Spectroscopic investigation of the structural and electronic properties of oxygen-deficient tantalum oxide thin films

Oxygen-deficient tantalum oxide (TaO_x, x < 2.5) thin films are often used as switching films for memristors with various switching mechanisms. The ion transport (and related switching mechanism) and filament stability strongly depend on the chemical composition and structure of the films. In order to scrutinize the structure-property relationship, the local and electronic structures of TaO_x thin films with various degrees of oxygen deficiencies (x = 1.0, 1.5, 2.1, and 2.5) are investigated by spectroscopic methods, including X-ray absorption spectroscopy and X-ray/ultraviolet photoelectron spectroscopy. To ensure the oxygen deficiency is accommodated as intended, the films are capped with Au layers in situ. The results of the analyses on the Ta valence, Ta-O bond characteristics, and electronic structure indicate that TaO_1.0 is stable and metallic despite the oxidation. Meanwhile, TaO_x with x ≥ 1.5 is almost fully coordinated by oxygen with a robust local structure, while the bond lengths and disorders are reduced gradually. The resemblance in the chemistry and electronic structure of TaO_1.0 to the Ta metal (and of TaO_2.1 to Ta₂O₅) implies that the intermediate chemical states in the Ta-TaO_x heterojunction, though present, would not degrade the memristive functionality, accounting for the superior endurability of the Ta-based memristors.