Physica C 469 (2009) 1574–1577

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Physica C

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Fabrication and characterization of superconducting MgB2 thin films grownby rf sputtering and thermal evaporation method

S.C. Park a, K.J. Song b, S.G. Kang a, Y.J. Lim a, J.-K. Chung a, C.J. Kim a,*

a Engineering Research Institute, Gyeongsang National University, Gajwa-dong 900, Jinju, Gyungnam 660-701, South Koreab Chonbuk National University, Deokjin-dong 1 Ga 664-14, Jeonju, Jeonbuk 561-756, South Korea

a r t i c l e i n f o

Article history:Available online 31 May 2009

PACS:74.70.Ad74.78.Db

Keywords:MgB2

Co-depositionThin film

0921-4534/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.physc.2009.05.220

* Corresponding author. Present address: i-Cube CAdvanced Materials Science and Engineering, GyeongsGajwa-dong, Jinju, Gyeongnam 660-701, South Korea

E-mail address: cjk@gnu.ac.kr (C.J. Kim).

a b s t r a c t

We have grown MgB2 thin films by co-deposition method. Magnesium (Mg) and Boron (B) were simul-taneously deposited on the substrates using the radio frequency (rf) sputtering and thermal evaporation.The deposition conditions were varied by changing Mg evaporation rate, B sputtering rate, substrate tem-perature, deposition time, and types of substrates (c-Al2O3, r-Al2O3, LaAlO3, MgO). The MgB2 layers had400–500 nm in thickness and occasionally MgB12 was formed as a second phase. Superconducting tran-sition temperatures have been measured around 20–37 K depend on the types of substrates and growthconditions. The relative deposition rate of Mg and B was important to maintain the stoichiometry ofMgB2. The structural features such as second phases, crystal orientation across the interface, and defectsin the cross-sectional direction were characterized with XRD, SEM, and EDS.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

Magnesium diboride (MgB2) was first reported with the transi-tion temperature of 39 K [1] in 2001. The simple crystal structure,higher superconducting transition temperature Tc compared tothose of Nb-based metallic superconductors, longer coherencelength of about 50 Å [2] and the relatively cheaper cost of rawmaterials than those of cuprate-based high-Tc superconductorsare all the characteristics that facilitate the application of MgB2

for current carrying wires and various devices. MgB2 can be easilyobtained in polycrystalline phase form by direct reaction of the ele-ments at high temperatures (800–900 K) under vacuum or inertatmosphere and its superconducting properties can be improvedby various doping [3–5]. However, polycrystalline MgB2 bulk con-tains large volume fraction of pores due to high volatility of Mg atthe reaction temperature, which greatly decrease the connectivityof the grains and deteriorate the current carrying capacity.

Thin film techniques could almost annihilate the pore forma-tion, thus a variety of thin film process have been tried to growMgB2 thin films, comprising hybrid physical–chemical vapor depo-sition (HPCVD) [6], molecular beam epitaxy (MBE) [7], reactiveevaporation [8], ultra high vacuum–molecular beam epitaxy(UHV–MBE) [9], electron beam evaporation (EBE) [10], ultra high

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enter, Division of Nano andang National University, 900,. Tel.: +82 055 751 5331.

vacuum–electron beam evaporation (UHV–EBE) [11] and pulsedlaser deposition (PLD) [12–13]. Since all of these methods normallyadopt ex situ or in situ annealing process during MgB2 formationreaction where big difference in vapor pressure of Mg and B areinevitable, it is not easy task to get smooth, robust, high-qualitythin film. Mg also reacts strongly with oxygen which can contam-inate the thin film and adversely affect the superconducting prop-erties. Among the various processing methods, HPCVD method hasbeen mostly successful to grow smooth and dense MgB2 thin filmsand in principle this process can be scaled to large substrates. Butdiborane toxicity imposes strictly controlled conditions and fabri-cation of multilayer with this technique is not straightforward.

In this study, considering these problems, we have tried co-deposition method as an alternative processing method for MgB2

thin film. This method is relatively simple and low-cost processcompared to the other techniques mentioned above. Especially,the vapor pressure of Mg and the impinging flux of B can be con-trolled separately by heater temperature of Mg block and rf sput-tering power, respectively. Usually, critical temperature (Tc) wereappeared in 37 K, 40 K [6], 35 K [7], 33.6 K [11], 34.2 K [13] byco-deposition, HPCVD, MBE, UHV EBE, PLD method. Also, crystal-linity and surface morphology are best in HPCVD method.

We have grown MgB2 thin films with various processing condi-tions including Mg evaporation rate, B sputtering rate, substratetemperature, deposition time, and types of substrates (c-Al2O3, r-Al2O3, LaAlO3, MgO). The structural features such as second phases,crystal orientation across the interface, and defects in the cross-sectional direction were characterized with XRD, SEM, and EDS.

S.C. Park et al. / Physica C 469 (2009) 1574–1577 1575

2. Experiments

Mg and B were simultaneously deposited on the c-Al2O3, r-Al2O3, LaAlO3, and MgO substrates using the rf sputter and thermalheater, respectively. Pure metals of 99.9% Mg and 99.99% B wereused as the deposition sources, and the thin films grew under anAr pressure 2 � 10�2 Torr. To reduce the oxygen contaminationduring deposition, the flushing Ar gas was filtered through the oxy-gen adsorption system which is composed of Mg granules (80–635 lm). The residual oxygen in Ar gas is easily adsorbed on thesurface of Mg granule to form MgO and the vapor pressure of oxy-gen could be greatly reduced inside the deposition chamber, thusreducing the contamination of MgB2 by oxygen. The depositionrate of Mg and B were fixed �16.5 Å/s and �3.6 Å/s, respectively,by controlling the sputtering rate of B and thermal evaporationrate of Mg. In the most of thin film process for MgB2, it is crucialto secure the enough Mg vapor pressure, especially in the processinvolving thermal evaporation of Mg. To increase of Mg vapor pres-sure during growth, a funnel-like guide has been adopted justabove the boat containing the Mg block as shown in Fig. 1 [14].The deposition condition is summarized in Fig. 2. The thicknessof MgB2 thin films were about 400–500 nm.

The phase analysis of the grown MgB2 thin films were measuredwith X-ray diffraction (XRD) system of D8 DISCOVER with GADDS(General Area Detector Diffraction System, Bruker) and XRD (Rik-agu D/Max-3C, Japan) with Cu Ka radiation (k = 1.5418 Å). Micro-

Fig. 1. Schematic diagram of the co-deposition chamber with a funnel-like guidefor Mg vapor.

Fig. 2. Profile of co-deposition for MgB2 thin film.

structure and compositional analyses were performed by ascanning electron microscope (SEM: JEOL JSM-6400, Japan)equipped with an energy dispersive X-ray spectroscopy (EDS) sys-tem. Superconducting critical temperature of MgB2 thin films havebeen measured by physical property measurement system (PPMS9T, Quantum Design) and Jc was calculated from the width of themagnetic hysteresis based on the extended Bean model [15–17].

3. Results and discussion

Since Mg was evaporated using a thermal heater by tungstenboat, and B by a sputtering system, the deposition rate of each ele-ment has to be controlled separately to obtain stoichiometricMgB2. The sintered B target was magnetron sputtered at the rfpower of �200 W, which corresponded to the deposition rate of�3.6 Å/s. With the deposition rate of B fixed, the vapor pressureof Mg was controlled by varying the temperature of tungsten boatwith heating element control unit between 100 and 900 �C. The va-por pressure and the deposition rate of Mg increased linearly withtemperature [14]. By adopting funnel-like guide for Mg vapor, en-ough Mg vapor pressure can be secured for MgB2 formation.

To observe the effect of substrates on the MgB2 phase forma-tion, Mg and B were deposited on the various substrates whileall the other experimental conditions fixed. Fig. 3 shows the grownMgB2 thin films XRD patterns on c-Al2O3, r-Al2O3, LaAlO3, and MgOsubstrates, and Fig 4 shows the corresponding SEM micrographs,respectively. In Fig. 3a, where strong Mg peaks are shown togetherMgB2 main phase. Boron might exist in the sample but did notclearly appear in the XRD pattern due to the low atomic scatteringfactor of B. In Fig. 3b, Mg peaks are almost undetectable and MgB12

peaks started to appear. This apparent difference in the XRD pat-tern of the samples with the different substrate are also clearlyseen in the morphologies as in Fig. 4a and b. The grain size ofMgB2 is larger when using c-Al2O3 than using r-Al2O3 and the grainorientation of MgB2 looks different with each other. That is, MgB2

grains grew with c-axis perpendicular to the c-Al2O3 surface(Fig. 4a) while parallel to the r-Al2O3 surface (Fig. 4b). Since thegrowth rate of MgB2 is different along the crystallographic direc-tion, the morphology and the diffraction intensities of MgB2 phaseappeared differently as in Fig. 3 and 4. The relative orientation ofthe MgB2 grains of the samples with the different substrates canbe compared using the (1 0 0), (1 0 1), (0 0 2), and (1 1 0) peaksat 2h = 33.52, 42.46, 51.87, and 59.33, respectively. While excess

Fig. 3. XRD patterns of MgB2 thin films deposited at 800 �C/30 min on the differentsubstrates. (a) c-Al2O3, (b) r-Al2O3, (c) LaAlO3, and (d) MgO.

Fig. 4. Surface morphologies of MgB2 thin films grown by co-deposition on the substrate of (a) c-Al2O3, (b) r-Al2O3, (c) LaAlO3, and (d) MgO.

1576 S.C. Park et al. / Physica C 469 (2009) 1574–1577

Mg was detected with c-Al2O3 substrate, MgB12 phase appeared asa second phase with r-Al2O3 substrate. Schmitt et al. [18] reportedthat the MgB12 could be obtained as the main phase at the temper-ature range between 800 �C and 1000 �C during MgB2 formation.This implies that shorter deposition time and low reaction temper-ature is necessary to avoid MgB12 formation (Fig. 3b) when using r-Al2O3 substrate.

Fig. 3c and d shows the XRD patterns and Fig. 4c and d showsthe SEM micrographs of MgB2 thin films on LaAlO3 and MgO sub-strate, respectively. Unlikely the case with Al2O3 substrates, MgB2

formation did not proceed easily, resulting in very fine particlesof MgB2 with the average grain size of �200 nm or very poor crys-tallinity as can be seen in the XRD pattern (Fig. 3c). Especially,MgB2 did not form easily on the MgO substrate since MgO is verystable and non-reactive [19].

Depending on the kind of the substrate, a large variety of micro-structure could be developed during thin film formation of MgB2,

Fig. 5. Resistivity vs. temperature curves of the grown MgB2 thin film on thevarious substrates.

from the well developed hexagonal grains with the c-axis perpen-dicular to the substrate surface (Fig. 4a), smaller grains with c-axisparallel to the substrate surface (Fig. 4b), very tiny and poorly crys-tallized grains (Fig. 4c), and even negligible reaction (Fig. 4d).

Fig. 5 shows the resistivity vs. temperature curves of MgB2 thinfilms formed at the various substrates. The critical temperatureranges from the highest �37 K to the lowest �20 K in the sampleswith c-Al2O3 substrate and with LaAlO3 substrate, respectively. Theresistivity at room temperature of the MgB2 thin film on c-Al2O3

substrate and r-Al2O3 substrate was �73 lX cm and �56 lX cm,respectively, which is inconsistent with the measured Tc valuesof �37 K and �28 K. This implies that the crystalline nature suchas orientation, grain size, impurity phases such as un-reacted Mgor MgB12 affect the superconducting properties. In the case of c-Al2O3 substrate, showed the best special quality. Owing to the welldeveloped hexagonal grains (�500 nm) with the c-axis perpendic-ular to the substrate surface, Tc value of about 37 K was obtained.

Fig. 6. The external field dependence of Jc at 5 K and 20 K of the MgB2 thin filmgrown on the c-Al2O3 substrate.

S.C. Park et al. / Physica C 469 (2009) 1574–1577 1577

These films were promising for applications in MgB2 Josephsonjunction device.

Fig. 6 shows the external field dependence of Jc at 5 K and 20 Kof the sample on the c-Al2O3 substrate. The highest Jc result wascalculated to be �63,800 A/cm2 at 5 K, 1 T and �26,000 A/cm2 at20 K, 1 T.

4. Conclusion

We synthesized MgB2 thin films using co-deposition method onthe c-Al2O3, r-Al2O3, LaAlO3, and MgO substrates. The enough Mgvapor pressure for MgB2 formation could be obtained by incorpo-rating funnel-like guide. The morphology and the crystallinity ofMgB2 thin film were quite different depending on the substrate.Even with the same Al2O3, the crystallographic orientation of thesubstrate affects the growth of the MgB2 grains. The highest Tc

was observed at �37 K with the MgB2 thin film on the c-Al2O3 sub-strate Jc was calculated as �63,800 A/cm2 at 5 K, 1 T and�26,000 A/cm2 at 20 K, 1 T.

Acknowledgments

This work was supported by the Korea Research FoundationGrant funded by the Korean Government (MOEHRD, Basic ResearchPromotion Fund) (KRF-2008-314-D00200) and partly by the 2ndstage of BK21 program.

References

[1] J. Nagamatsu, N. Nakagawa, T. Muranaka, Y. Zenitani, J. Akimitsu, Nature 410(2001) 63.

[2] D.K. Finnemore, J.E. Osterson, S.L. Bud’ko, G. Lapertort, P.C. Canfield, Phys. Rev.Lett. 86 (2001) 2420.

[3] X.L. Wang, S. Soltanian, M. James, M.J. Qin, J. Horat, Q.W. Yao, H.K. Liu, S.X. Dou,Physica C 408–410 (2004) 63.

[4] Q.W. Yao, X.L. Wang, J. Horvat, S.X. Dou, Physica C 402 (2004) 38.[5] S. Keshavarzi, M.J. Qin, S. Soltanian, H.K. Liu, S.X. Dou, Physica C 408–410

(2004) 601.[6] X.X. Xi, A.V. Pogrebnyakov, S.Y. Xu, K. Chen, Y. Cui, E.C. Maertz, C.G. Zhuang, Q.

Li, D.R. Lamborn, J.M. Redwing, Z.K. Liu, A. Soukiassian, D.G. Schlom, X.J. Weng,E.C. Dickey, Y.B. Chen, W. Tian, X.Q. Pan, S.A. Cybart, R.C. Dynes, Physica C 456(2007) 22.

[7] Y. Harada, T. Takahashi, H. Iriuda, M. Kuroha, Y. Nakanishi, M. Yoshizawa,Physica C 426–431 (2005) 1453.

[8] B.H. Moeckly, W.S. Ruby, Supercond. Sci. Technol. 19 (2006) L21.[9] Y. Harada, M. Uduka, Y. Nakanishi, N. Yoshimoto, M. Yoshizawa, Physica C

412–414 (2004) 1383.[10] S. Yata, G. Shimizu, Y. Yamada, S. Kubo, A. Matsushita, Physica C 388–389

(2003) 155.[11] H.M. Zhu, Y.B. Zhang, X.L. Sun, W.J. Xiong, S.P. Zhou, Physica C 452 (2007) 11.[12] W.N. Kang, H.-J. Kim, E.-M. Choi, C.U. Jung, S.-I. Lee, Science 292 (2001) 1521.[13] Y.S. Wu, Y. Zhao, D. Wexler, J.H. Kim, S.X. Dou, Physica C 468 (2008) 218.[14] S.C. Park, J.-K. Chung, S.G. Kang, K.J. Song, C.-J. Kim, C.J. Kim, Physica C 468

(2008) 1879.[15] P.S. Swartz, C.P. Bean, J. Appl. Phys. 39 (1968) 4991.[16] A. Gupta, H. Narayan, D. Astill, D. Kanjilal, C. Ferdeghini, M. Paranthaman, A.V.

Narlikar, Supercond. Sci. Technol. 16 (2003) 951.[17] Z.W. Zhao, S.L. Li, Y.M. Ni, H.P. Yang, Z.Y. Liu, H.H. Wen, W.N. Kang, H.J. Kim,

E.M. Choi, S.I. Lee, Phys. Rev. B 65 (2002) 064512.[18] R. Schmitt, J. Glaser, T. Wenzel, K.G. Nickel, H.-J. Meyer, Physica C 436 (2006)

38.[19] Z.-J. Liu, S.H. Zhou, X.X. Xi, Z.-K. Liu, Physica C 397 (2003) 87.