Ultra-low thermal conductivities in large-area Si-Ge nanomeshes for thermoelectric applications

Abstract

In this work, we measure the thermal and thermoelectric properties of large-area Si0.8Ge0.2 nano-meshed films fabricated by DC sputtering of Si0.8Ge0.2 on highly ordered porous alumina matrices. The Si0.8Ge0.2 film replicated the porous alumina structure resulting in nano-meshed films. Very good control of the nanomesh geometrical features (pore diameter, pitch, neck) was achieved through the alumina template, with pore diameters ranging from 294 ± 5nm down to 31 ± 4 nm. The method we developed is able to provide large areas of nano-meshes in a simple and reproducible way, being easily scalable for industrial applications. Most importantly, the thermal conductivity of the films was reduced as the diameter of the porous became smaller to values that varied from κ = 1.54 ± 0.27 W K−1m−1, down to the ultra-low κ = 0.55 ± 0.10 W K−1m−1 value. The latter is well below the amorphous limit, while the Seebeck coefficient and electrical conductivity of the material were retained. These properties, together with our large area fabrication approach, can provide an important route towards achieving high conversion efficiency, large area, and high scalable thermoelectric materials.

Advances in Scanning Thermal Microscopy Measurements for Thin Films

“…One way of reducing the thermal conductivity of a bulk material without affecting the transport properties is through reducing its dimensionality [36], such as preparing thin films. This thermal conductivity reduction has been reported for several materials, such as SiGe [37][38][39]…”

A Review on Principles and Applications of Scanning Thermal Microscopy (SThM)

“…Perez‐Taborda et al reported Si 0.8 Ge 0.2 nanomeshes with thermal conductivity down to 0.55 ± 0.10 W m −1 K −1 and n‐type Ag 2 Se film with 0.64 ± 0.10 W m −1 K −1 , being both obtained by 3ω‐SThM at room temperature. Si and SiGe nanowires were discovered to have size effects on thermal conductivity at 50 nm and 480 nm diameters respectively, when measured by 3ω‐SThM 147b…”

Nanoporous Ca3Co4O9 Thin Films for Transferable Thermoelectrics

“…1−3 The thermoelectric efficiency of any material system is related to dimensionless thermoelectric figure of merit ZT (= S 2 T /ρκ), where S , ρ, κ, and T are the Seebeck coefficient, electrical resistivity, thermal conductivity, and absolute temperature, respectively. High thermoelectric efficiency requires high Seebeck coefficient simultaneously with low electrical resistivity and thermal conductivity.…”