Fabrication and Structural Analysis of Trilayers for Tantalum Josephson Junctions with Ta$_2$O$_5$ Barriers

Tantalum (Ta) has emerged as a promising low-loss material, enabling record coherence times in superconducting qubits. This enhanced performance is largely attributed to its stable native oxide, which may host fewer two-level system (TLS) defects, which are the key contributors to decoherence in superconducting circuits. Nevertheless, aluminum oxide remains the predominant choice for Josephson junction (JJ) barriers in most qubit architectures. Here, we investigate techniques for forming high-quality oxide layers on $α$-phase tantalum films to develop tantalum-oxide JJ barriers. We explore thermal oxidation in a tube furnace, rapid thermal annealing, and plasma oxidation of both room-temperature and heated Ta films, characterize the resulting structures using X-ray techniques and electron microscopy, and propose a mechanistic picture of the oxidation pathways. We find that plasma oxidation provides the smoothest Ta$_2$O$_5$ layers, is compatible with in situ Ta deposition, and offers thickness control through the annealing temperature, advantageous for JJ fabrication. Lastly, we evaluate methods for growing Ta/TaO$_x$/Ta trilayers. All trilayers showed c-axis-oriented columnar growth of the bottom Ta layer, with sapphire substrates producing larger, better-aligned grains yet higher dislocation densities than silicon. Nucleation of c-axis-oriented $α$-Ta on tantalum-oxide required an Nb seed layer, as direct Ta deposition yielded amorphous Ta. These results demonstrate the feasibility of $α$-Ta/Nb/TaO$_x$/$α$-Ta stacks for JJs with clean interfaces.

💡 Research Summary

This paper investigates the feasibility of using tantalum oxide (Ta₂O₅) as the tunnel barrier in superconducting Josephson junctions (JJs) to exploit the low‑loss properties of α‑phase tantalum (α‑Ta). Three oxidation techniques—tube furnace annealing, rapid thermal annealing (RTA), and oxygen plasma oxidation—were applied to α‑Ta films deposited on sapphire or silicon substrates (the latter with a 6 nm Nb seed layer). The goal was to produce thin, uniform, and low‑defect Ta₂O₅ layers suitable for sub‑10 nm JJ barriers, while also enabling subsequent growth of a top α‑Ta electrode to form a complete α‑Ta/Ta₂O₅/α‑Ta trilayer stack.

Oxidation Process Comparison

1. Tube Furnace Oxidation: Conducted at 400 °C for 10–60 min under a high O₂ flow (20 000 sccm). X‑ray reflectometry (XRR) showed oxide thicknesses increasing from ~37 nm to ~60 nm with longer exposure, and atomic force microscopy (AFM) revealed surface roughness (Rₐ) between 0.5 nm and 1.7 nm. The resulting layers are relatively thick and rough, making them unsuitable for tunneling barriers that must be thinner than the superconducting coherence length (≈20–50 nm).

2. Rapid Thermal Annealing (RTA): Performed at 700 °C with 5 000 sccm O₂ for 1–10 min. The high thermal gradients and large lattice mismatch between Nb/Ta (a ≈ 3.3 Å) and Si (a ≈ 5.43 Å) induced severe stress, causing delamination and cracking of the Ta/Nb/Si stack. Consequently, XRR data could not be obtained, and the method was deemed impractical for reliable barrier formation.

3. Plasma Oxidation: Utilized a 100 W RF plasma with 20 sccm O₂, performed at substrate temperatures of 25 °C (room temperature), 200 °C, 300 °C, and 400 °C. The oxide thicknesses were ~7–8 nm at room temperature, ~10 nm at 200 °C, and ~15 nm at 400 °C, with little dependence on oxidation duration (1 h vs 2 h). AFM measured ultra‑smooth surfaces (Rₐ = 0.10–0.34 nm). X‑ray photoelectron spectroscopy (XPS) showed only Ta⁵⁺ (Ta₂O₅) and metallic Ta⁰ peaks, indicating complete oxidation of the top ~10 nm. Small binding‑energy shifts (0.7–0.8 eV) with temperature suggest variations in oxygen‑vacancy concentration, which could affect dielectric loss. Plasma oxidation therefore offers precise, temperature‑controlled thickness and excellent surface quality while being compatible with in‑situ Ta deposition.

Trilayer Growth and Structural Characterization
α‑Ta films were grown either at 500 °C on sapphire (yielding large, well‑aligned grains but higher dislocation density) or at room temperature on Si with a Nb seed layer (producing columnar α‑Ta with lower defect density). Direct deposition of α‑Ta on Ta₂O₅ without Nb resulted in amorphous Ta, whereas the Nb interlayer promoted epitaxial α‑Ta growth. After plasma oxidation of the bottom α‑Ta layer, a second α‑Ta layer was deposited, completing the α‑Ta/Nb/Ta₂O₅/α‑Ta stack. X‑ray diffraction and cross‑sectional transmission electron microscopy confirmed sharp interfaces and the preservation of crystalline order across the barrier.

Implications for Quantum Circuits
The study demonstrates that plasma‑oxidized Ta₂O₅ can be grown with nanometer‑scale thickness control, smooth morphology, and a predominantly Ta⁵⁺ chemical state, all of which are essential for low‑loss JJ barriers. The Nb seed layer is crucial for achieving crystalline α‑Ta on both sides of the barrier, ensuring superconducting continuity and minimizing interfacial TLS. Compared with the conventional AlOₓ barrier, Ta₂O₅ offers a more ordered, less defect‑prone interface, potentially reducing dielectric loss and extending qubit coherence times.

Future Directions
The authors propose fabricating actual Josephson junction devices using the α‑Ta/Nb/Ta₂O₅/α‑Ta architecture, measuring critical current density, I‑V characteristics, and TLS spectra, and benchmarking against state‑of‑the‑art Al‑based JJs. Further optimization may involve fine‑tuning plasma parameters, exploring alternative seed layers, and integrating the process into full qubit fabrication flows. If successful, Ta‑based JJs could become a new standard for high‑coherence superconducting quantum processors.