InternationalJournalofNanoelectronicsandMaterialsVolume11,No.2,Apr2018[127‐134]

NiobiumPentoxideThinFilmPreparedusingSimpleColloidalSuspensionforOptoelectronicApplication

M.K.Abood1,2,E.T.Salim1andJ.A.Saimon1

1DepartmentofAppliedScience,UniversityofTechnology,10001Baghdad,Iraq.

2EnergyandRenewableEnergiesTechnologiesCenter.

Received27July2017;Revised14September2017;Accepted6December2017

ABSTRACT

In this study, thin‐film microstructures of polymorphous niobium pentoxide weresuccessfully prepared at a low temperature. Spin coatingwas employed to deposit thecolloidal suspension before and after ultrasonic vibration. The deposited films werecharacterised and analysed by X‐ray diffraction, scanning electron microscopy, andspectrophotometry foroptical investigation.Results show that the filmspreparedbeforeultrasonic vibration possess a smooth and uniform surface. By contrast, films thatunderwent furtherultrasonicvibrationexhibitanearth rock‐like structureandahighlyoriented, void‐ and crack‐free surface. Both film structures indicate the formation of amonoclinic structure. Low energy gap values of approximately 2.5 eV and 2.25 eV aresuitable forheterojunctionoptoelectronicapplications; thisconditionwasprovenby thetypicalI‐Vbehavioursofthepreparedheterojunctiononthesiliconsubstrate.Keywords: Nb2O5, Thin Films, Scanning Electron Microscopy (SEM), Sol–Gel Method,NiobicAcid,ColloidalSolution.

1. INTRODUCTION

AsaGroupFivetransitionelement,niobiumfundamentallysubsistsinaminimumoffourstablestoichiometric oxide forms, namely NbO, Nb2O3, NbO2 and Nb2O5. Each of these oxide formspossesses several electrical properties ranging from conduction to semiconduction andinsulation[1,2].Thermodynamically, niobium pentoxide (Nb2O5) is a highly stable form [3]. This materialpossessessemiconductingpropertyandisknownasann‐typesemiconductorwithabandgapofapproximately3.4eV,dependingonthestructureandthepreparationmethod[4‐6].Nb2O5has gained considerable research attention because of its numerousdistinct physicochemicalproperties and structural isotropy [7‐14]; these properties make Nb2O5 appropriate fornumerousapplications,suchascatalysts,solarcells,gassensors,opticalfilters,electrochromicand magnetic devices, biotechnology and nanotechnology, and photoelectrodes andoptoelectronics[15‐17].Nb2O5 subsists innumerouspolymorphic shapesdependingon the temperature atwhich thematerial is crystallised. The amorphous phase starts to crystallise in the TT‐Nb2O5 (pseudo‐hexagonal) form at a low temperature of approximately 500°C; the T‐Nb2O5 (orthorhombic)structureisachievedatahighpreparationtemperaturerangingfrom600°Cto800°C;H‐Nb2O5(monoclinic structure) phase originates from heating to approximately 1100°C in air. The

*CorrespondingAuthor:evan_tarq@yahoo.com

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crystallineattitudeof thismaterial isaffectedby therawmaterialsused,synthesisprocesses,potential impurities and any interaction with other components [18‐20]. Oxygen content isanother critical property that influences the prepared Nb2O5 film structure. At low oxygencontent,thefilmstructureisintheTTcrystallinephase,whereasH‐Nb2O5presentsthehighestoxygencontent[21].Thedevelopmentofniobiumoxidethinfilms[22]ornanoparticles[6,23]show extensive potential applications, so different preparation methods such as pulse laserdeposition, electrodeposition, and sol–gel method have been employed [24, 25]. The sol–gelmethod offers numerous advantages compared to other methods, such as the flexibility toproduce thin or thick films depending on the deposition condition. Examples of sol–gelapplication include thin films used as bond coating to increase adhesion between layers andthickfilmsusedasacorrosionprotectionlayer,whichproduceacomplexshapegeometryingelstate.Thus,thesol–gelmethodisasimple,economical,andefficientmethodtoproducehigh‐purityfilms[26].Ultrasonic vibration can be employed to improve the structural, morphological, and opticalproperties of the prepared thin film. This improvement can be achieved by increasing thehomogeneity,stoichiometry,andcrystallinityofthecolloidalsolution.Thismethodcanalsobeused for cleaning because it can rapidly reach a high temperature and pressure duringultrasonication,andultrasoundcausesaresonantimpingingeffectonthesolids[27,28].Inthiswork, aniobic acid‐based colloidal solutionwasobtainedby sol–gelmethodand successfullydepositedbyspincoating.Thisstudy focusesona low‐temperaturepreparationofH‐Nb2O5 thin filmswithoutpostheattreatment inaddition to theuseof anewrawmaterial,whichhasnotbeenapplied inNb2O5thin‐filmpreparationtothebestofourknowledge.2. EXPERIMENT

The required Nb2O5 colloidal solution was obtained using M‐Nb2O5 powder (ultra purity,99.99%), hydrofluoric acid, and ammonia without further purification. The reagents weremixed at a 1:1:4:8 molar ratio of DIW, ethanol, HF and NH3OH, with 0.2g Nb2O5 powder.Hydrofluoric acid and Nb2O5 were mixed while being heated and stirred for 1 hour until atransparentsolutionwasobtained.Afterward,ammonia,water,andethanolwereaddedwhilebeing continuously stirred for 1 hour in a 100°Cwater bath. Amilky colloidal solutionwasobtained. An ultrasonic vibration instrument (Microelectronic brain control 3560) was usedwithanoperatingpowerof50Wfor30min.Chemicalreactionsinpreviousstudiesusuallytakethefollowingchemicalformula[29,30]:

OHNbOFHHFONb 25252 3210 (1)

OHFNHONbOHNHNbOFH 252452 741010][2 (2)

SpincoatingwasemployedtoobtainhomogeneousNb2O5thinfilms.Thecolloidalsolutionwasdeposited on quartz substrates at a rotational speed of 2500 rpm for 1min. The filmswerepreparedbeforeandafterultrasonicvibration.Afterdeposition,filmsweredriedat100°Cfor5min in static air.No furtherheat treatmentwas conducted.AT60UV–vis spectrophotometerwas used to measure the optical properties of the prepared films. The energy gap wasdeterminedusingthefollowingequation:

2/1)()( EghAh (3)

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whereαistheabsorptioncoefficient,hνistheenergyoftheincidentphoton,Aisaconstant,andEgistheopticalbandgap.ThevalueofαcanbeobtaineddependingonthefilmthicknesstanditstransmittanceTbyusingthefollowingequation:

Tt

1ln

1 (4)

The band gap of the prepared thin films was obtained by extending the straight line of the(αhν)2–hνplotwiththeincidentphotonenergy.StructuralpropertieswereobtainedusingCu‐kαX‐raysourcefromShimadzu6000.GrainsizeD,densityofdislocationδ,andstrainεweremeasuredbasedontheX‐raydiffraction(XRD)resultbyusingthefollowingequations:

cos

KD

(5)

2

1

D (6)

4 tan

(7)

whereλistheusedwavelength,βisthefullwidthathalfmaximumandθisBragg’sangle[31].A scanning optical reflectometer (Filmetrics F20, USA) was used for film thicknessmeasurement.Then,morphologicalpropertieswereobtainedusinganAA‐3000‐typescanningelectronmicroscope.3. RESULTSANDDISCUSSION

Figure1 shows theabsorption spectraof theNb2O5 thin filmspreparedonquartz substratesbeforeandafterultrasonicvibration.Theplottedabsorptionspectrumwithinaspectralrangeof 300–1000nm increases after ultrasonic vibration owing to the increase in film thickness,which may be attributed to the increase in the number of Nb2O5 microparticles. Thus, thedensityintheunitvolumeofthecolloidalsuspension(whichwassubsequentlydepositedonaquartzsubstrate)increases.Theseresultsareconsistentwiththoseofpreviousstudies[32,33].The filmthicknesswasmeasured tobeapproximately430and460nmforsamplesdepositedbefore and after ultrasonic vibration, respectively; this result indicates the increase in theabsorbanceresultsofthepreparedfilms.OpticalresultsshowasharpbandedgeabsorptionintheUVregionforallpreparedfilms,whichindicatesthattheobtainedenergygapvaluesrelyonadirectbandgapsemiconductormathematicalmodel,asexpressedinequation3andshowninFigure2.Figure2showstheobtainedEgvalues.Thevaluesarelowcomparedwiththebandgapvaluesof the corresponding Nb2O5 films obtained using other chemical methods. The values wereapproximately 2.5 and 2.25 eV for samples prepared before and after ultrasonic vibration,respectively. This result may be attributed to the presence of a sup band in the electronicstructure associated with the prepared microstructure films. This result coincides with thescanningelectronmicroscopy(SEM)results,whichindicatethe formationofamicrostructurethin film; aprevious studyalsoobtained similar results [34].Theenergygapvaluegenerallyincreaseswithmolarconcentration;inthiscase,thedecreaseinenergygapvaluescorresponds

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totheincreaseinparticlegrainsizeafterultrasonicvibration,whichisinagreementwiththeSEMresults.

Figure1.AbsorptionasafunctionofwavelengthforNb2O5thinfilmspreparedbeforeandafterultrasonicvibration.

Figure2.EstimatedenergygapvaluesforpreparedNb2O5thinfilmsbeforeandafterultrasonicvibration.TheXRDresultsofthepreparedfilmsonquartzsubstratebeforeandafterultrasonicvibrationarepresentedinFigure3.AllpeaksofNb2O5filmscorrespondtothepeaksofJCPDFcardfile00‐037‐1468.Thediffractionpeaks in both cases suggest the successful formation of H‐Nb2O5 thin films with a clearpolymorphous structure. The synthesis of this structure may be attributed to the abundantoxygeninthesuspensioncomingfromthedecompositionofH2Oandethanolduringchemicalreaction; the abundant oxygen thus increases the oxygen content in the prepared structure,producingH‐Nb2O5;apreviousstudyobtainedsimilarresults[21].ThisconditionmayalsobecorrelatedwiththetransformationoftherawmaterialstructurefromtheM‐Nb2O5structureoftheusedpowder,whichisassumedtobeadisorderedorpoorlycrystallineHinH‐Nb2O5afteranappropriateheattreatmentinwaterbathduringproduction;thisresultwasconfirmedbyapreviousstudy[35].

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The diffraction peaks at 2θ = (14.2, 17, 18.4, 20.4, 37.2 and 43.2) are ascribed to the Nb2O5diffraction plane values of 203, 301, 403, −304, −610 and 017, respectively. The moderatediffractionpeak forbothsamplespreparedbeforeandafterultrasonicvibration isat the301diffractionplane,suggestingtheformationofamonocliniccrystallinestructure.Figure3showssignificantimprovementinthestructureofthepreparedfilmsafterultrasonicvibration,wherethe Nb2O5 diffraction peak appears at the 301 diffraction plane with doubled intensitycomparedwith other diffractionpeaks corresponding to the samematerial. This condition isassociatedwitha relative reduction inNbOdiffractionpeaks comparedwith the relatedXRDpeakbelongingtotheNb2O5diffractionplaneforthesamesampleafterultrasonicvibration.

Figure3.X‐raydiffractionresultsofpreparedthinfilmsafterandbeforeultrasonicvibrationprocess.As shown inTable 1,D, ε,β andδ improve the crystallisationproperty and the reduced filmdefectsbyincreasingthegrainsizeandreducingthedislocationdensityandstrainvaluesafterultrasonicvibration[36].

Table1Structuralparametersofpreparedfilms

Beforeultrasonicvibrationprocess Afterultrasonicvibrationprocess

PeaksB

(deg)D

(nm)(δ)*103‐(line/m2) ε

B(deg)

D(nm)

(δ)*10‐3(line/m2) Ε

14.2 1.0 8.4 14.29 10.04 0.6 13.9 5.1 6.0

17.0 1.0 8.7 13.07 0.74 0.7 12.0 7.0 0.5

18.4 0.1 18.0 3.09 0.24 0.4 21.0 2.3 0.7

Average 0.7 11.7 10.15 3.67 0.6 15.7 4.8 2.4Themorphological properties of both Nb2O5 thin films prepared before and after ultrasonicvibrationareshownintheSEMmicrographinFigures4aand4brespectively.Themicrographsclearlyshowthatfilmhomogeneitydecreasesafterultrasonicvibration,owingtotheincreaseintheamountofNb2O5microparticlesinthecolloidalsuspension,whichtendtoaggregateandformamaterialwithalargegrainsize.Microstructureswithanearthrock‐likestructureformedalong the film surface. Such structure is anormal characteristicof filmsderived fromsol–gelmethod [37]. The average grain sizes obtained from the SEM micrographs increased afterultrasonic vibration, owing to the increase in aggregation associated with Nb2O5materialconcentrationperunitvolumeofthecolloidalsuspension,andconsequently,withfilmdensity

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10 20 30 40 50 60 70 802 Ɵ (deg)

Inte

nsi

ty (

a. u

.)

After

Before

Nb 2

O5

(203

)

Nb 2

O5

(301

) N

bO (

403)

Nb 2

O5(

-610

)

Nb 2

O5(

017

)

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perunitarea.Thegrainsbecamedenserafterultrasonicvibration, resulting inan increase inthethicknessofthepreparedfilms.

Figure4.Scanningelectronmicroscopemicrographrepresentspreparedfilmsbefore(a)and

after(b)ultrasonicvibrationalprocess.Tocharacterisethedeviceasheterojunction,wepreparedNb2O5filmsonthesiliconsubstratebeforeandafterultrasonicvibration.Figure5revealsaclearenhancement inthe forwardandreversecurrentcharacteristicsafterultrasonicvibration.Thecurrent–voltagecharacteristicintheforwardcurrentisshownintwosignificantregions,namelyrecombinationandtunnellingcurrentregions.Theforwardcurrentafter ultrasonic vibration significantly increased in the two regions; this increase may beattributedtotheincreaseinthenumberofelectron–holepairsgeneratedduetotheincreaseincolloidaldensity,thatis,thenumberofmoleculesperunitvolume.Consequently,theincreaseresults in the increase of the number of electron–hole pairs generated and drifting in thespecifically applied bias. The significant increase in the forward current is also due to theincreaseinfilmthicknessafterultrasonicvibration,whichisrelatedtothereducedthicknessofthe depleted region. The apparent barrier height is attributed to the increase in saturatedcurrentdensity.Apreviousstudyconfirmedthisresult[38].Similartotheforwardcurrent,thereversecurrent increasedsubstantiallyafterultrasonicvibration; thecurvealsopresentstworegions. The generated current slightly increasedwith the applied voltage as a result of thegeneration of electron–hole pairs at low voltage bias. The increase in the second region iscorrelatedwiththediffusionofminoritycarriersthroughthejunction.Devicerectificationalsoincreasedduetotheincreaseintheinterstitialsite,whichresultedinlargegeneration–recombinationanddiffusioncurrents.

(a)

(b)

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Figure5.Current–voltagecharacteristicsofNb2O5thinfilmsdepositedintosiliconsubstratesbeforeand

afterultrasonicvibration.4. CONCLUSIONS

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