Depth Profiling of Oxygen Migration in Ta/HfO₂ Stacks during Ionic Liquid Gating

Area of Science:

• Materials Science
• Nanotechnology
• Solid-state Physics

Background:

• Ionic liquid (IL) gating controls material properties via ion motion at interfaces.
• Oxygen distribution is crucial for stable control in magneto-ionic systems, but its depth profiles and voltage-dependence are poorly understood.
• Understanding oxygen incorporation is key for designing advanced nanoionic devices.

Purpose of the Study:

• To quantify oxygen depth profiles and oxide formation in Si/SiO2/Ta(15)/HfO2(t) films after IL gating.
• To investigate the effect of gate voltage and HfO2 capping thickness on oxygen migration.
• To provide insights for designing magneto-ionic and nanoionic devices.

Main Methods:

• Utilized X-ray reflectivity (XRR) and X-ray photoelectron spectroscopy (XPS) to analyze nanoscale films.
• Applied ionic liquid gating with varying gate voltages and HfO2 capping thicknesses (2 and 3 nm).
• Quantified oxygen depth profiles and tantalum oxide (Ta2O5) formation.

Main Results:

• Identified a threshold electric field of ≈-2.8 MV/cm for oxygen migration from HfO2 into Ta.
• Observed a linear increase in Ta2O5 thickness with gate voltage, reaching up to 4 nm at -3 V.
• Found that thicker HfO2 layers increase the barrier for oxygen migration, while thinner layers enhance oxidation.
• Confirmed atomically sharp Ta/Ta2O5 interfaces, indicating sequential oxide layer formation without deep diffusion into Ta.
• Detected indium migration from the electrode, relevant for surface-sensitive applications.

Conclusions:

• Precise control over oxygen incorporation in nanoscale stacks is achievable via IL gating.
• The study provides quantitative data on oxygen migration thresholds and oxide formation kinetics.
• Findings advance design principles for magneto-ionic and nanoionic devices, emphasizing the role of interface engineering and capping layer thickness.