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)

Outline Motivation for implantation studies in perovskites Objectives and work plan Implantation damage annealing in SrTiO 3 : –RBS/C, 89 Sr emission channeling, PL Fe in SrTiO 3 : –emission channeling lattice location, –magnetic moments (SQUID) 67 Cu and 111 Ag emission channeling in SrTiO 3 Rare earth 169 Yb in SrTiO 3 Conclusions Outlook

What are perovskites? Perovskites are metal oxides of the form ABO 3 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: SrTiO 3 Perovskites are ionic compounds:e.g. Sr 2+ Ti 4+ O 2  3

Perovskites and doping Perovskites (SrTiO 3, CaTiO 3, BaTiO 3, KTaO 3,...) 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.

SrTiO 3 : great variety of possible dopants Possible electrical dopantsPossible electrical dopants Some magnetic dopantsSome magnetic dopants Some optical dopantsSome optical dopants But little is known about ion implantation doping: lattice sites ? damage annealing ?

Motivation: ion implantation in perovskites T recrystallization Amorphization threshold temperature [K]

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

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

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

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

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!

Annealing of radiation damage: RBS/C SrTiO 3 implanted with 56 Fe at doses of 1  5  cm  2 2 MeV 4 He + RBS/C minimum yield  min measured as function of T A  only for T A >1000°C remaining damage reaches values similar to a virgin crystal

Annealing of radiation damage: 89 Sr emission channeling  as was expected, 89 Sr (60 keV, 8  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 T A could not be derived (problem of angular resolution, mosaicity of implanted sample!)  analysis of experiment still in progress! S Sr S Ti following T A =1000°C  max ~ substitutional fraction

Annealing of radiation damage: PL Sample implanted with 89 Sr at a dose of ~10 14 cm  2 PL with 325nm excitation (above band edge 387 nm = 3.2 eV ) measured at 1.6 K as function of T A  near-band edge luminescence restored only for T A >800°C

Motivation: SrTiO 3 in the focus as dilute magnetic semiconductor (room-temperature ferromagnetism?)

Fe in SrTiO 3 As 3d transition metal element, Fe is a candidate for magnetic doping of SrTiO 3 (cf Mn) which lattice site does Fe occupy following ion implantation?  study by means of   emission channeling using the radioactive isotope 59 Fe (t 1/2 =44 d) low dose implantations (60 keV, 5   1  cm  2 ) What are the magnetic properties of Fe-implanted SrTiO 3 ?  study by means of SQUID high dose implantations (60 keV,  cm  2 )

Emission channeling lattice location: basic principle

The cubic perovskite lattice of SrTiO 3 u 1 (Sr)=.077 Å u 1 (Ti)=.061 Å u 1 (Sr)=.085 Å

59 Fe lattice sites in SrTiO 3 as function of T A  as-implanted: octahedral interstitial I 8 prominent + substitutional Ti  Fe on I 8 decreases fast  no damage recovery stage  no (or little) Fe on Sr sites  Fe prefers displaced Ti sites (  Å) + Ti sites

  emission channeling patterns, 59 Fe 60 keV cm  2 in SrTiO 3, as-impl. displ. S Ti I 8 sites

  emission channeling patterns, 59 Fe 60 keV cm  2 in SrTiO 3, T A =900°C displ. S Ti ideal S Ti sites

  emission channeling patterns, 59 Fe 60 keV cm  2 in SrTiO 3, T A =900°C displ. S Ti ideal S Ti ideal S Sr

SQUID magnetic moment of SrTiO 3 :Fe (H) 56 Fe 60 keV 5  cm  2, T A =900°C, SQUID measurement at 10 K Diamagnetism of SrTiO 3 substrateDiamagnetism of SrTiO 3 substrate Weak paramagnetism of implanted Fe and contaminants (0.5 ppm Cu 2+ ?) Ferromagnetism of implanted Fe

SQUID magnetic moment of SrTiO 3 :Fe (T) Paramagnetic component becomes obvious from temperature dependence: 1/T Brillouin paramagnetic vs constant diamagnetic + ferromagnetic

small ferromagnetic signals from virgin SrTiO 3 1  cm  2 Fe implanted magnetization of 5  cm  2 Fe implanted SrTiO 3   7.5  B ferromagnetic signature SQUID results for different Fe fluences  Note: 60 keV 1  cm  2 corresponds to [Fe] max = 1.8% /unit cell  implanted Fe exhibits ferromagnetism in SrTiO 3

Sources of magnetism in Fe-implanted SrTiO 3

Cu and Ag in SrTiO 3 As group Ib elements, Cu and Ag on Sr sites are candidates for p-type (acceptor) doping of SrTiO 3 which lattice site do they occupy following ion implantation?  study by means of   emission channeling using the radioactive isotopes 67 Cu (t 1/2 =63 h) and 111 Ag (7.5 d) Note: low dose implantations (60 keV, 5   1  cm  2 )

67 Cu and 111 Ag lattice sites in SrTiO 3 as function of T A  as-implanted: substitutional + octahedral interstitial I 8 sites  Cu and Ag on I 8 disappears fast  broad recovery stage of damage  °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 Sr ÅCu ÅAg Å Ti Å Cu ÅAg Å 67 Cu 111 Ag

Motivation: SrTiO 3 in the focus as red phosphor material flat panel displays

  emission channeling patterns, 169 Yb cm  2 in SrTiO 3, T A =900°C S Sr S Ti

169 Yb lattice sites in SrTiO 3 as function of T A  no octahedral interstitial I 8 sites (in contrast to TMs Fe, Cu, Ag)  also broad recovery stage of damage  °C,  Yb is also amphoteric (found on both Sr and Ti sites)  Yb prefers Ti sites

Overview: SrTiO 3 emission channeling lattice location experiments Electrical dopantsElectrical dopants Magnetic dopantsMagnetic dopants Optical dopantsOptical dopants Emission channeling lattice location experiments undertaken so far Emission channeling lattice location experiments undertaken so far Experiments foreseen or possible Experiments foreseen or possible Experiments attempted Experiments attempted

Conclusions SrTiO 3 relatively resistant against implantation damage damage annealing starts above °C... but is only complete at ~ °C  annealing is less efficient than expected lattice location experiments in SrTiO 3 difficult to analyze: –Cu, Ag, Yb are amphoteric impurities: multiple sites –Fe also shows site distribution: 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

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