Preparation of Ca-Si Films on (001) Al2O3 Substrates by an RF Magnetron Sputtering Method and Their Electrical Properties

Abstract

The constituent phases, electrical conductivity, and Seebeck coefficient of Ca-Si films deposited on (001) Al 2 O 3 substrates by a radio frequency magnetron sputtering method using a Mg disk target with Ca and Si chips are investigated. X-ray diffraction analysis indicates that the films consist of a single phase of CaSi 2 , CaSi or Ca 5 Si 3 that are deposited together with the films consisting of a mixture of CaSi 2 and CaSi. Films with a CaSi 2 or CaSi single phase exhibit a metallic behavior. In contrast, films with a Ca 5 Si 3 single phase show p-type conduction and their Seebeck coefficient reaches 90 lV/K at 400°C.

Giant Enhancement of Seebeck Coefficient by Deformation of Silicene Buckled Structure in Calcium‐Intercalated Layered Silicene Film

“…The CaSi 2 films with metastable deformed silicene exhibit ≈3000 times enhancement of S 2 σ (≈40 μW cm −1 K −2 ) from that of reported CaSi 2 film at room temperature (RT). [34] This S 2 σ value is the highest among silicide-based TE films, [35][36][37] higher than that of Bi 2 Te 3 [38] and comparable to the state-of-the-art TE materials (SnSe, [39] Bi 0.5 Sb 1.5 Te 3 , [40] AgPbSbTe, [41] PbTe-SrTe, [42] and SiGeAu [43,44] ). This study demonstrates that S 2 σ is greatly enhanced by the deformation of silicene buckled structure, opening an avenue for realizing eco-friendly TE film.…”

“…e) S 2 σ of CaSi 2 thin films, the other silicide thin films and bulk, the well‐known rare metal‐based TE thin films, and the state‐of‐the‐art TE bulks at RT. [ 34–44,48 ]…”

“…This S 2 σ value is ≈3000 times larger than previously‐reported polycrystalline CaSi 2 thin films. [ 34 ] Although a metal with ultrahigh σ exhibits low | S | of <10 μV K −1 in general, semimetal CaSi 2 films with pseudo gap exhibited not only ultrahigh σ but also relatively high | S | of ≈40 μV K −1 presumably due to the manipulation of energy dispersion relation originating from the deformation of buckled structure. As a result, remarkably high S 2 σ was obtained in CaSi 2 films composed of only eco‐friendly elements, whose value is much higher than those of silicide thin films (FeSi 2 , MnSi 1.73 , Mg 2 Si) and bulk (CaSi 2 ), [ 35–37,48 ] and even higher than those of well‐known TE thin film materials (Bi 2 Te 3 and SnSe), [ 55,56 ] as shown in Figure 4e.…”

“…e) S 2 σ of CaSi 2 thin films, the other silicide thin films and bulk, the well-known rare metalbased TE thin films, and the state-of-the-art TE bulks at RT. [34][35][36][37][38][39][40][41][42][43][44]48] much larger than the calculated values at E−E F = 0 of 6R-CaSi 2 with the perfect crystal structure.…”

“…The crystal structure in the present CaSi 2 thin films with average Δz 1 = 0.969 Å (17.7 %) and Δz 2 = 0.960 Å (−0.56 %) is similar to SD-6R-CaSi 2 with Δz 1 = Δz 2 = 0.965 Å compared to 6R-CaSi 2 with perfect structure (Δz 1 = 0.823 and Δz 2 = 0.965 Å). e) S 2 σ of CaSi 2 thin films, the other silicide thin films and bulk, the well-known rare metalbased TE thin films, and the state-of-the-art TE bulks at RT [34][35][36][37][38][40][41][42][43][44]48]…”