ISSN1330–0008CODEN FIZAE4

OPTICAL CHARACTERISTICSOFBISMUTH SULFIDE(Bi2S3) THIN FILMS

S.MAHMOUD, A. H. EID, andH. OMAR

PhysicsDepartment,NationalResearch Centre, Cairo, Egypt

Received19May 1997

Revisedmanuscriptreceived15 September1997

UDC 538.975

PACSnumbers73.40.–c,78.66.–w

Thin filmsof bismuthsulfide(Bi2S3) weregrownby two depositiontechniques,by thermalevaporationandbychemicaldeposition.Thethermallydepositedreactionsconsistedin de-positingtheindividualelements,namelybismuthandsulfur, sequentiallyfrom a tungstenboatsourceandallowing thelayersto interdiffuseto form thecompoundduringtheheat-treatment.Thechemicaldepositionwasbasedonthereactionbetweenthetriethanolaminecompex of Bi3

�ionsandthioureain basicmedia.ScanningelectronmicroscopeandX-ray

diffractionanalysisweremadeon as-depositedandon annealedfilms to determinetheirstructure.Thedifferentelectronictransitionsandtheopticalconstantsaredeterminedfromthe transmisionandreflectiondataof thesethin films for normalincidence.The opticalgapsof Bi2S3 films show a remarkabledependenceon thepreparationmethod.

1. Introduction

The knowledgeof the optical propertiesof thin films is very importantin many sci-entific, technologicalandindustrialapplicationsof thin films suchasphoto-conductivity,solarenergy, photography, andnumerousotherapplications.Directbandgapsemiconduc-torswith badgapsin therange1.2to 1.7eV arewell suitedto convert light into electricity.In this respect,bismuthsufide(Bi2S3) seemsto be a promisingmaterial,sinceit shows

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a strongabsorptionof light of wavelengthsshorterthan900nm. TheopticalbandgapofBi2S3 films is in the range1.2 to 1.7 eV [1–7]. Sincethe photovoltaic propertiesaredi-rectly relatedto the materialproperties,the choiselies in both the preparationmethodandthe characterizationtechniques.Only a few methodshave beenusedto prepareandcharacterizethebismuthsulfidematerial,bothin singlecrystalandpolycrystallineforms,anddifferentaspectsof elecrochemicalbehaviour have beenreported[8–10]. Peter[11]showedthattheelectrodepositedBi2S3 films havetwo transitionsoninteractionwith light,anindirectoneat 1.25eV, anda directoneat 1.7eV. Thelatteris particularlyinteresting,sincesatisfactoryadsorptionof visible light via a direct transitioncanbe obtainedwithfilms of a just few micronsin thickness.The chemicalprocessfor the preparatinof thinfilms offerstheadvantagesof economy, convenienceandtheability to depositlargeareas.

In the presentwork, the mostfavorableresultsareobtainedwith thin films of Bi2S3formed by two depositiontechniques(thermalevaporationand chemicalmethod).Thematerialobtainedwascharacterizedby opticalmeasurementsandtheresultsarediscussedin connectionwith the crystalstructureof the film. As far aswe know, no reporton theoptical propertiesof polycryctallinestoichiometricfilms of Bi2S3, preparedby thermalevaporation,hasbeenpublished.

2. Experimentalprocedure

Thedepositionbathhasbeenpreparedfollowing theproceduresreportedin previouspapers[3,12–14].Thedetailedprocedureis: 1 M solutionsin distilled water, of thioureaandof bismuthnitratewereprepared.A definitevolumeof the bismuthnitratesolutionwascomplexedby theadditionof triethanolamine.Thiswasfollowedby theadditionof 1M thiourea,and17N ammonia(aq.)andwater. Thebasicoverall reactionis [9]:

2Bi[N(CH2CH2OH3]3� =C

�� ��� �

NH2

NH2

� Bi2S3 + 3 O = C

�� ��� �

NH2

NH2

+ (A)

whereA is thecomplexing agent,N (CH2CH2OH)3.

For a freshly preparedcomplex solutionandat a depositiontemperaturearound373K, goodquality depositsareobtainedfor pH around9.5. After the deposition,the filmswerecleanedby flushingwith distilled waterandthendried.Thethicknessof the layers,measuredby weightdifference-densityconsiderations,liesin the500to900nmrange.Thestructurestudiesweremadeby usingJSM-T20 scanningelectronmicroscopeandaPhilipsX-ray diffractometermodelPW1390with theCuKα target.Transmissivity andreflectivityof thesefilms in thewavelengthrange300–3000nm weremeasuredat normalincidenceusingadoublebeamscanningspectrophotometertypeUV-3101PCfrom Schimadzu.Theaccuraciesof thesemeasurementswerebetterthan0.3%and0.5%for transmittanceandreflectance,respectively, in almostthewholemeasuredspectrum.All measurementwereperformedon thefilm depositedon thesurfaceof theglassslidewhich facedthewall ofthebeaker. Thesefilms weresmoothandof uniform thickness.

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Also, Bi2S3 films werepreparedby the thermalevaporationtechniquein which theindividualelements(bismuthandsulfur)wereevaporatedfrom a tungstenboatundervac-uumconditionsof 10� 4 Pa.Thecompoundfilms weredepositedonglasssubstratekeptatnearlyroomtemperature.Thefilms wereheat-treatedat453K for 5 hoursin air. Thesam-plesweregivenanoverlayerof carbonin anattemptto reducethepossiblere-evaporationduringheat-treatment.Thethicknessof thefilm wasmeasuredby theTolansky’stechnique[15].

3. Resultsanddiscussion

3.1. Chemicallydepositedfilms

Thematerialwasfurthercharacterizedby structuralandopticaltechniques.Thechem-icaly depositedthin films werefoundto beamorphousin theas-preparedform. After airannealingnear523K, thethin films becomecrystalline,asfoundby XRD andmicroscopicobservations(Figs. 1 and2). Theseobservationsarein closeagreementwith the resultsreportedby otherinvestigators[4,9,16]. The transformationof the predomenantlyamor-phousstateof the ”as-prepared”sampleto crystallinestateis alsoevident in the opticaldensity[14], wheretheabsorptionedgeis shiftedto a lower wavelength.This behaviourmayindicatea dropin theopticalbandgap.

Fig. 1. X-ray diffractionpatternsof chemicallydepositedBi2S3 films: a) asdeposited,b)annealedat523K for 35min.

Figure3 illustratestheexperimentaltransmission(T) andreflection(R) of anon-heatedsample590nmthick. It is clearfrom thecurvesthattheabsorbanceis high for photonsofenergy greaterthanthebandgap.On theotherhand,beyondthegapedgetheabsorbanceis very smallandthetransmittanceis high,which indicatesthattheobtainedsampleis oflow impurity andhasfew latticedefects.

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MAHMOUD ET AL .: OPTICAL CHARACTERISTICS OF BISMUTH SULFIDE . . .

Fig.2.Scanningelectronmicrographof non-heatedandheatedchemicallydepositedBi2S3films, eachof a thicknessd � 860nm: a) nonheated,10 500� , b) heatedat 523K for 10min., 10 500� , c) heatedat 523 K for 35 min., 6000� , d) heatedat 573 K for 10 min.,6000� , ande)heatedat573K for 35min., 6000� .

Theabsorptioncoefficient (α) andtheopticalconstants(n, k) aredeterminedfrom thetransmissionandreflectionspectrumbasedon thefollowing relations[17]:

T �1 R� 2e� αd

1 R2e� 2αd 1�

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whered is thefilm thicknessandα is relatedto theextinctioncoefficientk by:

k � αλ � 4π � � 2�

Fig. 3. Transmission,T andreflection,R, spectrumof a typical Bi2S3 thin film (d � 590nm).

Therefractive index (n) canbedeterminedfrom Randk usingtherelation:

R � � n � 1� 2 � k2

� n � 1� 2 � k2 � � 3�

Theresultingvaluesof theopticalconstants(n � k) asfunctionsof photonenergy arepre-sentedin Fig. 4. Thefiguredemonstratethattheextinctioncoefficient(k) hasits minimumvalueatlow energyandincreaseswith increasingphotonenergy, while therefractiveindex(n) approximatelyconstant.

Theabsorptiondataat longwavelengthsareparticularlyinterestingfor thedetermina-tion of thebandgapof Bi2S3 with reasonableaccuracy. Theenegy-absorbancespectrumisshown in Fig. 5, asa testfor indirectanddirectinterbandtransitions.Theabsorptionedgeis muchbroaderthanexpectedfor adirect-bandgap-typematerial.Thiscanbeascribedtothegrain-boundarydiscontinuityeffect in thestructureandlackof stoichiometrygenerallyobservedin polycrystallinematerials[4,9].

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MAHMOUD ET AL .: OPTICAL CHARACTERISTICS OF BISMUTH SULFIDE . . .

Fig. 4. Variationof theopticalconstants(n � k) with photonenergy � hν ��� d � 590nm).

Fig. 5. Variationof optical absorptioncoeficient(α) with photonenergy (hν ��� d � 590nm).

For a largenumberof semiconductors,in both crystallineandamorphousforms, thedependenceof theabsorptioncoefficient (α) on thephotonenergy (hν), for optically in-ducedtransitions,takestheform:

α � A � hν � Eg� m � � 4�

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whereEg is theopticalenegy gap,A is a constantandm � 1 2 or 2 for analloweddirect-or indirect-transitionenegy gap,respectively.

Fig. 6. Variationof ! α vs.hν " d # 590nm).

Clearly, the transitionat $ 1 % 28 eV is indirect sincethe plot of α1& 2 vs. hν is linear(Fig. 6). This valueagreeswell with otherreports,althoughno detailedanalysisof thetransitionappearsto have beenpreviously reported[1,2,11]. On the otherhand,thereisevidencethatanadditionaldirect transitionmaybegin at 1.68eV, sincetheplot of α2 vs.hν is linearabovethisphotonenergy (Fig. 7). Thisvalueagreeswell with thevalue( $ 1 % 7eV) for amorphousfilms [3–5]. It hasbeenfoundfrom theX-ray analysis(Fig.1a)thattheBi2S3 films areamorphous.Thedirecttransitionat1.68eV wouldmakeBi2S3 particularlysuitablefor solarenergy convesionsincerelatively thin films couldbeused.

3.2. Thermallydepositedfilms

Thereflectionandtransmissiondatawererecordedfor many Bi2S3 thin films of dif-ferentthicknesses[18]. Theabsorptioncoefficient (α) is obtainedfrom theratio of trans-mission(T1' 2) of two thin films of differentthicknessesaccordingto therelation:

ln " T1 T2 ( � α∆d ) " 5(

where∆d is thethicknessdifferenceof thefilms.

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MAHMOUD ET AL .: OPTICAL CHARACTERISTICS OF BISMUTH SULFIDE . . .

Fig. 7. Variationof α2 vs.hν * d + 590nm).

Fig. 8. Variationof the optical constants(n , k) with photonenergy (hν) - thermallyde-positedfilm (d + 500nmheatedat453K).

Figure8 showstheopticalconstantscalculatedfrom Eqs.(2), (3) and(5) for theheatedsampleat 453K. Theestinctioncoefficient (k) increasesgraduallywith photonenergy up- 1 . 5 eV andthenrapidly increasesfor hν / 1 . 5 eV. Thevaluesof therefractiveindex (n)agreewith thevaluesgivenin Ref.6.

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Thedirectenergy gapis calculatedfrom therelation

0lnT1 1 T2 2 2 3 0 α∆d 2 2 3 A

0hν 4 Eg 2 5 0

62

Analysisof the shift in the bandedgeof absorptionin termsof conductionandvalencebandextremaat thecentreof Brillouin zonerevealedtheexistenceof thedirecttransition.The direct energy gapis found from the interceptof the straightline portion of the plotof0lnT1 1 T2 2 2 vs. hν (Fig. 9). Theextrapolationof thegraphgivesthe magnitudeof the

bandgapof 1.58 eV, which is in goodagreementwith the valuesobtainedby the otherauthors[3,4,7,15,18].

Fig. 9.0lnT1 1 T2 2 2 vs.hν for thermallydepositedBi2S3 film (d 6 500nm).

4. Conclusion

Amorphousphasesof any materialwith thesameco-ordinationasthatof thecrystallinephasehaveapproximatelythesamebandgap.In otherwords,thebandgapvaluedoesnotdependon whetherthefilm is crystallineor not, but only on theco-ordination.Sincethebandgapvaluein thethermalmethod(1.58eV) is lowerthanthatof chemicalmethod(1.68eV), it mustbeconcludedthattheamorphousphase(chemicalmethod)hasa differentco-ordinationthanthecrystallinephase(thermalmethod).Ordinarily, in amorphousphases,the averageco-ordinationwill be low [19,20]. Thus, the low valueof bandgap in thethermalmethod(1.58eV) maybedueto thehighervaluesof co-ordinationnumberthanthatof amorphousphase(chemicalmethod).

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OPTICKA SVOJSTVA TANKIH SLOJEVA BISMUT SULFIDA Bi2S3

Tanki su slojevi bismutsulfida(Bi2S3) nacinjeni dvjemametodama,vakuumskimnapar-avanjemi kemijskim talozenjem.Ti su se slojevi istrazivali sustavno-pretraznomelek-tronskom mikroskopijom i difrakcijom rentgenskog zracenja, prije i nakon termickogopustanja.Nasaose je niz elektronskihprijelazai odredilese opticke konstantena os-novi transmisijskihi refleksijskihpodatakazaokomituupadnusvjetlost.Opticki procijepiBi2S3 pokazujuprili cnuovisnosto metodipripremanja.

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