Oxygen vacancy effects on an amorphous-TaOx-based resistance switch: a first principles study

Abstract

Amorphous TaO_x (a-TaO_x) 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-TaO_x is controlled by the O concentration, we perform a systematic investigation on the evolution of structures and electronic properties of a-TaO_x (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-TaO_x. For a single O vacancy in a-TaO_2.5, 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-TaO_0.75, which suggests that not O vacancies, but the Ta–Ta bonding mainly contributes to the CF in a-TaO_x 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-TaO_x/Pt heterostructures further confirm the conductive nature of the Ta–Ta bonding in a-TaO_x, and also reveal the different conduction types in switching on (metallic contact) and off (electron hopping) states.