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LaVO x （ x=3-4 ）薄膜の内殻光電子分光

LaVO x （ x=3-4 ）薄膜の内殻光電子分光. 発表者. 堀田　育志　（ Hwang 研）. 共同研究者の皆様. Ｈｗａｎｇ研. 椋木　康滋、. 須崎　友文、. Harold Y. Hwang. 藤森研. 和達　大樹、. 藤森　淳. 科研費基盤研究 A 　　「単結晶薄膜化により物性を制御した強相関系遷移金属酸化物の電子構造の研究」研究会 . ２００５年６月１７日（金）　１３時～１８時 　東京大学　柏キャンパス　基盤棟２階複雑理工講義室. Crystal structure and energy diagram of LaVO 3.

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LaVO x （ x=3-4 ）薄膜の内殻光電子分光

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  1. LaVOx（x=3-4）薄膜の内殻光電子分光 発表者 堀田　育志　（Hwang研） 共同研究者の皆様 Ｈｗａｎｇ研 椋木　康滋、 須崎　友文、 Harold Y. Hwang 藤森研 和達　大樹、 藤森　淳 科研費基盤研究A 　　「単結晶薄膜化により物性を制御した強相関系遷移金属酸化物の電子構造の研究」研究会 ２００５年６月１７日（金）　１３時～１８時 　東京大学　柏キャンパス　基盤棟２階複雑理工講義室

  2. Crystal structure and energy diagram of LaVO3 Crystal structure Energy diagram Orthorhombic unit cell dn+1 Mott gap Ueff : ～1 eV V 3d ⇒λ : ～1240nm (Near-IR) dn-1 ～4 eV Direct observation of Mott gap by optical spectro- scopy using near-IR dnL O 2p Perovskite unit cell JPSJ 64, (1995) 2488. Structural phase transition : 141K Orthorhombic → Monoclinic a=5.555Å b=5.553Å c=7.849Å a=5.594Å b=7.760Å c=5.565Å γ=90.125° P. Bordet, et. al., J. Solid State Chem. 106, 253 (1993) 1/18

  3. Motivation of this study Energy diagram of interface between Mott and band insulator LaVO3/LaAlO3 superlattice hν ○ × La 5d e- e- Ti 3d 5.6 eV V 3d 3.2 eV LaAlO3 LaVO3 O 2p Thick LVO SrTiO3 LaAlO3 LaVO3 ? Band insulator Band insulator Mott insulator 2e- Thin LVO 5.6 eV 2e- 3.2 eV The QWs have sub band structures? Quantum confinement? 2/18

  4. SrTiO3/LaTiO3 superlattice A. Ohtomo et al., Nature 419 (2002) 378 3/18

  5. Purpose of XPS measurement To confirm valence state of vanadium in LaVO3 layer e- X-ray e- La3+ Al3+ O2-3 Interface vanadium La3+ V3+ O2-3 La3+ Al3+ O2-3 SrTiO3 LaVO3 layer embedded in aAlO3 layers maintains Mott gap? Valence state of vanadium ion is modulated at the interface? 4/18

  6. Experimental condition Growth condition Pulsed Laser Deposition system Tg : 500～900℃ PO2 : 5×10-8～1×10-2 Torr Target : LaVO4 polycrystals Growth rate : ～1.5 nm/min Thickness : 40 nm (100 u.c.) Fluence : ～3 J/cm2 (4 Hz) Characterization RHEED XRD AFM XPS 5/18

  7. RHEED, XRD, and AFM measurement Sample structure : LaVO3(100ML)/SrTiO3 (100) Growth temperature : Tg = 600℃ Oxygen partial pressure : PO2 >10-5 Torr 10-3 Torr 10-4 Torr 10-5 Torr 10-4 Torr 10-6 Torr 10-6 Torr <10-5 Torr 10-7 Torr 5μm 6/18

  8. Growth phase diagram of LaVOx on SrTiO3 Phase diagram of LaVOx We mapped out the growth phase diagram of LaVOx (x=3-4) film. LaVO4 (V5+ phase) PO2 > 10-3 Torr → LaVO4 (x=4) PO2 = 10-5～ 10-4 Torr → Competing phase PO2 < 10-6 Torr → LaVO3 (x=3) Competing phase No structural phases corresponding to V4+ (x=3.5) LaVO3 (V3+ phase) The optimal condition for atomically flat epitaxial LaVO3 films were obtained. Optimal condition for 2D growth 7/18

  9. STO LVO LAO LAO LVO LAO STO LAO LVO LAO STO Sample structure Thick LaVO3 with cap Quantum-well (QW) structure LaAlO3(3ML) LaAlO3 (3ML) LaAlO3 (3ML) LaVO3 (1ML?) LaVO3 (3ML?) LaVO3 (50ML) LaAlO3 (30ML) LaAlO3 (30ML) SrTiO3 (5ML) SrTiO3 (5ML) SrTiO3 (5ML) SrTiO3 (100) SrTiO3 (100) SrTiO3 (100) RHEED oscillation data RHEED oscillation data 8/18

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

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

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

  13. LaVO3(50ML)/SrTiO3(100) Core level fitting QW structure (３ML) Thick LaVO3 film （５０ML) No cap Binding energy (eV) Binding energy (eV) Binding energy (eV) Table : Fitting parameter 12/18

  14. Vanadium core level spectra QW structure (３ML) Thick LaVO3 film (50ML) No cap The role of capping layer to preserve V3+ is obviously shown. The vanadium ion could access valence state of 3+ in the structure. 13/18

  15. LaVO3 layer-thickness dependence To change LVO thickness : [LaAlO3]3ML/ [LaVO3]xML/ [LaAlO3]30ML/SrTiO3(100) RHEED intensity data Surface morphology 5ML 4ML 3ML 1ML Sample structure STO (100) LAO 30ML LVO 5ML LAO 10ML LVO 5ML LAO 10ML LVO xML LAO 3ML 14/18

  16. ML dependence A systematic variation was shown. O1s The V2p peak intensity increases with increasing the LVO layer thickness The spectral shapes from 1M to 5ML show a good agreement with each other. /SrTiO3(100) V2p3/2 V2p1/2 The 50ML sample shows more broad peak, indicating the existence of V4+ and V5+. Normalized by the O1s peaks Normalized by the spectral area LaVO3(50ML)/ SrTiO3 (100) /SrTiO3(100) V3+ V5+ V4+ V3+ V5+ Satellite (Kα5) V4+ 15/18

  17. Angular dependence 16/18

  18. Angular dependence VO2 V2O5 LaVO4 Where did V4+ (or V5+) come from? XPS results indicate that it comes from the capping layer region. VO2- LaO+ AlO2- When LaVO3 growth was interrupted, high-oxidized V phases could appear at the surface. →However, no evidence! Ex.) TEM image of LVO/STO superlattice STO LaVO3 Growth direction LVO STO Observed by L. Fitting and D. A. Muller in Cornell Univ. 17/18

  19. Conclusion LaVO3の成膜条件の最適化 ２次元結晶成長条件 : Tg = 600℃, PO2 = 10-6 Torr LaAlO3/LaVO3（５～１ML)/LaAlO3超格子のXPS測定 LaVO3の膜厚に対してV 2p3/2ピーク形状の変化なし →LaVO3１MLの場合でもV3+が主な価数 角度分解XPS測定 LaVO3５～１MLの試料で同様の傾向 →　上部の界面近傍により価数の高いバナジウムイオンが存在 →LaVO3１MLの場合にも角度依存性あり ※上部の界面でより酸化されたバナジウム相が混在？ 18/18

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