1. The study of ferroelectric switching using x-ray synchrotron radiation
Carol Thompson
Science with Microbeams
APS Scientific Advisory Cross-cut Review
January 21, 2004
This work was supported by the U. S. Department of Energy, Basic Energy Sciences, under contract W ENG-38, and the State of Illinois, USA, under Higher Education Collaborative Act.

2. Outline Collaborators Introduction
What is a ferroelectric
Concentrate on epitaxial films
Oxide perovskite system
Structural response of epitaxial ferroelectric to electric field
examples of dynamic studies
Summary and Conclusions
Domain studies, device studies, future studies require microbeams.
Collaborators
Chris Gunderson (Physics, NIU)
Marian Aanerud (Masters 2002, Physics, NIU)
Stephen Streiffer (MSD, ANL)
Brian Stephenson (MSD, ANL)
G. -R Bai (MSD,ANL)
W. K. Kee (XFD-XRP,ANL)
Armon McPherson, (currently at Sandia)
applications in ultra large scale integrated dynamic random access memories (ULSI-DRAM's) and non-volatile ferroelectric memories (NVFRAM's), surface acoustic wave (SAW) delay lines, IR-optical FET's, electro-optical switches and modulators.
C. Thompson, et al.

3. What are ferroelectrics?
Spontaneous permanent electric polarization.
Unit cell of crystal is non-centrosymmetric (charges separated)
A macroscopic sample with net zero polarization
combination of microscopic polarized domains.
E (kV/cm)
P(C/cm2)
Showing oxide perovskite structure (which is our material system)
Application of an electric field switches the polarization
Bulk Polarization P(E) exhibits a hysteresis loop with similar shape to a hysteresis loop of magnetisation in ferromagnets.
No iron or magnetism implied. Old literature refers to Seignette-electricity because seignette (Rochelle Salt) was the first material found to exhibit this behavior
Like the hysteresis loop for the ferromagnet, it impllies that at the microscopic level, the system is formed of polarized domains, and reversal is a process of nucleation and or growth ot domains
C. Thompson, et al.

4. What are ferroelectrics?
E (kV/cm)
P(C/cm2)
top
electrodes
substrate
electrode
active film
packaging
top
electrodes
Symmetry change
substrate
electrode
active film
E (kV/cm)
strain
C. Thompson, et al.

5. Synchrotron techniques are well matched to the study of the ferroelectric systems
Structure-property relationships control:
dielectric, ferroelectric, piezoelectric, electrostrictive, pyroelectric and electro-optical properties
for actuators, sensors, electro-optical switches, non-volatile memory elements, hi-K dielectric, detectors…
Scattering and diffraction examine the structural aspects that control the properties
Symmetry changes, orientation, lattice parameters, domains configurations
What are some questions for thin films?
Thin films: role of strain
Domains
domain wall
size limits
equilibrium
non-equilibrium
Dynamics
intrinsic speed
Electric measurements suggest that the intrinsic switching speed has not yet been measured (<1 nsec)
Various other mechanisms may contribute at other time scales
C. Thompson, et al.

6. Scattering example: fingerprints domain evolution
Time-resolved scattering
40 nm Pb(Ti,Zr)O3 film
200 Hz
Scattering profile can fingerprint the domain configuration in epitaxial films
P-V
SrRuO3
PbTiO3
Difficult
Easy
q00l
C. Thompson, et al.

7. High speed time-resolved Methods (BESSRC 12-ID-D)
At each voltage, collect all scattering (area detector)
Utilizes rocking curve of sample to “scan” q
Chopper synchronized (Hybrid fill: Singlet produces <100 psec x-ray probe pulses
Electrical stimulation of device synchronized/delayed so that sample is in particular electrical state during exposure
Purely c-axis films grown in horizontal MOCVD reactors on 2° vicinal (100) SrRuO3/SrTiO3
Pb(Mg1/3Nb2/3)O3-PbTiO3 Tgrowth ~775°C
~250 nm thick)
Top electrodes: 50 m, 100 m, 250 m, 750 m diameter dots
Pt deposited at 350°C
C. Thompson, et al.

8. Close-up photograph of sample manipulation and contact region
50 m capacitor
Purely c-axis films grown in horizontal MOCVD reactors on 2° vicinal (100) SrRuO3/SrTiO3
Pb(Mg1/3Nb2/3)O3-PbTiO3 Tgrowth ~775°C
~250 nm thick)
Top electrodes: 50 m, 100 m, 250 m, 750 m diameter dots
Pt deposited at 350°C
X-ray spot must be smaller than the device.
And x-ray spot must be aligned with the device under electrical stimulation.
Spot size used:5m x 5m
K-B mirror focus
C. Thompson, et al.

9. More Pictures
C. Thompson, et al.

10. Reciprocal Space Map 001
Initial experiments: Focus on position of film Bragg peak region and its immediate neighborhood.
Scattering shown for epitaxial films (thickness~250nm) of PMN and PMN-PT
STO
STO
SrTiO3
SrRuO3
PMN-PT hf
detector 
SRO
SRO
Pb(Mg1/3 Nb2/3)O3
Relaxor ferroelectric
PMN7PT3
PMN
C. Thompson, et al.

11. PMN7-PT3 Structural Response to a Step Voltage
Applied Voltage
+9V
+10V
-10V
-11V
10nsec
Response (speed) limited by size of device, not by how fast we can measure with x-rays yet
Smaller devices – smaller beams
C. Thompson, et al.

12. Summary and Conclusion
Structural techniques available at synchrotrons well suited to ferroelectric systems
And it’s a growing field: see also other groups doing exciting studies of ferroelectric films and crystals using microdiffraction, x-ray topography, and reciprocal space mapping.
Examples from our work:
Progress in development of techniques to study structural response at 100 psec time scale
Need to go to smaller devices, embedded devices
Progress in switching studies: to 50m ‘play’ device: switching speed limited to ~10nsec
Smaller devices allow faster switching
Need for microbeam capabilities
C. Thompson, et al.

13. Preferred Domain Pinning
Piezo-response atomic force microscopy:
Recent direct observation of preferred domain pinning in fatigued ferroelectric films is reported using piezo-response atomic force microscopy.
phase amplitude
AFM- Piezoreponse image
After apply
+3V
-3V
Direct observation of inversely polarized frozen nanodomains in fatigued, ferroelectric memory capacitors, E. L. Colla, I. Stolichnov, P. E. Bradely, and N. Setter, Appl Phys. Lett. 82, 1604 (2003).
Samples: Pt-PZT-Pt films.
C. Thompson, et al.

14. Time-Resolved Synchrotron X-Ray Scattering
Data taken on 250 nm thick PMN-PT film (PT ~30-35%)
C. Thompson, et al.

15. Lattice response: time-resolved x-ray diffraction
PMN7-PT3
Lattice response on different time scales
pulse with ~15 nsec rise time
6.3 kHz triangle wave (“80 sec rise time”)
C. Thompson, et al.