三村 和仙

Abstract To investigate the Ta<sup>5+</sup>-substitution effects on crystal structure and ferroelectric property in HfO<sub>2</sub>-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.