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.