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.