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

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1 Aberration-corrected TEM studies on interface of multilayered-perovskite systems By Lina Gunawan ( ) Supervisor: Dr. Gianluigi Botton November 1, 2006 MSE 702(1) Presentation

2 Outline Literature Review Perovskite double-oxides Issues of interface in thin films TEM Features & Role of C S -corrector Objectives Characterization across interface experimental & simulation Assessment physical properties changes structural changes Experimental Techniques 2

3 Perovskite Double Oxide ABO 3 (e.g. SrTiO 3, BaTiO 3 ) A n+ B n+ O 2- Stacking sequence of ABO 3 perovskite double oxide system : AO BO 2 3

4 Perovskite Double Oxides Properties & 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 4

5 Why interface? Differences from bulk in terms of: 1. Roughness 2. Atomic arrangement 3. Electronic bonding / valences (charge transfer) 4. Strain & Misfit Dislocations Different structure different physical properties Oxygen Vacancies Concentration ~ Resistivity Strain ~ Bandgap Energy 5

6 1. Roughness ~ Cation Intermixing a) LAO/STO multilayer Progressive interface roughness with growth b) Schematic of the cation intermixing Muller, et al., Nature Materials 5 (2006)

7 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) (1-4) 7

8 3. Charge Transfer in Multivalent Oxide LAO/STO Muller, et al., Nature Materials 5 (2006) ρ is the electric charge, E electric field and V electric potential 8

9 4. Misfit Dislocations at Interface of Epitaxial SrTiO 3 on Si Yang, G. Y., et al., J. Mater. Res. 17 (2002) Lattice mismatch of STO[100]/Si[110] ~ 1.6 % Strain relaxation misfit dislocations Hao, et al., Appl. Phys. Lett. 87 (2005) (1-3) 9

10 Dislocations at Grain Boundary HAADF LAADF Nb-doped SrTiO 3 10 SrTiO 3

11 Detailed Investigation at Interface Many Interesting Phenomena at Interface New insight into unexpected physical properties conductivity = f (V O concentration) Ways to control and modify the interface New generation C S -corrected TEM (sub-å probe) Atomic scale characterization Qualitative & quantitative information o HREM and Z-contrast images o EELS spectra 11

12 Transmission Electron Microscope Features TEM Imaging Diffraction Analytical Bright Field - Phase Contrast Dark Field Z-Contrast EELS EDX EFTEM SAD SAD : Selected Area Diffraction CBED : Convergent Beam Electron Diffraction EELS : Electron Energy Loss Spectroscopy CBED EDX : Electron Diffraction X-ray Spectroscopy EFTEM : Energy Filtered TEM 12

13 Better HRTEM Performance Directly interpretable (point resolution) limit : r = A C S 1/4 λ 3/4 at optimum (Scherzer) defocus: f = - B (C S λ) 1/2 Two ways for improving resolution: λ ~ V acc : reached practical limit & expensive C S : compensate positive C S value of objective lens with C S corrector (total C S ~ 0) 13

14 Role of Cs-Corrector Hetherington, Materials Today 7 (2004) Function: Compensate the positive C S of objective lens to zero and even to total negative C S value Push the spatial resolution to sub-å regime 14

15 Optics of C S -Corrector 1) Series of Octupole-Quadrupole Lenses 2) Series of Hexapole Lenses Bleloch, A., and Lupini, A., Materials Today, 7(2004) Hetherington, McMaster Materials Univ. -Today MSE 702(1) 7 Presentation (2004) 50-55

16 Better Resolution at Interface Haider M., Nature 392 (1998) 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 16

17 Detection and Quantification of Occupancy Level of Low-Z Element a) Experimental HREM image b) Simulated HREM image I 1&2 in (A) similar to I 3&4 in (B): the oxygen occupancy of 85 % and 80% respectively. Jia et al., Science 299 (2003)

18 Channeling Effect Voyles, P. M., et al., Microsc. Microanal. 10 (2004) 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 18

19 Channeling Effect (Cont d) 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 19 Voyles, P. M., et al., Ultramicroscopy 96 (2003)

20 Trade off in C S -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 VS. Channeling Intensity variation = f (t, probe size & crystal orientation) Non-homogeneous atomic columns complicates the quantitative interpretation of EELS spectra 20

21 Objectives Characterize the interface of multi-layered perovskite system Assess physical properties structural change at interface Experimental images & spectra Simulations Easier Interpretation Distinguish Instrumental s Artifact Quantitative analysis of images Structural atomic arrangement (exact chemistry) Electronic environment (atomic bonding & valences) Strain measurement 21

22 Image Formation in HRTEM Reinhard Otto

23 Exit Wave Reconstruction: Through Focal Series 23

24 Example of Artifacts (1) Multislice Simulation of STO thickness = 50 Å C s = 1.3 mm f = 698 Å C s = 1.3 mm f = 1066 Å C s = 0.05 mm f = 137 Å 24

25 Example of Artifacts (2) Multislice Simulation on STO C s = 0.05 mm f = 137 Å t = 50 Å t = 150 Å t = 250 Å 25

26 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 26

27 Potential Perovskite Systems for Multilayer Integration LaAlO 3 /(Sr,La)O 3 SrRuO 3 /cuprate (La,Ca)CuO 3 /cuprate Cuprate/manganate SrTiO 3 /LaMnO 3 /La(Al, Sc)O 3 LaAlO 3 /SrTiO 3 27

28 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 28

29 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 C S -corrected HRTEM will be utilized for qualitative and quantitative characterization across nano-scale interfaces 29

30 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) 30

31 References Haider M., Nature 392 (1998) Yang, G. Y., et al., J. Mater. Res. 17 (2002) Voyles, P. M., Grazul, J. L., Muller, D. A., Ultramicroscopy 96 (2003) Hu, et al., Appl. Phys. Lett. 82 (2003) Falke, U.; Bleloch, A.; Falke, M., Phys. Rev. Lett. 92 (2004) (1-4) Voyles, P. M., Muller, D. A., Kirkland, E. J., Microsc. Microanal. 10 (2004) Hao, et al., Appl. Phys. Lett. 87 (2005) (1-3) Hetherington, Materials Today 7 (2004) Muller, et al., Nature Materials 5 (2006) Yu, et al., Thin Solid Films (2004) He, et al., J. Appl. Phys. 97 (2005) (1-6) 31

32 Acknowledgement Dr. Gianluigi Botton Dr. Carmen Andrei Dr. Christian Maunders Nadi, Yang & Kai Fred Pearson Dr. Maria Varela (Oak Ridge Nat l Lab) 32

33 Thank You Questions???