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Ion implantation doping of perovskites and related oxides

Ion implantation doping of perovskites and related oxides. Ulrich Wahl Instituto Tecnológico e Nuclear (ITN), Sacavém, Portugal. Collaborators: João Guilherme Correia (Senior Res., ITN & CERN) Eduardo Alves (Senior Res., ITN) Ana Claudia Marques (PhD student, U Lisbon & CERN)

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Ion implantation doping of perovskites and related oxides

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  1. Ion implantation doping of perovskites and related oxides Ulrich Wahl Instituto Tecnológico e Nuclear (ITN), Sacavém, Portugal Collaborators: João Guilherme Correia (Senior Res., ITN & CERN) Eduardo Alves (Senior Res., ITN) Ana Claudia Marques (PhD student, U Lisbon & CERN) Carlos Pedro Marques (PhD student, U Lisbon) Karl Johnston (Post-Doc, U Saarbrücken & CERN) João Pedro Araújo (Prof., U Porto) Lino Pereira (PhD student, U Porto)

  2. Outline • Motivation for implantation studies in perovskites • Objectives and work plan • Implantation damage annealing in SrTiO3: • RBS/C, 89Sr emission channeling, PL • Fe in SrTiO3: • emission channeling lattice location, • magnetic moments (SQUID) • 67Cu and 111Ag emission channeling in SrTiO3 • Rare earth 169Yb in SrTiO3 • Conclusions • Outlook

  3. What are perovskites? Perovskites are metal oxides of the form ABO3 A forms a simple cubic lattice B forms a simple cubic lattice A and B together form a bcc lattice O occupies the faces of the cube Best studied perovskite: SrTiO3 Perovskites are ionic compounds:e.g. Sr2+Ti4+O2-3

  4. Perovskites and doping • Perovskites (SrTiO3, CaTiO3, BaTiO3, KTaO3, ...) are multifunctional materials... • ...whose electrical, magnetic and optical properties can be drastically changed by doping • Many possible applications of perovskites rely on doping these materials.

  5. SrTiO3: great variety of possible dopants • Possible electrical dopants • Some magnetic dopants • Some optical dopants But little is known about ion implantation doping: lattice sites ? damage annealing ?

  6. Motivation: ion implantation in perovskites Amorphization threshold temperature [K] Trecrystallization

  7. In short: • perovskites are hard to amorphize and... • ...recrystallize at moderate temperatures Þ should be attractive systems for ion implantation

  8. Major objectives of this project: • investigate basic possibilities for ion implantation doping of perovskite oxides and related materials in order to modify their optical, magnetic and electrical properties • implantation of magnetic impurities such as Fe or Mn, optically active dopants such as rare earth elements, and prospective electrically active donor and acceptor impurities • study lattice location of impurities and annealing of implantation damage using nuclear techniques • complementary characterization of optical, magnetic and electrical properties for certain implanted impurities

  9. Secondary objectives: • promoting the use of nuclear techniques in the production and characterization of novel materials in the technologically relevant fields of microelectronics, optoelectronics and spintronics • transfer of know-how in the application of nuclear techniques to the other participants of the Coordinated Research Project • training and formation of PhD and undergraduate students on a national level in materials science and in the application of nuclear techniques

  10. Work plan: • Year 1 (2005-06): lattice location and damage annealing studies in SrTiO3, first measurements of optical and magnetic properties • Year 2 (2006-07): lattice location and damage annealing studies extended to other perovskites, e.g. KTaO3, measurements of optical and magnetic properties under optimized conditions (dose, annealing…) • Year 3 (2007-08): set priorities for the study of those systems where, according to outcome of research in years 1+2, best results were obtained

  11. Foreseen methods of study • stable and radioactive ion implantation • nuclear methods using radioactive isotopes • - Emission Channeling (EC) lattice location • - Perturbed Angular Correlation (PAC) • -Photoluminescence (PL) • Rutherford Backscattering / Channeling (RBS/C) • conventional Photoluminescence (PL) • macroscopic magnetic moments by means of SQUID ...work in progress, presentation includes several experiments that are not yet fully analyzed!

  12. Annealing of radiation damage: RBS/C • SrTiO3 implanted with 56Fe at doses of 1-5´1015 cm-2 • 2 MeV 4He+ RBS/C minimum yield cmin measured as function of TA Þ only for TA>1000°C remaining damage reaches values similar to a virgin crystal

  13. Annealing of radiation damage: 89Sr emission channeling • analysis of experiment still in progress! <211> following TA=1000°C cmax ~ substitutional fraction SSr STi • as was expected, 89Sr (60 keV, 8´1014 cm-2) occupies mainly Sr sites • broad damage recovery stage »200-1000°C • annealing probably not yet finished at 1000°C • information on rms displacement of Sr atoms as function of TA could not be derived (problem of angular resolution, mosaicity of implanted sample!)

  14. Annealing of radiation damage: PL • Sample implanted with 89Sr at a dose of ~1014 cm-2 • PL with 325nm excitation (above band edge 387 nm = 3.2 eV ) measured at 1.6 K as function of TA Þ near-band edge luminescence restored only for TA>800°C

  15. Motivation: SrTiO3 in the focus as... ... dilute magnetic semiconductor (room-temperature ferromagnetism?)

  16. Fe in SrTiO3 • As 3d transition metal element, Fe is a candidate for magnetic doping of SrTiO3 (cf Mn) • which lattice site does Fe occupy following ion implantation? • study by means of b- emission channeling using the radioactive isotope 59Fe (t1/2=44 d) low dose implantations (60 keV, 5´1012- 1´1013 cm-2) • What are the magnetic properties of Fe-implanted SrTiO3? • study by means of SQUID high dose implantations (60 keV, 1015- 1016 cm-2)

  17. Emission channeling lattice location: basic principle

  18. The cubic perovskite lattice of SrTiO3 u1(Sr)=.077 Å u1(Ti)=.061 Å u1(Sr)=.085 Å

  19. 59Fe lattice sites in SrTiO3 as function of TA • as-implanted: octahedral interstitial I8 prominent + substitutional Ti • Fe on I8 decreases fast • no damage recovery stage • no (or little) Fe on Sr sites • Fe prefers <100> displaced Ti sites (D»0.4-0.8 Å) + Ti sites

  20. b- emission channeling patterns, 59Fe 60 keV 1013 cm-2in SrTiO3, as-impl. <100> displ. STi I8 sites

  21. b- emission channeling patterns, 59Fe 60 keV 1013 cm-2in SrTiO3, TA=900°C <100> displ. STi ideal STi sites

  22. b- emission channeling patterns, 59Fe 60 keV 1013 cm-2in SrTiO3, TA=900°C <100> displ. STi ideal STi ideal SSr

  23. SQUID magnetic moment of SrTiO3:Fe (H) • 56Fe 60 keV 5´1015 cm-2, TA=900°C, SQUID measurement at 10 K • Diamagnetism of SrTiO3 substrate • Weak paramagnetism of implanted Fe and contaminants (0.5 ppm Cu2+?) • Ferromagnetism of implanted Fe

  24. SQUID magnetic moment of SrTiO3:Fe (T) • Paramagnetic component becomes obvious from temperature dependence: 1/T Brillouin paramagnetic vs constant diamagnetic + ferromagnetic

  25. SQUID results for different Fe fluences magnetization of 5´1015 cm-2 Fe implanted SrTiO3 m»7.5 mB small ferromagnetic signals from virgin SrTiO3 1´1015 cm-2 Fe implanted ferromagnetic signature • Note: 60 keV 1´1015 cm-2 corresponds to [Fe]max = 1.8% /unit cell Þ implanted Fe exhibits ferromagnetism in SrTiO3

  26. Sources of magnetism in Fe-implanted SrTiO3

  27. Cu and Ag in SrTiO3 • As group Ib elements, Cu and Ag on Sr sites are candidates for p-type (acceptor) doping of SrTiO3 • which lattice site do they occupy following ion implantation? • study by means of b- emission channeling using the radioactive isotopes 67Cu (t1/2=63 h) and 111Ag (7.5 d) • Note: low dose implantations (60 keV, 5´1012- 1´1013 cm-2)

  28. 67Cu and 111Ag lattice sites in SrTiO3 as function of TA 111Ag 67Cu • as-implanted: substitutional + octahedral interstitial I8 sites • Cu and Ag on I8 disappears fast • broad recovery stage of damage »300-900°C, • Cu and Ag are both amphoteric (found on both Sr and Ti sites) • Cu prefers Ti sites while Ag prefers Sr sites! Possible explanation: ionic radii Sr2+ 1.13 Å Cu1+ 0.96 Å Ag1+ 1.26 Å Ti4+ 0.68 Å Cu2+ 0.69 Å Ag2+ 1.08 Å

  29. Motivation: SrTiO3 in the focus as... ... red phosphor material flat panel displays

  30. b- emission channeling patterns, 169Yb 1014 cm-2in SrTiO3, TA=900°C SSr STi

  31. 169Yb lattice sites in SrTiO3 as function of TA • no octahedral interstitial I8 sites (in contrast to TMs Fe, Cu, Ag) • also broad recovery stage of damage »300-900°C, • Yb is also amphoteric (found on both Sr and Ti sites) • Yb prefers Ti sites

  32. Overview: SrTiO3 emission channeling lattice location experiments • Electrical dopants • Magnetic dopants • Optical dopants • Emission channeling lattice location experiments undertaken so far Experiments foreseen or possible Experiments attempted

  33. Conclusions • SrTiO3 relatively resistant against implantation damage • damage annealing starts above 200-300°C... • but is only complete at ~1000-1100°C Þannealing is less efficient than expected • lattice location experiments in SrTiO3 difficult to analyze: • Cu, Ag, Yb are amphoteric impurities: multiple sites • Fe also shows site distribution: <100> diplaced Ti + Ti • generally doping is complicated by amphoteric nature • Ag more promising Sr-site acceptor than Cu, • Fe was shown to act as ferromagnetic dopant

  34. Performance against original work plan: • Year 1 (2005-06): lattice location and damage annealing studies in SrTiO3, first measurements of optical and magnetic properties • Year 2 (2006-07):lattice location and damage annealing studies extended to other perovskites, e.g. KTaO3(samples have now been bought),measurements of optical and magnetic properties under optimized conditions (dose, annealing…) ü • Year 3 (2007-08):set priorities for the study of those systems where, according to outcome of research in years 1+2, best results were obtained

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