One of the general features of ferroelectric systems is a complex nature of polarization reversal, which involves domain nucleation and motion of domain walls. Here, time‐resolved nanoscale domain ...imaging is applied in conjunction with the integral switching current measurements to investigate the mechanism of polarization reversal in yttrium‐doped HfO2 (Y:HfO2)—currently one of the most actively studied ferroelectric systems. More specifically, the effect of film microstructure on the nucleation process is investigated by performing a comparative study of the polarization switching behavior in the epitaxial and polycrystalline Y:HfO2 thin film capacitors. It is found that although the epitaxial Y:HfO2 capacitors tend to switch slower than their polycrystalline counterparts, they exhibit a significantly higher nucleation density and rate, suggesting that this is a rate‐limiting mechanism. In addition, it is observed that under the external fields approaching the activation field value, the switching kinetics can be described equally well by the nucleation limited switching and the Kolmogorov‐Avrami‐Ishibashi models for both types of capacitors. This signifies convergence of two different mechanisms implying that the polarization reversal proceeds via a homogeneous nucleation process unaffected by the film microstructure, which can be considered as approaching the intrinsic switching limit.
Snapshots of instantaneous domain patterns arising during polarization reversal in polycrystalline and epitaxial Y:HfO2 capacitors obtained by stroboscopic piezoresponse force microscopy. Convergence of Kolmogorov‐Avrami‐Ishibashi and nucleation limited switching models at high fields close to the thermodynamic activation value suggests a homogeneous nucleation process unaffected by film microstructure, which can be considered as approaching the intrinsic switching limit.
Ferroelectricity and Curie temperature are demonstrated for epitaxial Y-doped HfO2 film grown on (110) yttrium oxide-stabilized zirconium oxide (YSZ) single crystal using Sn-doped In2O3 (ITO) as ...bottom electrodes. The XRD measurements for epitaxial film enabled us to investigate its detailed crystal structure including orientations of the film. The ferroelectricity was confirmed by electric displacement filed - electric filed hysteresis measurement, which revealed saturated polarization of 16 μC/cm(2). Estimated spontaneous polarization based on the obtained saturation polarization and the crystal structure analysis was 45 μC/cm(2). This value is the first experimental estimations of the spontaneous polarization and is in good agreement with the theoretical value from first principle calculation. Curie temperature was also estimated to be about 450 °C. This study strongly suggests that the HfO2-based materials are promising for various ferroelectric applications because of their comparable ferroelectric properties including polarization and Curie temperature to conventional ferroelectric materials together with the reported excellent scalability in thickness and compatibility with practical manufacturing processes.
Abstract
Ferroelectric thin films are important because of their great potential for use in various electric devices such as ferroelectric random-access memory. It was expected that Bi
2
SiO
5
, a ...Si-containing ferroelectric material, would show improved ferroelectricity by targeting a film with the (001)-orientation (polar-axis) on the substrate. Although there was a narrow process window for the deposition of the (010)/(001)-oriented Bi
2
SiO
5
thin film, it was successfully prepared on a (101)-oriented TiO
2
single substrate using the pulsed layer deposition technique. The optimum film deposition conditions and film thickness were found, and in this material, the volume fraction of the (001)-oriented domain reached about 70%. By controlling film orientation to the polar axis, the remanent polarization value of this film was 4.8 μC cm
−2
, which is the highest value among reported Bi
2
SiO
5
.
Epitaxial films of (K,Na,Li)NbO3 with 10 µm thickness and various Li contents were fabricated at 240 °C on (001)La:SrTiO3 substrates by a hydrothermal method, and their crystal structures and ...piezoelectric properties were investigated. The film thickness was controlled by varying the deposition time (up to 3.5 h) and the number of deposition cycles. Scanning electron microscopy observations showed that dense thick films were formed. X-ray diffraction (XRD) measurements showed that {001}c-oriented epitaxial films were deposited, and the out-of-plane lattice constant changed with an increase in the nominal composition A = LiOH/(KOH + NaOH + LiOH) of the alkaline source solution. High-temperature XRD measurement revealed that with an increase in A, the Curie temperature increased, while the orthorhombic–tetragonal phase transition temperature decreased from 210 to 120 °C. These structural changes indicate that the Li content in the thick films can be controlled by varying A. The dielectric properties depend on the measurement frequency, and the minimum relative dielectric permittivity at all frequencies was observed at A = 0.02. Curves of field-induced strain vs. electric field showed that the maximum normalized strain of Smax/Emax = 40 pm/V was observed at A = 0.03, indicating that Li substitution is an effective way to improve the piezoelectricity of the hydrothermally deposited (K,Na)NbO3 thick films. Interestingly, all thick films exhibited a piezoelectric response despite the applied electric field being lower than the coercive field. Moreover, the Smax/Emax value did not change significantly with an increasing applied electric field. These results suggest that the hydrothermally deposited (K,Na,Li)NbO3 thick films adopt a self-polarized state without poling treatment.
Preparation of orthorhombic Y-doped TaON film Mimura, Takanori; Shimizu, Takao; Funakubo, Hiroshi
Journal of the Ceramic Society of Japan,
07/2022, Letnik:
130, Številka:
7
Journal Article
Recenzirano
Odprti dostop
Y-doped TaON films were grown on (001)YSZ and (012)Al2O3 substrates by radio frequency (RF) magnetron reactive sputtering deposition from Ta metal target and Y metal chips under the mixture gases of ...N2, O2 and Ar gases. Films with various Y content were prepared by changing area ratio of Y/(Y + Ta) of the target. The pure monoclinic phase was obtained for TaON films on both substrates by controlling the partial pressure of N2 and O2 gasses. Crystal structure changed from the monoclinic phase to the orthorhombic one with the lattice parameters of a = 0.518 nm, b = 0.488 nm, and c = 0.504 nm with the increase in Y content in the films. Our results reveal that TaON could be used as a novel mother fluorite-like monoclinic structure to prepare orthorhombic phase.
Effects of alkaline solution concentration and deposition temperature on the film thickness, crystal structure, and ferroelectric properties were investigated for epitaxial PbTiO3 films grown on ...(100)cSrRuO3//(100)SrTiO3 substrates by hydrothermal method. The (001)-oriented PbTiO3 films were grown at 150 °C with KOH solution concentrations from 1 to 10 mol/L. Potassium incorporation was not detected for the films deposited at 10 mol/L. c-axis lattice parameters continuously decreased with the increase in the KOH solution concentration. A 250-nm-thick film grown at 8 mol/L showed lattice parameters expected from bulk PbTiO3 and showed good ferroelectric properties; a remanent polarization and an average coercive field (average of plus and minus coercive field, Ec) of about 74 µC/cm2 and about 173 kV/cm, respectively. Film thickness was successfully increased by raising the deposition temperature to 200 °C at 8 mol/L. Lattice parameters were almost unchanged by changing the deposition temperature. The relationship between Ec and the film thickness (t) agreed with the semiempirical scaling law (Ec ∼ t−2/3). These data suggest that the increase in the deposition temperature is one of the effective ways to deposit thick PbTiO3 films.
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