Enhanced critical temperature in epitaxial ferroelectric Pb(Zr_0.2Ti_0.8)O₃ thin films on silicon

Article

Reference

Enhanced critical temperature in epitaxial ferroelectric Pb(Zr

_0.2

Ti

_0.8

)O

₃

thin films on silicon

SAMBRI, Alessia, et al.

Abstract

The structural and electrical properties of epitaxialPb(Zr0.2Ti0.8)O3thin filmsgrown on 2 in.

(001) silicon wafers were investigated. Using x-ray diffraction, the lattice behavior of the heterostructure has been studied as a function of temperature, suggesting a 250 °C increase of the Pb(Zr0.2Ti0.8)O3 ferroelectric-paraelectric transition temperature with respect to the bulk value. This significant enhancement of the critical temperature is understood in terms of a two-dimensional clamping effect.

SAMBRI, Alessia, et al . Enhanced critical temperature in epitaxial ferroelectric Pb(Zr

_0.2

Ti

_0.8

)O

₃

thin films on silicon. Applied Physics Letters , 2011, vol. 98, no. 1, p. 012903

DOI : 10.1063/1.3532110

Available at:

http://archive-ouverte.unige.ch/unige:46979

Disclaimer: layout of this document may differ from the published version.

1 / 1

Enhanced critical temperature in epitaxial ferroelectric Pb „ Zr

_0.2

Ti

_0.8

… O

₃

thin films on silicon

A. Sambri,^1,a兲 S. Gariglio,¹A. Torres Pardo,¹J.-M. Triscone,¹O. Stéphan,²J. W. Reiner,³ and C. H. Ahn³

1Department of Condensed Matter Physics (DPMC), University of Geneva, 24 quai Ernest-Ansermet, 1211 Geneva 4, Switzerland

2Laboratoire de Physique des Solides, Université Paris-Sud, CNRS-UMR 8502, Orsay 91405, France

3Department of Applied Physics, Yale University, P.O. Box 208284, New Haven, Connecticut 06520-8284, USA

共Received 26 July 2010; accepted 3 December 2010; published online 7 January 2011兲

The structural and electrical properties of epitaxial Pb共Zr0.2Ti_0.8

兲O

3thin films grown on 2 in.

共001兲

silicon wafers were investigated. Using x-ray diffraction, the lattice behavior of the heterostructure has been studied as a function of temperature, suggesting a 250 ° C increase of the Pb共Zr0.2Ti_0.8

兲O

3

ferroelectric-paraelectric transition temperature with respect to the bulk value. This significant enhancement of the critical temperature is understood in terms of a two-dimensional clamping effect. ©2011 American Institute of Physics.

关doi:10.1063/1.3532110兴

The push toward size and voltage downscaling has been driving a large effort aiming at the integration of functional materials into silicon technology. From this perspective, epitaxial thin films are of particular interest due to their characteristics and performances.^1,2 For ferroelectric/

piezoelectric materials, examples of applications that will benefit from this achievement are ferroelectric random ac- cess memories, surface acoustic wave devices, and piezoelectric-driven microelectromechanical systems

共piezo-

MEMS兲.

The epitaxial growth of ferroelectric oxides on silicon is, however, challenging due to the difference in chemical bond- ings, lattice parameters, and thermal expansion coefficients.

These issues, together with the problems related to the ther- modynamic instability and chemical reactivity of the metal- oxygen-silicon system, limit the choice of oxides that can be epitaxially grown on silicon while avoiding the formation of an amorphous SiO₂layer. A possible way to achieve epitaxial oxides on silicon is to use buffer layers, acting as structural templates.^3,4

Among them, SrTiO₃

共STO兲

is the most studied oxide since it can be grown with a very high degree of crystallinity.⁵The choice of STO determines the lattice mis- match and the strain state of the resulting heterostructure.

For ferroelectric materials, it is well known that the mechani- cal boundary conditions may affect substantially the ferro- electric properties because of the strong strain-polarization coupling often found in these compounds.^6–11

In this letter, we address the role of epitaxy on the ferro- electric and structural properties of Pb共Zr0.2Ti_0.8

兲O

3

共PZT兲

thin films grown on STO-buffered silicon wafers. In particu- lar, using temperature-dependent x-ray diffraction

共

XRD

兲

, we study the lattice evolution of the PZT thin films. The results are analyzed using theoretical predictions of the effect of a two-dimensional clamping.¹²

The investigated epitaxial heterostructure is composed of a ferroelectric PZT layer grown on top of a metallic SrRuO₃

共SRO兲

film used as a bottom electrode. This bilayer is depos-

ited onto a thin STO film epitaxially grown on a 2 in.

共001兲

silicon wafer. Such STO layer, besides being a good lattice template for SRO and PZT, also provides a barrier for Pb diffusion into the silicon wafer.^13–15 Typical thicknesses of the final stack are PZT共100 nm兲/SRO共30 nm兲/STO共6 nm兲/Si.

The growth of epitaxial STO films on silicon wafers is per- formed using molecular beam epitaxy through a complex multistep procedure monitored in situ using reflection high energy electron diffraction.^4,5,16,17Successively, the epitaxial SRO layer is grown by off-axis magnetron sputtering in 130 mTorr of an oxygen/argon mixture at a substrate temperature of 500 ° C. Finally, with the same technique, a PZT thin film is deposited at a growth temperature of 450 ° C in a working pressure of 200 mTorr.

The out-of-plane orientation and the cube-on-cube ar- rangement of the oxide stack on silicon are verified through

␪^-2␪ ^and ␾-scan diffractograms, performed with a high- resolution PANalytical X’Pert diffractometer, equipped with a four-bounce asymmetric Ge共220兲 monochromator for CuK␣1radiation. Using the pseudocubic notation for SRO, the measurements confirm the following crystallographic relations: PZT

关

001

兴

//SRO

关

001

兴

//STO

关

001

兴

//Si

关

001

兴

and PZT关100兴//SRO关100兴//STO关100兴//Si关110兴, as shown in Fig.

1. Finite size oscillations around the

共001兲

PZT and

共001兲

SRO Bragg peaks indicate the high crystalline coherence of the oxide layers.

a兲Electronic mail: alessia.sambri@unige.ch.

FIG. 1. 共Color online兲 共a兲X-ray diffraction,␪-2␪scan of an epitaxial het- erostructure: PZT共100 nm兲and SRO共30 nm兲layers(top)and STO共6 nm兲 buffer layer (bottom).共b兲Azimuthal␾-scans of PZT共101兲and Si 共202兲, showing the 45° rotated cube-on-cube epitaxy.

APPLIED PHYSICS LETTERS98, 012903共2011兲

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To quantify the amount of epitaxial strain present in our heterostructure, we measured the in-plane lattice parameters of each layer. At room temperature, bulk STO

共cubic, 3.905

Å兲 has a lattice mismatch on silicon

共cubic, 5.43 Å

/

冑

2

= 3.84 Å

兲

of ⫺1.7%. However, structural characterizations by x-ray diffraction and transmission electron microscopy

共TEM兲

reveal a fully relaxed STO layer. In the case of SRO

共in the pseudocubic structure

a= 3.93 Å兲, a fully coherent layer would have a lattice mismatch on STO of ⫺0.6%. In- stead, for the considered thickness, we observe a partially relaxed SRO layer with a= 3.918 Å. For a c-axis oriented PZT

共tetragonal,

a= 3.952 Å andc= 4.148 Å兲 on SRO, the in-plane lattice mismatch is expected to be⫺0.5%. The lat- tice parameters calculated from the

共

00l

兲

and

共

101

兲

PZT Bragg diffraction peaks indicate values of a= 3.965 Å and c= 4.146 Å. Reciprocal space maps around PZT共⫺103兲and SRO共⫺103兲 reflections confirm the relaxation of the PZT thin film on the SRO layer.

TEM characterization

关

Fig.2

共

a

兲兴

, performed on a JEOL JEM2010 microscope, reveals the presence of a thin SiO₂ amorphous layer of about 4.5 nm at the interface between silicon and the oxide multilayer. This layer does not affect the well ordered PZT/SRO/STO structure, as revealed by high-resolution TEM

共

HRTEM

兲

image shown in the inset of Fig. 2共a兲. The formation of this SiO2 layer occurs after the growth of the STO/Si interface template through oxygen dif- fusion, thus not hindering the epitaxy.^18–20

As shown in Fig. 2

共

b

兲

, polarization-voltage loops, per- formed at room temperature with a tester TF analyzer 2000 on 100⫻100 ␮m² Cr/Au top electrodes, reveal a remanent polarization and a coercive field of about 70 ␮^{C cm−2 and} 250 kV cm⁻¹, respectively.

In order to probe the structural phase transition of the ferroelectric PZT film, the evolution of the lattice parameters in temperature has been determined by XRD measurements between 30 and 800 ° C. As shown in Fig. 3共a兲, a linear expansion of the in-plane lattice parameter is observed for PZT, SRO, and Si. From their lattice parameter versus tem- perature slope, we evaluate the thermal expansion coeffi- cients ␣PZT= 1.1⫻10⁻⁵ ° C⁻¹, ␣SRO= 1.2⫻10⁻⁵ ° C⁻¹, and

␣Si= 4⫻10⁻⁶ ° C⁻¹. These values are in good agreement with literature data^21,22and indicate a mechanical decoupling be- tween the PZT/SRO layers and the silicon substrate. The amorphous SiO₂ layer

共

␣SiO₂= 5⫻10⁻⁷ ° C⁻¹

兲, observed by

TEM at the interface between the oxide stack and the silicon

substrate, might be at the origin of this decoupling, accom- modating the different expansions of the two lattices.²³

Increasing the temperature, the PZTc-axis decreases up to

⬃710 ° C, and then becomes

T independent up to

⬃810 ° C, the limit of our measurement. The corresponding

evolution in temperature of the PZT tetragonality is shown in Fig. 3共b兲. The behaviors of thea- and c-axes for bulk PZT are also shown in Fig. 3共a兲as a reference. The reversibility of the lattice thermal behavior has been checked by perform- ing heating-cooling cycles. For our experimental set-up

共

an- nealing in air兲, the room temperature lattice parameters were fully recovered for temperature cycles up to 700 ° C. Above this temperature, the crystal quality estimated from the dif- fracted intensity is progressively affected by the annealing process.²⁴

These measurements strongly suggest that the structural change, associated with the ferroelectric-paraelectric phase transition,^7,8 occurs at or above 700 ° C, which is 250 ° C higher than in the bulk. We also note that the system remains tetragonal above the transition temperature, as already re- ported for other ferroelectric thin films.^10,11This observation, together with the measured increase of the critical tempera- ture, can be explained as the result of the unit cell clamping between the PZT film and the other crystalline oxide layers.

Indeed, according to the model proposed by Pertsev,⁶ the

FIG. 2. 共Color online兲 共a兲Cross-sectional TEM image, revealing the pres- ence of a 4.5 nm thick amorphous SiO₂layer at the interface between silicon and the oxide multilayer. Inset: cross-sectional HRTEM picture showing the epitaxy of the oxide stack.共b兲Current-voltage loop共red䉱兲and correspond- ing polarization-voltage loop共black쎲兲at 100 Hz共top-top measurements兲 for a 100 nm ferroelectric PZT epitaxial thin film on SRO/STO/Si.

FIG. 3.共Color online兲 共a兲Evolution in temperature of the lattice parameters for the silicon substrate共green쐓兲, SROa-axis共blue䉭兲, PZTa-axis共red䊊兲, and c-axis 共red쎲兲. The dotted lines represent the bulk PZT temperature evolution 共data from Ref.11兲:c-axis in red anda-axis in black.共b兲The temperature dependence of the tetragonality共c/a兲shows a kink at around 700 ° C.

012903-2 Sambriet al. Appl. Phys. Lett.98, 012903共2011兲

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in-plane constraint of the substrate induces a strain which modifies the Gibbs energy of the ferroelectric film and leads to a change in the paraelectric-ferroelectric phase transition temperatureTcand the ferroelectric polarization. Such strain, which takes into account the two-dimensional mechanical clamping, is defined asmisfit strain Sm

共T兲

and can be calcu- lated as

Sm

共T兲

=af−ao

af

+

共

␣s−␣o

兲共T

−Tg

兲, 共1兲

wherea_o is the equivalent cubic cell parameter of the free- standing film, af is the effective substrate parameter,²⁵ both calculated at the growth temperature Tg, and ␣s and ␣o are the thermal expansion coefficients of the substrate and of the bulk compound, respectively. The first term represents the strain arising at the growth temperature, while the second term takes into account the stress occurring during the cool- ing due to the thermal expansion mismatch between the film and the substrate.

In the case of oxides grown on silicon, this last term is expected to be significant. However, as revealed by the ther- mal expansion coefficients experimentally determined for this structure, the oxide stack expands independently from the silicon substrate. As a consequence, the relevant misfit strain will only be given by the first term in Eq.

共

1

兲

, which leads to Sm= −7.5⫻10⁻³. According to the phase diagram calculated by Pertsev¹²for Pb共Zr0.2Ti_0.8

兲O

3thin films, such a misfit strain increasesTcto a value of about 660 ° C, a value relatively close to the experimentally measured one.

Through the strain/polarization coupling expressed by the equation

共T

⬍Tc

兲 共Refs.

26and27兲

P

共

T

兲 ⬃ 冑

a^c

共

T

兲

− c

a

共

T_c

兲

,

共

2

兲

it is possible to estimate the temperature evolution of the remanent polarization. A rough estimation of the polarization at 300 ° C yields values of several tens of ␮^{C cm−2. Such} large values suggest that epitaxial PZT thin films on silicon could be suitable for high temperature applications.

In conclusion, the effect of the mechanical boundary conditions on the properties of high quality epitaxial PZT/

SRO/STO heterostructures on silicon has been investigated.

Temperature-dependent XRD measurements show a change in the slope of the PZT tetragonality at around 700 ° C, an increase of about 250 ° C with respect to the bulk, in agree- ment with theoretical predictions for two-dimensional clamping. Our measurements also reveal excellent ferroelec- tric properties at room temperature with a remanent polariza- tion value of 70 ␮^{C cm−2}. These results confirm the poten- tial of a fully epitaxial approach for oxide growth on silicon for the fabrication of novel piezoelectric/ferroelectric devices.^28,29In particular, the possibility to obtain high qual- ity epitaxial PZT films exhibiting a transition temperature much higher than the bulk can be an advantage in terms of thermal budget for device microfabrication processing and operation.

The authors gratefully acknowledge P. Zubko and C.

Cancellieri for many fruitful discussions, M. Lopes for the technical support, F. Guy at the TIN-HEPIA, A. Gloter, and

K. March. This work was supported by the Swiss National Science Foundation through the National Center of Compe- tence in Research “Materials with Novel Electronic Properties-MaNEP” and the EU Project Oxides. The work at Yale was supported by the NSF Grant No. MRSEC DMR 0520495, NSF Grant No. DMR 1006256, ONR, and FENA.

1R. Ramesh and D. G. Schlom, MRS Bull. 33, 1006共2008兲.

2Y. Wang, C. Ganpule, B. T. Liu, H. Li, K. Mori, B. Hill, M. Wuttig, R.

Ramesh, J. Finder, Z. Yu, R. Droopad, and K. Eisenbeiser, Appl. Phys.

Lett. 80, 97共2002兲.

3K. J. Hubbard and D. G. Schlom,J. Mater. Res. 11, 2757共1996兲.

4R. A. McKee, F. J. Walker, and M. F. Chisholm,Phys. Rev. Lett. 81, 3014 共1998兲.

5J. W. Reiner, A. M. Kolpak, Y. Segal, K. F. Garrity, S. Ismail-Beigi, C. H.

Ahn, and F. J. Walker,Adv. Mater.共Weinheim, Ger.兲 22, 2919共2010兲.

6N. A. Pertsev, A. G. Zembilgotov, and A. K. Tagantsev,Phys. Rev. Lett.

80, 1988共1998兲.

7K. J. Choi, M. Biegalski, Y. L. Li, A. Sharan, J. Schubert, R. Uecker, P.

Reiche, Y. B. Chen, X. Q. Pan, V. Gopalan, L.-Q. Chen, D. G. Schlom, and C. B. Eom,Science 306, 1005共2004兲.

8J. H. Haeni, P. Irvin, W. Chang, R. Uecker, P. Reiche, Y. L. Li, S.

Choudhury, W. Tian, M. E. Hawley, B. Craigo, A. K. Tagantsev, X. Q.

Pan, S. K. Streiffer, L. Q. Chen, S. W. Kirchoefer, J. Levy, and D. G.

Schlom,Nature共London兲 430, 758共2004兲.

9D. G. Schlom, L.-Q. Chen, C.-B. Eom, K. M. Rabe, S. K. Streiffer, and J.-M. Triscone,Annu. Rev. Mater. Res. 37, 589共2007兲.

10S. Gariglio, N. Stucki, J.-M. Triscone, and G. Triscone,Appl. Phys. Lett.

90, 202905共2007兲.

11P.-E. Janolin, B. Fraisse, F. Le Marrec, and B. Dkhil,Appl. Phys. Lett. 90, 212904共2007兲.

12N. A. Pertsev, V. G. Kukhar, H. Kohlstedt, and R. Waser,Phys. Rev. B67, 054107共2003兲.

13D. S. Shin, S. T. Park, H. S. Choi, I. H. Choi, and J. Y. Lee,Thin Solid Films 354, 251共1999兲.

14Ø. Nordseth, C. C. You, E. Folven, S. Gariglio, A. Sambri, J.-M. Triscone, J. W. Reiner, C. H. Ahn, T. Tybell, and J. K. Grepstad,Thin Solid Films

518, 5471共2010兲.

15E. Tokumitsu, K. Itani, B.-K. Moon, and H. Ishiwara,Jpn. J. Appl. Phys., Part 1 34, 5202共1995兲.

16J. W. Reiner, K. F. Garrity, F. J. Walker, S. Ismail-Beigi, and C. H. Ahn, Phys. Rev. Lett. 101, 105503共2008兲.

17J. Lettieri, J. H. Haeni, and D. G. Schlom,J. Vac. Sci. Technol. A 20, 1332 共2002兲.

18G. J. Norga, C. Marchiori, C. Rossel, A. Guiller, J. P. Locquet, H. Sieg- wart, D. Caimi, J. Fompeyrine, J. W. Seo, and Ch. Dieker,J. Appl. Phys.

99, 084102共2006兲.

19L. V. Goncharova, D. G. Starodu, E. Garfunkel, T. Gustafsson, V. Vaithy- anathan, J. Lettieri, and D. G. Schlom,J. Appl. Phys.100, 014912共2006兲.

20V. Shutthanandan, S. Thevuthasan, Y. Liang, E. M. Adams, Z. Yu, and R.

Droopad,Appl. Phys. Lett. 80, 1803共2002兲.

21D. M. Kim, C. B. Eom, V. Nagarajan, J. Ouyang, R. Ramesh, V. Vaithy- anathan, and D. G. Schlom,Appl. Phys. Lett. 88, 142904共2006兲.

22J. Cheng, L. He, S. Yu, and Z. Meng,Appl. Phys. Lett. 88, 152906共2006兲.

23D. E. McCready, Y. Liang, V. Shutthanandan, C. M. Wang, and S. The- vuthasan, JCPDS-International Centre for Diffraction Data ISSN 1097- 0002共2006兲.

24P. Paruch and J.-M. Triscone,Appl. Phys. Lett. 88, 162907共2006兲.

25J. S. Speck and W. Pompe,J. Appl. Phys. 76, 466共1994兲 共note that the effective substrate in-plane lattice parameter is here thea-PZT film lattice parameter atT_g兲.

26U. Waghmare and K. Rabe,Phys. Rev. B 55, 6161共1997兲.

27M. Dawber, N. Stucki, C. Lichtensteiger, S. Gariglio, P. Ghosez, and J.-M.

Triscone,Adv. Mater.共Weinheim, Ger.兲 19, 4153共2007兲.

28D. Isarakorn, A. Sambri, P. Janphuang, D. Briand, S. Gariglio, J.-M.

Triscone, F. Guy, J. W. Reiner, C. H. Ahn, and N. F. de Rooij,J. Micro- mech. Microeng. 20, 055008共2010兲.

29D. Isarakorn, D. Briand, A. Sambri, S. Gariglio, J.-M. Triscone, F. Guy, J.

W. Reiner, C. H. Ahn, and N. F. de Rooij, Sens. Actuators B 共to be published兲, 10.1016/j.snb.2010.10.009.

012903-3 Sambriet al. Appl. Phys. Lett.98, 012903共2011兲

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