title: Oxygen vacancy effects on an amorphous-TaOx-based resistance switch: a first principles study.
abstract: Amorphous TaOx (a-TaOx) based resistance switches have recently demonstrated outstanding performance and are being considered as one of the most promising candidates for next-generation memory cells. However, the origin of the switching mechanism is still under debate, especially on the component of the conduction filament (CF). Since the resistance change of a-TaOx is controlled by the O concentration, we perform a systematic investigation on the evolution of structures and electronic properties of a-TaOx (0.75 ≤ x ≤ 2.85) from first principles. Our results reveal the strong correlation among Ta/O coordination numbers, Ta-Ta/Ta-O bond lengths, and O concentrations in a-TaOx. For a single O vacancy in a-TaO2.5, the Ta-Ta dimer structure is found to be the most stable, and the energy position of its defect state agrees well with experiments. With the decrease of O concentration, Ta atoms tend to merge together and finally form a continuous Ta-rich region in a-TaO0.75, which suggests that not O vacancies, but the Ta-Ta bonding mainly contributes to the CF in a-TaOx based resistance switches. Our molecular dynamics simulation suggests that in the CF, Ta atoms prefer to arrange in a layer structure, and hence the phase transformation to crystalline α-Ta with interstitial O atoms is proposed. In addition, the calculations on Pt/a-TaOx/Pt heterostructures further confirm the conductive nature of the Ta-Ta bonding in a-TaOx, and also reveal the different conduction types in switching on (metallic contact) and off (electron hopping) states.
authors.affiliation: Department of Materials Engineering, The University of Tokyo, Tokyo, Japan. xiaobo@cello.t.u-tokyo.ac.jp.
authors.forename: Bo
authors.initials: B
authors.lastname: Xiao
authors.forename: Satoshi
authors.initials: S
authors.lastname: Watanabe