We firstly prepared SrSi 2 thin films on insulating substrates and measured their thermoelectric properties. Thin films of SrSi system were deposited on (0001) Al 2 O 3 substrates by radio frequency magnetron sputtering method at various deposition temperatures and under various total deposition pressure. Constituent phases primary depend on the deposition temperature. The films deposited below 600°C consisted of amorphous or the metastable CaSi 2 structure phase. CaSi 2 structure phase was obtained at 600°C irrespective of the pressure and finally stable ¡-SrSi 2 (¡-phase) above 700°C. The films with CaSi 2 structure phase had low power factor below 10¯W m ¹1 K ¹2 for the temperature range between 100 and 400°C. On the other hand, the film with ¡-phase showed p-type conduction and good thermoelectric power factor beyond 700¯W m ¹1 K ¹2 at room temperature. This value is larger than the reported value of (111) one-axis-oriented Mg 2 Si films prepared by the same deposition process, maximum 130¯W m ¹1 K ¹2 at 300°C. The present result shows that ¡-phase is one of the promising candidates as thermoelectric materials.
In this work, (Ba x Sr 1-x )Si 2 thin films were prepared by the co-sputtering method at various deposition temperatures. The constituent phase of the films primarily depended on the deposition temperature and the composition x. The composition to make a solid solution was expanded by lowering the deposition temperature, compared to that of bulk-sintered bodies. Further lowering the deposition temperature produced a metastable phase, which was a layered structure (trigonal, EuGe 2 -type structure), with a low thermoelectric power factor. Substitution with Ba led to an increase in the temperature showing the highest power factor. The samples with Ba concentrations over 17% showed the maximum power factor around room temperature. © 2020 The Japan Society of Applied Physics 2. Experimental (Ba x Sr 1-x )Si 2 films 1.0-2.5 μm in thickness were prepared on (001) Al 2 O 3 substrates by the RF magnetron co-sputtering method under Ar with 5% H 2 . Sintered bodies of SrSi 2 and BaSi 2 were used as sputtering targets. The deposition temperature was 600, 650, and 700 ºC, while the deposition pressure was kept at 100 mTorr. In order to control the film composition x, the RF power for BaSi 2 was changed from 0 to 50 W, while the power for the SrSi 2 target was fixed to 50 W. The target-substrate distance was maintained at 60 mm.The Ba/(Ba + Sr) ratio, the value of x, of the deposited film was measured by X-ray fluorescence spectrometry (XRF; PANalytical, PW4400). The constituent phase of the films was characterized by X-ray diffraction (XRD; Bruker AXS, D8 DISCOVER). Scans of the 2θ-θ mode were repeated by changing the sample inclination angle (ψ) under the sample rotation conditions. Integrated 2θ-θ scans were obtained by integration along the ψ direction. 27)
The {100}-oriented (CaxSr1-x)Si2 thin films have been prepared by co-sputtering method at various deposition temperatures. Constituent phase of the films primarily depends on the deposition temperature and the composition x. Although CaSi2 films consisted of layered structure regardless of deposition temperature, the phase was changed by the deposition temperature: the majority phases of the film deposited at 600°C, 650°C and 700°C were 1T layered structure, 1T layered structure + 2H layered structure and 1T layered structure + 6R layered structure, respectively. When the (CaxSr1-x)Si2 films deposited at 700°C, the α-SrSi2-type phase was mainly confirmed below x = 0.17, which is the most stable phase of SrSi2. However, the main phase of all CaxSr1-xSi2 films deposited at 600°C changed to be 1T-type layered structure. Substitution with Ca below x = 0.50 in the film deposited at 600°C led to the decrease in the electrical resistivity compared with that of pure SrSi2.
SrSi2 composite films exhibit a significant improvement in thermoelectric performance through the mixture of the stable cubic phase with a metastable layered phase. The phase fraction of the cubic and layered phases in SrSi2 films is controlled by changing the deposition temperature. The film deposited at 700 °C, which is composed of a semimetallic cubic phase and a small portion of a metallic layered phase, shows a high figure of merit, ZT, beyond 0.4 at around room temperature. This value is eight times larger than the previously reported one for cubic SrSi2 bulk polycrystals. These large values originate from the microstructures of the films, which reduce thermal conductivity without deterioration of the power factor.
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