Takanori Mimura

<jats:p>(ZrxTa1−x)2O5−x was prepared by a solid-state reaction of ZrO2 and Ta2O5, and the L′-Ta2O5 phase was obtained by cooling the H-Ta2O5 phase. High-temperature x-ray diffraction measurements showed that the starting materials, ZrO2 and low-temperature L-Ta2O5, formed the high-temperature H-Ta2O5 phase when heated above 1360 °C. Upon cooling, this phase sequentially transformed into L″-Ta2O5, the high-temperature L′-Ta2O5 phase, and L′-Ta2O5 phases. As the Zr content, x, decreased, the transition from the H-Ta2O5 phase to the L″-Ta2O5 phase slowed. The temperature dependence of the dielectric constant revealed a maximum value, which is attributed to the phase transition from L′-Ta2O5 to L″-Ta2O5. This transition temperature decreases by approximately 50 °C for every 0.01 increase in the x value. The L′-Ta2O5 phase exhibited negative volumetric thermal expansion (NTE) behavior near the phase transition temperature. As x decreased, the NTE coefficient increased from −1.09 × 10−6/K (77–127 °C) for x = 0.10 to −2.06 × 10−5/K (327–427 °C) for x = 0.05. The substitution of Zr into Ta2O5 stabilized the non-centrosymmetric L′-Ta2O5 phase and controlled the phase transition temperature and thermal expansion behavior.</jats:p>

<jats:p>A dielectric material with a noncentrosymmetric L-Ta2O5-related structure, Zr0.10Ta0.90O2.45, was synthesized through a solid-state reaction using Ta2O5 and ZrO2 powders, followed by a 1700 °C heat treatment. The structure was determined to have a C-centered orthorhombic symmetry [a = 6.3717(2) Å, b = 10.8003(4) Å, c = 3.87058(12) Å], and is denoted as L′-Ta2O5. The possible space groups are C222, Cmm2, C2mm, or Cm2m. The L′-Ta2O5-type Zr0.10Ta0.90O2.45 has a strong second-harmonic generation signal and higher dielectric constant of 55, compared to conventional L-Ta2O5-related structures. High-temperature x-ray diffraction shows the phase transition to the L″-Ta2O5 phase with a pseudo-hexagonal structure around 400 K. The temperature dependence of the dielectric constant reveals that the phase has a maximum value of 60, which is attributed to the phase transition. Zr0.10Ta0.90O2.45 is a potential candidate for application in complementary metal–oxide–semiconductor-compatible devices using noncentrosymmetric materials.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Thin films of ferroelectric materials have been investigated for various applications because of their high dielectric constants, as well as piezoelectric and ferroelectric properties. Ferroelectricity has been explored for memory applications because of its two stable states after releasing an electric field, depending on the direction. Perovskite-based ferroelectrics have been studied for the last 30 years for these applications and have already been commercialized. However, the degradation of their ferroelectricity with decreasing film thickness (below about 30 nm) makes high-density memory applications difficult. A recent “discovery” of novel ferroelectrics, e.g., fluorite-type structure HfO<jats:sub>2</jats:sub>-based films and wurtzite structure AlN-, GaN-, and ZnO-based films, have enabled significant reductions in film thickness without noticeable degradation. In this article, we discuss the status and challenges of these novel non-perovskite-based ferroelectric films mainly for memory device applications.</jats:p>

Abstract To investigate the Ta&amp;amp;amp;lt;sup&amp;amp;amp;gt;5+&amp;amp;amp;lt;/sup&amp;amp;amp;gt;-substitution effects on crystal structure and ferroelectric property in HfO&amp;amp;amp;lt;sub&amp;amp;amp;gt;2&amp;amp;amp;lt;/sub&amp;amp;amp;gt;-based films, Ta _x Hf_1-x O_2+δ films with various film thicknesses and Ta content were prepared. The ferroelectric orthorhombic phase was formed in a wide film thickness range of 20-100 nm while in a narrow composition range of x = 0.10-0.14. These thickness-insensitive and composition-sensitive characteristics of Ta⁵⁺-substituted HfO₂ film are similar to Y³⁺ rather than Zr⁴⁺. The X-ray photoelectron spectroscopy measurement suggests that the ionic state of Ta is not reduced and Ta _x Hf_1-x O_2+δ film has an excess oxygen state. The excess oxygen may consist of a combination of oxygen vacancies and more interstitial oxygens. These defects facilitate the formation of the ferroelectric phase, while decreasing the breakdown voltage and increasing the leakage current in Ta⁵⁺-substituted HfO₂ films. On the other hand, the generation of excess oxygen indicates the possibility of controlling oxygen vacancies which deteriorate fatigue and retention properties.

Ferroelectricity in binary oxides including hafnia and zirconia has riveted the attention of the scientific community due to the highly unconventional physical mechanisms and the potential for the integration of these materials into semiconductor workflows. Over the last decade, it has been argued that behaviours such as wake-up phenomena and an extreme sensitivity to electrode and processing conditions suggest that ferroelectricity in these materials is strongly influenced by other factors, including electrochemical boundary conditions and strain. Here we argue that the properties of these materials emerge due to the interplay between the bulk competition between ferroelectric and structural instabilities, similar to that in classical antiferroelectrics, coupled with non-local screening mediated by the finite density of states at surfaces and internal interfaces. Via the decoupling of electrochemical and electrostatic controls, realized via environmental and ultra-high vacuum piezoresponse force microscopy, we show that these materials demonstrate a rich spectrum of ferroic behaviours including partial-pressure-induced and temperature-induced transitions between ferroelectric and antiferroelectric behaviours. These behaviours are consistent with an antiferroionic model and suggest strategies for hafnia-based device optimization.

<jats:p>The influence of biaxial stress on the maximum and remanent polarizations of 10 nm thick hafnium zirconium oxide thin films in metal–ferroelectric–metal capacitor structures has been quantified. In the as-prepared state with a nominal biaxial tensile strain of 0.20% and no applied extrinsic stress, remanent and maximum polarizations of 7.6 and 13.1 μC/cm2, respectively, were measured using a 2 MV/cm applied electric field. Reducing the intrinsic strain by 0.111% through the application of a compressive uniaxial stress results in a decrease in the remanent and maximum polarizations to 6.8 and 12.2 μC/cm2, respectively. The polarization dependence on strain is nearly linear between these values. The observed variation in polarization with strain is consistent with strain impacting ferroelastic switching whereby in-plane tension increases the fraction of the short polar axis orienting out-of-plane, hence increasing out-of-plane polarization. In contrast, reducing the in-plane strain through compression results in an increase in the fraction of the long non-polar axis orienting out-of-plane, thereby decreasing out-of-plane polarization.</jats:p>

Abstract The asymmetry in the capacitance–voltage (C–V) curves obtained from a ferroelectric material can provide information concerning the internal microstructure of a specimen. The present study visualized nanoscale switching of a HfO₂-based ferroelectric thin film in real space based on assessing asymmetry using a local C–V mapping method. Several parameters were extracted from the local C–V curves at each point. The parameter V_i, indicating the lateral shift of the local C–V curve, was employed as an indicator of local imprint. In addition, the differences in the areas between the C–V curves for the forward and reverse sweeps, S_f − S_r, provided another slightly different indicator of nanoscale switching asymmetry. These parameters obtained from asymmetric C–V curves are thought to be related to internal electric fields and local stress caused by defects in the film. The work reported here also involved a cluster analysis of the extracted parameters using the k-means method.

<jats:sec><jats:label/><jats:p>Y‐doped HfO<jats:sub>2</jats:sub> ferroelectric films of ≈1 μm thick are deposited without heating by a radio frequency magnetron sputtering method. {100}‐oriented epitaxial films with orthorhombic phase are grown on (100)ITO//(100)YSZ substrates without heating. Their crystal structure is almost unchanged after postheat treatment at 800 °C. Ferroelectricity is confirmed for the no‐heating‐deposited films by polarization−electric field (<jats:italic>P−E</jats:italic>) curves, and their remanent polarization (<jats:italic>P</jats:italic><jats:sub>r</jats:sub>) and coercive fields are 12 μC cm<jats:sup>−2</jats:sup> and 1.5 MV cm<jats:sup>−1</jats:sup>, respectively. These values are also almost unchanged after the postheat treatment. However, the postheat‐treated films show a lower breakdown electric field compared to the as‐deposited films without heat treatment. Approximately 1 μm‐thick films are also prepared without heating on (111)ITO/(111)Pt/TiO<jats:sub><jats:italic>x</jats:italic></jats:sub>/SiO<jats:sub>2</jats:sub>/(100)Si and (111)Pt/TiO<jats:sub><jats:italic>x</jats:italic></jats:sub>/SiO<jats:sub>2</jats:sub>/(100)Si substrates. Almost pure orthorhombic/tetragonal phase is deposited on both substrates without heating. The <jats:italic>P</jats:italic><jats:sub>r</jats:sub> value of the film on the (111)ITO/(111)Pt/TiO<jats:sub><jats:italic>x</jats:italic></jats:sub>/SiO<jats:sub>2</jats:sub>/(100)Si substrate is about 1.5 times larger than that on the (111)Pt/TiO<jats:sub><jats:italic>x</jats:italic></jats:sub>/SiO<jats:sub>2</jats:sub>/(100)Si substrate due to the better crystallinity of the film lattice‐matched with the underlying ITO layer. The effective piezoelectric constant (<jats:italic>d</jats:italic><jats:sub>33,<jats:italic>f</jats:italic></jats:sub>) of the film deposited on the (111)ITO/(111)Pt/TiO<jats:sub><jats:italic>x</jats:italic></jats:sub>/SiO<jats:sub>2</jats:sub>/(100)Si substrate without heating is estimated to be 4 pm V<jats:sup>−1</jats:sup>. The present low process temperature leads us to expect novel applications, especially for low‐heat‐resistance applications.</jats:p></jats:sec>

<jats:p> Electronic conduction pathways in dielectric thin films are explored using automated experiments in scanning probe microscopy (SPM). Here, we use large field of view scanning to identify the position of localized conductive spots and develop an SPM workflow to probe their dynamic behavior at higher spatial resolution as a function of time, voltage, and scanning process in an automated fashion. Using this approach, we observe the variable behaviors of the conductive spots in a 20-nm-thick ferroelectric Hf<jats:sub>0.54</jats:sub>Zr<jats:sub>0.48</jats:sub>O<jats:sub>2</jats:sub> film, where conductive spots disappear and reappear during continuous scanning. There are also fresh conductive spots that develop during scanning. The automated workflow is universal and can be integrated into a wide range of microscopy techniques, including SPM, electron microscopy, optical microscopy, and chemical imaging. </jats:p>