Vacancy defect detection and characterization in SrTiO 3 thin films by positron lifetime...
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Vacancy defect detection and characterization in SrTiO3 thin films by positron lifetime spectroscopy
David J. KeebleCarnegie Laboratory of Physics, University of Dundee
Dundee, DD14HN, Scotland, UK
Sebastian Wicklein and Regina DittmannPeter Grünberg Institute, Forschungszentrum Jülich,
52425 Jülich, Germany
Bharat Jalan and Susanne StemmerMaterials Department, University of California, Santa Barbara,
California 93106-5050, USA
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 Kanda
Non-stoichiometry in thin film SrTiO3
A-site
2SrV
B-site
4TiV
Cation Vacancies?
VSr inferred from inhomogenous TEM contrast modulations Ohnishi, et al. J. Appl. Phys. 103, 103703, (2008).
Ti – rich Amorphous TiO2
Sr – rich Ruddlesden-Popper SrO layer phases
Extreme cation non-stoichiometry
VSr 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).
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
20
1r c dn
r rr
V+ positiveNegligible e+ trapping
V0 neutralGood e+ trapping
V− negativeRydberg states
Excellent e+ trapping
B-site4−
VO: 2+
A-site
2−
E+
EB
Defect Free Bulk Lattice
B
e+ Positron source
Annihilation
Thermalization
Annihilation Radiation
1BULK
B
B
Defect
TrappingkD
D
Positron Annihilation Lifetime Spectroscopy
E+
EB
Lifetime 1Value less than bulk lifetime:reduced bulk lifetime
Lifetime 2‘fixed’ at the defect value
1 1 2 21 2
( ) 1 1exp( ) exp( )
dn tI t I t
dt
1 2
1 2 21
1 1
1
B D D
D
B D
I I I
Standard Trapping Model (STM)
The bulk positron lifetime is a characteristic of a given material
511 keV
511 keV
D D
D Defect specific trapping coefficient
2 222
1 1 1 1
dd
d d
dI
I d
Defect concentration [D]
Reduced bulk Vacancy 1 Vacancy 2
Annihilation Radiation
Defect Free Bulk Lattice
Defect 1
B D1
kD1
e+ Positron source
Trapping
Annihilation
Defect 2
kD2
D2
Thermalization
Two Defect – STM
21
1
D
32
1
D
11 2
1
B
1 2 31 ( ) I I I 12
1 1 2
D
B D D D
I 23
2 1 2
D
B D D D
I
15 12 10 .D s at Saturation trapping occurs for 50D ppm
What if the concentration of one/both vacancy is ‘very’ large?
Saturation trapping occurs: t1 and I1 tend to zero
Positron Annihilation Lifetime Spectroscopy
B-site
4TiV
4−2OV
2+
A-site
2SrV
2−
DFT-MIKATorsti, et al., Phys. Status Solidi B 243, 1016 (2006)
t (VTi) = 195 ps t (VSr) = 280 ps
O ion relaxation: +5.2 %Sr ion relaxation: - 8.4 % Tanaka et al. Phys. Rev. B 68 205213 (2003)
t (VTi)relax = 189 ps
O ion relaxation: +3.7 %Ti ion relaxation: - 2.1 %
t (VSr) relax = 281 ps
t (VO) = 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) = 152ps
Variable Energy - Positron Annihilation Spectroscopy
5 × 108 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 106 counts / spectrum
NEPOMUC beam line
Variable Energy - Positron Annihilation Lifetime Spectroscopy (VE-PALS)
0.511 MeV
Stop
e+
Start
e+
Acceleration 0.5 – 21 keV
SrTiO3 Film
SrTiO3 Substrate
Un-doped Pulsed Laser Deposited (PLD) SrTiO3 on SrTiO3 Thin Films
Ti-poor Sr-poor
Strontium (Sr) excess
HR x-ray diffraction [002]
Sebastian Wicklein and Regina Dittmann (Jülich)
SrTiO3 SrTiO3 Substrate
Un-doped PLD SrTiO3 on SrTiO3 Thin Films
DFT-MIKA (ps)
Bulk 152
VO 159
VTi 189
VSr 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-poor
Un-doped PLD SrTiO3 on SrTiO3 Thin Films
F = 2.00 J cm-2F = 1.50 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
tVSr = 280 ps
tVTi = 183 ps
tCluster 400 ps
t1 < tBulk 155 ps
La-doped Hybrid MBE SrTiO3 on SrTiO3 Thin Films [La] 3 x 1019 cm-3
tVSr = 280 ps
tVTi = 183 ps
tCluster 400 ps t1 < tBulk 155 ps
Hybrid MBE SrTiO3:La - estimate of cation vacancy concentration
Reduced bulk lifetime component, t < t B (155 ps), due to annihilation events with perfect lattice.
tB(STM) = 157(8) ps
E = 4.5 – 8 keV:
tB(STM) = 154(7) ps
E = 4.5 – 7 keV:
tB(STM) = 155(4) ps
Single crystal SrTiO3 [Mackie PRB 2009 79 014102]
Sr
Sr
VSr
V
V
Assume: m = 5 x 1015 s -1
SrV ?
No value measured in oxides, estimated values for negative vacancies in Si 2–29× 1015 s ̶ 1
[VSr] 5.4(6) x 1016 cm -3
[VSr] 1.7(5) x 1016 cm -3
1 1Sr Sr
Sr
V VVRB
I
k[VSr] = 5.1(1.5) x 109 s -1k[VSr] = 1.6(2) x 1010 s -1
[La] 3 x 1019 cm-3[La] 8 x 1017 cm-3
Un-doped Pulsed Laser Deposited (PLD) SrTiO3 on SrTiO3 Thin Films
Strontium (Sr) excess
Ti-poor Sr-poor
Sebastian Wicklein and Regina Dittmann (Jülich)
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 2-term fit3-term fit 3-term fit
1.33 Jcm-2 1.17 Jcm-2
Un-doped PLD SrTiO3 on SrTiO3 Thin Films
tCluster 420 ps
VPbVTi3VO
DFT 344 ps
Un-doped PLD SrTiO3 on SrTiO3 Thin Films
tCluster 420 ps
VPbVTi3VO
DFT 344 ps
430 ps 10-14 vacancies355 ps 5 vacancies
Hakala, PRB 57, 7621 (1998)Staab, PRB 65, 115210 (2002)
Silicon4TiV
2SrV
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):
tCluster 420 psTi-poor
4TiV
2SrV
4TiV
Conclusions
Hybrid-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 SrTiO3 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-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.
The strontium vacancy, VSr , is the dominant cation vacancy
tVSr = 280(4) ps