Dr. JOSÉ A. PARDO

Department of Materials Science and Technology,

& Aragón Institute of Nanoscience

University of Zaragoza

Pulsed laser deposition of oxide epitaxial thin films.

Recent results on Sr4Fe6O13

Pulsed Laser Deposition (PLD)

High-vacuum chamber

Substrate on substate heater

Rotating target (sintered ceramic)

O2 pressure control

Pulsed Laser Deposition (PLD)Advantages:

• Stoichiometric transfer of material (Complex oxides: YBa2Cu3O7-)

• Direct relation number of pulses- thickness ( 0.1-0.3 Å/pulse)

• Few experimental parameters (T, PO2)

PLA + D

Disadvantages:

• “Splashing” (solid particulates and liquid droplets)

• Angular distribution of ablated material cosn, n10 (small area or inhomogeneous thickness)

Pulsed laser-matter interaction

Roughly: I 104 - 105 W/cm2: heating I 105 – 107 W/cm2: melting I 107 – 1010 W/cm2: vaporization and plasma formation

Wavelength Pulse duration Energy per pulse EFocused on area S

Fluence = E/S

Peak power Pp = E/

Intensity I = Pp/S

S

Optical absorptivityThermal diffusivityOther properties...

PL-matterinteraction

Congruent ablation

Single target

> threshold

No target degradation

PLA-PLD: 10 ns 10 J/cm2

I 1 GW/cm2

UV excimerQ-switched Nd:YAG

D. B

ÄU

ER

LE

: “Laser P

rocessing and Chem

istry”. Springer (2000)

Thin film nucleation and growth

Deposited atom (adatom)

Hot atom

Diffusion to clusterDimer

2D-island

Atom reevaporation

Dissociation from cluster

3D-island

Cluster

Models for epitaxial growth

Free-energy:

s: substrate free surfacef: film free surfacei: substrate-film interface

f

s i

Models for epitaxial growth

Frank-Van der Merwe(2-D layer-by-layer)

s > f + i

Volmer-Weber(3-D islands)s < f + i

Stranski-Krastanov

Features of (epitaxial) thin films• “Single crytals”:

- Anisotropy- Very low density of high-angle grain boundaries

• High surface-to-volume ratio (surface effects)

• Some particualr growth-induced defects (stacking faults, misfit dislocations, buffer layers...)

• Epitaxial strain

• Influence of substrate (diffusion, chemical reactions at substrate/film interface...)

• Miniaturization (nanotechnology, sensors...)

• Alternated thin films: Multilayers and heterostructures (planar technology devices, magnetic tunnel junctions…)

MATERIALS WITH NEW PROPERTIES!

Epitaxial strain

Deformation of film lattice to match the substrate lattice

Strain: ≈ 1%

Hooke´s law: = E = F / Ao: stress, = l / lo: strain, Young modulus

Oxides: E ≈ 1011 Pa → Epitaxial stress: ≈ 1 GPaSubstrate choice:

• Compressive (af>as) or tensile (af<as) strain• Modulation of strain by substrate lattice parameter• Modulation of the film properties

Commensurate epitaxyCoherent strains

fs

aaa

mLattice mismatch:

mc·tc ≈ constant

La1.9Sr0.1CuO4 superconductors

PLDTc values:

Bulk LSCO: 25 K

LSCO/SrTiO3 (c): 10 K

LSCO/SrLaAlO4 (t): 49.1 K !!!

Multilayers of ionic conductors

MBE

Space charge region ≈ 2LD

PLD of Sr4Fe6O13 epitaxial films

PEOPLE INVOLVED:

• Barcelona - ICMAB: J. A. Pardo, J. Santiso, C. Solís, G. Garcia, M. Burriel, A. Figueras (PLD, CVD, XRD, XRR, SEM, Impedance) • Antwerp - EMAT: G. Van Tendeloo & M. D. Rossell

(TEM, HREM and ED)• Sacavém - ITN: J. C. Waerenborgh (Mössbauer)• Barcelona - ICMAB: X. Torrellas (Synchrotron) • Lisbon - FCUL: M. Godinho (Magnetism)

Sr4Fe6O13±

Parent member of the mixed conducting family Sr4Fe6-xCoxO13

Perovskite-type layer Sr-Fe-O

Fe-O double layer

a

b

c Intergrowth structure

Orthorhombic Iba2

a = 11.103 Å b = 18.924 Å c = 5.572 Å (A.. YOSHIASA et al., Mater.

Res. Bull. 21 (1986) 175)

x = 2: very high oxygen conductivity = el + i

Sr4Fe6O13/SrTiO3(100) filmsb-oriented. Cube-on-cube epitaxy

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160

XR

D in

tens

ity

(cps

)

100

101

102

103

104

105

106

2 (degrees)

(0

2 0)

SrT

iO3 (

0 0

1)

SrT

iO3 (

0 0

3)

SrT

iO3 (

0 0

2)

(0

4 0) (

0 6

0)

(0

8 0)

(0

10 0

)

(0

12 0

)

(0

14 0

)

(0

16 0

)

(0

18 0

)

(0

20 0

)

(0

22 0

) (0

24 0

)

SrT

iO3 (

0 0

4)

J. A. PARDO et al., Journal of Crystal Growth 262 (2004) 334

13 13.5 14 14.5 15XR

D in

tens

ity

(a.u

.)

(degrees)

0.3º

Lattice parameters vs. thickness

t < 30 nmfully strained films

t > 170 nmrelaxed films

Sr4Fe6O13/SrTiO3

Thickness range:t ≈ 15 – 300 nm

1,895

1,900

1,905

1,910

1,915

1,920

Ou

t-of

-pla

ne

par

amet

er (

nm

)b

o

SFO

out-of-plane

0 50 100 150 200 250 300 3500,390

0,391

0,392

0,393

0,394

Thickness (nm)

in-planeaSTO

do

(201)SFO

In-p

lan

e p

aram

eter

(n

m)

Epitaxial strain vs. thickness

~ t -1

for misfit dislocation-mediated plastic deformation

Sr4Fe6O13/SrTiO3(100)

10 1000,1

1

out-of-plane in-plane

Str

ain

(

)

Thikckness (t)

~ t -0.6

tc

Fullystrained

Relaxed

J. SANTISO et al., Applied PhysicsLetters 86 (2005) 132105

Oxygen content vs. thickness

Strained ( -0.8%)

Relaxed ( < -0.2%)

1,100 1,105 1,110 1,1150,40

0,41

0,42

0,43

0,44

0,45

12.82

12.86

12.84

Parameter a (nm)

12.88

Oxygen

conten

t 13-

Strained ( -0.8%)

Relaxed ( < -0.2%)

Sr4Fe6O13±/SrTiO3

films deposited underthe same O2 pressure

Oxygen superstructure with modulation vector

q = am*

13- = 12+2

Strain relaxation through change in oxygen superstructure

M. D. ROSSELL et al., Chem.Mater. 16 (2004) 2478

Conductivity measurements

10-7

10-6

10-5

10-4

10-3

10-2

0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

1000/T (K-1)

SrTiO3

NdGaO3

LaAlO3

(S

/cm

)

NdGaO3

substrates

Pt electrodes and wires

Impedance spectroscopyFurnace up to 800 ºCControlled atmosphere: O2, Ar…

Impedance analyzerHP-4192A (5 Hz - 13 MHz)

Sr4Fe6O13/NdGaO3(100) films

Plane matrix of Sr4Fe6O13±

Needle-like precipitates of SrFeO3-z

b-oriented films. Cube-on-cube epitaxy

1

10

102

103

104

105

0 20 40 60 80 100

Inte

nsit

y (c

.p.s

.)

2 (degrees)

(0 2

0)

(0 4

0)

(0 8

0)

(0 1

0 0)

(0 1

2 0)

(0 1

4 0)

(0 1

6 0)

(0 1

8 0)

(0 6

0)

Conductivity of SFO/NGO in O2

2

4

6

8

10

12

1.2 1.6 2 2.4

10 nm56 nm156 nm313 nmCeramic

ln

T (

-1 c

m-1

K)

1000/T (K-1)

O2

Strong dependence conductivity-thickness

J. A

. PA

RD

O e

t al.

Soli

d St

ate

Ioni

cs(s

ubm

itte

d)

Effect of stress on conductivity

Small polaron hopping: (T) = (A/T) exp(-Ea/kT)

0.1

1

10 100

A (

106

-1 c

m-1

K)

Thickness (nm)

Conductivity increases under compressive epitaxial stress

0.1

1

10 100

In-p

lan

e st

rain

xx

(%

)Thickness (nm)

SrTiO3

NdGaO3

Summary

• PLD is a versatile technique for the deposition of high-quality epitaxial thin films of oxides.

• The conductivity of epitaxial thin films of Sr4Fe6O13/NdGaO3(100) strongly depends on the film thickness.

• This dependence is most probably due to the effect of compressive epitaxial stress.