NIOBIUM NITRIDE THIN FILMS AND MULTILAYERS FOR SRF APPLICATIONS

Transcription

1 NIOBIUM NITRIDE THIN FILMS AND MULTILAYERS FOR SRF APPLICATIONS William M. Roach Advisor: R. Ale Lukaszew Department of Applied Science The College of William and Mary Abstract Superconducting thin films have the potential to overcome fundamental limitations of bulk niobium technology that is currently implemented in linear particle accelerators. Before these thin films can successfully be implemented, systematic studies on structureproperty correlations are necessary. Here, we present the characterization of niobium nitride thin films and multilayers. We explore the differences between niobium and niobium nitride surfaces. Additionally, we demonstrate that niobium nitride multilayers are capable of shielding an underlying niobium film from magnetic fields larger than the lower critical field of bulk niobium. I. Introduction Particle accelerator facilities are used to study sub atomic particles as well as the properties of materials. In order to carry out radiobiology studies to simulate exposure to space radiation, NASA created the Space Radiation Laboratory at Brookhaven National Laboratory [1,2]. Results from these studies can be used to protect astronauts from the adverse effects of being subjected to space radiation such as genetic mutations and cancer. The current technology implemented in linear particle accelerators at facilities like Brookhaven National Lab and the Thomas Jefferson National Accelerator Facility is based on the use of bulk Nb superconducting radio frequency (SRF) cavities [3]. However, individual cavities have been fabricated that operate near the fundamental accelerating gradient limit of ~50 MV/m [4]. This limit is closely related to the critical field values for Nb. In order to overcome the limits imposed by the use of bulk Nb, a theoretical model has been proposed that involves depositing alternating superconducting-insulatingsuperconducting (SIS) layers onto a Nb surface where the superconductor has a larger thermodynamic critical field, H C, and transition temperature, T C, than Nb [5]. Candidates to be used in these SIS layers include Nb based compounds such as NbN, Nb(Ti)N, and Nb 3 Sn as well as novel superconductors such as MgB 2. This SIS structure would shield the Nb cavity from higher magnetic fields which would allow higher field gradients to be achieved. The incorporation of this SIS model may allow for an increase in the capability of accelerator experiments such as those carried out at the Space Radiation Laboratory. Before, the SIS model can be implemented, fundamental studies on proof of principle SIS layers correlating surface morphology, structure, and superconducting properties are needed. Here, a comparison of Nb and NbN surfaces is presented. Additionally, evidence of shielding in SIS multilayers is presented. II. Surface Morphology One of major differences between the current SRF technology and the SIS model is that the inner surface of the accelerator cavity will be a different material. Since the quality of the Roach 1

2 surface is an important factor that dictates SRF performance, it is necessary to understand how a Nb surface would differ from the surface of one of the aforementioned candidate materials for the SIS layers. Thus, Nb and NbN films of similar thickness ( nm) were deposited onto MgO(100) substrates using DC magnetron sputtering and their structural and superconducting properties were examined [6]. The surface morphology of these films was studied using atomic force microscopy (AFM) and associated software [7]. Representative AFM scans are shown in Figure 1. When the root mean square (RMS) roughness values of the films were compared, it was found that the NbN films consistently grew smoother than their Nb counter parts. This is also evident in the representative line scans shown in Figure 2. In general, smoother cavity surfaces lead to better SRF performance. For this reason, it is encouraging that NbN films generally had smoother surfaces. In order to better understand why these films had lower roughness values, further analysis was carried out on the surface morphology images. nm, 1.21 nm, and 2.45 nm. Image from reference [6]. Figure 2: Representative line scans for Nb(110), Nb(100), and NbN surfaces. Image from reference [6]. The images displayed in Figure 1 were studied using power spectral density (PSD) analysis. For this analysis, the two-dimensional fast Fourier transform (FFT) of the surface morphology is obtained. From the FFT images, the PSD is then calculated for each spatial frequency, k, as shown in Figure 3. Figure 1: Surface morphology images for a NbN surface (left), a Nb(100) surface (top right), and a Nb(110) surface (bottom right). The respective RMS roughness values are <1 Figure 3: PSD vs. k for NbN, Nb(100), and Nb(110) surfaces. Image from reference [6]. For the PSD curves corresponding to the Nb(100) and Nb(110) surfaces, noticeable peaks are present near -1.5 nm -1 and -1.9 nm -1

3 respectively, while no such peaks are present in the NbN curve. The presence of these peaks in the Nb curves indicates that there is wavelength selection associated with the presence of a step edge diffusion barrier affecting the surface morphology [8]. The absence of the peak in the NbN curve indicates that no such barrier is present during film growth and that the surface features are forming in a self-affine manner [9]. III. NbN Multilayers In order to test the shielding ability of the SIS structure, several NbN/MgO/Nb/MgO(100) multilayers were fabricated and characterized. A cross sectional view of these structures is shown in Figure 4. In these multilayers, the Nb film represents the bulk Nb to be shielded. Figure 4: Cross sectional view of proof of principle SIS samples fabricated to test the viability of the SIS model. The superconducting properties of these SIS multilayers were studied using superconducting quantum interference device (SQUID) magnetometry. In particular, the magnetic moment as a function of applied field was examined. When superconductors are in the Meissner state and magnetic fields are expelled, the magnetic moment will increase linearly with increasing applied field. When the moment deviates from this linear regime, magnetic vortexes have penetrated the material indicating that the superconductor is no longer in the Meissner state. Magnetic field penetration is one of the mechanisms that limits the performance of bulk Nb SRF cavities. Once vortex penetration occurs, the overall efficiency of the cavity decreases significantly. If the field at which vortex penetration occurs can be increased using the SIS model, larger accelerating gradients can be achieved. The first evidence of magnetic shielding in SIS multilayers was presented by Antoine et al. [10,11]. In their study, Nb films as well as NbN/MgO/Nb multilayers were deposited onto sapphire substrates. When the penetration fields were measured, the Nb/sapphire film had a penetration field of 180 Oe while the multilayer had a penetration field of 960 Oe. The fact that the penetration field increased from 180 Oe to 960 Oe with the addition of the NbN multilayer indicates that magnetic shielding occurred. While this work was promising, a penetration field of 960 Oe is still less than the lower critical field of bulk Nb (1700 Oe). In our experiment, we were able to achieve shielding beyond the lower critical field of bulk Nb up to a penetration field of 2000 Oe [12]. Figure 5 shows a magnetization curve for a 50 nm NbN / 15 nm MgO / 250 nm Nb / MgO(100) multilayer where the deviation from the linear behavior occurs at H = 2000 Oe. This indicates that it is possible to use an SIS multilayer to shield Nb beyond its lower critical field value.

4 gradient that is achievable using SRF technology. Figure 5: Magnetization curve for an SIS multilayer showing a penetration field of H P = 2000 Oe. Image adapted from reference [12]. It is worth noting that the SQUID measurements ultimately represent an underestimate of the shielding capable with an SIS structure. Aligning the sample such that it is perfectly parallel to the applied field is necessary to measure the true magnetic behavior. Even though the sample was aligned as well as experimentally possible, even a slight misalignment will cause a perpendicular component and lower the field at which penetration is seen. Additionally, the SQUID measurements were carried out at 4-5 K while the critical field value of 1700 Oe is calculated for 0 K. If the measurements were done at lower temperatures, one would expect that the penetration would have been delayed to even higher fields. IV. Conclusions and Future Work The work presented here highlights some of the important differences, in particular surface morphology, between Nb and other candidate materials that need to be further explored before SIS multilayers can be successfully implemented in SRF cavity applications. Additionally, we have achieved magnetic shielding larger than the lower critical field of niobium, indicating that the SIS model may allow for an increase in the accelerating Work is currently underway to provide further information regarding which candidate material will provide optimal performance in the SIS structure. Initial work on MgB 2 thin films has shown promise by being able to achieve penetration fields much larger than bulk niobium [13]. The fabrication and characterization of MgB 2 multilayers is ongoing. V. Acknowledgements The author would like to thank everyone who has contributed to the work presented here. These people include members of the Lukaszew research group at the College of William and Mary: Dr. Ale Lukaszew, Dr. César Clavero (now at Lawrence Berkeley National Laboratory), Dr. Jonathan Skuza (now at the National Institute of Aerospace), Douglas Beringer, and Zhaozhu Li. The author is also grateful for input and thoughtful discussion with members of the SRF institute at Jefferson Lab: Dr. Charles Reece, Dr. Larry Phillips, Dr. Xin Zhao, Anne-Marie Valente- Feliciano, and Josh Spradlin. Funding for this work has been provided by the Defense Threat Reduction Agency (HDTRA ) and the Department of Engergy (DE-AC05-06OR23177). Last and certainly not least, I would like to extend my appreciation to the Virginia Space Grant Consortium for allowing me the great opportunity to serve as a graduate fellowship recipient for the last two years. It has been a wonderful experience for me.

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