Enhancement of ferroelectricity and orientation in solution-derived hafnia thin films through heterogeneous grain nucleation

Abstract

In this work, we demonstrate an approach for improving ferroelectric properties of La:HfO2 thin films by shifting the grain growth regime toward heterogeneous nucleation. A dilute 0.083 M instead of a 0.25 M solution together with an annealing step after every spin-coating cycle film gives rise to a significant improvement of ferroelectric properties. While a remanent polarization of 7 μC/cm2 was found for randomly oriented conventional films, the value of 15 μC/cm2 for the dilute solution is a result of a mixed 111 and 002 preferential orientation. A more than 50% improved breakdown voltage stems from a global density improvement from 8.0 to 8.4 g/cm3 as obtained from x-ray reflectivity (XRR). We also find superstructure peaks in XRR hinting on periodic alternations of the local density throughout the film thickness. Scanning transmission electron microscopy and secondary ion mass spectrometry confirm this periodicity. The sensitivity of XRR for this periodicity was leveraged to gain further insights in the origin of this superstructure with additional experiments. We conclude that both orientation and the superstructure are caused by a “layered structure” according to Schuler's microstructural zone model. However, our data also provide evidence for parallel chemical effects of cap formation in each stacked sub-layer. While this work shows a significant enhancement of ferroelectric properties, it also provides insights into further optimization potential of solution deposition of HfO2/ZrO2 thin films. Our XRR-based approach supplemented with suitable additional analysis can be of great value for the optimization of other solution-derived thin films beyond the material class studied here.