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 Peter Grünberg Institute, Forschungszentrum Jülich, Jülich, Germany Bharat Jalan and Susanne Stemmer Materials Department, University of California, Santa Barbara, California , USA

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

Non-stoichiometry in thin film SrTiO 3 A-siteB-site Cation Vacancies? V Sr inferred from inhomogenous TEM contrast modulations Ohnishi, et al. J. Appl. Phys. 103, , (2008). Ti – richAmorphous TiO 2 Sr – richRuddlesden-Popper SrO layer phases Extreme cation non-stoichiometry V Sr inferred from modelling O-K edge ELNES spectra Mizoguchi, et al. Appl. Phys. Lett. 87, (2005) Tokuda, et al. Appl. Phys. Lett. 99, (2011).

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

Positron Lifetimes Positron lifetime sensitive to electron density V + positive Negligible e + trapping V 0 neutral Good e + trapping V − negative Rydberg states Excellent e + trapping B-site 4− V O : 2+ A-site 2− E+E+ EBEB

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

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

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

Variable Energy - Positron Annihilation Spectroscopy 5 × 10 8 e + s -1 at 1 keV Variable Energy – Positron Annihilation Lifetime Spectroscopy (VE-PALS) MeV Stop e+e+ Start e+e+ Experiment station Acceleration 0.5 – 21 keV > 5 x 10 6 counts / spectrum NEPOMUC beam line

Variable Energy - Positron Annihilation Lifetime Spectroscopy (VE-PALS) MeV Stop e+e+ Start e+e+ Acceleration 0.5 – 21 keV SrTiO 3 Film SrTiO 3 Substrate

Un-doped Pulsed Laser Deposited (PLD) SrTiO 3 on SrTiO 3 Thin Films Ti-poorSr-poor Strontium (Sr) excess HR x-ray diffraction [002] Sebastian Wicklein and Regina Dittmann (Jülich)

SrTiO 3 SrTiO 3 Substrate Un-doped PLD SrTiO 3 on SrTiO 3 Thin Films DFT-MIKA (ps) Bulk152 VOVO 159 V Ti 189 V Sr 281 deconvolved e + states Keeble et. al. Phys. Rev. Lett (2010) 280 ps 183 ps 280 ps 183 ps Sr-poor

Un-doped PLD SrTiO 3 on SrTiO 3 Thin Films F = 2.00 J cm -2 F = 1.50 J cm -2 ALL films show saturation e + trapping [V A/B ] > ppm

La-doped Hybrid MBE SrTiO 3 on SrTiO 3 Thin Films Bharat Jalan and Susanne Stemmer (UCSB) [La]  8 x cm -3 [La]  3 x cm -3

La-doped Hybrid MBE SrTiO 3 on SrTiO 3 Thin Films [La]  8 x cm -3  VSr = 280 ps  VTi = 183 ps  Cluster  400 ps  1 <  Bulk  155 ps

La-doped Hybrid MBE SrTiO 3 on SrTiO 3 Thin Films [La]  3 x cm -3  VSr = 280 ps  VTi = 183 ps  Cluster  400 ps  1 <  Bulk  155 ps

Hybrid MBE SrTiO 3 :La - estimate of cation vacancy concentration Reduced bulk lifetime component,  <  B (155 ps), due to annihilation events with perfect lattice.  B (STM) = 157(8) ps E = 4.5 – 8 keV:  B (STM) = 154(7) ps E = 4.5 – 7 keV:  B (STM) = 155(4) ps Single crystal SrTiO 3 [Mackie PRB ] Assume:  = 5 x s -1 ? No value measured in oxides, estimated values for negative vacancies in Si 2–29× s ̶ 1 [V Sr ]  5.4(6) x cm -3 [V Sr ]  1.7(5) x cm -3  [V Sr ] = 5.1(1.5) x 10 9 s -1  [V Sr ] = 1.6(2) x s -1 [La]  3 x cm -3 [La]  8 x cm -3

Un-doped Pulsed Laser Deposited (PLD) SrTiO 3 on SrTiO 3 Thin Films Strontium (Sr) excess Ti-poorSr-poor Sebastian Wicklein and Regina Dittmann (Jülich)

Un-doped Pulsed Laser Deposited (PLD) SrTiO 3 on SrTiO 3 Thin Films Sebastian Wicklein and Regina Dittmann (Jülich) Ti-poorSr-poor

Un-doped PLD SrTiO 3 on SrTiO 3 Thin Films 2-term fit 3-term fit 1.33 Jcm Jcm -2

Un-doped PLD SrTiO 3 on SrTiO 3 Thin Films  Cluster  420 ps V Pb V Ti 3V O DFT 344 ps

Un-doped PLD SrTiO 3 on SrTiO 3 Thin Films  Cluster  420 ps V Pb V Ti 3V O DFT 344 ps 430 ps vacancies 355 ps 5 vacancies Hakala, PRB 57, 7621 (1998) Staab, PRB 65, (2002) Silicon

Conclusions SrTiO 3 thin films grown by PLD with varying laser fluence (F): Exhibit saturation trapping e + to both V Ti and to V Sr defects for all films in the range 1.5 ≤ F ≤ 2.0 Jcm -2 Good agreement between MIKA calculated relaxed structure e + lifetimes for V Ti and to V Sr (189 ps and 281 ps) defects and experiment (183 ps and 280 ps) ‘Stoichiometric‘ F = 1.5 Jcm -2 (  c = 0.0 pm) film: e + trapping dominated by V Ti, likely due to higher defect specific trapping coefficient ‘Sr-poor’ (  c = 0.2 pm) F = 2.0 Jcm -2 film: e + trapping dominated by V Sr Sr-poor

Conclusions SrTiO 3 thin films grown by PLD with varying laser fluence (F):  Cluster  420 ps Ti-poor

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