Chaojing Lu

The presence of scarce (101)-oriented F:SnO ₂ grains in the fluorine-doped SnO ₂ layer with a slight (100) texture seems insufficient to drive predominantly (001)-oriented growth of a BiVO ₄ photoanode through the alleged microscale epitaxy.

In their Progress Report, Sytnyk et al. claimed the epitaxial deposition of metal halide perovskites by inkjet printing on various substrates. The claims of epitaxy are doubtful due to a lack of convincing experimental evidence. Both optical microscopy and scanning electron microscopy images alone are insufficient to prove epitaxy. Even if the images suggest that epitaxial growth might have been achieved, direct evidences from X‐ray diffraction (XRD) or electron diffraction are still needed. Among the six perovskite‐substrate systems, only two CH₃NH₃PbI₃ deposits on (111)PbTe/BaF₂ are characterized with XRD. According to their XRD patterns, actually, it is estimated that each of their MAPbI₃ deposits contains only 2. None of the MAPbI₃ deposits can be considered epitaxial. It is critical to point out the errors in the methodology for verifying epitaxy, so that researchers would not be misled by Sytnyk's paper. Importantly, a new methodology to quantify the volume fraction of each epitaxial component in a thin film from XRD measurements is proposed. It is reliable to estimate the degree of texture according to the proposed equations, quite different from using Lotgering degree of orientation.

Recently, doped HfO₂ thin films have attracted considerable attention because of promising applications in complementary metal–oxide–semiconductor (CMOS)‐compatible ferroelectric memories. Herein, the ferroelectric properties and polarization fatigue of La:HfO₂ thin‐film capacitors are reported. By varying the substrate lattice constant and film thickness, a robust remanent polarization of ≈16 μC cm⁻² is achieved in a 12 nm‐thick Pt/La:HfO₂/La_0.67Sr_0.33MnO₃ capacitor. Fatigue measurements are conducted using designed pulse sequences, in which the voltage, pulse width, and interval time are changed to observe the evolution of switchable polarization with increasing cycles. Severe fatigue is observed when the La:HfO₂ capacitors are partially switched and the interval between the bipolar switching is elongated. These behaviors may be ascribed to the domain wall pinning scenario, in which domain switching is blocked by the migration and aggregation of charges on non‐electroneutral walls. Further analysis of the fatigue behaviors with a nucleation‐limited‐switching model shows that the mean time and activation field for polarization switching are increased in fatigued La:HfO₂ capacitors because electrical stimuli are required to disperse the aggregated charges before the domains are set free. These results facilitate the design and fabrication of HfO₂‐based ferroelectric memories with improved device reliability.

Bi₄Ti₃O₁₂‐based thin films of a/b‐axes orientation grown on standard‐type Pt/Ti/SiO₂/Si substrates are desirable for designing reliable ferroelectric devices. Herein, a/b‐axes‐oriented and randomly oriented Bi₄Ti₃O₁₂ thin films are deposited on Pt/Ti/SiO₂/Si through a sol–gel process. As expected, the a/b‐axes‐oriented films show a higher remnant polarization than that of randomly oriented films. The characteristic time τ of domain reversal in 74% a/b‐axes‐oriented Bi₄Ti₃O₁₂ films is 1.1 ms, notably shorter than that (τ > 2.5 ms) for randomly oriented films. The dynamic hysteresis scaling relations of the randomly oriented 56% and 74% a/b‐axes‐oriented Bi₄Ti₃O₁₂ films take the forms of ⟨A⟩ ∝ f^−0.106E₀^1.667, ⟨A⟩ ∝ f^−0.035E₀^0.952, and ⟨A⟩ ∝ f^−0.026E₀^0.600, respectively. The difference indicates that domain reversal in Bi₄Ti₃O₁₂ films of predominant a/b‐axes orientation exhibits higher stability compared with randomly oriented Bi₄Ti₃O₁₂ films under varying electric fields E(E₀, f). The enhancing speed and stability of polarization reversal in predominantly a/b‐axes‐oriented Bi₄Ti₃O₁₂ films can be understood in terms of their layered‐perovskite structure, large columnar grains, oxygen vacancies mobility, domain states, and polarization switching mechanism.