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Description
The design of a transparent thermoelectric material is a promising technology for touch–screen displays and solar cell applications, rendering a more sustainable powering of the device. In order to enhance the thermoelectric performance, the films must have a high Seebeck coefficient, high electrical but low thermal conductivity [1]. For this purpose, ZnO thin films were deposited by magnetron sputtering, and doped with Ga or Al and Bi. The approach consists in introducing Bi ions, a higher mass element, into the ZnO metal-oxide matrix, in order to hinder phonon mediated heat conduction, consequently reducing the thermal conductivity [2]. This work focuses on the Bi doping effect on ZnO, ZnO:Ga and ZnO:Al thin films. From Atom Probe Tomography (APT) analysis we determine the composition and investigate the cation and anion segregations to interfaces and grain boundaries. Based on thermal conductivity results obtained by Frequency Domain Thermoreflectance it is concluded that the doping of ZnO films with Al or Ga has a significant effect on thermal conductivity in contrast to the doping with Bi. This result can be understood by the fact that the Bi, unlike Al and Ga, is segregated at grain boundaries and does not substitute Zn in the crystal lattice. This last claim is unequivocally demonstrated by measurement results of APT and Transmission Electron Microscopy.
References
[1] R. Venkatasubramanian, et al, Thin-film thermoelectric devices with high room-temperature figures of merit, Nature. 413 (2001) 597–602. doi:10.1038/35098012.
[2] F.C. Correia, et al, Effect on the electrical and morphological properties of Bi incorporation into ZnO:Ga and ZnO:Al thin films deposited by confocal magnetron sputtering, Vacuum. 152 (2018) 252–260. doi:10.1016/j.vacuum.2018.03.033.
