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-31-109-ENG-38, and the State of Illinois, USA, under Higher Education Collaborative Act.

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.

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.

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.

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.

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.

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.

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.

More Pictures C. Thompson, et al.

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.

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.

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.

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.

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

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.