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EffectofpolardiscontinuityonthegrowthofLaNiO /LaAlO superlattices 3 3 Jian Liu1, M. Kareev1, S. Prosandeev1, B. Gray1, P. Ryan2, J. W. Freeland2 and J. Chakhalian1 1Department of Physics, University of Arkansas, Fayetteville, AR 72701 and 2Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439 Wehaveconductedadetailedmicroscopicinvestigationof[LaNiO (1u.c.)/LaAlO (1u.c.)] superlattices 3 3 N grownon(001)SrTiO andLaAlO toexploretheinfluenceofpolarmismatchontheresultingelectronicand 3 3 structuralproperties. Ourdatademonstratethattheinitialgrowthonthenon-polarSrTiO surfaceleadstoa 3 roughmorphologyandunusual2+valenceofNiinthefirstLaNiO layer,whichisnotobservedaftergrowth 3 onthepolarsurfaceofLaAlO .Anewlydevisedmodelsuggeststhatthepolarmismatchcanberesolvedifthe 3 perovskitelayersgrowwithanexcessofLaO,whichalsoaccountsfortheobservedelectronic,chemical,and structuraleffects. 0 1 0 Recently complex oxide ultra-thin films have been draw- thin SLs, we used a recently developed interrupted growth 2 ing enormous attention due to the possibility of stabilizing method[10,13].Thismethodconsistsofarapidablationanda n unusualquantumphasesandbuildinginterface-controlledde- prolongeddelaybetweentwosuccessiveunitcelllayers. The a vices [1, 2]. Towards this goal, the problem of polar in- usualoscillationofRHEEDspecularintensity(RSI)foreach J terfaces is of fundamental importance, since a polar crys- unit cell occurs within a short flux-on period seen as a sharp 6 tal structure grown on a non-polar substrate would be un- dip of RSI, followed by a further slow recovery (see inset in stable due to the depolarizing fields [3, 4]. To avoid this is- Fig. 1). While RSI would have a full recovery in the ideal ] sue, some mechanism must set in to compensate the poten- LBLgrowth,thisfirstlayergrownonSTOalwaysexhibitsa i c tial jump by forcing electronic, structural or chemical (non- characteristic drop as seen in Fig. 1, regardless of whether s - stoichiometry)changes. Inparticular, theactiveworkonthe theinitiallayerisLAOorLNO.Thisobservationisalsocon- l LaAlO /SrTiO system[5–7]hassuggestedanelectrontrans- sistentwiththeroughinitialgrowthofthepolarlayeronthe r 3 3 t ferbetweenatomiclayersthatdecreasesTivalenceby0.5at non-polarSTOsurface(i.e.polarmismatch)ascorroborated m the expense of 0.5e charge in the terminating atomic plane. byAFMimagingofthesurface.Conversely,thisphenomenon . t Additionally,analloyeffecthasbeenobservedattheinterface isabsentduringtheinitialgrowthonLAO,whereafullrecov- a [8], and first-principles computations revealed a contribution erycanbeseenafterthefirstdepositedlayer. m ofionicdisplacements[9]. - Toelucidatethemicroscopicoriginofthisphenomenon,we d In this letter, we report on the microscopic observation of performedresonantsoftx-rayspectroscopy(XAS)ontheNi on asunpuenrluastutiaclevsa(lSeLnc)egrsotawtenoofnNnio2n+-pinolualrtr(aotrhwineLakalNyipOo3l/aLr)aA(0l0O13) Lfin3,g2e-repdrgine.teBdyfoursisnagmXpAleSs,atshtehNiniavsalaensicnegslteatuencitanceblle.dBiryecuttliy- c [ TiO2-terminatedSrTiO3 (STO)substrates. Thisisinmarked lizingthisexquisitesensitivitytotheNichargestate,wewere contrast to identical SLs grown on polar LaAlO3 (LAO), able to detect and trace the effects of charge transfer, local 1 whichalwaysexhibitthe3+valenceofthebulk-likeLaNiO3 symmetry and non-stoichiometry across the layers. Due to v (LNO). To clarify the origin of these phenomena, we have theoverlappingLaM -edge, thespectrumattheNiL -edge 4 4 3 trackedthechangesoftheNivalenceasafunctionoftheSL isstronglydistortedwhichisthereasonforfocusingontheNi 1 8 thicknessanddevelopedanexperimentalproceduretomoni- L2-edge. Fig. 2showstheNiL2spectrameasuredinthebulk 0 torandcontroltheelectronicstateofatransitionmetalion. sensitiveFYmodeforthe[LaNiO (1u.c.)/LaAlO (1u.c.)] 3 3 N 1. High-qualityepitaxialLNO/LAOSLsweregrownon(001) (SLs([1/1]N thereafter)withavaryingnumberofrepeats,N, 0 TiO2-terminatedSTOsinglecrystalsubstratesbypulsedlaser anddifferentgrowthsequences. AthickerSLcomposedof5 0 depositionwithinsitumonitoringbyRHEED[10]. Tomin- u.c. thickLNOlayersisalsoincludedasaNi3+referencefor 1 imize induced defects, STO substrates were prepared by our a stoichiometric sample in the metallic phase[12]. The bulk v: recently developed chemical wet-etch procedure (‘Arkansas like Ni3+ valency of this superlattice was further confirmed i treatment’) [11]. A complementary set of SLs were also by comparison with the bulk LaNiO3. By direct comparison X grown on LAO single crystal substrates. Detailed spectro- of the absorption spectra in Fig. 2, the sharp difference be- ar scopic information was acquired in the soft x-ray regime in tweentheNichargestateinthe[1/1]N SLsandthebulklike both fluorescence yield (FY) mode and total electron yield Ni3+spectracanbeseen. (TEY) mode at the Ni L absorption edge at the 4ID-C 3,2 Afewimportantobservationsaredue.First,the[1/1] sam- 3 beamline of the Advanced Photon Source, ANL. To obtain plewithLNOgrowndirectlyontheSTOsurfaceclearlyex- precise information on the Ni charge state, all spectra were hibitsacharacteristicdoubletwitha0.3eVshifttoloweren- aligned by simultaneously measuring a NiO (Ni2+) standard ergy, which matches the Ni2+ XAS spectra of the NiO stan- with the SLs. Synchrotron-based x-ray diffraction has con- dard at the L edge. This implies that the initial growth re- 2 firmed the high structural quality and the full epitaxy of the sults in a phase analogous to heavily oxygen deficient bulk SLs[10]. LaNiO [14], such as LaNiO with a Ni valence of 2+. 3−x 2.5 Inordertomaintainthemorphologicalqualityandachieve AsN isfurtherincreased,theNivalencefortheSLsonSTO the layer-by-layer (LBL) growth of the single unit-cell movesprogressivelytowardsNi3+ asnotedbythedifference 2 Ni2+ 1 NiO u.) [1/1]x3 on STO a. d ( 0 LAO/[1/1]x3 on STO el e yi [1/1]x6 on STO c n -1 e [1/1]x20 on STO c s e r o [1/1]x6 on LAO u Fl -2 [1/1]x20 on LAO [5/5]x20 on STO Ni3+ -3 866 868 870 872 874 876 878 880 Energy (eV) FIG. 2: X-ray absorption spectra at the Ni L -edge of the [1/1] 2 N superlatticesgrownonSTOandLAO.Thetopblackcurverepresents theNi2+standardfromNiO. step using a 4-terminal geometry. This experiment revealed FIG.1: RHEEDspecularintensityoftheinitialgrowth(markedby that prolonged annealing in O of the as-grown samples has 2 blackarrows)ofLNO(A)andLAO(B)layersonanon-polarSTO no sizable effect on the resistivity. In contrast, the reduction substrate. InsetofA:Growthofasinglerepeatof(LNO/LAO)on in nitrogen had strongly increased the resistivity of the sam- thepolarLAOsubstrate. plesbyseveralordersofmagnitude,whilethesubsequentre- oxidation successfully restored it back to the as-grown level, indicatingthattheconductivityisnotrelatedtosimpleoxygen between the 6-repeat and 20-repeat samples. This result im- deficiency. Thisexperimentprovidesstrongevidencethatthe plies that growing the polar SLs on a non-polar or weakly- observeddeviationofNivalencyfrom3+isnotduetoinsuf- polar substrate such as STO results in a massive chemical ficientoxidationduringthedepositionbutislikelyrelatedto compensationandelectronicreconstructionduringtheinitial theformationofacceptor-likecomplexdefectscausedbythe growth. Incontrast,theXASspectraforthesamesetofSLs polardiscontinuity. grown on LAO show that Ni valence is very close to 3+ as Fundamentally,thephaseofamaterialdepositedonthesur- directly evident from XAS on the SLs with N=6 and 20. To faceisdeterminedbyenthalpy,H. WhileH iscontrolledby excludethecompensatingchargetransferbetweenTiofSTO the environmental thermodynamic parameters (e.g. pressure and Ni of LNO, we compared the Ti L-edge spectrum in a andtemperature),italsointimatelydependsontheatomicin- TEY mode (not shown) to the Ti spectrum taken on a bare teractions on the surface (i.e. wettability). Thus, the elec- STO substrate[11]. The measurement unambiguously con- trostatic interactions within a structure consisting of oppo- firms that no significant change of Ti4+ valence takes place sitely charged alternating atomic planes will result in a large nearthefilm-substrateinterface,likelyduetothehighoxygen increase to H due to polar misfit[5], and render the struc- pressureduringthedeposition. ture unstable. Experimentally, the probability to grow such Next we investigated a possibility to preserve the fragile a structure from the vapor phase is negligibly small. In- electronicstateofNi3+ ionsbyfabricatingaseriesofSLson stead, it has been proposed that such a polar mismatch can a2u.c. LAObufferlayerdepositedonSTO.Itisclearfrom be generally resolved by transferring 0.5e from the surface the data shown in Fig. 2 for the N=3 SL with a LAO buffer to the polar-nonpolar interface[4, 6]. However, the experi- thatthebuffergreatlyaidsinrestoringtheelectronicstateof mentalobservationofthelargeamountofNi2+presentinthe Nitowards3+. ultra-thin SLs contradicts this model. To circumvent the is- Conductivity of bulk LNO has been shown to be sensi- sue, we propose that the presence of Ni2+ is likely associ- tive to oxygen deficiency which tightly correlates with Ni atedwiththepolarcompensationbytheexcessgrowthofan valence[14]. To clarify if Ni2+ is connected to oxygen defi- additional LaO plane. As illustrated in Fig. 4, at the initial ciency,weannealedtheSLsat900oCinflowingultra-pureO stage of growth, this would result in a tri-layer structure like 2 at1Atmfor12hours. SubsequentreductioninN followed .../Ti4+O2−/La3+O2−/Ni2+O2−/La3+O2−,whichcontainsa 2 2 2 by re-oxidation in O was carried out under the same condi- perovskite unit cell plus an extra LaO plane and remarkably 2 tions.DCresistivitymeasurementswereperformedaftereach possessesnototalchargeordipolemoment.Consequently,Ni 3 approaching that of the N=20 with no sign of a static Ni2+ component[17]. In practice, other mechanisms can certainly set in compli- cating the situation. In particular, non-stoichiometry such as atomic vacancies may play an important role. For instance, theelectrostaticpotentialjumpmaydecreasetheenthalpyof formationofLavacanciesinLaOplanesandputrelatedoxy- FIG. 3: The model tri-layer structure consisting of three atomic planes(chargedensitiesontheright)resultedfromtheexcessLaO genvacanciesontoNi(Al)O2 planestoreducethedipolemo- growthattheinitialstageofdepositiononSTO. ment. Notice,thelocalfieldfromtheatomicvacanciesisalso capableofcreatingNi2+duetoasmallchargetransferenergy of the Ni2+O− exciton. The precise correlation between the is promoted to the 2+ charge state by the strong decrease of polarityproblemandcompensationbyatomicdefectsrequires theelectronpotentialduetothetwoLaOplanes. furtherinvestigation. As the growth continues, the newly deposited [LaO/Ni(or In summary, by monitoring the Ni charge state as the Al)O ] (m = 0,1,2,...) layers must be accommodated by control parameter, we have investigated the effect of polar 2 m adjusting the plane charge densities (the charge per u.c. in mismatch on the electronic and structural properties of [1uc unitsof|e|)ofthebottomNiO planeandthesurfaceplane. LNO/1ucLAO] SLs.Theinitialgrowthstageisfoundtoex- 2 N If the excess LaO plane remains on the surface, based on perience a structural reconstruction accompanied by marked the charge neutrality and dipole compensation[15], one can changes of the Ni electronic structure. We suggest that to show that its charge density equates to (m+1)/(2m+1), a large extent these results can be explained by the new while the Ni valence in the bottom NiO plane becomes tri-layer growth model, which corroborates the experimental 2 3 − (m + 1)/(2m + 1). Note, setting m to zero will cor- data. GrowingLAObufferonSTOisfoundtoefficientlycir- respond to the initial tri-layer structure. On the other hand, cumventthepolarmismatchissueandallowstopreservethe if the terminating plane is Ni(Al)O , one can deduce that its electronicallyactiveNiionsfromchangingthevalence.These 2 charge density equates to −m/2(m+1) with Ni valence in important findings are relevant for a wide range of complex- the bottom NiO plane being 3−(m+2)/2(m+1) (here oxide materials and should pave a way to growth of novel 2 m = 0willcorrespondtoaNiO planedepositedonthetop ultra-thin heterostructures with controlled electronic state of 2 ofthetri-layer,andeachofthefollowingLaO/Ni(Al)O unit transitionmetalions. 2 cellswillincreasembyone). Ineithercase,whenm → ∞, J.C. was supported by DOD-ARO under the Contract No. the charge density of the bottom NiO plane changes from 0402-17291 and NSF Contract No. DMR-0747808. 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