J Supercond Nov Magn (2013) 26:1563–1568DOI 10.1007/s10948-012-1969-3

O R I G I NA L PA P E R

MgB2 Thin Films on Metal Substrates for Superconducting RFCavity Applications

Chenggang Zhuang · Teng Tan · Alex Krick ·Qingyu Lei · Ke Chen · X.X. Xi

Received: 7 November 2012 / Accepted: 1 December 2012 / Published online: 5 January 2013© Springer Science+Business Media New York 2013

Abstract Magnesium diboride is a promising material forsuperconducting RF (SRF) cavity applications. Comparedto the currently used superconductor for SRF cavities Nb,MgB2 has the potential to achieve lower RF loss and higheracceleration field due to its higher critical temperature andthermodynamic critical magnetic field. Since the RF fieldonly penetrates a few penetration depths into a superconduc-tor, a superconducting coating of several hundred nanome-ters on a metal cavity is sufficient for superb SRF cavity per-formances. In this work, we report the properties of MgB2

thin films deposited by the hybrid physical–chemical vapordeposition (HPCVD) technique on different metal substratesincluding Nb, Mo, Ta, and stainless steel. All the films werepolycrystalline, as indicated by X-ray diffractometry andscanning electron microscopy, and showed Tc ∼ 39 K, de-termined by resistance versus temperature, magnetic suscep-tibility, and dielectric resonator measurements. MgB2 filmsdeposited on Nb substrates polished to various degrees ofsmoothness exhibit similar Tc. The result is a promising stepin the investigation of using MgB2 as an alternative to Nb forSRF cavities.

Keywords Superconducting RF cavity · Magnesiumdiboride · HPCVD · Metal substrates

1 Introduction

The performance of superconducting RF cavities made ofNb have experienced spectacular progress over the years [1].

C. Zhuang · T. Tan (�) · A. Krick · Q. Lei · K. Chen · X.X. XiDepartment of Physics, Temple University, Philadelphia, 19122PA, USAe-mail: phys.tan@temple.edu

For Nb, the maximum accelerating field is predicted theo-retically to be 50 MV/m [2], which is being approached [3].The thermodynamic critical field Hc of a superconductor de-termines its maximum accelerating field. An RF peak break-down field of 170–180 mT at 1.3 GHz has been demon-strated [4], while Hc ∼ 200 mT for Nb. For even higher ac-celerating field, new SRF materials are being investigated.

MgB2 is considered a possible candidate material forSRF cavities [5–7]. Since RF field decays exponentiallyfrom the surface of a superconductor, the cavity propertiesare determined by only a thin (a few penetration depths)layer of the superconductor on the cavity surface. So far,most exploratory work on MgB2 for SRF applications hasbeen on epitaxial films on single-crystal substrates. For theeventual application to SRF cavities, an investigation ofMgB2 films grown on suitable metallic materials is neces-sary. In this work, we have selected Nb, Mo, Ta, and stain-less steel as the substrates and deposited MgB2 films byHPCVD. Superconducting and surface morphology proper-ties are presented.

2 Experimental Details

Table 1 shows the properties of the metallic substrates usedin this work. Any substrate material for MgB2 coating needsto be stable at the deposition temperature of about 700 °Cagainst reaction or alloying with Mg vapor. Good thermalconductivity is desirable for SRF cavity material because ef-fective dissipation of the heat due to RF losses provides bet-ter thermal stability for the SRF cavity. Further, the substratematerial’s thermal expansion coefficient should be close tothat of MgB2, which is 5.5 × 10−6/K at room temperature,so that the coating will not suffer from cracking or peelingoff upon thermal cycling. Nb, Mo, and Ta, in particular Mo,

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Table 1 Properties of themetallic substrates used in thiswork

Materials Reactionwith Mg

Melting point Thermalconductivity

Thermalexpansion

Niobium [8] Stable 2477 53.7 7.02

Molybdenum [8] Stable 2623 138 5.43

Tantalum [8] Stable 3017 57.5 7.02

316 stainless steel [10] Stable 1375–1400 16.3 15.9

show suitable properties for MgB2 coating. Stainless steelwas also used in this work as a reference material. Althoughthe high thermal conductivity of Cu (17.6 × 10−6/K) [8] ishighly desirable for SRF cavities, it alloys with Mg above500 ◦C [9], making it very difficult to coat MgB2 on Cu.

The Nb, Mo, and Ta substrates used in this work were0.1 mm-thick foils, obtained from Johnson Matthey, withroot-mean-square (RMS) roughness ranging from 44 and130 nm. The stainless steel foils have a thickness of 0.2 mmand an RMS roughness of about 40 nm. After ultrasonicbath cleaning with diluted Micro-90TM cleaning solution,acetone, and isopropyl alcohol, they were loaded into thereactor for MgB2 deposition without further treatment.For Nb, 1 mm-thick wafers were also used, which weretreated with different levels of chemical-mechanical polish-ing (CMP) [11]. The 240 grit abrasive paper, 15 μm lappingfilm, 1 μm lapping film, and 50 nm slurry were used in theCMP process.

The MgB2 films were deposited by HPCVD, which hasbeen described elsewhere [12]. Different from the HPCVDsystem in [12], we employed a resistance heater in a water-cooled stainless steel reactor. The surface morphology ofthe MgB2 films was measured with scanning electron mi-croscopy (SEM) and atomic force microscopy (AFM). Thetransition temperature Tc was measured by DC transport,magnetic susceptibility, and dielectric resonator methods.The critical current density Jc was obtained using the Beanmodel from the hysteresis loops in the M–H measurementsusing a QuantumDesign PPMS system.

3 Results

3.1 MgB2 Films on Nb, Mo, Ta, and Stainless Steel Foils

The MgB2 films on the Mo, Nb, Ta, and stainless steel foilsare polycrystalline as shown by X-ray diffraction. Figure 1shows the SEM images for four 300 nm-thick MgB2 filmson Mo, Nb, Ta, and stainless steel foils, respectively. Theyshow MgB2 crystallites with random orientations, a surfacemorphology consistent with the polycrystalline film struc-ture. AFM measurements of the films show RMS roughnessranging from 66–187 nm.

Fig. 1 SEM images for four 300 nm MgB2 films on (a) Mo, (b) Nb,(c) Ta, and (d) stainless steel foils, respectively. The scale bars indicate1 μm for all the figures

Because the metallic substrates are conducting, it is diffi-cult to obtain the resistivity of the MgB2 films by the trans-port measurement even though Tc can be determined. InFig. 2, the magnetic susceptibility vs. temperature curvesfor four MgB2 films on Nb, Mo, Ta, and stainless steel foils,respectively, are shown. All the films show sharp supercon-ducting transitions with Tc around 39 K. These Tc values arein agreement with those obtained by the transport measure-ment, and indicate good quality in the MgB2 films on thesemetal substrates.

The upper critical field Hc2 was obtained by transportmeasurement under different applied field using a Tc onsetcriterion. Figure 3 shows Hc2 as a function of temperaturefor three MgB2 films on Ta, Mo, and Nb foils, respectively.The results for the three substrates almost completely over-lap with each other. They all show Hc2(0) of about 7 T. Thislow value is similar to that in the clean MgB2 films on SiCor sapphire single-crystal substrates [13], an indication ofweak scattering in the films.

From the M–H hysteresis loop measurements, we ob-tained the Jc(H) values using the Bean model and the resultis shown in Fig. 4 for four 450 nm-thick MgB2 films on(a) Ta, (b) Nb, (c) Mo, and (d) stainless steel substrates atdifferent temperatures. All the films show self-field Jc val-ues around or above 107 A/cm2 even at 20 K, indicatinghigh quality in these films. The Jc values are suppressedrapidly by the applied magnetic field, another indication of

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Fig. 2 AC susceptibility versustemperature for MgB2 films ondifferent metallic foils:(a) 300 nm film on Mo,(b) 300 nm film on Nb,(c) 300 nm film on Ta, and(d) 450 nm film on stainlesssteel

Fig. 3 Upper critical field versus temperature for MgB2 films on Ta,Mo, and Nb foils

the cleanness of the films such that they lack vortex pinningcenters.

The low electron scattering in the superconductor is de-sirable for SRF cavities. According to the BCS theory, thesurface resistance Rs is lower for superconductors withlower residual resistivity and higher energy gap [14]. Thus, aclean superconductor is necessary for low RF loss and highquality factor Q in SRF cavities. The result of clean MgB2

films on various metal substrates with high Tc and weakscattering is very encouraging for their application to SRFcavities.

3.2 MgB2 Films on Nb Wafers with Different Degrees ofSmoothness

Figures 5(a)–5(d) show optical microscope images of an as-received commercial Nb wafer and Nb wafers CMP polishedwith 15 μm and 1 μm lapping pads and 50 nm slurry, re-spectively. Figures 5(e)–5(f) are AFM images for Nb wafersCMP polished with 1 μm and 50 nm slurry, respectively. Theas-received stock Nb is rough, and CMP polishing producessmoother films as the particles’ size on the abrasive pad, orin the slurry, decreases. However, Nb is soft and scratchesare evident when the slurry size decreases to 1 μm [seeFig. 5(c)]. For the slurry size of 50 nm, some particles in theslurry are embedded in the Nb wafer [Fig. 5(d)]. The RMSroughness of the Nb wafers shown in Figs. 5(e) and 5(f) is6.3 nm and 2.3 nm, respectively. Similar results have beenreported in CMP polished Nb in the literature [15].

Figure 6 shows SEM images of different magnificationsfor four 300 nm-thick MgB2 films grown on Nb wafersof different degrees of surface smoothness, polished using240 grit abrasive paper, 15 μm lapping pad, 1 μm lappingpad, and 50 nm slurry, respectively. At low magnifications(the first three columns), a smoother substrate leads to asmoother MgB2 film. At the highest magnification (the lastcolumn), on the other hand, the morphology of the films aresimilar, showing randomly oriented MgB2 crystallites withsimilar dimensions. The RMS roughness measured by AFMis large for the films on rough substrates: 180 nm for a filmon Nb wafer polished with 15 μm lapping pad. For smoother

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Fig. 4 Jc(H) at differenttemperatures for four 450nm-thick MgB2 films ondifferent metallic substrates:(a) Mo, (b) Nb, (c) Ta, and(d) stainless steel

Fig. 5 Optical images of Nbwafers with different grades ofCMP polishing: (a) stock Nb,(b) 15 μm lapping pad polished,(c) 1 μm lapping pad polished,and (d) 50 nm slurry polished.AFM images of finely polishedNb: (e) 1 μm lapping padpolished, and (f) 50 nm slurrypolished

substrates, polished with 1 μm grade lapping pad or 50 nmslurry, the RMS roughness saturates at about 55 nm.

The Tc of MgB2 films grown on Nb wafers of differ-ent roughness was measured by the dielectric resonatormethod [16]. A sapphire puck was sandwiched between two

identically grown MgB2 films at the two ends, shown inthe inset, forming a resonator with a resonance frequencyat 23.33 GHz when the films are normal. The resonance fre-quency shifted when the film became superconducting, andthe onset temperature for this shift signifies the Tc of the

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Fig. 6 SEM images of variousmagnification of MgB2 films onNb substrates of differentpolishing grades

Fig. 7 Temperature dependence of the frequency shift of a dielectricresonator at around 23.33 GHz for four MgB2 thin films on Nb wafersof different roughness. The onset temperature of the shift signifies Tc ,which is are 38 K for all the four films

films. Figure 7 shows the results for four MgB2 films on Nbwafers with different degrees of polishing. All the four filmsshow Tc at about 38 K. Evidently, the roughness of the Nbsubstrates does not affect Tc significantly.

4 Conclusion

Using the HPCVD technique, we have grown MgB2 filmson Nb. Mo, Ta, and stainless steel foils and on Nb waferswith different degrees of polishing. All the films on metalfoils are polycrystalline and show good superconductingproperties indicated by high Tc and high Jc. The films areclean, demonstrated by low Hc2 and weak vortex pinning.The roughness of the Nb surface influences the morphologyof the MgB2 films grown on them, but this does not affect

the Tc of the films. The properties of the MgB2 films on dif-ferent metal substrates are encouraging for the applicationof MgB2 films in SRF cavities.

Acknowledgement This work is supported by US Department ofEnergy under grant No. DE-SC0004410.

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