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Microstructural,opticalandelectricalpropertiesofCr-dopedZnOShubraSingh,*E.
SenthilKumarandM.
S.
RamachandraRaoDepartmentofPhysicsandMaterialsScienceResearchCentre,IITMadras,Chennai600036,IndiaReceived27September2007;revised6November2007;accepted2January2008Availableonline12January2008UndopedandCr-dopedZnOpolycrystallinesamplesweresynthesizedchemically.
OpticalmeasurementsconrmedthepresenceofCrintheZnOlattice.
Microstructuralstudiesofbulksamplesrevealedanetworkofpetal-likestructuresfoundtobeuniquetoCr-dopedsamples.
ElementalmappingofthesesamplesrevealeduniformdistributionofCrandZnatoms.
Crdopingwasseentosuppresstherecombinationradiationnear385nmandincreasethestabilityofZnOlmsinO2atmosphere.
2008ActaMaterialiaInc.
PublishedbyElsevierLtd.
Allrightsreserved.
Keywords:ZnO;Crdoping;Microstructure;Optical;Sol–gelWidebandgapII–VIsemiconductorshavebeenthefocusofinterestofmanyresearchgroupsduringthepastfewyearsduetothepossibilityoftheirapplicationsinlight-emittingdiodes(LEDs)andlaserdiodes.
Thereareavastnumberofbinarycompounds,suchasGaAs,ZnS1ySeyandquaternaryZnxCdyMg1xySealloys,whicharecurrentlyemployedinthedevelopmentofLEDs.
Asaresultoflargereectivemassandlowersta-ticdielectricconstant,thebindingenergyofexcitonsinII–VIsemiconductorsislargecomparedwithsimilarbandgapIII–Vmaterials.
OneofthemostimportantmaterialsinthisfamilyisZnO.
WurtziteZnOhasbeenwidelyusedinopticalapplicationsasithasawidebandgapof3.
4eV.
Itistransparentandhasalargeexcitonbindingenergyof60meV.
Apartfromthevariousopti-calandoptoelectronicapplications,ZnOisalsousedwidelyasacosmeticandantibioticmaterial.
However,forthisparticularapplication,therecombinationradia-tionnear385nmisadeterrent.
ToeliminatetheeectofthisrecombinationradiationintheUVregion,itisnec-essarytodopetransitionmetalionsinZnOinordertoenvisagethepossibilityofshiftingtherecombinationwavelengthtohigherwavelengths.
Inlinewiththis,wehavedopedCrinZnO.
VariousgroupshaveperformedmagneticstudiesonCr-dopedZnOsamples[1,2].
First-principlescalculationsbySatoetal.
indicatethattheferromagneticstateofZnO:Crwouldbemorestablethanaspinglassstate[3],anditwouldalsobemoreenergeticallyfavorablethantheferromagnetisminCo-dopedZnO.
Inthiswork,wepresentthestructural,opticalandtransportpropertiesofCr-dopedandundopedZnO.
High-purityZnOandCr2O3wereusedtoprepareZn1xCrxO(x=0.
01)bychemicalprocessing.
Thechemicalingredientswereweighedinstoichiometricpro-portions,dissolvedinabout50mlofdeionizedwaterandstirredcontinuouslyonamagneticstirrer.
Citricacidwasaddedand,tomaketheresultingsolutioncom-pletelytransparent,afewdrops(about3ml)ofHNO3wereadded.
Thetemperatureofthesolutionwasraisedinsmallsteps(5°Cafterevery2h).
Whenagelwasformed,thetemperaturewasfurtherraisedto100°Ctoboiloanywater.
Afterheattreatmentat600°Ctheresultantsubstancerevealedanetwork-likestruc-ture.
Scanningelectronmicroscopy(SEM)studieswereperformedonthissubstance.
Apartofthesamplewassubsequentlygroundintonepowderandthepowderwasthenmadeintodisc-shapedpellets(8mmindiame-terand2mminthickness)byuniaxialpressing.
Inordertodensifythetargets(usedforthinlmgrowth),thediscsweresinteredat1200°Cfor24h.
ForpulsedlaserdepositiongrowthofCr-dopedZnOlms,asolid-state(NdYAG)laser(laseruence2Jcm2,repetitionrate10Hz)wasused.
Thinlmsweregrownonquartzsub-strateatanoxygenpartialpressureof5106mbarandasubstratetemperatureof500°C.
ThephasepurityofZnOandCr-dopedZnOwasstudiedbyX-raydirac-tion(XRD)usingCuKa1radiation(k=1.
5405A).
Thediusereectancespectra(DRS)atnormalincidenceofpowdersamplesweremeasuredwitha1359-6462/$-seefrontmatter2008ActaMaterialiaInc.
PublishedbyElsevierLtd.
Allrightsreserved.
doi:10.
1016/j.
scriptamat.
2008.
01.
008*Correspondingauthor.
Tel.
:+914422575910;e-mail:shubra@physics.
iitm.
ac.
inAvailableonlineatwww.
sciencedirect.
comScriptaMaterialia58(2008)866–869www.
elsevier.
com/locate/scriptamatspectrophotometer(OceanOpticsUSB2000).
Photolu-minescenceofsampleswasmeasuredusingaFluorogspectrophotometer.
Resistivityofsampleswasmeasuredbylinearfourprobetechnique.
Figure1showsXRDpatternsofdopedandundopedZnO.
Theaveragecrys-tallitesize(Debye–Scherrerformula,Dhkl=kk/(bcosh))forCr-dopedZnOsamples(fullwidthhalfmaximum(FWHM)=0.
26°)wasfoundtobe$385nm,whereasitwas703nmforundopedZnO(FWHM=0.
14°).
Asmallportionoftheas-preparedsample,calcinedat600°C,wascharacterizedbySEM.
Figure2apresentstheSEMimagesofbulkCr-dopedZnO.
Intheinsetahoneycomb-likestructurewithfourbranchesateachnodecanbeseen.
Itwasobservedthatthenetworkcon-sistedofpetal-likestructureswithtaperingends,eachhavinganaveragesizeof$5lm.
Eachpetal-likestruc-turehasasubstructureconsistingofaclusteroflongrodsjoinedtogethertoformlargerpetals.
ThisstructurewasobservedonlyforCr-dopedZnOandnotfortheundopedsample(Fig.
2b),preparedusingthesametech-niqueasthedopedsample.
ThisresultcanbeexplainedonthebasisofionicfractionofthebondinCr-dopedZnO.
Themorphologyofaparticledependsonthevalueoftheionicfractionofthebond.
Takeshietal.
usedtheconceptoftheelectronegativity(EN)(theratioFigure1.
XRDpatternsofundopedandCr-dopedZnObulksamples.
representstheimpuritypeakcorrespondingtoCr2O3.
Figure2.
(a)SEMimagesofCr-dopedZnOatdierentmagnications.
TheinsetshowsthemicrostructureofCr-dopedZnO.
(b)SEMimageofundopedZnO.
Figure3.
ElementalmappingofCr-dopedZnOsampleheatedat600°Cshowingthepresenceof(a)Crand(b)Znions,respectively.
Figure4.
(a)Diusereectancespectra.
(b)EmissionspectraofundopedandCr-dopedZnObulksamples.
Figure5.
(a)Ramanspectra.
(b)RTresistivityofdopedandundopedthinlmsbeforeandafterannealinginO2.
S.
Singhetal.
/ScriptaMaterialia58(2008)866–869867ofnegativeiontoelectrondensitiesofelements)toesti-matetheionicfractionofthebond.
TheENofZnis1.
65onthePaulingscale,whilethatofCris1.
66onthesamescale,i.
e.
theionicfractionislow($1),givingrisetolongitudinalstructures[4].
ItwasdemonstratedbyTakeshietal.
thatafurtherincreaseintheelectro-negativityofthedopantelement(foraslowas1at.
%doping)canchangethemicrostructurefromlongitudi-naltospherical.
Theseresultsindicatethatthemorphol-ogyoftheparentcompoundwasalteredbythedopingelement.
ElementalmappingoftheCr-dopedsamplere-vealedauniformdistributionofvariousionsinthesam-ple(Fig.
3).
Figure4aandbshowsthediusereectance(DR)andphotoluminescence(PL)spectraofundopedanddopedsamples.
ThevariationofDRnearthebandedgeforthedopedsamplesascomparedwiththeundopedsampleisanindicationofincorporationofCrionsintheZnOlattice.
Themid-band-gapstatesarisingduetoabsorptionat460and580nmcorrespondtothed–dtransitionbandsofCrions.
Thebroadabsorptionbandaround580nmcorrespondstothe4A24T2tran-sition[5].
However,thereectanceintensitywasfoundtoreduceongrindingthenetwork-likestructureandtheresolutionbetweenthetwotransitionsimproved.
Thiscanbeattributedtotheincreasedabsorptioncausedbytheincreaseinsurfaceareaduetothegrind-ing.
ThePLspectrumshowsthatultraviolet(UV)radi-ationwithawavelengthofabout385nm(therecombinationradiationduetotheexciton[6])issup-pressedforthedopedsample.
ThisindicatesthattheCrdopingincreasesthenonradiativerecombinationprocessandthattheexistenceoftransitionmetalionslikeCrcancontroltheexcitonicrecombinationradiation.
Figure5ashowsthenormalizedRamanspectraofthebulkundopedandCr-dopedcompound.
Thespectrarevealsstandardphononmodesat331,381,407,437and579cm1correspondingtoundopedZnO[7,8].
Themodeat655cm1isattributedtoan(E2L+B1H)combinedphononmode[8].
Forthedopedsampletherewasashifttowardsthelow-frequencyside,whichdependsontheresidualstress,structuraldisorderandcrystaldefectofthesamples.
The579cm1modeseemtomergewithanadditionalmodeat609cm1.
Therea-sonfortheappearanceoftheadditionalmodeisnotclearatpresent,butcanbeattributedtothechangesinducedbythedoping.
Weobservedanincreaseinresistivity(Fig.
5b)forCr-dopedZnOsamplesascom-paredwiththeundopedlmsforaslowas1mol.
%dopedZnO(bothincaseofthinlmandbulk).
ThemobilityoftheundopedZnOthinlmswasfoundtobe145cm2V1s1,whileforCr-dopedZnOlmsitwas90cm2V1s1.
Thiscanbeattributedtothepres-enceoftheCrintheZnO,whichaectsgraingrowth(wellknownasCrpoisoning)anddecreasesthecarriermobility(aswecanseefromtheaboveresults)duetocarrierscatteringatthegrainboundaries[9].
Thismayleadtoanincreaseinresistivityforbulkaswellasdopedlms.
TEMimagesofas-preparedundopedandCr-dopedZnOthinlms(Fig.
6aandb)alsoshowthattheaverageparticlesizeissmallerinthecaseoftheCr-dopedthinlms.
UndopedandCr-dopedZnOlmswerefurtherannealedinanoxygenatmosphereexsitufor5htoobserveanychangesinresistivity.
Figure5balsoshowsacomparisonofroomtemperatureresistivityvaluesofdopedandundopedthinlmsbeforeandafteranneal-ing.
Theresistivityofundopedlmincreasedconsider-ablyuponannealinginoxygenwhereasCr-dopedthinlmdidnotshowanyconsiderablechange.
Theincreaseinresistivityfortheundopedsampleinanoxygenatmo-spheremaybeattributedtochangesinthepotentialheightatthegrainboundaries[9],whichshowsthatCrdopingimprovesthestabilityofgrainboundariesun-derexposuretotheoxygenatmosphere.
Energy-disper-siveX-rayanalysis(EDX)onsamplesalsoshowedthatboththeundopedandCr-dopedsamplesareoxy-gendecient(EDXonlmsrevealedthepresenceof16.
55%ofoxygenbyweightinundopedZnOand18.
06%ofoxygenbyweightinCr-dopedZnO),andthatitmaynotberesponsibleforsuchalargevariationinelectricalbehaviourbetweenundopedanddopedsam-pleswhenannealedinoxygenatmosphere.
Inthisworktransitionmetal(Cr)wasdopedintotheZnOlatticeandthiswasconrmedbyopticalstudiessuchasPL,aswellasDRSstudies.
BydopingtransitionmetalionsintoZnOmatrix,UVradiationwithawave-lengthshorterthan385nmwasalmosteliminated.
Themicrostructureofthedopedsampleat600°Cexhibitsanetwork-likestructure.
WealsondthatCrdopingin-creasesthestabilityofZnOlmsunderexposuretoanoxygenatmosphere.
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