Abstract
Accurate determination and comprehensive understanding of the intrinsic c-axis thermal conductivity κc of thermoelectric layered Sb2Te3 is of high importance for the development of strategies to optimize the figure of merit in thin film devices via heterostructures and defect engineering. We present here high precision measurements of κc of epitaxial Sb2Te3 thin films on Al2O3 substrates grown by physical vapor deposition in the temperature range of 100 K to 300 K. The Kapitza resistances of the involved interfaces have been determined and subtracted from the film data, allowing access to the intrinsic thermal conductivity of single crystalline Sb2Te3. At room temperature, we obtain κc = 1.9 W/m K, being much smaller than the in-plane thermal conductivity of κab = 5 W/m K and even lower than the thermal conductivity of nano crystalline films of κnc ≈ 2.0–2.6 W/m K published by Park et al. [Nanoscale Res. Lett. 9, 96 (2014)]. High crystallinity and very low defect concentration of the films were confirmed by x-ray diffraction and high resolution transmission electron microscopy. Our data reveal that the phonon mean free path lmfpT is not limited by defect scattering and is of intrinsic nature, i.e., due to phonon-phonon scattering similar to other soft van der Waals type bonded layered systems.
Highlights
Tetradymite-type materials such as Sb2Te3 and Bi2Te3 are thermoelectric materials with promising high figure of merit ZT close to 1 in a temperature range of 300–400 K which is promising for technical applications.2 The thermoelectric efficiency of a material is given by the dimensionless figure of merit ZT 1⁄4 (S2r/j)T [S 1⁄4 Seebeck coefficient, r 1⁄4 specific electrical conductivity, j 1⁄4 thermal conductivity 1⁄4 sum of electronic jel and lattice jla contributions, T 1⁄4 absolute temperature (K)].3 In order to achieve a high ZT, high Seebeck coefficients (S), high electrical conductivity (r) and low thermal conductivity (j) are required
We present here high precision measurements of jc of epitaxial Sb2Te3 thin films on Al2O3 substrates grown by physical vapor deposition in the temperature range of 100 K to 300 K
The same author claimed a high ZT value of 2.4 for p-type Sb2Te3/Bi2Te3 multilayer structures.6. This result could not be reproduced by other research groups and was later questioned since the superlattices have been shown to be unstable at elevated temperatures
Summary
Tetradymite-type materials such as Sb2Te3 and Bi2Te3 are thermoelectric materials with promising high figure of merit ZT close to 1 in a temperature range of 300–400 K which is promising for technical applications. The thermoelectric efficiency of a material is given by the dimensionless figure of merit ZT 1⁄4 (S2r/j)T [S 1⁄4 Seebeck coefficient, r 1⁄4 specific electrical conductivity, j 1⁄4 thermal conductivity 1⁄4 sum of electronic jel and lattice jla contributions, T 1⁄4 absolute temperature (K)].3 In order to achieve a high ZT, high Seebeck coefficients (S), high electrical conductivity (r) and low thermal conductivity (j) are required. The thermoelectric efficiency of a material is given by the dimensionless figure of merit ZT 1⁄4 (S2r/j)T [S 1⁄4 Seebeck coefficient, r 1⁄4 specific electrical conductivity, j 1⁄4 thermal conductivity 1⁄4 sum of electronic jel and lattice jla contributions, T 1⁄4 absolute temperature (K)].3. In order to achieve a high ZT, high Seebeck coefficients (S), high electrical conductivity (r) and low thermal conductivity (j) are required. The same author claimed a high ZT value of 2.4 for p-type Sb2Te3/Bi2Te3 multilayer structures.. The same author claimed a high ZT value of 2.4 for p-type Sb2Te3/Bi2Te3 multilayer structures.6 This result could not be reproduced by other research groups and was later questioned since the superlattices have been shown to be unstable at elevated temperatures.. For many thermoelectric relevant materials the lowering of lattice thermal conductivity by the introduction of grain boundaries or interfaces in heterostructures have been demonstrated. Reduction of thermal conductivity down to j 1⁄4 0.22 W/m K by a factor of 7–8 was reported for Sb2Te3/Bi2Te3 multilayer structures by Venkatasubramanian et al. The same author claimed a high ZT value of 2.4 for p-type Sb2Te3/Bi2Te3 multilayer structures. This result could not be reproduced by other research groups and was later questioned since the superlattices have been shown to be unstable at elevated temperatures. Here, reduced cross-plane thermal conductivity (0.55 to 0.6 W/m K) is due to nanoalloying of the superlattice at elevated temperatures was observed, while no measurable reduction of the thermal conductivity by the superlattice-type 2D nanostructuring was found (see the overview in Ref. 8) pointing against simultaneous low thermal and high electrical cross plane conductivities in such systems
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