BiFeO3 thin films have been deposited on (001) SrTiO3 substrates by adsorption-controlled reactive molecular-beam epitaxy. For a given bismuth overpressure and oxygen activity, single-phase BiFeO3 films can be grown over a range of deposition temperatures in accordance with thermodynamic calculations. Four-circle x-ray diffraction reveals phase-pure, epitaxial films with w rocking curve full width at half maximum values as narrow as 29 arc sec (0.008°). Multiple-angle spectroscopic ellipsometry reveals a direct optical band gap at 2.74 eV for stoichiometric as well as 5% bismuth-deficient single-phase BiFeO3 films

Chen, Y. B.

Collins, R. W.

Heeg, T.

Ihlefeld, J. F.

Li, J.

Liu, Z. K.

Musfeldt, J. L.

Pan, X. Q.

Podraza, N. J.

Rai, R. C.

Ramesh, R.

Schlom, D. G.

Schubert, J.

Xu, X.

English

DigitalCommons@University of Nebraska

University of Nebraska - LincolnDigitalCommons@University of Nebraska - LincolnXiaoshan Xu Papers Research Papers in Physics and Astronomy2008Optical band gap of BiFeO3 grown by molecular-beam epitaxyJ. F. IhlefeldPennsylvania State UniversityN. J. PodrazaPennsylvania State UniversityZ. K. LiuPennsylvania State UniversityR. C. RaiUniversity of TennesseeX. 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.Ihlefeld, J. F.; Podraza, N. J.; Liu, Z. K.; Rai, R. C.; Xu, X.; Heeg, T.; Chen, Y. B.; Li, J.; Collins, R. W.; Musfeldt, J. L.; Pan, X. Q.;Schubert, J.; Ramesh, R.; and Schlom, D. G., "Optical band gap of BiFeO3 grown by molecular-beam epitaxy" (2008). Xiaoshan XuPapers. 29.https://digitalcommons.unl.edu/physicsxu/29AuthorsJ. 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. SchlomThis article is available at DigitalCommons@University of Nebraska - Lincoln: https://digitalcommons.unl.edu/physicsxu/29Optical band gap of BiFeO3 grown by molecular-beam epitaxyJ. F. Ihlefeld,1,2 N. J. Podraza,1 Z. K. Liu,1 R. C. Rai,3 X. Xu,3 T. Heeg,4 Y. B. Chen,5J. Li,6 R. W. Collins,6 J. L. Musfeldt,3 X. Q. Pan,5 J. Schubert,4 R. Ramesh,2,7 andD. G. Schlom1,a1Materials Research Institute, Pennsylvania State University, University Park, Pennsylvania 16802, USA2Department of Materials Science and Engineering, University of California, Berkeley, California94720, USA3Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, USA4Institute for Bio- and Nano-Systems (IBN1-IT), Research Centre Jülich, D-52425 Jülich, Germany5Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48019,USA6Department of Physics and Astronomy, University of Toledo, Toledo, Ohio 43606, USA7Department of Physics, University of California, Berkeley, California 94720, USAReceived 2 January 2008; accepted 19 February 2008; published online 10 April 2008BiFeO3 thin films have been deposited on 001 SrTiO3 substrates by adsorption-controlled reactivemolecular-beam epitaxy. For a given bismuth overpressure and oxygen activity, single-phaseBiFeO3 films can be grown over a range of deposition temperatures in accordance withthermodynamic calculations. Four-circle x-ray diffraction reveals phase-pure, epitaxial filmswith  rocking curve full width at half maximum values as narrow as 29 arc sec 0.008°.Multiple-angle spectroscopic ellipsometry reveals a direct optical band gap at 2.74 eV forstoichiometric as well as 5% bismuth-deficient single-phase BiFeO3 films. © 2008 AmericanInstitute of Physics. DOI: 10.1063/1.2901160BiFeO3 is the only known material that is both ferroelec-tric TC1083 K and antiferromagnetic TN625 K atroom temperature.1 Recent reports of a large spontaneouspolarization 100 C /cm2 in thin films,2 bulk ceramic,3and single crystals4 of BiFeO3 have led to an explosion ofinterest in its growth and properties.The ferroelectric and multiferroic properties of BiFeO3are of interest for a number of applications including devicesthat utilize heterojunction effects where knowledge of theBiFeO3 band gap is crucial for device design. To date, thereis limited and conflicting information on the band gap andoptical properties of BiFeO3, with existing reports limited topolycrystalline films5–7 or nanowires.8We have previously reported the deposition of BiFeO3films via adsorption-controlled reactive molecular-beam epi-taxy MBE on 111 SrTiO3 substrates.9 In this letter, wereport the adsorption-controlled growth of BiFeO3 on 001-oriented SrTiO3 and the resulting crystalline quality, micro-structure, optical dielectric functions, and band gap.Single-crystalline, TiO2-terminated10 SrTiO3 substratesaligned within 0.5° of 001 were used as substrates. Filmswere grown under conditions described previously.The parameter space for the adsorption-controlledgrowth of BiFeO3 was calculated through the CALPHADmethod11 and was empirically established usingin situ reflection high-energy electron diffraction RHEEDand confirmed by ex situ four-circle x-ray diffraction XRD.Figure 1 shows a calculated Ellingham diagram representingthe phase stability regions of I BiFeO3+-Fe2O3, IIBiFeO3, and III BiFeO3+Bi2O2.5 as a function of substratetemperature and O2 overpressure. Analogous phase stabilitydiagrams have been calculated from thermodynamic data forthe adsorption-controlled growth of III-V compounds,12–15MgB2,16 PbTiO3,17 and Bi4Ti3O12.12 In the case of BiFeO3and Bi2O2.5, the Gibbs energies of formation have not beenreported. The boundaries between regions I, II, and III werecalculated with the Gibbs energy functions of the gas phasecontaining various Bi and Bi–O species and the stable andmetastable iron and bismuth oxides, all taken from the SGTEdatabase.18 We consider two scenarios for Bi2O2.5 andBiFeO3. The enthalpy of formation of Bi2O2.5 is assumed tobe +100 or +4500 J /mol of Bi2O2.5 with respect to theBi–Bi2O3 tie line. The enthalpy of formation of BiFeO3 isassumed to be −1000 or −5000 J /mol with respect to theBi2O3–Fe2O3 tie line. The phase stability region was calcu-lated using THERMOCALC Ref. 19 with the partial pressureof bismuth fixed at 6.710−10 atm, which corresponds to thepressure at the plane of the substrate for an incident bismuthflux of 1.41014 Bi /cm2 s.20 The solid lines in Fig. 1 boundaElectronic mail: schlom@ems.psu.edu.FIG. 1. Calculated Ellingham diagram and RHEED patterns collected alongthe 110 azimuth of SrTiO3 during Bi–Fe–O deposition at different tem-peratures and BixOy gas overpressures. Solid lines represent phase bound-aries using +100 and −1000 J /mol formula unit free energies for Bi2O2.5and BiFeO3, respectively, specifying the narrowest growth window possible,and dashed lines for +4500 and −5000 J /mol formula unit, indicating theapproximate uncertainty in width of the growth window. Phase stabilitybetween BixOy gases and BiFeO3+-Fe2O3, BiFeO3, and BiFeO3+Bi2O2.5condensed phases is represented by Regions I, II, and III, respectively.APPLIED PHYSICS LETTERS 92, 142908 20080003-6951/2008/9214/142908/3/$23.00 © 2008 American Institute of Physics92, 142908-1the BiFeO3 region with formation enthalpies of +100 and−1000 J /mol for Bi2O2.5 and BiFeO3, respectively, specify-ing the narrowest growth window possible, while the dashedlines represent the stability for +4500 and −5000 J /mol, in-dicating the approximate uncertainty in the growth windowdue to the lack of relevant free energy data.The thermodynamic predictions were verified by inves-tigating a horizontal slice through this diagram at constantbismuth Bi flux=1.41014 Bi /cm2 s and oxygen O2+10% O3 background pressure=110−6 Torr overpressureduring the deposition of Bi–Fe–O over a temperature rangeof 375–475 °C and a fixed Bi:Fe flux ratio of 8:1. Thein situ RHEED patterns collected along the 110 azimuthaldirection of 001-oriented SrTiO3 delineating the three re-gions are superimposed in Fig. 1. Above 460 °C, RHEEDstreaks associated with BiFeO3 and additional spots are ob-served. These spots can be indexed to diffraction from 111-oriented -Fe2O3, the presence of which was verified byex situ XRD. For these growth parameters, this temperaturerepresents the boundary between regions I and II. Many au-thors have observed iron oxide inclusions in BiFeO3 filmsgrown by other techniques, e.g., -Fe2O3 by off-axis rfsputtering21 and -Fe2O3 by pulsed laser deposition at lowoxygen pressures.22,23 Between 415 and 460 °C, phase-purefilms can be grown indicative of region II. At a temperaturebelow 415 °C, additional spots form, which have been in-dexed by ex situ XRD as 001- and 110-oriented Bi2O2.5.The occurrence of these spots corresponds to the phaseboundary separating regions II and III. A change of the bis-muth flux or the oxygen activity results in a shift of thegrowth window for single-phase BiFeO3, as indicated by theEllingham boundaries of the adsorption-controlled growthwindow in Fig. 1. The O2 overpressures calculated comparewell with what is expected given the enhanced activity of O3and our directed gas inlet that locally increases the oxygenpressure at the substrate surface.20It has been reported that the single-phase field of BiFeO3grown by MBE is broad, with single-phase films as much as8% Bi-deficient being grown.9 Two films were grown at dif-ferent points in region II to investigate the growth tempera-ture dependence on composition. One sample was grown inthe middle of region II and the second was grown near thephase boundary between regions II and III. For a Bi:Fe fluxratio of 7:1, substrate temperatures of 405 and 375 °C, re-spectively, corresponded to these points in the Ellinghamdiagram. The sample grown at 375 °C had a stoichio-metric composition within the 3% measurement accuracy ofRutherford backscattering spectroscopy and displayed aminimum channeling yield min of 11%. The film grown at405 °C was Bi-deficient with a 0.95 0.03 :1.00 Bi:Fe ra-tio and a min of 16%. Our results indicate that stoichiometricsingle-phase BiFeO3 films may be prepared at the Bi-richend of region II, i.e., near the boundary with region III.An XRD scan of the 30 nm thick BiFeO3 / 001 SrTiO3stoichiometric sample is shown in Fig. 2a. The film isphase-pure 101¯2¯-oriented BiFeO3 hexagonal indices areused throughout this letter for BiFeO3. Figure 2b shows ahigh-resolution scan of the 101¯2¯ peak exhibiting clear thick-ness fringes indicative of a smooth film with high crystallinequality. A  rocking curve from the same film is shown inFig. 2c. The full width at half maximum FWHM of theBiFeO3 film, 29 arc sec 0.008°, is identical to that of theunderlying substrate indicating that the film crystallinity issubstrate limited and is comparable to the narrowest re-corded for a BiFeO3 film.9 Figure 2d shows a  scan of the112¯3¯ family of peaks. Four separate peaks are seen, demon-strating that the film is epitaxial with rhombohedral or lowersymmetry. Films of cubic or tetragonal symmetry would notexhibit diffraction at these peak positions.21Cross-sectional transmission electron microscopy TEMspecimens were imaged within a JEOL 3011 high-resolutionTEM HRTEM that has 0.17 nm point-to-point resolution.Dark-field and HRTEM imaging not shown reveals 71° and109° domain walls, as have been previously observed infilms grown by other techniques on nonvicinal 001 SrTiO3substrates.24,25 HRTEM verifies the 101¯2¯ orientation and re-veals an atomically abrupt interface.As stoichiometry can play a strong role in materialproperties, we investigated the optical properties and bandgaps for the two films grown in different locations withinregion II. Room temperature ellipsometric spectrain 	,  were collected ex situ at three angles of incidence,i=55°, 70°, and 85°, using a variable-angle rotating-compensator multichannel spectroscopic ellipsometer26,27over a spectral range from 0.75 to 6.5 eV for the stoichio-metric BiFeO3 film and at i=45°, 60°, and 75° over a spec-tral range from 0.75 to 5.0 eV for the Bi-deficient film. Thedielectric function spectra 1 ,2 shown in Fig. 3 were ex-tracted using a least squares regression analysis and aweighted root mean square error function28 to fit the experi-mental ellipsometric spectra to a four-medium optical modelconsisting of a semi-infinite SrTiO3 substrate/bulk film/surface roughness/air ambient structure where free param-eters correspond to the bulk and surface roughness thick-nesses of the BiFeO3 film and a parameterization of theBiFeO3 dielectric function. The dielectric function param-eterization of BiFeO3 consists of four Tauc–Lorentzoscillators29 sharing a common Tauc gap and a constant ad-ditive term to 1 represented by . The optical properties ofthe surface roughness layer are represented by a BruggemanFIG. 2. Color online a -2 X-ray diffraction pattern of a 30 nm thick101¯2¯-oriented BiFeO3 film grown on 001 SrTiO3. b Shows a close-upof the 101¯2¯ peak and thickness fringes. c Superimposed -rocking curvesof the 202¯4¯ film and 002 substrate peaks. d Azimuthal  scan of 112¯3¯=64.7°  diffraction peaks. =90° aligns the diffraction vector to be per-pendicular to the plane of the substrate. =0° corresponds to the projectionof the substrate 100 in-plane direction.142908-2 Ihlefeld et al. Appl. Phys. Lett. 92, 142908 2008effective medium approximation30 consisting of a 50% bulkfilm/50% void mixture. Although BiFeO3 is rhombohedraland exhibits uniaxial optical anisotropy, these thin films maybe treated as isotropic due to their mixed domain structurei.e., the four twin variants in these films distribute the opticaxes along the four 111 pseudocubic BiFeO3 directionswithin the macroscopic region sampled in the spectroscopicellipsometry measurement. The experimental real andimaginary dielectric spectra are a combination of the ordi-nary and extraordinary dielectric functions but, due to thedistribution in optical axis orientation, it is impossible toseparate the respective contributions.The onset of optical absorption is observed at2.070.14 eV and 2.110.06 eV for the bismuth-deficientand stoichiometric BiFeO3 films, respectively. The directband gap, obtained from a linear extrapolation of E2inset, Fig. 3 for the stoichiometric film is invariantat 2.74 eV. Band gap measurements on five differentMBE-grown BiFeO3 films on 001 SrTiO3, 001LaAlO30.3-SrAl0.5Ta0.5O30.7 LSAT, and 111 SrTiO3Ref. 31 revealed a direct band gap in all cases with Eg=2.770.04 eV. The invariance of the band gap energy withfilms of differing strain states suggests that the band gap isrelatively insensitive to these effects. This value is consistentwith predictions.32 Previous reports have suggested an indi-rect gap at lower energies in addition to the direct gap.5,6 Inour data, the lack of the characteristic shape of the E1/2versus energy plot indicating the required phonon participa-tion argues against an indirect gap.33 It is prudent to considerthe optical absorption onset as a joint density of states effectthat is very small and likely insignificant in ac conductivity.While the two films studied exhibit stoichiometry differ-ences, the dielectric function spectra show similar absorptiononsets and direct band gaps. This is to be expected, as theidentical crystal structure and, thus, bonding is present re-sulting in minute variations in the density of states and bandgap. A shift in the resonance energies to lower energy isobserved in the dielectric function for the nonstoichiometricfilm.This work was supported by the Office of Naval Re-search through Grant No. N00014-04-1-0426 monitored byDr. Colin Wood, by NSF through Grant Nos. DMR-0213623and IIP-0737759, and by the U.S. DOE through Grant Nos.DE-AC02-05CH11231 and DE-FG02-01-ER45885 UT.1G. A. Smolenskii and I. E. Chupis, Sov. Phys. Usp. 25, 475 1982.2J. Wang, J. B. Neaton, H. Zheng, V. Nagarajan, S. B. Ogale, B. Liu, D.Viehland, V. Vaithyanathan, D. G. Schlom, U. V. Waghmare, N. A. Spal-din, K. M. Rabe, M. Wuttig, and R. Ramesh, Science 299, 1719 2003.3V. V. Shvartsman, W. Kleemann, R. Haumont, and J. Kreisel, Appl. Phys.Lett. 90, 172115 2007.4D. Lebeugle, D. Colson, A. Forget, M. Viret, P. Bonville, J. F. Marucco,and S. Fusil, Phys. Rev. B 76, 024116 2007.5V. Fruth, E. Tenea, M. Gartner, A. Anastasescu, D. Berger, R. Ramer, andM. Zaharescu, J. Eur. Ceram. Soc. 27, 937 2007.6T. P. Gujar, V. R. Shinde, and C. D. Lokhande, Mater. Chem. Phys. 103,142 2007.7T. Kanai, S. Ohkoshi, and K. Hashimoto, J. Phys. Chem. Solids 64, 3912003.8F. Gao, Y. Yuan, K. F. Wang, X. Y. Chen, F. Chen, and J. M. Liu, Appl.Phys. Lett. 89, 102506 2006.9J. F. Ihlefeld, A. Kumar, V. Gopalan, D. G. Schlom, Y. B. Chen, X. Q.Pan, T. Heeg, J. Schubert, X. Ke, P. Schiffer, J. Orenstein, L. W. Martin, Y.H. Chu, and R. Ramesh, Appl. Phys. Lett. 91, 071922 2007.10G. Koster, B. L. Kropman, G. Rijnders, D. H. A. Blank, and H. Rogalla,Appl. Phys. Lett. 73, 2920 1998.11L. Kaufman, CALPHAD: Comput. Coupling Phase DiagramsThermochem. 25, 141 2001.12R. Heckingbottom, G. J. Davies, and K. A. Prior, Surf. Sci. 132, 3751983.13H. Seki and A. Koukitu, J. Cryst. Growth 78, 342 1986.14J. Y. Tsao, J. Cryst. Growth 110, 595 1991.15J. Y. 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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 2008.32S. J. Clark and J. Robertson, Appl. Phys. Lett. 90, 132903 2007.33J. Bardeen, F. J. Blatt, and L. H. Hall, in Photoconductivity Conference,edited by R. G. Breckenridge, B. R. Russell, and E. E. Hahn Wiley, NewYork, 1956, pp. 146–154.FIG. 3. Color online Dielectric function spectra obtained from spectro-scopic ellipsometry analysis of stoichiometric solid lines and Bi-deficientdashed lines 30 nm thick BiFeO3 / 001 SrTiO3 over a spectral range from0.75 to 6.5 eV for the stoichiometric film and from 0.75 to 5.0 eV for theBi-deficient film.142908-3 Ihlefeld et al. Appl. Phys. Lett. 92, 142908 2008

Optical band gap of BiFeO3 grown by molecular-beam epitaxy

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Optical band gap of BiFeO3 grown by molecular-beam epitaxy

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