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Physica B 363 (2005) 102–109

www.elsevier.com/locate/physb

Characterization of CdSexTe1�x sintered films forphotovoltaic applications

Lokendra Kumara, Beer Pal Singhb,�, Aparna Misrac,S.C.K. Misrac, T.P. Sharmab

aDepartment of Physics, University of Allahabad, Allahabad 211 002, IndiabDepartment of Physics, Ch. Charan Singh University, Meerut 250 004, India

cNational Physical Laboratory, New Delhi 110012, India

Received 15 September 2004; received in revised form 2 March 2005; accepted 6 March 2005

Abstract

Polycrystalline thin films of CdSexTe1�x ternary system with variable compositions have been deposited onto highly

cleaned glass substrates by sintering process. The optical, structural and electrical properties of CdSexTe1�x sintered

thin films have been examined. The optical band gap and optical constants of these films have been determined by using

reflection spectra of these films recorded by spectrophotometer (model U-3400) in a wavelength range of 700–880 nm.

The crystal structure and composition of CdSexTe1�x ternary system have been determined by X-ray diffraction

patterns applying Vigard’s law using a Philips X-ray diffractometer with CuKa radiation. The Schottky junction ofCdSe0.4Te0.6 with silver (Ag) has been made and barrier height has been determined using current–voltage

characteristics.

r 2005 Elsevier B.V. All rights reserved.

PACS: 78.30.Cl; 78.50.Ge; 78.66.�W; 78.66.H.f

Keywords: Sintering; Optical band gap; Reflection spectra; Crystal structure; Barrier height

1. Introduction

The II–VI group compound semiconductors arewidely used in solid-state devices such as infrareddetector, photovoltaic cells, nuclear radiation

e front matter r 2005 Elsevier B.V. All rights reserve

ysb.2005.03.008

ng author. Tel.: +91121 2762022;

62838.

ss: beerpal_singh@yahoo.co.in (B.P. Singh).

detector and windows for IR laser, etc. Highabsorption coefficients, high efficiency of radiativerecombination and nearly matching band gapswith the visible region of the solar spectrum are themain causes for the popularity of II–VI semicon-ductors. These are the important semiconductorsfor optoelectronic devices [1]. Cadmium selenideand cadmium telluride belonging to II–VI groupare the most widely used optoelectronic materials

d.

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L. Kumar et al. / Physica B 363 (2005) 102–109 103

for CdSe/CdTe and CdS/CdTe hetero-junctionphotovoltaic devices [2–4]. Ternary materialsprovide a possibility of tailoring their propertiesas per requirements and hence project themselvesas important semi-conducting materials for theapplications in the field of device fabrication [5].Ternary thin films of CdSexTe1�x alloy of CdSeand CdTe or Cd, Se and Te find extensiveapplication in photo-electrochemical solar cells[6], transistors [7], photoconductors [8], solarcontrol applications [9], etc. CdSexTe1�x is apromising ternary system because of the tenabilityof its physical parameters by controlling itsstoichiometry. The band gap of this material canbe varied from 1.44 to 1.74 eV by an appropriatechoice of atomic composition (x) from 0 to 1[10,11].Thin films of cadmium chalcogenide alloys have

been studied, depositing the films using varioustechniques such as thermal evaporation [12–14]electroplating [15], slurry painting [16], two sourceevaporation [17] of CdSe and CdTe, three-sourceelemental evaporation [18], electron beam eva-poration [19], hot wall deposition [20,21].The optical, structural and electrical properties

of thin films mainly depend upon the depositiontechniques and thickness of the film. The deposi-tion technique and its associated process para-meters have a characteristic effect on nucleationand growth dominated microstructure of a thinfilm and thereby its physical properties. Consider-able amount of work have been done onCdSexTe1�x ternary system by various researcher,depositing the films by several techniques. Sinter-ing is very simple and viable technique for filmdeposition compared to other cost intensivetechniques [22–24]. In this paper, optical, structur-al and electrical characterizations of polycrystal-line CdSexTe1�x sintered films have been carriedout for photovoltaic applications.

2. Experimental details

2.1. Synthesis of CdSexTe1�x films

The films of CdSexTe1�x (x ¼ 0:2; 0.4, 0.6, 0.8)ternary alloys were prepared by the screen-printing

technique followed by the sintering method. Tomake different compositions of CdSexTe1�x, CdSeand CdTe of 99.999% purity were used.CdSexTe1�x alloy was prepared by taking thestoichiometric ratio of CdSe and CdTe com-pounds. The different compounds are taken asfollows:Weight of CdSe ¼ 191:36 (x) grams,Weight of CdTe ¼ 240:00 (1�x) grams,

Weight of CdCl2 ¼ 10% weight of

ðCdSeþ CdTeÞ grams.

As these weights are large in quantity, we reducethem in the same proportion. All the threecompounds were mixed thoroughly and few dropsof ethylene glycol (CH2OH �CH2OH) were addedto make it a paste. Cadmium chloride was used asan adhesive and ethylene glycol as a binder. Thepaste thus prepared was screen printed on highlycleaned glass substrates. The glass substrates werecleaned in aquaregia, washed in distilled water andisopropyl alcohol (IPA). The samples thus pre-pared were dried at 120 oC for 4 h in open air. Thereason for drying the samples at lower temperaturewas to avoid the cracks in the samples. Theremoval of organic materials takes place at about400 oC, so sintering temperature cannot be lessthan 400 1C. We optimized the sintering tempera-ture and sintering time by performing the experi-ment for different values of these two parametersand concluded that the samples would be sinteredat 440 1C for 15min in a temperature controlledfurnace in air atmosphere. To minimize theproblem of diffusion due to temperature gradientduring the cooling of films after sintering, the filmswere covered with the glass plates of substrate size[25]. All the films of CdSexTe1�x ternary alloyswere deposited under same experimental condi-tions.

2.2. Techniques of characterization

The thickness of the films was determined by anoptical interference technique. The thickness ofvarious films was found to be in the order of 5 mm.The reflection spectra (optical reflectance versuswavelength) of these sintered films were recorded

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L. Kumar et al. / Physica B 363 (2005) 102–109104

in the wavelength range 700–880 nm using adouble beam spectrophotometer (Hitachi U-3400,Japan). The XRD patterns were obtained using aPhilips PW 1140/09 X-ray diffractometer withCuKa (l ¼ 1:5405 (A) radiation. The Schottkyjunction of CdSexTe1�x (x ¼ 0:4) with Ag wasfabricated and barrier height was determined usingcurrent–voltage characteristics. For I–V measure-ments, voltage was applied by keithley program-mable voltage source and current was measured bykeithley programmable electrometer. The electricalmeasurements were made in dark in vacuum of theorder of 10�3 Torr.

Fig. 1. Reflectra spectra of ternary CdSexTe1�x (x ¼ 0:2; 0.4,0.6 and 0.8) sintered films.

3. Results and discussion

3.1. Optical studies

The reflection spectra of ternary CdSexTe1�x

(x ¼ 0:2; 0.4, 0.6, 0.8) sintered films were recordedat room temperature in the wavelength range700–880 nm using a Hitachi U-3400 UV–VIS–NIRdouble beam spectrophotometer. The optical bandgaps of these films were determined with the helpof these reflection spectra. Almost all the II–VIgroup compounds are direct band gap semicon-ductors. According to Tauc’s relation [26], theabsorption coefficient for direct band gap materialis given by

ahn ¼ Aðhn� EgÞ1=2, (1)

where hn is photon energy, Eg is band gap and A isconstant which is different for different transi-tions. The absorption coefficient a may be writtenin terms of reflectance as [27]

2at ¼ ln½ðRmax � RminÞ=ðR � RminÞ�, (2)

where t is the thickness of the film and R is thereflectance for any intermediate photon energy.The reflectance falls from Rmax to Rmin due to theabsorption of light by the material. The crossing ofthe curves of reflection spectra is due to fact thatfilms have somewhat variable thickness. A graphwas plotted between (ahn)2 or the square of hn ln[(Rmax�Rmin)/(R�Rmin)] (as ordinate) and hn (asabscissa), a straight line is obtained. The extra-

polation of straight line to (ahn)2 ¼ 0 axis gives thevalue of the band gap of the film material.Fig. 1 represents the reflection spectra of ternary

CdSexTe1�x (x ¼ 0:2; 0.4, 0.6, 0.8) sintered films.In Fig. 2, we plotted a graph between the square ofhn ln [(Rmax�Rmin)/(R�Rmin)] and hn; for thedetermination of optical band gap. The band gapof ternary CdSexTe1�x (x ¼ 0:2; 0.4, 0.6, 0.8)sintered films varies from 1.50 (for x ¼ 0:2)to1.69 eV (for x ¼ 0:8). The composition depen-dence of the band gap shows a discontinuity inlinear variation of band gap is shown in Fig. 3.This discontinuity in band gap is due to phasetransformation after x ¼ 0:4: The ternaryCdSexTe1�x films have zinc blend structure up tox ¼ 0:4 and a wurtzite structure for x40.4. Thediscontinuity in the curve shows the existence oftwo phases of CdSexTe1�x.The mode of optical transitions in these films is

of band-to-band direct type. This has beenconfirmed by plotting ln (ahn) versus ln (hn�Eg)for direct allowed type transitions [28] as shown inFig. 4. The variation yields a straight line with aslope equal to 1

2: As given above, the absorption

coefficient a of the film material is proportional toln½ðRmax � RminÞ=ðR � RminÞ: The absorption coef-ficient was estimated for all the compositions of

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Fig. 3. Variation of energy band gap (Eg) with composition (x)

of ternary CdSexTe1�x (x ¼ 0:2; 0.4, 0.6 and 0.8) sintered films.

Fig. 4. Plot of ln (ahn) versus ln (hn�Eg) of CdSe0.2Te0.8sintered film.

Fig. 2. Energy band gap determination of ternary CdSexTe1�x

(x ¼ 0:2; 0.4, 0.6 and 0.8) sintered films from reflection spectra.

L. Kumar et al. / Physica B 363 (2005) 102–109 105

CdSexTe1�x ternary system. The magnitude of a ishigh and is of the order of 104 cm�1. Theabsorption coefficient increases sharply with

photon energy beyond the fundamental absorp-tion edge. The high absorption coefficient, tailoredand nearly matching band gaps with the visibleregion of the solar spectrum of ternaryCdSexTe1�x sintered films are great advantageswith respect to their applications in photovoltaicdevices.

3.2. XRD studies

Figs. 5–8 shows the X-ray diffraction pattern ofCdSe0.2Te0.8, CdSe0.4Te0.6, CdSe0.6Te0.4 andCdSe0.8Te0.2 sintered films. These traces confirmedthe formation and composition of the alloys ofCdSexTe1�x ternary system. The experimental d-values for the different compositions ofCdSexTe1�x ternary system are calculated usingthe Bragg’s law for each composition and com-pared with the d* (ASTM) values obtained fromVegard’s law for CdSexTe1�x system.According to Vegard’s law, the d* for ternary

system will be the linear function of concentrationsof the constituent crystals for CdSexTe1�x and areexpressed as [29]. For CdSexTe1�x ternary system,

d¼ ðxÞdðCdSeÞ þ ð1� xÞdðCdTeÞ. (3)

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Fig. 5. X-ray diffraction pattern of CdSe0.2Te0.8 sintered film.

Fig. 6. X-ray diffraction pattern of CdSe0.4Te0.6 sintered film.

Fig. 7. X-ray diffraction pattern of CdSe0.6Te0.4 sintered film.

L. Kumar et al. / Physica B 363 (2005) 102–109106

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Fig. 8. X-ray diffraction pattern of CdSe0.8Te0.2 sintered film.

Table 1

X-ray diffraction data of ternary CdSexTe1�x (x ¼ 0.2, 0.4, 0.6

and 0.8) sintered films

Sr.No. 2y (deg) d (A1) Exp. d* (A1) ASTM hkl

CdSe0.2Te0.81 24.0 3.704 3.695 111

2 39.8 2.262 2.262 220

3 47.0 1.932 1.929 311

4 57.6 1.599 1.599 400

5 72.2 1.308 1.307 422

CdSe0.4Te0.61 24.40 3.645 3.649 111

2 40.20 2.241 2.234 220

3 47.60 1.908 1.906 311

4 58.20 1.583 1.579 400

5 64.00 1.453 1.451 331

CdSe0.6Te0.41 24.4 3.645 3.606 002

2 34.0 2.634 2.626 102

3 40.8 2.209 2.209 110

4 49.0 1.857 1.848 201

5 62.0 1.495 1.442 210

6 69.8 1.346 1.274 300

CdSe0.8Te0.21 25.0 3.559 3.558 002

2 34.6 2.590 2.590 102

3 41.2 2.189 2.180 110

4 48.6 1.871 1.866 112

5 50.0 1.822 1.823 201

6 63.0 1.475 1.475 301

7 65.4 1.425 1.426 210

8 75.4 1.259 1.258 300

L. Kumar et al. / Physica B 363 (2005) 102–109 107

The experimental d-values and ASTM d*-valuesare in good agreement (Table 1) and show zincblend structure for CdSe0.2Te0.8 and CdSe0.4Te0.6and wurtzite structure for CdSe0.6Te0.4and CdSe0.8Te0.2 system. The discontinuity inthe linear variation of band gap with compositionis due to phase transformation after x ¼ 0:4:Similar results are reported by Strauss et al.[30] for CdSexTe1�x films prepared by spraytechnique.

3.3. Electrical studies

The current–voltage (I–V) characteristics ofCdSe0.4Te0.6 sintered film with silver contact werestudied using a Keithley SMU-32 setup. Thestandard Schottky barrier theory should explainthe saturation current Js of the general I–Vcharacteristics as a function of temperature (T)[31,32]:

J ¼ JseqV=ðkT�1Þ, (4)

where J is the current density, V is the voltage, q isthe electronic charge and k is the Boltzmannconstant. In the Schottky theory, Js(T) is mainlydetermined by the barrier height fB and theeffective Richardson constant A*

Js ¼ AT2e�qfB=kT . (5)

The barrier height may be written as

fB ¼kT

q

� �ln

AT2

Js

� �. (6)

A plot of ln J versus V gives a straight line. The

saturation current density Js can be obtained by

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Fig. 9. Current–Voltage characteristics of Ag/CdSe0.4Te0.6Schottky junction.

Fig. 10. Forward current density (ln JF) versus forward voltage

(VF) plot of Ag/CdSe0.4Te0.6 junction.

L. Kumar et al. / Physica B 363 (2005) 102–109108

extrapolating the straight line to V ¼ 0: Knowingsaturation current density Js, Richardson constantA* and the temperature T, the barrier height canbe determined. The I–V characteristics of thejunction shown in Fig. 9 and can be analyzed interms of the thermo-ionic emission model ofSchottky barrier.These characteristics indicate a diode behavior

of the hetrojunction and a build up of charges atthe Ag/CdSeTe interface. The I–V characteristicsshow an appreciable increase in the forward biasconditions and a small reverse saturation underreverse bias conditions. The forward I–V char-acteristics are linear in the low-field regionindicating a Schottky behavior. At higher fieldsthe I–V characteristics becomes non-linear. Thenon-linearity of the I–V characteristics over thewide voltage range is associated with the grainboundary effects within the sintered films as thepolycrystalline films are generally characterized bythe presence of moderately large grains, which areoften comparable with the mean free path of thecharge carriers. As the applied voltage increases acertain value, the density of interface trap states atthe grain boundary region decreases, i.e. the traps

in the semiconductor start filling [33]. Thisphenomenon is commonly observed in materialshaving conducting grains in non-conducting ma-trix. Fig. 10 shows a plot between the forwardcurrent density ln JF and forward voltage VF formetal–semiconductor contact Ag/CdSe0.4Te0.6.The extrapolated value of current density JF tozero voltage gives the saturation current density Js.The barrier height of the Ag/CdSe0.4Te0.6 Schottkyjunction is obtained by using Eq. (6) and has thevalue 0.547 eV.

4. Conclusion

The method of preparing thin films ofCdSexTe1�x ternary system by sintering techniqueis cost effective, reasonably accurate and efficient.The optical, structural and electrical studies ofthese films indicate that the films are quite suitablefor photovoltaic device fabrication as the energyband gap of the films match that of solar radiationand films have good absorption in this wavelengthrange. The band gap of the CdSexTe1�x ternarysystem can be tailored from 1.50 eV (for x ¼ 0:2)

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L. Kumar et al. / Physica B 363 (2005) 102–109 109

to 1.69 eV (for x ¼ 0:8) by varying composition(x). All the films of CdSexTe1�x ternary system arefound to be polycrystalline in nature and changethe structure after x ¼ 0:4 from zinc blend towurtzite structure. The presence of a barrier in thehetrojunction device configuration with Ag indi-cates the utility of these films for photovoltaicfabrication.

Acknowledgments

The authors are thankful to the Department ofPhysics, C.C.S. University, Meerut and NationalPhysical Laboratory for providing the technicalsupport to pursue this work.

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