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David J. Keeble Carnegie Laboratory of Physics, University of Dundee Dundee, DD14HN, Scotland, UK

Vacancy defect detection and characterization in SrTiO 3 thin films by positron lifetime spectroscopy. David J. Keeble Carnegie Laboratory of Physics, University of Dundee Dundee, DD14HN, Scotland, UK Sebastian Wicklein and Regina Dittmann

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David J. Keeble Carnegie Laboratory of Physics, University of Dundee Dundee, DD14HN, Scotland, UK

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  1. Vacancy defect detection and characterization in SrTiO3 thin films by positron lifetime spectroscopy David J. Keeble Carnegie Laboratory of Physics, University of Dundee Dundee, DD14HN, Scotland, UK Sebastian Wicklein and Regina Dittmann Peter Grünberg Institute, ForschungszentrumJülich, 52425 Jülich, Germany Bharat Jalan and Susanne Stemmer Materials Department, University of California, Santa Barbara, California 93106-5050, USA

  2. Acknowledgements University of Dundee Ross Mackie, Gurmeet Kanda FRMII-NEPOMUC beamline ChristophHugenschmidt (Technische Universität München, ZWEFRM 11) European Commission Programme RII3-CT-2003-505925 FRMII-NEPOMUC VE-PALS instrument station Werner Egger (Universität Bundeswehr München) TEM L. Jinand C. L. Jia, Peter Grünberg Institute, Research Centre Jülich

  3. Non-stoichiometry in thin film SrTiO3 A-site B-site Extreme cationnon-stoichiometry Cation Vacancies? Ti – rich Amorphous TiO2 VSr inferred from inhomogenous TEM contrast modulations Sr – rich Ruddlesden-Popper SrO layer phases Ohnishi, et al. J. Appl. Phys. 103, 103703, (2008). VSr inferred from modelling O-Kedge ELNES spectra Mizoguchi, et al. Appl. Phys. Lett. 87, 241920 (2005) Tokuda, et al. Appl. Phys. Lett. 99, 033110 (2011).

  4. Positrons trap at missing atom defects, open volume defects: antimatter traps at sites of missing matter Positron annihilation spectroscopy (PAS) methods have ppm-level sensitivity PAS methods, combined with DFT, can detect and identify vacancy defects Three PAS methods: here we report positron lifetime spectroscopy measurements

  5. Positron Lifetimes Positron lifetimesensitive to electron density E+ EB Negligible e+ trapping V+ positive • VO: 2+ Good e+ trapping V0 neutral A-site B-site • 4− • 2− V− negative Excellent e+ trapping Rydberg states

  6. Positron Annihilation Lifetime Spectroscopy e+ Standard Trapping Model (STM) Positron source Thermalization Defect Free Bulk Lattice E+ kD Trapping EB Defect Annihilation B D B 511 keV Annihilation Radiation 511 keV Lifetime 1 Value less than bulk lifetime: reduced bulk lifetime Lifetime 2 ‘fixed’ at the defect value The bulk positron lifetime is a characteristic of a given material

  7. Positron Annihilation Lifetime Spectroscopy e+ Positron source Two Defect – STM Thermalization Defect concentration [D] Defect Free Bulk Lattice kD1 kD2 Trapping Defect specific trapping coefficient Defect 2 Defect 1 B D1 D2 Annihilation What if the concentration of one/both vacancy is ‘very’ large? Annihilation Radiation Vacancy 2 Vacancy 1 Reduced bulk Saturation trapping occurs: t1 and I1 tend to zero Saturation trapping occurs for

  8. B-site A-site • 4− • 2+ • 2− DFT-MIKA Torsti, et al., Phys. Status Solidi B 243, 1016 (2006) Mackie et al. Phys. Rev. B 79 014102 (2009) Keeble et al. Phys. Rev. Lett. 105226102 (2010) t(VTi) = 195 ps t(VSr) = 280 ps t(bulk) = 152ps t(VO) = 161 ps t(VTi)relax = 189 ps t(VSr)relax = 281 ps e+enhancement: AP : Arponen and E. Pajanne, Ann. Phys. (N.Y.) 121, 343 (1979); B. Barbiellini, et al Phys. Rev. B 53, 16201 (1996). O ion relaxation: +5.2 % O ion relaxation: +3.7 % Sr ion relaxation: - 8.4 % Tanaka et al. Phys. Rev. B68 205213 (2003) Ti ion relaxation: - 2.1 %

  9. Variable Energy - Positron Annihilation Spectroscopy Start 5 × 108e+ s-1 at 1 keV NEPOMUC beam line Acceleration 0.5 – 21 keV Stop e+ e+ > 5 x 106 counts / spectrum 0.511 MeV Experiment station Variable Energy – Positron Annihilation Lifetime Spectroscopy (VE-PALS)

  10. Variable Energy - Positron Annihilation Lifetime Spectroscopy (VE-PALS) SrTiO3 Film SrTiO3 Substrate Start Acceleration 0.5 – 21 keV Stop e+ e+ 0.511 MeV

  11. Un-doped Pulsed Laser Deposited (PLD) SrTiO3 on SrTiO3 Thin Films Sebastian Wicklein and Regina Dittmann (Jülich) HR x-ray diffraction [002] Ti-poor Sr-poor Strontium (Sr) excess

  12. Un-doped PLD SrTiO3 on SrTiO3 Thin Films Sr-poor deconvolvede+ states deconvolvede+ states 280 ps 280 ps 183 ps 183 ps SrTiO3 SrTiO3 Substrate Keebleet. al. Phys. Rev. Lett. 105226102 (2010)

  13. Un-doped PLD SrTiO3 on SrTiO3 Thin Films F = 1.50 J cm-2 F = 2.00 J cm-2 ALL films show saturation e+ trapping [VA/B] > 50-100 ppm

  14. La-doped Hybrid MBE SrTiO3 on SrTiO3 Thin Films Bharat Jalan and Susanne Stemmer (UCSB) [La]  8 x 1017 cm-3 [La]  3 x 1019 cm-3

  15. La-doped Hybrid MBE SrTiO3 on SrTiO3 Thin Films [La]  8 x 1017 cm-3 tCluster 400 ps t1 < tBulk 155ps tVSr = 280 ps tVTi = 183 ps

  16. La-doped Hybrid MBE SrTiO3 on SrTiO3 Thin Films [La]  3 x 1019 cm-3 t1 < tBulk 155ps tCluster 400 ps tVSr = 280 ps tVTi = 183 ps

  17. Hybrid MBE SrTiO3:La - estimate of cation vacancy concentration Reduced bulk lifetime component, t < t B (155 ps), due to annihilation events with perfect lattice. [La]  8 x 1017 cm-3 [La]  3 x 1019 cm-3 k[VSr] = 1.6(2) x 1010 s -1 k[VSr] = 5.1(1.5) x 109 s -1 No value measured in oxides, estimated values for negative vacancies in Si 2–29× 1015s ̶ 1 ? Assume: m = 5 x 1015 s -1 [VSr]  5.4(6) x 1016 cm -3 Single crystal SrTiO3 [Mackie PRB 2009 79 014102] E = 4.5 – 8 keV: E = 4.5 – 7keV: tB(STM) = 155(4) ps [VSr]  1.7(5) x 1016 cm -3 tB(STM) = 157(8) ps tB(STM) = 154(7) ps

  18. Un-doped Pulsed Laser Deposited (PLD) SrTiO3 on SrTiO3 Thin Films Sebastian Wicklein and Regina Dittmann (Jülich) Ti-poor Sr-poor Strontium (Sr) excess

  19. Un-doped Pulsed Laser Deposited (PLD) SrTiO3 on SrTiO3 Thin Films Sebastian Wicklein and Regina Dittmann (Jülich) Ti-poor Sr-poor

  20. Un-doped PLD SrTiO3 on SrTiO3 Thin Films 2-term fit 3-term fit 2-term fit 3-term fit 1.17 Jcm-2 1.33 Jcm-2

  21. Un-doped PLD SrTiO3 on SrTiO3 Thin Films VPbVTi3VO DFT 344 ps tCluster 420 ps

  22. Un-doped PLD SrTiO3 on SrTiO3 Thin Films tCluster 420 ps VPbVTi3VO DFT 344 ps Silicon 355 ps 5 vacancies 430 ps 10-14 vacancies Hakala, PRB 57, 7621 (1998) Staab, PRB 65, 115210 (2002)

  23. Conclusions SrTiO3 thin films grown by PLD with varying laser fluence (F): Exhibit saturation trapping e+to both VTi and to VSr defects for all films in the range 1.5 ≤ F ≤ 2.0 Jcm-2 Good agreement between MIKA calculated relaxed structure e+lifetimes for VTi and to VSr (189 psand 281 ps) defects and experiment (183 psand 280 ps) ‘Stoichiometric‘ F = 1.5 Jcm-2 (Dc = 0.0 pm) film: e+ trapping dominated by VTi , likely due to higher defect specific trapping coefficient ‘Sr-poor’ (Dc = 0.2 pm) F = 2.0 Jcm-2 film: e+ trapping dominated by VSr Sr-poor

  24. Conclusions SrTiO3 thin films grown by PLD with varying laser fluence (F): tCluster 420 ps Ti-poor

  25. Conclusions Hybrid-MBE SrTiO3shows a reduced bulk lifetime – a fraction of positrons annihilate from perfect lattice. Previous measurements of laser ablated SrTiO3 thin films have observed saturation positron trapping. Near-surface 50 nm contains small vacancy cluster defects. tVSr = 280(4) ps The strontium vacancy, VSr, is the dominant cation vacancy The concentrations were estimated to be 5.4(6) x 1016 cm -3for the [La]  8 x 1017 cm-3 film and 1.7(5) x 1016 cm -3 for the [La]  3 x 1019cm-3film. These vacancy concentrations are at least an order of magnitude lower than the La concentrations.

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