Radiation Defects

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• Radiation Defects
• in Alkali Halides and Oxides
• A.I. Popov
• Institute of Solid State Physics, University of
  Latvia, LV

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REI-15, Padova, Sept.1, 2009

• Basic Properties of Radiation-Induced Point
  Defects in Halides and Oxides
• A.I. Popov, Max Planck Institute, Stuttgart and
  Institute of Solid State Physics, University of
  Latvia, LV
• E.A. Kotomin, Max Planck Institute, Stuttgart and
  Institute of Solid State Physics, University of
  Latvia
• J. Maier, Max Planck Institute, Stuttgart

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The is no doubt that F center in AHC may be
decribed as an electron trapped on anion vacancy.
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Optical absorption by F centers in alkali
halides
1.Shape of the band is single Gaussian in almost
all AHC KK0exp-a(h?max -h?)2 2. The half-width
depends on T H(T)/H(0)2cothh?/2kT) 3. It was
found experimentally that in alkali halides for
F-band absorption the relation Eabs 16.75 eV/(a
Å)1.772 holds quite well!
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Radiation Defects
Ionizing 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 monotonically in the series Vc, Va and
F center 1.19 eV, 1.44 eV and 1.64 eV In 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 monotonically in the series Vc, Va, F
and F center (2.43, 2.50, 2.72, and 3.13 eV,
respectively).
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Radiation Damage Processes
1. Electronic processes 2. Elastic collisions
Five types of radiation may produce displaced
atom or ions (1) ?- rays, (2) energetic
electrons, (3) thermal neutrons, (4) fast
neutrons, (5) energetic atoms or ions 3.
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
ion
The available energy, Egap (in fact Ex lt Egap)
gt 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) site Non-radiative transitions,
allowing an efficient kinetic energy transfer to
an atom, must prevail over radiative
transitions
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• Could work in
• alkali halides
• (anions and cations)
• alkaline-earth halides
• Difficult in
• oxides

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Elastic collisions
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 maximum energy 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 1.022)/A
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Displacement energy
Other materials II-VI ZnS 7-9/15-20 ZnSe
7-10/6-8 CdTe 6-9/5-8 CdSe 6-8/8-12 III-V GaAs
9/9.4 InP 6.7/8.7 InAs 6.7/8.3 Group IV C
25 graphite 35-80 diamond Si
13 Ge 13-16
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F-H pair Formation in alkali halides
Self-trapped Exciton ? F-H pair
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Resistant and sensitive materials

• Resistant
• Metals, semi-conductors.
• crystalline Oxides
• metastables (SrTiO3, MgO, Al2O3, c-SiO2)
• Sensitive
• Alkali halides
• Alkaline-earth halides CaF2, MgF2, SrF2
• KMgF3, BaFBr, LiYF4
• Silver halides AgCl AgBr
• Amorphous solids a-SiO2 , a-As2Se3, a-As2S3,
  a-Se, a-As
• Water and organic mater (bio matter)

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Radiolysis versus ballistic damage

• Radiolysis is not universal, not easily
  predictable
• 2) Is in essence temperature dependent
• 3) Spans over a wide time scale
• 4) Occurs generally on one sub-lattice (anions)
• 5) Radiolysis occurs occasionally
• when it occurs, it is with a good energetic
  efficiency. Elastic damage occurs every time
• but with a relatively poor energetic efficiency.

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Charge-carriers self-trapping
Self trapping of charge carriers results from a
competition between deformation and polarisation
of the lattice
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Radiation Defects
1.Electronic defects, which involve changes in
valence states Examples KClTl Tl hole
? Tl2 Tl electron ? Tl0 MgOFe
etc Fe2 hole ? Fe3 Fe3 electron ?
Fe2
n-irradiated MgO
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In this talk

• 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 materials

Type Self-trapping Self-trapping Formation of defects Formation of defects Some
exciton hole Single excitation Dense excitation examples
1 no no no yes MgO, CaO
2 yes yes yes yes Alkali halides
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RABIN AND KLICK DIAGRAM
P D Townsend 1973 J. Phys. C Solid State Phys. 6
961-966
Data for Cs-halides with CsCl-structute are
absent !!!
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CsI

• 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 CsITl crystals with Tl ion
  concentration of about 1017 ion/cm3 have been
  supplied by Dr. P. Schotanus (SCIONIX, Holland).
• The highly doped CsITl 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.

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F centre production in CsI crystals under ion
irradiation at 15 K86Kr ions (8.63 MeV/amu)
Evolution of the optical absorption spectra of
CsI under irradiation at 15 K with fluences 1011
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/D0.43 CsBr - 8  102
eV/centre S/D0.32 CsI - 2.5  107
eV/centre S/D0.17
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EXTENSION of The Rabin and Klick diagram
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Photoconversion 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.
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Hole Centeres in MgO
V- center - hole trapped on an oxygen
neighboring a cation vacancy. They are produced
by UV-light, X-rays, or low-energy ions Optical
absorption band at 2.3 eV A half-life time at
RT 2-7 year
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Hole Centeres in MgO
V0 center - two hole trapped on an oxygens
neighboring a cation vacancy. Optical
absorption band at 2.36 eV A half-life time at
RT 10 hours
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Hole 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)
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Hole Centeres in MgO
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Photoconversion of F centers in
neutron-irradiated MgO
3296 cm-
3323 cm-
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Differential spectrum of the n-irradiated MgO
crystals before and after UV irradiation for 50
min.
Fe2 h ? Fe3.
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During thermal annealing conversion F center??
colloid band NaCl, KCl, KBr etc 350 ?? T ? 500
K MgO, Al203 etc ?????
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MgO 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.

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MgO vacancy diffusion
MgO- The activation energy for diffusion is
found to increase monotonically in the series Vc,
Va, F and F center (2.43, 2.50, 2.72, and 3.13
eV, respectively).
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Dynamics of F-center annihilation in TCR MgO
F concentration (a) sample N-1 2 x1017 cm-3
Activation energy 1.9 eV (a) sample N-2
2 x1017 cm-3 Activation energy 2.5
eV (c) sample N-3 5 x1018 cm-3
Activation energy 3.4 eV To 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 14001650 K. Eact is 3.4
eV which agrees well with the theoretical energy
(3.1 eV) of the F-center elementary jump Eact
values of 1.9 and 2.5 eV are significantly lower
and hence can not be attributed to 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 dislocations and grain boundaries)
or with more mobile defects (such as magnesium
vacancies) Mg vacancy F-center ? ionised F
center
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Dynamics of F-center annihilation in TCR MgO
F concentration (a) sample N-2 2 x1017
cm-3 (b) (c) sample N-3 5 x1018
cm-3 Normalised concentration of (a) F centers
in sample MgO II, (b) F centers in sample MgO
III, (c) 3.593.35 eV absorption band in MgO III
against isochronal annealing temperature. Assumin
g a first order kinetics, an activation energy
for F-center diffusion was estimated for sample
III to be 3.4 ? 0.6 eV, in good agreement with
theoretical calculations
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Dynamics of F-center annihilation in TCR MgO
5 x1018 cm-3 Unexpected Results brown
coloration due to a broad extinction band
centered at 3.59 eV (345 nm).
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Nanocavities 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.
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?exp345 nm From Mie theory ?exp320 nm This
extinction band has been attributed to Mie
scattering from nano-size cavities with typical
dimensions of 3 nm, coated with magnesium metal.
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Specimens 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
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Optical absorption by F centers in alkali
halides with NaCl structure
F center in AHC was decribed as an electron
trapped on anion vacancy It was found
experimentally that in AHC for F-band
absorption the relation Eabs 16.75 eV/(a
Å)1.772 holds quite well! Particlein-a-box type
model E3.14(i2j2k2)/2a2 Transition energy
from GS(ijk1) to the first excited
state (2,1,1) (1,2,1) or (1,1,2) is given as Ea
3(3.14)2/ 2a2 Particlein-a-box type model
---gt Electron in halogen vacancy
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Optical absorption by F centers in alkali
halides with CsCl structure
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Comparison of LiF and MgO
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Mollwo-Ivey rule (extension)
It was found experimentally that in alkali
halides for F-band absorption the relation Eabs
16.75 eV/(a Å)1.772 holds quite well! It works
also for oxides (MgO, SrO, CaO) sulfids (CaS,
SrS, BaS) This confirm Particlein-a-box type
model ---gt Electron in halogen (or oxygen, or
sulphur) vacancy
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Optical absorption spectra of MgO crystal 1)
after TCR 2) after subsequent uv
irradiation 3) after neutron-irradiation MgO
crystal up to a dose of 6.91018 neutrons/cm2
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Conclusion

• 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

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• Thank you very much for your attention