging
when the sputtering-target is composed by different elements. Here,
the potential of pulsed power utilization is demonstrated for compositional
control of Ge<sub>2</sub>Sb<sub>2</sub>Te<sub>5</sub> thin films via
pulse-reversal time and pulse-frequency engineering in pulsed DC-magnetron
sputtering process. When annealed at 400 °C for 1 h in vacuum
conditions, amorphous thin films (of 200 nm thickness, deposited on
glass substrate) crystallize in to face centered cubic phase with
average nanocrystallite size ∼10 nm. Power density enhancement
to 5.56 W/cm<sup>2</sup> at low pulse reversal time induces maximum
process throughput as 450 nm/min. Increase in either of pulse frequency
or pulse reversal time decreases the discharge voltage and plasma
density. As a consequence, kinetic energy of ions and ionization of
plasma species are sequentially controlled to improve the stoichiometry
of film and eventually; the electronic transport. The optimization
of pulse plasma engineering yields maximum thermoelectric power factor
value as 1.35 μW cm<sup>–1</sup> K<sup>–2</sup> with process throughput more than 300 nm/min. The obtained values
are promising for applications in the automobile and microelectronics
industry