Comparative study of pyroelectric response of PZT-

1ComparativestudyofpyroelectricresponseofPZT-film/Si,PZT-film/Por-Si/SiandPVDF-film/SistructuresS.Bravinaa,N.Morozovskya,E.A.Eliseevb,E.Cattanc,D.RemienscandA.GrosmandaInstituteofPhysicsofNASofUkrainre,46ProspectNauki,03028Kiev,UkraineE-mail:[e-mail]fMaterialsScience,NationalAcademyofScienceofUkraine,Krjijanovskogo3,03142Kiev,Ukraine,E-mail:[e-mail],UMR8520UniversitedeValenciennesetduHainaut-Cambresis,ZIdelapetitesavate,59600Maubeuge,FrancedGroupedePhysiquedesSolides,UMR7588,UniversitesParis6&7,2PlaceJussieu,75251ParisCedex05,FranceThecomparativeinvestigationofpyroelectricresponsemodulationfrequencydependencesofstructuresof“polaractivefilm-Sisubstrate”typebasedonPZTandPVDFandofstructuresof“polaractivefilm-bufferlayer-Sisubstrate”typebasedonPZTandporoussilicon(por-Si)hasbeencarriedout.Byphotopyroelectricmodulationmethodthethermowavepyroelectric“under-surface”probingwasperformedandamplitude-to-frequencyandphase-to-frequencydependencesofpyroelectricresponseofinvestigatedsystemsinthevoltageandcurrentmodeswereobtained.ByperformingtheanalysisofobtaineddependencesthethermaldiffusivityvaluesofinvestigatedPZTandPVDFfilms,andalsopor-Siinterlayerwereestimated.Theresultsoftheoreticalconsiderationofthestructuresunderinvestigationareinagoodagreementwiththeexperimentaldata.Theproblemofthermaldecouplingandself-decouplingfrompositionofchain“frequency-thickness-material”andquestionofcompleteandincompletethermallinkagearebrieflydiscussed.Theobtainedresultsshowthatapor-SilayerisasuitablematerialforeffectivethermaldecouplingofpolaractivefilmandheatremovingSi-substratewhichrealizedinsignificantincreaseofpyroelectricresponsevalueandapproachittothatofcharacteristicforafreesensitiveelement.Keywords:ferroelectricmaterials,pyroelectricdetectors.PACS:77.70.+a,77.80.-e,77.84.Dy.21.IntroductionCeramicsofPb(ZrxTi1-x)O3,(PZT)andpolymerpolyvinilydenefluoride(-CH2-CF2-)n,n≥104,(PVDF)arewell-knownpolar-activematerials[1-7].ApplicationofPZTceramicsstartedinthesixtiesandveryquicklyspreadfirstlyintotheareaofpiezoelectrictransducers[5]thenforplaneandcavitypyroelectricconverters[3,6]andnowisspreadingintotheareaofmicro-andelectromechanicsandmemoryelements[2,7].ApplicationofPVDF,withtemperaturedependentdipolarmomenthigherincomparisonwiththeotherpolymers,hasstartedsincetheseventies[1,3,4].AtpresentonthebaseofPVDFaremanufacturedpiezo-andpyroelectricconvertersforvariouspurposesandofvariousconfigurations[3,6]inbothplaneandcavitydesign[8].CeramicsofPZTtypeareobtainedbyhightemperaturesynthesisandcrystallizationoffurnacechargeofdefinitecomposition(forPZTceramicsZr/Tiratiodefinesthetemperaturesofphasetransitionandvicinitytomorphotropicboundary)[5].PiezoelectricandpyroelectricpropertiesofPZTceramicsareacquiredastheresultofhightemperatureelectric-fieldtreatment.ThickfilmsofPVDFacquirepolarpropertiesinthecourseofmechano-thermo-electricaltreatment(stretchinginratiofrom1:4to1:20,polarizationinelectricalfield~100MV/mundertemperatureТt=80-120оС)resultsinformationandfixingofpolarβ-phase[4,9].Polarpropertiesofthin(0.01-1mm)PZTplatesandthick(10-100µm)PVDFfilmsareremainedalsointhinner(≤1µm)filmsofthesematerials[6,9].FilmtechnologyofbothPZTandPVDFinprincipaliscompatiblewithSi-basisofmodernmicroelectronics.Inparticular,piezo-andpyroactivestructuresof“polaractivefilmonSi-substrate”typebasedonPZTfilmscanbeobtainedbysol-gel[10]andRFsputteringtechniques[11]andthosebasedonPVDFfilmsaremanufacturedbyvacuumdeposition[12]andbysol-gelspincoating[13].PZTfilmsdepositedonSi-substrateacquirepolarpropertiesaftertemperaturetreatmentatTt=550-650оСduetoastableperovskitephaseformation,whichisachievedundercoolingtoroomtemperature.ToacquirepiezoandpyroelectricpropertiesofPVDFfilmsonSi-substratethestructuresaresubjectedtoelectric-fieldtreatmentatTt=80-120оC.TheadvantageofPZTfilmsoverPVDFfilmsarehighvaluesofpyroelectriccoefficient(≈3⋅10-4C/m2K)andthetemperatureofreversibleretentionofpolaractivity(≈200оС),butratherhighvalueofdielectricpermittivity(from≈300to≈1000fordifferent3Zr/Tiratios)).TheadvantageofPVDFfilmsareratherlowvaluesofdielectricpermittivity(≈11)andthermaldiffusivity(~10-7m2/c)andalsohighmechanicalflexibilityandlowfabricationcostsbutratherlowvalueofpyroelectriccoefficient(≈3⋅10-5C/m2K)andthetemperatureofkeepingpolaractivity(≈60оС).Thebothconsideredpolaractivematerialsarethebasisofsensitiveelements(SE)ofmostmodernSi-integratedpyroelectricconvertersincludingpyroelectricdetectors(PyED).ItisknownthatefficiencyofconversionofPyEDisdeterminednotonlybypropertiesofSEandsubstratematerialbutalsocanbecontrolledbyusingofintermediatebufferlayer.Thislayerwithhigh(heatsink)orlow(heatinsulator)valueofthermalconductivity,whichplacedbetweensensitivelayeranditssubstrateincaseoflowvalueofthermalconductivityplaysroleofthermalcouplingordecouplingbetweensensitivepyroactivelayeranditssubstrate.Atpresent,thewell-knownSi–derivedmaterialswiththermalconductivityreducedincomparisonwithcrystallineSi,namelySiOx,SiO2-Si3N4systems,amorphousSiandporousSiar