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The recent observation of ferroelectricity in the metastable phases of binary metal oxides, such as HfO2, ZrO2, Hf0.5Zr0.5O2, and Ga2O3, has garnered a lot of attention. These metastable ferroelectric phases are typically stabilized using epitaxial strain, alloying, or defect engineering. Here, we propose that hole doping plays a key role in the stabilization of polar phases in binary metal oxides. Using first-principles density-functional-theory calculations, we show that holes in these oxides mainly occupy one of the two oxygen sublattices. This hole localization, which is more pronounced in the polar phase than in the nonpolar phase, lowers the electrostatic energy of the system, and makes the polar phase more stable at sufficiently large concentrations. We demonstrate that this electrostatic mechanism is responsible for stabilization of the ferroelectric phase of HfO2 aliovalently doped with elements that introduce holes to the system, such as La and N. Finally, we show that spontaneous polarization in HfO2 is robust to hole doping, and a large polarization persists even under a high concentration of holes.
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Interface effects in the phase determination of Hf0.5Zr0.5O2 epitaxial thin films
HfO2-based ferroelectrics show tremendous potential for applications in computing technologies, but questions remain as to what dictates the stabilization of the desired phase. Here, it is demonstrated that the substrate the film is grown on is more influential than factors such as thickness, defect content, and strain. The presence of different possible polymorphs of Hf0.5Zr0.5O2 are observed to vary widely for different substrate materials—with La0.67Sr0.33MnO3, (LaAlO3)0.3(Sr2AlTaO6)0.7, and Al2O3 being (more) optimal for stabilizing the ferroelectric-orthorhombic phase. This substrate effect is found to be more influential than any changes observed from varying the film thickness (7.5–60 nm), deposition environment (oxygen vs argon), and annealing temperature (400–600 °C) in vacuum (10−5 Torr). X-ray diffraction and scanning transmission electron microscopy verify the phases present, and capacitor-based studies reveal ferroelectric behavior (or lack thereof) consistent with the phases observed. First-principles calculations suggest that forming oxygen vacancies in Hf0.5Zr0.5O2 lowers its work function, driving electrons away and helping to stabilize the ferroelectric phase. Substrates with a high work function (e.g., La0.67Sr0.33MnO3) facilitate this electron transfer but must also have sufficient ion conductivity to support oxygen-vacancy formation in Hf0.5Zr0.5O2. Together, these observations help clarify key factors essential to the stabilization of HfO2-based ferroelectrics.
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- PAR ID:
- 10565140
- Publisher / Repository:
- American Institute of Physics
- Date Published:
- Journal Name:
- APL Materials
- Volume:
- 13
- Issue:
- 1
- ISSN:
- 2166-532X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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