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
21 3
Nucleation and Growth in Solution Derived PZT Thin Films
Introduction• Lead zirconate titanate
(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)
Nucleation and Growth in Solution Derived PZT Thin Films
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
2K. Nittala et al, Journal of Materials Science 2011.
Phase evolution during crystallization of PLZT based thin films heated at 5°C/min.2
Nucleation and Growth in Solution Derived PZT Thin Films
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
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.
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
6 T. Tani, PhD Thesis (UIUC, Urbana - Champaign, 1993).7 P. Muralt, J. Appl. Phys. 100 (2006) 051605.
Nucleation and Growth in Solution Derived PZT Thin Films
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.
Nucleation and Growth in Solution Derived PZT Thin Films
Film deposition• Solutions prepared through
IMO process9
• All solutions prepared with 10% excess Pb content
• Films pyrolyzed at 300°C
• Films in situ crystallized at APS
40 nm
~ 1mm
350 nm
Ti
Pt
SiO2
Sisubstrate
PZT
170 nm
400 nm
Film layer stack
9 R. A. Assink and R. W. Schwartz, Chem. Mater. 5 (1993) 511.
Nucleation and Growth in Solution Derived PZT Thin Films
APS: Experimental setup
Detector Sample/Stage Shutter X-ray beam
path
• 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.
Detector image
Sample heated with IR lamp
Nucleation and Growth in Solution Derived PZT Thin Films
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
Nucleation and Growth in Solution Derived PZT Thin Films
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
Nucleation and Growth in Solution Derived PZT Thin Films
Method of texture representation
• 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.
Intensity at 90° indicates (100) texture
Intensity at 36° and 144° indicates (111) texture
Nucleation and Growth in Solution Derived PZT Thin Films
PZTPe Pe Pe Pe Pe Pe
F F F F
Pt
PtxPb
Pe: PerovskiteF: Fluorite
• Fastest heating rate: ~100°C/s
• No overlap in PtxPb and perovskite phases
• Fluorite phase directly precedes perovskite phase
Nucleation and Growth in Solution Derived PZT Thin Films
■ ■ ■ ■ ■ ■
● ● ● ●
PtxPb
▼
■ ■ ■
● ● ● ●
▼
PtxPb
▼ Pt ■ Perovskite ● Fluorite
Fix error bars~5°C/s
~100°C/s• No overlap in PtxPb and perovskite phases
• Amount of PtxPb formed decreases with heating rate
Heating rate influences phase evolution
Nucleation and Growth in Solution Derived PZT Thin Films
Heating rate influences phase evolution
■ ■ ■ ■ ■●
PtxPb
▼
■ ■ ■ ■ ■ ■● ● ● ● ●
▼▼ Pt ■ Perovskite ● Fluorite
• Crystallinity of fluorite phase changes with heating rate
• No PtxPb formation is observed
~0.5°C/s
~1°C/s
Nucleation and Growth in Solution Derived PZT Thin Films
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.
Nucleation and Growth in Solution Derived PZT Thin Films
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.
𝑓ℎ𝑘𝑙 = 𝐼ℎ𝑘𝑙𝐼100 +𝐼111 +𝐼𝑟𝑎𝑛𝑑𝑜𝑚
*S. Y. Chen and I. W. Chen, J. Am. Ceram. Soc. 81 (1998) 97.
Nucleation and Growth in Solution Derived PZT Thin Films
Microstructure and chemistry100°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
Pt
Ti
Pt
Ti5°C/second• Large Zr/Ti compositional
gradient through film• Ti preference at interface,
< 50 nm thick
Nucleation and Growth in Solution Derived PZT Thin Films
Pt
Microstructure and chemistry
Ti• 1°C/second• Some Zr/Ti segregation
through thickness, but less than faster ramp rates
• No preferential Ti near interface
Pt
Ti • 0.5°C/second• No preferential Ti near
interface• Some Zr/Ti segregation
through thickness, but less than faster ramp rates.
Nucleation and Growth in Solution Derived PZT Thin Films
Origin of Ti segregation at fast heating
Ti-segregation near bottom electrode interface at fast heating rates.
1. PbTiO3-rich nuclei at fast heating rates– PbTiO3-rich PZT requires lower energy for
nucleation
2. 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
Ti maps 100°C/s 5°C/s
1°C/s 0.5°C/s
F. Calame and P. Muralt, Applied Physics Letters 90 (2007).P. Muralt et al., J. Appl. Phys. 83 (1998) 3835.
Nucleation and Growth in Solution Derived PZT Thin Films
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
Nucleation and Growth in Solution Derived PZT Thin Films
Evolution of Fluorite phase100 °C/s
5 °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
Nucleation and Growth in Solution Derived PZT Thin Films
Evolution of Fluorite phase
B. A. Tuttle et al. J. Mater. Res. (1992)
Nanoscale regions fluorite type phase formed after pyrolysis
Nucleation and Growth in Solution Derived PZT Thin Films
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
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
21 3
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.
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.
Nucleation and Growth in Solution Derived PZT Thin Films
PtxPb: Pole Figures
• Pt • PtxPb
Nucleation and Growth in Solution Derived PZT Thin Films
No evidence for texture in Fluorite
100°C/s 5°C/s
1°C/s 0.5°C/s