Aberration-corrected TEM studies on interface of - Course Notes

Aberration-corrected TEM studies on interface of multilayered-perovskite systems

ByLina Gunawan (0326114)

Supervisor: Dr. Gianluigi BottonNovember 1, 2006

MSE 702(1) Presentation

Nov 1, 2006McMaster Univ. - MSE 702(1) Presentation2

Outline

� Literature Review� Perovskite double-oxides � Issues of interface in thin films� TEM Features & Role of CS-corrector

� Objectives� Characterization across interface

• experimental & simulation

� Assessment • physical properties changes ⇔ structural changes

� Experimental Techniques


Perovskite Double Oxide ABO3 (e.g. SrTiO3, BaTiO3)

An+

O2-

Bn+

AO

BO2

Stacking sequence of ABO3 perovskite

double oxide system:


Perovskite Double OxidesProperties & Applications

� Physical properties:� insulator, semiconductor, conductor

superconductor� optical, magnetic, dielectric, piezoelectric,

and ferroelectric

� Applications:� Magnetic field sensors� Hard disk read heads� Tunable dielectrics� Non-volatile Memory (EEPROM & Flash)

� Multi-layered Capacitor

� Smaller ~ Better vs. Finite Size Effect

http://www.apple.com/ca/ipodnano/

http://en.wikipedia.org/wiki/Hard_disk


Why interface?

� Differences from bulk in terms of:1. Roughness2. Atomic arrangement3. Electronic bonding / valences (charge transfer)4. Strain & Misfit Dislocations

� Different structure → different physical properties� Oxygen Vacancies Concentration ~ Resistivity� Strain ~ Bandgap Energy


1. Roughness ~ CationIntermixing

b) Schematic of the cation intermixing

Muller, et al., Nature Materials 5 (2006) 204-209

a) LAO/STO multilayerProgressive interface roughness with growth


2. Unexpected Interfacial Atomic Arrangement

� Nanoscale dimension (a few atomic monolayers):� interfacial atomic arrangement ≠ bulk atomic arrangement � new physical phenomena

Falke, U.; Bleloch, A.; Falke, M., Phys. Rev. Lett. 92 (2004) 116103(1-4)


3. Charge Transfer inMultivalent Oxide LAO/STO

Muller, et al., Nature Materials 5 (2006) 204-209

ρ is the electric charge, E electric field and V electric potential


4. Misfit Dislocations at Interface of Epitaxial SrTiO3 on Si

Yang, G. Y., et al., J. Mater. Res. 17 (2002) 204-213

� Lattice mismatch of STO[100]/Si[110] ~ 1.6 % � Strain relaxation → misfit dislocations

Hao, et al., Appl. Phys. Lett. 87 (2005) 131908(1-3)


Dislocations at Grain BoundaryHAADF

SrTiO3Nb-doped

SrTiO3

http://www.ccmr.cornell.edu/facilities/imagewinners06/LenaFitting2.html

LAADF


Detailed Investigation at Interface� Many Interesting Phenomena at Interface� New insight into unexpected physical

properties � conductivity = f (VO concentration)

� Ways to control and modify the interface � New generation CS-corrected TEM (sub-Å

probe) � Atomic scale characterization� Qualitative & quantitative information

o HREM and Z-contrast images o EELS spectra


Transmission Electron Microscope Features

EDX

Analytical

EELS EFTEM

TEM

Imaging Diffraction

Dark Field –Z-Contrast

Bright Field -Phase Contrast

SAD CBED

SAD : Selected Area DiffractionCBED : Convergent Beam Electron DiffractionEELS : Electron Energy Loss Spectroscopy

EDX : Electron Diffraction X-ray SpectroscopyEFTEM : Energy Filtered TEM


Better HRTEM Performance

Directly interpretable (point resolution) limit :

r = A CS1/4 λ3/4

at optimum (Scherzer) defocus:

∆f = - B (CS λ)1/2

Two ways for improving resolution:• ↓ λ ~↑ Vacc : reached practical limit & expensive• ↓ CS : compensate positive CS value of objective lens with CS corrector (total CS ~ 0)


Role of Cs-CorrectorHetherington, Materials Today 7 (2004) 50-55

Function:• Compensate the positive CS of objective lens to zero

and even to total negative CS value• Push the spatial resolution to sub-Å regime


Optics of CS-Corrector

Bleloch, A., and Lupini, A., Materials Today, 7(2004) 42-48

Hetherington, Materials Today 7 (2004) 50-55

1) Series of Octupole-Quadrupole Lenses2) Series of Hexapole Lenses


Better Resolution at Interface

Haider M., Nature 392 (1998) 768-769

Uncorrected TEM image at

optimum “Scherzer”defocus

Uncorrected TEM image at disk of least confusion “Lichte” defocus

Cs-corrected TEM image at optimum“Scherzer” defocus


Detection and Quantification of Occupancy Level of Low-Z Element

a) Experimental HREM image

b) Simulated HREM image

Jia et al., Science 299 (2003) 870-873

I1&2 in (A) similar to I3&4 in (B): the oxygen occupancy of 85 % and 80% respectively.


Channeling Effect

� Channeling effect = f (thickness)

� Prominent in crystalline materials by using a sub-A probe

� Nano-scale interface & e-interaction has not yet been thoroughly investigated and simulated

Voyles, P. M., et al., Microsc. Microanal. 10 (2004) 291-300


Channeling Effect (Cont’d)

Voyles, P. M., et al., Ultramicroscopy 96 (2003) 251-273

� Probe starts between the two Si dumbells

� No intensity below the initial probe position at t= 145 Å and 430 Å

� Crucial factor for EELS signal analysis


Trade off in CS-corrected TEM

Better Resolution• Pinpoint the exact location of interface

• Reveal the unexpected interfacial atomic arrangement

• Detection of Low-Z element in vicinity of High-Z elements

• Quantification of degree of occupancy

Channeling• Intensity variation = f (t, probe size & crystal orientation)

• Non-homogeneous atomic columns complicates the quantitative interpretation of EELS spectra

VS.


Objectives

� Characterize the interface of multi-layered perovskite system� Assess physical properties ⇔ structural change at interface

Experimentalimages & spectra

Simulations

�Quantitative analysis of images

�Structural atomic arrangement (exact chemistry)

�Electronic environment (atomic bonding & valences)�Strain measurement

Distinguish Instrumental’s Artifact

Easier Interpretation


Image Formation in HRTEM

Reinhard Otto - http://www.physik.hu-berlin.de/~rotto/autumn_school


Exit Wave Reconstruction:Through Focal Series


Example of Artifacts (1)Multislice Simulation of STO

Cs= 1.3 mm ∆f = 698 Å Cs= 1.3 mm ∆f = 1066 Å Cs= 0.05 mm ∆f = 137 Å

thickness = 50 Å


Example of Artifacts (2)Multislice Simulation on STO

Cs= 0.05 mm ∆f = 137 Å

t = 50 Å t = 150 Å t = 250 Å


Experimental Techniques� Conventional and Cs-Corrected HRTEM

� Image (BF or HAADF) – sub-Å resolution: » the atomic arrangement, strain measurement

� SAD: » unit cell structure and phase

� EDX: » chemical composition

� EELS: » electronic environment: oxidation state, atomic valences, and interatomic bondings

� CBED: » strain measurement & lattice parameter determination at interface


Potential Perovskite Systems for Multilayer Integration

• LaAlO3/(Sr,La)O3

• SrRuO3/cuprate

• (La,Ca)CuO3/cuprate

• Cuprate/manganate

• SrTiO3/LaMnO3/La(Al, Sc)O3

• LaAlO3/SrTiO3


Future Works

� Experimental TEM & analytical characterization of the interface of multilayered perovskite double oxides.

� Simulations:� Multislice image simulation:

• Distinguishing artifacts from the real materials’ features (e. g. unexpected atomic arrangement)

• Observing e- propagation at interface� Spectrum simulation:

• Analytical interpretation of the interfacial atoms• Valences/chemistry, composition, and bonding


Summary

� The different environment of interfacial atoms: � Roughness � Atomic Structure� Electronic Structure (charge transfer)� Strain & Misfit dislocations

induces physical properties that differs from bulk

� Characterization is needed for:further understanding → tailor and modify the wanted properties across interface

� Sub-Å resolution CS-corrected HRTEM will be utilized for qualitative and quantitative characterization across nano-scale interfaces


Summary

� Trade-offs in Cs-corrected HRTEM:� Better Resolution vs. Channeling Effect

� Simulation is needed for:

� quantitative analysis of experimental images and spectra

� distinguishing artifacts from real features (intensities-atomic position relationship cannot be directly interpreted)


References

� Haider M., Nature 392 (1998) 768-769

� Yang, G. Y., et al., J. Mater. Res. 17 (2002) 204-213

� Voyles, P. M., Grazul, J. L., Muller, D. A., Ultramicroscopy 96 (2003) 251-273

� Hu, et al., Appl. Phys. Lett. 82 (2003) 203-205

� Falke, U.; Bleloch, A.; Falke, M., Phys. Rev. Lett. 92 (2004) 116103(1-4)

� Voyles, P. M., Muller, D. A., Kirkland, E. J., Microsc. Microanal. 10 (2004) 291-300

� Hao, et al., Appl. Phys. Lett. 87 (2005) 131908(1-3)

� Hetherington, Materials Today 7 (2004) 50-55� Muller, et al., Nature Materials 5 (2006) 204-209

� Yu, et al., Thin Solid Films 462-463 (2004) 51-56

� He, et al., J. Appl. Phys. 97 (2005) 104921(1-6)


Acknowledgement

� Dr. Gianluigi Botton� Dr. Carmen Andrei� Dr. Christian Maunders� Nadi, Yang & Kai� Fred Pearson� Dr. Maria Varela (Oak Ridge Nat’l Lab)

Thank You

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Aberration-corrected TEM studies on interface of - Course Notes