Thermally stable dielectric responses in uniaxially (001)-oriented CaBi4Ti4O15 nanofilms grown on a Ca2Nb3O10− nanosheet seed layer

To realize a high-temperature capacitor, uniaxially (001)-oriented CaBi4Ti4O15 films with various film thicknesses were prepared on (100)cSrRuO3/Ca2Nb3O10− nanosheet/glass substrates. As the film thickness decreases to 50 nm, the out-of-plane lattice parameters decrease while the in-plane lattice ones increase due to the in-plane tensile strain. However, the relative dielectric constant (εr) at room temperature exhibits a negligible degradation as the film thickness decreases to 50 nm, suggesting that εr of (001)-oriented CaBi4Ti4O15 is less sensitive to the residual strain. The capacitance density increases monotonously with decreasing film thickness, reaching a value of 4.5 μF/cm2 for a 50-nm-thick nanofilm, and is stable against temperature changes from room temperature to 400 °C irrespective of film thickness. This behaviour differs from that of the widely investigated perovskite-structured dielectrics. These results show that (001)-oriented CaBi4Ti4O15 films derived using Ca2Nb3O10− nanosheets as seed layers can be made candidates for high-temperature capacitor applications by a small change in the dielectric properties against film thickness and temperature variations.

Scientific RepoRts | 6:20713 | DOI: 10.1038/srep20713 To realize novel dielectrics with a large capacitance density and small change in ε r at elevated temperatures, we previously proposed bismuth layer-structured dielectrics (BLSDs) 11 . BLSDs have a natural superlattice structure along the c-axis [(001)-orientation], which consists of two kinds of two-dimensional monolayers (i.e., a bismuth oxide (Bi 2 O 2 ) 2+ layer and a pseudoperovskite layer generally described as (A m−1 B m O 3m+1 ) 2− , where m is the number of BO 6 octahedra in the pseudoperovskite layer) 12,13 . Compared to BaTiO 3 -based materials, some BLSDs exhibit a high Curie temperature and their ε r shows small temperature dependence along the stack direction (i.e., the c-axis) up to the high temperature region [14][15][16] . These materials also have important additional features in which the degradation in ε r as the film thickness decreases is small (i.e., a small "size effect"), allowing a high capacitance density to be realized 17 . This behaviour showing the small strain sensitivity of ε r , which is attributed to the structural two-dimensionality of BLSDs, differs from conventional perovskite-based dielectrics.
We have already investigated the dielectric properties of 500-nm-thick (001)-oriented epitaxial films of CaBi 4 Ti 4 O 15 , which are BLSDs prepared on (100) c SrRuO 3 /(100)SrTiO 3 . These films not only have an ε r of about 190 at room temperature, but they also display a stable capacitance against the applied electric field as well as temperature variations up to 500 °C 11 . Moreover, the ε r for epitaxial thin films of SrBi 4 Ti 4 O 15, which is another BLSD, does not drastically degrade as the film thickness decreases to 15 nm 17 . These reports clearly show that the ε r of thin-film capacitors of epitaxial BLSDs have a superior temperature-dependent performance compared to BaTiO 3 -based ones and may overcome issues observed in conventional BaTiO 3 -based materials in high-temperature applications.
However, the dielectric properties of BLSDs include a strong anisotropy due to the crystal structure of BLSDs 16 . In fact, our group has previously reported that the dependence of the film thickness on the dielectric property in BLSDs is strongly influenced by the tilting angle of the c-axis from the substrate normal 18 . Therefore, reproducible growth of (001)-oriented BLSDs films (i.e., the stack direction of the layers) is a critical issue to achieve a stable ε r as both the film thickness and temperature decrease.
From the viewpoint of practical applications, reproducible fabrication of uniaxially (001)-oriented BLSDs films on various common substrates, including amorphous glasses and (100) Si wafers, is a critical issue for practical applications even if the in-plane orientation of these films is more random than that of epitaxial films. Shibata et al. reported the {100} orientation control of SrTiO 3 films on amorphous glass using a seed layer of Ca 2 Nb 3 O 10 − nanosheets 19 . Based on their work, we envisioned that a Ca 2 Nb 3 O 10 − seed layer could control the (001) orientation of BLSD films because (001)-oriented BLSD films were grown on (100)SrTiO 3 single crystals 15 .
In the present study, we sought to enhance the capacitance density of Pt/(001)-oriented CaBi 4 Ti 4 O 15 /(100) c Sr-RuO 3 capacitors by decreasing the film thickness of the CaBi 4 Ti 4 O 15 layer. This should allow the capacitor size to be scaled down or the total capacitance to be increased. In the case of (Ba, Sr)TiO 3 film capacitors, ε r is reported to change drastically not only with film thickness but also with temperature, which makes it difficult to design a capacitor with a small capacitance change over a wide temperature range [8][9][10] . For (001)-oriented CaBi 4 Ti 4 O 15 films, the capacitance density reaches 4.5 μ F/cm 2 as the film thickness is decreased to 50 nm, and the capacitance has a small temperature coefficient from room temperature to 400 °C despite the increase in the residual strain

Results and discussion
Crystal structure and microstructure. The out-of-plane XRD θ-2θ patterns for CaBi 4 Ti 4 O 15 films with various thicknesses show only the h00 c and 00 l diffraction peaks of SrRuO 3 and CaBi 4 Ti 4 O 15 ( Fig. 1(a)), respectively, suggesting a (001) single orientation of CaBi 4 Ti 4 O 15 regardless of the film thickness. The X-ray pole figure measurement at a fixed 2θ angle corresponding to CaBi 4 Ti 4 O 15 119 (2θ = 30.6°) shows strong ring-shape-peaks at Psi ≑ 50° for 780-nm-thick films (inset of Fig. 1(a)), indicating a uniaxial (001)-orientation along the substrate surface normal with an in-plane random orientation. Figure 1(b) presents the enlarged XRD patterns around the CaBi 4 Ti 4 O 15 0020 peak in Fig. 1(a). The diffraction angles of the CaBi 4 Ti 4 O 15 0020 peak become higher as the film thickness decreases, indicating that the c-axis lattice parameters along the substrate normal decrease. Moreover, the in-plane XRD patterns around CaBi 4 Ti 4 O 15 220 are shown in Fig. 1(c). The CaBi 4 Ti 4 O 15 220 peaks shift toward a lower diffraction angle with decreasing film thickness, suggesting an increase in the in-plane lattice parameter (i.e., the a(b)-axis). Figure 2(a,b) plot the out-of-plane c-axis and in-plane a(b)-axis lattice parameters calculated from the diffraction data shown in Fig. 1(a-c) as functions of film thickness, respectively. Additionally, reference data for the CaBi 4 Ti 4 O 15 powder are shown 14 . The in-plane XRD measurements confirm that clear peak splitting is not detected for the 200/020 peak irrespective of the film thickness because the lattice parameters of the a-and b-axes are very close in the CaBi 4 Ti 4 O 15 crystal lattice (data not shown). The out-of-plane and in-plane lattice parameters for the 780-nm-thick film are almost the same as the reported data for strain-free CaBi 4 Ti 4 O 15 powder. As the film thickness decreases, the out-of-plane c-axis lattice parameter decreases and drops drastically below 200 nm, whereas the in-plane a-axis and b-axis lattice parameters increase from 5.42 to 5.46 Å.
To    Figure 4(a) shows the ε r for the (001)-oriented CaBi 4 Ti 4 O 15 films measured at room temperature and 100 kHz as functions of the film thickness. The ε r of these films is approximately 210, which is a negligible degradation as the film thickness decreases to 50 nm. This is almost equivalent to the results for epitaxial films 17 . As a reference of ε r , the ε r values for (Ba 0.7 Sr 0.3 )TiO 3 films reported by Parker et al., which continuously degrade as the film thickness decreases below 600 nm, are also plotted in Fig. 4(a) 9 . The ε r value of (Ba 0.7 Sr 0.3 )TiO 3 films becomes smaller than that of CaBi 4 Ti 4 O 15 with a film thickness below 80 nm, suggesting that a CaBi 4 Ti 4 O 15 layer less than 80-nm thick achieves a thin-film capacitor with a higher capacitance density than the conventional (Ba 0.7 Sr 0.3 )TiO 3 one. Figure 4(b) shows the capacitance density at room temperature as a function of film thickness, where the dashed line indicates the estimated data assuming that the CaBi 4 Ti 4 O 15 film has an almost constant ε r value (ε r = 210, which is equivalent with that of the 780-nm-thick specimen) as the film thickness changes. The theoretical capacitance density is generally proportional to the inverse of the film thickness. The capacitance densities of the obtained (001)-oriented CaBi 4 Ti 4 O 15 films are almost identical to the estimated values. As the film thickness decreases, the density increases and reaches a value of 4.5 μ F/cm 2 at a thickness of 50 nm.

Dielectric property.
It is noteworthy that ε r remains almost constant despite the increase in the residual strain as the thickness is scaled down to 50-nm (Fig. 2). Thus, we confirm that ε r of the (001)-oriented CaBi 4 Ti 4 O 15 films is free from the size-effect of the capacitance. This is an important feature not only for epitaxial (001)-oriented BLSD films, as already mentioned, but also for uniaxially (001)-oriented ones. Figure 5 shows the frequency dependencies of the capacitance density and dielectric loss, tan δ , measured from room temperature to 400 °C for 70 and 140 nm-thick CaBi 4 Ti 4 O 15 films. The capacitance density is almost independent of the measurement frequency from 100 to 10 kHz irrespective of the measurement temperature up to 400 °C. On the other hand, tan δ decreases with increasing measurement frequency from 100 to 10 kHz, but increases again above 10 kHz for 70 nm thick films as shown in Fig. 5(a). The relatively large dielectric loss at low frequency may originate from the leakage of the capacitor. The tan δ value increases as the measurement temperature increases due to the contribution from leakage current. However, the tan δ of 140 nm thick films remains at a low value below 4%, almost independent of the frequency from 10 to 10 kHz, even at the highest measurement temperature of 400 °C, as shown in Fig. 5.
It must be mentioned that no noticeable peel-off of CaBi 4 Ti 4 O 15 films was detected, either in the case of as-deposited films or in the case of films at the highest measurement temperatures of up to 400 °C irrespective of the film thickness. Figure 6 plots the temperature dependence of the capacitance density and tan δ for (001)-oriented CaBi 4 Ti 4 O 15 films with different thicknesses. The capacitance density of these films has a negative slope versus temperature, but shows a small temperature coefficient of capacitance , which is based on the defini-  tion of the Electronic Industries Alliance) in the temperature range from 25 to 400 °C regardless of film thickness down to 50 nm 23 . These TCC values fall within the range from − 350 to − 120 ppm/°C; this characteristic is almost equivalent to that of the epitaxial films in our previous work 11 . These results imply that uniaxially (001)-oriented CaBi 4 Ti 4 O 15 films with an in-plane random crystal orientation have dielectric and insulating properties with a small temperature dependence similar to epitaxial films. The capacitance density increases with decreasing film thickness for all temperature regions ( Fig. 6(a)). On the other hand, the tan δ value increases with the temperature due to the increase of the leakage current, as shown in Fig. 6(b). However, it decreases with increasing film thickness, especially if it is below 10% up to 400 °C for the films above 100 nm in thickness.
To analyse the effect of thickness on the capacitance density at elevated temperatures, Fig. 7(a) compares the capacitance densities for (001)-oriented CaBi 4 Ti 4 O 15 films measured between 150 °C and room temperature. The dashed line indicates the case without a degraded capacitance density up to 150 °C. The measured data are almost located on the dashed line, indicating a negligible difference in the capacitance change between these two temperatures. Figure 7(a) also plots the data for (Ba 0.7 Sr 0.3 )TiO 3 films as a reference; as the temperature increases to 150 °C, the capacitance density for thicker films drastically decreases and has a negative TCC value 9 . In addition, Fig. 7 − nanosheets/glass substrates. All films exhibited a (001) single orientation along the substrate surface normal, but had a random in-plane orientation. The continuous increase in the residual tensile strain as the film thickness decreases leads to a reduction in the out-of-plane "c-axis" lattice parameters and an increase in the in-plane "a-and b-axes" ones. However, the change in ε r is unremarkable as the film thickness decreases to 50 nm. Consequently, the monotonous increase in the capacitance density is proportional to the inverse of the film thickness. The capacitance densities of the obtained films show small TCC values up to a temperature of 400 °C and are almost independent of film thickness between 50 and 780 nm. By contrast, conventional (Ba, Sr)TiO 3 -based films do not exhibit these behaviours. These results indicate that uniaxially (001)-oriented CaBi 4 Ti 4 O 15 thin films prepared on Ca 2 Nb 3 O 10 − nanosheet-buffered substrates have potential in high-temperature capacitor applications.

Methods
Uniaxially (001) − nanosheets)/glass (Corning#1737) by the RF-magnetron sputtering method at substrate temperatures of 600 and 550 °C, respectively. The 1-2 unit-thick Ca 2 Nb 3 O 10 − nanosheet layers were coated onto glass substrates by the Langmuir-Blodgett process 20,24,25 . We chose CaBi 4 Ti 4 O 15 as the dielectric layer because its high Curie temperature of 790 °C enables a small temperature dependence of the capacitance at elevated temperatures. The deposition time controlled the CaBi 4 Ti 4 O 15 film thickness between 50 and 780 nm. Details of the deposition are described elsewhere 11 . After preparing circular Pt top electrodes (100 μ mφ) by electron-beam deposition, the Pt/CaBi 4 Ti 4 O 15 /SrRuO 3 capacitors were annealed at 400 °C for 30 min under O 2 gas flow.
The constituent phase and crystal orientation of the deposited films were identified by X-ray diffraction (XRD) using a Philips X'pert MRD with Cu Kα radiation. The residual strain state was also estimated by in-plane XRD measurements using a Rigaku Smart-lab diffractometer with Cu Kα radiation. The electrical properties under various temperatures were measured using an impedance analyser (HP4194A, Agilent) and a sample-heating stage.