Studying the Effect of Cation Stoichiometry and Thickness Gradients on Epitaxial BaTiO3 Films Using Off-Center Pulsed Laser Deposition

The growth parameters used in complex oxide thin film deposition are typically tuned to minimize heterogeneity. However, composition gradients in film thickness and stoichiometry can be created intentionally by taking advantage of the anisotropic expansion of the plasma plume in pulsed laser deposition (PLD). Here, we apply off-center PLD to study epitaxially grown BaTiO₃ (BTO) thin films on (001) oriented SrTiO₃ single crystal substrates. Instead of aligning the substrate symmetrically in front of the ablated target area, we deliberately offset the substrate by several millimeters from the central position while keeping the target and substrate surfaces parallel. The Ba:Ti ratio was measured as a function of laser fluence by microfocus X-ray photoemission spectroscopy at multiple positions along the sample length. We managed to obtain a stoichiometric film, i.e. 1:1 cation ratio, all across the 10 mm long sample once the fluence was increased above a threshold value. Microfocus X-ray diffraction shows a monotonic decrease in the out-of-plane lattice parameter when measuring further away from the central position at which the plume impinges on the substrate at the shortest distance. Furthermore, scanning transmission electron microscopy was used to elucidate the microstructural defects present in the nonstoichiometric samples and their localized effect on the lattice parameter by comparing lamella obtained from different locations along the sample length. This study highlights the potential of off-center PLD as an efficient way to study the effect of compositional gradients on the material properties of BTO and other multi-cation functional oxides.