Presentation on theme: "in Alkali Halides and Oxides"— Presentation transcript:
1in Alkali Halides and Oxides Radiation Defectsin Alkali Halides and OxidesA.I. PopovInstitute of Solid State Physics, University of Latvia, LV
2REI-15, Padova, Sept.1, 2009Basic Properties of Radiation-Induced Point Defects in Halides and OxidesA.I. Popov, Max Planck Institute, Stuttgart and Institute of Solid State Physics, University of Latvia, LVE.A. Kotomin, Max Planck Institute, Stuttgart and Institute of Solid State Physics, University of LatviaJ. Maier, Max Planck Institute, Stuttgart
3The is no doubt that F center in AHC may be decribed as an electron trapped on anion vacancy.
4Optical absorption by F centers in alkali halides 1.Shape of the band is singleGaussian in almost all AHCK=K0exp[-a(hmax -h)22. The half-width depends on T[H(T)/H(0)]2=coth[h/2kT)3. It was found experimentally thatin alkali halides for F-band absorptionthe relationEabs= eV/(a Å)1.772holds quite well!
5Radiation DefectsIonizing radiation produces a variety of vacancy and intersitial type of point defects:In alkali halides: vacancy defects includes bare cation and anion vacancy, as well as halogen vacancy with one electron ( or F center).KCl - The activation energy for diffusion is found to increase monotonicallyin the series Vc, Va and F center 1.19 eV, 1.44 eV and 1.64 eVIn simple oxides: vacancy defects includes bare cation and oxygen vacancy, as well as oxygen vacancy with one or two electrons (F+ and F center).MgO- The activation energy for diffusion is found to increase monotonicallyin the series Vc, Va, F+ and F center (2.43, 2.50, 2.72, and 3.13 eV, respectively).
7Radiation Damage Processes 1. Electronic processes2. Elastic collisionsFive types of radiation may produce displaced atom or ions (1) - rays, (2) energetic electrons, (3) thermal neutrons, (4) fast neutrons, (5) energetic atoms or ions3. Radiolysis(1) Electronic excitation creation of an electronic defects(2) Conversion of this energy into kinetic energy of a lattice ion ion moves(3) The motion and stabilization of the ionThe available energy, Egap (in fact Ex < Egap)> the formation energy of the Frenkel pair.the radiolysis can only occurs in insulators or wide band-gap semiconductors.The excitation must be localised on one atomic (or molecular) siteNon-radiative transitions, allowing an efficient kinetic energy transfer to an atom,must prevail over radiative transitions
8Could work inalkali halides(anions and cations)alkaline-earth halidesDifficult inoxides
9Elastic collisions Td =2147.7E(E + 1.022)/A Defect Production rate as a function of irradiation energy for MgO under electron irradiation.The damage rate is strongly dependent on the energy.Threshold for radiation damage.For relativistic particles such as electrons, the maximumenergy Td (in eV) transferable from an incident electron of energy E (in MeV) to a lattice ion of mass number A is given by:Td =2147.7E(E )/A
10Displacement energy Other materials: II-VI ZnS 7-9/15-20 ZnSe 7-10/6-8 CdTe 6-9/5-8CdSe 6-8/8-12III-VGaAs 9/9.4InP /8.7InAs /8.3Group IVC graphite35-80 diamondSiGe
13Radiolysis versus ballistic damage Radiolysis is not universal, not easily predictable2) Is in essence temperature dependent3) Spans over a wide time scale4) Occurs generally on one sub-lattice (anions)5) Radiolysis occurs occasionallywhen it occurs, it is with a good energetic efficiency. Elastic damage occurs every timebut with a relatively poor energetic efficiency.
14Charge-carriers self-trapping Self trapping of charge carriers results froma competition between deformation and polarisation of the latticeSTE:BeO-YAGMgO, Al2O3
15Radiation Defects1.Electronic defects, which involve changes in valence statesExamples: KCl:Tl+Tl hole Tl2+Tl electron Tl0MgO:Fe etcFe hole Fe3+Fe electron Fe2+n-irradiated MgO
16In this talk: Type Self-trapping Formation of defects Some exciton F center production in Cs-halides. Show the extension of Rabin-Klick diagram for all AHC.Discuss differences between F center in AHC and F+ and F center in oxide materials (MgO as an example)Discuss whether common and famous Mollwo-Ivey rule could be extended for oxide materialsTypeSelf-trappingFormation of defectsSomeexcitonholeSingleexcitationDenseexamples1noyesMgO, CaO2Alkali halides
17Data for Cs-halides with CsCl-structute are absent !!! RABIN AND KLICK DIAGRAMP D Townsend 1973 J. Phys. C: Solid State PhysData for Cs-halides with CsCl-structute are absent !!!
18CsI Three different types of CsI crystals were studied in this paper. Nominally pure CsI crystals have been grown in the Laboratoire de Spectroscopie Atomique (CNRS/ISMRA, Caen).The low-doped CsI–Tl crystals with Tl+ ion concentration of about 1017 ion/cm3 have been supplied by Dr. P. Schotanus (SCIONIX, Holland).The highly doped CsI–Tl with Tl+ ion concentration of about 1019 ion/cm3 was obtained Institute of Solid State Physics, University of Latvia.Crystals have been irradiated at GANIL on the medium-energy beam line (SME) with 86Kr ions (8.63 MeV/amu).In this study, both the irradiation and in-situ measurements were done at 15 K.
19F centre production in CsI crystals under ion irradiation at 15 K 86Kr ions (8.63 MeV/amu) Evolution of the optical absorption spectra of CsI under irradiation at 15 K with fluences1011 ions/cm2 (1);3 × 1011 ions/cm2 (2); 6 × 1011 ions/cm2 (3); 9 × 1011 ions/cm2 (4); 1.2 × 1012 ions/cm2 (5); 1.6 × 1012 ions/cm2 (6); 2.0 × 1012 ions/cm2 (7).Production efficiency (eV/centre) of F band absorption for all cesium halides.CsCl - 7 × 103 eV/centre S/D=0.43CsBr - 8 × 102 eV/centre S/D=0.32CsI × 107 eV/centre S/D=0.17
22Photoconversion of F+ centers in neutron-irradiated MgO Experiments and theory demonstrate that photon excitation of the positively charged anion vacancies at 5.0 eV releases holes that are subsequently trapped at V-type centers, which are cation vacancies charge-compensated by impurities, such as Al3+, F−, and OH− ions. A photoconversion mechanism occurs very likely via electron transfer to F+ centers from the quasi-local states which are induced in the valence band. INDO quantum chemical simulations of F+ centers confirmed the appearance of two induced quasi-local states located at 1.2 and 2.0 eV below the top of the valence band.
23Hole Centeres in MgO V- center - hole trapped on an oxygen neighboring a cation vacancy.They are produced byUV-light, X-rays, or low-energy ionsOptical absorption bandat 2.3 eVA half-life time at RT:2-7 year
24Hole Centeres in MgO V0 center - two hole trapped on an oxygens neighboring a cation vacancy.Optical absorption bandat 2.36 eVA half-life time at RT:10 hours
25Hole Centeres in MgO Impurity-related V center holes are trapped oxygens neighboring a cation vacancy, which are charge compensators for impurities (OH-, F-, Al3+,Si4+ etc)
30MgO TCR samples The MgO crystals used were grown at the Oak Ridge National Laboratory using the arc fusion technique.The starting material was MgO powder from the Kanto Chemical Company, Japan.TCR was performed in a tantalum chamber at 2000 K and 7 atmospheres of magnesium vapor, followed by rapid cooling. This process produces anion oxygen vacancies, due to a stoichiometric excess of cations.
31MgO: vacancy diffusion MgO- The activation energy for diffusion is found to increase monotonicallyin the series Vc, Va, F+ and F center (2.43, 2.50, 2.72, and 3.13 eV, respectively).
32Dynamics of F-center annihilation in TCR MgO F concentration(a) sample N x1017 cm Activation energy = 1.9 eV(a) sample N x1017 cm Activation energy = 2.5 eV(c) sample N x1018 cm Activation energy = 3.4 eVTo explain these observations, we suggest that a direct manifestation of the intrinsic diffusion of F centers is their diffusion-controlled aggregation to ultimately form nano cavities in the temperature range of 1400±1650 K.Eact is 3.4 eV which agrees well with the theoretical energy (3.1 eV) of the F-center elementary jumpEact values of 1.9 and 2.5 eV are significantly lower and hence can not be attributedto migration of single F-centers. Thus, in samples MgO I and MgO II oxygen vacancies are annihilated either by forming dimer centers with selected impurities, which favours their joint diffusion to internal sinks (such as dislocationsand grain boundaries) or with more mobile defects (such as magnesium vacancies)Mg vacancy + F-center ionised F center
33Dynamics of F-center annihilation in TCR MgO F concentration(a) sample N-2 2 x1017 cm-3(b) (c) sample N x1018 cm-3Normalised concentration of(a) F centers in sample MgO II,(b) F centers in sample MgO III,(c) 3.59±3.35 eV absorptionband in MgO III against isochronal annealing temperature.Assuming a first order kinetics, an activation energy for F-center diffusion was estimated forsample III to be 3.4 0.6 eV, in good agreement with theoreticalcalculations
34Dynamics of F-center annihilation in TCR MgO 5 x1018 cm-3Unexpected Results:brown coloration due to abroad extinction band centered at 3.59 eV (345 nm).
35Nanocavities formation in MgO As the annealing temperature increased, the band became more intense, as it shifted toward lower energy. The band ultimately peaked at 3.35 eV It reached maximum intensity at 1673 K.
36exp=345 nmFrom Mie theory:exp=320 nmThis extinction band has been attributed to Mie scattering from nano-size cavities with typical dimensions of 3 nm, coated with magnesiummetal.
37Specimens for TEM studies were prepared by mechanical grinding, dimpling, and argon ion-milling with an acceleration voltage of 5 kV and an incident angle of 10°.TEM, x-ray microanalysis, and electron diffraction studies were carried out in a Philips CM200 field-emission analytical electron microscope operated at 200 kV and equipped with a Be specimen holder.Electron microscopy: TCR sample after annealing at 1673K in a reducing atmosphere.Areas with a high concentration of dislocations were separated by regions in which only small rectangular features are observed
38Optical absorption by F centers in alkali halides with NaCl structureF center in AHC was decribed asan electron trapped on anion vacancyIt was found experimentally thatin AHC for F-band absorptionthe relationEabs= eV/(a Å)1.772holds quite well!Particle–in-a-box type model:E=3.14(i2+j2+k2)/2a2Transition energy from GS(i=j=k=1)to the first excited state(2,1,1); (1,2,1) or (1,1,2) is given asEa = 3(3.14)2/ 2a2Particle–in-a-box type model --->Electron in halogen vacancy
39Optical absorption by F centers in alkali halides with CsCl structure
41Mollwo-Ivey rule (extension) It was found experimentally that in alkali halides for F-band absorptionthe relationEabs= eV/(a Å)1.772holds quite well!It works also foroxides (MgO, SrO, CaO)sulfids (CaS, SrS, BaS)This confirm:Particle–in-a-box type model --->Electron in halogen (or oxygen, or sulphur)vacancy
45Conclusion:F center production in Cs-halides. Show the extension of Rabin-Klick diagram for all AHC.Discuss differences between F center in AHC and F+ and F center in oxide materials (MgO as an example)Show that famous Mollwo-Ivey rule could be extended for some simple oxide and sulfide materials with NaCl structure