Development and characterization of Nb3Sn/Al2O3 superconducting multilayers for high- performance radio-frequency applications

Superconducting radio-frequency (SRF) resonator cavities provide extremely high quality factors > 10 at 1-2 GHz and 2K in large linear accelerators of high-energy particles. The maximum accelerating field of SRF cavities is limited by penetration of vortices into the superconductor. Present state-of-the-art Nb cavities can withstand up to 50 MV/m accelerating gradients and magnetic fields of 200-240 mT which destroy the low-dissipative Meissner state. Achieving higher accelerating gradients requires superconductors with higher thermodynamic critical fields, of which Nb3Sn has emerged as a leading material for the next generation accelerators. To overcome the problem of low vortex penetration field in Nb3Sn, it has been proposed to coat Nb cavities with thin film Nb3Sn multilayers with dielectric interlayers. Here, we report the growth and multi-technique characterization of stoichiometric Nb3Sn/Al2O3 multilayers with good superconducting and RF properties. We developed an adsorption-controlled growth process by co-sputtering Nb and Sn at high temperatures with a high overpressure of Sn. The cross-sectional scanning electron transmission microscope images show no interdiffusion between Al2O3 and Nb3Sn. Low-field RF measurements suggest that our multilayers have quality factor comparable with cavitygrade Nb at 4.2 K. These results provide a materials platform for the development and optimization of high-performance SIS multilayers which could overcome the intrinsic limits of the Nb cavity technology.


Introduction
For decades, Nb has been the material of choice for the radio-frequency superconducting (SRF) resonators for high-energy particle accelerators. Technological advances have resulted in the development of Nb cavities which can exhibit extremely high quality factors Q > 10 10 @ 1-2 GHz and 2 K while sustaining accelerating gradients up to 50 MV/m [1][2][3] . Such exemplary performance and low RF losses can only be achieved if the cavities operate in a Meissner state which can persist up to the maximum magnetic field at the inner cavity surface reaches the superheating field Bs = 240 mT [1][2][3] . At B = Bs the low-dissipative Meissner state becomes absolutely unstable with respect to dissipative penetration of vortices, causing an explosive increase of RF power and thermal quench of the cavity. The state-of-the-art Nb cavities can already operate at the peak magnetic field close to Bs, thus, increasing accelerating gradients beyond the intrinsic limits of Nb requires materials with higher Bs. There are many such materials but all of them are type-II superconductors with lower critical field Bc1 smaller than 1 ≈ 170 − 180 mT of Nb which makes high-Bs superconductors prone to detrimental penetration of vortices at low fields 4,5 . To overcome this problem, it was proposed to nanostructure the inner surface of Nb cavities by coating it with multilayers of thin superconductors (S) separated by dielectric insulating (I) layers (   Figure 1) 6 . Here the S-layer material has a superheating field Bs higher than Bs0 of Nb, whereas the thickness d of S layers is smaller than the London penetration depth , and the thickness of I layers can be a few nm to suppress the interlayer Josephson coupling. Such SIS structures greatly increase barriers for penetration of vortices in the bulk of the cavity which could potentially withstand the RF fields limited by the superheating field of S-layer. For instance, using Nb3Sn with = 480 mT could nearly double the maximum accelerating gradient as compared to the best Nb cavities. The multilayer approach is based on the lack of thermodynamically stable parallel vortices in thin decoupled S screens at < 1 where 1 is strongly enhanced in films with < 6-10 . Because the inner surface of the Nb cavity is partially screened by multilayers, both Q(H) and the breakdown field can be increased due to lower surface resistance Rs and higher Hc of the layer material 6 .
The multilayer coating, which opens up a principal opportunity to break the Nb monopoly in SRF cavities, has been tested by several groups using MgB2, Nb3Sn, NbN, NbTiN, and dirty Nb as coating materials. These experiments have shown an increase of the dc field onset of penetration of vortices on Nb surfaces coated with different SIS structures 4,[11][12][13][14][15][16][17][18][19] , although such key SRF characteristics as the surface resistance and quality factors of SIS multilayers under high-amplitude RF fields have been investigated to a much lesser extent. The first results on low-field Q measurements on NbN/MgO multilayers 13,19 have shown that SIS multilayers can have lower Rs than bulk Nb. However, the SRF performance of Nb3Sn, the current material of choice for the next generation coating material 20 , has not yet been investigated in SIS structures. The development of SIS structures requires overcoming many materials science and technological challenges to achieve good superconducting properties are SRF performance while providing optimal stoichiometry and morphology of the layers and the interfaces and transparency of grain boundaries to extremely high RF current densities. In this work we report results on growth and characterizations of Nb3Sn/Al2O3 multilayers which exhibit good superconducting properties and low-field SRF performance on par with the cavity-grade Nb.

Multilayer growth
We developed a technique of high-temperature confocal sputtering of Nb and Sn from elemental targets to grow stoichiometric Nb3Sn multilayers with Al2O3 interlayers. Details are given in the Supplemental Information. Thin films and multilayers of different thicknesses were grown on different sapphire single crystal substrates for the subsequent characterizations. For instance, 60 nm thick Nb3Sn films were grown on 10 x 10 mm sapphire substrates for transport, scanning tunneling spectroscopy and electron microscopy characterizations. For RF tests, we grew Nb3Sn/Al2O3 multilayers on 2" diameter sapphire wafers (R-plane, 300 μm thick). These multilayers had up to three 60 nm Nb3Sn layers separated by 6 nm Al2O3. The thickness of the Nb3Sn layers was chosen to be smaller than the London penetration depth 5,6 . A 200 nm thick Nb film was deposited on the backside of the wafers to prevent leakage of RF field during cavity measurements. The geometry of multilayer samples used in our RF measurements of quality factors is shown in Figure 1.
The Nb-Sn phase diagram contains several line compounds. For instance, Nb3Sn and Nb6Sn5 coexist in the region marked in Figure 2a. Here a low-Tc Nb6Sn5 phase is clearly undesirable in these films 21 . Within the Nb3Sn phase region extending from 17-25 % Sn, the critical temperature Tc degrades steeply as stoichiometry moves away from a 3:1 ratio 22 . These two conditions demand that Nb3Sn films should contain 25% of Sn. This was accomplished by providing processing conditions reflecting the field in the upper right of the phase diagram in Figure 2a, a two-phase region containing only stoichiometric Nb3Sn and liquid Sn. Films were grown by confocal sputtering of Nb and Sn from elemental targets. By providing a large over-pressure of Sn at high growth temperatures, it has been found that the ratio of Nb:Sn can be pinned at 3:1. The abundance of Sn drives the material into the two-phase region, where excess Sn re-evaporates from the film, avoiding the formation of Sn precipitates 23,24 . To achieve the high temperatures (>930 °C) required for this growth, sapphire substrates were heated from behind with a SiC radiative heater. Radiation passed through the substrate and heated the depositing metal directly. Growth temperature was measured by pyrometer. Details of the film growth are given in the Supplemental information.
A series of films was grown with fixed Nb flux (0.7 Å/s) and varying Sn flux (0.4-2.5 Å/s), and low-temperature resistance measurements were carried out to find the window for this selfregulating adsorption-controlled process. Shown in Figure 2b are the dependencies of the critical temperature Tc and transition width ΔTc on the deposition rate of Sn which clearly saturate at ~1 Å/s. Given the dependence of Tc on Sn content in Nb3Sn, this growth rate roughly corresponds to the boundary between two processing regimes. At lower flux, Sn that is necessary to form stoichiometric Nb3Sn evaporates before it can be incorporated into the film. At higher flux, the sufficient Sn is provided to react with all available Nb, and excess Sn re-evaporates.
The dielectric Al2O3 interlayers were grown after allowing Nb3Sn to cool down to <400 °C, using a single stoichiometric target with RF power at a rate of 1.8 nm/min without any further heating applied to the substrate. Depositing under these conditions protects the SiC heater element from oxygen evolved during the sputtering process and prevents undesired reactions with the Nb3Sn surface. This Nb3Sn/Al2O3 stack was then heated again to 900+ °C, which allows the Al2O3 to crystallize, and the process was repeated to grow heterostructures of up to three Nb3Sn layers. The chamber setup and growth steps are depicted in Structural characterization A SIS sample with three Nb3Sn layers was prepared for analysis by cross-sectional scanning transmission electron microscopy (STEM). A low-magnification image (   Figure 4a) represents the morphology and nanostructure of the stack. Each Nb3Sn layer is polycrystalline with irregular interfaces and grain size is 20-100 nm along the film surface direction. The Al2O3 layers conform closely to the layer below but are discontinuous along the Nb3Sn/Al2O3 interface. Despite the repeated thermal cycling during stacking, it appears that the lower layers have not degraded in comparison to the top layer.
The chemical stability of these films is further confirmed by compositional mapping with energy dispersive spectroscopy (EDS) (  Figure 4b are due to the slight oxidation of the TEM specimen surface. As our RF cavity measurements show, these Al2O3 layers do not contribute significantly to surface resistance at low fields. A higher-magnification image of the S-I interface is shown in Figure 4c. The atomic structure of Nb3Sn is well-preserved at the interface, suggesting that there is almost no diffusion or intermixing from the Al2O3. The lower Nb3Sn grain orients the [023] direction normal to the film surface, and this direction is also preserved in the upper Nb3Sn grain. This can occur when the upper Nb3Sn layer deposits with the same epitaxial relationship to the underlying Al2O3 as the lower layer has with the Al2O3 substrate. This structure can also form when a Nb3Sn grain nucleates on top of a Nb3Sn surface exposed by breaks in the discontinuous Al2O3 layer. X-ray diffractometry indicates that Nb3Sn grains in the second layer have more random crystallographic orientation compared to the first layer (see the Supplemental material).
Superconducting properties. Our dc transport measurements have shown that the Nb3Sn films capped with Al2O3 and annealed with no further deposition exhibit good superconducting properties. For instance, the superconducting resistive transitions of a bare Nb3Sn film and a Nb3Sn/Al2O3 structure annealed at 900 °C for 10 minutes are shown in Figure 5a. Here the critical temperature of the annealed sample is about 0.25 K higher than of the unannealed sample, and residual resistivity ratio (RRR), an indicator of crystalline and metallic quality, is improved from 3.5 to 4.26. On the other hand, Nb3Sn films annealed without the Al2O3 cap, even under high Sn flux to prevent evaporative loss, have degraded superconducting properties compared to an un-annealed film.
The superconducting properties essential for the RF performance were characterized by the scanning tunneling spectroscopy (STS) which measures the differential tunneling conductance dI/dV proportional to the quasiparticle density of states (DOS), N(E). Shown in Figure 5b is a representative tunneling spectrum measured in the center of a Nb3Sn grain at 4 K. The DOS curves, which clearly show the superconducting gap Δ at the Fermi surface, were fit using the conventional Dynes model 25,26 : where the phenomenological parameter Γ accounts for the broadening of the DOS peaks due to a finite lifetime of quasiparticles, and N0 is the DOS in the normal state. The fit was done with Γ = 0.4 meV and Δ ≈ 3.1 meV, consistent with the conventional gap value for a stoichiometric Nb3Sn 4 . The ratio Γ Δ ≈ 13 ⁄ % in our samples turns out to be about 2-3 times larger than the values observed by tunneling spectroscopy on 1-2 m thick Nb3Sn films for rf applications 27 and Nb coupons 28 . The deviations of the STM data from the Dynes model at low energies < Δ may indicate the effects of local non-stoichiometry, gap anisotropy and strain 22 , scattering of quasiparticles on magnetic impurities, and a thin layer with deteriorated superconducting properties at the surface 27,28,29,30 . In turn, the subgap quasiparticles states which appear at |E| <  due to a finite  contribute to a temperature-independent residual surface resistance Ri at kBT << Δ 5,29 Here μ0 is the permeability of free space, is the normal-state resistivity, is the magnetic penetration depth, and = 2 is the circular RF frequency 5 . For λ = 120 nm, = 3.0x10 -7 Ωm, and the fit parameters Δ = 3.1 meV and Γ = 0.4 meV, we obtain Ri ≈ 5.0 nΩ at f = 1.3 GHz. This estimate is of the order of Ri ≈ 5-10 nΩ for large-grain Nb cavities 31 . Below a few nm thick surface layer but well within the rf penetration depth λ ≈ 120 nm, the gap peaks in the DOS are likely much sharper. There are other essential contributions to Ri most notably due to non-stoichiometric regions in the bulk 27 , grain boundaries and trapped vortices 32 .

Low-field RF characterization
Multilayer samples grown on 2" sapphire wafers were tested in a hemispherical Nb-coated cavity at SLAC National Accelerator Laboratory. The experimental setup was described previously 33 . A rendering of this cavity is shown in Figure 6bError! Reference source not found.. The cavity operates in a TE032-like mode at 11.4 GHz, with a pocket on the flat face for mounting 2"-diameter samples (shown in purple). The overall cavity quality factor is measured, and the properties of the wafer can be deduced by comparison with known samples. The geometry of the cavity is engineered such that the magnetic field is strongest at the sample surface, limiting the contribution of the cavity material to the overall cavity loss. According to simulations, the participation factor is 0.33 for the 2"-diameter sample. Crucially, the magnetic field at the sample is in the radial direction and parallel to the sample surface, making it possible to measure RF properties of the sample without interference from the perpendicular component of the field.
The SRF performance of two Nb3Sn samples were compared in this system to a cavity-grade bulk Nb coupon. A 500 nm (~4λ) Nb3Sn film intended to completely screen out the RF magnetic field, and a 3x60 nm Nb3Sn/Al2O3 trilayer were tested under the RF field. Both samples were coated with a 200 nm Nb film on the backside of the wafer to prevent leakage of magnetic field as shown in Figure 1. The quality factor of the cavity with each sample, measured at low power with a network analyzer, is plotted in Figure 6a. The abrupt increase in Q at about 15 K corresponds to the superconducting transition of Nb3Sn, followed by an increase of Q(T) at Tc = 9K of the Nbcoated host cavity.
As shown in Figure 6a, the thick Nb3Sn film and the trilayer have nearly identical Q at T < 9K, indicating that Al2O3 dielectric layers and interfaces do not contribute significantly to the RF dissipation. We would expect the thick Nb3Sn film to have a higher Q, as magnetic field is more fully screened before reaching the substrate and backside, so this result suggests that the maximum Q of these films and multilayers is limited by the quality of the Nb3Sn material rather than by the interfaces with Al2O3. The quality factors of both the film and the trilayer samples exceed Q(T) of Nb at T > 6K due to the higher Tc of Nb3Sn and is about 2 times smaller than Q of Nb at 4K.

Discussion
The results of this work show that, despite the obvious non-stoichiometry and inhomogeneity of superconducting properties, grain boundaries, Nb inclusions, and incomplete Al2O3 layers, our multilayers exhibit the quality factors on par of those of cavity-grade bulk Nb at 4K and low RF power. The significant local non-stoichiometry of thick (a few micron) polycrystalline Nb3Sn coatings of Nb cavities 20,27 , as well as Sn depletion at grain boundaries in Nb3Sn [34][35][36][37] have been well documented in the literature. Yet, despite these materials issues which are also characteristic of 1-3 m thick Nb3Sn films used in SRF cavities 38 , our Nb3Sn SIS structures exhibit higher low-field Q values than Nb at T > 6K 20 , consistent with the larger superconducting energy gap Δ Nb3Sn ≈ 2Δ Nb and a lower BCS surface resistance ∝ 2 1/2 −Δ/ of Nb3Sn. These experimental results not only show a remarkable resilience of low-field quality factors of Nb3Sn to the significant non-stoichiometry and materials imperfections but also suggest that the SRF performance of Nb3Sn coatings can be further improved by materials treatments. Our Nb3Sn multilayers exhibit a similar resilience of the low-power SRF performance to the materials imperfections.
The slopes of Q(T) for both the Nb3Sn film and multilayer shown in Fig. 5 tend to level off at 4-5 K and are clearly smaller than the slope of Q(T) for Nb. This indicates that Q(T) of the Nb3Sn samples at T = 4 -5 K is not limited by the BCS surface resistance for which the slope of Q(T) ∝ Δ/ for Nb3Sn would be larger than for Nb because Δ Nb3Sn ≈ 2Δ Nb . The behavior of Q(T) of the Nb3Sn samples at 4-5 K is thus indicative of a significant residual surface resistance caused by the multiphase structure of the films and multilayers and trapped vortices. Yet Q 0 ≃ 10 7 observed on our Nb3Sn multilayers at 11.4 GHz and 4 K suggests values of Q 0 ∼ 10 9 at 4K and 1 GHz given the frequency dependence Q ∝ −2 which comes from the BCS surface resistance 1-3 , ohmic losses in metallic precipitates smaller than the RF skin depth and perhaps Josephson vortices trapped on grain boundaries 39 .
SRF performance at high RF fields and breakdown fields of Nb3Sn/Al2O3 multilayers are yet to be explored. Generally, the effects of nonstoichiometry, proximity-coupled normal precipitates and weakly-coupled grain boundaries become more pronounced at higher RF fields. For instance, nonstoichiometric grain boundaries in Nb3Sn have been identified as prime pinning centers for vortices in Nb3Sn wires for high-field dc magnets 39 . However, weakly-coupled grain boundaries in Nb3Sn coating layers would block RF currents and cause dissipative penetration of Josephson vortices at fields well below the superheating field 40 , and sub-stoichiometric regions in Nb3Sncoated Nb cavities are suspected to play an important role in RF cavity quench 27 . At the same time, significant meandering and breaks in Al2O3 layers shown in Figure 4 may not be detrimental for SRF performance as the layers can still provide their main role of intercepting and pinning small vortex loops originating at surface structural defects 5,8 since the pinholes sizes 10-50 nm in the Al2O3 layers are smaller than magnetic size of the vortex ≃ 100 − 200 nm of Nb3Sn. Though Al2O3 layers do not fully separate Nb3Sn layers, we found that a 500 nm thick Nb3Sn film had a quality factor identical to a multilayer with three 60 nm Nb3Sn layers separated by 6 nm Al2O3, and both had Q approximately 2x lower than a cavity-grade Nb reference. This is strong evidence that the Al2O3 and the oxide-metallic interfaces do not contribute to surface resistance samples prepared with this process.

Conclusions
In summary, we have developed a self-regulating, adsorption-controlled process for growth of Nb3Sn films and Nb3Sn/Al2O3 multilayers. We have produced and characterized multiple multilayer samples with up to four superconducting layers. Despite the detrimental effects of nonstoichiometry, grain boundaries and breaks in the meandering Al2O3 interlayers, the SRF performance of our multilayers turned out to be on par with that of Nb films. The growth technique reported in this work provides a platform for further optimizations of the SRF properties of SIS high-performance multilayers for superconducting resonator applications.

Methods
Nb3Sn films were sputtered from elemental Nb (99.95%) and Sn (99.99%) targets in 3 mTorr of Ar at a distance of 15.5 cm from the substrate. DC power to the sputter guns was current-controlled, and deposition rate was measured with an in situ quartz crystal monitor prior to growth. Pyrometer reading of the SiC heating element at the beginning of growth was ~1250 °C, and dropped to around 905 °C after 60 nm was deposited. Al2O3 was sputtered from a stoichiometric 2" diameter ceramic target after the pyrometer reading fell below 400 °C. After deposition, the temperature was ramped back up to a pyrometer reading of 905 °C over the course of 10 minutes. These two steps were repeated to produce the multilayers.
Scanning transmission electron microscope (STEM) imaging and elemental analysis were performed in a probe-corrected JEOL JEM-ARM200cF with an Oxford X-Max N 100TLE SDD energy dispersive X-ray spectroscopy (EDS) detector.
Superconducting transitions were measured in a closed-loop He cooler using 4-point van der Pauw geometry on 10x10 mm samples. The critical temperature Tc is defined as the temperature at which the sheet resistance falls below 1% of its normal state value at 18 K. The transition width ΔTc is defined as a difference between Tc and the point at which the lines drawn through the normal-state resistance and transition region intersect.
Low-temperature scanning tunneling microscopy/spectroscopy (STM/S) measurements were carried out in a Unisoku-1300 STM system at 4K using polycrystalline PtIr tips. The dI/dV spectra were acquired using standard lock-in technique by applying a bias modulation of 0.2 mV (r.m.s.) at 732 Hz.