MOCVD生长的GaN_Mg外延膜的光电性质_英文_

半　导　体　学　报

JOURNALOFSEMICONDUCTORS

Vol.29　No.1

Jan.,2008

OpticalandElectricalPropertiesofGaN:MgGrownbyMOCVD3

WangLili󰂍,ZhangShuming,YangHui,andLiangJunwu

(InstituteofSemiconductors,ChineseAcademyofSciences,Beijing　100083,China)

Abstract:Mg2dopedGaNlayerspreparedbymetalorganicchemicalvapordepositionwereannealedattemperaturesbe2tween550and950℃.Roomtemperature(RT)Hallandphotoluminescence(PL)spectroscopymeasurementswereper2formedontheas2grownandannealedsamples.Afterannealingat850℃,ahighholeconcentrationof8×1017cm-3andaresistivityof018Ω・cmareobtained.Twodominantdefect2relatedPLemissionbandsinGaN:Mgareinvestigated;thebluebandiscenteredat218eV(BL)andtheultravioletemissionbandisaround3127eV(UVL).TherelativeintensityofBLtoUVLincreasesafterannealingat550℃,butdecreaseswhentheannealingtemperatureisraisedfrom650to850℃,andfinallyincreasessharplywhentheannealingtemperatureisraisedto950℃.Theholeconcentrationincreaseswithin2creasedMgdoping,anddecreasesforhigherMgdopingconcentrations.Theseresultsindicatethatthedifficultiesinachie2vinghighholeconcentrationof1018cm-3appeartoberelatednotonlytohydrogenpassivation,butalsotoself2compensa2tion.

Keywords:Halleffect;photoluminescence;p2GaNPACC:7280;7850G;7855

CLCnumber:TN30412+3　　　Documentcode:A　　　ArticleID:025324177(2008)01200292041　Introduction

Wellcontrolledp2typedopingisoneofthefore2mostobstaclesintheprogressofdevicedevelopmentofⅢ2nitridesforfabricatingvisibleandultravioletlight2emittingdevices.Lowenergyelectron2beamirradia2tion[1]orthermalannealinginN2atmosphere[2]isnec2essarytoobtainsignificantholeconcentrationsforGaNgrownbymetal2organicchemicalvapordeposition(MOCVD).Althoughtheunderlyingmechanismfortheseactivationprocessesisnotyetfullyunderstood,itiswidelybelievedthathydrogenimpuritiescreatedinthegrowthpassivateMgacceptorsbyformingMg2Hneutralcomplexes.Observationofalocalvibrationalmode(LVM)fortheMg2N2Hcomplexbyinfrared(IR)absorption[3]andRamanscattering[4]inas2grownsamplesgivesconvincingevidenceforthishypothesis.Therefore,apostgrowthtreatmentisrequiredtoacti2vatetheMgacceptorsthroughdissociationofMg2Hcomplexes.

However,thereisstrongevidencethatp2typedo2pingofGaNisamorecomplexprocessandMg2Hcom2plexesmaynotbetheonlypassivatingcenters[4,5].Theoreticalcalculationspredictthatnitrogenvacanciesarethedominantpointdefectsinp2typeGaNbecauseoftheirlowformationenergy[6,7].Moreover,duetothehighMgionizationenergy,highdopingconcentrationsinthe1019cm-3rangearerequiredtoachieveholecon2

centrationsinthelow1017cm-3range.Saarinenetal.appliedpositronannihilationspectroscopy(PAS)toi2dentifyMgGa2VNcomplexesasimportantcompensatingcentersinMg2dopedGaNlayers[5].Despitethat,westilllackanunderstandingofthedefectsresponsiblefordeterminingtheelectricalandopticalpropertiesofMg2dopedGaNlayers.

Inthiswork,weperformHallandphotolumines2cence(PL)spectroscopymeasurementsofMg2dopedGaN:MglayerspreparedbyMOCVD.Twocloselyconnectedeffectsareobserved.First,aRTPLbandpeakedaround218eVdominatesthePLspectra.Sec2ond,theholeconcentrationasafunctionofMgconcen2trationreachesamaximumvalue.Thestabilityofde2fectsinheavilyMg2dopedGaNlayersisalsoinvestiga2ted.TherolethatFermilevelplaysondefectstabilityinp2typeGaN:Mglayersistakenintoaccount.

2　Experiment

p2typeGaN:MglayersweregrownonsapphiresubstrateusingahorizontalMOCVDreactor.Trimeth2yl2gallium(TMGa)andammonia(NH3)wereusedasGaandNprecursors,respectively,andbis2cyclopenta2dienylmagnesium(Cp2Mg)asthep2typedoping

μmsemi2insulatingGaN(n<5×1016source.First,a4

cm-3)layerwasgrownonsapphirefollowedbyaμmGaN:Mglayerat1040℃.Themoleratioof115

Cp2MgtoTMGa([Cp2Mg]/[TMGa])variedbe2

3ProjectsupportedbytheNationalNaturalScienceFoundationofChina(Nos.60506001,60576003,60476021)󰂍Correspondingauthor.Email:wangll@red.semi.ac.cn

　Received31May2007,revisedmanuscriptreceived16August2007Ζ2008ChineseInstituteofElectronics

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半　导　体　学　报第29卷

Fig.2　HoleconcentrationandresistivityinGaN:Mglayersas

determinedbyHallmeasurementversustheannealingtempera2ture

Fig.1　(a)PLspectraforGaN:Mglayerbeforeandafterannea2lingatdifferenttemperatures;(b)RelativeintensityofBLtoUVLasafunctionofannealingtemperature

tween01004and01015.Toachievep2typecharacteris2ticsintheGaN:Mglayer,rapidthermalannealing(RTA)wascarriedoutundervariousconditions.Aone2stepRTAprocesswasperformedonthesamplespreparedwitha[Cp2Mg]/[TMGa]ratioof010065attemperaturesof550,650,750,850,and950℃inN2at2mospherefor20min.ThesampleswerestudiedbyPLandRTHallmeasurementsintheas2grownconditionandaftereachannealingstep.Photoluminescencewasexcitedwiththe325nmlineofaHe2Cdlaser(10mW).

3　Resultsanddiscussion

Figure1(a)showsthetypicalPLspectraat295KofGaN:Mglayersbeforeandafterannealingatdiffer2enttemperatures.Thepeakcenteredat～315eVcorre2spondstothebandedgeemissionoftheunderlyingGaNlayer.Anapparentintensityultravioletemission

bandaround312eV(UVL)isobservedintheas2grownsample,whichwaspreviouslyattributedtodo2nor2acceptorpair(DAP)emissioninvolvingashallowdonorofunknownoriginandaMgacceptor[8].Afterannealing,theintensityofthisDAPemissionbandisdecreasedbyoneorderofmagnitudeandthepeakposi2tiongraduallyshiftsto3127eV(UVL).Theshiftinthepeakpositionmaybeexplainedbythereductionofpotentialfluctuations[9]asaresultofscreeningfromthecarriers.Athirdbandwithapeakaround218eV(BL)isobservedandtheintensityofthispeakchangessignificantlyafterannealing.Toclearlydemonstratethechangeoftheintensityoftheobservedpeaks,Fig2ure1(b)showstherelativeintensityofBLtoUVL(BLintensity/UVLintensity)asafunctionofannea2lingtemperature(T).BLintensity/UVLintensityin2creasesafterannealingat550℃,butdecreaseswhentheannealingtemperatureiselevatedfrom650to850℃,andfinallyincreasessharplywhentheannealingtemperatureisraisedto950℃.

TheobservedchangeinthePLspectrauponan2nealingcanbeattributedtochangesinthedefectcon2centrations.Priortoannealing,shallowdonorsarepresentinhighconcentrations,leadingtoUVL.Uponannealingat550℃innitrogen,theshallowdonorsareeliminatedandtheintensityofUVLdecreasessharply.SincetheUVLisquenchedbyannealingatarelativelylowtemperatureof550℃,itmustinvolveadonorthathasahighdiffusivity.Onepossibilityishydrogen,whichhasahighdiffusivityinGaNandhasbeenpre2dictedtobeadonor[10].IRabsorption[3]andRamanscatteringdata[4]showclearevidencethatH2relatedcomplexesarepresentinas2grownGaN:Mglayers.TheBLthatbecomesdominantuponannealinghasbeenpreviouslyattributedtodeepdonorshallowac2ceptor(DDAP)emission,where,inthiscase,thedonorisdeepandtheacceptorisshallow[9].

Figure2showstheholeconcentrationandresis2tivityinGaN:Mglayersannealedatdifferenttempera2

第1期WangLilietal.:　OpticalandElectricalPropertiesofGaN:MgGrownbyMOCVD

31

Fig.4　Schematicenergybanddiagramtoillustratethepeaksob2

servedinMg2dopedGaN　Dindicatesthedonorstate,andDDin2dicatesthedeepdonorstate.

Fig.3　DependenceoftheintensityofthePLbands(a),andtheholeconcentrationofGaN:Mglayerson[Cp2Mg]/[TMGa]ratio(b)

tures.Thelayersshowpoorp2typeconductivitybeforeannealing.Thermalannealingfrom550to950℃for20minhasnoobviouseffectonourGaN:Mglayerswithhighcrystalquality,asdemonstratedbyX2raydif2fraction.

Afterannealingat550℃,theholeconcentrationandtheresistivitychangelittle.Astheannealingtem2peratureincreasesfrom550to850℃,theholeconcen2trationcontinuouslyincreasesfrom1016cm-3to8×1017cm-3andtheresistivitydecreasesfrom9to018Ω・cm.However,theholeconcentrationdropsdramatical2lyaftertheannealingtemperatureisraisedto950℃,andtheresistivityincreasesto4Ω・cm.

Figure3(a)showsthedependenceoftheintensityofthePLbands,andFigure3(b)showstheholecon2centrationofGaN:Mglayersonthe[Cp2Mg]/[TM2Ga]ratio.Thep2typeGaN:Mglayersunderinvestiga2tionwereannealedat750℃for20min.Theincrementofthe[Cp2Mg]/[TMGa]ratiocorrespondstothein2creaseinMgdopingconcentration[11].

ThedatainFig.3(b)showthatinitiallytheholeconcentrationincreasesasMgdopingincreases,butde2creaseslaterforhigherMgdopingconcentrations.Theresistivityshowstheoppositetrend.Thedifficultiesinachievinghighholeconcentrationabove8×1017cm-3andlowresistivitybelow018Ω・cmappeartobepri2marilyrelatedtoself2compensationofMg[12].Asindi2catedinFig13(a),formoderatelyMg2dopedp2typeGaN,theUVLisseenasaweakpeak,comparedtothe

BL.ForheavilyMg2dopedGaN,however,theUVLquenches,andtheBLdominantsthespectra.ItseemsthattheintensityoftheBL,isnotdirectlyrelatedtothep2typeconductionmechanism[13],butincreasesastheMgdopingconcentrationincreases.ThisbandhasthereforebeenassignedtotherecombinationinvolvingisolatedMgacceptorsanddeepdonorspresumablyin2ducedbyself2compensation.AcandidateforthedeepdonorisMgGaassociatedwithanitrogenvacancy(VN),whichistheonlynativedefectwitharelevantconcentrationinp2GaN[7],andhastheoppositechargeofMgGa.Thus,neutralMgGa2VNcomplexesareexpec2tedtoform.Atagrowthtemperaturearound1000℃theconstituentsareoppositelychargedandVNismo2bile.

Afterthermalannealingat550～850℃theMgGa2VNcomplexesdissociateandVNmigratestothesur2face.Asaresult,theholeconcentrationincreases,whiletheintensityofBLdecreases,asshowninFigs.1and2.TheMgGa2VNcomplexesareobservedinas2grownMOCVDGaN:Mg,butnotinthematerialgrownbyMBE[5].Therelevantdifferenceisrelatedtothepres2enceofhydrogeninthegrowthenvironment.Theas2grownMOCVDGaN:Mgcommonlyshowspoorp2typeconductivitybecauseMgacceptorsarepassivatedbyhydrogen.ApostgrowththermalannealisrequiredtoactivatetheMgacceptorsandobtainsignificantholeconcentration,asillustratedinFig12.Hydrogenisab2sentintheMBEgrowthandtheas2grownmaterialal2readypossesseswellp2typeconductivity.TheseresultssuggestthattheMgGa2VNcomplexesarestableatthegrowthtemperaturearound1000℃onlyiftheFermilevelisclosetothemidgap;otherwisethecomplexesdissociate.Thereby,asshowninFigs.1and2,whentheannealingtemperatureisraisedfrom850to950℃,largeamountsofVNareexpectedtoform.Asaconse2quence,thenetholeconcentrationdrops,andtheFermilevelmovesawayfromthemaximumofthevalenceband,whichresultsintheformationofMgGa2VNcom2plexesagain.Basedontheanalysisabove,thedramaticincreaseintheintensityofBLinFig.1isunderstanda2

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半　导　体　学　报第29卷

ble.

Insummary,Figure4showsaschematicenergy

banddiagramtoillustratetheDAPpeak(UVL)andDDAPpeak(BL)observedinMg2dopedGaNlayers.TheshallowdonorresponsibleforUVLisattributedtohydrogen,whereasthedeepdonordefectresponsibleforBLisattributedtonitrogenvacancycomplexesas2sociatedwithMgacceptors.

References

[1]　AmonoH,KitoM,HiramatsuK,etal.P2typeconductioninMg2

dopedGaNtreatedwithLEEBI.JpnJApplPhysPart2,19,28:L2112

[2]　NakamuraS,IwasaN,MukaiT,etal.Thermalannealingeffectson

p2typeMg2dopedGaNfilms.JpnJApplPhysPart1,1992,31:L139

[3]　G󰂪tzW,JohnsonN,BourD,etal.Localvibrationalmodesofthe

Mg2HacceptorcomplexinGaN.ApplPhysLett,1996,75:1383

[4]　ReboredoFA,PantelidesST.Noveldefectcomplexesandtheir

roleinthep2typedopingofGaN.ApplPhysLett,1999,82:1887

[5]　HautakangasS,OilaJ,AlataloM,etal.Vacancydefectsascom2

pensatingcentersinMg2dopedGaN.PhysRevLett,2003,90:137402

[6]　NeugebauerJ,VandeWalleCG.Theoryofpointdefectsandcom2

plexesinGaN.MaterResSocSympProc,1996,395:5

[7]　NeugebauerJ,VandeWalleCG.Atomicgeometryandelectronic

structureofnativedefectsinGaN.PhysRevB,1994,50:8067

[8]　ViswanathAK,ShinE,LeeJI,etal.Magnesiumacceptorlevelsin

GaNstudiedbyphotoluminescence.JApplPhys,1998,83:2272

[9]　ReshchikovM,YiGC,WesselsBW.Behaviorof2.82and3.22eV

photoluminescencebandsinMg2dopedGaNatdifferenttempera2turesandexcitationdensities.PhysRevB,1999,59:13176

[10]　VandeWalleCG.Interactionsofhydrogenwithnativedefectsin

GaN.PhysRevB,1997,56:R10020

[11]　SugiuraL,SuzukiM,NishioJ.P2typeconductioninas2grownMg2

dopedGaNgrownbymetalorganicchemicalvapordeposition.ApplPhysLett,1998,72:1748

[12]　OblohH,BachemKH,KaufmannU,etal.Self2compensationin

Mgdopedp2typeGaNgrownbyMOCVD.JCrystGrowth,1998,195:270

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4　Conclusion

Afterannealingat850℃,ahighholeconcentra2tionof8×1017cm-3ofandaresistivityof018Ω・cmareobtained.PLandHallresultsonMg2dopedGaNlayersindicatethatthedifficultiesinachievinghigherholeconcentrationappeartoberelatedtoself2compen2sationandhydrogenpassivation.Theobserveddepend2enceoftherelativeintensityofBLtoUVLonannea2lingtemperaturescanbeexplainedintermsofhydro2gendonors.Uponannealing,thehydrogenrelatedDAPdefectsdissociateandtheassociatedluminescencebandquenches,whereasMgacceptorsareactivatedandholeconcentrationincreases.Thereisevidencethatdeepdo2nors,whichareassignedtoMgGa2VNcomplexes,areformedastheMgdopingconcentrationincreases.TheFermilevelpositionmaybeacriticalfactorinthesta2bilizationofMgGa2VNcomplexes.WhentheFermilevelpositionisnearthemaximumofthevalenceband,re2sultingfromhighholeconcentration,MgGa2VNcomple2xesarenotstable,anddissociateafterannealingbe2tween550and850℃.

MOCVD生长的GaN:Mg外延膜的光电性质3

王莉莉󰂍　张书明　杨　辉　梁骏吾

(中国科学院半导体研究所,北京　100083)

摘要:用MOCVD技术生长GaN:Mg外延膜,在550～950℃温度范围内,对样品进行热退火,并进行室温Hall、光致发光谱(PL)测

试.Hall测试结果表明,850℃退火后空穴浓度达到8×1017cm-3以上,电阻率降到018Ω・cm以下.室温PL谱有两个缺陷相关发光峰,位于218eV的蓝光峰(BL)以及3127eV附近的紫外峰(UVL).蓝光峰对紫外峰的相对强度(BL/UVL)在550℃退火后升高,之后

)而下降,继续提高退火温度至950℃,BL/UVL急剧上升.空穴浓度先随着Mg掺杂浓度的增加而随着退火温度的升高(650～850℃

升高;但继续增加Mg掺杂浓度,空穴浓度反而下降.这些结果表明要实现空穴浓度达1018cm-3,不仅要考虑H的钝化作用,还要考虑Mg受主的自补偿效应.

关键词:霍尔效应;光致发光;p型GaNPACC:7280;7850G;7855

中图分类号:TN30412+3　　　文献标识码:A　　　文章编号:025324177(2008)0120029204

3国家自然科学基金资助项目(批准号:60506001,60576003,60476021)󰂍通信作者.Email:wangll@red.semi.ac.cn　2007205231收到,2007208216定稿

Ζ2008中国电子学会