Characterization of Berthelot-type behaviors of InGaN_GaN se

Correspondingauthor.Tel.:+88632118800x5790;

fax:+88632118507.

E-mailaddress:rmlin@mail.cgu.edu.tw(R.-M.Lin).

T.-E.Neeetal./JournalofCrystalGrowth287(2006)468–471

469

Berthelot-typedynamicalmechanismsinInGaN/GaNheterosystems.Thecorrelationbetweenrelaxationprocessandstructuralirregularitywasanalyzedaswell.

2.Experiments

Thesamplesinvestigatedinthisworkweregrownonc-planesapphiresubstratesinametal-organicvaporphaseepitaxy(MOVPE)process.Thelayerstructureofthesamplesconsistedofa20-nm-thickGaNbufferlayer,a3mmthicklayerGaN:Si,followedbyanundopedGaNlayerwith veperiodsofmultiplequantumwells(MQWs)anda100nmthicklayerGaN:Mg.IntheMQWs,eachwellisa2-nm-thickIn0.15Ga0.85Nlayerforallthesamples.ButinsteadofaconventionalGaNbarrier,thethreekindsofheterostructureswithdifferentheterobarriersconsistedof vepairsofInxGa1ÀxN/GaNnanostructuresnamedmultiplequantumbarriers(MQBs)withxvaryingfrom0.005to0.02werepreparedinthisexperiments.Ahigh-resolutionX-raydiffraction(HRXRD)measurementwasperformedusingaBedeD1systemtostudytheepilayerpropertiesofMQWs.Becausethe(002)rockingcurvescontaintheinformationofbothlatticemismatchandoverlayercomposition,o/2yspectraweremeasuredforeachsample.ThesplittingduetolatticemismatchandtheIncontentbetweenInGaNandGaNwerethereforededucedfromtheserockingcurves,respectively.Forcurrent–voltagemeasurements,aKeithley2430asacurrentsourcedrovethesamplesmountedinaclosed-cycleheliumcryostat.TheelectroluminescencesignaldispersedbyanActonSpectraPro500imonochromatorweredetectedbyaSiphotodiodeandprocessedviaastandardlock-intechnique.3.Resultsanddiscussion

ThesplittingbetweenthediffractionpeaksoftheInGaNandtheGaNareshowninFig.1,fromwhichtheheterostructureswithlow,medium,andhighIncontentbarrierscanbequalitativelyanalyzed.Theobservedhigh-ordersatellitepeaksemergingfromtheInGaNthinlayerimplytheInincorporationintheheterobarriers,whilethemainpeakis(002)fromthethickGaNlayer.Sincethe(002)diffractionpeakre ectsthelatticeconstantinthegrowthdirectiononly,largersplittingmeanshigherIncontentforthesamples.ThexvaluesoftheInxGa1ÀxN/GaNMQBsforlow-,medium-,andhigh-In-contentsamplesare0.005,0.01,and0.02,respectively.ThebroadenedX-raydiffractionlinewidthsarealsoobservedastheIncompositioninthebarriersfurtherincreased,indicatingthecrystallinerandomization,theintermixing,and/orinterfaceroughnessinInGaN/GaNnanostructures.Differencesbetweenheterostructuresinvestigatedaresubstantiallyre http://doc.guandang.netparedtotheELat160K,thechangesintheemissionpeaksasafunctionoftemperatureforthesethreeheterodevicesfrom160to300K,respectively,areplottedinFig.2.Allspectral

(a)

low indium

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medium indium

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high indium

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/ 2θ scan (arcsec)Fig.1.High-resolutionX-raydiffraction(HRXRD)o/2y-scanspectrafor

heterostructureswithlow-,medium-andhigh-Incontentinthemultiplequantumbarriers.

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V0em( )K60614(hp2 E )T0(hpE6420

160180200

220240260280300

Temperature (K)

Fig.2.Changesintheemissionpeaksasafunctionoftemperatureforthesethreeheterodevicesfrom160to300K,respectively,comparedtotheELat160K.

positionsblueshiftattemperatureslowerthantheturning-pointtemperature,whichisdesignatedasTt,leadingtotheso-calledBerthelot-typeprocesses.ItisfoundthatthelineshiftdecreaseswithincreasingtheInNmolarfractioninthenanobarrier.Furthermore,asthearrowsshowninFig.2,theobservedTtforthelow-,medium-andhigh-In-contentsamplesareabout200,220,and240K,respec-tively,indicatingthestoichiometry-relatedopticalrelaxa-tions.Withincreasingtemperatureupto300K,theredshift-temperatureevolutionsarestronglycontingentontheInGaN/GaNcon gurations.Theseexperimental

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T.-E.Neeetal./JournalofCrystalGrowth287(2006)468–471

ndingsareconsistentwiththeaspectthattheinhomoge-neousGaussianbandtailinthedisorderingheterosystemsgovernsthespectralanomaliesoftheluminescenceinnanostructuredsemiconductors[5].However,boththelatticemismatchstrainandthespinodaldecompositionfacilitatetheformationofthequasi-three-dimensionalInGaNmicroclustersduringtwo-dimensional(2D)activeregionepitaxialgrowth.Asiswell-known,thequantum-dotconformationsinherentlydeterminenonequilibriumandequilibrium llingsofquantumstates[11,12].Thesigni cantblueshiftinthespectralpositionwithtempera-turemightberesponsiblefortheimproveddot-sizeuniformity.ThecarrierdistributionfunctionsintheIn0.15Ga0.85Nlayersrenormalizedbythesurroundingdot-likebarrierswithhighIn-compositionisthusattrib-utedtothesmallerblueshiftandthehigherTt,comparedwiththelow-In-contentsample.BeyondthetemperatureofTt,we ndthatthesamplewithIn0.005Ga0.995N/GaNheterobarrierexhibitsafastertemperature-dependentred-shiftofÀ0.048meV/KthandothesampleswithIn0.01-Ga0.99N/GaNheterobarrierandwithIn0.02Ga0.98N/GaNheterobarrierofÀ0.039andÀ0.019meV/K,respectively.BecausetheelasticstiffnessconstantsofGaNarelargerthanthoseofInN,thelow-In-contentpotentialbarrierisexpectedtoresultinthermalexpandingoftheInGaNwellregion.

CloserinspectionsoftheBerthelot-typebehaviorsofthephotonpeakpositionsforthesesamplesatthetemperaturerangeof160–300KareshowninFig.3,employingtheanalyticalformulationsdevelopedbySinghandJohn[8,9].Undertheassumptionofthebandgapshrinkagewithalinearthermalcoef cientofC1,thetemperaturedepen-denceoftheabnormalopticalpropertiesisuniquelycharacterizedby

ðEphÀEgÞTÀ1¼ÀC1ÀC2TÀ3,

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Fig.3.Plotof(EphÀEg)/Tvs.TÀ3forthetemperature-dependentELpeakpositionofsampleswithdifferentbarriers.

whereEphisthephotonenergy,Egisthegapenergyat0K,C2isastatic-microbarrier-relatedparameterofadis-orderedInGaNlayer,Tistheabsolutetemperature.KnowingtheC1allowsTttobeestimatedfromtheextracte …… 此处隐藏：7591字，全部文档内容请下载后查看。喜欢就下载吧 ……