We have developed the means to grow BiMnO3 thin films with unparalleled structural perfection by reactive molecular-beam epitaxy and determined its band gap. Film growth occurs in an adsorption-controlled growth regime. Within this growth window bounded by oxygen pressure and substrate temperature at a fixed bismuth overpressure, single-phase films of the metastable perovskite BiMnO3 may be grown by epitaxial stabilization. X-ray diffraction reveals phase-pure and epitaxial films with w rocking curve full width at half maximum values as narrow as 11 arc sec (0.003°). Optical absorption measurements reveal that BiMnO3 has a direct band gap of 1.1±0.1 eV

Heeg, T.

Ihlefeld, J. F.

Ke, X.

Lee, J. H.

Liu, Z. K.

Mei, Z. G.

Misra, R.

Musfeldt, J. L.

Roeckerath, M.

Schiffer, P.

Schlom, D. G.

Schubert, J.

Xu, X. S.

English

DigitalCommons@University of Nebraska

University of Nebraska - LincolnDigitalCommons@University of Nebraska - LincolnXiaoshan Xu Papers Research Papers in Physics and Astronomy2010Adsorption-controlled growth of BiMnO3 films bymolecular-beam epitaxyJ. H. LeeCornell UniversityX. KePennsylvania State UniversityR. MisraPennsylvania State UniversityJ. F. IhlefeldPennsylvania State UniversityX. S. XuUniversity of Nebraska-Lincoln, xiaoshan.xu@unl.eduSee next page for additional authorsFollow this and additional works at: https://digitalcommons.unl.edu/physicsxuPart of the Atomic, Molecular and Optical Physics Commons, Condensed Matter PhysicsCommons, and the Engineering Physics CommonsThis Article is brought to you for free and open access by the Research Papers in Physics and Astronomy at DigitalCommons@University of Nebraska -Lincoln. It has been accepted for inclusion in Xiaoshan Xu Papers by an authorized administrator of DigitalCommons@University of Nebraska -Lincoln.Lee, J. H.; Ke, X.; Misra, R.; Ihlefeld, J. F.; Xu, X. S.; Mei, Z. G.; Heeg, T.; Roeckerath, M.; Schubert, J.; Liu, Z. K.; Musfeldt, J. L.;Schiffer, P.; and Schlom, D. G., "Adsorption-controlled growth of BiMnO3 films by molecular-beam epitaxy" (2010). Xiaoshan XuPapers. 28.https://digitalcommons.unl.edu/physicsxu/28AuthorsJ. H. Lee, X. Ke, R. Misra, J. F. Ihlefeld, X. S. Xu, Z. G. Mei, T. Heeg, M. Roeckerath, J. Schubert, Z. K. Liu, J. L.Musfeldt, P. Schiffer, and D. G. SchlomThis article is available at DigitalCommons@University of Nebraska - Lincoln: https://digitalcommons.unl.edu/physicsxu/28Adsorption-controlled growth of BiMnO3 films by molecular-beam epitaxyJ. H. Lee,1,2 X. Ke,3 R. Misra,3 J. F. Ihlefeld,2 X. S. Xu,4 Z. G. Mei,2 T. Heeg,1M. Roeckerath,5 J. Schubert,5 Z. K. Liu,2 J. L. Musfeldt,4 P. Schiffer,3 and D. G. Schlom1,a1Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853, USA2Department of Materials Science and Engineering, Pennsylvania State University, University Park,Pennsylvania 16802, USA3Department of Physics, Pennsylvania State University, University Park, Pennsylvania 16802, USA4Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, USA5Institute of Bio- and Nano-Systems (IBN1-IT), JARA-Fundamentals of Future Information Technology,Research Centre Jülich, D-52425 Jülich, GermanyReceived 20 February 2010; accepted 4 June 2010; published online 2 July 2010We have developed the means to grow BiMnO3 thin films with unparalleled structural perfection byreactive molecular-beam epitaxy and determined its band gap. Film growth occurs in anadsorption-controlled growth regime. Within this growth window bounded by oxygen pressure andsubstrate temperature at a fixed bismuth overpressure, single-phase films of the metastableperovskite BiMnO3 may be grown by epitaxial stabilization. X-ray diffraction reveals phase-pureand epitaxial films with  rocking curve full width at half maximum values as narrow as 11 arc sec0.003°. Optical absorption measurements reveal that BiMnO3 has a direct band gap of1.10.1 eV. © 2010 American Institute of Physics. doi:10.1063/1.3457786Multiferroic materials are being studied for their richproperties, fascinating physics, and application potential.1Bismuth manganite, BiMnO3, is a well-known, but contro-versial multiferroic. Early first-principles calculations sug-gested that BiMnO3 might be simultaneously ferroelectricand ferromagnetic, arising from bismuth 6s lone pairs andmagnetic ordering of manganese.2 Subsequent experimentsconfirmed multiferroism in thin films.3–5 The prediction of itsferroelectricity was based on the reported noncentrosymmer-tic C2 space group of BiMnO3.6–8 Recent calculations andexperiments, however, find that BiMnO3 belongs to the cen-trosymmetric C2 /c space group and therefore stoichiometricBiMnO3 may not be a multiferroic at all.9,10To better establish the properties of BiMnO3, we havegrown films of it by reactive molecular-beam epitaxyMBE using adsorption-controlled growth conditions.Adsorption-controlled growth conditions are the standardway of growing phase-pure compound semiconductors;11 ithas also been applied to the growth of ferroelectric oxidethin films.12,13 In this letter, we first describe the adsorption-controlled growth of single phase BiMnO3 films by control-ling bismuth overpressure, substrate temperature, and oxi-dant partial pressure. The ferromagnetic properties andoptical absorption spectrum of the grown films are then re-ported.The pressure-temperature region for the adsorption-controlled growth of BiMnO3 was calculated using theCALPHAD approach.14 It was empirically established using insitu reflection high-energy electron diffraction RHEED andconfirmed by ex situ four circle x-ray diffraction XRD.Figure 1 shows a calculated Ellingham diagram representingthe phase stability regions of I BiMnO3+Mn3O4, IIBiMnO3, and III BiMnO3+Bi2O2.5 as a function of sub-strate temperature and O2 overpressure. The boundaries be-tween regions I, II, and III were calculated with the Gibbsfree energy functions of the gas phase containing variousbismuth and Bi–O species and the stable and metastablemanganese and bismuth oxides.15 Unfortunately there is noGibbs free energy of formation reported for BiMnO3 orBi2O2.5. First-principles calculations, using the generalizedgradient approximation GGA and GGA plus Hubbard UGGA+U method, predicted the enthalpy of formation ofBiMnO3 to be between +400 J /mol and +4000 J /mol ofBiMnO3 with respect to Bi2O3 and Mn2O3, respectively.16Since there is no detailed crystal structure of Bi2O2.5 avail-able for first-principles calculations, we assume its enthalpyof formation to be +100 J /mol or +4500 J /mol with respectto bismuth and Bi2O3, as assumed in Ref. 13. The phasestability region was calculated using THERMOCALC17 with thepartial pressure of bismuth fixed at 2.610−10 atm, whichaElectronic mail: schlom@cornell.edu.FIG. 1. Color online Calculated Ellingham diagram and RHEED patternscollected along the 110 azimuth of SrTiO3 during the deposition ofBiMnO3 at different temperatures and oxygen partial pressures. Solid linesrepresent phase boundaries using +4000 J /mol and +100 J /mol formulaunit Gibbs free energies for BiMnO3 and Bi2O2.5, respectively, specifyingthe narrowest growth window possible. Dashed lines are for +400 J /moland +4500 J /mol formula unit, indicating the approximate uncertainty inwidth of the growth window. Phase stability between BixOy gases andBiMnO3+Mn3O4, BiMnO3, and BiMnO3+Bi2O2.5 condensed phases is rep-resented by regions I, II, and III, respectively.APPLIED PHYSICS LETTERS 96, 262905 20100003-6951/2010/9626/262905/3/$30.00 © 2010 American Institute of Physics96, 262905-1corresponds to the pressure at the plane of the substrate foran incident bismuth flux of 5.51013 atom / cm2 s.18BiMnO3 thin films were deposited on buffered-HFtreated 001 SrTiO3 Ref. 19 and 110 DyScO3 substratesin a similar manner to previous adsorption-controlledoxides by MBE.12,13 The volatile bismuth and oxygencomponents were supplied continuously and the nonvolatilecomponent manganese was supplied in doses correspond-ing to individual MnO2 monolayers making up theBiO–MnO2–BiO–MnO2–¯ stacking sequence along the001 pseudocubic growth direction of the BiMnO3 perov-skite structure. Due to the narrow growth window for phase-pure BiMnO3 growth region II in Fig. 1, the substrate tem-perature was monitored in situ by measuring the absorptionedge of the SrTiO3 substrate.20 The thermodynamicpredictions were verified by investigating a horizontal slicethrough Fig. 1 at constant bismuth bismuth flux=5.51013 Bi / cm2 s and oxygen O2+ 10% O3 backgroundpressure=110−6 Torr overpressure during the depositionof Bi–Mn–O over a temperature range of 580–690 °C and afixed Bi:Mn flux ratio of 3:1. The in situ RHEED patternscollected along the 110 azimuthal direction of 001-oriented SrTiO3 delineating the three regions are superim-posed in Fig. 1. Above 650 °C region I, RHEED spots areobserved and can be indexed to diffraction from 101-oriented Mn3O4. The presence of this phase was verified byex situ XRD. Between 610 and 640 °C region II, phase-pure BiMnO3 can be grown. Below 610 °C region III, ad-ditional spots form, which can be indexed by ex situ XRD as001-and 110-oriented Bi2O2.5. The occurrence of thesespots corresponds to the phase boundary separating region IIand III. A change in bismuth flux or oxygen activity resultsin a shift in the growth window of single-phase BiMnO3region II in Fig. 1. The calculated O2 overpressures com-pare well with what is expected given the enhanced activityof O3 and our directed gas inlet that locally increases theoxygen pressure at the substrate surface.18The structure of two 29 nm thick BiMnO3 films depos-ited on 001 SrTiO3 was characterized. Both films weregrown under adsorption-controlled conditions region II inFig. 1. -2 XRD scans of both films in Fig. 2a revealthem to be phase-pure and epitaxial. The full width at halfmaximum FWHM of the  rocking curve of the pseudocu-bic 002 film peaks are equal to 80 arc sec 0.02° and 36arc sec 0.01°, which are the same as the underlying SrTiO3substrates. Both films show clear thickness fringes Fig.2b, corresponding to a film thickness of 292 nm. Theout-of-plane lattice constant was calculated to be4.010.01 Å for the BiMnO3 films grown in 10% ozoneand 3.980.01 Å for the film grown in distilled ozone. Theextended out-of-plane lattice constants result from the latticemismatch between the SrTiO3 substrate a=3.905 Å andthe BiMnO3 film apseudocubic=3.95 Å.10 The 0.03 Å dif-ference in out-of-lattice constants between the two samples isa result of the different oxidating atmosphere during growth.The films grown in 10% ozone show a longer lattice con-stant than the films grown in distilled ozone. Rutherfordbackscattering spectrometry confirms that the ratio of Bi:Mnof these films is 1:1 within the 5% experimental error of themeasurements.Magnetic measurements on the same two BiMnO3 filmsare shown in Fig. 3. Both films exhibit clear ferromagnetichysteresis loops. The oxygen-rich films tend to show lowerTC and saturation magnetization than the oxygen-stoichiometric films. BiMnO3 films grown in 10% ozoneshowed a ferromagnetic transition at 105 K, which isclose to TC of bulk BiMnO3, and a magnetization value of2.5 B /Mn at 5 K. In contrast, films grown in distilledozone showed a TC at 95 K and a magnetization value of1.9 B /Mn at 5 K. A depression of TC has been reported infilms grown by pulsed-laser deposition.3,21 We attribute it tooxygen stoichiometry since the Bi:Mn ratio was observednot to influence the transition temperature. Measurements onbulk BiMnO3+x 0x0.16 samples also observed this de-pression of TC and magnetization in samples with higheroxygen content.10The optical spectra of BiMnO3 films were measured intransmittance using a Perkin–Elmer Lambda-900 spectrom-eter 3000–190 nm and 0.41–6.53 eV with bare substratesas references. The film on DyScO3 had a rocking curveFWHM of 11 arc sec 0.003°, comparable to the underlyingDyScO3 substrate and over 100narrower than any reportedBiMnO3 film.3–5 Absorption was calculated as E=−1 /dlnTE, where d is the film thickness, T is the mea-sured transmittance, and E is the energy of the light.Figure 4 displays the absorption spectrum of BiMnO3at 300 and 4 K. The onset of optical absorption in BiMnO3is 0.75 eV 1650 nm,22 much lower than the 2.2 eVFIG. 2. Color online a -2 XRD scans of BiMnO3 films grown on 001SrTiO3 at an oxidant background pressure of 110−6 Torr and the 3:1Bi:Mn flux ratio stated above. The difference was that one film was grownin 10% ozone directly out of an ozone generator at Tsub=590 °C whereasthe other was grown using distilled ozone 80% ozone at Tsub=630 °C.Substrate peaks are marked with an asterisk . b A close-up of the 001 peak,showing a clear difference in the out-of-plane lattice constant.FIG. 3. Color online Field-cooled and zero field-cooled magnetization ofthe same BiMnO3 films as Fig. 2 grown using 10% ozone and distilledozone. Inset Magnetic hysteresis of these films at 5 K.262905-2 Lee et al. Appl. Phys. Lett. 96, 262905 2010560 nm onset in BiFeO3 thin films.13,23,24 Peaks occur at1.6 eV 780 nm and 5.5 eV 230 nm, with a broadshoulder near 4 eV. The near-infrared feature in perovskitemanganites has been studied extensively.25–28 Relevant pre-dictions from first-principles electronic structure calculationsfor BiMnO3 include strong Mn–O hybridization, a stere-ochemically active Bi lone pair that mixes Bi and O states,and insulating behavior in calculations that employ strongexchange correlation.2,9,29,30 Several very different assign-ments might reasonably account for the 1.6 eV near infraredexcitation in BiMnO3: i on-site excitations between crystalfield split 3d levels, ii charge transfer excitations involvingstrongly hybridized Mn3+ and oxygen states, or iii transi-tions between localized Hubbard bands. Based upon com-parison with electronic structure calculations,2,9,23,30 the ex-perimental spectrum of chemically similar materials withMn3+ centers in locally distorted octahedral environmentssuch as LaMnO3,25–28 and the overall intensity 1.0510−5 cm−1, we assign the near infrared peak at 1.6 eVto charge transfer excitations between mixed O 2p andMn 3d states. The limited temperature dependence of the 1.6eV structure supports this assignment. A plot of E2 versusenergy inset, Fig. 4 places the 300 K charge gap in BiMnO3at 1.10.1 eV.22 Our fitting indicates that BiMnO3 has adirect band gap. The relatively small direct band gap ofBiMnO3 could be relevant to ferroelectric solar cell applica-tions, e.g., creating a solid solution with BiFeO3 with Eg=2.7 eV Refs. 13, 23, and 24 could result in a band gapideally matched to the solar spectrum.According to electronic structure calculations,30 thecharge excitations are predicted to be strongly spin polarized.The gap is determined by majority carriers, and minoritystates are involved only above 3 eV. Higher energy excita-tions above 3.7 eV 330 nm are assigned as O 2p→Mn 3d charge transfer, with some mixing of Bi 6p levelsabove 3 eV. The aforementioned electronic excitationssharpen and blueshift slightly with decreasing temperatureFig. 4. We find no obvious anomalies at the 105 K Curietemperature within our sensitivity, indicative of modestcharge-spin interactions.We gratefully acknowledge the financial support fromthe National Science Foundation through Grant No. DMR-0507146 and the MRSEC program Grant No. DMR-0820404 and from the U.S. DOE through Grant No. DE-FG02-01-ER45885UT.1W. Eerenstein, N. D. Mathur, and J. F. Scott, Nature London 442, 7592006.2N. A. Hill and K. M. Rabe, Phys. Rev. B 59, 8759 1999.3A. Moreira dos Santos, A. K. Cheetham, W. Tian, X. Pan, Y. Jia, N.Murphy, J. Lettieri, and D. G. Schlom, Appl. Phys. Lett. 84, 91 2004.4A. Sharan, J. Lettieri, Y. Jia, W. Tian, X. Pan, D. G. Schlom, and V.Gopalan, Phys. Rev. B 69, 214109 2004.5J. Y. Son and Y.-H. Shin, Appl. Phys. Lett. 93, 062902 2008.6T. Atou, H. Chiba, K. Ohoyama, Y. Yamaguchi, and Y. Syono, J. SolidState Chem. 145, 639 1999.7A. Moreira dos Santos, A. K. Cheetham, T. Atou, Y. Syono, Y. Yamagu-chi, K. Ohoyama, H. Chiba, and C. N. R. Rao, Phys. Rev. B 66, 0644252002.8K. Kodama, S. Iikubo, S. Shamoto, A. A. Belik, and E. Takayama-Muromachi, J. Phys. Soc. Jpn. 76, 124605 2007.9P. Baettig, R. Seshadri, and N. A. Spaldin, J. Am. Chem. Soc. 129, 98542007.10A. A. Belik, T. Kolodiazhnyi, K. Kosuda, and E. Takayama-Muromachi, J.Mater. Chem. 19, 1593 2009.11J. Y. Tsao, Materials Fundamentals of Molecular Beam Epitaxy Aca-demic, Boston, 1993.12D. G. Schlom, J. H. Haeni, J. Lettieri, C. D. Theis, W. Tian, J. C. Jiang,and X. Q. Pan, Mater. Sci. Eng., B 87, 282 2001.13J. F. Ihlefeld, N. J. Podraza, Z. K. Liu, R. C. Rai, X. Xu, T. Heeg, Y. B.Chen, J. Li, R. W. Collins, J. L. Musfeldt, X. Q. Pan, J. Schubert, R.Ramesh, and D. G. Schlom, Appl. Phys. Lett. 92, 142908 2008.14L. Kaufman, CALPHAD: Comput. Coupling Phase Diagrams Thermo-chem. 25, 141 2001.15Scientific Group Thermodata Europe, in Thermodynamic Properties ofInorganic Materials, Landolt-Börnstein, New Series, Group IV Vol. 19,edited by Lehrstuhl für Theoretische Hüttenkunde Springer, Berlin,1999.16Z. G. Mei, S. Shang, Y. Wang, D. G. Schlom, and Z. K. Liu unpublished.17J. O. Andersson, T. Helander, L. H. Hoglund, P. F. Shi, and B. Sundman,CALPHAD: Comput. Coupling Phase Diagrams Thermochem. 26, 2732002.18D. G. Schlom and J. S. J. Harris, in Molecular Beam Epitaxy: Applicationsto Key Materials, edited by R. F. C. Farrow Noyes, Park Ridge, NJ,1995, pp. 505–622.19G. Koster, B. L. Kropman, G. Rijnders, D. H. A. Blank, and H. Rogalla,Appl. Phys. Lett. 73, 2920 1998.20kSA BandiT, k-Space Associates, Ann Arbor, MI.21S. Fujino, M. Murakami, S.-H. Lim, L. G. Salamanca-Riba, M. Wuttig,and I. Takeuchi, J. Appl. Phys. 101, 013903 2007.22The 0.1 eV error bars on the gap and the background absorption inBiMnO3 come from the small film thickness and the associated chal-lenges in growing BiMnO3 films with thicknesses 	33 nm. Fortunately,they do not obscure the most important physics.23S. R. Basu, L. W. Martin, Y. H. Chu, M. Gajek, R. Ramesh, R. C. Rai, X.Xu, and J. L. Musfeldt, Appl. Phys. Lett. 92, 091905 2008.24A. Kumar, R. C. Rai, N. J. Podraza, S. Denev, M. Ramirez, Y.-H. Chu, L.W. Martin, J. Ihlefeld, T. Heeg, J. Schubert, D. G. Schlom, J. Orenstein, R.Ramesh, R. W. Collins, J. L. Musfeldt, and V. Gopalan, Appl. Phys. Lett.92, 121915 2008.25K. Tobe, T. Kimura, Y. Okimoto, and Y. Tokura, Phys. Rev. B 64, 1844212001.26M. A. Quijada, J. R. Simpson, L. Vasiliu-Doloc, J. W. Lynn, H. D. Drew,Y. M. Mukocskii, and S. G. Karabashev, Phys. Rev. B 64, 224426 2001.27M. W. Kim, P. Murugavel, S. Parashar, J. S. Lee, and T. W. Noh, New J.Phys. 6, 156 2004.28N. N. Kovaleva, A. V. Boris, C. Bernhard, A. Kulakov, A. Pimenov, A. M.Balbashov, G. Khaliullin, and B. Keimer, Phys. Rev. Lett. 93, 1472042004.29R. Seshadri and N. A. Hill, Chem. Mater. 13, 2892 2001.30P. Baettig, R. Seshadri, and N. A. Spaldin, private communicationSeptember 26, 2008.FIG. 4. Color online Optical absorption spectrum of 33 nm thick BiMnO3films grown on 001 SrTiO3 and 110 DyScO3 at Tsub=640 °C in a back-ground pressure of 110−6 Torr of distilled ozone were combined to createa composite absorption curve and were identical in the overlap regime.Inset Charge gap extraction from 300 K absorption.262905-3 Lee et al. Appl. Phys. Lett. 96, 262905 2010

Adsorption-controlled growth of BiMnO3 films by molecular-beam epitaxy

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Adsorption-controlled growth of BiMnO3 films by molecular-beam epitaxy

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