Microstructure and interface studies of ${\mathrm{LaVO}}_3$/${\mathrm{SrVO}}_3$ superlattices

The structure and interface characteristics of (${\mathrm{LaVO}}_3$)${}_{6m}$(${\mathrm{SrVO}}_3$)${}_m$ superlattices deposited on a (100)-SrTiO${}_3$ substrate were studied using transmission electron microscopy (TEM). Cross-section TEM studies revealed that both ${\mathrm{LaVO}}_3$ (LVO) and ${\mathrm{SrVO}}_3$ (SVO) layers are good single-crystal quality and epitaxially grown with respect to the substrate. It is evidenced that LVO layers are made of two orientational variants of a distorted perovskite compatible with bulk ${\mathrm{LaVO}}_3$, while SVO layers suffers from a tetragonal distortion due to the substrate-induced stain. Electron energy loss spectroscopy investigations indicate changes in the fine structure of the V $L_{23}$ edge, related to a valence change between the ${\mathrm{LaVO}}_3$ and the ${\mathrm{SrVO}}_3$ layers.

Figure 1

Top: Schematic showing the first two terms of an [$(X){}_{6m}(Y){}_m$] series of superlattice. The composition is identical in both cases but the periodicity of the superlattice is different. Bottom: HREM images (JEOL 2010F) for the first three terms of the superlattices (LaVO${}_3$)${}_{6m}$(SrVO${}_3$)${}_m$, i.e., samples $m=1$, $m=2$, and close to $m=3$, with a compound 17/3 (denoted $m~3$).

Figure 2

(a) Low-magnification HAADF-STEM image of an $m=3$ (18/3) superlattice. (b) High-resolution HAADF-STEM image at the film/substrate interface. The SrVO${}_3$ (SVO) layer is marked by its darker contrast compared to the surrounding LaVO${}_3$ (LVO) layers. Note that there is some amount of roughness or atomic steps at the LVO/SVO interfaces.

Figure 3

(a) [100]${}_p$ SAED patterns from an $m=1$ superlattice. The most intense reflections can be indexed in the perovskite subcell. Along [001]${}_p$* (white arrows), satellite reflections appear systematically associated with reflections of the perovksite subcell. (b) Enlargement of the area within the white rectangle in (a). The measured periodicity is $27.1\AA \approx 7a_p$ and is therefore compatible with an $m=1$ (LaVO${}_3$)${}_6$/(SrVO${}_3$)${}_1$ superlattice. (c) Enlargement of part of the [100]${}_p$ SAED patterns from the $m~3$ (LaVO${}_3$)${}_{17}$/(SrVO${}_3$)${}_3$ superlattice with a periodicity of $77.3\AA \approx 20a_p$.

Figure 4

(a) Central area of a [100]${}_p$ SAED pattern from an $m=1$ superlattice and its schematic representation. The rows of additional reflections indicated by white arrows can be decomposed into two parts: (b) the first, located at the $0\frac{k}{2}\frac{l}{2}$ positions of the perovskite subcell, are compatible with a [010] zone axis pattern for LaVO${}_3$ (orthorhombic Pnma bulk reference); (c) the second, located at the $0\frac{k}{2}l$ positions of the perovskite subcell, are consistent with a [101] zone axis pattern.

Figure 5

HREM images of the film/substrate interface for two samples in the series (LaVO${}_3$)${}_{6m}$(SrVO${}_3$)${}_m$, with (a) $m~3$ and (b) $m=7$. For each sample, Fourier transforms performed on the circled areas show that LaVO${}_3$ layers are formed by the imbrication of LVO${}_{\mathrm{I}}$- and LVO${}_{\mathrm{II}}$-oriented domains.

Figure 6

(a) HREM image of the film/substrate interface for a 135-nm-thick $m=7$ superlattice and the local deformations obtained by the GPA method along the in-plane (${\varepsilon }_{\mathit{xx}}$) and out-of-plane (${\varepsilon }_{\mathit{yy}}$) directions. Reference is made to the STO substrate. Compared to the substrate chosen as the reference area, green coloring indicates no change; blue, a negative deformation; and red-orange, a positive deformation. (b) HRTEM image of LVO/SVO interfaces (middle of the film) and the associated local deformations ${\varepsilon }_{\mathit{xx}}$ and ${\varepsilon }_{\mathit{yy}}$. Reference is made here to the LVO layer. The profile across the $~$3-nm-thick SVO layer indicates a deformation of about $-2.2$%.

Figure 7

Probe scanning EELS spectra in STEM mode for SrVO${}_3$ and LaVO${}_3$ layers from an $m=3$ (18/3) superlattice. (a) SrVO${}_3$ and LaVO${}_3$ spectra with the background removed before the V $L_{23}$ edge and the energy aligned by cross-correlation to the La $M_5$ peak (present in the SrVO${}_3$ layer as well because of the spatial resolution limitations and signal delocalization). (b) SrVO${}_3$ and LaVO${}_3$ spectra (V $L_{23}$ edge and O $K$ edge) after plural scattering removal. The SrVO${}_3$ spectrum is subtracted from the weighted LaVO${}_3$ spectrum to eliminate the effect of overlap.