www.advmat.de www.MaterialsViews.com Probing Interfacial Electronic Structures in Atomic Layer LaMnO3 and SrTiO3 Superlattices

By Amish B. Shah, Quentin M. Ramasse, Xiaofang Zhai, Jian Guo Wen, Steve J. May, Ivan Petrov, Anand Bhattacharya, Peter Abbamonte, James N. Eckstein, and Jian-Min Zuo* COMMUNICATION

An epitaxial interface between two strongly correlated transition Growing atomically sharp interfaces and controlling the metal oxides can lead to emergent electronic states at the stoichiometry and defects have been shown to be critical for interface, because the explicit breaking of translational symmetry achieving desired interfacial properties.[1,14–18] Atomic resolution can nucleate new electronic phases.[1–4] Recent studies have characterization is essential to determine the interface structure shown that new interfacial states in oxides can be designed and and to map interfacial electronic states. This can be achieved by constructed using physical vapor deposition techniques (for a electron energy loss spectroscopy (EELS) in combination with review, see Reference [5]), and when the interface is repeated in atomic resolution scanning transmission electron microscopy close spacing (a few unit cells), it can lead to bulk-like (STEM) using an annular dark-field (ADF) detector.[19] Here, we properties.[6,7] Some interfaces that have attracted considerable report the interfacial electronic structure characterization of a attention recently are those formed between two perovskites of m (LaMnO3)/n (SrTiO3) superlattice based on aberration- [8] [6,7,9] LaTiO3 and SrTiO3 or LaMnO3 and SrMnO3. These corrected STEM and EELS. This superlattice was designed to constituents are all insulating individually in the bulk phase. enhance the optical absorption using interfacial states by However, the interfaces become highly conductive via different contacting a band insulator, SrTiO3 (STO), with a [20] mechanisms; the interface of LaTiO3-SrTiO3 has layer-to-layer Mott-insulator LaMnO3 (LMO). This system is also interesting [10,11] charge transfer and the interface of LaMnO3-SrMnO3 is because of the presence of La1-xSrxMnO3-SrTiO3 and [6,9,12] between two antiferromagnetic insulators. Some strongly Sr1-xLaxTiO3-LaMnO3 interfaces shown in the layering model correlated perovskites also exhibit competing orders involving of Figure 1a. La1-xSrxMnO3 in bulk phase is a spin-polarized and charge and spin, resulting in a heightened sensitivity to ferromagnetic and has received considerable attention recently [13] [21] temperature, external fields, and strain. By combining for spin-valve applications. La doping makes Sr1-xLaxTiO3 different perovskites in an epitaxial superlattice, the interruption metallic for x as small as <0.1[22] and superconducting. We have of translational lattice symmetry and strain can be used to favor grown a 2 2 superlattice with m ¼ n ¼ 2 over a large thickness of one competing order over others. 63 1 layers of LMO and STO. The structure consists of one unit

[*] Prof. J. M. Zuo, A. B. Shah Department of Materials Science and Engineering and Fredrick Seitz Materials Research Laboratory University of Illinois at Urbana-Champaign Urbana, Illinois 61801, USA E-mail: [email protected] Dr. Q. M. Ramasse National Center for Electron Microscopy Lawrence-Berkeley National Laboratory Berkeley, CA 94720 USA Dr. X. Zhai, Prof. P. Abbamonte, Prof. J. N. Eckstein Department of Physics and Fredrick Seitz Materials Research Laboratory University of Illinois at Urbana-Champaign Urbana, Illinois, 61801, USA Dr. J. G. Wen, Dr. I. Petrov Fredrick Seitz Materials Research Laboratory University of Illinois at Urbana-Champaign Urbana, Illinois, 61801, USA Dr. S. J. May, Dr. A. Bhattacharya Materials Science Division and Center for Nanoscale Materials, Figure 1. The atomic structure of an oxide superlattice characterized by Argonne National Laboratory, electron imaging and diffraction. a) A model of the 2 LaMnO3/2 SrTiO3 Argonne, Illinois 60439, USA superlattice. b) A HAADF-STEM image shows atomic level sharpness of films c) Nanoarea electron diffraction pattern shows the strong funda- DOI: 10.1002/adma.200904198 mental reflections and weak satellite reflections (q) from the superlattice.

1156 ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2010, 22, 1156–1160 COMMUNICATION 1157 , þ ratio in 2 L / 3 edges. a) The edges inside L www.advmat.de edge has been K 2,3 . b) The integrated K 5 M edges obtained from 2,3 L peaks. Mn sites 1 and 2 give used sites 1 and 2 with little or no Ti edges and La 2 2,3 2,3 L L and L edge. The change in the 2 3 edge. The Ti atomic layer of the Mn2 site. To look for 2 used to identify sites 1 and 2, which have little or K 2,3 L However, in these previous studies, the EELS [27,28] Atomic scale mapping ELNES of Ti and Mn L cations, and O as 2- anion, the superlattice is neutral and þ edge peak than the L [25,26] 3 The chemical structure that we concluded from the EELS data evidences from the O Figure 2. of Figure 2a and HAADF-STEMin Figure images 3a. is There shown are twoin schematically different the Mn and superlattice Ti sitesatomic with respectively layers. In Ti2 the simple andTi ionic as picture 4 Mn2 with La between andthere LaO Mn is a as dipole and 3 across 2.5 SrO unitabove cells the between Ti2 the and LaO MnO atomic layer no Mn mixing,integrated and edge intensity site of the 3 Mn with significant Mn mixing. c) Similarly, the mixing, and site 3 withsites Ti mixing. identified d,e) in Averaged b) spectra obtained and from c). the possible effect of this dipole,the superlattice we and examined in the the O STOshown substrate. The O tosuperlattices. be sensitive to the electronic structure in oxide similar ELNES.spectrum A obtained from noticeable site 3in difference characterized L by a is larger increase observed in the edge intensity of the Ti site 3 have a smaller intensity andother the same two ELNES features sites. as the columns The is comparison for shownminimum two in between different the Figure L Mn 1e, atomic which is normalized to the HAADF-STEM image marked with theEELS spectral position maps of for the Ti EELS and line Mn scan and Mn oxides, instate. general, indicates a change in the oxidation . g ˚ unit 92 A . The : 2 3 and e Details 3 2g ¼ edges are TiO [23] SL 0.5 c 2,3 L La 0.5 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 0.25. Thus, for this ß ) along (001) are clear ¼ q 5.2%, corresponding to q edges respectively. In each 2,3 , STO and Sr L An example of the superlattice 3 edges, while the spectra from the [24] 3 MnO sites (Mn and Ti) in the perovskite L 0.5 B Sr and 0.5 2 2 supercell and change of both cations on the L unit cell precision can be identified. 1156–1160 3 22, 2 superlattice should have , edges of individual atomic columns. Figure 2a shows 2,3 2010 L -axis is elongated by 0.4% compared to the STO substrate. edge obtained from a separate linescan over the STO c edges obtained from the three sites. For comparison, the Ti We use the atomic layer-by-layer MBE (ALL-MBE) technique to The high spatial resolution of EELS allows us to examine the Ti sites (La and Sr) and 2,3 2,3 substrate is also plotted.edge onsets Its are energy at the loss samenormalized value axis for was comparison. above The shifted spectra the are sobetween 470 the edge and 490 threshold eV. Tisimilar sites in 1 near and the 2 edge in energy the structure. superlattice However, window show these Ti Adv. Mater. peaks for both the cell each of LMO, La www.MaterialsViews.com profile, sites wheremaximum the are identified integratedlabeled as edge as predominantly 1 intensity Ti and is 2, ormixed Ti respectively. Mn and near Sites Mn where sites signals the are there and L identified are as significant 3. Figure 2d plots the Ti L an EELS line scan background subtractedacross with eight an supercells. exponential The law integrated edgeFigure intensity 2b is plotted and in 2c for Ti and Mn The distance of the firstAn order reflection, ideal q, 2 is measured at 0.237. different from thatsuperlattice of have the slightly more substrate. intensity The between Ti the t spectra from the substrate has apeaks. sharper The drop difference is in notresolution; the both caused spectra by minimum were a obtained between changesetting. with the in these same EELS This instrument energy will be further discussed next together with in the pattern.substrate, we Using measured the the c-axis length calibrationcell of the in average obtained the perovskite superlattice from from Figure the 1c and STO obtained nanoarea electron diffraction patterndiffraction pattern is was also shown recorded from in thethe STO Figure substrate near superlattice 1c. A diffraction (not spot shown) indexedWeak and as first order (001) superlattice used reflections is ( for the film calibration. growth The direction. structure make this superlattice particularlysynthesis challenging and for both characterization. Theallows aberration EELS core corrected loss STEM finescale. structure to be analyzed on an atomic grow theALL-MBE oxide can be superlattice. used to Past grow oxide studies thin-films have with 10 shown the small repeating 2 A for growth arecollaborators presented in in this issue. the Thefirst paper growth 6 was layers by started of by Zhai,superlattice. STO depositing Eckstein, as The a and growth bufferperiodic layer, after followed for by the the the first remainderHAADF growth STEM of of 3 image the of supercells the growth.which upper remained shows Figure supercells that of epitixial 1b growth the displays of superlattice, theatomic films a sharpness. was The carried superlattice out structure with was quantifiednanoarea using electron diffraction. cell precision ingradients composition, to and providesample making a with range 10 use of of integrated beam doses, flux regions in a The particular sample, there is a deviation of an incommensurate latticeaveraged of repeat mixed of 2 4.2 times to the 3 perovskite unit unitand cells Mn cell. with an 1158 COMMUNICATION nryi the if energy O the from 2p O transition The the position. same from the originates at edge meet energy onsets The the scan. so line shifted STO was separate the scale a from in spectrum obtained averaged was an which each 3b substrate, Figure on in plotted spectra also we comparison, For 4 law. exponential an to using subtracted background 3 of average B n n n,a aee na.A vrgdO averaged SrTiO An a). the in from an labeled with as Mn2, subtracted and background Mn1 were O maps b) fit. spectral exponential The scan. line same on otepeeko h O the of prepeak the to point cni niae ytedwwr ro.Fgr bposteEELS O the plots of 3b Figure spectrum arrow. O downward the the by of indicated is position scan The 3a. Figure in vdne yteaoeT n nLeg data. edge L Mn as and Ti correction above aberration the with by improved evidenced significantly resolution is spatial the EELS study, this O of In the basis. cell examine unit to individual an enough on high not was resolution spatial 3. Figure ihntelnt faspratc ntcl.TeHAADF-STEM The cell. unit Ti the superlattice and a image of length the within the of Ti model the structural and a image with HAADF-STEM together the shown superlattice, is map The superlattice. the O the a) superlattice. the www.advmat.de stsa lutae nFgr a h pcrmpotdi an is plotted spectrum The 3a. Figure in illustrated as -sites iue3eaie h LE fteO the of ELNES the examines 3 Figure tts hseg spse pt h otnu thigher at continuum the to up pushed is edge This states. h apn fO of mapping The 3 2p usrt sas hw o eeec.Tedw arrows down The reference. for shown also is substrate K L ttsaeflyocpe sfrO for as occupied fully are states K deotie nteT1 i,M1adMn2 and Mn1 Ti2, Ti1, the on obtained edge 2,3 desetao ordfeetaoi ie,T1 Ti2, Ti1, sites, atomic different four on spectra edge de r hw oehrwt h O the with together shown are edges K dempfo iears igesprelof supercell single a across line a from map edge K K defrT n naoi oun inside columns atomic Mn and Ti for edge dea icse ntext. in discussed as edge K B rpa nteEL line EELS the in prepeak st.Tesetawere spectra The -site. K ß K L desetu obtained spectrum edge 2,3 00WLYVHVra mH&C.Ka,Weinheim KGaA, Co. & GmbH Verlag WILEY-VCH 2010 deta smapped is that edge de bandi the in obtained edges 1s ounoccupied to 2- os The ions. K K edge edge K fteeg ne.Teoeli Ti overlaid The onset. edge the of tmclyri eitiue nSOadteitrailM2 The Mn2. LaO interfacial the the from and STO electron in the redistributed that is indicate layer atomic sites Ti both at decrease ihS dadL 5 La and 2 4d O Sr to with attributed peak a third 3 the by Figure in 4b in broadening observed Figure with as features in ELNES marked similar show is arrow) position white (the film superlattice STO the the from inside spectrum The film. layer 5 buffer STO of between deposited O difference average appreciable overlapped an no is was the There plotted case. each spectrum layer in STO The spectra the superlattice. and buffer the STO the in substrate, STO the from taken xlrdi iue4,wihposteEL pcrmo O of spectrum EELS the further plots is which trend 4c, This Figure substrate. in STO explored the as same the is buffer STO ul cuidi h pnu ad tteitraeo between of interface LaO-MnO the At of band. layers up spin atomic the in occupied fully h O The ihTi shown are with layer O buffer the and STO 4a the Figure line of in the position with the of marked and layers position image few scan HAADF-STEM a The first STO. to and layer buffer LMO STO deposited the to substrate oadlwreeg ososre nteO the in Ti observed the loss for observed energy shift chemical same lower The layer. of toward buffer average STO an and and substrate superlattice STO the the between is 4d Figure in plotted B 5 or Sr of ncuid n h eki eks nLOwt t 3 its with the LMO in where weakest is substrate peak STO the and the unoccupied, in strongest is h rpa t51e sa niainof indication an is eV 531 at prepeak the euto ftepeekitniy hl h O the while intensity, prepeak the of reduction a O The law. power a with subtracted hti bevdfrL oe T yXryabsorption X-ray by STO doped La for observed spectroscopy. to similar is is shape spectral what The spectra). the in resolved distinctly lcrncsrcue nte2 O the in structure structures fine electronic the in changes hl h pcrmfo h T hnfimhsaslight a has film thin STO the from the in difference spectrum appreciable broadening no the show also 4d while Figure in substrate STO ueltiestsa La as O sites the superlattice of peak second the for nSOcrepnigt oeigo h odcinbn edge. band conduction the of lowering a to corresponding occurs STO spectroscopy in absorption optical the by observed alignment esrda .2e,wihare ihteotclabsorption optical the with data. agrees which spectroscopy eV, 0.22 at measured rpa nest,smlrt h eairo rdpn nLMO. in doping Sr of behavior the to similar intensity, prepeak oiebywae hntepeeki h T substrate, STO the Ti in of states prepeak are unoccupied of Both number the similar. lowered are a than indicating sites Ti2 weaker and Ti1 noticeably at intensity prepeak The eatiue ote3 ttso h no Ti or Mn the O of the states 3d of the peaks to attributed main be The states. metal transition and O observed hne ntespratc )asitt oe nryls o the for loss energy 2 lower O of to hybridization shift to a attributed peak 1) superlattice the in changes t 2g st aina aee nFgr 3b. Figure in labeled as cation -site h bv bevdceia hf fO of shift chemical observed above The iue4sostetasto fO of transition the shows 4 Figure ado h ueltie eas oieasgicn ekshift peak significant a notice also We superlattice. the of band K L d rpa hw nices tteM2stsada and sites Mn2 the at increase an shows prepeak 2,3 ttso aa the at La of states K ,Mn [22] dei h euto yrdzto ewe h 2 O the between hybridization of result the is edge [20] t d 2g K esitnepeek n hmclshift chemical a and prepeak, intense less a , L L - 2,3 2,3 e K 5d defo h T usrt n the and substrate STO the from edge h ESrslsso htteband the that show results EELS The g dempi hw nFgr btogether 4b Figure in shown is map edge pitn fteL the of splitting de h muto hmclsitis shift chemical of amount The edge. ad i ihSr with mix bands as l pcr eebackground were spectra All maps. 2 K SO esea nraei the in increase an see we -SrO, A deatiue oteSr the to attributed edge LaMnO st ain and cation, -site L 2,3 K [29–31] l on onwinterfacial new to point all de ftebfe ae and layer buffer the of edges p K 3 d.Mater. Adv. ihS 4 Sr with d /2 K bn filling; -band demprvasclear reveals map edge h eaiesrnt of strength relative The K 2 www.MaterialsViews.com B de rmteSTO the from edges ek h difference The peak. 4d n iL Ti and st cation, -site SrTiO (The d d 2010 4s bn slargely is -band K p rL 5 La or K 3 and hybridization dei also is edge e 2,3 , superlattice. g dei the in edge [31] 22, K eki not is peak de and edges dt 4p decan edge 3d 4d 1156–1160 4d d h peak the 2g n 2) and , K fthe of nthe in states nall on band edge [32] p COMMUNICATION 1159 ) s 13 C , 378. J. Am. Science 419 , 196404. , To record 99 , [35] Phys. Rev. Lett. 2002 www.advmat.de 2007 , 016802. Phys. Rev. B: Condens. Nature 99 , 054428. , 79 states. Electron probe , 1196. , The microscope has been , 423. 2g 2007 317 , 630. Received: December 8, 2009 [34] 2009 , 427 . ˚ , 428 Phys. Rev. Lett. 1A , 2007 spacing between two Ga atomic 2004 Published online: February 10, 2010 ˚ 2004 Phys. Rev. B Science Phys. Rev. Lett. Nature Nature order probe forming spherical aberration ( rd , 2429. and helpful discussions. 91 , , 174409. 77 2008 , , 257203. , 1942. 2008 100 313 , , C. W. Schneider, T. Kopp, A. S. Ruetschi,J. D. Jaccard, M. M. Triscone, Gabay, D. J. A. Muller, Mannhart, Ceram. Soc. J. M. Zuo, M. R. Fitzsimmons, S. D. Bader, J. N. Eckstein, 2008 X. Zhai, J. N. Eckstein, S.Matter D. Bader, A. Bhattacharya, J. N. Eckstein, S. D. Bader, J. M. Zuo, 2006 Advanced Materials 165 pA. (The image shown in Figure 1 differs from the other figures in [1] S. Okamoto, A. J. Millis, [2] N. Reyren, S. Thiel, A. D. Caviglia, L. F. Kourkoutis, G. Hammerl, C. Richter, [4] B. R. K.[5] Nanda, D. S. G. Satpathy, Schlom, L. Q. Chen, X. Q. Pan, A. Schmehl, M. A. Zurbuchen, [6] A. Bhattacharya, S. J. May, S. Velthuis, M. Warusawithana, X. Zhai, B. Jiang, [3] R. Pentcheva, W. E. Pickett, [7] S. J. May, A. B. Shah, S. G. E. t, Velthuis,[8] M. R. A. Fitzsimmons, Ohtomo, D. J. A. M.[9] Muller, J. Zuo, L. S. Grazul, Smadici, H. P. Y. Abbamonte, Hwang, A. Bhattacharya, X. F. Zhai, B. Jiang, A. Rusydi, states from hybridization with the Ti 3d t Research is supportedMaterials by the SciencesNational U.S. under Department Laboratory ofDE-AC02-06CH11357 grant Energy, under Digital Division DEFG02-91ER45439 of theout Synthesis at and the FWP. U.S. Frederick Seitz MicroscopyUniversity Materials Argonne of Department Illinois, Research was which Laboratory is Central partially of carried supportedEnergy Facilities, by under the U.S. grants Energy Department of DE-FG02-07ER46453the and grant DE-FG02-07ER46471, National and Centerwhich for is Electron supportedDE-AC02-05CH11231. JMZ Microscopy, thanks by Lawrence Dr. Maria theof Berkeley Varela for U.S. Lab, sharing a Department preprint of Energy under grant Experimental After growth,prepared cross by mechanical sectional wedge polishingcarried samples and ion out milling. for Microscopy on was equipped electron with a microscopy a 200 CEOS kV were 3 JEOL JEM2200FS STEM. This microscope is Acknowledgements pA). The EELS spectrapower are law background-subtracted with to an removeelectrons exponential the through or the contribution sample. of random multiple scattering of EELS, the energy dispersion is obtainedfilter. For using EELS an spectroscopy, in-column we omega-energy usedand a a 26 mrad 41 probe mrad convergence collection angle angle. Thethat probe the current semi-convergence was angle is measured 16 mrad to and be the probe current is aberration correction was essential for themapping high of spatial interfacial resolution electronic states. shown to becolumns viewed able along to the [120] resolve direction in 1.04 A GaN thin films. corrector to reduce the probe size to [10] A. Ohtomo, H. Y. Hwang, [11] S. Thiel, G. Hammerl, A. Schmehl, C. W. Schneider, J. Mannhart, 3 þ 2,3 3 L and Ti SrTiO K /2 3 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ß LaMnO edge shifts towards lower K edge within individual unit which reported mostly a Ti K [33] edge in STO is too small compared 2,3 1156–1160 22, (see Reference [8]). The amount of electron transfer STO superlattice, , þ 4 2010 a) HAADF STEM image after a linescan shows the buffer layer of spectra averaged over five integration points from the substrate, K and Ti edge prepeak in STO, which is attributed to unoccupied O 2p LMO/2 edges, was observed in ultrathin STO. The electron transfer þ In summary, we have demonstrated direct EELS evidences of K 3 2,3 Adv. Mater. L superlattice. The superlatticealignment, is in the epitaxially form of strained. a The 0.22 eV band chemical shift of O interfacial electronic stateselectron for transfer, or band charge leakage, alignment in and a 2 interfacial oxidation state in STO. Thislayer, superlattice which has an is extra not LaO present atomic in our case. change we observed in Ti L and O buffer layer, and a STO film indicated by the horizontal white arrow. Figure 4. STO indicated by the bracketthin over films the are STO slightly substrate. misoriented Thedisappears from buffer in the layer the substrate. and third Thiswith supercell. misorientation a b) power EELS law spectra fit. background The subtracted broad peak in the O energy loss in the STO thin films, but not in the STO buffer layer. c) Ti www.MaterialsViews.com to the difference in theTi fine edge structures that one expects for from LMO to STO isO evidenced by a reduction in the intensity of to Tipredominant likely change is incells the small, suggests O that muchtransition metal electron and less oxygen bonding. transfer Theinto than small STO is electron observed transfer here achieved one is in17 through contrast electron. to the a recent The EELS study on a 1160 COMMUNICATION 2]M .Wrswtaa .V ol,J .Ekti,M .Weissman, B. M. Eckstein, N. J. Colla, V. E. Warusawithana, P. M. [23] 1]N aaaa .Y wn,D .Muller, Warusawithana, A. D. M. Hwang, Y. Soukiassian, H. Nakagawa, A. Jaime, N. M. 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[32] 3]Z .Zag .Sge .Krz .Ruhle, M. Kurtz, W. Sigle, W. Zhang, L. Z. [31] Hofer, F. Engel, W. Sauer, H. Brydson, R. [30] 2]F .F ero,J ae,J .M ihes .T zzk .Abbate, M. Czyzyk, T. M. Michiels, M. J. J. Faber, J. Degroot, F. M. F. [29] 2]A .Sa,X hi .Jag .G e,J .Ekti,J .Zuo, M. J. Eckstein, N. J. Wen, G. J. Jiang, B. Zhai, X. Shah, B. A. [28] 2]J ebek .I eee,G a edlo .Mercey, B. Tendeloo, Van G. Lebedev, I. O. Verbeeck, J. [27] 2]D .Pasn .C h,B Fultz, B. Ahn, C. C. Pearson, H. D. Petrov, I. [25] Twesten, R. Li, Q. B. Tao, J. Gao, M. Zuo, M. J. [24] 2]L .J ave .J Craven, J. A. Garvie, J. A. L. [26] .G hbatn,J hn,K a,J .Zo .Msia .Aoki, T. Mishina, S. Zuo, M. J. Ran, K. Zhang, J. submitted Chobpattana, G. V. 21 .Vrl,S .Pnyok .Lo,J Santamaria, J. Leon, C. Pennycook, J. Garcia-Herna S. Varela, M. M. Nemes, M. N. .Dme .Kid,G .Swtk,M aao .Tkd,H Eisaki, H. Takeda, Y. Takano, M. Sawatzky, A. Uchida, G. S. Kaindl, G. Domke, M. 3429. .C Fuggle, C. 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