Recently simulation groups have reported the lanthanide series elements as the dopants that have the strongest effect on the stabilization of the ferroelectric non-centrosymmetric orthorhombic phase in hafnium oxide. This finding confirms experimental results for lanthanum and gadolinium showing the highest remanent polarization values of all hafnia-based ferroelectric films until now. However, no comprehensive overview that links structural properties to the electrical performance of the films in detail is available for lanthanide-doped hafnia. La:HfO appears to be a material with a broad window of process parameters, and accordingly, by optimization of the La content in the layer, it is possible to improve the performance of the material significantly. Variations of the La concentration leads to changes in the crystallographic structure in the bulk of the films and at the interfaces to the electrode materials, which impacts the spontaneous polarization, internal bias fields, and with this the field cycling behavior of the capacitor structure. Characterization results are compared to other dopants like Si, Al, and Gd to validate the advantages of the material in applications such as semiconductor memory devices.
A polyethylene oxide-based composite solid polymer electrolyte filled with one-dimensional ceramic Li0.33La0.557TiO3 nanofibers was designed and prepared.
the formation of a non-centrosymmetric Pca2 1 orthorhombic phase (o-phase). [1][2][3][4][5][6][7] For increasing doping concentrations, ALD HfO 2 films undergo a phase transition from a non-ferroelectric m-phase to ferroelectric orthorhombic phase and for higher concentrations to the tetragonal phase (t-phase; space group: P4 2 /nmc) if the dopants are smaller than Hf like Si and Al, or to the cubic phase if the dopants are larger than Hf like Gd, La, Sr, and Y. [8] Besides the influence of doping, four other factors are known to promote the stabilization of the ferroelectric phase: surface or interface/grain boundary energy, film stress, and the presence of oxygen vacancies. [9][10][11][12][13] Oxygen vacancies and the related defect states play an important role in the so-called wake-up effect. [14] Wake-up describes the increase of the remanent polarization during electrical field cycling with opening of an initially pinched polarization-voltage hysteresis. [11] In Hf 1−x Zr x O 2 films, Materlik et al. suggested that the bulk and surface free energy of the o-phase is located between those of the m-phase and t-phase. As a result, the o-phase is stabilized in a specific film thickness and grain size region. This suggestion matches well Thin film metal-insulator-metal capacitors with undoped HfO 2 as the insulator are fabricated by sputtering from ceramic targets and subsequently annealed. The influence of film thickness and annealing temperature is characterized by electrical and structural methods. After annealing, the films show distinct ferroelectric properties. Grazing incidence X-ray diffraction measurements reveal a dominant ferroelectric orthorhombic phase for thicknesses in the 10-50 nm range and a negligible non-ferroelectric monoclinic phase fraction. Sputtering HfO 2 with additional oxygen during the deposition decreases the remanent polarization. Overall, the impact of oxygen vacancies and interstitials in the HfO 2 film during deposition and annealing is correlated to the phase formation process.
Dual-source evaporation approach is applied to deposit AgBi2I7, AgBiI4 and Ag2BiI5 films; a planar junction AgBiI4-solar cell is demonstrated.
A synthetic route has been discovered to thermodynamically unstable, i.e., metastable, Sn(II)−perovskite oxides that have been highly sought after as lead-free dielectrics and small bandgap semiconductors. A highly facile exchange of Sn(II) is found by using a low melting SnCl 2 /SnF 2 peritectic flux, yielding mixed A-site (Ba 1−x Sn x )ZrO 3 and mixed A-and B-site (Ba 1−x Sn x )(Zr 1−y Ti y )O 3 solid solutions that exhibit a very high metastability, with up to 60% Sn(II) cations and a calculated reaction energy for decomposition of up to −0.3 eV atom −1 . Kinetic stabilization of the higher Sn(II) concentrations is achieved by the high cohesive energy of the perovskite compositions containing Zr(IV) and mixed Zr(IV)/Ti(IV) cations. Significantly red-shifted bandgaps are found with increasing Sn(II) substitution, enabling the optical absorption edge to be broadly tuned from ∼3.90 to ∼1.95 eV. Percolation pathways are calculated to occur for BSZT compositions with >12.5% Sn(II) and >25% Ti(IV) cations. High photocatalytic rates are found for molecular oxygen production for compositions which exceed the percolation thresholds, wherein extended diffusion pathways should "open up" across the structure and the charge carriers become delocalized rather than trapped. These results establish the critical importance of synthetically accessing metastable semiconductors for the discovery of advanced optical and photocatalytic properties.
Next-generation wearable electronics calls for flexible non-volatile devices for ubiquitous data storage. Thus far, only organic ferroelectric materials have shown intrinsic flexibility and processibility on plastic substrates. Here, we discovered that by controlling the heating rate, ferroelectric hafnia films can be grown on plastic substrates. The resulting highly flexible capacitor with a film thickness of 30 nm yielded a remnant polarization of 10 μC/cm 2 . Bending test shows that the film ferroelectricity can be retained under a bending radius below 8 mm with bending cycle up to 1,000 times. The excellent flexibility is due to the extremely thin hafnia film thickness. Using This article is protected by copyright. All rights reserved. the ferroelectric film as a gate insulator, a low voltage non-volatile vertical organic transistor was demonstrated on a plastic substrate with an extrapolated date retention time up to 10 years.
crystalline phase in doped HfO 2 thin film. [17,18] However, this phase cannot be observed in the phase diagram of bulk HfO 2 and ZrO 2 , [19,20] even when doped with elements that are used in the thin film counterparts. [21] Therefore, multiple factors were suggested as origin of the stabilization of the ferroelectric phase. Materlik et al. suggested that the o-phase can be stabilized due to surface energy effects, [22] and Park et al. comprehensively examined the surface energy model and their experimental observations. [23] In the latter study, it was confirmed that the o-phase can be stabilized within the nuclei formed during the atomic layer deposition (ALD) process, but the interface/grain boundary effect is not sufficient to stabilize the o-phase within the final grain size after annealing. [23] Therefore, it was suggested that the crystalline phase of the initial nuclei can remain even after crystallization, implying that the phase transformation to the monoclinic phase (m-phase, space group: P2 1 /c) can be kinetically suppressed. [23,24] Stress in thin films was also suggested as a possible origin, [1,25,26] and Shiraishi et al. experimentally proved that the polymorphism of Hf 0.5 Zr 0.5 O 2 thin films is strongly affected by the thermal expansion coefficient (TEC) of the used substrate. [27] However, the detailed mechanism of the formation of the unexpected o-phase is not yet resolved.It is generally known that the structure and electrical properties of perovskite ferroelectrics are strongly coupled. [28][29][30] The ferroelectric properties of doped HfO 2 thin films are also believed to be determined by their crystalline structure. ParkThe ferroelectricity in fluorite oxides has gained increasing interest due to its promising properties for multiple applications in semiconductor as well as energy devices. The structural origin of the unexpected ferroelectricity is now believed to be the formation of a non-centrosymmetric orthorhombic phase with the space group of Pca2 1 . However, the factors driving the formation of the ferroelectric phase are still under debate. In this study, to understand the effect of annealing temperature, the crystallization process of doped HfO 2 thin films is analyzed using in situ, high-temperature X-ray diffraction. The change in phase fractions in a multiphase system accompanied with the unit cell volume increase during annealing could be directly observed from X-ray diffraction analyses, and the observations give an information toward understanding the effect of annealing temperature on the structure and electrical properties. A strong coupling between the structure and the electrical properties is reconfirmed from this result.
Applications for integrated energy storage and pulse-power devices may have found opportunities in the emergence of the ferroelectric hafnium-zirconium oxide thin film system. To explore the boundaries of this material thin film system, 10 nm thick binary Hf0.5Zr0.5O2 (HZO) thin films are doped with Al or Si (Al or Si-doped HZO). The added dopants provide a distinct shift in behavior from ferroelectric to antiferroelectric characteristics. Si-doped Hf0.5Zr0.5O2 thin films exhibited a larger than 50 J/cm3 energy storage density with an efficiency of over 80%. The Si-doped Hf0.5Zr0.5O2 thin films were cycled 109 times up to 125 °C and maintained a robust 35 J/cm3 energy storage density and greater than 80% efficiency. Al-doped Hf0.5Zr0.5O2 thin films exhibited a larger switching field, leading to a smaller energy storage density and less robust cycling properties than Si-doped Hf0.5Zr0.5O2.
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