Theoretical Studies on the Reaction Mechanism of AsCl3 with H2 in the Vapor Phase Epitaxy of GaAs

1、Theoretical Studies on the Reaction Mechanism of AsCl3 with H2 in the Vapor Phase Epitaxy of GaAs263则2007.3结构化学(JIEGOUIfUAXUE)ChineseJ.Sfruct.Chem.VbI.26.No.3267272TheoreticalStudiesontheReactionMechanismofAsCl3withH2intheVaporPhaseEpitaxyofGaAsMALinGUANWen.JiaXUChangZhiSUNRenAnat9(Schoolo./Chemistr

2、)andChemicalEngineering,LiaoningNormalUniversity,Dalian116029,Chinn,(&hoolofPharmaceuticalScience,SunYat-senUniversity,Guangzhou510080,China)ABSTRACTThereactionmechanismofAsCI3withH2hasbeenstudiedbyusingthemethodofBHandHLYPinDensityFunctionalTheoryfDFT)atthe63IlGbasisset.Thetransitionstateofeach

3、reactionisverifiedviatheanalysisofvibrationmodeandIntrinsicReactionCoordinate(IRC).Meanwhile,singlepointenergyhasbeencalculatedattheQCISD(T)/63llGleve1.andthezeropointenergycorrectionhasbeenmadetothetotalenergyandreactionenergybarrieItshowsthatAsCI3reactswithH2tofirstresultinAsHCI2whichmayinclinetos

4、elf-decomposeandfinallyaffordtheproductAs2,orcontinuetoreactwithH2toprovidetheproductAsH3.Thecomputingresultdemonstratesthattheformeristhemainreactionchanne1.Keywords:AsC13/H2,transitionstate,DFTlIJ,JTRoDUCTIoNGaAsisoncofthemostimportantandversatilesemi-conductivematerialsamongsemi.conductivechemica

5、lcompounds.Itsforbiddenbandandelectronmobilityarelargerthanthoseofsiliconandgermanium.Therefore,thedevicesmadebyGaAsmaterialpossesssuchexcellentpropertiesasnicefrequencyresponse,fastspeed,andhighworkingtemperature【.GaAsvaporphaseepitaxy,basedondisproportionationreaction,widelyutilizesGa/AsClffH2syst

6、em,whosefundamentalprincipleisthatGaismigratedintheformofGaCI,andentersthevaporphase.Whenitistransportedtothelowtemperaturesite,thedisproportionationreactionofGaClOccursintheAssteamatmosphere.thusob.tainingGaAs,andthenGaAsmonocrystalmembranegrowsonthesubstrateaccordingtopredeterminedofientation.Repo

7、rtsathomeandabroadaremuchemphasizedonthethermodynamicanddynamicstudiesofthecrystalgrowthsurfaceinexperimentI引.whereasthestudiesonreactionmechanismarerare.Inthispaper,quantumchemistrymethodisemployedtodiscussthereactionmechanismofAsCI3withH2invaporphase.Moreover,thetransitionstate,reactionenergybarri

8、erandreactionenthalpyofeachreactionarefoundv/atheoreticalcalcula.tionssoastorevealthereactionmechanismofAsChwithH2aswellastheproductionofAssourcetheoretically.2CALCULATIoNMETHoDSThegeometricalconfigurationsofreactants(R),products(P)andtransitionstates(TS)aswellastheReceivedI7July2006;accepted8Septem

9、ber2006r1)Theproject吣supportedbyLiaoningProvincialDepartmentofEducation(990321076)Correspondingaufllor.male.Professor.E-mail:268MAL.P,a1.:TheoreticaIStudiesontheReactionMechanismofAsCI3withH2intheVaporPhaseEpitaxyofGaAsNo.3intermediate0M)areoptimizedbyusingthemethodofBERNYenergygradie

10、ntandDFTatBhandHLYP/63llGleve1.Thetransitionstateisveilfledviathevibrationmodeanalysis.Thecalculationofvibrationfrequencyexhibitsthattheobtainedreactant,productandintermediateafteroptimizationaretheenergyminimalpointsonthepotentialenergysurface,hndtheirfrequenciesarepositive,butthetransitionstateonl

11、yhasanimaginaryfrequency.Startingfromthepositiveandnegativedirectionsoftheonlyimaginaryfrequencyvibrationmodeintransitionstate,theintrinsicreactioncoordinate(IRC)calculationisperformedrespectivelyforproductandreactanttoobtaintheminimumenergypath(MPE)andfurtherverifythetransitionstate.Finally,thesing

12、lepointenergies.ofoptimizedreactant,productandtransitionstateaswellasintermediatearecalculatedattheQCISD(T)/631IGleve1.andthezeropointenergycorrectionhasbeenmadetothetotalenergyandreactionenergybarrier.H2AsCI3.3RESUISANDDISCUSSION3.1AnalysisofthereactionmechanismTheprobablereactionchannelsforthetitl

13、ereactionareshowninFig.1.Thegeometricalconfigurationsandmainparametersofreactants(R),transitionstates(TS),intermediate(IM)andproducts(P)aredepictedinFig.2.Inparticular,theexperimentalordocumentvaluest,l0】ofAsCI1,AsHCI2,AsH2CIandAsH3aregiveninthebrackets.Thesefigurespresentthatthemaximumabsoluteerror

14、ofbondlengthis0.018Aandthebondangle0.9.,whichsuggeststhatthemethodandbasissetadoptedinthisworkarereliable.Meanwhile,alltheimaginaryfrequencyvibrationmodesoftransitionstatearedemonstratedinthesefigures.ThetransitionstateonlyhasanimaginaryfrequencywiththevaluelistedinTable1.Fig.1.Mechanismofthewholere

15、actionForthereactionofAsCI3withH2,threeprobablereactionchannelsaredesigned:Channela:thetwoHatomsofH,moleculere.spectivelyattacktheAsatomandoneClatominAsCI3,obtainingtheproductsofAsHCI2andHCIv/atransitionstateTSlChannelb:thetwoHatomsofH2moleculeattacktwoClatomsinAsCl3atthesametime,andtheproductsofAsC

16、1and2HCIareobtainedthroughtransitionstateTS2Channelc:oneHatomofH2moleculeattacksoneCIatominAsCI3,providingtheproductsofAsCI2,HCIandHatomviatransitionstateTS3Thefollowingenergyanalysisinsection3.2showsthattheenergybarrierofchannelaisminimumsothatitshouldbethemainreactionchanne1.Onthebasisofthis,thefo

17、llowingdiscussionsarefocusedontheprobablereactionsofthisreactionchannelproducingAsHCI2.Becauseinthevaporphase,AsHCI2mayinclinetoself-decomposeorcontinuetoreactwithH2,thefollowingsixprobablereactionchannelsaredesigned:Channelal:AsHCI2decomposestoproduceproductsAsCIandHCIviathetransitionstateTSI.I.Cha

18、nnelalJ:AsCIcontinuestoreactwithH2toproduceAsHandHClthroughthetransitionstate2007V(I.26结构化学(JIEGOUHUAXUE)ChineseStruct.Chem.一TS1.1.IChannelaIll:byreactingAsHwithAsCI,As,andHCIareobtainedviatheintermediateIMandtransitionstateTSIlI1.Channela2:twoHatomsofH2moleculerespectivelyattackAsandCIatomsinthemol

19、eculefromonesideofAsCICIsurfaceofAsHCI2molecule,andAsH,CIandHClareobtainedviathetransitionstateTS12Channela3:twoHatomsofH2moleculerespectivelyattackAsandCIatomsinthemoleculefromonesideofAsHCIsurfaceofAsHC12molecule,givingAsH2CIandHCIVthetransitionstateTSI_3Channela4:twoHatomsofH2moleculerespectively

20、attackHandClatomsintheAsHCI2,andproductsAsCI,H2andHCIareobtainedviathetransitionstateTSI_4ChanneIas:twoHatomsofH2moleculeattackthetwoCIatomsintheAsHCI2atthesametime,whichaffordstheproductsofAsHand2HClviathetransitionstateTSI一5Channela6:oneHatomofH2moleculeattacksoneCIatomintheAsHCI2,andproductsAsHCI

21、,HCIandHatomareobtainedviathetransitionstateTS1.6Thefollowingenergyanalysisinsection3.2showsthatchannelsa2andatarethetwomutuallycompetitivereactionchannels,SOwhenthedecompositionofAsHCI2isdiscussed,theproductAsH2CIobtainedfromthereactionofAsHCI2andH2hasalsobeenstudied.AimedattheconditionthatHatomofH

22、2moleculeattacksdifferentatomsinAsH2CI,fourprobablereactionchannelsaredesignedasfollows:Channela21:twoHatomsinH2moleculerespectivelyattackAsandCIatomsintheAsH2CI,andAsHandHCIareobtainedvmthetransitionstateTS1-21Channela22;twoHatomsinH2moleculeattackHandClatomsintheAsH2CIatthesametime,obrainingAsH,H2

23、andHCIthroughthetransitionstateTS122.Channela23:twoHatomsinH2moleculeattacktwoHatomsintheAsH2CIatthesametime,whichgivesAsCIand2H2vthetransitionstateTSI一2-3Channela24:theHatominH2moleculeattacksoneCIatominAsH2CItogiveAsH,HCIandHatomthroughthetransitionstateTSl一2_4ThedecompositionofAsHCl2andtheformofA

24、s2270MAL.Ptnf.:TheoreticalStudiesontheReactionMechanismofAsCI3withH2intheVaporPhaseEpitaxyofGaAsNo.3AsHC12+H2AsH2CI+H2Fig.2.GeometriesandrelativeenergiesofR,TSandP(bondlengths:A,bondangle:degree)Intrinsicreactioncoordinate(C)calculationsarerespectivelyperformedforallabovetransitionstatesSOastoobtain

25、theminimumenergypath(MEP).Theresultspresentthatthetransitionstatesareallcorrespondingtothereactantsandproducts,whichindicatesthatthetransitionstatesfoundarecorrect.TheIRCgraphsofseveralmainreactionchannelsthatshouldbepassedthroughwhenpro-ducingtheAs2areshowninFig.3.AsCl3+H2_Ia2+HaAsHCIz-*AsCI+HCIAsC

26、I+Hz-*AsH+HCIAsH+AsCI-*As2+HCIFig.3.IRCaboutthemainpathwaysattheBHandHLYP/6-311Glevel3.2AnalysisoftheenergyThecalculatedimaginaryfrequencyvaluesIMGoftransitionstateattheBHandHLYP/6-311Glevel,thefreeenergiesAGofreactantandproductaswellasthefreeenergychangeAGoftheoverallreaction,zero-pointenergyZPE,th

27、esingle-pointenergyEcalculatedattheQCISDfI3/6-311Glever.andtherelativeenergyEafterzero-pointenergycorrectionarelistedinTable1.2007v,1.26结构化学(JIEGOUIt兰!:翌!:!竺:!Alongthereactionpath,therelativeenergychangesofreactant,transitionstate,intermediateandproductofeachreactionalealsoshowninFig.2.Table1present

28、sthatthechangesofreactionfreeenergyproducingAsHCI2,AsH2CIandAsH3arenegative,whichsuggeststhisreactionISeasytoundergothermodynamically.ThedecompositionfreeenergychangeofAsHCI2ispositive,indicatingthatthisreactionisnonspontaneousandshouldperformathightemperature.Hence,thetemperatureofGaAsvaporphaseepi

29、taxialtechniqueisgenerallyat750850.ThefreeenergychangetOproduceAs2IS一104.94kcal/mol,showingthermodynamicallyitisagreattrendthatAsCIreactswithAsHtOgiveAs2.Dynamically,whenAsCI3andH2reactwiththemixtureproductsofAsHCI2andHCl,thereactionchannelenergybarrieriStheminimumandthereactionisexothermal,whilethe

30、energybarriersoftheotherreactionchannelsaremuchhigherandthereactionsareendotherma1.Thereby,AsHCI2isregardedasthemainproduct.Inthevaporphase,AsHCI2maybeinclinedtodecomposeorfurtherreactwithH2.Thedifferencebetweenbothenergybarriersisonly1.75kcal/mo1.Thetworeactionsarecompetingreactionsathightemperatur

31、e.Thusbothproductsunderthesetwoconditionsshouldbe272MAL.P,口,:TheoreticaIStudiesontheReactionMechanismofAsCIswithH2intheVaporPhaseEpitaxyofGaAs.No.3studied.ThedecompositionreactionproducesAsClwhichreactswithH2toaffordAsH,andinthesetworeactionsthenecessaryenergybarrierisonly19.68kcal/mol,whichrevealst

32、hatthereactioniseasytoperform.ProductAs2isobtainedbyreactingAsCIandAsHwiththeenergyofonly34.62kcal/mo1.TheenergybarrierofAsHCI2reactingwithH2toobtainAsH2CIviathetransitionstateTSI一2isminimum,indicatingitiseasyfortheH2moleculetoattackfromtheAsCICIsideinAsHCI2molecule.AsH2CIcontinuestoreactwithH2toaff

33、ordthemainproductAsHswiththeenergybarrierof61.81kcal/mol,whichismuchhigherthanthoseoftworeactionsthatshouldbepassedthroughtoproduceAs2.AlthoughthereactionofAsH2CIandH2toobtainAsH3iseasytoundergothermodynamically,quitehighdynamicalenergybarrierisneeded.Thus,AsH3shouldberegardedasthebyproduct.Tosumup,

34、inthetechniqueofGaAsvaporphaseepitaxy,varioushydrideproductscomeintobeing,suchasAsHCI2,AsH2CIandAsH3.FinallythetitlereactiongetsAs2asthemainproductandAsH3asthebyproduct.ThusthereactionofAsCI3andH2canofferAssourcetotheproductionofGaAs,whichisconsistentwiththeexperiment.Intheprocessofproducingthemainp

35、roduct,theproducingenergybarrier(56.91kcal/mo1)ofAsHCI2isthehighest.soAsCI3+H2_AsHCI2+HCIistheratedeterminingstep.4CoNCLUSIoNThereactionmechanismofAsCI3andH2inGaAsvaporphaseepitaxyhasbeenstudiedbyusingDensityFunctionalTheory(DFT)attheBhandHLYP/631IGlevelTheresulshowthatbyreactingAsCI3withH2,theAsHCI2is