This project aims at developing a technological pathway to enhance the performances of superconductors-based devices with a new nano-heterostructures synthesized by ALD that can be scaled and tested to real world objects.
Since their discovery at the beginning of the twentieth century, the unique properties of superconductivity have been used in a wide variety of applications from powerful electromagnets, used in MRIs and fusion reactors, to high sensitivity particle detectors, next generation electronic fast digital circuits (Quantum-bits) and particle accelerators. Major causes for performance limitations in a superconductor originate from its interaction with external electro-magnetic fields; when a magnetic field is applied, both screening supercurrents develop and vortices appear in the superconductor, which when set into motion, result in dissipation: the superconductor loses its superconducting properties. In addition, their interaction with deleterious defects (native oxides, impurity phases) further limit the maximal current a superconductor can carry, and induce premature loss of its superconducting properties. These effects combined are responsible for the entire electromagnetic behavior of applied superconducting materials and drastically impedes device performances.
We propose an original approach to mitigate the superconducting dissipation originating from deleterious vortices: a new superconducting multilayer as efficient screening structures to inhibit the vortices entry into the bulk superconductor. The synthesis and design of these nano hetero-structures will be optimized and tailored to drastically improve the performance of a superconductor-based device: superconducting radio frequency (SRF) cavities. SRF cavities are macroscopic quantum resonators that confine an RF electromagnetic field used to accelerate charged particles (figure 1). This RF magnetic field induces oscillating superconducting and normal screening currents within a depth of a few penetration depth, λ, as well as vortices motion into the material that induces dissipation at finite temperature and ultimately a quench to the normal state. The Quality factor, Q, of a cavity is a measure of this surface dissipation. A cavity performance is therefore extremely sensitive to interactions between the superconducting properties, the defects and intense RF fields within the first ~ 100 nm of the superconductor and, as such, are ideal tools to test and optimize the multilayer screening efficiency.
This project is a synergistic approach between synthesis, design, characterization and performance tests of the most effective screening hetero-structures based on the superconducting nitride alloys NbN, NbTiN, MoN and insulating materials AlN, MgO, Al2O3, Y2O3 in order to provide a technological breakthrough towards unprecedented superconductor performances for superconducting resonators. This 3 years thesis program will focus on three research thrusts or work packages:
- WP1- Explore synthetic routes to deposit innovative hetero-structures. Years 1-2.
- WP2- Tailor hetero-structure properties to optimize superconductor performances. Years 2-3
- WP3- Test optimized hetero-structure on superconducting Nb resonators. Year 3.
The multilayer approach for SRF cavities demands a fine-tuning of the nano-films’ superconducting properties and a uniformity down to a few atomic layers on large surface with complex shapes. To our knowledge, only one deposition technique is able to fulfill these requirements: the Atomic Layer Deposition (ALD). ALD functionalizes surfaces by synthesizing materials atomic layer by atomic layer, with an atomic control of the films chemical composition. They grow films conformally on arbitrary complex-shaped structures and are reproducible synthesis techniques that produce clean surfaces that require no additional processing. ALD can also be easily scaled to mass production as it has already been done in industry for gate dielectrics, diffusion barriers, and anti-reflective coatings.
One of the major challenges is the synthesis of new, high quality superconducting/insulating alloys for optimal multilayers screening efficiencies. The superconducting properties in particular are extremely sensitive to defects such as non-stoichiometry, poor crystallinity, contamination…that are to be studied, understood and mastered for this project to succeed. Several key challenges will be investigated for the promising s-wave superconducting alloys we choose to focus on during this project: NbN, NbTiN, MoN.
1-Binary and ternary alloys composition and stoichiometry:
New compounds that show inherent growth imperfections (NbN, MoN) will require the design and synthesis of new precursor candidates that must satisfy several key criteria that are necessary to insure stoichiometric and impurities-free materials. Moreover, unexpected chemical interactions between precursor molecules and surface species (inter-compound etching) can deviate the final film composition from the targeted one. This process can be overcome in ternary alloys by adjusting growth sequences and can be used to our advantage: sacrificial precursors can be used to reduce impurity content in the film, benign dopant can stabilize the stoichiometry and superconducting crystalline phases. The synthesis of superconducting alloys will be explored using commercially available and candidate custom made molecules as well as tailored growth sequences and dopants.
2-Interfaces and nucleation:
The optimization of the crystalline structure requires finding a compatible insulator-superconductor pair to insure the best possible epitaxial conditions, and avoiding deleterious cross-contamination by diffusion of atoms from one layer to the next. The candidate insulating layers selected for optimal epitaxy condition with each superconducting alloys are AlN, MgO Al2O3, Y2O3 that have been grown by ALD. The nucleation is a determinant step in thin film properties and growth: it can delay growth significantly, drive the structural properties of the whole film by limiting the grain sizes as well as producing deleterious phases that cause dissipation when exposed to an RF field. Based on the team’s unique experience, we propose to investigate the synthesis of suitable seed layers to alleviate this problem.
3- Thickness dependence and screening properties:
As the film’s thickness increases during the growth process the film’s structural and electronic properties will evolve: On the one hand, the superconducting parameters such as the gap Δ, HC, TC etc. will improve until a maximum will be reached for thicknesses > ξ. On the other hand, the predicted screening efficiency is obtained for thicknesses < λ. For the alloys of interest λ>>ξ, so optimal thicknesses have to be found for each heterostructure that will provide both the best superconducting properties and screening efficiency. Once this optimum is found for one multilayer, the same process will have to be repeated for the total number of multilayers.
Deposition: Atomic Layer Deposition
- Structural : Diffraction rayon X (XRD, XRR), scanning electron microscope (SEM). Microscope électronique a transmission (MET) (collaboration SIMAP/LMPG).
- Chemical : SIMS (Secondary ion mass spectroscopy), XPS (X-ray photoemission spectroscopy).
- Electronic (superconducting): Magnetometry, Tunneling spectroscopy, 4 points transport measurements.
“Maximum screening fields of superconducting multilayers structures” A. Gurevich. AIP advances, 5, 017112 (2015). http://dx.doi.org/10.1063/1.4905711.
“Atomic layer deposition: An overview”. S.M. George, Chemical Reviews, 110, 111-131 (2010). http://pubs.acs.org/doi/abs/10.1021/cr900056b.
“Tunneling spectroscopy of superconducting MoN and NbTiN grown by atomic layer deposition” N.R. Groll, J.A. Klug, C. Cao, S. Altin, H. Claus, N.G. Becker, J.F. Zasadzinski, M. Pellin and T. Proslier, Applied Physics Letters, 104, 092602 (2014). http://dx.doi.org/10.1063/1.4867880.
CEA: Thomas Proslier, email@example.com, 01 69 08 87 11
CEA: Claire Antoine, firstname.lastname@example.org, 01 69 08 73 28