Eng, L. M.

Ganpule, C.

Pertsev, N. A.

Roelofs, A.

Schlaphof, F.

Waser, R.

Juelich Shared Electronic Resources

APPLIED PHYSICS LETTERS VOLUME 80, NUMBER 8 25 FEBRUARY 2002Depolarizing-field-mediated 180° switching in ferroelectric thin filmswith 90° domainsA. RoelofsInstitut fu¨r Werkstoffe der Elektrotechnik, RWTH Aachen, D-52056 Aachen, GermanyN. A. Pertseva) and R. WaserElektrokeramische Materialen, Institut fu¨r Festko¨rperforschung, Forschungszentrum Ju¨lich, D-52425 Ju¨lich,GermanyF. Schlaphof and L. M. EngInstitut fu¨r angewandte Photophysik, Technische Universita¨t Dresden, D-01062 Dresden, GermanyC. Ganpule, V. Nagarajan, and R. RameshMaterials Science Research and Engineering Center, University of Maryland, College Park, Maryland 20742~Received 13 August 2001; accepted for publication 5 December 2001!Switching of the out-of-plane and in-plane polarizations in polydomain epitaxial PbZr0.2Ti0.8O3 thinfilms is studied using three-dimensional piezoresponse force microscopy ~PFM!. It is found that,under an electric field induced between the PFM tip and the bottom electrode, the 180° switchingoccurs in both c and a domains. After the removal of this field, the spontaneous reversal of theout-of-plane and in-plane polarizations back to the initial orientations takes place, evolving viaheterogeneous development of antiparallel 180° domains. The switching of in-plane polarizationinside a domains and the preferential formation of reversed 180° domains at 90° domain walls areexplained by the effects of the depolarizing fields caused by transient polarization charges appearingon these domain walls. © 2002 American Institute of Physics. @DOI: 10.1063/1.1448653#Recent advances in the application of the scanning forcemicroscopy to the studies of ferroelectric films made pos-sible the nanoscale imaging of polarization reversal.1 A pow-erful tool to investigate and manipulate the domain structureis a modification of the scanning probe microscope2 ~SPM!termed piezoresponse force microscope ~PFM!.3 Up untilnow, PFM was applied mainly for the detection of the polar-ization components normal to the film surface.4–6 Recently,it was shown that this technique enables the observation ofin-plane polarization as well.7In this letter, we report PFM investigations of the polar-ization switching in epitaxial PbZr0.2Ti0.8O3 ~PZT! thin filmscontaining multiple 90° domains ~twins!. Changes of bothout-of-plane and in-plane polarizations in the film are moni-tored. A phenomenological theory is used to explain the ex-perimental observations.The ferroelectric heterostructure used in this study con-sisted of a 400 nm-thick PZT film epitaxially grown on a@001# single crystalline SrTiO3 ~STO! substrate with an in-termediate 50 nm-thick La0.5Sr0.5CoO3 ~LSCO! oxide layeremployed as the bottom electrode. PZT films were fabricatedby pulsed laser deposition at Tg5650 °C and a partial oxy-gen pressure of 100 mTorr ~see Ref. 8 for details!. At roomtemperature, as-grown PZT films displayed the presence of atwo-dimensional grid of 90° domains embedded into theferroelectric matrix with the tetragonal c axis being orientedorthogonal to the substrate.5a!Permanent address: A. F. Ioffe Physico-Technical Institute, Russian Acad-emy of Sciences, 194021 St. Petersburg, Russia; electronic mail:pertsev@domain.ioffe.rssi.ru1420003-6951/2002/80(8)/1424/3/$19.00Downloaded 15 Dec 2006 to 134.94.122.39. Redistribution subject toNanoscale visualization of the film domain structure wascarried out with the aid of a commercial Digitial InstrumentsNanoscope IIIA Multimode SPM. Details of the PFM tech-nique can be found elsewhere.3 This technique enables us todifferentiate between the c and a domains and to determinetheir polarization directions. Investigation of the as-grownfilm showed that the out-of-plane polarization in c domainspoints preferentially towards the bottom electrode @see Fig.1~a!#. In the further discussion, such domains exhibiting adownward polarization will be termed c2 domains. From theasymmetry of the piezoresponse signal across a particular adomain, it can be deduced at which angle ~45° or 135°! theboundaries of this a domain are inclined to the substrate.5The inspection of the piezoresponse images shows that theobserved configuration is always of the ‘‘head-to-tail’’ type~see Ref. 9 for detailed analysis!.The native c2 domains were switched to 180° into c1domains by applying a negative voltage of 28 V to the con-ductive cantilever and ground to the bottom electrode whilescanning a square pattern of approximately 434 mm2. Thisarea then was rescanned with a zero dc field between the tipand electrode. From the piezoresponse image shown in Fig.1~c!, we see that the poled c2 domains transformed almostcompletely into the c1 ones. Moreover, the in-plane polar-ization in most parts of the a domains was observed toswitch by 180° as well @see Fig. 1~d!#, though the electricfield is applied mainly in the film-thickness direction.5Figure 2 demonstrates the evolution of the piezoresponseimages during the time period after poling. It can be seenthat the polarization configuration gradually relaxes back tothe initial state. Several needle-like c2 domains appear near90° walls and then grow laterally in the film plane, as shown4 © 2002 American Institute of Physics AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp1425Appl. Phys. Lett., Vol. 80, No. 8, 25 February 2002 Roelofs et al.by circles in Figs. 2~a! and 2~c!. Similar results were recentlyreported for ferroelectric BaTiO3 ceramics under compres-sive stress.10 The backswitching of the polarization may alsodevelop via the expansion of the residual c2 domains adja-cent to nonswitched a domains @see Fig. 1~c! and inset#. Tak-ing a closer look at Fig. 2~c! in the area denoted by blackarrows, we also find that the a domain represents an obstaclefor the in-plane growth of the c2 domains. Remarkably,when the switched area eventually penetrates through theobstacle @white arrow in Fig. 2~c!#, the adjacent part of the adomain appears to be switched back to its initial state @com-pare the regions shown by black arrows in Figs. 2~b! and2~d!#.Now, let us analyze the experimental data with the aid ofa phenomenological theory. The spontaneous polarization Psand equilibrium domain structure of epitaxial ferroelectricfilms should strongly depend on the misfit strain Sm betweenthe film and substrate.11–13 The misfit strain at T535 °C canbe evaluated as Sm’(ab2a0)(T2Tg), where ab and a0 arethe thermal expansion coefficients ~TECs! of the substrateand the film prototypic cubic phase, respectively.14 Takingfor TECs of STO and PZT 20/80 the average values of ab>1131026 K21 and a0>631026 K21,15 we obtainedSm>2331023. At this misfit strain, as follows from thenonlinear thermodynamic theory,11 a well-defined polydo-main c/a/c/a state should form in PZT 20/80 films. More-over, the formation of this domain pattern in the PZT/LSCO/STO epitaxial system during the cooling is predicted toproceed via the following two-stage process: paraelectricFIG. 1. Piezoresponse images of the polydomain ~twinnerd! PZT film in theas-grown states @~a! and ~b!# and after poling @~c! and ~d!#. The ‘‘out-of-plane’’ PFM signal is shown in ~a! and ~c!, whereas the ‘‘in-plane’’ signal in~b! and ~d!. Regions with bright, dark, and intermediate contrast correspondto different orientations of the out-of-plane and in-plane polarizations inferroelectric domains, as shown schematically in the sketches below thePFM images. The ellipse in ~c! and ~d! demonstrates a specific region, wherethe polarization orientation remains unchanged after poling. The inset in ~c!shows that from this region the polarization backswitching starts.Downloaded 15 Dec 2006 to 134.94.122.39. Redistribution subject tophase→single-domain ferroelectric c phase→polydomainc/a/c/a state. Therefore, the a domains nucleate within analready polarized matrix. ~Self-polarization of ferroelectricfilms has been reported several times in literature and attrib-uted to the presence of an internal field in the ferroelectric orat the interface.!16 Since the out-of-plane polarization P3 hasa preferential orientation in our films, it is energetically mostfavorable to form uniformly polarized a domains with thehead-to-tail orientation of the in-plane polarization P1 ~orP2! with respect to the matrix polarization P3, as illustratedin Fig. 3~a!. This conclusion is supported by the analysis ofstability of the plate-like a domains, which have ‘‘head-to-FIG. 2. Evolution of the polarization relaxation after poling. Images repre-senting the out-of-plane @~a! and ~c!# and in-plane @~b! and ~d!# PFM signalswere taken after 104 s @~a! and ~b!# and after 2.63104 s @~c! and ~d!#. Thecircles show reversed needle-like c2 domains near 90° domain walls. Theblack arrows in ~c! indicate that the a domain is an obstacle for the growingc2 domain, within which the polarization backswitching takes place. Thepenetration of the switched area through the obstacle @white arrow in ~c!#also includes 180° switching of the a domain back to its initial polarizationstate @black arrow in ~d!#.FIG. 3. The expected scenario of polarization switching and relaxation inpolydomain PZT films: ~a! initial c/a/c domain state with the native down-ward polarization in the c2 domains, ~b! imaginary transient domain con-figuration during the tip-induced local 180ßD switching, which shows po-larization charges on the section of 90° domain wall swept by moving 180°wall ~related image charges in the electrode are shown as well!, ~c! forwardgrowth of the residual c2 domain after the removal of the applied field ~firststage of the back-switching!, ~d! lateral motion of the 180° domain bound-aries during the second stage of backswitching. AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp1426 Appl. Phys. Lett., Vol. 80, No. 8, 25 February 2002 Roelofs et al.tail’’ or ‘‘tail-to-tail’’ polarization configurations relative toadjacent c ones. Using the nonlinear thermodynamictheory,11 we found that such a domains are completely un-stable in PZT 20/80 films at room temperature. This instabil-ity is due to the effect of depolarizing field.The following scenario may be proposed now for thepolarization switching caused by the inhomogeneous electricfield E(r) induced between the PFM tip and an extendedbottom electrode. Inside c2 domains, the 180° switchingeasily occurs because the out-of-plane field E3(r) is ratherhigh @the mean value ^E3(r)&;20 MV/m#, and the depolar-izing field of the needle-like c1 domain formed under the tippromotes its penetration through the whole film thickness.During further polarization reversal, the c1 domain freelygrows in lateral direction, following the displacement of thePFM tip, until its boundary reaches the c/a domain wall. The90° domain wall represents an obstacle for the moving 180°wall, because once they join, a net polarization charge r5div P appears at the 90° boundary @Fig. 3~b!#.Near the upper film surface, the local driving forceF(r)52PsE3(r) acting on a 180° wall is very high so that iteasily passes through the a domain. This proceeds via thedepolarizing-field-induced 180° switching of the a domain.Indeed, after the switching of the c2 domain, the adjacentunswitched a domain becomes thermodynamically unstable,which ensures its 180° switching even in the absence of anyin-plane applied field.In the vicinity of the bottom electrode, however, the out-of-plane electric field E3(r) becomes relatively small5 so thatthe force F(r) strongly reduces. Moreover, the in-plane fieldEa(r) ~a51,2! disappears at the bottom electrode because itforms an equipotential surface. Therefore, the bottom edge othe 180° wall remains electrostatically pinned at the junctionwith the c/a wall. Since the upper part of the 180° boundarystill follows the moving PFM tip, the wall bends in the filmcross section and becomes S-shaped. At some critical curva-ture, the bent wall loses its stability and splits into a sepa-rated pinned domain boundary and the moving 180° wall@Fig. 3~c!#. As a result, three residual 180° domains are leftnear the junction of the c/a wall with the interface: ~i! in-plane antiparallel domain inside the a domain, and ~ii! twosmall c2 domains adjacent to the 90° walls and surroundedby the c1 domains. As soon as the tip moves away, theresidual c2 domain transforms into a needle-like one @Fig.3~c!# in order to reduce the depolarizing field. Under theaction of internal electric field, the needle-like c2 domainthen grows in the vertical direction until it reaches the uppersurface @Fig. 3~d!#. In the next stage of the polarization re-versal, the c2 domain grows more slowly in the film plane.To check whether the proposed evolution of the residualdomain is possible, we studied the energetics of the c2 do-main. The total energy U of the 180° domain having a vol-ume V and a boundary area S may be evaluated from therelation17U5Udep1gS12PsEVE3~r!dV ,Downloaded 15 Dec 2006 to 134.94.122.39. Redistribution subject towhere Udep is the energy associated with the depolarizingfield, and g is the domain-wall energy per unit area. Assum-ing the domain to be half of a prolate spheroid and usingEqs. ~3.5! and ~4.1! derived in Ref. 17, we studied the de-pendence of U on the domain height h. ~Owing to the elec-trostatic interaction between the c2 domain and the adjacenta one, the transverse size t of the former is fixed at thebottom electrode.! The calculation shows that, for a reason-able values of g, Ps , and the dielectric constants of PZT, thec2 domain has an optimum height h*@t in the absence ofelectric field. This result proves that the transformation of theresidual c2 domain into a needle-like one may be energeti-cally favorable even at the expense of the surface-energyincrease.The aforementioned scenario explains the most remark-able experimental observations, namely, the 180° switchingof the in-plane polarization in a domains near the freesample surface, and the preferential nucleation of the re-versed c2 domains near 90° walls during polarization relax-ation.1 K. Franke, J. Besold, W. Haessler, and C. Seegebarth, Surf. Sci. Lett. 302,L283 ~1994!; T. Hidaka, T. Maruyama, I. Sakai, M. Saitoh, L. A. Wills, R.Hiskes, S. A. Dicarolis, J. Amano, and C. M. Foster, Integr. Ferroelectr.17, 319 ~1997!; O. Auciello, A. Gruverman, H. Tokumoto, A. S. Prakash,S. Aggarwal, and R. Ramesh, MRS Bull. 23, 33 ~1998!; M. Abplanalp, L.M. Eng, and P. Gu¨nter, Appl. Phys. A: Mater. Sci. Process. 66A, S231~1998!.2 G. Binnig and H. Rohrer, Helv. Phys. Acta 55, 726 ~1982!.3 A. Roelofs, F. Schlaphof, S. Trogisch, U. Bo¨ttger, R. Waser, and L. M.Eng, Appl. Phys. Lett. 77, 3444 ~2000!.4 A. Gruverman, O. Auciello, and H. Tokumoto, Annu. Rev. Mater. Sci. 28,101 ~1998!; A. Gruverman, A. Kholkin, A. Kingon, and H. Tokumoto,Appl. Phys. Lett. 78, 2751 ~2001!.5 C. S. Ganpule, V. Nagarajan, H. Li, A. S. Ogale, D. E. Steinhauer, S.Aggarwall, E. Williams, and R. Ramesh, Appl. Phys. Lett. 77, 292 ~2000!.6 C. S. Ganpule, V. Nagarajan, S. B. Ogale, A. L. Roytburd, E. D. Williams,and R. Ramesh, Appl. Phys. Lett. 77, 3275 ~2000!.7 L. M. Eng, H. J. Gu¨ntherodt, G. A. Schneider, U. Ko¨pke, and J. Mun˜ozSaldan˜a, Appl. Phys. Lett. 74, 233 ~1999!.8 S. Madhukar, S. Aggarwal, A. M. Dhote, R. Ramesh, A. Krishnan, D.Keeble, and E. Poindexter, J. Appl. Phys. 81, 3543 ~1997!.9 C. S. Ganpule, B. K. Hill, S. P. Alpay, V. Nagarajan, A. L. Roytburd, E. D.Williams, R. Ramesh, A. Roelofs, R. Waser, and L. M. Eng, J. Appl. Phys.~submitted!.10 J. Munez-Saldana, G. Schneider, and L. M. Eng, Surf. Sci. 480, L402~2001!.11 N. A. Pertsev and V. G. Koukhar, Phys. Rev. Lett. 84, 3722 ~2000!.12 J. S. Speck and W. Pompe, J. Appl. Phys. 76, 466 ~1994!.13 S. P. Alpay, V. Nagarajan, L. A. Bendersky, M. D. Vaudin, S. Aggarwal, R.Ramesh, and A. L. Roytburd, J. Appl. Phys. 85, 3271 ~1999!.14 N. A. Pertsev, A. G. Zembilgotov, S. Hoffmann, R. Waser, and A. K.Tagantsev, J. Appl. Phys. 85, 1698 ~1999!.15 Y. S. Touloukian, R. K. Kirby, R. E. Taylor, and T. Y. R. Lee, ThermalExpansion, Nonmetallic Solutions, Thermophysical Properties of MatterVol. 13 ~Plenum, New York, 1997!.16 A. Kholkin, K. Brooks, D. Tayler, S. Hiboux, and N. Setter, Integr. Fer-roelectr. 22, 525 ~1998!; J. Frey, F. Schlenkrich, and A. Scho¨necker, Integr.Ferroelectr. 35, 105 ~2001!.17 R. Landauer, J. Appl. Phys. 28, 227 ~1957!. AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp

Depolarization-field-mediated 180° switching in ferroelectric thin films with 90 ° domains

http://juser.fz-juelich.de/record/25965/files/17362.pdf

Depolarization-field-mediated 180° switching in ferroelectric thin films with 90 ° domains

Abstract

Similar works

Full text

Available Versions

Juelich Shared Electronic Resources