Sandra Schujman

Characterization of Hydroxyls in Surface Oxide of Superconducting Tantalum and Their Mitigation in Quantum Circuits

  1. Ekta Bhatia,
  2. Nicholas Pieniazek,
  3. Aleksandra Biedron,
  4. Sandra Schujman,
  5. Hunter Frost,
  6. Zhihao Xiao,
  7. Jakub Nalaskowski,
  8. Kevin Musick,
  9. Thomas Murray,
  10. and Satyavolu Papa Rao
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.
arxiv pdf

Temperature-Dependent Dielectric Function of Tantalum Nitride Formed by Atomic Layer Deposition for Tunnel Barriers in Josephson Junctions

  1. Ekta Bhatia,
  2. Aaron Lopez Gonzalez,
  3. Yoshitha Hettige,
  4. Tuan Vo,
  5. Sandra Schujman,
  6. Kevin Musick,
  7. Thomas Murray,
  8. Kim Kisslinger,
  9. Chenyu Zhou,
  10. Mingzhao Liu,
  11. Satyavolu S. Papa Rao,
  12. and Stefan Zollner
We report the dielectric functions of insulating tantalum nitride (TaN) films, deposited using atomic layer deposition (ALD) on 300 mm Si/SiO2 substrates, to demonstrate their suitability
as tunnel barriers in tantalum-based Josephson junctions (JJ) for superconducting quantum circuits. The temperature-dependent ellipsometric angles were measured using ALD TaN films with nominal thicknesses of 13 nm and 25 nm at an incidence angle of 70 degrees, across photon energy ranges of 0.03 eV to 0.7 eV (80-300 K) and 0.5 eV to 6.5 eV (80-600 K). This data was used to develop a dispersion model for insulating ALD TaN films that incorporates a Tauc-Lorentz oscillator with a band gap of 1.5-1.8 eV to model the interband optical transitions. The extracted dielectric function of ALD TaN films shows an insulating behavior (mid-infrared transparency) at all temperatures and for both film thicknesses tested. ALD TaN does not exhibit infrared absorption due to free carriers, even at elevated temperatures, demonstrating its insulating nature, which is required for the tunnel barrier of the JJ in quantum applications. The results of transmission electron microscopy, including selected area electron diffraction, and X-ray diffraction are also discussed. Sputter depth-profile X-ray photoelectron spectroscopy (XPS) shows an N/Ta ratio of ~1.2 throughout the film. The lower band gap, low roughness, and thermal stability of ALD TaN compared to AlOx suggest the possibility of fabricating JJs with thicker barriers while achieving critical current densities required for qubits, better control of thickness and composition, reduced topography, and resistance to aging.
arxiv pdf