3 thin films by positron lifetime spectroscopy David J Keeble Carnegie Laboratory of Physics University of Dundee Dundee DD14HN Scotland UK Sebastian Wicklein and Regina Dittmann ID: 462890
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Slide1
Vacancy defect detection and characterization in SrTiO3 thin films by positron lifetime spectroscopy
David J.
Keeble
Carnegie Laboratory of Physics, University of Dundee
Dundee, DD14HN, Scotland, UK
Sebastian
Wicklein
and Regina
Dittmann
Peter
Grünberg
Institute,
Forschungszentrum
Jülich
,
52425
Jülich
, Germany
Bharat
Jalan
and Susanne Stemmer
Materials Department, University of California, Santa Barbara,
California 93106-5050, USASlide2
L. Jin and C. L. Jia, Peter Grünberg Institute, Research
Centre Jülich TEM
Acknowledgements
European
Commission
Programme
RII3-CT-2003-505925
FRMII-NEPOMUC
beamline
Christoph Hugenschmidt (Technische Universität München, ZWEFRM 11)
FRMII-NEPOMUC VE-PALS instrument station
Werner Egger (Universität Bundeswehr München)
University of Dundee
Ross Mackie,
Gurmeet
KandaSlide3
Non-stoichiometry in thin film SrTiO3
A-site
B-site
Cation
Vacancies?
V
Sr
inferred from
inhomogenous
TEM contrast modulations
Ohnishi, et al.
J. Appl. Phys.
103
, 103703, (2008).
Ti – rich
Amorphous TiO
2
Sr
– rich
Ruddlesden
-Popper
SrO
layer phases
Extreme
c
ation
n
on-stoichiometry
V
Sr
inferred from
modelling
O-
K
edge ELNES
spectra
Mizoguchi
,
et al.
Appl. Phys.
Lett
.
87, 241920 (2005
)
Tokuda
,
et al.
Appl. Phys.
Lett
. 99, 033110 (2011).Slide4
Positrons trap at missing atom defects, open volume defects: antimatter traps at sites of missing matterPositron annihilation spectroscopy (PAS) methods have ppm-level sensitivity PAS methods, combined with DFT, can
detect and identify vacancy defectsThree PAS methods: here we report positron
lifetime
spectroscopy measurementsSlide5
Positron LifetimesPositron lifetime sensitive
to electron density
V
+
positive
Negligible
e
+
trapping
V
0
neutral
Good
e
+
trapping
V
−
negative
Rydberg
states
Excellent
e
+
trapping
B-site
4−
V
O
: 2+
A-site
2−
E
+
E
BSlide6
Defect Free Bulk Lattice
B
e
+
Positron source
Annihilation
Thermalization
Annihilation Radiation
B
Defect
Trapping
k
D
D
Positron Annihilation Lifetime Spectroscopy
E
+
E
B
Lifetime 1
Value less than bulk lifetime:
reduced
bulk lifetime
Lifetime 2
‘fixed’ at the defect value
Standard Trapping Model (STM)
The bulk positron lifetime is a characteristic of a given material
511
keV
511
keVSlide7
Defect specific trapping
coefficient
Defect concentration [D
]
Reduced bulk
Vacancy 1
Vacancy 2
Annihilation Radiation
Defect Free Bulk Lattice
Defect 1
B
D1
k
D1
e
+
Positron source
Trapping
Annihilation
Defect 2
k
D2
D2
Thermalization
Two Defect
–
STM
Saturation trapping
occurs for
What
if
the concentration of one/both
vacancy
is ‘very’ large?
Saturation trapping
occurs:
t
1
and
I
1
tend to zero
Positron Annihilation Lifetime Spectroscopy Slide8
B-site
4−
2+
A-site
2−
DFT-MIKA
Torsti,
et
al
., Phys. Status Solidi B 243, 1016 (2006)
t
(
V
Ti
) = 195 ps
t
(
V
Sr
) = 280 ps
O
ion
relaxation
: +5.2 %
Sr
ion
relaxation
: - 8.4 %
Tanaka
et
al.
Phys. Rev.
B
68
205213
(
2003)
t
(
V
Ti
)
relax
= 189 ps
O
ion
relaxation
: +3.7 %
Ti
ion
relaxation
: - 2.1 %
t
(
V
Sr
)
relax
= 281 ps
t
(
V
O
) = 161 ps
Keeble
et
al.
Phys. Rev.
Lett
.
105
226102 (2010)
Mackie
et al
.
Phys. Rev. B
79
014102 (
2009)
e
+
enhancement
:
AP
:
Arponen
and E.
Pajanne
, Ann. Phys. (N.Y.)
121
, 343 (1979); B.
Barbiellini
, et al Phys. Rev. B
53
, 16201 (1996).
t
(
bulk
) = 152psSlide9
Variable Energy - Positron Annihilation Spectroscopy
5 × 10
8
e
+
s
-1
at 1 keV
Variable Energy – Positron Annihilation Lifetime Spectroscopy (VE-PALS)
0.511
MeV
Stop
e
+
Start
e
+
Experiment station
Acceleration 0.5 – 21
keV
> 5 x 10
6
counts / spectrum
NEPOMUC beam lineSlide10
Variable Energy - Positron Annihilation Lifetime Spectroscopy (VE-PALS)
0.511
MeV
Stop
e
+
Start
e
+
Acceleration 0.5 – 21
keV
SrTiO
3
Film
SrTiO
3
SubstrateSlide11
Un-doped Pulsed Laser Deposited (PLD) SrTiO3 on SrTiO3 Thin Films
Ti-poorSr
-poor
Strontium (
Sr
)
e
xcess
HR x-ray diffraction [002]
Sebastian Wicklein and Regina
Dittmann (Jülich) Slide12
SrTiO
3
SrTiO
3
Substrate
Un-doped
PLD
SrTiO
3 on SrTiO
3 Thin Films
DFT-MIKA
(
ps)
Bulk
152
V
O
159
V
Ti
189
V
Sr
281
deconvolved
e
+
states
deconvolved
e
+
states
Keeble
et. al.
Phys. Rev.
Lett
.
105
226102 (2010)
280
ps
183
ps
280
ps
183
ps
Sr
-poorSlide13
Un-doped PLD SrTiO3 on SrTiO3
Thin Films
F = 2.00 J cm
-
2
F = 1.50 J cm
-2
ALL
films show saturation
e
+
trapping
[V
A/B
] > 50-100 ppm Slide14
La-doped Hybrid MBE SrTiO3 on SrTiO3 Thin Films Bharat Jalan and Susanne Stemmer (UCSB)
[La]
8 x 10
17
cm
-3
[La] 3 x 10
19 cm-3Slide15
La-doped Hybrid MBE SrTiO3 on SrTiO3 Thin Films [La]
8 x 1017 cm-3
t
VSr
= 280
ps
t
VTi
= 183 pst
Cluster 400 ps
t
1 < tBulk
155 psSlide16
La-doped Hybrid MBE SrTiO3 on SrTiO3 Thin Films [La]
3 x 1019 cm-3
t
VSr
= 280
ps
t
VTi
= 183 pst
Cluster 400 ps
t1 <
tBulk 155 psSlide17
Hybrid MBE SrTiO3:La - e
stimate of cation vacancy concentration Reduced bulk lifetime component,
t
<
t
B
(155
ps), due to annihilation events with perfect lattice.
t
B(STM) = 157(8) ps
E = 4.5 – 8 keV:
t
B(STM) = 154(7) ps
E = 4.5 –
7
keV
:
t
B
(STM) = 155(4)
ps
Single crystal
SrTiO
3
[
Mackie
PRB 2009 79 014102]
Assume
:
m
= 5 x 10
15
s
-1
?
No value measured in oxides, estimated values for negative vacancies in Si
2–29
× 10
15
s
̶ 1
[
V
Sr
]
5.4(6) x 10
16
cm
-3
[
V
Sr
]
1.7(5) x 10
16
cm
-3
k
[
V
Sr
] = 5.1(1.5) x 10
9
s
-1
k
[
V
Sr
] = 1.6(2) x 10
10
s
-1
[La]
3 x 10
19
cm
-3
[La]
8 x 10
17
cm
-3Slide18
Un-doped Pulsed Laser Deposited (PLD) SrTiO3 on SrTiO3 Thin Films
Strontium (Sr) excess
Ti-poor
Sr
-poor
Sebastian Wicklein and Regina
Dittmann (Jülich) Slide19
Un-doped Pulsed Laser Deposited (PLD) SrTiO3 on SrTiO3 Thin Films Sebastian Wicklein and Regina
Dittmann (Jülich)
Ti-poor
Sr
-poorSlide20
Un-doped PLD SrTiO3 on SrTiO3 Thin Films
2-term fit
2-term fit
3
-term fit
3
-term fit
1.33 Jcm
-2
1.17 Jcm
-2Slide21
Un-doped PLD SrTiO3 on SrTiO3
Thin Films
t
Cluster
420
ps
V
Pb
V
Ti
3V
O
DFT 344
psSlide22
Un-doped PLD SrTiO3 on SrTiO3
Thin Films tCluster
420
ps
V
Pb
V
Ti
3VO
DFT 344 ps
430
ps
10-14 vacancies
355
ps
5 vacancies
Hakala
,
PRB 57
, 7621 (1998
)
Staab
,
PRB
65, 115210 (2002
)
SiliconSlide23
ConclusionsSrTiO3 thin films grown by PLD with varying laser fluence (F):
Exhibit saturation trapping e+ to both V
Ti
and to
V
Sr
defects for
all films in the range 1.5 ≤ F ≤ 2.0 Jcm
-2Good agreement between MIKA calculated relaxed structure e+
lifetimes for VTi and to VSr (189 ps and 281 ps
) defects and experiment (183 ps and 280
ps)‘Stoichiometric‘ F = 1.5 Jcm-2 (Dc
= 0.0 pm) film: e+ trapping dominated by
V
Ti
, likely due to higher defect specific trapping coefficient
‘
Sr
-poor’ (
D
c
= 0.2 pm) F = 2.0 Jcm
-2
film:
e
+
trapping dominated by
V
Sr
Sr
-poorSlide24
ConclusionsSrTiO3 thin films grown by PLD with varying laser fluence (F):
tCluster
420
ps
Ti-poorSlide25
ConclusionsHybrid-MBE SrTiO3 shows a reduced bulk lifetime – a fraction of positrons annihilate from perfect lattice.
Near-surface 50 nm contains small vacancy cluster defects. Previous measurements of laser ablated SrTiO
3
thin films have observed saturation positron trapping.
The concentrations were
estimated
to be
5.4(6) x 1016 cm
-3 for the [La] 8 x 1017
cm-3film and 1.7(5) x 1016 cm -3 for the [La] 3 x 1019
cm-3 film.
These vacancy concentrations are at least an order of magnitude lower than the La concentrations.The strontium vacancy,
VSr , is the dominant cation
vacancy
t
VSr
= 280(4)
ps