Transcript No Slide Title

Radiation Defects
in Alkali Halides and Oxides
A.I. Popov
Institute of Solid State Physics, University of
Latvia, LV
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
The is no doubt that F center in AHC may be decribed as an electron trapped on anion vacancy.
Optical absorption by F centers in alkali halides
1.Shape of the band is single
Gaussian in almost all AHC
K=K0exp[-a(hmax -h)2
2. The half-width depends on T
[H(T)/H(0)]2=coth[h/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!
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).
Type of Defect
Alkali Halides
Oxide
Other II-VI
Anion Vacancy
V+ = simple vacancy
V0 = F
V- = F 
V++ = simple vacancy
V+ = F +
V0 = F
Cation
Vacancy
V- = simple vacancy
V0 = V F
V2- = simple vacancy
VV0
F, F+ and simple
vacancy reported
(MgS, CaS, SrS,
BaS
V2- = simple
vacancy
VV0
Polyvacancies
Divacancy
M = F2
M += F2 +
M = F2R = F3
R+ = F 3 +
R = F3N = F4 etc
colloids
I0 = H center
I+ = simple cation
intersitial
I = simple anion
intersitial
Anion vacancy
aggregates
F2
F2 +
F22+
nanocavities
Self-trapped hole
Self-trapped exciton
V2 and V3 centers
Small polarons in
some systems
Intersitials
Other intrinsic
defects
O2- in fluorite
structure
oxides
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 < 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) site
Non-radiative transitions, allowing an efficient kinetic energy transfer to an atom,
must prevail over radiative transitions
Could work in
alkali halides
(anions and
cations)
alkaline-earth
halides
Difficult in
oxides
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
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
F-H pair Formation in alkali halides:
Self-trapped Exciton  F-H pair
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)
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.
Charge-carriers self-trapping
Self trapping of charge carriers results from
a competition between deformation and polarisation of the lattice
STE:
BeO-YAG
MgO, Al2O3
Radiation Defects
1.Electronic defects, which involve changes in valence
states
Examples: KCl:Tl+
Tl+ + hole  Tl2+
Tl+ + electron  Tl0
MgO:Fe etc
Fe2+ + hole  Fe3+
Fe3+ + electron  Fe2+
n-irradiated MgO
In this talk:
1.
2.
3.
F center production in Cs-halides. Show the extension of RabinKlick 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
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
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 !!!
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 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.
F 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
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/D=0.43
2
CsBr - 8 × 10 eV/centre
S/D=0.32
7
CsI - 2.5 × 10 eV/centre
S/D=0.17
absorption (arb.units)
100
80
15 K
C s I (p u re )
60
C s I-T l (lo w -d o p e d )
40
20
C s I-T l (h ig h ly -d o p e d )
0
0,0
5,0x10
11
1,0x10
12
1,5x10
12
fluence
2,0x10
12
EXTENSION of The Rabin and Klick diagram
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.
Hole Centeres in MgO
V- center hole trapped on an oxygen
neighboring a cation
vacancy.
They are produced by
UV-light, X-rays, or lowenergy ions
Optical absorption band
at 2.3 eV
A half-life time at RT:
2-7 year
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
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)
Hole Centeres in MgO
Centre
VV0
VAl
VF
VOD
VOH
Na0
OA(eV)
2.33
2.36
2.33
Half-life
years
10 h
10 -15 h
hours
2.21
1.51
10 h
Photoconversion of F+ centers in neutron-irradiated MgO
3296 cm3323 cm-
Differential spectrum of the n-irradiated MgO crystals before and after UV
irradiation for 50 min.
Fe2+ +h+ → Fe3+.
During thermal annealing
conversion
F center colloid band
NaCl, KCl, KBr etc
350  T  500 K
MgO, Al203 etc ?????
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.
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).
Dynamics of F-center annihilation in TCR MgO
F concentration
(a) sample N-1
(a) sample N-2
(c) sample N-3
2 x1017 cm-3
2 x1017 cm-3
5 x1018 cm-3
Activation energy = 1.9 eV
Activation energy = 2.5 eV
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 1400±1650 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
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 cm3
Normalised concentration of
(a) F centers in sample MgO II,
(b) F centers in sample MgO III,
(c) 3.59±3.35 eV absorption
band in MgO III against
isochronal annealing
temperature.
Assuming 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
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).
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.
exp=345 nm
From Mie theory:
exp=320 nm
This extinction band has been
attributed to Mie scattering
from nano-size cavities with
typical dimensions of 3 nm,
coated with magnesium
metal.
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 fieldemission 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
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!
Particle–in-a-box type model:
E=3.14(i2+j2+k2)/2a2
Transition energy from GS(i=j=k=1)
to the first excited state
(2,1,1); (1,2,1) or (1,1,2) is given as
Ea = 3(3.14)2/ 2a2
Particle–in-a-box type model --->
Electron in halogen vacancy
Optical absorption by F centers in alkali
halides
with CsCl structure
optical density
1,0
CsCl
Energy (eV)
2.2
CsBr
15 K
1 - C sI-Tl (low -doped)
0,8
2 - C sI - pure
0,6
0,4
CsI
0,2
2.0
1
2
1,5
1,6
1,7
1,8
1,9
energy (eV)
1.8
1.6
4
4,5
Lattice constant (Å )
5
Comparison of LiF and MgO
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:
Particle–in-a-box type model --->
Electron in halogen (or oxygen, or sulphur)
vacancy
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 6.9·1018
neutrons/cm2
Conclusion:
1.
2.
3.
F center production in Cs-halides. Show the extension of RabinKlick 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
Thank you very much for your attention