Transcript THIN LAYERS OF TRANSITION METAL OXIDES Tjipke Hibma Materials Science Centre, University of Groningen, The Netherlands.

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
PVD
Thermal
Chemical Deposition
CVD
Energetic
MBE
PLD
ALL-MBE
ALE
UHV PLD
most clean and precise
deposition techniques
SPUTTERING
MOCVD
LACVD
PECVD
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
Deposition
Main Growth Parameters
Desorption
Arrival rates Fn
Energies En
Diffusion
Mixing
Growth
Nucleation
Temperature T
Atomic layer-by-layer growth
Control of
Growth
MBE
Parameters
PLD
Stoichiometry
Relative Flux Fn
Difficult for n>2,
ALL-MBE
Loss of volatile
components.
Surface
“Chemistry”
Temperature T
Energies En
Tsubstrate
Tsubstrate
thermal <0.1 eV
0.1-10eV (Pback)
Epitaxy
Substrate
RHEED
(RHEED)
Nucleation rate
(lbl growth mode) (Fn/Dn )a
Morphology
Thickness
(nr of layers)
Absolute Flux Fn RHEED,
ALE
# Pulses,
(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 :
1 6
2 0 0
3
/A l2O
5 1
O
2 7
1 0 0
O
O
1 8
0
2
3 0 0
4 0 0
V O
3
I n te n s ity (a r b . u n its )
I n t e n s it y ( a r b .u n it s )
V
V
A l
5 0 0
6 0 0
7 0 0
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)
200
18
V O
16
220
240
O
18
260
x
x
capped
O
uncapped
O
280
B a c k s c a tte r in g e n e r g y (k e V )
VOx film on MgO (100)
300
Stoichiometry Control
18O-
RBS of VOx
1 .4
x -O x y g e n c o n te n t
1 .3
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
O x y g en p ressu re (m V )
8
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
strain:
 ||  f
2
  
 ||   f
1 
(misfit f = Da/a)
a||film  a||substrate
Epitaxy
Dislocation formation
Energy E →
Critical Thickness
Strain
energy
Dislocation
energy
tc
Critical thickness:
F
IJ
G
HK
b(1   cos2  )
tc
tc 
ln
8 f o (1   ) sin  cos
b
thicknes t →
Epitaxy
Critical Thickness of CoO/MgO(001)
k
K
k’
Strain in the CoO/MgO
2q
Experiment
Theory
0.010
2
10000
5
0.008
2
1000
5
2
0.006
100
5
2
10
0.004
5
36.0
0.002
0.000
0
200
400
600
800
Layer thickness (A)
1000
37.0
38.0
39.0
40.0
41.0
42.0
43.0
44.0
45.0
46.0
(002)- reflections of
film and substrate
Epitaxy
Non-specular diffraction spots
coherent growth
Reciprocal space
K
K
k
k’
2q
Epitaxy
Non-specular diffraction spots
relaxed growth
Reciprocal space
K
k
k’
2q
Morphology
The three growth modes
“Wetting Criterion”
Layer-by-layer
b g
 film   int erface
erface   substrate
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
Reciprocal
Lattice
Lattice
Rods
Rods
Allowed
Reciprocal
Lattice
Vectors
First
Order
Second
Order
Perfectly flat
surface
Reciprocal
rods have no
width
First
Order
Second
Order
Surface with
monolayer
roughness.
Broadened
rods.
Reciprocal
lattice
points
First
Order
Second
Order
Surface with
large
roughness.
Transmission
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
Intensity (arb units, log. scale)
1
Reflectivity (experiment)
Simulation
0
-1
k
-2
K
k’
-3
2q
-4
-5
-6
-7
0
1
2
Theta (degr.)
3
4
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)
(-1-1 3)
LEED pattern of
CrOx/MgO(001)
(-2-2 2)
Refinement of
data collected
at ID10,ESRF:
1/3 Cr-sites
are vacant
(0 0 2)
c
Areal XRD picture
of CrOx/MgO(001).
(-1-1 1)
z
y
(-2-2 0)
x
a
b
Epitaxial Strain
Coherent VOx layers on MgO and STO
(002)MgO
(004)MgO
(002)VOx
VO(113)
VOx on MgO
(aMgO=4.21 Å)
tensile strain
(004)VOx
MgO(113)
MgO(113)
(002)STO
STO(113)
(004)STO
VO(113)
(002)VOx
(004)VOx
(004)VOx
2Theta-omega scan
VOx on STO
(aSTO=3.90 Å)
compressive strain
Epitaxial Strain
MI-transition in strained VOx layers
(A.D.Rata)
SC
M
MgO
(4.213 Å)
MgAl2O4
(4.041×2 Å)
SrTiO3
(3.903 Å)
Epitaxial Strain
Compressed metallic VOx shows upturn
of  and positive MR at low T
-3
x =
0 .8 2
1 5 0
2 0 0
R e s is t iv it y ( o h m
c m )
1 0
1 0
-4
1 0
-5
H
H
0
5 0
=
=
0 T
5 T
1 0 0
T (K )
2 5 0
3 0 0
Epitaxial Strain
eg
t2g
L=0, S=3/2
dxy

dxz,dyz
Stretched CoO layer,
(MnO)10 (CoO)7(MnO)50/Ag
eg
t2g
L=1, S=3/2
“Bulk” CoO: very small effect
dxz,dyz
dxy

Total electron yield (arb.units)
Compressed CoO layer,
(CoO)50/Ag
(S. Csiszar, M. Haverkort, H. Tjeng)
Total electron yield (arb.units)
XMLD of strained CoO
1.0
Co
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
-0.1
770
L3-edge
775
50ML CoO on Ag
T=77K
grazing
normal
difference
780
785
Photon energy h
2.5
2.0
Co L3-edge
1.5
1.0
CoO sandw. on Ag
T=77K
grazing
normal
difference
0.5
0.0
-0.5
-1.0
770
775
780
Photon energy h
785
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)