Search

The analyses of thermal transport properties can help improve the performance of high-tech semiconducting devices, such as thermoelectric generators and electronic devices. However, methods to measure thermal transport properties are limited, especially the group velocity and phonon mean free path (MFP) of thin films. Herein, we propose a simple technique to measure the group velocity and phonon MFP based on nanoindentation using Si and SiO2/Si wafers. The group velocities, including the longitudinal, transverse, and average group velocities, were estimated using Young's modulus, shear modulus, and Poisson's ratio. The phonon MFP was estimated from the average group velocity, lattice thermal conductivity, and specific heat. The determined group velocities (∼6050 m/s) and phonon MFP (∼80 nm) when the Si wafer was used as a bulk material were in good agreement with the corresponding reference data. The influence of the underlying Si substrate increased when the indentation depth was increased when the SiO2/Si wafer was used as a thin film (SiO2 layer thickness, 200 nm). The influence of the underlying Si substrate existed even when the smallest indentation depth of 50 nm and the determined group velocities (∼4750 m/s) and phonon MFP (∼0.5 nm) were slightly different from those of the reference data. Therefore, although the accuracy of the measurement technique can be improved, this study verified that nanoindentation can be used to measure the group velocity and phonon MFP of materials. Furthermore, this technique opens a pathway for investigating the thermal transport properties of nanostructured materials. [ABSTRACT FROM AUTHOR]

High-quality thermoelectric thin films formed on flexible substrates are important for the development of flexible thermoelectric generators as power supplies for wireless sensors and wearable devices. However, the thermoelectric properties of the films formed on flexible substrates are inferior to those of films formed on rigid substrates because flexible substates shrink during the processes of film deposition and post-thermal annealing. To overcome this problem, we propose a modification during these processes. Bismuth telluride thin films were deposited on polyimide substrates using radio-frequency magnetron sputtering. The substrates were bent to a convex or concave shape, followed by thermal annealing under the same bending conditions. The crystallite size and crystal orientation improved when the films were set to a concave shape during the film deposition and post-thermal annealing processes. This phenomenon occurred presumably because the substrate shrinkage was counteracted by the stretching of the substrate, and the strains did not accumulate much in the films, resulting in improved crystal growth. Consequently, the electrical conductivity was increased, and the resulting power factor of 16.1 μW/(cm K2) is 1.4 times higher than that of the Bi 2 Te 3 film formed on a flat substrate. [Display omitted] • Bi 2 Te 3 thin films were deposited on polyimide substrates using RF sputtering. • Three substrate shapes (flat, convex, and concave) were used for deposition. • Post-thermal annealing performed for two substrate shapes (convex and concave). • Substate shape during deposition and annealing affected thermoelectric properties. • Highest power factor was 16.1 μW/(cm K2) for the convex-shaped substrate. [ABSTRACT FROM AUTHOR]