功能材料3：基础(2)2013

无机功能材料InorganicFunctionalMaterials（AdvancedElectronicMaterials）材料科学与工程学院郭露村第二章功能材料基础ChapterIIFundamentalsofFunctionalMaterials§2.1半导体基础知识§2.2光电子材料基础§2.3功能材料界面现象1功函数与电子亲和力2肖特基势垒3隧穿效应4场发射效应5晶界效应§2.3电子材料界面现象InterfacePhenomenaRelatedtoElectronicMaterials1.功函数与电子亲和力Workfunction&Electronaffinity费米能级及金属表面光电子效应、热电子效应功函数、电子亲和力EF费米能级及金属表面Fermilevel&MetalSurface金属表面VacuumVacuumWhy?【光电子效应】Photoelectriceffect入射波长与光电子能量的关系Einsteinsolvedthisparadox!Whenametallicsurfaceisexposedtoelectromagneticradiationaboveacertainthresholdfrequency,thelightisabsorbedandelectronsareemitted.In1902,PhilippEduardAntonvonLenardobservedthattheenergyoftheemittedelectronsincreasedwiththefrequency,orcolor,ofthelight.ThiswasatoddswithJamesClerkMaxwell'swavetheoryoflight,whichpredictedthattheenergywouldbeproportionaltotheintensityoftheradiation.In1905,Einsteinsolvedthisparadoxbydescribinglightascomposedofdiscretequanta,nowcalledphotons,ratherthancontinuouswaves.BaseduponMaxPlanck'stheoryofblack-bodyradiation,Einsteintheorizedthattheenergyineachquantumoflightwasequaltothefrequencymultipliedbyaconstant,latercalledPlank'sconstant.Aphotonaboveathresholdfrequencyhastherequiredenergytoejectasingleelectron,creatingtheobservedeffect.ThisdiscoveryleadtothequantumrevolutioninphysicsandearnedEinsteintheNobelPrizein1921.Millikan'sAttemptstoDisproveEinstein'sTheoryIfweacceptEinstein'stheory,then,thisisacompletelydifferentwaytomeasurePlanck'sconstant.TheAmericanexperimentalphysicistRobertMillikan,whodidnotacceptEinstein'stheory,whichhesawasanattackonthewavetheoryoflight,workedfortenyears,until1916,onthephotoelectriceffect.Heevendevisedtechniquesforscrapingcleanthemetalsurfacesinsidethevacuumtube.Forallhiseffortshefounddisappointingresults:heconfirmedEinstein'stheory,measuringPlanck'sconstanttowithin0.5%bythismethod.OneconsolationwasthathedidgetaNobelprizeforthisseriesofexperiments.【热电子发射】ThermionicemissionHerewhatwehavedoneisheatthecathodebyapplyingavoltagebetweenthetwocathodeheaterleads,H1andH2.Wehavealsoconnectedthecathodeandanodetogetherviaaresistor.Ifwedothiswegetasurprisingresult.Wefindthattheanodedevelopsanegativepotentialwithrespecttothecathode,andsomecurrentwillflowthroughtheresistor.Notethatinthiscasethepositiveandnegativesignsshownontheabovediagramdon’trepresentvoltageswehaveappliedfromanexternalsource.Theyindicatewhatthevalvegenerates!ThermionicemissionThermionicemissionistheflowofchargecarriersfromasurfaceoroversomeotherkindofelectricalpotentialbarrier,causedbythermalvibrationalenergyovercomingtheelectrostaticforcesrestrainingthechargecarriers.Thechargecarrierscanbeelectronsorions,andaresometimesreferredtoasthermions.Thetotalchargeoftheemittedcarriers(eitherpositiveornegative)willbeequalinmagnitudeandoppositeinsigntothechargeleftintheemittingregion.Themostclassicalexampleofthermionicemissionistheemissionofelectronsfromahotmetalcathodeintoavacuum(archaicallyknownastheEdisoneffect),butthetermisnowusedtorefertoanythermallyexcitedchargeemissionprocess,evenwhenthechargeisemittedfromonesolid-stateregionintoanother.Thisprocessiscruciallyimportantintheoperationofavarietyofelectronicdevicesandcanbeusedforpowergenerationorcooling.Themagnitudeofthechargeflowincreasesdramaticallywithincreasingtemperatureandforvacuumemissionfrommetalstendstoonlybecomesignificantfortemperaturesover1000K.Thesciencedealingwiththisphenomenonisthermionics.Unlessweknowwhatishappeninginsidethediodethisresultispuzzlingasweseemtohavecreatedelectricalpowerfromnowhereandperhapsviolatedthelawofenergyconservation!Isthisthesolutiontoourglobalenergycrisisandwecangiveupfossilfuels?...Afraidnot.Whatishappeningisthatwhenweheatthecathodewecreateacloudofelectronsinthevacuumnearthesurfaceofthecathode.Mostoftheseelectronswillstaynearthecathode.Butafewwillhaveenergytoleapfarenoughfromthecathodetobeabletocrossthevacuumandstriketheanode.Sincetheanodeisn’theated,itwillgrabanyelectronsthathitit,andtheywon’thaveenoughkineticenergytoescapeitagain.Asaresultanumberofelectronsendupsittingontheanodethathavecrossedthevacuumtoreachit.Ifwedon’tconnecttheanodetoanything,thenassomeelectronsgatherontheanodetheygiveitanegativecharge.Thiswouldtendtoproduceanegativepotential,whichthentendstorepelanyotherelectronsthatapproachtheanode.Henceunlesswegivethearrivingelectronssomewaytoescape,theybuildupuntiltheyrepelanyfurtherborders!Howeverifweconnecttheanodebacktothecathodeviaanexternalresistor,thearrivalscanflowback‘home’tothecathodeviathepaththroughtheresistor.Astheydothis,theanodepotentialrelativetothecathodebecomeslessnegativeandsomemoreelectronswillhaveenoughenergytocrossthevacuumgap,thuscontinuingtheflow.Thebrokenbluelineswitharrowheadsintheabovediagramshowthedirectionoftheelectronflow.Howevernotethatfordafthistoricalreasonswedefine‘conventional’currenttobeintheoppositedirectiontotheactualelectronflow.Thusinconventionaltermswe’dsayacurrentflowsthroughtheresistorfromCtoA(positivetonegativepotential).Theinventionofvacuumtubediode1904:BritishengineerJohnAmbroseFleminginventsandpatentsthethermionicvalve,thefirstvacu