The hafnate perovskites PbHfO3(antiferroelectric) and SrHfO3(“potential” ferroelectric) are studied as epitaxial thin films on SrTiO3(001) substrates with the added opportunity of observing a morphotropic phase boundary (MPB) in the Pb1−
Since 2011, ferroelectric HfO2has attracted growing interest in both fundamental and application oriented groups. In this material, noteworthy wake‐up and fatigue effects alter the shape of the polarization hysteresis loop during field cycling. Such changes are problematic for application of HfO2to ferroelectric memories, which require stable polarization hystereses. Herein, electrical and structural techniques are implemented to unveil how cyclic switching changes nanoscale film structure, which modifies the polarization hysteresis. Impedance spectroscopy and scanning transmission electron microscopy identify regions with different dielectric and conductive properties in films at different cycling stages, enabling development of a structural model to explain the wake‐up and fatigue phenomena. The wake‐up regime arises due to changes in bulk and interfacial structuring: the bulk undergoes a phase transformation from monoclinic to orthorhombic grains, and the interfaces show changes in and diminishment of a nonuniform, defect rich, tetragonal HfO2layer near the electrodes. The evolution of these aspects of structuring contributes to the increase in
- NSF-PAR ID:
- 10086016
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Advanced Electronic Materials
- Volume:
- 2
- Issue:
- 9
- ISSN:
- 2199-160X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract x Srx HfO3system. The resulting (240)‐oriented PbHfO3(Pba 2) films exhibited antiferroelectric switching with a saturation polarization ≈53 µC cm−2at 1.6 MV cm−1, weak‐field dielectric constant ≈186 at 298 K, and an antiferroelectric‐to‐paraelectric phase transition at ≈518 K. (002)‐oriented SrHfO3films exhibited neither ferroelectric behavior nor evidence of a polarP 4mm phase . Instead, the SrHfO3films exhibited a weak‐field dielectric constant ≈25 at 298 K and no signs of a structural transition to a polar phase as a function of temperature (77–623 K) and electric field (–3 to 3 MV cm−1). While the lack of ferroelectric order in SrHfO3removes the potential for MPB, structural and property evolution of the Pb1−x Srx HfO3(0 ≤x < 1) system is explored. Strontium alloying increased the electric‐breakdown strength (E B) and decreased hysteresis loss, thus enhancing the capacitive energy storage density (U r) and efficiency (η). The composition, Pb0.5Sr0.5HfO3produced the best combination ofE B = 5.12 ± 0.5 MV cm−1,U r = 77 ± 5 J cm−3, and η = 97 ± 2%, well out‐performing PbHfO3and other antiferroelectric oxides. -
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Abstract As an emerging nonvolatile memory technology, HfO2‐based ferroelectrics exhibit excellent compatibility with silicon CMOS process flows; however, the reliability of polarization switching in these materials remains a major challenge. During repeated field programming and erase of the polarization state of initially pristine HfO2‐based ferroelectric capacitors, the magnitude of the measured polarization increases, a phenomenon known as “wake‐up”. In this study, the authors attempt to understand what causes the wake‐up effect in Hf0.5Zr0.5O2(HZO) capacitors using nondestructive methods that probe statistically significant sample volumes. Synchrotron X‐ray diffraction reveals a concerted shift in HZO Bragg peak position as a function of polarization switching cycle number in films prepared under conditions such that they exhibit extremely large (≈3000%) wake‐up. In contrast, a control sample with insignificant wake‐up shows no such peak shift. Capacitance – voltage measurements show evolution in the capacitance loop with switching cycle number for the wake‐up sample and no change for the control sample. Piezoresponse force microscopy measurements are utilized to visualize the domain switching with wake‐up. The combination of these observations clearly demonstrates that wake‐up is caused by a field‐driven phase transformation of the tetragonal phase to the metastable ferroelectric orthorhombic phase during polarization switching of HZO capacitors.
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While ferroelectric HfO2shows promise for use in memory technologies, limited endurance is one factor that challenges its widespread application. Herein, endurance is investigated through field cycling W/Hf0.5Zr0.5O2/W capacitors above the coercive field while manipulating the time under field using bipolar pulses of varying pulse duration or duty cycle. Both remanent polarization and leakage current increase with increasing pulse duration. Additionally, an order of magnitude decrease in the pulse duration from 20 to 2 μs results in an increase in endurance lifetime of nearly two orders of magnitude from 3 × 106to 2 × 108cycles. These behaviors are attributed to increasing time under field allowing for charged oxygen vacancy migration, initially unpinning domains, or driving phase transformations before segregating to grain boundaries and electrode interfaces. This oxygen vacancy migration causes increasing polarization before creating conducting percolation paths that result in degradation and premature device failure. This process is suppressed for 2 μs pulse duration field cycling where minimal wake‐up and lower leakage before device failure are observed, suggesting that very short pulses can be used to significantly increase device endurance. These results provide insight into the impact of pulse duration on device performance and highlight consideration of use of conditions when endurance testing.
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Abstract Knowledge about phase transitions in doped HfO2and ZrO2‐based films is crucial for developing future ferroelectric devices. These devices should perform in ambient temperature ranges with no degradation of device performance. Here, the phase transition from the polar orthorhombic to the nonpolar tetragonal phase in thin films is of significant interest. Detailed electrical and structural characterization is performed on 10 nm mixed HfxZr1‐xO2binary oxides with different ZrO2in HfO2and small changes in oxygen content. Both dopant and oxygen content directly impact the phase transition temperature between the polar and nonpolar phase. A first‐order phase transition with thermal hysteresis is observed from the nonpolar to the polar phase with a maximum in the dielectric constant. The observed phase transition temperatures confirm trends as obtained by DFT calculations. Based on the outcome of the measurements, the classification of the ferroelectric material is discussed.
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