V_3 Si: an alternative thin film material for superconducting RF cavities.

V3Si: AN ALTERNATIVE THIN FILM MATERIAL FOR SUPERCONDUCTING RF CAVITIES∗ C. Benjamin1†, R. Valizadeh1, D. Seal1, O. B. Malyshev1, J. Conlon. ASTeC, STFC Daresbury Laboratory, Sci-Tech Daresbury, Warrington, UK G. B. G. Stenning. STFC RAL, Rutherford Appleton Laboratory, Didcot, UK 1also at the Cockcroft Institute, Sci-Tech Daresbury, Warrington, UK Abstract Superconducting materials such as: V 3 Si, NbN, NbTiN and Nb 3 Sn, are potential alternatives to Nb for next generation thin film SRF cavities. In comparison to the Nb, their relatively high critical temperature (T 𝑐 ) could allow for operation at higher temperatures ( ≥ 4 K) and the higher critical field could lead to higher accelerating gradients. The deposition of thin film V 3 Si using single target high-power impulse magnetron sputtering is investigated on sapphire and niobium substrates. We report on the T 𝑐 and relate it to thin film characterisation using X-ray photoelectron spectroscopy, scanning electron microscopy and grazing incidence X-ray diffraction. INTRODUCTION Current generation bulk Nb superconducting RF (SRF) cavities are approaching their theoretical limit for performance and next generation accelerators are demanding higher accelerating gradients at higher operational temperatures. Alternative superconducting materials, such as, V 3 Si and Nb 3 Sn are promising candidates due to their relatively high critical temperatures of T 𝑐 = 18 and 17 K, and upper critical fields (H 𝑐 ) of 720 and 540 mT respectively [1]. These properties may allow for production of thin film SRF cavities that can operate at higher temperatures ( ≥ 4 K) and higher accelerating gradients. This would reduce the complexity of the cryogenic systems and the infrastructure of the particle accelerator [2]. V 3 Si is a cubic A15 phase superconductor. In bulk form, Hardy et. al. measured a critical temperature (T 𝑐 ) of 17.1 K whilst also reporting the effect of impurities of less than 0.1 % can drastically reduce T 𝑐 [3]. V 3 Si thin films have been manufactured following a few different methods with varying levels of success: Zhang et. al. deposited a vanadium (V) thin film on a silicon (Si) layer and annealed the sample in high vacuum at temperatures ranging from 650 ∘ C to 900 ∘ C, forming V 3 Si by interdiffusion of the V into the Si, resulting in a T 𝑐≥ 13 K [4]. Deambrosis et. al. deposited using dc magnetron co-sputtering and reactive sputtering techniques introducing silane to the process gas to improve Si stoichiometry. On sapphire substrates they reported a T 𝑐 of 16.8 K, while on Nb substrates no superconducting phase was observed, due to the large diffusion rate between V and Nb [5]. ∗ This work has been supported by: the IFAST collaboration which has received funding from the European Union’s Horizon 2020 Research and Innovation programme under Grant Agreement No 101004730. †Christopher[email protected] A series of V 3 Si thin films were deposited on both Nb and sapphire substrates and at varying temperatures using high impulse power magnetron sputtering (HiPIMS). The films are characterised using surface characterisation techniques: Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and grazing incidence X-ray diffraction (GI-XRD). Superconducting performance was characterised by four-point probe (FPP) and SQUID vibrating-sample magnetometer (SQUID-VSM). EXPERIMENTAL DETAILS Sample Preparation Nb foil and sapphire substrates were cleaned in isopropanol and acetone ultrasonic baths removing any adventitious carbon and other contaminates that may be present. The samples were loaded into an ultrahigh vacuum (UHV) load lock and heating to deposition temperature, ≥ 700 ∘ C, and left to thermally stabilise and for the sample stage to outgas over 12 hrs. Sample is then transferred into the deposition chamber at a pressure of <1 × 10 −8 mbar, whilst substrate is heated. Depositions were conducted using a HiPIMS power supply using the deposition parameters shown in Table 1. Kr was used as the process gas for all depositions at a partial pressure of 3 × 10−3 mbar. Sample Characterisation Structural characterisation was performed via GI-XRD using a Rigaku SmartLab with a 9 kW rotating anode Cu source. Thin film composition was obtained by XPS. Spectra were acquired using a non-monochromated Al K 𝛼 x-ray source and a Thermo Alpha 110 hemispherical analyser. Survey and core region spectra were acquired with a pass energy of 50 and 20 eV respectively. Surface morphology was imaged by Hitachi SU500 SEM. T 𝑐 characterisation was conducted using a cryogenic four point probe for sapphire [6] and Quantum Design MPMS 3 SQUID-VSM. Table 1: HiPIMS Deposition Parameters Deposition Parameters Units Average Power 300 [W] Frequency 1000 [Hz] Pulse Length 10 [µs] Duty Cycle 10 [%] Deposition Length 3 [hr] 15th International Particle Accelerator Conference,Nashville, TN JACoW Publishing ISBN: 978-3-95450-247-9 ISSN: 2673-5490 doi: 10.18429/JACoW-IPAC2024-WEPS39 MC7.T07 Superconducting RF 2779 WEPS: Wednesday Poster Session: WEPS WEPS39 Content from this work may be used under the terms of the CC BY 4.0 licence (© 2024). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI. (a) V3Si thin film on sapphire (b) V3Si thin film on Nb Figure 1: SEM images of V 3 Si thin films on a) sapphire and b) Nb substrates. EXPERIMENTAL RESULTS AND DISCUSSION Figure 1 displays the SEM images of the deposited V 3 Si thin films on a) sapphire and b) Nb foil. On the sapphire substrate a V 3 Si crystallite structure is observed with small voids existing between grains. This porosity suggests that there is a preferential nucleation on preexisting V 3 Si sites instead of the sapphire. The overall topography is related solely to the growth of the film as the underlying sapphire wafer has a roughness of R 𝑎 <1 nm. In contrast, the V 3 Si crystallites are significantly smaller on the Nb foil which could be related to no preferential nucleation sites on the substrate. The larger structural features are due to the roughness of the Nb substrate used. XPS was utilised to determine bulk elemental composition. Figure 2 shows the core region spectra: a) V 2p and O 1s b) Si 2p and c) C 1s present after 4 hours of Kr + ion beam sputtering. XPS spectra for the V 2p 3/2 is at a binding energy (BE) of 512.3 eV which corresponds to reference data of metallic vanadium and the Si 2p is shifted by 0.7 eV to 98.7 eV from reference BE for silicon wafer. Contaminates are also present, two C 1s peaks are situated at 282.5 eV and 285.0 eV which aligns with carbide and C-C bonding. A broad O 1s peak is also present at 531.0 eV which couldn’t be defined [7]. The oxygen and C-C contamination is likely from the sample being exposed to atmosphere before being 510515520525530535 Binding Energy [eV] Intensity [CPS] O 1s V 2p3/2 V 2p5/2 (a) V 2p 9698100102104106 Binding Energy [eV] Intensity [CPS] V3Si (98.8 eV) 102.7 eV (b) Si 2p 280285290295 Binding Energy [eV] Intensity [CPS] Carbide Organic (c) C 1s Figure 2: XPS core region spectra of: V 2p, Si 2p, O 1s and C 1s of a V 3 Si film on Nb substrate deposited using HiPIMS. loaded into the XPS analysis system. Unfortunately, carbide is introduced during the deposition process and could be from the sputtering target, sample or system out gassing. Zhang et. al. [4], measured BEs of 513.0 eV and 99.0 eV for V 2p 3/2 and Si 2p respectively for multiple V-Si phases: V5Si3, VSi2and V3Si. 15th International Particle Accelerator Conference,Nashville, TN JACoW Publishing ISBN: 978-3-95450-247-9 ISSN: 2673-5490 doi: 10.18429/JACoW-IPAC2024-WEPS39 2780 MC7.T07 Superconducting RF WEPS39 WEPS: Wednesday Poster Session: WEPS Content from this work may be used under the terms of the CC BY 4.0 licence (© 2024). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI. 10 20 30 40 50 60 70 80 90 2 [ o ] Intensity [arb. u] (110) (200) (220) (211) (222) (320) (321) (400) Figure 3: Grazing incidence X-ray diffraction of a V 3 Si thin film on Nb (background subtracted). The labelled Miller indices correspond to V 3 Si and the unlabelled peaks correspond to the Nb substrate. 150 160 170 180 190 200 Substrate Heater Power [W] 10 12 14 16 18 Tc [K] Niobium (VSM) Sapphire (FPP) Figure 4: Measured T 𝑐 of a series of V 3 Si thin films deposited on Nb and sapphire using HiPIMS, at range of substrate heater power. Additionally, GI-XRD was conducted on the V 3 Si film on Nb (Fig. 3) and confirms the formation of the superconducting A15 phase of V 3 Si. The peaks associated with the V 3 Si are broad suggesting small crystallites of random orientation, which is in agreement with the SEM images previously discussed. The XRD shows no presence of other V-Si phases and the unlabelled sharp peaks are associated with the Nb substrate. Figure 4 shows the T 𝑐 of all the V 3 Si depositions on both Nb and sapphire. T 𝑐 was obtained using a four point probe for sapphire and vibrating sample magnetometer for Nb substrates. Samples were deposited at varying substrate temperatures, monitored by the heater power. Due to the high temperature required for the formation of superconducting A15 V 3 Si phase, the heater power was adjusted close to the systems maximum capability. The highest T 𝑐 measured was 14.85 K and 13 K for sapphire and Nb respectively. The lower T 𝑐 on the Nb substrates is likely related to the smaller crystallite structure. The oxygen and carbon contamination present in the film explains the T 𝑐 being lower than the theoretical maximum on both substrates. CONCLUSION Experiments conducted show promise for V 3 Si thin films deposited using HiPIMS. Superconducting V 3 Si thin films have been successfully grown on sapphire inline with previous research and on Nb substrates. SEM shows a small crystallite structure which is in agreement in GI-XRD. The T 𝑐 s measured are still below bulk value which could be as a result of the crystal growth or the contamination observed in the XPS. Additionally, the variation between sapphire and Nb substrates is related to the formation of preferential nucleation sites. FUTURE WORK The deposition parameters can be explored further by: adjusting of the duty cycle, introducing Bipolar HiPIMS and increasing substrate temperature. This may show improvement of the V 3 Si film by increasing crystallite size. UHV annealing techniques and post processing such as flash [8] or laser annealing is also desirable as it has the potential to improve film crystallisation and reduce defect states. REFERENCES [1] A. - M. Valente-Feliciano, “Superconducting rf materials other than bulk niobium: A review,” Superconductor Science and Technology, vol. 29, no. 11, p. 113 002, 2016. doi:10.1088/0953-2048/29/11/113002 [2] G. 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