Functional Dielectrics for Electronics: Fundamentals of Conversion Properties (Woodhead Publishing Series in Electronic and Optical Materials)

307 Pages·2020·18.489 MB·English

by  Yuriy Poplavko, Yuriy Yakymenko

Functional Dielectrics for Electronics
Woodhead Publishing Series in Electronic and
Optical Materials
Functional Dielectrics
for Electronics
Fundamentals of Conversion Properties
Yuriy Poplavko
Yuriy Yakymenko
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Preface
The authors of this book represent the microelectronics department of National
Technical University of Ukraine (Kiev Polytechnic Institute), founded 120years
agoandnamedafterIgorSikorsky,whotrainedatthisinstitute.Functionaldielectrics
havebeenstudied,elaborated,andtaughtinthemicroelectronicsdepartmentformany
years, and the authors have published hundreds of articles and several books in
this area.
Thetaskofthisbookistoprovideengineersspecializinginmaterialsscienceappli-
cation and research in electronics and information technology with up-to-date and
clearideasaboutthenatureofelectricalpolarization,dielectricnonlinearity,electrical
chargetransfermechanisms,thermalproperties,andthenatureofthermalstableper-
mittivityinlow-lossmicrowavedielectricsandotherfunctionaldielectrics.Thebook
describes in detail the intrinsic mechanisms of electrical polarization and energy
transformationsinnoncentrosymmetriccrystals,whichareresponsibleforconverting
thermal, mechanical, optical, and other actions into electrical signals. In order to
extend the use of functional dielectrics into the field of nanoelectronics, this book
contains a versatile and comprehensive presentation of the main physical processes
thatprovidetheelectrical,mechanoelectrical,thermoelectrical,andotherconversion
phenomena in polarcrystals.
Moredetaileddescriptionsaregiventoelectricalmanifestationsofpolarsensitivity
incrystals;interactionofpolarizationwithconductivityisdescribed,aswellasanom-
alies in thermal expansion coefficient and the main peculiarities of heat transfer in
polar-sensitive crystals. Some applications of polarity theory to pyroelectrics and
piezoelectrics are described. Microwave dielectrics are also of great importance in
modernelectronics,becausetheyarewidelyusedincommunicationdevices.Inthem,
smalldielectriclossescanbeachievedinsingle-phasecompositions,basedon“hard”
paraelectric material that resists polar structure formation. The thermal stability of
permittivity can be obtained when paraelectricity is suppressed by paramagnetism.
Nonlineardielectricsprovideanimportantopportunityforrapidcontrolbypermittiv-
ity, used in microwave phase shifters. Other frequency-agile devices are also
described: tunable dielectric resonators and phase shifters consisting of microwave
dielectricscoupledwithcontrolbyapiezoelectricairgapthatavoidsinsertedlosses.
The authors express their deep gratitude to their colleagues from “Igor Sikorsky
Kiev Polytechnic Institute” for their interest in this work and for their valuable
comments. The authors are also grateful to their students who pointed out hard-
to-understand concepts and asked us for clarification. The responsibility for any
remaining errors orshortcomings is, ofcourse,ours.
viii Preface
WewanttogivespecialthankstoProf.Dr.Sci.YuriyProkopenko,Ass.Prof.Dr.
VictorKazmirenko,Ass.Prof.Dr.DmitriyTatarchuk,andDr.YuriyDidenkofortheir
usefulcollaboration.
Finally, we are deeply grateful to the staff of Elsevier: to Kayla Dos Santos and
PeterAdamsonfortheirpatienceineliminatingtheshortcomingsofthemanuscript.
Dr. Phys.-Mat. Sci. Prof.Yuriy Poplavko
Academician of UAS, First Vice Rector, Dr. Sci.Prof. Yuriy Yakymenko
Introduction
The dielectrics intended for use inelectrical and electronic devices must have good
electrical insulating properties, including very low electrical conductivity and high
electrical strength. At the same time, in present-day electronics technology, other
properties of dielectrics have gained special importance, namely those that allow
for conversion of energy or information; these dielectrics may be considered
functional materials.
Thefunctional(oractive,oradaptive,orcontrolled,orsmart)dielectricsactively
reacttochangesoftemperature,pressure,mechanicalstress,electricalandmagnetic
fields, light illumination, and even smell. Functional dielectrics can be classified as
pyroelectrics, ferroelectrics, piezoelectrics, electrets, quantum-electronics materials,
microwave dielectrics with tunable permittivity, etc.
Active dielectrics perform different functions. For example, the piezoelectrics
convert mechanical energy into electrical energy (and vice versa) that is used in
piezoelectric filters, ultrasound emitters, piezoelectric transformers, piezoelectric
motors, etc. Pyroelectrics convert heat into electricity and are applied as sensitive
detectorsofradiation,thermal-visiondevices,etc.Thenonlinearpropertiesofferro-
electricsandparaelectricsandtheconstantelectricalfieldproducedbyelectretsallow
use of these functional dielectrics for modulation, detection, amplification, register-
ing, storing, displaying, and other types of electrical conversion of signals carrying
information. It is also necessary to mention the use of functional dielectrics in
multifunctionalelectronicdevices,alongwiththeactivesearchfornewtechnological
solutionstochallengesregardingapplicationofthesedielectricsinthefieldofinfor-
mationtechnology.
Themainpropertyofanydielectricisitselectricalpolarization,i.e.,theeffectof
separation of electrical charges that remain bonded in spite of their shifting. As a
result,anelectricalmomentappears(asaproductofchargemagnitudeanddisplace-
ment);thevolumetricdensityofthismomentispolarizationP.Oneunusualfeatureof
functionaldielectricsisthattheirelectricalpolarizationcanbeinducednotonlybyan
electrical field, but also by other causes. A comparison of conventional (nonpolar)
dielectricsandtwocategoriesofpolardielectricsareshowninTableI.1.Theessential
distinctionbetweenactive(functional)dielectricsandordinarydielectricsisobvious.
In many applications of functional dielectrics (such as sensors, actuators, filters,
transformers,motors,etc.),theyaresubjectedtoexternalthermal,electrical,mechan-
ical,andotherinfluences—scalar,vector,ortensortypes.TableI.1showsthatacon-
ventional dielectric electrically reacts only to electrical field action: E)P, while
polar piezoelectrics and pyroelectrics, besides the E)P response, are capable of
electrical response to other actions: mechanical X)P and thermal dT)dP.
x Introduction
TableI.1 ElectricalpolarizationPasaresponsetovariousactions.
Nonpolar
Actions crystals Piezoelectrics Pyroelectrics
Scalaraction:
TemperaturedT – – Pyroelectriceffect:
Pressuredp – – dP¼γdT
Volumepiezoelectric
effect:dP¼ζdT
Vectoraction:
ElectricalfieldE P¼ε εE P¼ε χE P¼ε χE+(e2/c)E
0 0 0
+(e2/c)E +(γ2Т)E/(ε C)
0
Tensoraction:
Mechanical – P¼dX P¼d0X
stressX
Note:Parameterγispyroelectriccoefficient;ζisvolumetricpiezoelectricmodulus;dispiezoelectricmodulus;cis
elasticstiffness;Cisspecificheat.Thistabledoesnottakeintoaccount:
(1) flexoelectricitypossibleinalldielectricsundernonhomogeneousmechanicalaction;
(2) actinoelectricitythatoccursinpiezoelectricsunderthegradTaction;
(3) photopolarizationeffectinpolar(noncentrosymmetric)dielectrics.
Atthesametime,piezoelectricsrespondtoelectricalactionnotbyordinarypolariza-
tiononly,ofP¼ε χE,butalsoproduceanelectromechanicalresponseP0¼(e2/c)E,
0
whilepyroelectrics,inaddition,alsoproduceanelectrothermalresponseP00 ¼(γ2Т)
E/(ε C).
0
Inviewofpossibleapplicationsofdielectricsasfunctional(converting)elements
inelectronics, oneshouldidentifyand describetheir properties notonly interms of
theirexclusivelyelectricalcharacteristicsbutalsotheircapabilitytomanifestvarious
electrical,mechanical,andthermalproperties.Thesematerialsareimportantalsofor
miniaturizationofmicrowaveandtelecommunicationsequipment.Inthiscase,adeci-
sive role is played by the value of permittivity (ε), since the planar dimensions of
microwavedevicesarereducedexactlybythefactorε.Insomefunctionaldielectrics
(paraelectrics),alargevalueofεcanbeobtainedtogetherwithlowdielectriclosses,
whichhasimportanttechnicalapplicationinhigh-frequencydevices.Somefunctional
dielectrics allow electrical control by permittivity that can be used for electrically
controlled microwave devices.
For all the reasons mentioned, interest in ferroelectrics, paraelectrics, piezoelec-
trics,andpyroelectricshasrecentlygrowninthefieldofelectronicsmaterialsscience,
preciselybecauseoftheirnewapplicationsininstrumentationengineeringandelec-
tronics, and also owing to significant progress in modern microelectronic and
nanoelectronic technologies.
Functionalmaterialsareparticularlyrelevanttomodernandfutureinstrumentation
basedonmicromachining.Inthistrend,microelectronicgrouptechnologyisusedina
varietyoftechnicalfields.Usingmodernequipment,micromachiningisorganically
Introduction xi
connectedwithmicroelectronicsandnanoelectronics.Amongcontemporaryapplica-
tionsoffunctionaldielectrics,oneshouldnotethefollowingmainareasofparticular
relevance: ferroelectric and paraelectric thin films integrated with semiconductors;
microsystems combining sensors, processors, and actuators; microwave micro-
electronics, based on functional dielectric components; nanodielectrics, which are
prospective technologies for sensors, computer memories, and electrical power
generation.
Engineers specializing in the application of materials science to electronics and
informationtechnologiesneedtoaccessthelatestknowledgeonthenatureofelectri-
cal polarization, electrical charge transfer mechanisms, thermal properties of func-
tional dielectrics, mechanisms of dielectric losses, as well as intrinsic mechanisms
ofpolarizationinnoncentrosymmetriccrystals,whichareresponsibleforconverting
thermal, mechanical, optical, and other actions into electrical signals. In order to
extendtheuseoffunctionaldielectricsintothefieldofnanoelectronics,itisnecessary
for designers and technologists to have a thorough and deep understanding of the
physical processes that provide electrical, mechanoelectrical, thermoelectrical, pho-
toelectrical andotherconversion phenomena inpolarcrystals.
Itmightbethatthemoststrikingfeaturesofpolardielectricsare,first,themutual
influenceofmechanical,electrical,andthermalpropertiesoneachotherand,second,
thedependenceofthesepropertiesontheconditionsinwhichpolarcrystalsarestud-
iedorapplied.Todemonstratethisinterdependence,thebasicmechanical,electrical,
andthermallineareffectsinthenonpolarandpolardielectricsaresymbolicallycom-
paredinFig.I.1.Itisseenthatintheordinarydielectricsthesepropertiesareindepen-
dent,but inthe polarcrystals they are connected by quite complex interactions.
Fig.I.1Asymbolicallyshowsthattheelasticmechanicaldeformationx(strain)ina
solid dielectric is proportional to the applied mechanical stress x¼s(cid:2)X, where s is
elasticcompliance.Notethatthislinearrelationshipcanalsobewritteninthereverse
direction,X¼c(cid:2)x,asHooke’slaw,whereciselasticstiffness.Similarly,inadielec-
tric,electricallyinducedpolarizationPisproportionaltothemagnitudeoftheapplied
electricalfield,P¼ε χ(cid:2)E,whereχisdielectricsusceptibilitywhileε istheelectrical
0 0
constantinSIunits.Ifelectricalpolarizationisinducednonelectrically,thenanother
recording of this linear connection will be more convenient: Е¼ε(cid:3)1ξ(cid:2)P where
0
Fig.I.1 Linkingdiagramformechanical,electrical,andthermaleffects:(A)Innonpolar
dielectric;(B)inpolardielectric.
xii Introduction
ξ¼χ(cid:3)1isdielectricimpermeability.Finally,thermalpropertiesaredescribedbythe
proportionalityoftheamountofheatδQinacrystalappearingduetoambienttem-
perature change: δQ¼C(cid:2)δT whereCisspecific heat.
In polar crystals, the mechanical, electrical, and thermal properties are
interdependent,soapossiblediagramoftheirinteractionlooksliketwosquarescon-
nectedbyoneside(Fig.I.1B).Thisschemeshowshowcomplicatedthedescriptionof
noncentrosymmetric (polar) crystal properties can be.
PiezoelectricityissymbolicallydescribedbytheleftsquareinFig.I.1B.Twohor-
izontalandtwocrossed-connectinglineswitharrowspresenteightlinearpiezoelectric
effectsthatmaybeobservedinpolarcrystalsunderdifferentconditions.Thenumber
ofpiezoelectriceffectsis8,sincethiseffectcanbedirectorconverse(2),thecrystal
canbeelectricallyopen-circuitedorshort-circuited(2),andthecrystalmechanically
canbefreeorclamped(2),so2(cid:4)2(cid:4)2¼8.Theseeffectsaredescribedbyfourpie-
zoelectric coefficients-moduli (d, e, g, h); further, simplified equations of the direct
effects are P¼d(cid:2)X, P¼e(cid:2)х, E¼(cid:3)g(cid:2)x, E ¼(cid:3)h(cid:2)X, and for the converse effect the
j
equations are х¼d(cid:2)Е, X¼e(cid:2)E, х¼g(cid:2)P, X¼h(cid:2)P. However, all four moduli men-
tioned can be calculated through each other and through known elastic constants,
for example, in simplified form: d ¼ε ε(cid:2)g¼e(cid:2)s¼ε ε(cid:2)h(cid:2)s, and so on. As can be
in 0 0
seen,thedescriptionofelectromechanicaleffectsinpolarcrystals(eveninthelinear
case)isnot aneasy task.
Inpolar-sensitivecrystals,thedescriptionofelectrothermaleffectsisalsonotsim-
ple.Thepyroelectriceffectoccurswhenthedisturbancefactoristhethermalactionon
acrystalwhile theresponse hasanelectricalnature. The electrocaloriceffectisthe
converseeffect:itariseswhentheelectricalfieldactsonthepyroelectric,whilethe
result is heating or cooling of the polar crystal. Both these effects are symbolically
presented inthe rightsquare inFig. I.1B.
First,two horizontallines and twocrossed-connectedlineswith arrows symboli-
cally characterize four options in the pyroelectric effect implementation: the polar
crystalcanbeelectricallyopen-circuitedorshort-circuited;inaddition,thepyroelec-
triceffectcanoccurintwodifferentthermalconditions:adiabaticwithδQ¼0oriso-
thermalwithδТ ¼0.Sothepyroelectriceffectmightbedescribedbyfourequations:
Р¼γ(cid:2)δТ, P ¼γ0(cid:2)δQ, E¼γ00(cid:2)δТ, and Е¼γ000(cid:2)δQ, in which different pyroelectric
i
coefficients correspond to various boundary conditions.
Second,fourlineswitharrows,shownintherightpartofFig.I.1B,areusedherefor
symbolicdescriptionoftheelectrocaloriceffect.Asapyroelectriceffect,thiseffect
may be described by four different linear relationships (depending on thermal and
electricalboundaryconditions).However,thiseffectundernormalconditions(attem-
perature(cid:5)300K)isratherweakandisnotconsideredhereinmoredetail.(However,
in Chapter 4, a large electrocaloric effect will be discussed, arising when an anti-
ferroelectricisswitchingelectricallyintoaferroelectricphase,whichcanbeconsid-
ered asa possible basis for an electrocaloric refrigerator.)
Animportantconsequenceoftheserelationshipsisthedependenceofthecrystal
fundamental parameters on thermal, electrical, or mechanical processes passing in
polarcrystalsatdifferentboundaryconditions.Forexample,dielectricsusceptibility
ofafreecrystalχX(whichmeansthestressX¼0)andofaclampedcrystalχx(that