Parametric study of barium titanate gradient thin films via off-center pulsed laser deposition

The deposition parameters used in complex oxide thin film growth are typically tuned to optimize functional properties with minimized heterogeneity, for instance in composition and crystalline structure. However, in pulsed laser deposition (PLD), gradients in film thickness and stoichiometry can be created deliberately by taking advantage of the anisotropic expansion of the plasma plume. Here, we use PLD with an off-center alignment to study barium titanate (BTO) thin films grown with a fluence sweep at constant pressure, and a pressure sweep at constant fluence. First, reflection high energy electron diffraction (RHEED) is used to distinguish between 2D layer-by-layer, multi-level, and 3D island growth by observing both diffraction patterns and positional RHEED scans. Next, X-ray diffraction (XRD) reveals that the out-of-plane lattice parameter increases with higher fluence values and decreases with higher pressure. Lastly, X-ray photoelectron spectroscopy (XPS) shows that Ba-rich phases can form either at low fluence conditions or within the pressure range greater than 0.02 mbar but less than 0.2 mbar. 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.