Effect of surface nanopatterning on the thermoelectric properties of bismuth antimony telluride films

Abstract

Nanostructuring is known to yield higher energy conversion efficiencies of thermoelectric materials. The ion beam method is a versatile technique to create such nanostructures which allows precise control over irradiation parameters such as ion energy, ion flux, incident angle, and bombardment time. We report the effect of sputtering angle (θ) on surface nanopatterning of bismuth antimony telluride (Bi0.6Sb1.4Te3) films prepared by ion beam sputtering. It was found that the nanostructure growth occurred for films sputtered at θ ≥ 45°. Atomic force microscopy images confirmed the evolution of surface topography with surface features up to 4.5 nm for the film deposited at 60°. Hall results showed that the electrical conductivity (σ) increased from ∼6 Ω−1cm−1 for the film deposited at 0°, to 10 Ω−1cm−1 for the film deposited at 60°. This was due to the increased in mobility from ∼2.5 cm2V−1s−1 to 6 cm2V−1s−1, respectively for these films in which the carrier density (∼2 × 1019 cm−3) remained invariant across the sputtering angles. Seebeck coefficient, |α|, for the film deposited at 0° was 390 µVK−1 which decreased to 194 µVK−1 for the film deposited at 60°. The highest power factor (α2σ = 90 µWm−1K−2) was found for the film deposited at 0° due to a high Seebeck coefficient. The results showed that surface nanopatterning of thermoelectric films can be simply achieved by varying the sputtering angle during ion beam sputtering.