The recently discovered high temperature superconductor F-doped LaFeAsO and
related compounds represent a new class of superconductors with the highest
transition temperature (Tc) apart from the cuprates. The studies ongoing
worldwide are revealing that these Fe-based superconductors are forming a
unique class of materials that are interesting from the viewpoint of
applications. To exploit the high potential of the Fe-based superconductors for
device applications, it is indispensable to establish a process that enables
the growth of high quality thin films. Efforts of thin film preparation started
soon after the discovery of Fe-based superconductors, but none of the earlier
attempts had succeeded in an in-situ growth of a superconducting film of
LnFeAs(O,F) (Ln=lanthanide), which exhibits the highest Tc to date among the
Fe-based superconductors. Here, we report on the successful growth of
NdFeAs(O,F) thin films on GaAs substrates, which showed well-defined
superconducting transitions up to 48 K without the need of an ex-situ heat
treatment

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Ikuta, H.

Kawaguchi, T.

Ohno, T.

Tabuchi, M.

Takeda, Y.

Takenaka, K.

Uemura, H.

Ujihara, T.

English

arXiv

T. Kawaguchi

H. Uemura

T. Ohno

M. Tabuchi

T. Ujihara

K. Takenaka

Y. Takeda

H. Ikuta

CiteSeerX

In-situ growth of superconducting NdFeAs(O,F) thin films by Molecular Beam Epitaxy

Superconducting NdFeAs(O,F) thin films were grown on GaAs substrates by molecular beam epitaxy. Films grown with a sufficiently long growth time exhibited a clear superconducting transition with an onset temperature up to 48 K and zero resistance temperature up to 42 K without the need of an ex situ annealing process. Electron probe microanalysis and Hall coefficient measurements indicated that the superconducting films are doped with fluorine, and depth-profile analysis by Auger electron spectroscopy revealed the formation of a NdOF layer near the surface, which is probably connected with the fluorine doping.journal articl

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In situ growth of superconducting NdFeAs„O,F… thin films by molecularbeam epitaxyT. Kawaguchi,1,3,a H. Uemura,1,3 T. Ohno,1,3 M. Tabuchi,2,3 T. Ujihara,1,3 K. Takenaka,1,3Y. Takeda,1,3 and H. Ikuta1,31Department of Crystalline Materials Science, Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan2Venture Business Laboratory (VBL), Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan3Transformative Research-Project on Iron Pnictides (TRIP), JST, Sanbancho 5, Chiyoda-ku,Tokyo 102-0075, JapanReceived 17 May 2010; accepted 23 June 2010; published online 30 July 2010Superconducting NdFeAsO,F thin films were grown on GaAs substrates by molecular beamepitaxy. Films grown with a sufficiently long growth time exhibited a clear superconductingtransition with an onset temperature up to 48 K and zero resistance temperature up to 42 K withoutthe need of an ex situ annealing process. Electron probe microanalysis and Hall coefficientmeasurements indicated that the superconducting films are doped with fluorine, and depth-profileanalysis by Auger electron spectroscopy revealed the formation of a NdOF layer near the surface,which is probably connected with the fluorine doping. © 2010 American Institute of Physics.doi:10.1063/1.3464171The recently discovered high temperature supercon-ductor F-doped LaFeAsO Ref. 1 and related compounds2–8represent a class of superconductors with the highest transi-tion temperature Tc apart from the cuprates. The studiesongoing worldwide are revealing that these Fe-based super-conductors are forming a unique class of materials that areinteresting from the viewpoint of applications. To exploit thehigh potential of the Fe-based superconductors for deviceapplications, it is indispensable to establish a process thatenables the growth of high quality thin films. Studies onthin film preparation started soon after the discovery ofFe-based superconductors,9–11 and many research groupshave been reporting on thin films of Co-doped AEFe2As2AE=Sr,Ba Refs. 10, 12, and 13 and iron-chalcogens,14–16the best films of which already possessing a Tc value exceed-ing 20 K. Quite recently, the fabrication of K-dopedBaFe2As2 with an onset Tc up to 40 K was also reported.17In contrast, growing thin films of LnFeAsOLn=lanthanide, the so-called 1111 family, which possessesthe highest Tc of 56 K among the Fe-based superconductorsknown to date, has been confronted with more difficulties.Hiramatsu et al.9 prepared La-1111 films by pulsed-laserdeposition PLD on oxide crystalline substrates. They havesucceeded in obtaining epitaxial films but the films did notshow a superconducting transition. Soon thereafter, anothergroup had succeeded in the preparation of F-doped La-1111thin films that exhibited superconducting transitions.11,18However, it was necessary to treat these films ex situ at sig-nificantly high temperatures to form the 1111 phase. Thenecessity of such heat treatment places a considerable burdenon the processing of superconducting devices, and the devel-opment of a procedure that does not require a subsequenthigh-temperature treatment is a necessary next step.While the above mentioned two groups had employedPLD methods, we have succeeded in growing Nd-1111 thinfilms by molecular beam epitaxy MBE in a previouswork.19 The films were grown on GaAs substrates, whichalso differentiates our work from the other studies. GaAs wasselected because of its good lattice matching with Ln-1111and the similarity in the atomic coordination around As. Wealso expect that various growth techniques established instudies on semiconductors, such as strain control by an alloybuffer layer, can be applied when GaAs is used as a sub-strate, and interesting devices may be realized by combininga superconductor with a semiconductor.The growth condition for obtaining a single-phased filmof Nd-1111 with a very high reproducibility was identifiedfrom a detailed study in our previous work.19 However, thefilm did not show a superconducting transition and the resis-tivity increased at low temperature. Because that film wasonly 15 nm thick, we speculated that it might have somestructural imperfections. Therefore, the effect of increasingthe film thickness was studied in the present work. As aresult, we succeeded in obtaining as-grown, superconductingthin films of F-doped Nd-1111. The highest onset tempera-ture Tconset of these films was 48 K with a zero resistancetemperature Tc0 of 42 K. This was achieved without an ex situheat treatment, and the observed Tc is the highest among theso-far reported thin films of Fe-based superconductors.The thin films were grown by a MBE method, the detailof which is reported in the previous paper.19 Briefly,GaAs001 was used as substrates, on which an about300 nm thick GaAs buffer layer was grown at 610 °C afterthe oxide layer on the substrate was removed by sublimation.Nd-1111 was grown by supplying all elements from solidsources charged in Knudsen cells; Fe, As, NdF3, and Fe2O3.The substrate temperature was 650 °C and the vapor pres-sures of Fe, As, NdF3, and oxygen were 1.910−6 Pa, 1.510−3 Pa, 2.710−6 Pa, and 210−5 Pa, respectively.The composition of the films was checked by electron probemicroanalysis EPMA using Nd-1111 powders as a refer-ence. Depth-profile analysis was performed using an Augerelectron spectroscopy combined with Ar ion sputteringJEOL JAMP-7800. The growth rate of the films was esti-mated to be 15 nm/h in our previous study.19 Resistivity wasmeasured by a four-probe method, and susceptibility wasaElectronic mail: kawaguchi@iku.xtal.nagoya-u.ac.jp.APPLIED PHYSICS LETTERS 97, 042509 20100003-6951/2010/974/042509/3/$30.00 © 2010 American Institute of Physics97, 042509-1Downloaded 07 Oct 2010 to 133.6.32.9. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionsmeasured using a superconducting quantum interference de-vice magnetometer Quantum Design MPMS-7. Hall coef-ficient was measured under a magnetic field of 9 T.Figure 1a shows out-of-plane -2 x-ray diffractionXRD patterns of Nd-1111 thin films grown with differentgrowth time tg. The optimal growth condition of our previousstudy19 was employed, while the substrate temperature waslowered slightly to 650 °C in an attempt to reduce the im-purities that were observed in films grown with long tg seebelow. Except for the growth time, all other parameterswere identical for the films shown in Fig. 1a. As can beseen, the films grown for tg3 h were single-phased and allXRD peaks except those arising from the substrate could beindexed as 00l reflections of the Nd-1111 phase. With in-creasing the growth time further, however, the growth be-came somewhat unstable and some impurity peaks were ob-served in the XRD patterns. In particular, the formation ofNdOF phase is obvious for the tg=5 and 6 h films. A fourfoldsymmetry was confirmed for the Nd-1111 phase in all filmsby measuring the azimuthal -scan of the 102 peak, asshown for example for the tg=6 h film in Fig. 1b. Theseresults indicate that the Nd-1111 phase was grown epitaxiallyon the substrate with the relation schematically shown inFig. 1c.Figure 2 shows the temperature dependence of resistivityT of the Nd-1111 films. The film thickness that wasnecessary to convert resistance data to resistivity was calcu-lated using the previously determined growth rate 15 nm/hRef. 19 and assuming that it does not depend on the growthtime. As shown in Fig. 2a, the resistivity of the films grownfor tg4 h increased at low temperature similarly to thefilm we reported in our previous study19 and did not show asuperconducting transition. In a stark contrast, however, thetg=5 and 6 h films shown in Fig. 2b exhibited clear super-conducting transitions. Tconset and Tc0 of the tg=6 h film wereslightly higher than the tg=5 h film, and were 48 K and42 K, respectively. The susceptibility of the tg=6 h film,which is shown in the inset to Fig. 2b, confirms also asuperconducting transition at about 40 K.The difference between the tg4 h and tg5 h filmscan be attributed to F-contents. Figure 3 shows the tempera-ture dependence of Hall coefficient of the Nd-1111 films. Allfilms exhibited a negative Hall coefficient for the tempera-ture range investigated. The magnitude of Hall coefficient ofthe tg4 h films, which did not show a superconductingtransition, increased steeply below about 200 K. On the otherhand, the Hall coefficient of the tg=5 and 6 h films had amuch weaker temperature dependence. This change in thebehavior of Hall coefficient corresponds quite well to thedifference between nondoped and F-doped single crystals ofNd-1111,20 suggesting that the films grown for tg=5 and 6 hwere doped with fluorine. Interestingly, the change in thebehavior of Hall coefficient was rather abrupt betweentg=4 and 5 h. The results of EPMA is consistent with thisobservation because no fluorine was found in the tg4 hfilms while the tg=5 and 6 h films contained a certainamount of fluorine.Next, we would like to discuss why fluorine was ob-served only in the films that were grown with long tg. Figure4 shows the Auger depth profile of the tg=6 h film, which isa plot of the intensities of the Auger signals as a function ofthe sputtering time tsp. The interface between the film and the0 20 40 60 80Intensity(arb.units)002 005004GaAsGaAs001 003 006007NdAsFeAsFe2O3NdOF**▼▼tg=1 htg=2 htg=3 htg=4 htg=5 htg=6 h2θ (deg.)++++●●●++▼●●●●(a)-180 -90 0 90 1800200400600Intensity(cps) tg=6 h (102) peakφ (deg.)(b)(c)GaAsNdFeAs(O,F)aaaaacGaNdO, FFeAsFIG. 1. Color online a Out-of-plane -2 XRD profiles of films grownwith different growth time tg. All growth parameters except the growth timewere identical for these films. The XRD profiles indicate that the Nd-1111phase was grown with the c-axis perpendicular to the substrate in all films.b Azimuthal -scan profile of the off-axis 102 reflection of the tg=6 hfilm. A clear fourfold symmetry can be confirmed. c A schematic drawingof the epitaxial relationship between the Nd-1111 phase and the GaAs sub-strate, which is consistent with the results of XRD analyses.100 200 3002468100ρ(mΩcm)tg = 1 h(a)T (K)2 h3 h4 h100 200 3000.51.01.50T (K)ρ(mΩcm)(b)H⊥ ctg = 6 h2 Oetg = 5 htg = 6 h0 20 40 60-1.0-0.504πχT (K)FCZFCFIG. 2. Color online Temperature dependence of resistivity of the Nd-1111films grown for a tg4 h and b tg5 h. The films grown for tg=5 and6 h exhibited clear superconducting transitions. Tconset and Tc0 of thetg=5 h film were 45 K and 38 K, respectively, while they were 48 K and42 K for the tg=6 h film. The inset shows the temperature dependence ofsusceptibility of the tg=6 h film.0 100 200 300-10-8-6-4-20RH(10-3cm3/C)tg=1 htg=2 htg=3 htg=4 htg=5 htg=6 hFIG. 3. Color online Temperature dependence of Hall coefficient of theNd-1111 films.042509-2 Kawaguchi et al. Appl. Phys. Lett. 97, 042509 2010Downloaded 07 Oct 2010 to 133.6.32.9. Redistribution subject to AIP license or copyright; see http://apl.aip.org/about/rights_and_permissionssubstrate is evidenced by a rise in the Ga signal, and it can beseen that the contents of Nd, Fe, As, and O are nearly con-stant above the interface up to a certain thickness. The pres-ence of fluorine is also confirmed, which is consistent withthe conclusion of Hall coefficient and EPMA measurements.Very interestingly, however, we observed steep increases inNd, O, and F contents near the surface tsp200 s, whichwas accompanied by depletion of Fe and As contents. Thisimplies that the NdOF phase, that was observed in the XRDanalyses of the tg=5 and 6 h films, was formed at the end ofthe film growth. Indeed, the reflection high-energy electrondiffraction pattern that was monitored during the growth sug-gested that Nd-1111 was grown until tg4 h, while a differ-ent phase, presumably NdOF, was dominant for tg	4 h.The change in the dominantly growing phase fromNd-1111 to NdOF at tg4 h was entirely unexpected for usbecause none of the processing parameters were changedduring the growth. However, we point out that the films weregrown in an atmosphere that was probably quite excessive influorine. This is because NdF3 was used as the source of Ndand F, which means that the supplied amount of F was threetimes larger than Nd. Therefore, what was unusual might benot the formation of NdOF but rather the Nd-1111 phaseduring the early stage of the film growth. It has been reportedthat fluorine can react with GaAs forming GaF3 when GaAsis exposed to a F-containing vapor and the formed GaF3sublimates above about 550 K about 280 °C.21 We thinkthat the same reaction took place at the early stage of the filmgrowth, and the GaF3 phase had immediately sublimated be-cause the substrate temperature was 650 °C. This had a con-sequence of regulating the amount of fluorine, and the Nd-1111 phase had grown. With the increase in the filmthickness, however, reaching the GaAs surface became in-creasingly difficult for fluorine, and some of the fluorine re-mained unconsumed. When the amount of fluorine exceededthan a certain level, the growth of NdOF was thermodynami-cally more favorable, causing the change in the dominantlygrowing phase.The present model can explain why the dominantlygrowing phase changed with the growth time. However, onewould expect then that a F-doped Nd-1111 film with noNdOF should be obtained when the growth is stopped at tg4 h. Nevertheless, the results of Hall coefficient andEPMA measurements indicate that this is not the case. Apossible explanation for the lack of fluorine in the tg4 hfilms is that fluorine may diffuse easily through Nd-1111 athigh temperature and had escaped from the film after thesupply of NdF3 was stopped to terminate the growth. TheNdOF phase that was grown in the tg5 h films may haveworked as a cap layer and had prevented the loss of fluorine.In this respect, it is interesting that Kidszun et al.18 had men-tioned that the formation of LaOF at the surface is a typicalfeature in their F-doped La-1111 films.In summary, we have grown superconducting films ofNd-1111 on GaAs substrates. The as-grown films exhibitedsuperconducting transitions when the growth time was suffi-ciently long with Tconset and Tc0 up to 48 K and 42 K, respec-tively. While the films grown for tg3 h were single-phased, we found a NdOF layer near the surface of the filmsthat exhibited superconducting transitions. We think that thisis because our films were grown in an excess supply of fluo-rine but the presence of the cap layer may have played animportant role to realize an as-grown superconducting film.1Y. Kamihara, T. Watanabe, M. Hirano, and H. Hosono, J. Am. Chem. Soc.130, 3296 2008.2G. F. Chen, Z. Li, D. Wu, G. Li, W. Z. Hu, J. Dong, P. Zheng, J. L. Luo,and N. L. Wang, Phys. Rev. Lett. 100, 247002 2008.3Z.-A. Ren, W. Lu, J. Yang, W. Yi, X.-L. Shen, Z.-C. Li, G.-C. Che, X.-L.Dong, L.-L. Sun, F. Zhou, and Z.-X. Zhao, Chin. Phys. Lett. 25, 22152008.4H. Kito, H. Eisaki, and A. Iyo, J. Phys. Soc. Jpn. 77, 063707 2008.5M. Rotter, M. Tegel, and D. Johrendt, Phys. Rev. Lett. 101, 1070062008.6J. H. Tapp, Z. Tang, B. Lv, K. Sasmal, B. Lorenz, P. C. W. Chu, and A. M.Guloy, Phys. Rev. B 78, 060505 2008.7F.-C. Hsu, J.-Y. Luo, K.-W. Yeh, T.-K. Chen, T.-W. Huang, P. M. Wu,Y.-C. Lee, Y.-L. Huang, Y.-Y. Chu, D.-C. Yan, and M.-K. Wu, Proc. Natl.Acad. Sci. U.S.A. 105, 14262 2008.8X. Zhu, F. Han, G. Mu, P. Cheng, B. Shen, B. Zeng, and H.-H. Wen, Phys.Rev. B 79, 220512 2009.9H. Hiramatsu, T. Katase, T. Kamiya, M. Hirano, and H. Hosono, Appl.Phys. Lett. 93, 162504 2008.10H. Hiramatsu, T. Katase, T. Kamiya, M. Hirano, and H. Hosono, Appl.Phys. Express 1, 101702 2008.11E. Backen, S. Haindl, T. Niemeier, R. Hühne, T. Freudenberg, J. Werner,G. Behr, L. Schultz, and B. Holzapfel, Supercond. Sci. Technol. 21,122001 2008.12T. Katase, H. Hiramatsu, H. Yanagi, T. Kamiya, M. Hirano, and H.Hosono, Solid State Commun. 149, 2121 2009.13K. Iida, J. Hanisch, R. Huhne, F. Kurth, M. Kidszun, S. Haindl, J. Werner,L. Schultz, and B. Holzapfel, Appl. Phys. Lett. 95, 192501 2009.14Y. Han, W. Y. Li, L. X. Cao, S. Zhang, B. Xu, and B. R. Zhao, J. Phys.:Condens. Matter 21, 235702 2009.15E. Bellingeri, I. Pallecchi, R. Buzio, A. Gerbi, D. Marre, M. R. Cimberle,M. Tropeano, M. Putti, A. Palenzona, and C. Ferdeghini, Appl. Phys. Lett.96, 102512 2010.16Y. Imai, T. Akiike, M. Hanawa, I. Tsukada, A. Ichinose, A. Maeda, T.Hikage, T. Kawaguchi, and H. Ikuta, Appl. Phys. Express 3, 0431022010.17N. H. Lee, S.-G. Jung, D. H. Kim, and W. N. Kang, Appl. Phys. Lett. 96,202505 2010.18M. Kidszun, S. Haindl, E. Reich, J. Hanisch, K. Iida, L. Schultz, and B.Holzapfel, Supercond. Sci. Technol. 23, 022002 2010.19T. Kawaguchi, H. Uemura, T. Ohno, R. Watanabe, M. Tabuchi, T. Ujihara,K. Takenaka, Y. Takeda, and H. Ikuta, Appl. Phys. Express 2, 0930022009.20P. Cheng, H. Yang, Y. Jia, L. Fang, X. Zhu, G. Mu, and H.-H. Wen, Phys.Rev. B 78, 134508 2008.21W. C. Simpson, T. D. Durbin, P. R. Varekamp, and J. A. Yarmoff, J. Appl.Phys. 77, 2751 1995.0 200 400 600 800Sputter time (sec.)Intensity(arb.units) OF AsGaNdtg=6 hFeFIG. 4. Color online Auger depth-profile analysis of the Nd-1111 filmgrown for 6 h. The intensity of the Auger signal is plotted as a function ofsputtering time.042509-3 Kawaguchi et al. Appl. Phys. Lett. 97, 042509 2010Downloaded 07 Oct 2010 to 133.6.32.9. 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In situ growth of superconducting NdFeAs(O,F) thin films by molecular beam epitaxy

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<i>In situ</i> growth of superconducting NdFeAs(O,F) thin films by molecular beam epitaxy

In-situ growth of superconducting NdFeAs(O,F) thin films by Molecular
  Beam Epitaxy

In-situ growth of superconducting NdFeAs(O,F) thin films by Molecular Beam Epitaxy

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