Dr. Simon Romani also analysed the 2nm SDC NBCO film where the SDC was deposited at a lower temperature to the NBCO (film 3 in Table 3.5). For this analysis he used the focused ion beam (FIB) technique.
Figure 3.38: (a) Large scale FIB image of the 2 nm multilayer film deposited with SDC at the lower temperature with a more zoomed in section in the yellow box shown in (b).
The STO substrate is shown at the bottom of Figures 3.38 (a) and (b). Figure 3.38 (b) is a zoomed in section of (a). The SDC layers are again much brighter than the NBCO layers, and the SDC layers appear to be of a more consistent thickness throughout the film.
Figures 3.39 and 3.40 show high angle annular dark field (HAADF) and inverse fast Fourier transform (IFFT) images on increasing magnification.
Figure 3.39: (a) Original HAADF Image and (b) IFFT Filtered (Annular Mask) on a 5nm scale.
Figure 3.40: (a) Original HAADF Image and (b) IFFT Filtered (Annular Mask) on a 2nm scale.
Figures 3.39 and 3.40 show the interfaces between the film and substrate as well as the SDC film and NBCO film layers. In part (a) of Figures 3.39 and 3.40 the darker perovskite layer shows varying thickness throughout the film. The IFFT filtering (b) gives a clearer image of the interfaces, and the perovskite → fluorite interfaces are observed to be not as sharp as the fluorite → perovskite interfaces.
Films 1 and 3 from Table 3.5 were also analysed by Prof. Joke Hadermann at Electron Microscopy for Materials Science (EMAT), University of Antwerp. Films 1 and 3 were both 2nm SDC NBCO films, with differing SDC growth temperatures (film 1 NBCO/SDC both at 850°C and film 3 is with SDC at 650°C).
Initially film 3 was analysed (for which Dr. Simon Romani performed the FIB analysis). Figure 3.41 indicates that the layers are of unequal thickness, in particular the darker perovskite layers, in agreement with the FIB analysis.
Figure 3.41: TEM of film 3 by Prof. Joke Hadermann to see if the thickness of the layers are equal throughout the film.
A magnified TEM image is shown in Figure 3.42 where the quality of the interfaces can be observed.
Figure 3.42: TEM of film 3 by Prof. Joke Hadermann to see the quality of the interfaces of the multilayer film.
The start of the first fluorite block is sharp throughout large parts of the film. Also, the first fluorite → perovskite interface is clear, although there occur steps where a layer has fluorite structure on one side of the film, and perovskite structure on the other side. For example in the yellow box in Figure 3.43, at the first fluorite → perovskite interface there is a layer that is definitely fluorite at the bottom of the image and turns to perovskite halfway up the image.
Figure 3.43: TEM of film 3 by Prof. Joke Hadermann to see the quality of the interfaces of the multilayer film.
In general throughout the rest of the film, the fluorite → perovskite interfaces look abrupt and the perovskite → fluorite interfaces do not, in agreement with the FIB analysis. The brightness gradient which is present in each perovskite block, going from bright dots at the A site positions to almost invisible ones may be caused by strain, as the difference between the four A site elements (Sm-62, Ce-58, Nd-60, Ba-56) is not that large that it could explain such large difference in the colour of the A site cation column.
The perovskite layer can be terminated with an AO or BO2 layer and from Figure 3.44 it can be seen that there is a mixture of both. The first two transitions from fluorite to perovskite (starting from the substrate at the right) occur with a CoO2 layer first, numbered 1 and 2 in Figure 3.44, while the third fluorite to perovskite transition starts with an AO layer numbered 3. The perovskite to fluorite interfaces are not clear enough to determine their termination.
Figure 3.44: TEM by Prof. Joke Hadermann to see the interfacial termination of the perovskite layer in the multilayer film. The initial fluorite to perovskite transitions are numbered from the STO substrate.
EDX analysis was performed to determine the elemental composition of each layer and to see if there was any chemical diffusion across the interfaces. The vague borders in the TEM (as shown in Figure 3.45 (a)) when going from perovskite → fluorite suggests that there is diffusion occurring.
Figure 3.45: EDX analysis by Prof. Joke Hadermann of the multilayer film (a) HAADF-STEM image of the area across which the EDX was performed (b) EDX profile across the film.
The EDX measurements are shown in Figure 3.45 (b). Figure 3.45 (a) shows a line of the intended area, over a HAADF-STEM image. Figure 3.45 (b) shows EDX spectra for the elements Co, Ba, Nd, Ce, and Sm. The HAADF-STEM image is of poorer quality due to the spot size being larger, to obtain sufficient counts for the EDX. The spectra do not precisely correspond to the line drawn on the image in Figure 3.45 (a), because during the measurement, which takes 15-30 minutes, a continuous shift of the sample occurs.
From Figure 3.45 (b) it can be seen that Ce-L has the opposite profile of Co-K, and has peaks where the HAADF-STEM profile has bright areas, indicating that Ce and Co are indeed in separate layers. Sm-L is not so clear, but this is not
unexpected since there is only 20% Sm mixed with the Ce, so the counts are low and noisy.
Comparing the HAADF-STEM profile to the Co-K profile it can be seen that they agree with the supposition that bright layers are SDC and the dark ones NBCO. Each bright region in the HAADF-STEM profile corresponds to a trough in the Co-K profile. The peaks in the Ba-L profile correspond to the peaks in the Co-K profile, although they are less clear because there is less Ba than Co in these layers. The high plateau of Ba at the end of the EDX is an artifact due to the exact overlap of the Ti curve with the Ba curve. In the film it is assumed that this curve is due to Ba and at the end where the substrate is encountered, it is assumed that this curve is Ti.
The Nd-L profile should also follow the Co-K profile if there are well defined NdBaCo2O5 perovskite blocks alternating with Sm0.2Ce0.8O2 fluorite blocks. However, the Nd does not show the same alternation as Co, and occasionally there is a Nd peak where there is a Co trough. This suggests that diffusion of Nd is occurring to the fluorite blocks. This is also observed with the second 2nm multilayer film (film 1 in Table 3.5) where SDC is deposited at 850°C.