A. Zaltron, M. Bazzan, N. Argiolas,

M.V. Ciampolillo

and

Cinzia Sada

University of Padova, Physics Department

Local doping of Lithium Niobate crystals

by Fe diffusion: a depth-resolved study

of photorefractive properties

SIF – L’Aquila 26/9/2011

introduction

Fe:LiNbO3 doping and characterization

Photorefractive properties and optical set-up

depth-resolved holographic measurements

conclusion

Outline

The material

Uniaxis Ferroelectric material with electro-optic effect

<001>

Z axis

Lithium Niobate(LiNbO3)

1. Optical modulators

2. Integrated waveguides

New goals: optical memories by holographic recording

thanks to photorefractivity

Li

Li

Nb

Nb

O

Ps

Dno = -1

2no

3r13Ez

Dne = -1

2ne

3r33Ez r13≈9.6 pm/V r33≈31 pm/V

The photorefractive effect

1. presence of donor/acceptor centers

2. photoexcitation process at suitable wavelength in the bright region

3. trapping of charges in the dark region

Fe is one of the most exploited donor/center dopant

Photorefractive properties in LiNbO3

The refractive index pattern (i.e grating) is correlated to the light pattern

Electro-optic effect

Variation of the refractive index n induced by

inhomogeneous illumination

Photoexcitation

Photorefractive properties: physics

Esc = Esc,sat 1-e-t/tsc( )

Esc,sat µ Fe3+éë

ùû

1

t sc

µs ph µ IFe2+é

ëùû

Fe3+éë

ùû

j phv =ee0

¶Esc

¶t t @ 0µ Fe2+é

ëùû

Electric space charge field Esc depends on the exposition time t

Charge migration is due to drift, diffusion and bulk photovoltaic effect

Once center model and inhomogeneous illumination in Fe:LiNbO3

jtot = jdrift + jdiffusion + jphotovoltaic

Photorefractive properties

Inhomogeneous illumination

I (z) = I0

1+msin(K ×z)[ ]

m=Imax- Imin

Imax+ Imin

The refractive index pattern (grating)

is correlated to the light pattern

h ºI d

I d + I t

= sin2 p DnL

l cos q( )

æ

èçç

ö

ø÷÷

Final goal: integrated optical sensor

8

writing reading

signal

reference

K

ksig

kref

waveguides

waveguide

signal

reference

hologramUnknown

input

Output

reference

It

Id

Grating efficiency η=Id/(Id+It)

Applications

9

Input

I

NH3

NH3CH3COOH CH3H6O C2H5OH

OK!

Selective identification of chemical subtances by optical methods

λ

I

NH3

Integrated optical memories

Fe doping

Integrated optical devices

Bulk Fe: LiNbO3 samples

Locally doped Fe:LiNbO3 samples

Few works on Fe-diffused LiNbO3 samples (waveguide configuration)

limited to estimate the photorefractive properties

Need to measure the real photorefractive parameters and correlation

of all the physical properties

Fe:LiNbO3

Sample cutting: polished LiNbO3 x-cut slabs

Fe film deposition: RF magnetron sputtering

deposition rate: ( 4.15 ± 0.02) x 1014 at /s·cm2

Fe film quality: X-Ray Reflectivity (film thickness)

Atomic Force Microscope

Film thickness 5-20 nm

Density 5x1022 at/cm3

Iron deposition

1. Diffusion process:

2. Reducing treatments:

- wet O2 atmosphere

- 1100°C

- duration: 20h

- wet Ar (96%) + H2 (4%)

- 500°C

- duration: 20 min-6h

Fe3+ Fe2+

Longer reducing treatments increasing reduction

degree [Fe2+]/[Fe3+]

Iron in-diffusion

Fe semi-gaussian concentration profile

Secondary Ion Mass Spectrometry: determination of the total Fe content

Csup = (18.0 ± 0.4) 1024 at/m3

wFe = (21.3 ± 0.5) mm

Reduction treatment does not affect Fe distribution

0 10 20 30 40 50 60 700,0

5,0x1024

1,0x1025

1,5x1025

2,0x1025

as diffused Fe:LN

reduced Fe:LNF

e c

on

cen

tra

tio

n (

at/

m3)

depth (mm)

Compositional characterization

cFe(x)=NFe

p DFe te

-x2

4DFe t

ω2 = DFe · 2t

Iron in-diffusion

Ciampolillo PhD thesis, 2010

D = D0e-EA

KT

Activation energy

* Ciampolillo, Zaltron, Sada et al. Applied. Physics A, vol.104 n.1 p 453, 2011

EA=(2.8±0.1)eV

Reduction degree = [Fe2+]/[Fe3+]

3440 3460 3480 3500 3520 35400,10

0,11

0,12

0,13

0,14

0,15

0,16

0,17

abso

rbance

wavelength number (cm-1)

crystal

Fe:LN as-diffused

Fe:LN reduced 4.6%

Reducing treatment does not increase significantly the

Hydrogen content

Compositional characterization

Fe3+ +O2- +1

2H2 «Fe2+ +OH -

Nb5+ +O2- +1

2H2 «Nb4+ +OH -

VB

CB

Fe2+/Fe3+

C band 3.1 eV

A band 1.1 eV

D band 2.6 eV

532 nm Fe2+

Reduction degree

[Fe2+]/ [Fe3+]342 nm Fetot*Ciampolillo, Zaltron, Bazzan, Argiolas and C. Sada. Applied Spectroscopy, 65 (2): 216, 2010.

Optical characterization

Compositional characterization by measuring the optical

transmission T

as-diffused: [Fe2+]/[Fe3+] 0.18 %

reduced: [Fe2+]/[Fe3+] 4.6 %

Optical characterization

D band intensity is proportional only to Fe2+

Fe2+éë

ùû fluence

=OD

sFe2+

OD = logTFeLN

TpureLN

æ

èçç

ö

ø÷÷

Optical Absorption: involving the total amount of Fe2+

Estimation of absorption coefficient

asup at the surfacesup

sup 3

Fe

cFluence

DensityOpticala

Peak Transition

1.1 eV (1128 nm) 5A – 5E (Fe2+)

2.6 eV (477 nm) Fe2+ - Nb5+

3.1 eV (400 nm) O2- - Fe3+

2.57 eV– 2.91 eV(483nm – 426 nm)

d- d (Fe3+)

OD = -log (TLN/TFeLN)

Fluence (Fe2+) = OD/s532

Optical characterization

Structural characterization: H-XRD

Reciprocal space map (220)

Rocking curve w-2q to determine the lattice mismatch ξ

Lattice mismatch

dsub Interplanar distance in the substrate

(330)

x^ =d - dsubstrate

dsubstrate

Structural characterization

x surface = m1CFesurface +m2asurface

a surface = ODC

Fesurface

Fe[ ]fluence

*Ciampolillo, Sada et al. J. Appl. Phys. 107, 084108 (2010)

The maximum lattice deformation ξ induced at the surface of iron doped lithium

niobate crystal by thermal diffusion depends on:

1. Fe concentration

2. reduction degree

Holographic technique Refractive index grating as diffraction grating

One center model

Kogelnik’s theory

q

cossin2 Ln

II

I

td

d

Photorefractive properties

q Angle between the incident beam and fringes plane

Fe:LiNbO3

substrate

X - axis

Z - axis

Scans along x axis

Fe concentration CFe(x)

In-depth profiles of

photorefractive properties

Local study of photorefractive properties by light-

induced formation of m-holograms

Photorefractive properties

m - holograms Focused beamsConfined interference area

Holographic set-up

Zaltron PhD thesis, 2011

Interference area: probe size

5-10 mm

50.6 mm

X - axis

Z - axis

Z- axis

Y- axis

d2 = 983.6 mm

2qind1 = 50.6 mm

Holographic parameters

Angle between interference

beams

2qin = (5.89 ± 0.01)°

Grating parametersK = Kz = (1.22 ± 0.07) 106 m-1

L = (5.15 ± 0.30) mm

Maximum intensity Imax = (35 ± 4) 104 W/m2

Holographic parameters

Monitoring time evolution of Id t Esc(t)

0 5 10 15 20 250,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5

Esc (

x 1

06

V/m

)

t (s)

Hologram recording: space charge field build-up

exponential fit

One center model

Holographic measurements

Esc =2Dn

ne

3reff

Dn = arcsin h( )l cos(q )

pL

reff = r33 cos2 (q )- r13 sin2(q )+ne - no

ne

r33 + r13( )sin2(q )

I= 35 W/cm2

300025002000150010005000

0,51,01,52,02,5

3,0

3,5

4,0

9060

4030201050

depth (mm)

0 mm

5 mm

10 mm

20 mm

30 mm

40 mm

60 mm

90 mm

Esc,sat

(x 106 V/m)

t (s)

Diffused Fe:LN: recording curves

Scans along the diffusion direction (x axis) • Esc,sat decreases

• tsc increases

-50 0 50 100 150 200 2500

5

10

15

20

25

30

35

40

45

depth (mm)

Esc,s

at (

x 1

04 V

/m)

Undoped sample

0 20 40 60 80 1000

1

2

3

4

5

Esc,s

at (x

10

6 V

/m)

depth (mm)

experimental space-charge field

gaussian fit

diffused Fe:LN sample

Esc in-depth profile of diffused FeLN

Fe2+/Fe3+=0.18 %

Esc in-depth profile of diffused FeLN

Fe2+/Fe3+=0.18 %

tsc≈120s

top - viewHolographic erasing time

Erasing time depends on the dark conductivity of the doped material

Esc in-depth profile similar to cFe:

wE = (23.5 ± 1.3) mm

wFe = (21.3 ± 0.5) mm

3FeEsc

Enhanced photorefractive effect in Fe-doped samples

LiNbO3 Esc,sat = (3.5 ± 0.1) 105 V/m |n| = (5.7 ± 0.1) 10-5

Fe:LiNbO3 Esc,sat = (4.3 ± 0.4) 106 V/m |n|0mm = (7.3 ± 0.9) 10-4

Esc in-depth profile of diffused FeLN

0 20 40 60 80 1000

1

2

3

4

5

6

7

8

9

experimental data

fit

photo

conductivity (

x 1

0-1

2

-1 m

-1)

depth (mm)

[Fe2+]/[Fe3+] is not constant

in the doped layer

ws = (3.2 ± 0.9) mm Fe2+ confined near

the surface of the sample

1

t sc

µs ph

s ph µ[Fe2+ ] / [Fe3+ ]

Photoconductivity in as-diffused Fe:LN

New results!

Conclusions

0 20 40 60 80 1000,0

5,0x1024

1,0x1025

1,5x1025

2,0x1025

Fe tot

Fe2+

Fe3+

Fe

co

nce

ntr

atio

n (

m-3)

depth (mm)

Photovoltaic current in-depth profile proportional to Fe2+

Optical absorption total Fe2+ amount [ Fe2+] = 2.4 · 1019 at/m2

Space charge field proportional to Fe3+

Conclusions

Study of photorefractive mechanisms in Fe-diffused samples

- new optical set-up based on m-hologram recording

- in-depth profiles of photorefractive properties

- new insights on iron reduction mechanism

Iron doping allows great enhancement of photorefractivity

- high reproducibility

- for permanent grating, need of thermal fixing process

Work in progress

Detailed investigation of reduction process

influence of T, cFe, intrinsic defects on photorefractive response of Fe-

diffused crystals

Development of integrated devices based on Fe-diffused LiNbO3

- waveguide integration,

- exploitation of holographic stage in all-optical devices

- study of thermal fixing process for permanent refractive index grating

Lithium niobate

Compound system Li2O – Nb2O5

ne=2.23 and no=2.31

Li – Nb - vacancy

native defects: niobium antisites NbLi

Congruent composition

Li deficiency

Electro-optic effect ..1

,2

l

lk

kijkl

k

kijk

ij

EEsErn

[Li]1-5x [NbLi]x [VLi]4x [NbNb] O3

Data analysis conditions

thick hologramKogelnik’ equation

Samples with

[Fe3+] ~ 18 · 1025 m-3

[Fe2+]/[Fe3+] ~ 4%

mVEphv /108 6

mVED /103 5

mVEq /104 8

mVEq

/109 9'

Simplified solutions

Esc,sat ~ Ephv

q

cossin2 Ln

II

I

td

d

holograms thickness

L = (199 ± 8) mm

Undoped LiNbO3 samples

Repeatability Measurements relative error ~ 13%

In-depth measures

250200

150100500

0,1

0,2

0,3

0,4

-20-10

010

60200

-20 mm

-10 mm

0 mm

10 mm

60 mm

200 mm

Esc (

x 1

06 V

/m)

depth (mm)

t (s)

Esc increases when

interference area is

inside the sample

Esc and tsc constant

inside the crystal

A. Zaltron - Ph.D. Thesis

-50 0 50 100 150 200 2500

5

10

15

20

25

30

35

40

45

-20 -15 -10 -5 0 5 10

05

10152025303540

depth (mm)

depth (mm)

Esc,s

at (

x 1

04 V

/m)

Esc,s

at (

x 1

04 V

/m)

-50 0 50 100 150 200 250

0

2

4

6

8

10

12

14

16

18

depth (mm)

photo

conductivity (

x 1

0-1

2

m

-1)

Esc,sat = |Ephv| = (35.1 ± 0.9) 104 V/m

|n| = (5.7 ± 0.1) 10-5

(Althoff et al.)

sph = (13.4 ± 0.4) 10-12 -1m-1

(Buse et al.)

Undoped LiNbO3 samples

Reduced Fe:LiNbO3 samples

Validity of the one-center model in Fe:LN

One-center model

I 104 W/m2 I > 106 - 107 W/m2

Two-centers modelThis work

Imax = 3.5 · 105 W/m2

s ph µ I 2IbIaph s

0 5 10 15 20 25 30 35 400

100

200

300

400

500

Co

nd

uctivity (

x 1

0-1

2

m

-1 )

Intensity (x 104 W/m

2)

cFe

= 1.8 x 1025

at/m3

A. Zaltron - Ph.D. Thesis

Band transport model

with one

photorefractive center

Photoconductivity in depth profile

Nominally pure LiNbO3

sph = (13.4 ± 0.4) 10-12 -1m-1

Undoped area (at 90 mm) of Fe:LiNbO3

sph = (0.06 ± 0.01) 10-12 -1m-1

During diffusion process electrons move toward the higher concentration of empty traps ?

Photoconductivity in as-diffused Fe:LN

0 20 40 60 80 100 120

0

100

200

300

400

500

ph

oto

co

nd

uctivity (

x 1

0-1

2

-1 m

-1)

I = 5 x 104 W/m

2

I = 11 x 104 W/m

2

I = 35 x 104 W/m

2

depth (mm)

[Fe2+]/[Fe3+] not constant

as-diffused: ws = (3.2 ± 0.9) mm

reduced: ws = (15.5 ± 3.0) mm

sph in-depth profile

at 0 mm [Fe2+]/[Fe3+] ~ 12%

at 50 mm [Fe2+]/[Fe3+] ~ 1%

Photoconductivity in reduced Fe:LN

s ph µ I Fe2+éë

ùû / Fe3+é

ëùû

Photoconductivity in reduced Fe:LN

The reducting treatments:

1. increase the amount of Fe2+ in the material;

2. promote the diffusion of the reducing electrons (first

captured by the Fe3+ near the surface and then

migrate inside the material).

Agreement with iron concentration profile

wE = (22.1 ± 1.3) mm and wFe = (21.3 ± 0.5) mm

Esc does not depend on light intensity

Esc in-depth profile of reduced FeLN

Esc,sat = (3.9 ± 0.2) 106 V/m |n| = (6.6 ± 0.9) 10-4

Fe2+/Fe3+=4.6%

Agreement with iron concentration profile?

n in-depth profile of reduced FeLN

Dark conductivity

Negligible with light: I 1 W/cm2 at room temperature

Without light: erasing of refractive index grating

Protonic conductivity

Electrons tunneling between Fe atoms

- above 70°- 80°C

- at room temperature for [Fe] 0.1% mol

- at room temperature for [Fe] 0.5% mol

- depends on reduction degree

Dark conductivity

0 1x105

2x105

3x105

4x105

5x105

6x105

1,0x106

1,5x106

2,0x106

2,5x106

3,0x106

3,5x106

4,0x106

4,5x106

5,0x106

Esc,s

at (

V/m

)

experimental data

exponential fit: t3.68 ± 0.65xs

linear fit: t4.06 ± 0.44xs

time (s)

sdark = (6.4 ± 1.3) 10-16 -1m-1

phdarkphtot

sc

sssst

0

stot≈10-12 -1 m-1

0 20 40 60 80 100 120

0,0

2,5

5,0

7,5

10,0

12,5

15,0

17,5

20,0I = 35 W/m

2

I = 11.1 W/m2

I = 5.5 W/m2

ph

oto

vo

lta

ic c

urr

en

t (A

/m2)

depth (mm)

][ 2 FeIjphv

[Fe2+] profile differs

from [Fe3+] one

wj = (10.8 ± 1.6) mm

Electrons concentrated

at the surface of the sample

Surface reduction degree ~ 10%: agreement with photoconductivity values

Photovoltaic current in reduced Fe:LN