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1、Mater.Res.Soc.Symp.Proc.Vol.918©2006MaterialsResearchSociety0918-H01-02ElectronicandAtomicStructureofGe2Sb2Te5phasechangememorymaterialJohnRobertson,KaXiong,andPaulPeacockEngineering,CambridgeUniversity,Cambridge,CB21PZ,UnitedKingdomAbstractElectronicstructurecalculationsarepresent
2、edforvariousmodelstructuresofthecrystallineandamorphousphasesofGe2Sb2Te5(GST).Thestructuresareallfoundtopossessabandgapoforder0.5eV,indicatingclosedshellbehaviour.ItispointedoutthatstructuralvacanciesinA7-likeGSTarenotelectronicallyactive.Inaddition,A7-likestructuresdonotsupportval
3、encealternationpair(VAP)defects,whichareonemodeloftheconductionprocessesintheglassyphaseinnon-volatilememories.IntroductionChalcogenide-basedphasechangememory(PCM)materialsareusedinre-writableopticalmemories[1].Theyusethedifferenceinopticalbandgapandreflectivitybetweenthecrystallin
4、e(c-)andamorphous(glassy)phases.Theywillalsobeusedinelectronicnon-volatilememorydevicesbecauseoftheirpossiblesuperiorscalingpossibilitiescomparedtoFlash[2].Thishasresultedinanincreasednumberofstudiesoftheirstructuralandthermalproperties[3-6].However,thelocalatomicstructureisstillth
5、ematterofdebate[7,8],particularlyfortheglassyphase,asaretheirelectronicproperties.TheprototypicalphasechangematerialisGST,Ge2Sb2Te5.Thecrystallinephaseisadistortedcubicrocksaltstructure,accordingtoX-raydiffraction,andthistransformsintoarelatedhexagonalcrystalabove~200C[6].Therehave
6、beentwostudiesofthelocalstructureofthecrystalandglassyphasesbyextendedx-rayfinestructure(EXAFS)[7,8].ThecrystalphaseisconsistentwiththestructurefoundbyX-raydiffraction.However,thereisdebateabouttheglassystructure,aseitheroneinwhichtheTeatomsformalatticelikeinthecubicphaseasinthemod
7、elofKolobovetal[7],orwhethertheyformastructureasinamorphousGeSe,withlowercoordinationsandGe-Gebonds,assuggestedbyBakeretal[8].Thisbringsintofocusthepointthatsulfidesandselenidesaregoodglass-formingchalcogenides,whereastelluridesaredifferent.Theyhavealowerglass-formingability.Thisis
8、whatmakesthephasechangebet
