Local structural investigation of hafnia-zirconia polymorphs in powders and thin films by X-ray absorption spectroscopy

Despite increasing attention for the recently found ferro- and antiferroelectric properties, the polymorphismin hafnia- and zirconia-based thin films is still not sufficiently understood. In the presentwork, we show that it is important to have a good quality X-ray absorption spectrum to go beyond ananalysis of the only the first coordination shell. Equally important is to analyze both EXAFS and XANESspectra in combination with theoretical modelling to distinguish the relevant phases even in bulk materialsand to separate structural from chemical effects. As a first step toward the analysis of thin films,we start with the analysis of bulk references. After that, we successfully demonstrate an approach thatallows us to extract high-quality spectra also for 20 nm thin films. Our analysis extends to the secondcoordination shell and includes effects created by chemical substitution of Hf with Zr to unambiguouslydiscriminate the different polymorphs. The trends derived from X-ray absorption spectroscopy agreewell with X-ray diffraction measurements. In this work we clearly identify a gradual transformation frommonoclinic to tetragonal phase as the Zr content of the films increases. We separated structural effectsfrom effects created by chemical disorder when ration of Hf:Zr is varied and found differences for theincorporation of the substitute atoms between powders and thin films, which we attribute to thedifferent fabrication routes. This work opens the door for further in-depth structural studies to shinelight into the chemistry and physics of these novel ferroelectric thin films that show high applicationrelevance.