Study of oxygen vacancies in m-HfO2: first-principles calculations

Abstract

HfO2 has high dielectric properties (k ~ 18), phase stability, and wide bandgap (~5.4eV) and is easy to deposit on Si thin film. Because of these characteristics, HfO2 is used not only as a high-k material in CMOS (Complementary Metal Oxide Semiconductor) but also in a ReRAM (Resistive Random Access Memory), a well-known type of next-generation memory. ReRAM refers to a device that can store states in a non-volatile manner using a resistance change phenomenon. As an active material of ReRAM, HfO2 is widely used in OxRAM (Metal Oxide Resistive Random Access Memory) that uses the resistance change phenomenon by using the conductive filament caused by the clustering of oxygen vacancies. Therefore, understanding interaction phenomenon between oxygen vacancies is imperative. However, the oxygen vacancy interaction phenomenon of HfO2 has yet to be strictly investigated based on electronic structural analysis. Our findings proved that oxygen vacancies do not prefer interaction since they repel each other when +2 is charged. The binding force converges to 0 meV regardless of the charge state when the oxygen vacancies are spaced apart by 5 Å or more, proving that there is no long-distance interaction between them. Also, when the oxygen vacancies are neutral, it was found that bonding is favored as the distance decreases when the oxygen vacancy distance is less than 4 Å. Meanwhile, it was interesting to find that the interaction between the neutral oxygen vacancies was the strongest at the vicinity of 4 Å, thus the most stable bonding form. Oxygen vacancy bonding near the 4 Å distance is the only form that can bond between oxygen vacancies and oxygen vacancies with Hf in the middle. The interaction characteristics of the LCAO (Linear Combination of Atomic Orbitals) type between oxygen vacancies and Hf 5dz2 orbitals were carried out through electronic structure analysis. In conclusion, it was found that HfO2 prefers interaction within 4 Å only when oxygen vacancies are neutral: the most stable configuration exists through linear interaction among Hf 5dz2 orbitals and oxygen vacancies through a first-principles calculation study. This study is meaningful in that it provides a physicochemical understanding of the interaction properties of oxygen vacancies in m-HfO2, which is difficult to identify experimentally, and reveals new interaction characteristics of LCAO-type based on this.