Structure and Formation Mechanisms in Tantalum and Niobium Oxides in Superconducting Quantum Circuits
Jin‐Su Oh, Rahim Zaman, Akshay A. Murthy, Mustafa Bal, Francesco Crisa, Shaojiang Zhu, Carlos G. Torres‐Castanedo, Cameron Kopas, J. Mutus, Dapeng Jing, John Francis Zasadzinski, Anna Grassellino, Alexander Romanenko, Mark C. Hersam, Michael J. Bedzyk, Matthew Joseph Kramer, Bi‐Cheng Zhou, L.W. Zhou
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.
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.
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).
Recent advances in superconducting qubit technology have led to significant progress in quantum computing, but the challenge of achieving a long coherence time remains. Despite the excellent lifetime performance that tantalum (Ta) based qubits have demonstrated to date, the majority of superconducting qubit systems, including Ta-based qubits, are generally believed to have uncontrolled surface oxidation as the primary source of the two-level system loss in two-dimensional transmon qubits. Therefore, atomic-scale insight into the surface oxidation process is needed to make progress toward a practical quantum processor. In this study, the surface oxidation mechanism of native Ta films and its potential impact on the lifetime of superconducting qubits were investigated using advanced scanning transmission electron microscopy (STEM) techniques combined with density functional theory calculations. The results suggest an atomistic model of the oxidized Ta(110) surface, showing that oxygen atoms tend to penetrate the Ta surface and accumulate between the two outermost Ta atomic planes; oxygen accumulation at the level exceeding a 1:1 O/Ta ratio drives disordering and, eventually, the formation of an amorphous Ta 2 O 5 phase. In addition, we discuss how the formation of a noninsulating ordered TaO 1−δ (δ < 0.1) suboxide layer could further contribute to the losses of superconducting qubits. Subsurface oxidation leads to charge redistribution and electric polarization, potentially causing quasiparticle loss and decreased current-carrying capacity, thus affecting superconducting qubit coherence. The findings enhance the comprehension of the realistic factors that might influence the performance of superconducting qubits, thus providing valuable guidance for the development of future quantum computing hardware.
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.
Recently, tantalum (Ta) has gained attention in superconducting quantum circuits due to the longer coherence times achieved when replacing niobium (Nb) in capacitor pads. Previous literature shows that surface oxides that form upon ambient exposure on superconducting metals such as Ta, Al, and Nb host two-level system (TLS) defects, which are a leading source of microwave loss and decoherence. While the surface oxides of Nb and Al have been extensively studied, Ta oxides remain less well understood. Using secondary ion mass spectrometry of alpha-Ta films deposited at 300 mm wafer scale, we show for the first time that hydroxyls accumulate in the Ta suboxide region above the underlying Ta. Angle-resolved X-ray photoelectron spectroscopy shows that the surface region is dominated by Ta2O5, with sub-stoichiometric TaOx present in between the Ta2O5 and underlying Ta. The thickness of the tantalum oxide is confirmed by transmission electron microscopy. We demonstrate that [OH] incorporation can be suppressed by replacing the native oxide with an oxide formed during chemical mechanical planarization of alpha-Ta films. Our findings support the hypothesis that TLS defects are non-uniform within the oxide thickness and suggest hydroxyls as a probable molecular origin of these loss channels. Furthermore, we show the feasibility of plasma nitridization as a method to decrease hydroxyl loading on alpha-Ta surfaces. The modulation of hydroxyl content through surface engineering of alpha-Ta can enable the fabrication of more robust, high-coherence superconducting quantum circuits by addressing a potential TLS source.
Superconducting qubits are among the most advanced platforms for quantum information, offering potential scalability and compatibility with standard device fabrication techniques [1-5]. Among the materials investigated for superconducting qubits, Tantalum (Ta) has emerged as a compelling candidate due to its self-terminating stoichiometric surface oxide and chemical resistance to harsh post-fabrication processes. Efforts to mitigate surface loss by treating the surface oxides and utilizing high-resistivity silicon substrates have led to Ta-based qubits with lifetime and coherence times exceeding 1 ms [3]. Guided by these successes in reducing dielectric loss in Ta-based devices, we are interested in exploring material engineering approaches that enhance the superconducting gap. Alloying Ta with another element to enhance the superconducting performance expands the range of viable material candidates available to tackle different loss channels in superconducting circuits [1]. Recently, we explore superconducting gap engineering in Ta-based devices as a powerful strategy for expanding the range of suitable host materials. By alloying 20 at.% hafnium (Hf) into Ta thin films, we achieve a superconducting transition temperature (Tc) of 6.09 K as observed in direct current (DC) transport measurements, reflecting an increase in the superconducting gap. Here we demonstrate an approach for controlling varying deposition conditions to control film orientation and transport properties of Ta and Ta-Hf alloy thin films. We investigate the microstructure and composition distribution of the thin films by using high-resolution scanning transmission electron microscopy (STEM) with super-X EDS system (no serious beam damage issue [6-8]). Our structural analysis using TEM diffraction and STEM verifies the BCC structure across all the films (Figure 1), and direct current (DC) transport measurements reveal a 40% enhancement in Tc relative to undoped Ta (Tc=4.3 K). We monitored oxide growth over an 18-day period following film deposition to better understand how Hf incorporation influences surface oxides. In both Ta and Hf spectra, the fraction of fully oxidized (Ta2O5 and HfO2) states gradually increased during the 18-day exposure, indicating progressive surface oxidation. To estimate the surface oxide thickness, we use cross-sectional TEM/EDS mapping (Figure 1c). The analysis reveals that the oxide thickness for the 1 ks Ta-Hf alloy film is approximately 3 nm in thickness, comparable to the 3 nm oxide observed on native Ta [2]. For the 2 ks Ta-Hf film, the oxide thickness increases slightly to about 4 nm (Figure 1). The measured surface loss tangent of the Ta-Hf alloy remains comparable to that of native Ta. This result suggests that the surface oxide of the Ta-Hf alloy, which contains a measurable fraction of HfOx, does not introduce additional dielectric loss. By engineering the alloy composition and film deposition, this study achieves both improved superconducting properties and compatibility with fabrication processes. These findings show that material doping and alloying can strategically expand the suite of viable materials for superconducting quantum devices. This approach provides a pathway for tailoring material properties to support the next generation of quantum technologies and offers broad relevance to researchers in materials science, quantum information, and device engineering [9]. STEM images on cross sections of 1 ks (a) and 2 ks (b) Ta-Hf thin films, both films were sputtered at 750 ◦C. (c) Cross-sectional EDS elemental mapping on Ta-Hf. Cross-sectional EDS mapping Ta (blue) and Hf (orange) confirmed a uniform Ta-Hf alloy with the Ta to Hf atomic ratios for the bulk to be (83% : 17%). The oxygen (red) map reveals a surface oxide layer with the estimated surface oxide thickness of around 4 nm. The 1 ks Ta-Hf film exhibits an oxide thickness of approximately 3 nm, similar to that of native Ta. The EDS resolution limit is 0.5 atomic percent.