Takanori Mimura

<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>

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.