Effects of RF Magnetron Sputtering Deposition Power on Crystallinity and Thermoelectric Properties of Antimony Telluride and Bismuth Telluride Thin Films on Flexible Substrates

Abstract

Thermoelectric materials carry significant promise for self-powering future generations of unattended microdevices and wearable devices. The current increased interest in such devices highlights the need for research to provide understanding of the basic material properties of thermoelectric materials, specifically in thin-film form, deposited on flexible polymer substrates. In this study, the surface topography, crystalline structure, and electrical properties of sputtered thin films of two of the most common thermoelectric materials, i.e., antimony telluride (Sb₂Te₃) and bismuth telluride (Bi₂Te₃), supported on silicon and polymer substrates were investigated. The study focuses on determining the effect of the sputtering power and underlying substrate on the crystal structure formation as well as grain size of the resulting thin film. Radiofrequency (RF) magnetron sputtering with power levels from 50 W to 200 W was used to deposit these layers on several test structures. The results demonstrate that increasing the RF sputtering power resulted in (i) an increase in the crystalline size (from 0.48 nm to 29.66 nm for Sb₂Te₃ and from 10.60 nm to 20.29 nm for Bi₂Te₃), (ii) a significant increase in the content of tellurium (Te) in the Sb₂Te₃ and Bi₂Te₃ thin films, (iii) an order-of-magnitude increase in the electrical conductivity of the Bi₂Te₃ thin film fabricated on silicon wafer, and (iv) a 150% increase in the Seebeck coefficient for both Bi₂Te₃ and Sb₂Te₃ samples. Furthermore, surface roughness analysis showed that deposition on polyimide substrate modestly increased the surface roughness (R_a), from 6.59 nm to 9.91 nm for Bi₂Te₃ and from 12.46 nm to 15.41 nm for Sb₂Te₃. The electrical resistivity of Bi₂Te₃ thin films on polyimide was found to be 2.72 × 10⁻³ Ω m, compared with 1.58 × 10⁻³ Ω m on silicon substrate, while for Sb₂Te_3,, the electrical resistivity on polyimide substrate increased to 580 × 10⁻³ Ω m as compared with 145 × 10⁻³ Ω m on silicon substrate. Taken together, the results of this work demonstrate that the use of high deposition power during RF sputtering of Sb₂Te₃ and Bi₂Te₃ thin films results in significant improvements in their crystallinity, conductivity, and Seebeck coefficient, which are key material properties of great importance for thermoelectric materials.