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