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