We investigate ferroelectric BaTiO3 capacitors with SrRuO3-based electrodes employing a (BaxSr1−x)TiO3 (BST) buffer layer on (100) SrTiO3 substrates prepared by pulsed laser deposition. Structural analysis by high-resolution X-ray diffraction reciprocal space mapping shows that the BST (x = 0.5 and 0.7) layers are relaxed and have their bulk in-plane lattices in the upper part owing to strain relief. The bottom electrodes on the buffer layers grow heteroepitaxially but cannot withstand a tensile stress and then show a diminution of the in-plane lattice. On the BST (x = 0.7) layer, compared with the capacitor with an SrRuO3 electrode, that with a Ba0.1Sr0.9RuO3 one has a reduced in-plane lattice relaxation of the BaTiO3 film and then exhibits a larger remanent polarization (Pr) of 34 μC/cm2 associated with a smaller shift of the hysteresis loop. The polarization hysteresis shift is attributed to a flexoelectric effect stemming from the coupling between out-of-plane polarization and a strain gradient in the BaTiO3 film. We conclude that a reduced misfit strain relaxation in the ferroelectric film achieved with the Ba0.1Sr0.9RuO3 electrode on the thicker BST (x = 0.7) buffer layer is the origin of an enhanced Pr with a smaller hysteresis shift.
Ferroelectric BaTiO3 (BT) thin-film capacitors with a buffer layer of (Ba1−
x
Sr
x
)TiO3 (BST) have been fabricated on (001) SrTiO3 (STO) single-crystal substrates by a pulsed laser deposition method, and the crystal structure and polarization hysteresis properties have been investigated. X-ray diffraction reciprocal space mapping shows that the BST buffer effectively reduces the misfit strain relaxation of the BT films on SrRuO3 (SRO) electrodes. The BT capacitor with the SRO electrodes on the BST (x = 0.3) buffer exhibits a well-saturated hysteresis loop with a remanent polarization of 29 µC/cm2. The hysteresis loop displays a shift toward a specific field direction, which is suggested to stem from the flexoelectric coupling between the out-of-plane polarization and the strain gradient adjacent to the bottom interface.
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