THIN LAYERS OF TRANSITION METAL OXIDES

Tjipke HibmaMaterials Science Centre, University of Groningen, The Netherlands

Contents

• Introduction to thin film deposition• Atomic layer-by-layer growth - Stoichiometry - Surface “chemistry” - Epitaxy - Morphology - Thickness

• Manipulation of properties, a few examples

Ultimate goal: Epitaxial growth of perfect thin layers with

atomic precision onto (selected parts of) a single crystalline substrate, in order to manipulate materials properties (or to design ultrathin devices).

IntroductionAtomic Layer-by-Layer Growth

• Substrate influence

• Finite size

• Epitaxial strain

• Artificial stacking

enforcement of geometric, magnetic and electronic structure (metastable phases, exchange bias, proximity effects, ..)

thickness < characteristic length(quantum wells, ballistic transport,..)

deformation(bandgap, level splittings)

new layered compounds or structures(high-Tc, new ferromagnetic(-electric) compounds)

IntroductionManipulation of materials properties by

K.Ueda, H.Tabata, T. Kawai, Science 280 (1998) 1064

Goodenough-Kanamori rules:Cr3+-O-Fe3+ (d3-d5) 180°-superexchange interaction is Ferromagnetic

LaCrO3-LaFeO3 Atomic Superlattices

Introduction

Physical DepositionPVD

Thermal Energetic

MBE PLD SPUTTERING

ALL-MBEALE

UHV PLD

Chemical DepositionCVD

MOCVDLACVDPECVD

Thin film deposition

most clean and precise deposition techniques

Advantages of MBE :• High purity elemental

sources• Abrupt interfaces • RHEED growth control • In-situ surface analysis

Disadvantages of MBE : • Slow• Sophisticated and

expensive UHV equipment

• Multi-element rate control difficult

Thin film depositionMolecular Beam Epitaxy (MBE)

Advantages of PLD :• Suitable for complex

materials • Fast and flexible• (RHEED growth control) • (In-situ surface analysis)

Disadvantages of PLD : • Particulates• Loss of volatile elements• Small area deposition

Thin film deposition(UHV-) Pulsed Laser Deposition (PLD)

Deposition

Diffusion NucleationGrowth

Desorption

Mixing

Arrival rates Fn

Temperature T

Main Growth Parameters

Energies En

Atomic layer-by-layer growthGrowth processes

Control of GrowthParameters

MBE PLD

Stoichiometry Relative Flux FnDifficult for n>2, ALL-MBE

Loss of volatile components.

Surface “Chemistry”

Temperature T Energies En

Tsubstrate

thermal <0.1 eV

Tsubstrate

0.1-10eV (Pback)

Epitaxy Substrate

Morphology(lbl growth mode)

Nucleation rate(Fn/Dn )

RHEED (RHEED)

Thickness(nr of layers)

Absolute Flux Fn RHEED, ALE

# Pulses,(RHEED)

Atomic layer-by-layer growth

Atomic Layer-by-layer MBE (ALL-MBE) (Eckstein and Bosovic, Annu. Rev. Mater. Sci., 25,679,1995)

Stoichiometry Control

Atomic absorption flux control and computer controlled shuttering of individual K-cell.

MBE of Binary Oxides

Stoichiometric MnOm

excess oxygen Nonstoichiometric MxOy

vary FM/FO, determine x afterwards:• Fe3-Moessbauer Spectroscopy• CrOx , XPS• VOx, TiOx (0.8<x<1.3), 18O-RBS

Stoichiometry Control

0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0

V2

O3

/ A l2

O3

2 7 A l

1 6 O1 8 O

5 1 V

In

te

ns

ity

(a

rb

.un

its

)

B a c k s c a t t e r i n g e n e r g y ( k e V )

2 0 0 2 2 0 2 4 0 2 6 0 2 8 0 3 0 0

1 6 O 1 8 O

V Ox

u n c a p p e d

In

te

ns

ity

(a

rb

. u

nit

s)

B a c k s c a t t e r i n g e n e r g y ( k e V )

1 8 O

V Ox

c a p p e d

V2O3 film on Al2O3 (0001) VOx film on MgO (100)

RBS spectra of 1.8 MeV He+ ions scattered from :

18O- RBS

Stoichiometry Control

3 4 3 2 3 0 2 8 2 6 2 4 2 2 2 0 1 8 1 6 1 4 1 2 1 0 8

0 . 4

0 . 5

0 . 6

0 . 7

0 . 8

0 . 9

1 . 0

1 . 1

1 . 2

1 . 3

1 . 4

x-

Ox

yg

en

co

nt

en

t

O x y g e n p r e s s u r e ( m V )

Stoichiometry Control18O- RBS of VOx

• Elements surface diffusion, nucleation• Binary Oxides diffusing species: M, O, MO ??• Complex Oxides ?????

Surface “Chemistry”

• Epitaxy = Well-defined orientation relationship

between substrate and film lattice.

• Coherent epitaxya afilm substrate

|| ||strain:

|| f

(misfit f = a/a)

Epitaxy

2

1 || f

Dislocation formation

Critical thickness:

tb

f

t

bco

c

FHG

IKJ

( cos )

( ) sin cosln

1

8 1

2

Epitaxy

Critical Thickness

Dislocation energy

Strain energy

tc thicknes t →Energ

y E

→

Epitaxy

Critical Thickness of CoO/MgO(001)

0 200 400 600 800 10000.000

0.002

0.004

0.006

0.008

0.010

Str

ain

in t

he

Co

O/M

gO

Layer thickness (A)

Experiment Theory

36.0 37.0 38.0 39.0 40.0 41.0 42.0 43.0 44.0 45.0 46.0

5

10

2

5

100

2

5

1000

2

5

10000

2

(002)- reflections of film and substrate

k k’K

2

Reciprocal space

K

coherent growth

Epitaxy

Non-specular diffraction spotsk k’

K

2

Reciprocal space

Epitaxy

Non-specular diffraction spots

relaxed growth

k k’K

2

Layer-by-layer

Layer + 3D islands

3D-islands

The three growth modes

“Wetting Criterion” film erface substrate C p p int ln / 0b gsupersaturation favors lbl growth

Morphology

film erface substrate int

MgO(001) Fe3O4/MgO(001)

MorphologyRHEED

k

k’

ReciprocalLattice

Rods

Reciprocallatticepoints

Perfectly flat surface

Reciprocal rods have no

width

Surface with monolayer roughness. Broadened

rods.

Surface with large

roughness. Transmission

features.

ReciprocalLattice

RodsAllowed

ReciprocalLattice Vectors

FirstOrderSecondOrder

FirstOrderSecondOrder

FirstOrderSecondOrder

MorphologyRHEEDPatterns

TiOx/MgAl2O4(001)vacancy ordered phase

MorphologyTransmission RHEED Pattern

Kinematic diffraction / Step density models

Do not explain - phase shifts !!! - in-/out of phase amplitude - damping due to dynamic and incoherent scattering effects)

only the ML period is reliable parameter

MorphologyRHEED Oscillations

Thickness

• in-situ: quartz monitor, RHEED oscillations

• ex-situ: X-ray Reflectivity, RBS

0 1 2 3 4-7

-6

-5

-4

-3

-2

-1

0

1

Inte

nsity

(ar

b un

its,

log.

sca

le)

Theta (degr.)

Reflectivity (experiment) Simulation

k k’K

2

• Substrate influence

• Finite size

• Epitaxial strain

• Artificial stacking

enforcement of geometric, magnetic and electronic structure (metastable phases, exchange bias, proximity effects, ..)

thickness < characteristic length(quantum wells, ballistic transport,..)

deformation(bandgap, level splittings)

new layered compounds or structures(high-Tc, new ferromagnetic(-electric) compounds)

Manipulation of properties

• Substrate influence

• Finite size

• Epitaxial strain

• Artificial stacking

- new phases, CrOx, TiOx ,Sr(N,O)- Anti-Phase Boundaries, Fe3O4

- Electronic structure of NiO- Superparamagnetism in Fe3O4

- MI-transition in VOx

- Tetragonal distortion in CoO

- OFeOFeO non-polar initial phase on Al2O3- new ferro-magnetic(electric) materials

Manipulation of propertiesTransition metal oxides TMO

• Chromium monoxide CrO does not exist as a bulk material, but can be grown on cubic substrates as CrxO (0.67<x<1) .• Cr2+/Cr3+ iso-electronic with Mn3+/Mn4+ (d4/d5) SCOO in Cr-oxides ?

(O. Rogojanu)

Substrate influence Metastable Chromium Monoxide CrxO

ab

c

xy

z

Areal XRD picture of CrOx/MgO(001).

LEED pattern of CrOx/MgO(001)

(-1-1 1)

(0 0 2) (-2-2 2)

(-1-1 3)

(-2-2 0)

(0 0 4) Refinement of data collected at ID10,ESRF:1/3 Cr-sites are vacant

(O. Rogojanu, S.Hak)“Rocksalt”-Cr2O3/MgO(001)

Substrate influence

VOx on STO(aSTO=3.90 Å)compressive strain

MgO(113)MgO(113)

VO(113)

(002)MgO

(002)VOx

(004)MgO

(004)VOx

(002)VOx

(004)VOx

(002)STO (004)STO

VO(113)

STO(113)

2Theta-omega scan

VOx on MgO(aMgO=4.21 Å) tensile strain

Epitaxial Strain Coherent VOx layers on MgO and STO

(004)VOx

(A.D.Rata)

SrTiO3

(3.903 Å)MgAl2O4

(4.041×2 Å)

MgO(4.213 Å)

M

SC

Epitaxial Strain MI-transition in strained VOx layers

0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0

1 0 - 5

1 0 - 4

1 0 - 3

x = 0 . 8 2

H = 0 T H = 5 T

Re

sis

tiv

ity

(o

hm

cm

)

T ( K )

Epitaxial Strain Compressed metallic VOx shows upturn of and positive MR at low T

770 775 780 785-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

T

otal

ele

ctro

n yi

eld

(arb

.uni

ts)

Photon energy h

50ML CoO on AgT=77K

grazing normal difference

770 775 780 785-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

T

ota

l ele

ctro

n y

ield

(a

rb.u

nits

)

Photon energy h

CoO sandw. on AgT=77K

grazing normal difference

Compressed CoO layer, (CoO)50/Ag

Stretched CoO layer, (MnO)10 (CoO)7(MnO)50/Ag

“Bulk” CoO: very small effect

L=1, S=3/2

L=0, S=3/2

Epitaxial Strain XMLD of strained CoO (S. Csiszar, M. Haverkort, H. Tjeng)

Co L3-edge

Co L3-edge

t2g

t2g

eg

eg

dxz,dyz

dxz,dyz

dxy

dxy

(0 ,3)(1 ,1)

b * a *

Endt ~ 105s

-Al2O3(0001)Fe3O4 (111)FeO typereciprocal lattice (111)

Artificial Stacking Nonpolar [OFeOFeO] stack ?

Startt = 0s

Final remarks

• The ideal of atomic layer-by-layer growth can be approached using MBE and UHV-PLD techniques.

However,

• Control of stoichiometry, completeness and structure of atomic layer during growth is still unsatisfactory.

• Knowledge of surface “chemistry” is almost fully lacking.

• Postgrowth characterisation of composition and structure is a tedious and tough job.

Tjipke HibmaHenk BruinenbergWilma EerensteinDiana RataSjoerd HakSzilard Csiszar

MSC-cooperations : Tjeng, Sawatzky (electron spectroscopy) Niesen, Boerma (Moessbauer spectroscopy, RBS) Palstra (transport measurements)

Inorganic Thin Layers Group