Fabrication, micro-structure characteristics and transport properties of co-evaporated thin films of Bi2Te3 on AlN coated stainless steel foils

Aziz Ahmed,Seungwoo Han

doi:10.1038/s41598-021-83476-7

Abstract

N-type bismuth telluride (Bi2Te3) thin films were prepared on an aluminum nitride (AlN)-coated stainless steel foil substrate to obtain optimal thermoelectric performance. The thermal co-evaporation method was adopted so that we could vary the thin film composition, enabling us to investigate the relationship between the film composition, microstructure, crystal preferred orientation and thermoelectric properties. The influence of the substrate temperature was also investigated by synthesizing two sets of thin film samples; in one set the substrate was kept at room temperature (RT) while in the other set the substrate was maintained at a high temperature, of 300 °C, during deposition. The samples deposited at RT were amorphous in the as-deposited state and therefore were annealed at 280 °C to promote crystallization and phase development. The electrical resistivity and Seebeck coefficient were measured and the results were interpreted. Both the transport properties and crystal structure were observed to be strongly affected by non-stoichiometry and the choice of substrate temperature. We observed columnar microstructures with hexagonal grains and a multi-oriented crystal structure for the thin films deposited at high substrate temperatures, whereas highly (00 l) textured thin films with columns consisting of in-plane layers were fabricated from the stoichiometric annealed thin film samples originally synthesized at RT. Special emphasis was placed on examining the nature of tellurium (Te) atom based structural defects and their influence on thin film properties. We report maximum power factor (PF) of 1.35 mW/m K2 for near-stoichiometric film deposited at high substrate temperature, which was the highest among all studied cases.

Highlights

N-type bismuth telluride (Bi2Te3) thin films were prepared on an aluminum nitride (AlN)coated stainless steel foil substrate to obtain optimal thermoelectric performance
The results of our SEM, energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) experiments showed that we fabricated well crystallized thin films differing in composition, microstructure and crystal orientation, making it possible to establish the relationship between the parameters of these materials and their thermoelectric properties
Polycrystalline thin films with columnar micro-structure and multi-oriented crystal structure were obtained in the case of the thin films deposited at high substrate temperatures, whereas the annealed stoichiometric thin films have a unique ordered microstructure consisting of in-plane layers preferentially stacked along the (00 l) c-axis orientation

Summary

Introduction

N-type bismuth telluride (Bi2Te3) thin films were prepared on an aluminum nitride (AlN)coated stainless steel foil substrate to obtain optimal thermoelectric performance. The electrical resistivity and Seebeck coefficient were measured and the results were interpreted Both the transport properties and crystal structure were observed to be strongly affected by non-stoichiometry and the choice of substrate temperature. We observed columnar microstructures with hexagonal grains and a multi-oriented crystal structure for the thin films deposited at high substrate temperatures, whereas highly (00 l) textured thin films with columns consisting of in-plane layers were fabricated from the stoichiometric annealed thin film samples originally synthesized at RT. Some papers have reported thermoelectric thin film deposition on polyimide substrates, which flexible, are not thermally conductive sufficiently for high power density applications, nor the homogeneity of thin films microstructure and physical properties[17,31,32]. The use of polyimide substrate limits the film processing temperature as they cannot withstand very high temperatures[17]

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