Ta⁵⁺-substitution effects on crystal structure and ferroelectric property in HfO₂-based films

Abstract To investigate the Ta 5+ -substitution effects on crystal structure and ferroelectric property in HfO 2 -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 Ta 5+ -substituted HfO 2 film are similar to Y 3+ rather than Zr 4+ . 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 phasebut also decrease the breakdown voltage and increase the leakage current in Ta 5+ -substituted HfO 2 films. On the other hand, the generation of excess oxygen indicates the possibility of controlling oxygen vacancies which deteriorate fatigue and retention properties.