Effect of Annealing

linion

man

SemiconductorNanocrystallineThin film

S3 thining

at a bath temperature 318 K. The prepared films are subsequently annealed at differenttemperatures for studying the effect of thermal treatment on the structural, surfacemorphology, optical and electrical properties of the films. The X-ray diffraction studies

ls havial app

trical switching, solar selective, decorative coatings etc. [4].

devices as its band gap of 1.7 eV in bulk material lies in the

4,11] etc. Amongn, chemical bathfor a large area

n films have beenide ion releasingd thioacetamide] deposited Bi2S3

films using thiourea as a sulphide ion source and triethano-

1.47 eV. Biswas et al. prepared Bi2S3 thin films using TEA and

Contents lists available at ScienceDirect

.els

Materials Science in Sem

Materials Science in Semiconductor Processing 21 (2014) 74811369-8001/$ - see front matter & 2014 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.mssp.2014.01.029thioacetamide as the complexing agent and sulphide ionsource respectively in an alkaline bath. The films are amor-phous with an optical band gap of 1.7 eV and resistivity of105107 cm. Lokhande and coworkers [12,1718] obtainedn Corresponding author.Among these VVI compounds semiconductor, bismuthsulphide (Bi2S3) is a promising candidate for optoelectronic

lamine (TEA) as a complexing agent in an alkaline bath (pH8). The films are reported to be amorphous with band gapsize, since the material properties are dependent upon thegrain size. These unique properties are required for thedevelopment of modern electronic devices. The membersof VVI compounds semiconductor are considered importantmaterials for their potential application in photosensitivity,photoconductivity and thermoelectric power [13]. Thesecompounds are widely used in optoelectronic devices, elec-

tion gas interface [10], spray deposition [these techniques of thin film preparatiodeposition is simple, economic and suiteddeposition. Chemical deposition of Bi2S3 thireported earlier by using different sulphsources such as thiosulfate, thiourea an[1214]. Pramanik and Bhattacharya [15cells and other electronic devices. The optical and electricalproperties of the materials changes by changing the grain

such as vacuum evaporation [6], cathodic electrodeposition[7], anodic electrodeposition [8], hot-wall method [9], solu-1. Introduction

Recently, nanostructured materiascientific research due to their potentsize of 14 nm to 28 nm. The scanning electron microscopy (SEM) images revealed that thefilms comprise of grains of spherical shape of unequal size. It is also observed that thesmall particles aggregate together to form a larger cluster. The average grain sizesdetermined from the TEM images are smaller than the crystallites size obtained fromthe XRD studies. The optical band gap of the films has been estimated to be 2.242.05 eVfor the as-prepared and annealed films, respectively. The electrical conductivity of the asprepared Bi2S3 films at room temperature is found to be in the order of 103 1 m1.

& 2014 Elsevier Ltd. All rights reserved.

e been focus oflications in solar

visible solar energy spectrum [5] and could be increased tohigher energy by reducing the grain size.

Several researchers have reported for the preparation ofnanocrystalline Bi2S3 thin films using different techniquesXRDSEM

affirmed that the deposited films are orthorhombic structures with average crystallitesEffects of annealing on nanocrystalprepared by chemical bath deposit

Amir Hussain n, Anayara Begum, Atowar RahDepartment of Physics, Gauhati University, Guwahati, Assam-781014, India

a r t i c l e i n f o

Available online 7 February 2014

Keywords:

a b s t r a c t

Nanocrystalline Bi2the solution contai

journal homepage: wwwe Bi2S3 thin films

n films are deposited on tin chloride treated glass substrate frombismuth nitrate, triethanolamine (TEA) and thioacetamide (TAM)

evier.com/locate/mssp

iconductor Processing

A. Hussain et al. / Materials Science in Semiconductor Processing 21 (2014) 7481 75amorphous Bi2S3 thin films from acidic as well as alkalinebaths using the disodium salt of ethylenediaminetetraaceticacid (EDTA) as a complexing agent and Na2S2O3, thiourea,as the sulphide ion source. The band gap energy is reportedto be 1.54 eV (indirect) and 1.74 eV (direct). Hussain et al.[19] and Begum et al. [20] reported to prepare Bi2S3 thinfilms in PVA matrix using Na2S as the sulphide ion source.Ubale et al. [21] have prepared Bi2S3 thin films by modifiedchemical bath deposition at room temperature and reportedtheir electrical and optical properties. Deshmukh et al. [22]studied electrical and optical properties of the films obtainedfrom aqueous alkaline bath using thiourea as a source ofsulphur ions and TEA as a complexing reagent.

In this paper, we report the preparation of nanostruc-tured Bi2S3 thin films by chemical bath deposition techni-que using thioacetamide (TAM) as sulphide ion source andtriethanolamine (TEA) as complexing agent. The preparedBi2S3 thin films are annealed in air atmosphere for 1 h atdifferent temperatures. The structural, morphological,optical and electrical properties of the as-prepared andannealed films are studied.

2. Experimental procedures

2.1. Substrate cleaning

The substrate cleaning is very important in the deposi-tion of thin film. The substrates of appropriate sizes2.52.5 cm2 are cut from the glass slide and washed withordinary detergent solution, then treated in a mixture ofnitric acid and isopropyl alcohol. The substrates are takenout from the solution and ultrasonically cleaned with de-ionized water for 1 h and wiped with acetone and heatedin an oven for drying. Finally, the chemically cleaned glasssubstrates are treated in 0.05 wt% of tin chloride solutionfor 20 min, rinsed in distilled water and heated at 473 K for20 min.

2.2. Preparation of Bi2S3 thin films

For the preparation of nanocrystalline Bi2S3 thin films,Bi(NO3)3 and C6H15NO3 (TEA) purchased from Merck Che-micals are used as Bi3 source and complex agent respec-tively and CH3CS NH2 (TAM) purchased from Loba Chemieis used as S2 ions sources. For this 5 ml of 0.5 M Bi(NO3)3dissolved in 2 ml of TEA and 4 ml of 1 M CH3CS NH2 ismixed together. The resultant solution is stirred for 20 minat room temperature to get uniform mixture solution.Finally, 39 ml of distilled water is added to the resultantsolution to obtain a total volume of 50 ml. The tin chloridetreated glass substrates are dipped vertically in the resul-tant solution supported by the wall of the beaker andheated at 318 K for 20 min. The resultant solution changesfrom brown to dark brown colour which indicates theinitiation of Bi2S3 film formation. The solution is kept atroom temperature for 2 h for further deposition. The glasssubstrates coated with Bi2S3 film are removed from thesolution and washed with distilled water several times. Theglass substrate facing towards the wall of the beaker isretained for further studies and the other side is removedwith dilute nitric acid. The deposited film is subjected toannealing at different temperature in the air atmospherewith the help of a hot oven (Oven Universal, NSW Indiaoperating up to 400 1C).

2.3. Reaction mechanism

The deposition process of Bi2S3 film is based on theslow release of Bi3 and S2 ions in the solution whichthen condenses ion by ion or cluster by cluster on thesurface of the substrates. The deposition of Bi2S3 thin filmsoccurs when the ionic product Bi3 and S2 ions exceedsthe solubility product of Bi2S3. The concentration of Bi3

and S2 ions in the solution controls the rate of Bi2S3formation. The rate of Bi3 ions is controlled by TEA,which forms a complex Bi[(TEA)n]3 with Bi3 . Thechemical reaction responsible for Bi2S3 film is given below

BiNO33 5H2OTEA-BiTEA35H2O3NO3 1

BiTEA3-Bi3TEA 2

In alkaline medium

CH3CSNH2OH-CH3CONH2SH 3

SH OH-S2H2O 4

Bi3S2-Bi2S3 5

2.4. Characterization of the prepared film

The structure of the film is obtained by using X-raydiffraction (XRD) XPERT-PRO Philips diffractometer with Cu-K radiation (1.5405 ) within the 2 range 20401 whichis operated at 40 kV and 20mA. The composition analysis ofthe prepared film is carried out by X-ray Fluorescence (XRF).Surface morphology of the film is studied using JEOL-JSM6360 operating at 20 kV. Transmission electron microscopy(TEM) of the Bi2S3 sample is carried out using JEM-2100operated at 200 kV to estimate the grain size. The averagethickness of the film is measured by the Tolansky methoddiscussed in our earlier paper [23]. The optical characteristicsare studied using Carry-300 scan UVVisible spectrophot-ometer to determine the optical band gap energy. Forelectrical conductivity measurement Al electrodes in a co-planar configuration separated by a small gap are evaporatedin vacuum on the surface of Bi2S3 thin films. A constantvoltage is applied across the sample and the current is notedusing a Keithley electrometer. The temperature on the samplesurface is measured by Instron (IN 303) electronic tempera-ture controller. The type of electrical conductivity is deter-mined by simple hot probe method [23].

3. Results and discussion

3.1. Structural analysis

The crystal structure, orientation and crystallites size ofthe Bi2S3 thin films of the as-prepared and annealedsamples at different temperature are investigated in the

range of angle 2 between 201 and 401 using X-ray diffrac-tion (XRD) patterns. Generally, with an increase in anneal-ing temperature, structure of the film changes fromamorphous to crystalline and grain growth occurs withincrease in mobility with temperature. The XRD patternsof the as-prepared and annealed at different temperatureof Bi2S3 thin films deposited on the treated glass substrateis shown in Fig. 1. It is observed that the as-prepared filmshave low intensity with very small peaks indicating theirpredominantly amorphous nature or consisting of smallgrains. The intensity of peaks is seen to increase indicatingan improvement in crystallinity with annealing tempera-ture. The observed d spacing and the respective promi-nent peak correspond to reflection (2 2 0), (1 0 1), (1 3 0),(0 2 1), (2 1 1), (0 4 0) and (4 2 0) planes which are in goodagreement with the JCPDS data file No. 17-0320. Therefore,it has been concluded that the deposited Bi2S3 thin filmsare polycrystalline in nature with orthorhombic structure.In Fig. 1, the intensity of (1 3 0) peak is most intense ascompared with the other peaks. This indicates that the

given in Table 1. The deviation in the values of the latticeconstant of the Bi2S3 films from the bulk value indicatesthe presence of strain in the films. The misfit stress is oneof the most important factor adversely affecting thestructural properties which is resulted from geometricmismatch at inter phase boundaries between crystallinelattices of films and substrates [14]. These stresses cancause microstrain in the films. The microstrain and dis-location density of the Bi2S3 films annealed at differenttemperature are calculated using Eqs. (8) and (9), respec-tively, and the calculated values are represented in Table 1.The microstrain and dislocation density decreases withincrease in annealing temperature as shown in Fig. 3.

3.2. X-ray fluorescence (XRF) and energy dispersive X-rayanalysis (EDAX) analysis

XRF spectra give the confirmation of the presence ofBi and S in the as-prepared film as shown in Fig. 4. Thepresence of Bi is indicated by the distinct peaks BiM and BiM1

A. Hussain et al. / Materials Science in Semiconductor Processing 21 (2014) 748176orientation of the grain growth is preferably along (1 3 0)direction. These results are in good agreement with thatobtained by Benramdane et al. [24] and Mizogushi et al.[25]. The crystallite size of the film is determined from theXRD spectra by using Scherrer's formula [26]

D 0:94 cos

6

where (1.5405 ) is the wavelength used, is the full widthat half maximum intensity in radians and is the Bragg'sangle. It is found that crystallite size increases from 14 nm to28 nm with increasing annealing temperature from 300 K to523 K as shown in Fig. 2. At high temperatures, atoms haveenough diffusion activation energy to occupy the energeticallyfavourable site in the crystal lattice and eventually grains withthe lower surface energy become larger [27]. This could beexplained by considering the thermal annealing inducedcoalescence of small grains by grain boundary diffusion whichcauses major grain growth [28].

Fig. 1. XRD spectra of the as-prepared and annealed Bi2S3 thin films.The lattice constant is calculated by using the relation

dhkl 1

h2

a2 k2

b2 l2c2

q 7

The microstrain is calculated by using the following for-mula [29]

cot 4

8

where is the full width at half maxima in radian and isthe Bragg,s angle.

The dislocation density, defined as the length ofdislocation lines per unit volume of the crystal and isdetermined by using the following formula [29]

nd2

9

where n is a factor which is equal to unity giving minimumdislocation density and D is the average crystallite size.

The lattice parameter is calculated using Eq. (7) fororthorhombic structure and the calculated values are

Fig. 2. Plots of crystallite size and optical band gap vs Temperature.

neale

Calcul

a11.149

11.256

11.273

A. Hussain et al. / Materials Science in Semiconductor Processing 21 (2014) 7481 77Table 1Calculated structural parameters of chemically prepared Bi2S3 thin film an

AnnealingTemperature (K)

Planes d-Spacing values () Average crystallitesize (nm)

JCPDS Experimental

300 101 3.748 3.753 14130 3.569 3.587021 3.253 3.237

373 220 3.967 3.976 18101 3.748 3.750130 3.569 3.599and S is indicated by SK and SK. The spectra also show anotherpeak for the presence of Cl which is due to the glass substrateas confirmed by taking the XRF spectra of the bare glasssubstrate (not shown here). The presence of Rh peak in thespectra comes from the substrate holder used in the XRFinstrument. The quantitative and qualitative compositionalanalysis of the as-deposited Bi2S3 film is carried out by EDAXtechnique to study the stoichiometry of the prepared Bi2S3 film.Fig. 5 shows a typical EDAX pattern and details of relativeanalysis for an as-deposited Bi2S3 thin film. The spectrumconfirms that Bi and S atoms are present in the prepared film.The extra peaks observed in the EDAX spectra correspond to C,

021 3.253 3.255423 101 3.748 3.735 19 11.266

130 3.569 3.557021 3.253 3.241211 3.118 3.100221 2.812 2.806240 2.521 2.506

473 220 3.967 3.968 23 11.218101 3.748 3.751130 3.569 3.561211 3.118 3.118221 2.812 2.802240 2.521 2.510

523 220 3.967 3.970 28 11.216101 3.748 3.757130 3.569 3.557021 3.253 3.252211 3.118 3.118040 2.524 2.822240 2.521 2.516

Fig. 3. Plots of Dislocation density and microstrain vs Temperature.d at different temperature.

ated lattice constant () Dislocation density1015 (lines/m2)

Averagemicrostrain103

b11.304

c3.981

11.280 3.952 5.102 12.267

11.310 3.980 3.086 10.195Si, Na, Ca, Mg etc. which is due to glass substrate or thesubstrate holder used in the EDAX instrument. There is nosource of these elements in the chemical used for the Bi2S3 filmsynthesis. We consider only the atomic % of Bi and S and theaverage atomic percentage of Bi and S is found to be 43.36 and56.54 respectively showing that the film is S deficient.

3.3. Scanning electron microscopy (SEM) analysis

Scanning electron microscopy (SEM) photographs are usedfor studying the surface morphology of the film. Fig. 6 shows

11.261 3.963 2.770 8.348

11.227 3.980 1.890 7.336

11.241 3.987 1.275 6.342

Fig. 4. XRF spectra of the as-prepared Bi2S3 thin film.

10,000 magnification SEM images of as-prepared andannealed films at different temperatures. It is observed thatthe films are continuous over the glass surface and are fairly

uniform. The grains of the films have different shapes andsizes but almost compact. There are no macroscopic defectssuch as voids, peeling or cracks. The SEM images revealincrease of grain size with increasing temperature.

3.4. Transmission electron microscopy (TEM) analysis

Transmission electron microscopy (TEM) images of the as-prepared and annealed (523 K) Bi2S3 films are shown in Figs. 7and 8, respectively. It reveals that small grains attach togetherand produce large grains. The grain in dark colour shown inFigs. 7 and 8(a) are the nanocrystallites Bi2S3 and thecalculated grain size is found to be in the range of 29 nmand 520 nm respectively which is smaller than the X-raydiffraction results. The discrepancy between the grain size andcrystallite size obtain from the TEM and XRD measurementsmay be due to the difference in the thickness of the samplesstudied for these measurements, since the TEM grid requiresvery thin layer deposition on the carbon coated copper grid.But for XRD characterization, film deposited on the glasssubstrate is used and hence the film is thicker as compared tothat made for the TEM characterization [30]. HRTEM is usedFig. 5. EDAX spectra of the as-prepared Bi2S3 thin film.

Fig. 6. SEM photographs (a) as-prepared (b) 3

py an

A. Hussain et al. / Materials Science in Semiconductor Processing 21 (2014) 748178Fig. 7. (a) TEM image (b) high-resolution transmission electron microsco73 K (c) 423 K (d) 473 K and (e) 523 K.

d (c) selected area electron diffraction images of as-prepared Bi2S3 film.

A. Hussain et al. / Materials Science in Semiconductor Processing 21 (2014) 7481 79to study the structure as well as to observe lattice imagesshowing different orientations of the nanocrystals of Bi2S3.The HRTEM image shown in Fig. 7(b) exhibits lattice fringeswith d-spacing of 0.351 nm, 0.371 nm and 0.398 nm corre-sponding to the (3 1 0), (1 0 1) and (2 2 0) planes of the Bi2S3

Fig. 8. (a) TEM image (b) high-resolution transmission electron microscopy and (

Fig. 9. (a) Plot of absorbance vs wavelength (b) (h)2 vs h of Bi2S3 thinfilms of different annealing temperature.c) selected area electron diffraction images of annealed (523 K) Bi2S3 film.orthorhombic phase respectively and Fig. 8(b) depicts latticefringe with d-spacing of 0.351 nm corresponding to (3 1 0)plane which are in good agreement with the lattice constantfor the Bi2S3 orthorhombic structure. Also intersection oflattice fringes are observed,indicative of the overlap of severalBi2S3 nanoparticles with different orientation. Selected areaelectron diffraction (SAED) image shown in Figs. 7 and 8(c)exhibits multiple diffractions ring with missing periodicitywhich is due to the random orientation of the polycrystalline.No significant difference is observed in both the images exceptthe difference in grain sizes. In both cases nearly sphericalBi2S3 nanoparticles are observed.

3.5. Optical properties analyses

The absorption spectra of as-prepared and annealedBi2S3 thin films at different temperature are shown inFig. 9(a). The absorption at higher wavelengths is low andan intense absorption can be seen at lower wavelength.Further, absorption increases as annealing temperaturerises from 300 K to 523 K. The band gap of the films iscalculated by plotting (h)2 vs h using the relation

ahEgn

h10

Fig. 10. Plot of log(s) vs 1000/T of of Bi2S3 films of different annealingtemperature.

associated with defect levels within the band gap. Electrical

i2S3 t

age g

A. Hussain et al. / Materials Science in Semiconductor Processing 21 (2014) 748180where a is a constant, h is the photon energy, Eg is theband gap and is the absorption coefficient. The value of is obtained from the relation

2:303At

11

where A is the absorbance and t is the thickness of thefilm. The thickness of the Bi2S3 thin films is found to be167 nm. The value of n is 1/2 or 2 depending on thepresence of the allowed direct and indirect transitions.Fig. 9(b) shows the plots of (h)2 vs h for the Bi2S3 filmsat different annealing temperatures. The nature of theplots is linear indicating that the transition is a direct bandgap. The band gap is determined by extrapolating thestraight line portion of the plot to the energy axis.The intercept on energy axis gives the value of band gapenergy for all the samples and it decreased from 2.264 eVto 2.036 eV on increasing the annealing temperature from300 K to 523 K as shown in Fig. 9(b). The presence ofdefects in the nanostructure films produces discrete statesin the band structure which is responsible for the highvalue of the energy gap in the case of the as-prepared film.However, for higher annealing temperature, the films aremore homogeneous and reduce the number of defects anddisorder which decreases the density of localized states inthe band structure and consequently decreases the opticalenergy gap.

3.6. Electrical conductivity analyses

The variation of log(s) with temperature for the as-prepared and annealed films of Bi2S3 are shown in Fig. 10.The plot reveals that there are two distinct region: onebelow 378 K, where the conductivity varies comparativelyslowly with temperature and the other above 378 K, where

Table 2Variation of crystallite size and band gap with annealing temperature of B

Temperature (K) Thickness of thefilms (nm)

Crystallite size fromXRD (nm)

Aver

TEM

300 167 14 29373 18423 19473 23523 28 520the conductivity varies abruply with temperature. Theregion below 378 K can be called the low temperatureregion and above 378 K can be called high temperatureregion. The thermal activation energy is calculated usingthe relation [19,20]

s soeEa2kT 12

where Ea is the activation energy, so is a constant, k is theBoltzman's constant and T is the absolute temperature. Thecalculated activation energies values are presented inTable 2. In the low temperature region the activation energyfor the conduction is found to be 0.262 eV to 0.125 eV.In the high temperature region the energy is found to beconductivity of a semiconductori controlled by the numberof charge carriers available for conduction. As the tempera-ture increases from absolute zero, transitions are takingplace between the defects level and the conduction bandand valence band [6]. The conductivity of the films arefound to increase with increasing annealing temperaturebecause the grain boundaries and the crystal lattice defi-ciencies of the thin films are reduced with annealingtemperature resulting in an increase of the mobility of thecarrier. The electrical conductivity of the as-prepared film isfound to be in the order of 103 1 m1. From thermoe.m.f measurements it is found that the polarity of thethermally generated voltage at the hot end is positiveindicating that the Bi2S3 thin films is n-type.

4. Conclusion

In the present investigation, the effect of annealing onBi2S3 thin film is studied. The XRD studies reveal thatprepared Bi2S3 films are polycrystalline in nature withorthorhombic structure. The crystallite size determinedfrom the XRD spectra is found to increase from 14 nm to28 nm with increasing annealing temperature. From SEMphotograph, it is observed that the films are dense, smoothand consist of grains of unequal shapes and sizes. TEMconfirms the formation of nanocrystalline Bi2S3 grains. Theoptical band gap, electrical conductivity and activationenergy are observed temperature dependent. The pre-pared Bi2S3 films are found to be n-type as determinedby hot probe method.

Acknowledgments0.511 eV to 0.336 eV. These energy levels are thought to be

hin films.

rain size (nm) Optical band gap (eV) Activation energy (eV)

Region I Region II

2.237 0.262 0.5112.207 0.201 0.4422.157 0.164 0.4022.115 0.141 0.3882.058 0.125 0.336We express our gratefulness to the Department ofinstrumentation & USIC, SAIF, Gauhati university, Guwa-hati for providing us the XRD and and SAIF, NEHU for TEMand SEM facilities.

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A. Hussain et al. / Materials Science in Semiconductor Processing 21 (2014) 7481 81

Effects of annealing on nanocrystalline Bi2S3 thin films prepared by chemical bath depositionIntroductionExperimental proceduresSubstrate cleaningPreparation of Bi2S3 thin filmsReaction mechanismCharacterization of the prepared film

Results and discussionStructural analysisX-ray fluorescence (XRF) and energy dispersive X-ray analysis (EDAX) analysisScanning electron microscopy (SEM) analysisTransmission electron microscopy (TEM) analysisOptical properties analysesElectrical conductivity analyses

ConclusionAcknowledgmentsReferences