Nucleation and Growth in solution derived PZT thin films: Effect of heating rate

1. Nucleation and Growth in solution derived PZT thin films: Effect of heating rate Krishna Nittala*1, Geoff L. Brennecka2, Bruce A. Tuttle2, Jon F. Ihlefeld2,Bryan Gauntt2, Douglas S. Robinson3, and Jacob L. Jones1 1 2 3

2. Introduction • Lead zirconatetitanate (PZT) based thin films are used in applications such as capacitors and FERAMs • Solution deposition is an attractive route for depositing these ferroelectric thin films Multilayer stack of PLZT thin films with film thickness of ~ 120 nm1. 1G. L. Brennecka et al., Journal of Materials Research 2008, 23, 176. Solution Preparation Spin Coating Pyrolysis (300 °C-400 °C) Crystallization (~700 °C)

3. Solution deposition • Processing conditions known to affect final texture in thin film • PZT anisotropy: desirable to control texture • Intermediate phases formed during crystallization proposed to affect texture. 111 texture: fast heating rate 100 texture: slow heating rate Phase evolution during crystallization of PLZT based thin films heated at 5°C/min.2 2K. Nittala et al, Journal of Materials Science 2011.

4. Proposed mechanisms for texture evolution Intermediate and Transient phases: • Nucleation in the presence of PtxPb leads to (111) orientation3 • degree of fluorite crystallinity controls the final film texture4 Adhesion layer • Pt3Ti 6 and TiO2 7 at the film-Pt interface seed (111) orientation Relative flux • Relative flux of oxides effects final orientation7 PtxPb phase forms at the interface of the Pt electrode and the thin film.5 Pyrolysis: 350°C, 10s (111) texture4 Pyrolysis: 450°C, 2 min (100) texture 3S. Y. Chen and I. W. Chen, J. Am. Ceram. Soc. 81 (1998) 97. 4G. J. Norga et al, J. Mater. Res. 18 (2003) 1232. 5 Z. Huang et al, J. Appl. Phys. 85 (1999) 7355. 6 T. Tani, PhD Thesis (UIUC, Urbana - Champaign, 1993). 7 P. Muralt, J. Appl. Phys. 100 (2006) 051605.

5. Methodology • in situ crystallization experiments to understand the inter-relationship between phase evolution and texture • TEM to characterize microstructure and chemical inhomogeneties in the crystallized film • X-ray diffraction to characterize texture 8R. D. Klissurska et al, J. Am. Ceram. Soc. 78 (1995) 1513.

6. Film deposition • Solutions prepared through IMO process9 • All solutions prepared with 10% excess Pbcontent • Films pyrolyzed at 300°C • Films in situ crystallized at APS Film layer stack 9 R. A. Assink and R. W. Schwartz, Chem. Mater. 5 (1993) 511.

7. APS: Experimental setup Detector image Sample/Stage Detector X-ray beam path Shutter Sample heated with IR lamp • Experiments were conducted at Advanced Photon Source (APS) (synchrotron X-rays) to study phase evolution at high heating rates. • Heating rates varied between ~100 °C/s to 1°C/s. • 2-D detector allows for characterization of texture. • Continuous diffraction patterns collected with 1s acquisition time.

8. Data Extraction • Diffraction intensities in a limited azimuthal (g) range (85° - 95°) were binned to generate 2q vs. Intensity data • 2q vs. Intensity data generated for each diffraction pattern 2q g

9. Data Extraction • Variation of peak intensity with g indicates texture. • Azimuthal (g) section at specific 2q were binned • Binning was done for each acquisition. • Data generated for each sample is represented as a contour plot. Alumina powder on substrate Diffraction pattern of a textured thin film

10. Method of texture representation Intensity at 90° indicates (100) texture Intensity at 36° and 144° indicates (111) texture • Variation of intensity of the (100) perovskite peaks with g is plotted against time. • Intensity vs gamma data for the (100) peak is similar to that obtained through a typical c-scan.

11. PZT Pe Pe Pe Pe Pe Pe F F F F • Fastest heating rate: ~100°C/s PtxPb • No overlap in PtxPb and perovskite phases • Fluorite phase directly precedes perovskite phase Pe: Perovskite F: Fluorite Pt

12. Heating rate influences phase evolution ~100°C/s ■ ■ ■ ■ ■ ■ PtxPb ■ ■ ■ • No overlap in PtxPb and perovskite phases PtxPb ● ● ● ● ● ● ● ● ▼ ▼ ~5°C/s • Amount of PtxPb formed decreases with heating rate Fix error bars ▼ Pt ■ Perovskite ● Fluorite

13. Heating rate influences phase evolution ~1°C/s • Crystallinity of fluorite phase changes with heating rate ~0.5°C/s ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ● ● ● ● ● ● PtxPb • No PtxPb formation is observed ▼ ▼ ▼ Pt ■ Perovskite ● Fluorite

14. Variation of PtxPb with heating rate • Maximum intensity of PtxPb increases with heating rate • No PtxPb observed to form in slowest heating rate • Observed stability of PtxPb is consistent with ex situ observations* • No overlap is observed between the PtxPb phase and the perovskite phase, indicating one probably does not template the other *S. Y. Chen and I. W. Chen, J. Am. Ceram. Soc. 77 (1994) 2332.

15. Texture consistent with literature* • (100) texture decreases with increasing heating rate • In fast heating rates, homogenous nucleation may dominate over heterogeneous nucleation and growth from the bottom electrode • No overlap between PtxPb and perovskite phase is observed • Crystallinity of fluorite phase seems to change with heating rate Fast heating rate achieved by directly placing sample in preheated furnace. *S. Y. Chen and I. W. Chen, J. Am. Ceram. Soc. 81 (1998) 97.

16. Pt Pt Microstructure and chemistry 100°C/second • Rosetta-type grain structure • Porosity observed in the middle of the film • Some Pb loss at surfaces • Ti (PbTiO3?) segregation at interface, < 50 nm thick Ti Ti • 5°C/second • Large Zr/Ti compositional gradient through film • Ti preference at interface, < 50 nm thick

17. Pt Pt Microstructure and chemistry • 1°C/second • Some Zr/Ti segregation through thickness, but less than faster ramp rates • No preferential Ti near interface Ti Ti • 0.5°C/second • No preferential Ti near interface • Some Zr/Ti segregation through thickness, but less than faster ramp rates.

18. Origin of Ti segregation at fast heating Ti maps Ti-segregation near bottom electrode interface at fast heating rates. • PbTiO3-rich nuclei at fast heating rates • PbTiO3-rich PZT requires lower energy for nucleation • Ti diffusion from adhesion layer at fast heating rates • Ti present in Si/SiO2/Ti/Pt adhesion layer could diffuse to the Pt-film interface to form TiO2 • Both processes need considerable diffusion • Initial work suggests that segregation is due to Ti diffusion from the adhesion layer 5°C/s 100°C/s 0.5°C/s 1°C/s F. Calame and P. Muralt, Applied Physics Letters 90 (2007). P. Muralt et al., J. Appl. Phys. 83 (1998) 3835.

19. Results from RC measurements • Seeding due to Ti segregation? • Work by collaborators suggests that considerable amount of Ti diffuses from the adhesion layer to the top of the electrode

20. Evolution of Fluorite phase 100 °C/s • The amorphous hump is observed to transform continuously to the fluorite phase • This trend is observed to be consistent for all the heating rates investigated • No preferred orientation was observed in the fluorite phase 0.5 °C/s 5 °C/s

21. Evolution of Fluorite phase B. A. Tuttle et al. J. Mater. Res. (1992) Nanoscale regions fluorite type phase formed after pyrolysis

22. Conclusions • PtxPb might not directly nucleate (111) texture in solution deposited PZT thin films • The crystallinity of the fluorite phase is observed to change with different heating rate. • RC measurement: conclusion • No evidence for textured fluorite phase was observed

23. Phase and texture evolution in solution deposited PZT thin films Krishna Nittala*1, Geoff L. Brennecka2, Bruce A. Tuttle2, Douglas S. Robinson3, Jon F. Ihlefeld2, Bryan Gauntt2, and Jacob L. Jones1 1 2 3 Sungwook Mhin and Katherine Dunnigan Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

24. PtxPb: Pole Figures • Pt • PtxPb

25. No evidence for texture in Fluorite 100°C/s 5°C/s 1°C/s 0.5°C/s