Effect of Grain Size and Mechanical Stress on Polarization Switching of Ferroelectrics

he polarization response such as ferroelectric and ferroelastic switching in ferroelectrics is the important feature for ferroelectric and electromechanical applications. In polycrystalline form ferroelectrics, effects of the microstructural parameters such as texture, grain size, and residual stress are there and have not fully been understood. Among these effects, (1) the origin of grain size effects on ferroelastic switching, (2) mechanical stress effects on polarization switching, and (3) ferroelectric switching kinetics and the relationship to grain boundaries are investigated. Firstly, the microscopic origin of ferroelastic switching suppression in smaller grains is discovered using a microscopic probing technique (piezoresponse force microscopy). It is demonstrated that there is no independent grain size effect on ferroelastic switching; the grain size affects the domain structure in a grain, and the domain structure plays an important role in the ferroelastic switching suppression. This result suggests that the grain size is not an independent critical parameter for the electromechanical property degradation in a grain \u3c 1 m as the ferroelastic switching is a dominant component for the electromechanical property. The study about the mechanical stress effects on the electric field induced polarization switching rationalizes the emergence of the electric field induced low-symmetry phases observed in tetragonal Pb(Zr,Ti)O3 and BaTiO3 ceramics after poling. It is demonstrated that a shear stress plays an important role in stabilizing the monoclinic phase in Pb(Zr,Ti)O3 whereas a normal stress along the polarization axis is a key for the monoclinic phase in BaTiO3 with a thermodynamic approach. It is suggested that the fraction of the low-symmetry phase, which is important for the large electromechanical property, can be engineered by applying an appropriate stress. For the work about ferroelectric switching kinetics, the first direct Barkhausen noise associated with ferroelectric switching is measured. The domain switching time is quantified by the frequency of the Barkhausen noise. It is discovered that the dominant domain wall pinning site is grain boundaries based on the domain wall jump distance between pinning sites calculated from the switching time. This result suggests that the technique is a good tool for understanding the relationship between microstructure – domain wall kinetics. In sum, the mechanisms of the polarization switching suppression due to domain structure and grain boundaries, and the emergence of the low symmetry phases due to stresses are revealed. These discoveries facilitate further improvements of the device performances with engineering the domain structure, grain boundaries and residual stress

High-Speed and High-Power Ferroelectric Switching Current Measurement Instrument for Materials with Large Coercive Voltage and Remanent Polarization

A high-speed and high-power current measurement instrument is described for measuring rapid switching of ferroelectric samples with large spontaneous polarization and coercive field. Instrument capabilities (±200 V, 200 mA, and 200 ns order response) are validated with a LiTaO3 single crystal whose switching kinetics are well known. The new instrument described here enables measurements that are not possible using existing commercial measurement systems, including the observation of ferroelectric switching in large coercive field and large spontaneous polarization Al0.7Sc0.3N thin films

High-Temperature Ferroelectric Behavior of Al0.7Sc0.3N

Currently, there is a lack of nonvolatile memory (NVM) technology that can operate continuously at temperatures > 200 °C. While ferroelectric NVM has previously demonstrated long polarization retention and >1013 read/write cycles at room temperature, the largest hurdle comes at higher temperatures for conventional perovskite ferroelectrics. Here, we demonstrate how AlScN can enable high-temperature (>200 °C) nonvolatile memory. The c-axis textured thin films were prepared via reactive radiofrequency magnetron sputtering onto a highly textured Pt (111) surface. Photolithographically defined Pt top electrodes completed the capacitor stack, which was tested in a high temperature vacuum probe station up to 400 °C. Polarization–electric field hysteresis loops between 23 and 400 °C reveal minimal changes in the remanent polarization values, while the coercive field decreased from 4.3 MV/cm to 2.6 MV/cm. Even at 400 °C, the polarization retention exhibited negligible loss for up to 1000 s, demonstrating promise for potential nonvolatile memory capable of high−temperature operation. Fatigue behavior also showed a moderate dependence on operating temperature, but the mechanisms of degradation require additional study

Switching it Up: New Mechanisms Revealed in Wurtzite-type Ferroelectrics

Wurtzite-type ferroelectrics have drawn increasing attention due to the promise of better performance and integration than traditional oxide ferroelectrics with semiconductors such as Si, SiC, and III-V compounds. However, wurtzite-type ferroelectrics generally require enormous electric fields, approaching breakdown, to reverse their polarization. The underlying switching mechanism(s), especially for multinary compounds and alloys, remains elusive. Here, we examine the switching behaviors in (Al,Sc)N alloys and new wurtzite-type multinary candidate compounds we recently computationally identified. We find that switching in these tetrahedrally-coordinated materials proceeds via a variety of non-polar intermediate structures and that switching barriers are dominated by the more electronegative of the cations. For (Al,Sc)N alloys, we find that the switching pathway changes from a collective mechanism to a lower-barrier mechanism enabled by inversion of individual tetrahedra with increased Sc composition. Our findings provide insights for future engineering and realization of wurtzite-type materials and open a door to understanding domain motion

Emerging Materials and Design Principles for Wurtzite-type Ferroelectrics

The energy demand for computing and data storage will continue to rise exponentially unless non-traditional computing architectures and innovative storage solutions are explored. Low-energy computing, including compute-in-memory architectures, has the potential to address these energy and environmental challenges and, in particular, tetrahedral (wurtzite-type) ferroelectrics are promising options for both performance and integration with existing semiconductor processes. The AlScN alloy is among the few tetrahedral materials that exhibit ferroelectric switching, but the electric field required to switch the polarization i.e., the coercive field, E_c, is on the order of MV/cm, which is about 1–2 orders of magnitude higher than more traditional oxide perovskite ferroelectrics (E_c 90% of the considered multinary materials, we identify unique switching pathways and non-polar structures that are distinct from the commonly assumed switching mechanism in AlN-based materials. Our results disprove the existing design principle based on reduction of wurtzite c/a lattice parameter ratio while supporting two emerging design principles – ionicity and bond strength