LaVOxx=3-4HwangHarold Y. HwangA

Crystal structure and energy diagram of LaVO3Crystal structureOrthorhombic Monoclinic Structural phase transition : 141K a=5.555 b=5.553c=7.849a=5.594b=7.760c=5.565P. Bordet, et. al., J. Solid State Chem. 106, 253 (1993)=90.125Perovskite unit cellOrthorhombic unit cellMott gap Ueff : 1 eVdn+1dn-1dnLO 2pV 3d : 1240nm (Near-IR)Direct observation of Mottgap by optical spectro-scopy using near-IREnergy diagramJPSJ 64, (1995) 2488.4 eV1/18

Motivation of this study5.6 eVLaAlO3LaVO3hLaVO3/LaAlO3 superlatticeO 2pTi 3dLa 5d3.2 eV5.6 eVSrTiO3LaAlO3LaVO3?Thick LVOThin LVOEnergy diagram of interface betweenMott and band insulatorBandinsulatorBandinsulatorMottinsulator3.2 eVQuantum confinement?2e-2e-The QWs have sub band structures?V 3de-e-2/18

SrTiO3/LaTiO3 superlatticeA. Ohtomo et al., Nature 419 (2002) 3783/18

Purpose of XPS measurementLa3+ Al3+ O2-3La3+ V3+ O2-3La3+ Al3+ O2-3Interface vanadiume-X-rayTo confirm valence state of vanadium in LaVO3 layer Valence state of vanadium ion is modulated at the interface?e-SrTiO3LaVO3 layer embedded in aAlO3 layers maintains Mott gap?4/18

Experimental conditionPulsed Laser Deposition systemPO2 : 510-8110-2 Torr Tg : 500900 Growth rate : 1.5 nm/minTarget : LaVO4 polycrystals Thickness : 40 nm (100 u.c.)Growth conditionFluence : 3 J/cm2 (4 Hz)CharacterizationXRDXPSAFMRHEED5/18

RHEED, XRD, and AFM measurementGrowth temperature : Tg = 60010-4 Torr 10-5 Torr 10-6 Torr 10-7 Torr 10-4 Torr 10-5 Torr 6/18

Growth phase diagram of LaVOx on SrTiO3LaVO3 (V3+ phase)LaVO4 (V5+ phase)Competing phaseOptimal condition for 2D growth No structural phases corresponding toV4+ (x=3.5) We mapped out the growth phasediagram of LaVOx (x=3-4) film.

PO2 > 10-3 Torr LaVO4 (x=4)PO2 = 10-5 10-4 Torr Competing phasePO2 < 10-6 Torr LaVO3 (x=3) The optimal condition for atomically flatepitaxial LaVO3 films were obtained. Phase diagram of LaVOx7/18

Sample structureSrTiO3 (100)SrTiO3 (100)LaVO3 (50ML)LaVO3 (3ML?)LaVO3 (1ML?)SrTiO3 (100)LaAlO3 (3ML)LaAlO3 (30ML)LaAlO3 (3ML)LaAlO3 (30ML)LaAlO3 (3ML)SrTiO3 (5ML)SrTiO3 (5ML)SrTiO3 (5ML)Thick LaVO3 with capRHEED oscillation dataQuantum-well (QW) structureRHEED oscillation data8/18

Wide scan and core level spectraThe La 3d spectrum in LaAlO3 layer showed a good agreement with its in La2O3.Ref.) Chem. Phys. 253 (2000) 27The energy position of Al 2p peak was assigned to valence state of 3+.9/18

Quantum-well structure[LaAlO3]3ML/ [LaVO3]xML/ [LaAlO3]30ML/SrTiO3(100)Thick LVOWell structure The curve shape of the V 2p3/2 spectrum in the 3ML-LVO-QW sample showedgood agreement with the result for LaVO3 bulk treated by filling in vacuum.Bulk : PRB 53, 1161 (1996)V 2p3/2Energy of electrons (eV)Escape depth ()This studyEscape depth for electron 10/18V3+V4+V5+

Core level fittingRef. J. Phys. Chem. Ref. Data, Vol.8, 329 (1979)Peak fitting of V5+ spectra The V 2p3/2 peak from LaVO4 is assignedas single component of V5+ state. Fitting parameterBinding energy (B-E)Peak intensity (INT) Lorentzian width (L-W) Gaussian width (G-W) Some parameters should be fixed to obtaina objective fitting result for the spectra. Core level fittings of the XPS spectra were performed to estimate the componentof V3+ state in the capped LaVO3 samples.11/18

V5+L-W 0.60 [Ref.]G-W 1.23

Binding energy (eV)Binding energy (eV)Binding energy (eV)Core level fittingTable : Fitting parameterThick LaVO3 film ML)QW structure (ML)No cap12/18

(LAO)3/(LVO)50/STOV3+V4+V5+515.131.770.601.870.36516.681.230.601.870.25517.530.360.601.230.07

(LAO)3/(LVO)3/(LAO)50/STOV3+V4+V5+515.131.960.601.870.46516.680.930.601.870.22517.530.000.601.230

B-EINTL-WG-WAREA

(LVO)50/STOV3+V4+V5+ 515.130.980.601.870.28516.681.820.601.870.53 517.530.110.601.230.03

QW structure (ML)Thick LaVO3 film (50ML)No capVanadium core level spectra The role of capping layer to preserveV3+ is obviously shown. The vanadium ion could access valence state of 3+ in the structure. 13/18

V3+V4+V5+AREA0.530.370.100.47

V3+V4+V5+AREA0.330.630.040.67

LAO30MLSTO(100)LVO5MLLVO5MLLVOxMLLAO10MLLAO10MLLAO3MLRHEED intensity dataSample structure5ML4ML3ML1MLLaVO3 layer-thickness dependenceSurface morphologyTo change LVO thickness : [LaAlO3]3ML/ [LaVO3]xML/ [LaAlO3]30ML/SrTiO3(100)14/18

ML dependenceNormalized by the O1s peaksLaVO3(50ML)/SrTiO3 (100)/SrTiO3(100)Normalized by the spectral areaO1sV2p1/2V2p3/2V3+V4+V5+V3+V4+V5+Satellite (K5) /SrTiO3(100)A systematic variation was shown.The V2p peak intensity increases withincreasing the LVO layer thickness The spectral shapes from 1M to 5ML showa good agreement with each other. The 50ML sample shows more broad peak,indicating the existence of V4+ and V5+. 15/18

Angular dependence16/18

Angular dependenceLaVO3VO2V2O5LaVO4Where did V4+ (or V5+) come from?When LaVO3 growth was interrupted,high-oxidized V phases could appear atthe surface. LaO+VO2-AlO2- XPS results indicate that it comes fromthe capping layer region. However, no evidence!Ex.) TEM image of LVO/STO superlatticeGrowth direction STOSTOLVOObserved by L. Fitting and D. A. Muller in Cornell Univ.17/18

ConclusionLaAlO3/LaVO3ML)/LaAlO3 XPSLaVO3 :Tg = 600, PO2 = 10-6 TorrLaVO3 V 2p3/2LaVO3 MLLaVO3 MLV3+XPSLaVO3 ML18/18