1. LaVO x （ x=3-4 ）薄膜の内殻光電子分 光 堀田 育志 （ Hwang 研） 和達 大樹、 椋木 康滋、須崎 友文、 Harold Y. Hwang 藤森 淳 藤森研 Ｈｗａｎｇ研 共同研究者の皆様 発表者 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 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

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

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

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

5. Purpose of XPS measurement La 3+ Al 3+ O 2- 3 La 3+ V 3+ O 2- 3 La 3+ Al 3+ O 2- 3 Interface vanadium e-e- X-ray To confirm valence state of vanadium in LaVO 3 layer Valence state of vanadium ion is modulated at the interface? e-e- SrTiO 3 LaVO 3 layer embedded in aAlO 3 layers maintains Mott gap? 4/18

6. Experimental condition Pulsed Laser Deposition system P O2 : 5×10 -8 ～ 1×10 -2 Torr T g : 500 ～ 900 ℃ Growth rate : ～ 1.5 nm/min Target : LaVO 4 polycrystals Thickness : 40 nm (100 u.c.) Growth condition Fluence : ～ 3 J/cm 2 (4 Hz) Characterization XRD XPS AFM RHEED 5/18

7. RHEED, XRD, and AFM measurement Growth temperature : T g = 600 ℃ 10 -4 Torr 10 -5 Torr 10 -6 Torr 10 -7 Torr 10 -4 Torr <10 -5 Torr 10 -3 Torr Oxygen partial pressure : P O2 Sample structure : LaVO 3 (100ML)/SrTiO 3 (100) 5μm 10 -6 Torr >10 -5 Torr 6/18

8. Growth phase diagram of LaVO x on SrTiO 3 LaVO 3 (V 3+ phase) LaVO 4 (V 5+ phase) Competing phase Optimal condition for 2D growth No structural phases corresponding to V 4+ (x=3.5) We mapped out the growth phase diagram of LaVO x (x=3-4) film. P O2 > 10 -3 Torr → LaVO 4 (x=4) P O2 = 10 -5 ～ 10 -4 Torr → Competing phase P O2 < 10 -6 Torr → LaVO 3 (x=3) The optimal condition for atomically flat epitaxial LaVO 3 films were obtained. Phase diagram of LaVO x 7/18

9. Sample structure SrTiO 3 (100) LaVO 3 (50ML) LaVO 3 (3ML?) LaVO 3 (1ML?) SrTiO 3 (100) LaAlO 3 (3ML) LaAlO 3 (30ML) LaAlO 3 (3ML) LaAlO 3 (30ML) LaAlO 3 (3ML) SrTiO 3 (5ML) Thick LaVO 3 with cap STOLVOLAO STO LVOLAO RHEED oscillation data STOLVOLAO Quantum-well (QW) structure RHEED oscillation data 8/18

10. Wide scan and core level spectra The La 3d spectrum in LaAlO 3 layer showed a good agreement with its in La 2 O 3. 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 [LaAlO 3 ] 3ML / [LaVO 3 ] xML / [LaAlO 3 ] 30ML /SrTiO 3 (100) Thick LVO Well structure The curve shape of the V 2p 3/2 spectrum in the 3ML-LVO-QW sample showed good agreement with the result for LaVO 3 bulk treated by filling in vacuum. Bulk : PRB 53, 1161 (1996) V 2p 3/2 Energy of electrons (eV) Escape depth ( Å ) This study Escape depth for electron 10/18 V 3+ V 4+ V 5+

12. L-W 0.60 [Ref.] G-W 1.23 Core level fitting Ref. J. Phys. Chem. Ref. Data, Vol.8, 329 (1979) Peak fitting of V 5+ spectra The V 2p 3/2 peak from LaVO 4 is assigned as single component of V 5+ state. Fitting parameter Binding energy (B-E) Peak intensity (INT) Lorentzian width (L-W) Gaussian width (G-W) Some parameters should be fixed to obtain a objective fitting result for the spectra. Core level fittings of the XPS spectra were performed to estimate the component of V 3+ state in the capped LaVO 3 samples. 11/18

13. (LAO) 3 /(LVO) 50 /STO V 3+ V 4+ V 5+ 515.13 1.77 0.60 1.87 0.36 516.68 1.23 0.60 1.87 0.25 517.53 0.36 0.60 1.23 0.07 (LAO) 3 /(LVO) 3 /(LAO) 50 /STO V 3+ V 4+ V 5+ 515.13 1.96 0.60 1.87 0.46 516.68 0.93 0.60 1.87 0.22 517.53 0.00 0.60 1.23 0 B-E INT L-W G-W AREA (LVO) 50 /STO V 3+ V 4+ V 5+ 515.13 0.98 0.60 1.87 0.28 516.68 1.82 0.60 1.87 0.53 517.53 0.11 0.60 1.23 0.03 Binding energy (eV) Core level fitting Table : Fitting parameter Thick LaVO 3 film （５０ ML) QW structure ( ３ ML) No cap LaVO 3 (50ML)/SrTiO 3 (100) 12/18

14. V 3+ V 4+ V 5+ AREA0.680.320 V 3+ V 4+ V 5+ AREA0.530.370.10 0.47 V 3+ V 4+ V 5+ AREA0.330.630.04 0.67 QW structure ( ３ ML) Thick LaVO 3 film (50ML) No cap Vanadium core level spectra The role of capping layer to preserve V 3+ is obviously shown. The vanadium ion could access valence state of 3+ in the structure. 13/18

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

16. ML dependence Normalized by the O1s peaks LaVO 3 (50ML)/ SrTiO 3 (100) /SrTiO 3 (100) Normalized by the spectral area O1s V2p 1/2 V2p 3/2 V 3+ V 4+ V 5+ V 3+ V 4+ V 5+ Satellite (Kα5) /SrTiO 3 (100) A systematic variation was shown. 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. The 50ML sample shows more broad peak, indicating the existence of V4+ and V5+. 15/18

17. Angular dependence 16/18

18. Angular dependence LaVO 3 VO 2 V 2 O 5 LaVO 4 Where did V4+ (or V5+) come from? When LaVO 3 growth was interrupted, high-oxidized V phases could appear at the surface. LaO + VO 2 - AlO 2 - XPS results indicate that it comes from the capping layer region. → However, no evidence! Ex.) TEM image of LVO/STO superlattice Growth direction STO LVO Observed by L. Fitting and D. A. Muller in Cornell Univ. 17/18

19. Conclusion LaAlO 3 /LaVO 3 （５～１ ML)/LaAlO 3 超格子の XPS 測定 LaVO 3 の成膜条件の最適化 ２次元結晶成長条件 :T g = 600 ℃, P O2 = 10 -6 Torr LaVO 3 の膜厚に対して V 2p 3/2 ピーク形状の変化なし → LaVO 3 １ ML 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？ → LaVO 3 １ ML の場合でも V 3+ が主な価数 角度分解 XPS 測定 LaVO 3 ５～１ ML 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 18/18