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arXiv:2410.13160 · Physical Review Applied

Suppressed paramagnetism in amorphous Ta 2 O 5 − x oxides and its link to superconducting-qubit performance

P. Graham Pritchard, James M. Rondinelli

Published May 28, 2025• 6 citations
arXivHTMLPDF

Abstract

Reduced transmon qubit $T_1$ coherence times have been linked to the amorphous oxide layers formed by thin film capacitors during processing. Because Ta or Ta capped Nb capacitors exhibit overall superior qubit performance to those fabricated with Nb capacitors, it has been hypothesized that the amorphous, non-stoichiometric Ta$_2$O$_{5-x}$ oxide is less lossy than its Nb$_2$O$_{5-x}$ counterpart. The origins of what makes amorphous Ta$_2$O$_{5-x}$ less susceptible to accepted decoherence channels is unknown. Here we establish the microscopic features of amorphous Nb$_2$O$_{5-x}$ and Ta$_2$O$_{5-x}$ using a combination of \textit{ab initio} molecular dynamics and density functional theory calculations. Our simulations establish that oxygen deficiency is less likely to occur in amorphous Ta$_2$O$_{5-x}$ than in Nb$_2$O$_{5-x}$ for $0\le x \le 0.25$ and that at a given level of oxygen deficiency the formation of metal Ta-Ta bonds is enhanced. These bonds, which are accommodated by structural flaws in the amorphous network, capture electrons better than in amorphous Nb$_2$O$_{5-x}$. These thermochemical differences quench or highly suppress magnetic moments in amorphous Ta$_2$O$_{5-x}$ and eliminate a potential source of quasiparticles and magnetic flux noise. Our calculations also show that hyperfine couplings between Nb nuclei and local magnetic moments in Nb$_2$O$_{5-x}$ could form "two-level systems" (TLS) or "two-level fluctuators" (TLF) with energy splittings of 100-1000 MHz or higher. This reveals a new TLS mechanism in amorphous Nb$_2$O$_{5-x}$ oxide layers that is unlikely in Ta$_2$O$_{5-x}$. Our work provides fundamental understanding of the materials chemistry and limitations imposed by native oxides of superconducting qubits, which can be used to guide materials selection and processing.

DOI
10.1103/6wyt-fxjg

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High Resolution Image Download MS PowerPoint Slide Improving the qubit’s lifetime (T 1 ) is crucial for fault-tolerant quantum computing. Recent advancements have shown that replacing niobium (Nb) with tantalum (Ta) as the base metal significantly increases T 1, likely due to a less lossy native surface oxide. However, understanding the formation mechanism and nature of both surface oxides is still limited. Using aberration-corrected transmission electron microscopy and electron energy loss spectroscopy, we found that Ta surface oxide has fewer suboxides than Nb oxide. We observed an abrupt oxidation state transition from Ta 2 O 5 to Ta, as opposed to the gradual shift from Nb 2 O 5, NbO 2, and NbO to Nb, consistent with thermodynamic modeling. Additionally, amorphous Ta 2 O 5 exhibits a closer-to-crystalline bonding nature than Nb 2 O 5, potentially hindering H atomic diffusion toward the oxide/metal interface. Finally, we propose a loss mechanism arising from the transition between two states within the distorted octahedron in an amorphous structure, potentially causing two-level system loss. Our findings offer a deeper understanding of the differences between native amorphous Ta and Nb oxides, providing valuable insights for advancing superconducting qubits through surface oxide engineering.
Qubits made from superconducting materials are a mature platform for quantum information science application, such as quantum computing. However, material-based losses are now a limiting factor in reaching the coherence times needed for applications. In particular, knowledge of the atomistic structure and properties of the circuit materials is needed to identify, understand, and mitigate material-based decoherence channels. In this work, we characterize the atomic structure of the native oxide film formed on Nb resonators by comparing fluctuation electron microscopy experiments to density functional theory calculations, finding that an amorphous layer is consistent with an Nb2O5 stoichiometry. Comparing x-ray absorption measurements at the Oxygen K edge with first-principles calculations, we find evidence of d-type magnetic impurities in our sample, known to cause impedance in proximal superconductors. This work identifies the structural and chemical composition of the oxide layer grown on Nb superconductors and shows that soft x-ray absorption can fingerprint magnetic impurities in these superconducting systems.
The performance of superconducting qubits is orders of magnitude below what is expected from theoretical estimates based on the loss tangents of the constituent bulk materials. This has been attributed to the presence of uncontrolled surface oxides formed during fabrication which can introduce defects and impurities that create decoherence channels. Here, we develop an ab initio Shiba theory to investigate the microscopic origin of magnetic-induced decoherence in niobium thin film superconductors and the formation of native oxides. Our ab initio calculations encompass the roles of structural disorder, stoichiometry, and strain on the formation of decoherence-inducing local spin moments. With parameters derived from these first-principles calculations we develop an effective quasi-classical model of magnetic-induced losses in the superconductor. We identify d-channel losses (associated with oxygen vacancies) as especially parasitic, resulting in a residual zero temperature surface impedance. This work provides a route to connecting atomic scale properties of superconducting materials and macroscopic decoherence channels affecting quantum systems.
Improving qubit lifetime is essential for fault-tolerant quantum computing. It is widely acknowledged that intricate processing techniques introduce defects at the interfaces and surfaces of superconducting quantum circuits, which can create sources of decoherence. Therefore, a precise understanding of how a material's structure contributes to decoherence is critical for enhancing the performance of superconducting qubits. This talk highlights our recent advancements in understanding surface oxides in resonators and ultrathin aluminum oxide barriers in Josephson junctions (JJs) using transmission electron microscopy and spectroscopy techniques [1-4]. The Superconducting Quantum Materials & Systems Center (SQMS) demonstrated that encapsulating niobium surfaces with a ∼10nm tantalum layer results in 2-5x longer T1​ than bare niobium [2]. Our collaborative investigation shows that this improvement is attributed to tantalum's less lossy surface oxide, which contains fewer sub-oxides than niobium oxide. Moreover, the amorphous Ta2O5 shows a bonding nature closer to crystalline form than amorphous Nb2O5, potentially reducing hydrogen diffusion toward the oxide/metal interface [1, 2]. Moreover, we will briefly discuss our preliminary study of the thin film structure under cryogenic conditions down to 20K, conducted using a liquid helium TEM holder in a Thermo Cs-corrected Titan. We also explored the effect of aluminum deposition rates on aluminum oxide microstructure and qubit lifetime. Although increasing the deposition rate from 0.5 Å/s to 5 Å/s significantly improved oxide layer roughness and thickness variation, the qubits’ coherence remained unchanged. This is likely due to Cooper pairs only tunneling through the thinnest parts of the barrier, making them less affected by the grain boundaries in the thin aluminum films. Additionally, we uncovered a stress-induced grain boundary sliding mechanism, which contributes to short-circuit failures in JJs [4]. Our discovery provides vital insight into oxide microstructure and superconducting qubits’ functionality [5].
Superconducting qubits have become one of the leading contenders in making quantum computing a reality because of their robustness and scalability [1]. Replacing the widely adopted Al and Nb, Ta has become the most promising candidate material for superconducting qubit fabrication due to its exceptionally enhanced thermal relaxation time (i.e., T1 time). This time can exceed 0.3 milliseconds which is three orders of magnitude higher than the previous methods [2]. However, during the fabrication process, exposure of tantalum to air leads to the formation of an amorphous surface oxide layer. The presence of this oxide layer can affect the superconducting properties of the tantalum film and, hence, the performance of the qubit. This native amorphous oxidized surface results in a dielectric loss through an energy reduction mechanism called a two-level system caused by structural and chemical defects [3]. Although Ta exhibits superior performance, particularly T1 time, compared to other superconducting materials, its value remains insufficient for quantum processor hardware. Therefore, understanding the structural and chemical profile of the native amorphous surface of the oxides for superconducting qubits is required to optimize their performance. In this study, we employed scanning transmission electron microscopy (STEM) with a combination of electron energy loss spectroscopy (EELS) and four-dimensional STEM (4D-STEM) to probe the electronic and crystallographic structure of the local surface areas. Furthermore, to circumvent the spectral and diffractometric profiles that overlap as a function of depth position caused by electron beam broadening, we applied Multivariate curve resolution (MCR) analysis that can separate complex mixtures into their basis components. In the subsequent experiments, we investigated the surface morphology of (110)-oriented α-Ta film grown on an A-plane sapphire substrate using scanning electron microscopy (SEM) (Figure 1a), and high-angle annular dark-field (HAADF) STEM imaging (Figure 1b and 1c). We found that the surface of the Ta film is bumpy, and made of an amorphous material with a thickness of about 3 nm. The film exhibits a columnar structure along the [001] growth direction. EELS fine structure analysis reveals that most of the amorphous regions are in the Ta2O5 phase (Figure 2a and 2b). By extracting valuable basis components of O-K fine structures, we were able to quantify the concentration of the chemical species with Å-scale at the interface using MCR analysis. Specifically, a depth dependent composition variation, especially that of oxygen concentration, was observed. We further investigated amorphous oxide films and interfaces with 4D-STEM based fluctuation electron microscopy. (Figure 2c and 2d) We obtained results that were consistent with the chemical profiles obtained from the O-K edge EELS mapping. We also revealed the crystalline ordering information of each chemical species. The detailed descriptions of the results will be presented at the conference. These STEM analysis findings were supported by first-principles calculations of oxygen concentration changes and phase formation energies. Overall, our research provides important insights into the formation and properties of the native amorphous surface oxides of a superconducting qubit tantalum film, which is critical for the development of next-generation quantum computing technologies [4]. Surface morphology of α-Ta film on A-plane sapphire. (a) Top-view SEM image. Cross-sectional HAADF STEM images at (b) low and (c) high magnification. To protect the film surface during the FIB process, the coating layer is on top of the bumpy surface of the Ta film with a thin native oxidized layer of gray contrast. Crystal orientation is indicated on the left-bottom corner in (a) and (c) for the top-view and cross-sectional view images, respectively. STEM-EELS and 4D-STEM analysis across the interfaces between the native oxidized amorphous and the crystalline Ta film. (a) HAADF STEM image of the region-of-interest, simultaneously acquired with EELS mapping. (b) Representative EELS spectra of O-K edge fine structures and (c, d) electron diffraction patterns from the native oxidized amorphous region (green) and the crystalline Ta film region (blue).
Superconducting thin films of niobium have been extensively employed in transmon qubit architectures. Although these architectures have demonstrated improvements in recent years, further improvements in performance through materials engineering will aid in large-scale deployment. Here, we use information retrieved from secondary ion mass spectrometry and electron microscopy to conduct a detailed assessment of the surface oxide that forms in ambient conditions for transmon test qubit devices patterned from a niobium film. We observe that this oxide exhibits a varying stoichiometry with NbO and NbO 2 found closer to the niobium film/oxide interface and Nb 2 O 5 found closer to the surface. In terms of structural analysis, we find that the Nb 2 O 5 region is semicrystalline in nature and exhibits randomly oriented grains on the order of 1–3 nm corresponding to monoclinic N–Nb 2 O 5 that are dispersed throughout an amorphous matrix. Using fluctuation electron microscopy, we are able to map the relative crystallinity in the Nb 2 O 5 region with nanometer spatial resolution. Through this correlative method, we observe that the highly disordered regions are more likely to contain oxygen vacancies and exhibit weaker bonds between the niobium and oxygen atoms. Based on these findings, we expect that oxygen vacancies likely serve as a decoherence mechanism in quantum systems.