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Ferroelectric zirconium oxide for piezo- and pyroelectric devices (Zeppelin)

Subject Area Electronic Semiconductors, Components and Circuits, Integrated Systems, Sensor Technology, Theoretical Electrical Engineering
Synthesis and Properties of Functional Materials
Term from 2020 to 2023
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 433647091
 
Final Report Year 2024

Final Report Abstract

The Zeppelin project’s primary goals were to make significant strides in the theoretical and experimental understanding of ferroelectricity, pyroelectricity, and piezoelectricity in ZrO 2 and Zr-rich Hf1-xZrxO2. In the context of the current state of the field of fluorite-structured ferroelectrics, ZrO2 is in general less studied and less well-understood than HfO2. Notably, the predominant tendency of ZrO2 to exhibit antiferroelectric behavior in thin film form distinguishes it from HfO2 and has considerable implications ferro-, piezo-, and pyroelectric devices. The origin of antiferroelectricity and its influence on the electrically active properties of ZrO2 films were objects of theoretical and experimental investigation within the project. On the experimental side, various film deposition techniques such as ALD, PVD, and CSD were used to grow ZrO2 and Hf1-xZrxO2. Growing thick ZrO2 films, which can be advantageous for sensing and actuating applications, were limited in ALD and PVD processes due to the growth of the undesirable low-k monoclinic phase, which could be overcome by using CSD. Process dependencies on the oxygen content and doping significantly impact the ZrO2 ferroelectric and pyroelectric, broadly transferrable to commercial applications, including memories and sensors. Piezoresponse force microscopy measurements confirmed the genuine antiferroelectric response of ZrO2, and the first images of the local piezoresponse of electric field-induced phase transitions were demonstrated. The use of Raman spectroscopy to unambiguously distinguish the polar orthorhombic from the nonpolar tetragonal phases in 45 nm ZrO2 film marks a significant experimental milestone in quantifying the phases in relatively thin fluorite films. This approach could also confirm the irreversible tetragonal to polar orthorhombic phase transition with wake-up cycling in ZrO2. Progress was made in better understanding the stress and strain dependency of ferroelectricity in ZrO2 films, which can be further built upon in future studies. Different to the ALD and PVD case, thick ferroelectric layers of undoped and Y-doped doped ZrO2 up to 500nm could be produced by CSD. With theoretical calculations, the polymorphism of ZrO2 was thoroughly investigated, and two unidentified low-energy phases were found so far. For doped ZrO2, only a small window for the support of the polar phase compared to the tetragonal phase was found. Oxygen interstitials substantially destabilized the tetragonal phase, reducing the kinetic energy barrier. The piezo- and pyroelectric constants were very different, genuinely negative, and large and positive, close to the phase transition. The Zeppelin project has significantly contributed to the knowledge of the ferroelectric, piezoelectric, and pyroelectric properties of ZrO2-based films. A number of follow-up investigations appear promising for further understanding and exploiting these properties for memory, neuromorphic, and sensing applications.

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