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, USA
Acknowledgements
University of Dundee
Ross Mackie, Gurmeet Kanda
FRMII-NEPOMUC beamline
Christoph Hugenschmidt (Technische Universität München, ZWEFRM 11)
European Commission Programme RII3-CT-2003-505925
FRMII-NEPOMUC VE-PALS instrument station
Werner Egger (Universität Bundeswehr München)
TEM
L. Jin and C. L. Jia, Peter Grünberg Institute, Research Centre Jülich
Non-stoichiometry in thin film SrTiO3
Extreme cation non-stoichiometry
Ti – rich
Sr – rich
B-site
Amorphous TiO2
Ruddlesden-Popper SrO layer phases
A-site
Cation Vacancies?
VSr inferred from inhomogenous TEM
contrast modulations
Ohnishi, et al. J. Appl. Phys. 103, 103703, (2008).
VSr inferred from modelling O-K edge ELNES spectra
4
VT i
Mizoguchi, et al. Appl. Phys. Lett. 87, 241920 (2005)
Tokuda, et al. Appl. Phys. Lett. 99, 033110 (2011).
2
VS r
Positrons trap at missing atom defects, open volume
defects: antimatter traps at sites of missing matter
Positron annihilation spectroscopy (PAS) methods
have ppm-level sensitivity
PAS methods, combined with DFT, can detect and
identify vacancy defects
Three PAS methods: here we report positron lifetime
spectroscopy measurements
Positron Lifetimes
Positron lifetime sensitive to electron density
1

  r0 c  
2


 r  n  r   dr
E+
EB
Negligible e+ trapping
V+ positive
Good e+ trapping
V0 neutral
Excellent e+ trapping
VO: 2+
V− negative
Rydberg states
B-site
4−
A-site
2−
Positron Annihilation Lifetime Spectroscopy
Standard Trapping Model (STM)
Positron source
e+
Thermalization

Defect Free Bulk Lattice
dn ( t )
dt
 I 1 1 exp( 
1
1
t )  I 2  2 exp( 
1
2
t)
kD Trapping
Defect
1 
B Annihilation
D
B
511 keV
Annihilation Radiation
511 keV
 B U LK 
1
B  k D
I1  1  I 2
2 
I2 
1
D
kD
 B  1  k D
1
B
The bulk positron lifetime is a
characteristic of a given material
E+
EB
Lifetime 1
Value less than bulk
lifetime:
reduced bulk lifetime
Lifetime 2
‘fixed’ at the
defect value
Positron Annihilation Lifetime Spectroscopy
Two Defect – STM
Positron source
e+
Thermalization
Defect concentration [D]
k  D D
Defect Free Bulk Lattice
kD1
 D  Defect specific trapping
coefficient
What if the concentration of
one/both vacancy is ‘very’ large?
1 
B
D1 D2 Annihilation
Annihilation Radiation
2 
B  k1  k 2
Reduced bulk
Defect 2
Defect 1
1
I1  1  ( I 2  I 3 )
kD2 Trapping
I2 
1
3 
 D1
k D1
I3 
 B   D1  k D1  k D 2
I2
I1

D 2
k D2
 B   D 2  k D1  k D 2
Vacancy 2
Vacancy 1
Saturation trapping occurs: 1 and I1 tend to zero
1
kd2
k d1

d 2 d2 
 d 1  d1 
15  1
 D  2  10 s at . Saturation trapping occurs for  D   50 ppm
B-site
A-site
4−
2+
4
2−
2
2
VT i
VS r
VO
DFT-MIKA
Torsti, et al., Phys. Status Solidi B 243, 1016 (2006)
Mackie et al. Phys. Rev. B 79 014102 (2009)
Keeble et al. Phys. Rev. Lett. 105 226102 (2010)
e+ enhancement: AP : Arponen and E. Pajanne, Ann. Phys. (N.Y.) 121, 343 (1979); B. Barbiellini, et al Phys. Rev. B 53, 16201 (1996).
 (VTi) = 195 ps
 (bulk) = 152ps
 (VO) = 161 ps
 (VTi)relax = 189 ps
O ion relaxation: +5.2 %
Sr ion relaxation: - 8.4 %
 (VSr) = 280 ps
 (VSr) relax = 281 ps
Tanaka et al. Phys. Rev. B 68 205213 (2003)
O ion relaxation: +3.7 %
Ti ion relaxation: - 2.1 %
Variable Energy - Positron Annihilation Spectroscopy
Start
5 × 108 e+ s-1 at 1 keV
NEPOMUC beam line
Acceleration 0.5 – 21 keV
Stop
e+
> 5 x 106 counts / spectrum
0.511 MeV
Experiment station
Variable Energy – Positron Annihilation Lifetime Spectroscopy
(VE-PALS)
Variable Energy - Positron Annihilation Lifetime Spectroscopy (VE-PALS)
SrTiO3
Film
Start
Acceleration 0.5 – 21 keV
Stop
e+
0.511 MeV
SrTiO3 Substrate
Un-doped Pulsed Laser Deposited (PLD) SrTiO3 on SrTiO3 Thin Films
Sebastian Wicklein and Regina Dittmann (Jülich)
HR x-ray diffraction [002]
Ti-poor
Sr-poor
Strontium (Sr) excess
Un-doped PLD SrTiO3 on SrTiO3 Thin Films
Sr-poor
DFT-MIKA
(ps)
Bulk
152
VO
159
VTi
189
VSr
281
deconvolved e+ states
deconvolved e+ states
280 ps
280 ps
183 ps
SrTiO3
SrTiO3 Substrate
Keeble et. al. Phys. Rev. Lett. 105 226102 (2010)
183 ps
Un-doped PLD SrTiO3 on SrTiO3 Thin Films
F = 1.50 J cm-2
F = 2.00 J cm-2
ALL films show saturation e+ trapping
[VA/B] > 50-100 ppm
La-doped Hybrid MBE SrTiO3 on SrTiO3 Thin Films
Bharat Jalan and Susanne Stemmer (UCSB)
[La]  8 x 1017 cm-3
[La]  3 x 1019 cm-3
La-doped Hybrid MBE SrTiO3 on SrTiO3 Thin Films
[La]  8 x 1017 cm-3
Cluster  400 ps
VSr = 280 ps
VTi = 183 ps
1 < Bulk  155 ps
La-doped Hybrid MBE SrTiO3 on SrTiO3 Thin Films
[La]  3 x 1019 cm-3
Cluster  400 ps
VSr = 280 ps
VTi = 183 ps
1 < Bulk  155 ps
Hybrid MBE SrTiO3:La - estimate of cation vacancy concentration
Reduced bulk lifetime component,  <  B (155 ps), due to annihilation events with perfect lattice.
[La]  8 x
1017
cm-3
k[VSr] = 1.6(2) x 1010 s -1
[La]  3 x
1019
k V  IV
cm-3
k[VSr] = 5.1(1.5) x
109
Sr
s
-1
[VSr]  1.7(5) x 1016 cm -3
[VSr]  5.4(6) x 1016 cm -3
Sr
 1
1



 V Sr
 RB
V Sr  
V
kV
Sr
V
Sr




?
No value measured in
oxides, estimated values
for negative vacancies in Si
2–29× 1015 s ̶ 1
Sr
Assume:  = 5 x 1015 s -1
E = 4.5 – 8 keV:
B(STM) = 157(8) ps
E = 4.5 – 7 keV:
B(STM) = 154(7) ps
Single crystal SrTiO3
[Mackie PRB 2009 79 014102]
B(STM) = 155(4) ps
Un-doped Pulsed Laser Deposited (PLD) SrTiO3 on SrTiO3 Thin Films
Sebastian Wicklein and Regina Dittmann (Jülich)
Ti-poor
Sr-poor
Strontium (Sr) excess
Un-doped Pulsed Laser Deposited (PLD) SrTiO3 on SrTiO3 Thin Films
Sebastian Wicklein and Regina Dittmann (Jülich)
Ti-poor
Sr-poor
Un-doped PLD SrTiO3 on SrTiO3 Thin Films
2-term fit
1.33 Jcm-2
3-term fit
2-term fit
1.17 Jcm-2
3-term fit
Un-doped PLD SrTiO3 on SrTiO3 Thin Films
VPbVTi3VO
DFT
344 ps
Cluster  420 ps
Un-doped PLD SrTiO3 on SrTiO3 Thin Films
Cluster  420 ps
VPbVTi3VO
DFT
4
VT i
344 ps
Silicon
355 ps 5 vacancies
430 ps 10-14 vacancies
Hakala, PRB 57, 7621 (1998)
Staab, PRB 65, 115210 (2002)
2
VS r
Conclusions
SrTiO3 thin films grown by PLD with varying laser fluence (F):
Exhibit saturation trapping e+ to both VTi and to VSr defects for all films in the range
1.5 ≤ F ≤ 2.0 Jcm-2
Good 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 VTi , likely due to higher
defect specific trapping coefficient
‘Sr-poor’ (Dc = 0.2 pm) F = 2.0 Jcm-2 film:
e+ trapping dominated by VSr
Sr-poor
Conclusions
SrTiO3 thin films grown by PLD with varying laser fluence (F):
Ti-poor
Cluster  420 ps
2
VS r
4
VT i
4
VT i
Conclusions
Hybrid-MBE SrTiO3 shows a reduced bulk lifetime – a fraction of positrons annihilate
from perfect lattice.
Previous measurements of laser ablated SrTiO3 thin films have observed saturation
positron trapping.
Near-surface 50 nm contains small vacancy cluster defects.
VSr = 280(4) ps
The strontium vacancy, VSr , is the dominant cation vacancy
The concentrations were estimated to be 5.4(6) x 1016 cm -3 for the [La]  8 x 1017 cm-3
film 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.