High-resolution electron energy-loss spectroscopy of $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3∕\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ multilayers

The dielectric properties of $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3$ thin films and multilayers are different from bulk materials because of nanoscale dimensions, interfaces, and stress-strain conditions. In this study, $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3∕\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ multilayers deposited on $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ substrates by pulsed laser deposition have been investigated by high-energy-resolution electron energy-loss spectroscopy. The fine structures in the spectra are discussed in terms of crystal-field splitting and the internal strain. The crystal-field splitting of the $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3$ thin layer is found to be a little larger than that of bulk $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3$, which has been interpreted by the presence of the internal strain induced by the misfit at the interface. This finding is consistent with the lattice parameters of the $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3$ thin layer determined by the selected area diffraction pattern. The near-edge structure of the oxygen K edge in $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3$ thin layers and in bulk $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3$ are simulated by first-principle self-consistent full multiple-scattering calculations. The results of the simulations are in a good agreement with the experimental results. Moreover, the aggregation of oxygen vacancies at the rough $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3∕\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ interface is indicated by the increased $\left[\mathrm{Ti}\right]∕\left[\mathrm{O}\right]$ element ratio, which dominates the difference of dielectric properties between $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3$ layer and bulk materials.

Figure 1

(a) Cross-sectional TEM dark-field image of the multilayers consisting of $\mathrm{STO}\left(50\mathrm{nm}\right)∕\mathrm{BTO}\left(10\mathrm{nm}\right)$. (b) Cross-sectional STEM bright-field image of the multilayers consisting of $\mathrm{STO}\left(100\mathrm{nm}\right)∕\mathrm{BTO}\left(10\mathrm{nm}\right)$. (c) Selected area diffraction pattern from the interface region between the $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3$ film and the $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ substrate. The reflections from the (400) and (004) planes are magnified 8 times in the insets. (d) EEL spectra line-scan acquisition across a BTO layer. The energy drift was continuously monitored at the marked position. The white arrow indicates the scanning direction. The upper and lower interfaces are indicated by dashed white lines. The raster image, consisting of a series of spectra, is shown in the inset.

Figure 2

(a) Cubic perovskite structure of bulk $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3$ above the Curie point. (b) Tetragonal perovskite structure of bulk $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3$ at room temperature. The labeled lattice parameters and bond lengths are given in Å.

Figure 3

Crystal-field splitting in the ${\mathrm{L}}_3$ and ${\mathrm{L}}_2$ edges across the $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3$ layer. The vertical dotted lines indicate the interface positions. Additionally, the measured values for bulk $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3$ are given (triangles).

Figure 4

Experimental spectra of bulk BTO and STO compared to $\mathrm{Ti}{\mathrm{O}}_2$ anatase, brookite, and rutile. The splitting of the $e_g$ peak in the ${\mathrm{L}}_3$ edge is indicated by vertical dotted lines. Satellite structures appear above the white lines.

Figure 5

(a) Comparison of the fine structure of the titanium ${\mathrm{L}}_{23}$ edge measured in STO, BTO, and at the STO/BTO interface. (b) Comparison of the fine structure of the oxygen K edge. The reference spectra measured in bulk BTO and STO were averaged by five spectra taken from different positions.

Figure 6

(a) Satellite structures of the Ti ${\mathrm{L}}_2$ edge of $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ calculated in the shell mode of the FMS method. The spectra are labeled by the total number of the atoms in the clusters. The number of atoms in the new shell with respect to the previous cluster is indicated in parenthesis. (b) The experimental satellite structures of the Ti ${\mathrm{L}}_{23}$ spectrum compared with the FMS calculations in $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$.

Figure 7

Experimental and simulated oxygen K edges.

Figure 8

$\left[\mathrm{Ti}\right]∕\left[\mathrm{O}\right]$ ratio across the $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3$ layer. The vertical dotted lines indicate the upper and lower interfaces. The abscissa denotes the relative distance from the middle of the BTO layer.