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THIN LAYERS OF TRANSITION
METAL OXIDES
Tjipke Hibma
Materials 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
Introduction
Atomic Layer-by-Layer Growth
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).
Introduction
Manipulation of materials properties by
• Substrate influence
enforcement of geometric, magnetic and electronic structure
(metastable phases, exchange bias, proximity effects, ..)
• Finite size
thickness < characteristic length
(quantum wells, ballistic transport,..)
• Epitaxial strain
deformation
(bandgap, level splittings)
• Artificial stacking
new layered compounds or structures
(high-Tc, new ferromagnetic(-electric) compounds)
Introduction
LaCrO3-LaFeO3 Atomic Superlattices
K.Ueda, H.Tabata, T. Kawai, Science 280 (1998) 1064
Goodenough-Kanamori rules:
Cr3+-O-Fe3+ (d3-d5) 180°-superexchange
interaction is Ferromagnetic
Thin film deposition
Physical Deposition Chemical Deposition
PVD CVD
Thermal Energetic MOCVD
LACVD
MBE PLD SPUTTERING PECVD
ALL-MBE
UHV PLD
ALE
most clean and precise
deposition techniques
Thin film deposition
Molecular Beam Epitaxy (MBE)
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 deposition
(UHV-) Pulsed Laser Deposition (PLD)
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
Atomic layer-by-layer growth
Growth processes
Main Growth Parameters
Deposition
Arrival rates Fn Desorption
Energies En
Growth
Diffusion Nucleation
Mixing Temperature T
Atomic layer-by-layer growth
Control of Growth MBE PLD
Parameters
Stoichiometry Relative Flux Fn Difficult for n>2, Loss of volatile
ALL-MBE components.
Surface Temperature T Tsubstrate Tsubstrate
“Chemistry” Energies En thermal <0.1 eV 0.1-10eV (Pback)
Epitaxy Substrate
Morphology Nucleation rate RHEED (RHEED)
(lbl growth mode) (Fn/Dn )a
Thickness Absolute Flux Fn RHEED, # Pulses,
(nr of layers) ALE (RHEED)
Stoichiometry Control
Atomic Layer-by-layer MBE (ALL-MBE)
(Eckstein and Bosovic, Annu. Rev. Mater. Sci., 25,679,1995)
Atomic absorption
flux control and
computer controlled
shuttering of
individual K-cell.
Stoichiometry Control
MBE of Binary Oxides
Stoichiometric MnOm
excess oxygen
Nonstoichiometric MxOy
vary FM/FO, determine x afterwards:
• Fe3-dO4, Moessbauer Spectroscopy
• CrOx , XPS
• VOx, TiOx (0.8<x<1.3), 18O-RBS
Stoichiometry Control
18O- RBS
RBS spectra of 1.8 MeV He+ ions scattered from :
V O /A l2O V O x
capped
2 3 3
I n t e n s it y ( a r b .u n it s )
I n te n s ity (a r b . u n its )
18
O
1 6
O
5 1
1 8 V
O
2 7
A l V O uncapped
x
16
O 18
O
0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 200 220 240 260 280 300
B a c k s c a tte r in g e n e r g y (k e V ) B a c k s c a tte r in g e n e r g y (k e V )
V2O3 film on Al2O3 (0001) VOx film on MgO (100)
Stoichiometry Control
18O- RBS of VOx
1 .4
1 .3
x -O x y g e n c o n te n t
1 .2
1 .1
1 .0
0 .9
0 .8
0 .7
0 .6
0 .5
0 .4
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
O x y g en p ressu re (m V )
Surface “Chemistry”
• Elements
surface diffusion, nucleation
• Binary Oxides
diffusing species: M, O, MO ??
• Complex Oxides
?????
Epitaxy
• Epitaxy
= Well-defined orientation relationship between
substrate and film lattice.
• Coherent epitaxy
a||film a||substrate
strain:
|| f
2
|| f
1
(misfit f = Da/a)
Epitaxy
Critical Thickness
Energy E →
Strain
Dislocation formation energy
Dislocation
energy
tc thicknes t →
Critical thickness:
tc
b(1 cos2 ) FI
tc
GJ
8 f o (1 ) sin cos
ln
HK
b
Epitaxy
Critical Thickness of CoO/MgO(001)
k K k’
2q
0.010 Experiment
Strain in the CoO/MgO
2
Theory 10000
5
0.008 2
1000
5
2
0.006 100
5
2
10
0.004 5
36.0 37.0 38.0 39.0 40.0 41.0 42.0 43.0 44.0 45.0 46.0
0.002
(002)- reflections of
0.000 film and substrate
0 200 400 600 800 1000
Layer thickness (A)
Epitaxy
K
Non-specular diffraction spots k’
k
coherent growth 2q
Reciprocal space
K
Epitaxy
K
Non-specular diffraction spots k’
k
relaxed growth 2q
Reciprocal space
Morphology
The three growth modes
“Wetting Criterion”
Layer-by-layer film b g
int erface substrate C ln p / p0
supersaturation favors lbl growth
Layer + 3D islands
3D-islands
Morphology
RHEED
MgO(001) Fe3O4/MgO(001)
k’
k
Morphology
RHEED
Patterns Reciprocal
Lattice
Reciprocal
Lattice
Rods Rods
Allowed
Reciprocal Reciprocal
Lattice lattice
Vectors points
First First First
Order Order Order
Second Second Second
Order Order Order
Perfectly flat Surface with Surface with
surface monolayer large
Reciprocal roughness. roughness.
rods have no Broadened Transmission
width rods. features.
Morphology
Transmission RHEED Pattern
TiOx/MgAl2O4(001)
vacancy ordered phase
Morphology
RHEED Oscillations
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
Thickness
• in-situ: quartz monitor, RHEED oscillations
• ex-situ: X-ray Reflectivity, RBS
1
Intensity (arb units, log. scale)
Reflectivity (experiment)
0
Simulation
-1
-2 k K k’
-3
-4 2q
-5
-6
-7
0 1 2 3 4
Theta (degr.)
Manipulation of properties
• Substrate influence
enforcement of geometric, magnetic and electronic structure
(metastable phases, exchange bias, proximity effects, ..)
• Finite size
thickness < characteristic length
(quantum wells, ballistic transport,..)
• Epitaxial strain
deformation
(bandgap, level splittings)
• Artificial stacking
new layered compounds or structures
(high-Tc, new ferromagnetic(-electric) compounds)
Manipulation of properties
Transition metal oxides TMO
• Substrate influence
- new phases, CrOx, TiOx ,Sr(N,O)
- Anti-Phase Boundaries, Fe3O4
• Finite size
- Electronic structure of NiO
- Superparamagnetism in Fe3O4
• Epitaxial strain
- MI-transition in VOx
- Tetragonal distortion in CoO
• Artificial stacking
- OFeOFeO non-polar initial phase on Al2O3
- new ferro-magnetic(electric) materials
Substrate influence
Metastable Chromium Monoxide CrxO
(O. Rogojanu)
• 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 ?
Substrate influence
“Rocksalt”-Cr2O3/MgO(001)
(O. Rogojanu, S.Hak)
(0 0 4) Refinement of
data collected
(-1-1 3) at ID10,ESRF:
1/3 Cr-sites
LEED pattern of are vacant
CrOx/MgO(001) (-2-2 2) (0 0 2)
c
(-1-1 1) z
Areal XRD picture y x
of CrOx/MgO(001). (-2-2 0) a
b
Epitaxial Strain
Coherent VOx layers on MgO and STO
(002)MgO
VOx on MgO
(004)MgO
(aMgO=4.21 Å)
(002)VOx VO(113) tensile strain
(004)VOx
MgO(113)
MgO(113)
(002)STO
STO(113) VOx on STO
(004)STO
(aSTO=3.90 Å)
VO(113) compressive strain
(002)VOx
(004)VOx
(004)VOx
2Theta-omega scan
Epitaxial Strain
MI-transition in strained VOx layers
(A.D.Rata)
SC
M
MgO MgAl2O4 SrTiO3
(4.213 Å) (4.041×2 Å) (3.903 Å)
Epitaxial Strain
Compressed metallic VOx shows upturn
of and positive MR at low T
-3
1 0
x = 0 .8 2
c m )
H = 0 T
R e s is t iv it y ( o h m
-4
1 0 H = 5 T
-5
1 0
0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0
T (K )
Epitaxial Strain
XMLD of strained CoO (S. Csiszar, M. Haverkort, H. Tjeng)
Compressed CoO layer, 1.0
Total electron yield (arb.units)
0.9
Co L3-edge 50ML CoO on Ag
T=77K
(CoO)50/Ag 0.8
grazing
0.7
normal
eg 0.6
0.5
difference
0.4
0.3
t2g 0.2
dxy 0.1
0.0
L=0, S=3/2 dxz,dyz
-0.1
770 775 780 785
Photon energy h
Stretched CoO layer, 2.5
Total electron yield (arb.units)
(MnO)10 (CoO)7(MnO)50/Ag 2.0 Co L3-edge CoO sandw. on Ag
T=77K
1.5 grazing
eg 1.0
normal
difference
0.5
t2g dxz,dyz
0.0
L=1, S=3/2 dxy -0.5
-1.0
770 775 780 785
“Bulk” CoO: very small effect Photon energy h
Artificial Stacking
Nonpolar [OFeOFeO] stack ?
a-Al2O3(0001)
Fe3O4 (111)
FeO type
reciprocal lattice (111)
(0,3)
(1,1)
b* a*
End
t ~ 105s
Start
t = 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.
Inorganic Thin Layers Group
Tjipke Hibma
Henk Bruinenberg
Wilma Eerenstein
Diana Rata
Sjoerd Hak
Szilard Csiszar
MSC-cooperations :
Tjeng, Sawatzky (electron spectroscopy)
Niesen, Boerma (Moessbauer spectroscopy, RBS)
Palstra (transport measurements)
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