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

Abstract

Despite increasing attention for the recently found ferro-and antiferroelectric properties, the polymorphism in hafnia-and zirconia-based thin films is still not sufficiently understood. In the present work, we show that it is important to have a good quality X-ray absorption spectrum to go beyond an analysis of the only the first coordination shell. Equally important is to analyze both EXAFS and XANES spectra in combination with theoretical modelling to distinguish the relevant phases even in bulk materials and 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 that allows us to extract high-quality spectra also for 20 nm thin films. Our analysis extends to the second coordination shell and includes effects created by chemical substitution of Hf with Zr to unambiguously discriminate the different polymorphs. The trends derived from X-ray absorption spectroscopy agree well with X-ray diffraction measurements. In this work we clearly identify a gradual transformation from monoclinic to tetragonal phase as the Zr content of the films increases. We separated structural effects from effects created by chemical disorder when ration of Hf:Zr is varied and found differences for the incorporation of the substitute atoms between powders and thin films, which we attribute to the different fabrication routes. This work opens the door for further in-depth structural studies to shine light into the chemistry and physics of these novel ferroelectric thin films that show high application relevance.

Ferroelectric Phase Content in 7 nm Hf(1−x)ZrxO2 Thin Films Determined by X‐Ray‐Based Methods

“…Similar peaks, valleys, lower peak amplitude, and multipeak second shell were all reported previously. [ 30,32 ]…”

“…The R factors are all in the range of 20–30% and the fit results estimated phase fractions without large disagreements. The R factors used to gauge the quality of fits may be higher than usually desired [ 30,32 ] (>20%) due to a few key factors. First, the crystal structure models for HZO of different compositions require fine tuning by theoretical simulations averaged over different configurations and energy considerations.…”

“…As the hafnia lattice parameters are slightly lower than those corresponding to ZrO 2 structure phases, the mixed oxides of different compositions are expected to be intermediates as reported by Materlik et al [ 37 ] However, these subtle differences in the lattice parameters are not expected to significantly change the crystal phase distribution in the samples discussed here. Schenk et al [ 32 ] used the combination of ab initio simulations, XANES and EXAFS to study high‐quality spectra from both 20 nm thin films deposited by ALD and reference powder samples of HZO of different compositions. Using the second shell analysis, they were able to study the structural and chemical effects.…”

“…All these large number of parameters needs a good initial guess and are fit to obtain a good fit by shell-by-shell analysis with an R factor of 5-10%. [30,32] However, LCF requires lesser number guessed parameters and is considered to be rigorous by constraining parameters for entire local structure of each phase as explained by McBriarty et al [33] For our study, we used LCF of the EXAFS spectra from different MIM stacks using a basis set of theoretical spectra corresponding to different phases and compositions generated from supercells as explained in the GI-XRD analysis earlier. Pca2 1 phase found in ultrathin films of hafnia-based dielectrics are nonexistent in the bulk form to be used as reference foil for acquiring model or a calibration spectra.…”

“…Similar peaks, valleys, lower peak amplitude, and multipeak second shell were all reported previously. [30,32] LCF of Zr edge data estimates consisted of 6.7% of orthorhombic phase along with 66.6% of tetragonal phase and 26.7% of monoclinic phase. While the Hf edge results estimated 4.3% of orthorhombic phase along with 67.1% of tetragonal phase and 28.6% of monoclinic phase.…”