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

Area of Science:

• Materials Science
• Solid State Physics
• Computational Materials Science

Background:

• Amorphous tantalum oxide (a-TaOx) resistance switches show promise for next-generation memory.
• The exact mechanism of the conduction filament (CF) in a-TaOx remains debated.
• Oxygen concentration is known to influence the resistance change in a-TaOx.

Purpose of the Study:

• To systematically investigate the structural and electronic properties of a-TaOx with varying oxygen concentrations (0.75 ≤ x ≤ 2.85).
• To elucidate the primary contribution to the conduction filament (CF) in a-TaOx based resistance switches.
• To understand the atomic arrangement and electronic behavior within the CF.

Main Methods:

• First-principles calculations to study structures and electronic properties of a-TaOx.
• Molecular dynamics simulations to analyze atomic arrangements in the CF.
• Calculations on Pt/a-TaOx/Pt heterostructures to confirm conductive properties.

Main Results:

• A strong correlation was found between Ta/O coordination, bond lengths, and oxygen concentration in a-TaOx.
• For oxygen vacancies in a-TaO2.5, Ta-Ta dimer structures are the most stable.
• Decreasing oxygen concentration leads to Ta atom merging and formation of Ta-rich regions, indicating Ta-Ta bonding as the main CF contributor.
• Molecular dynamics simulations suggest a layered structure for Ta atoms in the CF.
• Calculations confirm the conductive nature of Ta-Ta bonding and reveal distinct conduction mechanisms for the ON (metallic) and OFF (hopping) states.

Conclusions:

• Ta-Ta bonding, rather than oxygen vacancies, is the dominant factor in forming the conduction filament in a-TaOx resistance switches.
• A phase transformation to crystalline α-Ta with interstitial oxygen atoms is proposed for the CF.
• The study provides critical insights into the switching mechanism of a-TaOx, paving the way for improved memory device design.