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

TL;DR

This work addresses decoherence in superconducting qubits by targeting Ta-based Josephson junction barriers. It systematically compares tube-furnace, rapid-thermal, and oxygen-plasma oxidation of $\alpha$-Ta to form $Ta_2O_5$, characterizes the resulting oxides with XPS, XRR, AFM, TEM, and XRD, and uses DFT to explain oxidation mechanisms. Plasma oxidation yields the smoothest, most controllable oxide, enabling in-situ growth of epitaxial $\alpha$-Ta on oxidized Ta to form Ta/$Ta_2O_5$/Ta trilayers. These trilayers with clean interfaces offer a route to lower TLS loss barriers and improved Ta-based qubits, potentially surpassing current Al-based junctions in coherence and lifetime.

Abstract

Tantalum (Ta) has recently 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 is believed to host fewer two-level system (TLS) defects key $-$ contributors to decoherence in superconducting circuits. Nevertheless, aluminum oxide (AlO$_x$) remains the predominant choice for Josephson junction barriers in most qubit architectures. In this study, we systematically investigate various techniques for forming high-quality oxide layers on $α$-phase tantalum ($α$-Ta) thin films, aiming to develop effective Josephson junction barriers. We explore thermal oxidation in a tube furnace, rapid thermal annealing, as well as plasma oxidation of both room-temperature and heated Ta films, and propose a mechanistic picture of the underlying oxidation mechanisms. All methods yield Ta$_2$O$_5$, the same compound as tantalum's native oxide. Among these, plasma oxidation produces the smoothest and highest-quality oxide layers, making it particularly well-suited for Josephson junction fabrication. Furthermore, we demonstrate the successful epitaxial growth of $α$-Ta atop oxidized $α$-Ta films, paving the way for the realization of trilayer Ta/Ta-O/Ta Josephson junctions with clean, low-loss interfaces.

Figures (6)

• Figure 1: High-resolution Ta(4f) XPS spectra of tantalum films -- featuring a native oxide (blue curve, sample 1), as well as after tube furnace oxidation (red curve, samples 3), rapid thermal annealing (green curve, samples 6), and plasma oxidation (pink curves, samples 9,10,14) -- reveal the characteristic Ta$_2$O$_5$ doublet peaks at binding energies of approximately 26.2 eV (Ta 4f$_{7/2}$) and 28.1 eV (Ta 4f$_{5/2}$). For some samples, peaks corresponding to metallic tantalum appear near 21.4 and 23.3 eV due to XPS probing deeper than the oxide thickness. Dotted lines mark the positions of the metallic Ta peaks, which are notably weak in plasma-oxidized samples 9 and 10.
• Figure 2: (a) XRR measurements of Ta films subjected to native oxidation (control), tube furnace oxidation, and plasma oxidation treatments. Solid black lines show fitted curves used to extract the oxide thicknesses. The dotted portion of the sample 10 fit curve indicates poor fitting quality at high Q values. (b) Oxidation temperature vs Thickness. A clear trend of approximately equal thickness can be seen for different temperatures and times. The gray line is a guide for the eye.
• Figure 3: Surface morphology of native and oxidized tantalum films characterized by AFM, with mean roughness ($R_a$) annotated for each sample. Tantalum films grown on (a) sapphire at 500 $\degree$C (sample 2) and (b) silicon at room temperature (sample 1) display distinct grain structures. (c) Thermal oxidation (sample 5) resulted in an oxide with large grain sizes and moderate surface roughness. (d) Rapid thermal annealing (sample 8) resulted in films with high surface roughness across a 10 µm$^2$ scan region. Note that the pictured 1 µm$^2$ scan region displays a lower roughness locally. Films that underwent plasma oxidation at (e) room temperature (sample 10), (f) 200 $\degree$C (sample 11), (g) 300 $\degree$C (sample 13), and (h) 400 $\degree$C (sample 15) display smooth surfaces with much smaller grains. The color bar shown on the left of the figure corresponds to measured sample height in nm. The scale bar shown on the bottom-right of each panel corresponds to a length of 200 nm.
• Figure 4: (a) Thermodynamic stability of the TaO$_x$ films supported on the Ta(110) surface depending on the content and spatial distribution of the oxygen species. Isolated O impurities in the bulk Ta are the least stable. Their presence induces lattice relaxation and promotes layer-by-layer accumulation of oxygen, which is further stabilized by the Ta oxidation to Ta$_2$O$_5$ and accompanying amorphization. (b) Energies of oxygen adsorption on the TaO$_x$ surface and absorption into the TaO$_x$ films, depending on the proximity to the interface and film composition. Oxygen adsorbs and diffuses as O$^{2-}$ if the film is not fully oxidized or if the Ta interface is close enough to promote charge transfer, and as O$^0$ bound to a preexisting O$^{2-}$ and forming an O$_2^{2-}$ molecular ion otherwise. (c) and (d) Potential energy profiles for the diffusion of interstitial O$^{2-}$ in the case of partially oxidized TaO$_x$ (c) and nearly fully oxidized amorphous Ta$_2$O$_5$ ($a$-Ta$_2$O$_5$) (d).
• Figure 5: (a-c) X-Ray diffraction (XRD) patterns of the trilayer films. The stack order for each set is shown in the cartoon insets, with "x" indicating the thickness of the oxide in nm. (Bottom panel) Reference peak angles for the phases of interest: $\delta$-Ta$_2$O$_5$ ($36.9\degree$),Kabekkodu2024$\alpha$-Ta (110, $38.3\degree$),Wu2023 (211, $69.1\degree$),Ding2021 and (220, $82\degree$),Wu2023 Nb (211, $69.1\degree$),Chukwuike2021 and Si (004,$70.8\degree$).Face1987 as measured in our films.