Detection of oxygen vacancy migrations in Fe-

doped SrTiO3 by second harmonic

generation and photoluminance

Yuhang Ren and Steve Greenbaum

Physics and Astronomy

Hunter College, City University of New York

1

Collaboration with Clive Randall and Russell Maier from PSU

Outline

• Defect physics of oxygen migrations in Fe doped SrTiO3

• Principles and setups based on second harmonics generation (SHG) to detect defects in Fe:STO near the interface

• SHG signals from intrinsic, reduced, and oxidized Fe:STO samples with and without an applied electric field.

• Analysis of SHG results and discussions.

• Photoluminance in Fe:STO samples

• Future plan

2

Ferroelectric Perovskites

• Applications in capacitor, piezoelectric devices and sensors

• Excellent performances under high electric fields and high

temperature (high electrical resistivity, efficient supporter of

electrostatic fields)

• Defects play a fundamental role in determining failure mechanisms

Fe

Role of Interfaces and Defects

• Grain boundaries, domain walls and/or electrode interfaces contribute to the degradation and thermal breakdown process.

• Surfaces and interfaces in dielectric structures determine the underlying polarization switching and ionic properties that in turn, determine the overall ionic degradation and thermal breakdown processes in ferroelectric devices.

R. Sarathi, et al., Microstructure and Processing 2007; E. Pop, 2008.

Metal electrode

Metal electrode

Dielectric ceramic layer

at least 5-7 grains

P type n type Intrinsic

With applied voltage

+ -

Reduced and Oxidized SrTiO3 doped Fe

Both reduced and oxidized samples were annealed and equilibrated

under the conditions marked by the grey bar below. They have been

quenched to freeze-in the defect concentrations.

Oxidized Reduced

From Russell and Clive @PSU

Types of Iron Centers in SrTiO3

tetragonal distortion defect complex

a

a

c

a

a

c

a

a

c

Jahn-Teller

TiFe

TiFe Ti OFe V

non-centrosymmetric non-centrosymmetric centrosymmetric

From Russell and Clive @PSU

6

Second Harmonic Generation (SHG)

Contrast is found at interfaces or defect locations due to broken symmetry

2

𝑃𝑘 = 𝜀0(𝜒𝑖𝑘1

𝐸𝑖 + 𝜒𝑖𝑗𝑘2

𝐸𝑖𝐸𝑗 + 𝜒𝑖𝑗𝑘𝑙3

𝐸𝑖𝐸𝑗𝐸𝑙 + ⋯ )

The interface sensitivity of the SHG process

results from its coupling to the broken inversion

symmetry at the interface (e.g. defect induced),

while contributions from the centrosymmetric

bulk material are suppressed!

7

Polarization-dependent SHG set up

to extract 𝜒𝑧𝑥𝑥 𝜒𝑥𝑥𝑧 𝜒𝑧𝑧𝑧

𝜆/2 wave plate

polarizer

Apply a 𝜆/2 wave plate to continuously change polarization of the incident beam

Apply a polarizer to choose the SHG signals from three fixed output polarizations: p, s and d.

The SHG experimental apparatus: Ti:Sapphire pulsed laser (Tsunami, Spectra Physics)

is employed as the light source. The laser beam is sent to the SrTiO3 doped with 0.01%

Fe to create the SHG signal.

𝜒𝑧𝑥𝑥 𝜒𝑥𝑥𝑧 𝜒𝑧𝑧𝑧 are SHG components directly related oxygen vacancy migrations, defect clusters, and Jahn-Teller distortions

8

Symmetry in STO crystal is 4mm with z

as the fourfold axis perpendicular to the

interface. This symmetry allows only

three independent nonzero contributions

to the SHG susceptibility compared to 27

components in 𝜒𝑖𝑗𝑘:

(i) 𝜒𝑧𝑥𝑥= 𝜒𝑧𝑦𝑦,

(ii) 𝜒𝑥𝑥𝑧= 𝜒𝑥𝑧𝑥= 𝜒𝑦𝑦𝑧= 𝜒𝑦𝑧𝑦,

(iii) 𝜒𝑧𝑧𝑧. Allowed O(2p)-Ti(3d) 2ω transitions for

all three 𝜒𝑖𝑗𝑘 components within the 4mm

symmetry group

The three nonzero contributions 𝜒𝑧𝑥𝑥 𝜒𝑥𝑥𝑧 𝜒𝑧𝑧𝑧 are associated with symmetry-allowed O(2p)-Ti(3d) 2ω transitions relevant to defects

and they contribute to the second harmonic signals.

9

From A4-A7, we obtained the SHG vs. alpha

and we can subtract 𝜒𝑥𝑥𝑧, 𝜒𝑧𝑥𝑥, 𝜒𝑧𝑧𝑧 components by placing p, d, and s polarizations to understand the nature of

defects. 𝜒𝑧𝑧𝑧:

: orbital reconstruction, 𝜒𝑧𝑥𝑥, 𝜒𝑥𝑥𝑧: underlying polarization switching and ionic

properties. 𝜒𝑧𝑧𝑧 is much bigger than 𝜒𝑥𝑥𝑧 , 𝜒𝑧𝑥𝑥 .

10

Theoretical analysis of the SHG signal in perovskite oxides

A. Rubano, M. Fiebig et al., Phys. Rev. B 83, 155405 (2011). E. D. Mishina et al., JAP 93, 6216 (2003).

Multidomains

0.0

0.5

1.0

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0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

xxz

=0.0039

zzz

=0.32139

SH

G s

ign

al (a

.u.)

p_out fit

p_out sample

d_out fit

d_out sample

zxx

=-0.23759

Intrinsic STO

Theoretical analysis of the intrinsic STO sample to subtract three important fitting parameters

11

Our results show that the defects in the reduced sample have affected the SHG signals through 𝜒𝑥𝑥𝑧, 𝜒𝑧𝑥𝑥, 𝜒𝑧𝑧𝑧.

SHG of intrinsic (SrTiO3), oxidized, and reduced

SrTiO3:Fe (0.01wt%) Samples

0

2

4

0

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4

SH

G s

ignal (a

.u.)

p-out

d-out

s-out

intrinsic STO

0

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reduced STO

SH

G s

ign

al (a

.u.)

p-out

d-out

s-out

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oxidized STO

Pola

riza

tio

n o

f in

cid

en

t be

am

(d

eg

ree

)

p-out

d-out

s-out

12

0

5

10

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reduced STO

p-out

SH

G s

ign

al (a

.u.)

900 V

0

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15

0

30

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330

0

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reduced STO

p-out

SH

G s

igna

l (a

.u.)

1000 V

0

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10

0

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330

0

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10

reduced STO

p-out

SH

G s

igna

l (a

.u.)

1200 V

0

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10

0

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0

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reduced STO

p-out

SH

G s

igna

l (a

.u.)

1400 V

0

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10

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330

0

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10

reduced STO

p-out

SH

G s

igna

l (a

.u.)

1500 V

0

5

10

0

30

60

90

120

150

180

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240

270

300

330

0

5

10

SH

G s

ign

al (a

.u.)

0 V

reduced STO

p-out

Second symmetry/phase was introduced due to oxygen ion (O2-) migrations

when an electric filed is applied at the reduced Fe:STO anode: SHG phase

shift appears due to the appearance of a new distorted phase ( a rhombohedral

phase or monoclinic phase)

13

0

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reduced STO

p-out

SH

G s

ign

al (a

.u.)

0 V

900 V

1200 V

1500 V

Oxygen ion/vacancy migration dynamics

(1) When enough voltage is applied, oxygen vacancy begins to move and

accumulate in the STO side of interface resulting in additional asymmetry

observed in the SHG signal (phase shift); (2) Each oxygen ion/vacancy is

associated with a local dipole. The SHG intensity is then applied to describe as

the local dipole field.

14

Oxygen ion moving towards

anode side under applied

voltage Oxygen ion

accumulation induces the

formation of a distorted T/R

phase (SHG phase shift)

Further increase of applied

field introduces local dipole

field into the anode side

0

5

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20

0

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0

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oxidized STO

p-out

Pola

rization o

f in

cid

ent beam

(degre

e)

0 V

0

5

10

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20

0

30

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0

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oxidized STO

p-out

Pola

rization o

f in

cid

ent beam

(degre

e)

900 V

0

10

20

30

0

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330

0

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oxidized STO

p-out

Pola

rization o

f in

cid

ent beam

(degre

e)

1000 V

0

10

20

30

0

30

60

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150

180

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330

0

10

20

30

oxidized STO

p-out

Po

lari

za

tio

n o

f in

cid

en

t b

ea

m (

de

gre

e)

1200 V

0

10

20

30

40

0

30

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0

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oxidized STO

p-out

Pola

riza

tio

n o

f in

cid

en

t be

am

(d

eg

ree

) 1500 V

Oxidized sample consists of mixtures of all three types of defect centers. But only

SHG intensity changes are observed in the oxidized sample (due to crystal distortion

and formation of clusters (Prosandeev’s model): the SHG intensity is proportional to

oxygen vacancy/surface ion concentration and therefore the distributions of these

defects) 15

0

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oxidized STO

p-out

Pola

riza

tio

n o

f in

cid

en

t be

am

(d

eg

ree

)

0 V

900 V

1200 V

1500 V

0 400 800 1200 160010

15

20

25

30

35

Am

plit

ud

e o

f S

HG

(a

.u.)

Voltage Applied (V)

reduced STO

oxidized STO

16

Our observations: (1) the anode side of a reduced sample becomes more and more

oxidized with increasing applied field. But only small oxygen surface states are

formed; (2) Oxygen surface states in the oxidized sample show a significant change

with the oxygen accumulation at the anode side.

For the reduced sample,

the SHG intensity is

slightly decreased with

increasing applied

voltage. In contrast, the

SHG increases about

75% within the same

region of voltage for the

oxidized sample.

0 400 800 1200 1600

0

5

10

15

20

25

30

reduced STO

oxidized STO

Pha

se c

ha

ng

e o

f S

HG

(de

gre

e)

Voltage Applied (V)

17

We attribute that (1) a new distorted phase is introduced into the anode side of the

reduced sample with increasing applied field. The distortion of the crystal structure

due to oxygen accumulation near the surface changes the spontaneous polarization

away from the [001] direction; (2) the distortion in the oxidized sample is almost

negligible with adding more oxygen ions at anode surface.

For the oxidized sample,

the SHG phase is only

slightly changed with the

applied voltage. In contrast,

the SHG increases about 25

degree within the same

region of voltage for the

reduced sample.

Wavelength dependence of SHG in oxidized STO

0

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5

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15

Pola

riza

tio

n o

f in

cid

en

t be

am

(d

eg

ree

) 710 nm 1.74 eV 750 nm 1.65 eV

810 nm 1.53 eV

870 nm 1.42 eV

oxidized STO

SHG intensity in oxidized STO decreases with decreasing photon energy which is

the same as the results from PL measurements indicating a peak near 350 nm in

oxidized Fe(0.01wt%):STO (different excitation bands + resonant effect!).

18

Some important observations from SHG

• Oxygen vacancy/ion migration dynamics: oxygen ion moving under applied voltage the reduced/oxidized samples may be separated into p-type, n-type, and intrinsic regions (phase localization) charge gradient formation at high applied fields

• Crystal distortions under applied fields: coexistence of tetragonal and rhombohedral phases due to imposed epitaxial strains in both reduced and oxidized samples.

• Electronic structure mapping by a wavelength dependent SHG and photoluminescence studies.

19

A broad luminescence band is present in the range of 380–600 nm with a maximum

at 520 nm. The spectrum is interpreted as a disordered surface creating several

localized bandgap states. Two weak shoulders locate at 430 and 595 nm. The signal

at 595 nm originates from the Fe4+ ionization/oxygen surface states. The shoulder at

430 nm is attributed to surface point defects related to oxygen vacancies and

surface states. 20

Xu et al., J. Appl. Phys. 114, 154106 (2013). Hanzig et al., J. Appl. Phys. 110, 064107 (2011).

Oxygen vacancy state

Fe4+ state

Before the degradation, the oxygen vacancy/surface state can be observed in PL spectra both for reduced and oxidized samples. The peak is more significant in the oxidized sample than that in the reduced sample because of the surface oxygen population.

21

2.53.03.54.0

Pt-O Charge Transfer

(3.51eV)

Oxygen vacancy/Electron PCCD (2.97eV)

Self-Trapped Excitons (2.67eV)

Cumulative Fit

PL

In

ten

sit

y (

CP

S)

Emission Energy (eV)

Excitation @ 280nm

Reduced Fe(0.01):STO

BL: Oxygen vacancy/

Surface states

Yang et al., J. Am. Ceram. Soc., 94, 1811–1816 (2011)

22

With the applied field, the PL intensity for the surface oxygen states increases significantly and the Pt/oxygen cluster states are clearly identified at the anode side for the reduced samples. The peak is more significant in higher applied fields because of the increase of extrinsic defect population.

2.5 3.0 3.5 4.0

PL

In

ten

sit

y (

CP

S)

Surface oxygen states

Pt-O Charge Transfer States

Emission Energy (eV)

Reduced

Degraded Reduced

Oxidized

Pt/O

Anode with applied field

PCCD: principal charge

compensating defect

Yang et al., J. Am. Ceram. Soc., 94, 1811–1816 (2011)

+ -

23

After degradation, the 2.9 eV peak doesn’t show significant changes with applied field in both the cathode and anode sides in reduced sample. This is due to the degradation of Fe:STO structures. The degradation finally causes the diffusion of defects from anode to cathode side.

2.4 2.7 3.0 3.3 3.60.0

0.2

0.4

0.6

0.8

1.0

No

rmali

zed

PL

In

ten

sit

y [

a.u

.]

Photon Energy (eV)

0 V

700 V

1000 V

1300 V

Fe(0.01wt%):STO

Reduced Cathode

after degradation

Wojtyniak et al. J. Appl. Phys.

113, 083713 (2013)

Consistent results in photoluminescence (PL)

• Reduced sample: The oxygen vacancy and surface defect states around 2.9 eV are identified. The peak increases significantly at anode side when a high field was applied. Reduced oxidized transition

• Oxidized sample: The oxygen surface states are identified around 2.9 eV at both electrode sides. The peak gets more and more pronounced with an increasing applied field. Accumulation of surface oxygen ions.

Both oxygen vacancy and surface states are identified and the peak shows significant changes with the applied field before degradation, but do not change much after degradation

24

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K

M

KMHDk

M

KH

Ultrafast probing magnetic inhomogeneity in manganites

Y.Gong, Z. Zhang, D. Ascienzo, Y. Abranyos, H. B. Zhao, G. Lupke, Qi Li, Y. H. Ren, Europhysics Letters 108, 17010 (2014).

Outlooks:

26

Publications: (AFOSR support acknowledged) • Y.Gong, Z. Zhang, D. Ascienzo, Y. Abranyos, H. B. Zhao, G. Lupke, Qi Li, Y. H. Ren, Europhysics

Letters 108, 17010 (2014).

• “Experimental demonstration of 55-fs spin canting in photoexcited iron nanoarrays”, Yuhang Ren*, Wei

Lai, Zehra Cevher, Yu Gong, and G. P. Zhang, Nature Photonics (under review) 2015.

• “Optimization of the defects and nonradiative lifetime of undoped GaAs/AlGaAs double

heterostructures”, Z. Cevher, P. Folkes, H. Hier, B. VanMil, B. C. Connelly, W. Beck, and Y. H. Ren*,

(submitted).

• “Second harmonic detection of electric field induced oxygen migrations in Fe doped SrTiO3”, Haochen

Yuan, David Ascienzo, Onur Kurt, Steve Greenbaum, Russel Maier, Clive Randall, Yuhang Ren*, (in

preparation).

• “Probing the coupled breakdown processes near interfaces in dielectrics”, David Ascienzo, Onur Kurt,

Steve Greenbaum, Russel Maier, Clive Randall, Yuhang Ren*, (in preparation).

• Coupled breakdown dynamics by SHG and PL: imaging nanoscopic textures, crystallographic grains, and t the size and shape of the space charge regions

• Perovskite bicrystals for idealized grain boundary type behavior

• Ultrafast detection of elastic and magnetic clusters in dielectrics

Thanks for your attention

31

0 300 600 900 1200 1500

No

nlin

ea

r p

ara

me

ters

Applied Voltage (V)

d31

d15

d33

Oxidized Fe: STO