Antiferroelectric lead zirconate thin films by pulsed laser ablation

Materials Science and Engineering B64(1999)54–59Antiferroelectric lead zirconate thinfilms by pulsed laser ablationS.S.N.Bharadwaja,S.B.Krupanidhi*Materials Research Centre,Indian Institute of Science,Bangalore560012,IndiaReceived19March1999AbstractLead Zirconate(PbZrO3)thinfilms were deposited by pulsed laser ablation method.Pseudocubic(110)oriented in-situfilms were grown at low pressure.Thefield enforced anti-ferroelectric(AFE)to ferroelectric(FE)phase transformation behaviour was investigated by means of a modified Sawyer Tower circuit as well as capacitance versus applied voltage measurements.The maximum polarisation obtained was36m C cm−2and the criticalfield to induce ferroelectric state and to reverse the antiferroelectric states were65and90kV cm−1respectively.The dielectric properties were investigated as a function of frequency and temperature.The dielectric constant of the AFE lead zirconate thinfilm was190at100kHz which is more than the bulk ceramic value(120)with a dissipation factor of less than0.07.The polarisation switching kinetics of the antiferroelectric PbZrO3 thinfilms showed that the switching time to be around275ns between antipolar state to polar states.©1999Elsevier Science S.A. All rights reserved.Keywords:Antiferroelectric;Lead zirconate;Pulsed laser ablation1.IntroductionFerroclectric(FE)thinfilms have a wide range of applications in areas such as memory systems,infrared detectors and piezoelectric transducers.More recently, much attention is being focused on the piezoelectric, electrostrictive and charge storage properties of fer-roelectric materials for applications in the areas of microelectronics and microelectromechanical systems (MEMS)[1]and high charge storage devices[2].Ultra-sonic micromotors have been fabricated using lead zirconate titanate(PZT)micromachined thinfilms[3]. However,even higher strain values could be produced using phase switching compositions based on antifer-roelectric(AFE)materials[4,5].Recently,Lathanum modified AFE sol gel thinfilms materials were reported by Cross et al.[6].Thefield enforced AFE to FE phase transition could be utilised to produce discrete strain levels for the applications in the areas such as actuators and shape memory devices[7].AFE materials are char-acterised by rows of dipoles with the dipole moment of adjacent rows equal but anti-parallel so that in equi-librium there is no net spontaneous polarisation and lead zirconate(PbZrO3)was thefirst compound to be identified as an antiferroelectric material[8].The small free energy between the AFE and FE phases makes such switching possible[9],while thefield induced switching is accompanied by large strain changes[10]. The AFE phase can be recovered by the removal of the external appliedfield.Antiferroelectric PbZrO3thinfilms fabrication was reported by earlier workers[11–13]and both polycrys-talline and oriented PbZrO3thinfilms[14]exhibited excellent electromechanical response.Besides these me-chanical properties,it was suggested that the AFE materials could be used as very high charge storage capacitors[2].Whenever the applied electricfield is removed from the poled FE phase of lead zirconate,it reverts back to the low energy AFE phase,thereby releasing all of the polarisation charge at afixed voltage,and AFE materials act like a constant current source.In the present work,a detailed study was done on the processing of PbZrO3thinfilms deposited by laser ablation technique and it’s structural and dielec-tric properties in order to investigate reproducibility and compatibility in fabrication of microactuators,mi-crosensors and charge storage capacitors.*Corresponding author.Tel.:+91-80-331-1330;fax:+91-80-334-1683.E-mail address:sbk@mrc.iisc.ernet.in(S.B.Krupanidhi)0921-5107/99/$-see front matter©1999Elsevier Science S.A.All rights reserved. PII:S0921-5107(99)00163-4S.S.N.Bharadwaja,S.B.Krupanidhi/Materials Science and Engineering B64(1999)54–59552.ExperimentalLead Zirconate thinfilms were deposited by a KrF (248nm)pulsed excimer laser system(Lambda Physik). The schematic diagram of the deposition set-up was given elsewhere[15].The laser beam at a repetition frequency of7Hz was focused using a UV grade plano-convex lens of50cm focal length and was made incident on the ceramic target through a quartz window at an angle of45°.The energyfluence on the target surface was kept in the range of3–3.5J cm−2by adjusting the laser spot size on the target.The operat-ing pressure inside the chamber was controlled precisely with a MKS massflow meter,whereas the substrate temperature was monitored with an external biased heaterfixed inside the deposition chamber.A lead zirconate target was prepared by mixing the oxides,PbO(Aldrich,99.9%)and ZrO2(Aldrich, 99.9%)in molar ratio like in the conventional solid state reaction method.The calcinated PbZrO3powder was pressed into a pellet of diameter of2in.with PVA as binder and sintered at1100°C for2h to achieve above96%of theoretical density.The structural and compositional analysis of the target was confirmed prior to deposition by X-ray diffraction technique (XRD)and energy disperse analysis of X-rays(EDAX).A copper plate was glued on the rear side of the PbZrO3target in order to avoid thermal shock effects during ablation.The front surface of the target was polished and mounted on a motor driven rotator shaft with a continuous rotation at a constant speed,to ensure uniform ablation rate during deposition. (111)Pt/TiO2/SiO2/(100)Si wafers were cleanedfirstly with boiled water and then with isopropanol and they were then placed parallel to the target at a distance of 3cm on a substrate holder with a mechanical clamping system.The temperature of the substrate was controlled by means of an external voltage regulator.The lead zirconate thinfilms were deposited around550°C with various oxygen partial pressures.The structure of the films was confirmed by XRD.The microstructures and chemical composition of thefilms were examined by means of SEM and EDAX,respectively.To study the electrical and dielectric properties,gold electrodes with a radius of0.4mm were deposited on the top surface of thefilms by means of an evaporation technique.The dielectric properties were investigated at different temperatures by a computer controlled Keith-ley3000LCZ meter in conjunction with a Eurotherm temperature controller.The hysteresis properties were studied by a utilizing modified Sawyer Tower circuit. The switching kinetics of AFE lead zirconate was also investigated in order to evaluate the switching speed of field induced transition between AFE and FE phases.3.Results and discussion3.1.Structure and morphologyLead Zirconatefilms were deposited on Pt coated Si substrates.Fig.1shows the X-ray diffraction pattern for thefilms deposited at different pressures with a repetition rate of7Hz and3–3.5J cm−2fluence on the target for30min.Thefilm thickness was determined by means of cross sectional SEM and verified by stylus method and was found to be around1.0m m.At a low pressure of10mTorr,thefilms showed initiation of perovskite peaks with very less intensity of main peaks. As the pressure was increased to50mTorr,the peak intensities increased and further increase in the partial pressure to100mTorr tends to show a dominant pyrochlore phase along with perovskite phase.As the pressure was increased from50to100mTorr the scattering of cationic species also increased which in turn,led to off-stoichiometric cationic ratio in thefilms, thus leading to the dominance of the pyrochlore phase. At a very low pressure like10mTorr the oxidation of the cationic species matters in the enhancement of the perovskite phase.Earlier workers reported this type of behaviour in he case of pulsed excimer laser deposition of PZTfilms[16].The semi-quantitative analysis also revealed that the cationic ratio deviated from the stoi-chiometric ratio(1:1)in thefilms deposited at10and 100mTorr.Films deposited25°C below and25°C above550°C also showed only pyrochlore phase.This Fig.1.X-ray diffraction pattern of in-situ grown lead zirconate thin films with a substrate temperature of550°C,(a)10mTorr,(b)50 mTorr,(c)100mTorr.S.S.N.Bharadwaja,S.B.Krupanidhi/Materials Science and Engineering B64(1999)54–59 56Fig.2.(a)Scanning electron micrographs of surface image of lead zirconate thinfilm deposited at550°C with50mTorr.(b)Scanning electron micrograph of cross sectional image of lead zirconate thin film deposited at550°C with50mTorr.Fig.3.Dielectric constant and loss factor dispersion with frequency at room temperature.at100kHz and did not show much dispersion with frequency.This value was little higher than the bulk value( 120)reported by Cross et al.[17].The dielec-tric constant of the stoichiometric lead zirconatefilms showed a temperature dependent broad phase transi-tion around235°C,which is very close to the bulk crystal( 230°C),[8]as shown in Fig.4.The observed broadness of the peak and the higher transition temper-ature than the bulk ceramic could be attributed to either:(1)the stress betweenfilm and electrodes;(2) defects such as Pb vacancies and/or oxygen vacancies,explains that optimal operating parameters are very much necessary to get reproducible100%perovskite phase in the PvZrO3films and thefilms that were deposited at550°C with50mTorr were taken to study dielectric properties of the same,while this set of growth condition offered stoichiometricfilms.Fig.2(a)and(b)show the SEM data of the surface and cross sectional features of thefilm deposited at a temperature of550°C with50mTorr pressure.The micrographs of thefilm showed a very dense,fine grained columnar structure with less number of particu-lates.Micrographs of thefilms that were deposited at 10and100mTorr showed notable discontinuities with off stoichiometry in cationic ratio which was also re-vealed earlier from the XRD patterns.3.2.Electrical propertiesThe variation in the dielectric constant(K)and dissi-pation(tan d)factor were measured with50mV oscilla-tion level for1.0m m PbZrO3films deposited at550°C is shown in Fig.3.The dielectric constant was above190Fig. 4.Variation of dielectric constant with temperature in lead zirconate thinfilm.S .S .N .Bharadwaja ,S .B .Krupanidhi /Materials Science and Engineering B 64(1999)54–5957Fig.5.Polarisation versus voltage hysteresis behaviour of anti ferroelectric lead zirconate thin film.The thickness of the film is 1m m.associated with the film during deposition;or (3)to the finer grain size of the films or combinations of all the above mentioned factors.3.3.Hysteresis beha 6iourThe polarisation–voltage (P–V)measurements were done using a computer automated modified Sawyer Tower circuit on the lead zirconate thin films and results were shown in Fig.5.The forward AFE [FE and backward switching FE [AFE fields were 64and 90kV cm −1respectively.The saturated polarisation was 36m C cm −2which was a comparable to that reported by earlier workers [18,19].This behaviour confirmed the presence of the AFE phase in the laser ablated PbZrO 3films.The other way to demonstrate phase switching in the antiferroelectric material is by recording the variation of the capacitance of the film as a function of the d.c.bias.The change in the polarisation of a polar material with the applied electric field changes the dielectric constant as follows.K h(P(1)where K is the dielectric constant of the materials and (#P /#E )is the polarisation change with the change in the d.c.field.For a constant geometrical factor,(m 0A )/d (where m 0is the permitivity of the free space,A is the electrode area,d is the thickness of the film),the capacitance is proportional to the dielectric constant of the material.Thus,the typical capacitance versus voltage (C–V)curve for an AFE composition exhibits a double peak butterfly type characteristics [2].The capacitance variation with the d.c.field is shown in the Fig.6.the observation of ‘double butterfly’loop in the C–V curve clearly establishes the evidence for the field induced ferroelectric transition in the AFE leadzirconate films.The values of forward AFE [FE (E AFE )switching field and backward FE [AFE (E FE )switching field are 64and 90kV cm −1respectively and may be found in concurrence with the values observed from the P–V double hysteresis loop.The asymmetry in E AFE and E FE values in the forward and reverse directions might be attributed to:(1)the differences in work functions between top and bottom surfaces of the films,there by leading to the formation of the space charge layer between the electrode and the film sur-faces;(2)the lateral mechanical interaction between the film and the substrate which leads to a 2-D clamping effect of the dipoles [20];and (3)the growth and /or nucleation of ferroelectric and ferroelastic domain wall reorientations in the grain remains because of the possi-ble presence of space charges at the grain boundaries.Fig.6.Capacitance versus applied voltage in lead zirconate thin film.S.S.N.Bharadwaja,S.B.Krupanidhi/Materials Science and Engineering B64(1999)54–59 583.4.D.C.leakage propertiesIt is worth understanding the leakage current mecha-nism through the antiferroelectric PbZrO3thinfilms,since the leakage current directly effects the chargestorage in these materials.The leakage current mecha-nism can be explained by the following relation[21];I8V n(2)for a dielectric slab in between two electrodes,where Iis the current density and V is the applied voltage.Thevalue of the exponent varies depending up on thefieldstrength and microstructural inhomogeneities in thefilms.In lower voltage regime there is a linear relation-ship between leakage current and the d.c.bias,whichfalls under ohmic region.The ohmic regime is given byI8V(3)i.e.for n=1,the leakage current varies linearly with theapplied d.c.voltage,which is generally observed in thelow voltage regime,like in many other dielectrics.For,although at any voltage V,however small,there will beexcess charge injected into the insulator,from impuri-ties and structural defects present in thefilm.Thesecharged species excite into the conduction band even atlower voltages and are responsible for the leakagecurrent.For the values of n higher than1the leakagecurrent varies non-linearly with the applied d.c.stressand if the dielectric possesses discrete traps with differ-ent activation energies it follows space charge limitedconduction(SCLC)given by[22];I8V(s+1)d(2s+1)(4)where s=T t/T and d is the thickness of thefilm,T t is a temperature parameter characterising the trap distribu-tion and T is the absolute temperature.Fig.7shows the log I−log V characteristics of AFE lead zirconate thin films sandwiched between the Au and the Pt electrode. It may be seen that at lower temperatures(30–50°C) there was a sudden jump to the higher order in the magnitude of the current.As the temperature was increased to above a certain critical temperature,the abruptness in the change of order slowly vanished and followed a non-linear curvature.At higher temperature and higher voltages,the curves are showing a slope of approximately2,which means the current conduction mechanism through these PbZrO3thinfilms follow a square law.At higher voltages all the curves are merg-ing with one another this in turn indicates that the temperature does not have a critical role on the conduc-tion mechanism,and is in concurrence with the Space Charge Limited Conduction theory.The sudden shoot-ing up in the current below certain characteristic tem-perature in lowfield regime could be attributed to the releasing of certain discrete inherent traps with lesser Fig.7.Log current−log voltage(log I−log V)curves measured for lead zirconate thinfilms.activation energy as shallow traps present are in the film.As the temperature increased to above that char-acteristic temperature,the onset of distributed trap space charge limited conduction(SCLC)was holding in the moderate and highfield regimes.Further detailed studies are required to confirm this type of behaviour with thefilms of different thickness’which are in progress.3.5.Field induced switching kinetics from AFE to FE phaseAnother important parameter worth studying in the AFE thinfilms for microactuator applications is the switching speed offield induced phase transition be-tween the AFE to FE phases.In Fig.8,the second current maximum shows the switching behaviour of PbZrO3thinfilms under voltage pulses.The initial peak is the surge of capacitor at the time offield applied.The switching time is defined as the time at which the current decreases to10%of the value at the second maximum[23].With the applied signal voltage in-creased(see Fig.8),the switching time also increased. The shift in the discharge peak(first peak)with the voltage increase might be due to d.c.stress between the film and the electrode surfaces and/or the accumulation of the space charges between the electrode and thefilm. The switching time betweenfield induced FE to AFE phases was found to be about275ns,which was much less when compared to the speed offield induced transi-tion in the bulk La-modified antiferroelectric ceramics ( 2m s)[24]and comparable to that reported forS .S .N .Bharadwaja ,S .B .Krupanidhi /Materials Science and Engineering B 64(1999)54–5959Fig.8.Room temperature switching characteristics in lead zirconate thin films.AcknowledgementsAuthors acknowledge the financial support from De-partment of Science and Technology to carry out the present work.One of the authors (SSNB )also acknowl-edges UGC for the financial assistance.References[1]D.L.Polla,Microelec.Eng.29(1995)51–58.[2]B.Jaffe,Proc.of the IRE,(1960)1264–1267.[3]K.R.Udayakumar,J.Chen,K.G.Brooks,L.E.Cross,A.M.Flymn,D.J.Ehrlich,Mater.Res.Soc.Symp.Proc.49(1992)243.[4]W.Y.Pan,C.Q.Dam,Q.M.Zhang,L.E.Cross,J.Appl.Phys.66(1989)6014.[5]W.Pan,Q.Zhang,A.Bhalla,L.E.Cross,J.Am.Ceram.Soc.72(1998)571–578.[6]K.G.Brooks,J.Chen,K.R.Udayakumar,L.E.Cross,J.Appl.Phys.75(1994)1699.[7]K.Uchino,Proc.of the Materials Research Society Internationalmetting on Advanced Materials,vol.9,MRS,1989pp.489–503.[8]G.Shirane,S.Swaguchio,T.Tokagi,Phy.Rev.B 84(1951)476.[9]D.Berlincourt,IEEE Trans.Sonics Ultrason.SU-13(1966)116.[10]D.Berlincourt,H.H.A.Krueger,B.Jaffe,J.Phys.Chem.Solids.25(1964)659–674.[11]T.Tani,J.F.Li,D.Viehland,D.A.Payne,J.Appl.Phys.75(1994)3017.[12]K.Yamakawa,S.Trolier-McKinstry,J.P.Dougherty,S.B.Kru-panidhi,Appl.Phys.Lett.67(1995)2014.[13]K.D.Budd,S.K.Dey,D.A.Payne,Br.Ceram.Soc.Proc.36(1985)107.[14]K.Yamakawa,S.Trolier-McKinstry,J.P.Dougherty,Proc.ofthe 10th IEEE International Symposium of the Application of Ferroelectrics (ISAF 96),vol.II,1996,405–408.[15]S.B Krupanidhi,J.Vac.Sci.Tech.A 10(1992)1569.[16]D.Roy,S.B.Kruapnidhi,J.Mater.Res.7(1992)2521–2529.[17]M.J.Haun,T.J.Harvin,nagan,Z.Q.Zhuang,S.J.J.ang,L.E.Cross,J.Appl.Phys.65(1989)3173.[18]K.K.Li,F.Wang,G.H.Haertling,J.Mater.Sci.30(1995)1386.[19]T.Tani,J.F.Li,D.Viehland,D.A.Payne,J.Appl.Phys.75(1994)3017.[20]N.A.Pertsev,A.G.Zembilgotov,A.K.Tagantsev,Phys.Rev.Lett.80(1998)1988.[21]J.J.O’Dwyer,The theory of Electrical Conduction and Break-down in Solid Dielectrics,Clarendon Press,Oxford,1973.[22]mpert,P.Mark,Current Injection In Solids,AcademicPress,New York,1970.[23]W.Mertz,Phy.Rev.95(1954)690.[24]W.Y.Pan,W.Y.Gu,L.E.Cross,Ferroelectrics 99(1989)185–194.[25]K.G.Brooks,J.Chen,K.R.Udayakumar,L.E.Cross,J.Appl.Phys.75(1994)1699.sol-gel thin films [25].The kinetics is controlled by nucle-ation of new domains,grain size and defects present in the films.Further studies are in progress with the varia-tion of the film thickness,electrode padding to gain fur-ther understanding.4.ConclusionsIn-situ antiferroelectric (AFE)lead zirconate (PbZrO 3)thin films were successfully fabricated using pulsed laser ablation at a low pressure.PbZrO 3thin films with pseu-docubic (110)orientation were prepared on Pt coated Si substrate with TiO 2buffer layer.Films exhibited a dielectric constant of 190at 100kHz frequency.The field enforced ferroelectric (FE)–antiferroelectric (AFE)transition was confirmed at room temperature,both in polarisation versus voltage (P–V)and capaci-tance versus voltage (C–V)curves.The maximum polar-isation obtained was 36m C cm −2and the energies required for the both forward AFE [FE and backward FE [AFE switching field was 64and 90kV cm −1respectively.The switching time of field induced transi-tion between AFE and FE phase was about 275ns..。