Current Applied Physics 4 (2004) 419–425

www.elsevier.com/locate/cap

Characterization of CdSexTe1�x sintered films

Monika Sharma a, Sushil Kumar b, L.M. Sharma a, T.P. Sharma c, M. Husain b,*

a Department of Chemistry, N.A.S. College, Meerut 250001, Indiab Department of Physics, Jamia Millia Islamia, New Delhi 110025, India

c Department of Physics, C.C.S. University, Meerut 250004, India

Received 7 January 2003; accepted 30 September 2003

Available online 21 November 2003

Abstract

CdSexTe1�x is a promising ternary material which has received considerable attention due to its applications in the fabrication oflarge area economic solar cell, semiconductor-metal Schottky barrier cell, etc. This material possesses various advantages, princi-

pally the high absorption coefficient, optimum band gap and chemical stability, which make it attractive for this kind of devices.

CdSexTe1�x films with variable concentration (x ¼ 0 to 1) have been deposited onto ultra clean glass substrates by screen printingand then sintered. The optical, electrical and structural properties of CdSexTe1�x alloys have been studied, which were foundapplicable in photovoltaics. The optical band gap of these films were determined by reflectance measurements in the wavelength

range of 700–880 nm. The modification of band gap of intermixed CdSexTe1�x system has been described and was found suitable forefficient absorption in the visible region of the spectrum. Schottky barrier height and ideality factor for Al/CdTe and Al/CdSe

junctions were determined by current–voltage characteristics. X-ray diffraction patterns of these films were reported. The films were

of polycrystalline texture over the whole range studied and exhibit predominant cubic zinc blende structure. Sintering is very simple

and viable compared to other costly methods. It is a technique less time-consuming, of maximum material utility and less pollutant

and offers a suitable method for preparing films on large area substrates.

� 2003 Elsevier B.V. All rights reserved.

PACS: 78.20.Cl; 78.50.Ge; 78.66.)w; 78.66.HfKeywords: Sintering; Band gap; Barrier height; Crystal structure; Photovoltaic

1. Introduction

Pseudo-binaries of II–VI, IV–VI and III–V group

compounds are attracting a great deal of attention be-

cause of their potential abilities in a wide spectrum of

optoelectronic devices [1–7]. Polycrystalline semicon-ductor materials have come under increased scrutiny,

because of their potential use in cost reduction of devices

for photovoltaic applications [8–13]. The band struc-

tures, optical properties and crystal structures of both

CdSe and CdTe are very similar and therefore the sys-

tem CdSexTe1�x would not only result in the feasibilityof graded energy gap of a broad spectral sensitivity but

many more material characteristics would be altered andexcellently controlled by the system concentration x. The

* Corresponding author. Fax: +91-11-6830337.

E-mail address: mush_phys@rediffmail.com (M. Husain).

1567-1739/$ - see front matter � 2003 Elsevier B.V. All rights reserved.

doi:10.1016/j.cap.2003.10.002

optical band gap of this ternary alloy CdSexTe1�x can betuned from 1.45 to 1.70 eV by changing the relative

amounts of the chalcogenides in the alloy. CdTe in film

form is a very promising photovoltaic material due to its

near optimum energy gap of 1.45 eV. Currently CdTe-

based solar cells have achieved efficiencies greater than16%, in which CdTe absorber layer is prepared by close

space sublimation process [14]. One of the simplest

techniques for fabricating the polycrystalline films of

AIIBVI compounds is the sintering. Solid solutions of

CdSe–CdTe are one of the important materials for use

in optoelectronic devices [15,16]. These films find appli-

cation in electro-optic devices, photoelectrochemical

solar cells and solar control coatings. Solar control coat-ing refers to selective solar radiation filters applied on

architectural glazings of buildings situated in warm cli-

mates [17,18]. CdSexTe1�x films with x varying from 0 to1 exhibit good solar control properties for use in tropi-

cal climate [19–21]. It has attracted the attention of

420 M. Sharma et al. / Current Applied Physics 4 (2004) 419–425

workers because of wide tunability of band gap as wellas lattice parameter [22,23]. Study has been undertaken

to investigate the photoactive properties of CdSexTe1�xsintered films.

2. Experimental details

There are a variety of methods reported in the liter-

ature for the preparation of CdSe–CdTe pseudo-binary

thin films. Thermal evaporation of pre-reacted samples

in vacuum [24,25], co-evaporation of CdSe and CdTe

powders in required quantities [26,27], three sourceevaporation of the elements [28], electron beam evapo-

ration [29], laser-induced formation [30], flash evapo-

ration [31], chemical methods [32,33], spraying and

sintering suspended mixtures of CdSe and CdTe [34–37],

etc. are commonly used techniques for the synthesis of

CdSexTe1�x films.CdSexTe1�x films have been prepared by screen-

printing followed by sintering [38,39]. For the prepara-tion of the films of CdSexTe1�x (x ¼ 0:00, 0.25, 0.50,0.75, 1.00), all the three compounds (CdSe, CdTe and

CdCl2) were mixed properly according to the concen-

tration required and then added few drops of ethylene

glycol to form the paste. Cadmium chloride acts as an

adhesive and ethylene glycol as binder. The desired

materials were formed as

Weight of CdSe ¼ 191:36 � x gmWeight of CdTe ¼ 240:00 � ð1� xÞ gmWeight of CdCl2 �H2O ¼ 10% weight of

ðCdSeþ CdTeÞAs these weights are quite large, we can reduce them

in the same proportion. The paste thus formed wasscreen printed on glass substrate, which has been

cleaned by soap solution, embry powder, HCl and fi-

nally washed with distilled water. The samples thus

prepared were dried at 120 �C for 4 h in open air andthen heated at 400 �C for 15 min to remove the organicmaterial left. The reason of drying the samples at lower

temperature was to avoid the cracks in the samples.

After this the films were sintered at 550 �C for 10 min.The melting point of CdCl2 is 568 �C. However, theevaporation of CdCl2 starts above 400 �C. Cadmiumchloride is hygroscopic, so to get a stable sintered film,

organic material and cadmium chloride should not re-

main in the sample. Hence to get a stable sintered film,

the sintering temperature was maintained at 550 �C.

Fig. 1. Reflection spectra of CdSexTe1�x sintered films.

3. Characterization of samples and analysis

3.1. Optical characterization

The reflection spectra of CdSexTe1�x sintered filmswere recorded at room temperature in the wavelength

range of 700–880 nm using a Hitachi U3400 UV–VIS–NIR double beam spectrophotometer. The energy band

gaps of these films were determined by reflection spectra.

Almost all the II–VI compounds are direct band gap

semiconductors. According to the Tauc relation, the

absorption coefficient for direct band gap material is

given by [40]

ahm ¼ Aðhm � EgÞ1=2; ð1Þa may be written in terms of reflectance as [41]

2at ¼ ln½ðRmax � RminÞ=ðR� RminÞ�; ð2Þwhere R is the reflection for any intermediate energy

photons (hm), A is a constant which is different for dif-ferent transitions, Eg is the energy band gap and t is thethickness of the film.

When we plot a graph between ðahmÞ2 or the square ofhm ln½ðRmax � RminÞ=ðR� RminÞ� (as ordinate) and hm (asabscissa), a straight line is obtained. The extrapolation

(of straight line) to the energy axis, gives the value ofband gap of film material. Fig. 1 represents the reflection

spectra of CdSexTe1�x sintered films. In Fig. 2, weplotted a graph between the square of hm ln½ðRmax�RminÞ=ðR� RminÞ� and hm for the determination of theband gap. The band gaps of CdSexTe1�x sintered filmsare found to be 1.46, 1.58, 1.64, 1.65 and 1.71 eV for

x ¼ 0:00, 0.25, 0.50, 0.75 and 1.00, respectively, which

Fig. 2. Energy band gap determination of CdSexTe1�x sintered films.

M. Sharma et al. / Current Applied Physics 4 (2004) 419–425 421

are considered to be optimum values for their use in

highly efficient tandem solar cell structure.

Mangalhara et al. [29] have reported the various

optical properties of CdSexTe1�x thin films deposited byelectron bean evaporation. CdSexTe1�x thin films areuseful in many optoelectronic devices and their tunableband gap will be an added advantage in this respect.

3.2. Electrical characterization

The current flows in a Schottky barrier diode because

of charge transport from the semiconductor to the metal

or in the reverse direction. There are four different

transport mechanisms by which the carrier transport canoccur:

(a) thermionic emission of the electrons over the top of

the barrier;

(b) quantum mechanical tunneling through the barrier;

(c) carrier recombination in the depletion region;

(d) carrier recombination in the neutral region (minor-

ity carrier injection).

Process (a) is usually the dominant mechanism in

Schottky barrier junctions and leads to the ideal diode

characteristics. The thermionic emission theory predicts

the current–voltage characteristics given by the follow-

ing equation [42,43]:

J ¼ Js½expðeV =nkT Þ � 1� ð3Þand the saturation reverse current density Js is given by

Js ¼ AT 2 expð�e/B=kT Þ ð4Þ

where n is the ideality factor for Schottky barrier, A the

effective Richardson constant at 300 K and /B is theSchottky barrier height. The barrier height and idealityfactor of the junction are given by

/B ¼ ðkT=eÞ lnðAT 2=JsÞ; ð5Þ

n ¼ ðe=kT Þ½oV =oðln JÞ�: ð6Þ

For an ideal Schottky barrier, where the barrier height is

independent of the bias and current flows only due to

thermionic emission, n ¼ 1. Factors which make n largerthan unity are the field (bias) dependence of barrier

height, electron tunneling through the barrier and the

carrier recombination within the depletion region.

To fabricate Al/CdTe and Al/CdSe junctions, alu-

minium layer was thermally evaporated over the films of

CdTe and CdSe which were already deposited on the

conducting glass. The size of aluminium layer was

smaller than that of films. The sample was mounted in aspecially designed sample holder where a vacuum of

10�3 Torr could be maintained throughout the experi-ment. There was a provision of an optically plane glass

window through which light may incident on the sam-

ple. For measurements, two electrical contacts, one from

aluminium and other from conducting glass, were taken.

A d.c. voltage (0–20 V) was applied across the sample

and the resulting current was measured by a digitalelectrometer (Keithley, model 617). Measurements were

made at room temperature.

The dark and illuminated current–voltage character-

istics for Al/CdTe and Al/CdSe junctions are shown in

Figs. 3 and 4. When Al/CdTe and Al/CdSe junctions

were illuminated with a tungsten bulb of 100 W (the

radiant power incident on the sample was 127 Candela

approximately), the I–V characteristics showed an appre-ciable increase in the forward current.Under reverse

bias conditions, a small reverse current was observed.

There was a very minute increase in reverse current

under illumination.

When the radiation falls on the metal side of the

Schottky junction, positive and negative charges (elec-

tron–hole pairs) within the depletion region are created.

These electron–hole pairs are separated by the electricfield in the junction present due to the dissimilar work

functions and Fermi levels. These electrons and holes

give rise to photovoltage and hence photocurrent.

The non-linearity of the I–V characteristics over thewide voltage range may be associated with the grain

boundary effects within the sintered films. The poly-

crystalline films are generally characterized by the

Fig. 3. (a) Current–voltage characteristics of CdTe film with Al contact and (b) graph plotted between ln Jf and Vf for CdTe film with Al contact.

422 M. Sharma et al. / Current Applied Physics 4 (2004) 419–425

presence of moderately large grains which are oftencomparable with the mean free path of the charged

carriers. As the applied voltage increases upto a certain

value, the density of interface trap states at the grain

boundary region decreases, i.e., the traps in the semi-

conductor starts to be filled [44]. The non-linearity in the

dark as well as illuminated I–V characteristics may alsobe due to the barrier layer formed between Al contact

and the films of CdTe or CdSe.We calculated the Schottky barrier heights and the

ideality factors of Al/CdTe and Al/CdSe junctions using

Eqs. (5) and (6) and taking the value of ln Js and½oðln JÞ=oV � from Figs. 3 and 4, respectively. The barrierheights ð/BÞ of Al/CdTe and Al/CdSe Schottky junc-tions are found to be 0.61 and 0.47 V, respectively. The

ideality factors (n) of Al/CdTe and Al/CdSe Schottkyjunctions are found to be 1.17 and 1.21, respectively.Belyaev et al. [27,45] performed detailed studies on

the electrical properties of CdSexTe1�x pseudo-binarythin films. The value of conductivity depends on con-

centration x, i.e., the amount of CdSe in the film. Anincrease in CdSe percentage leads to an increase in film

conductivity. The type of conductivity is also a functionof concentration x.

3.3. Structural characterization

X-ray diffractograms of CdSexTe1�x sintered filmswere obtained using a Philips X-ray diffractometer PW

1140/09 with CuKa radiation ðk ¼ 1:5405 �A) with ascanning angle (2h) from 20� to 65�. X-ray diffractionpatterns of CdSe, CdTe and CdSe0:5Te0:5 are shown in

Fig. 5. XRD patterns of other samples of CdSexTe1�xhave almost similar trends and hence not shown here.

XRD traces confirmed the formation of the alloys of

CdSexTe1�x. XRD patterns showed no peaks of CdTe

or CdSe oxidation. X-ray diffractograms of CdSexTe1�xfilms revealed that the films are polycrystalline in nature.

The d-values for CdSe0:5Te0:5 were calculated fromthe Bragg’s relation 2d sin h ¼ nk (here n ¼ 1, k ¼1:5405 �A) by taking the values of h corresponding to thepeaks of XRD pattern. These calculated d values werecompared with the d values obtained from Vegard’s

law for CdSe0:5Te0:5. According to the Vegard’s law, the

Fig. 4. (a) Current–voltage characteristics of CdSe film with Al contact and (b) graph plotted between ln Jf and Vf for CdSe film with Al contact.

M. Sharma et al. / Current Applied Physics 4 (2004) 419–425 423

value of lattice parameters of mixed crystals obtained by

X-ray data, are a linear function of concentration of the

constituent crystals and are expressed as

r ¼ f1r1 þ f2r2

where r1 and r2 are the lattice parameters of the con-stituent crystals and r that of the mixed crystal. f1 and f2are the mole fractions of the two components in the

mixed crystal. For CdSe0:5Te0:5; we have f1 ¼ f2 ¼ 0:5,hence d ¼ 0:5 � dðCdSeÞ þ 0:5 � dðCdTeÞ. The d valuesof CdSe and CdTe are taken from ASTM data.

The d and d values are in well agreement (Table 1)and show the cubic zinc blende structure of CdSe0:5Te0:5.

The same is true for other samples.Uthana and Reddy [24] and Mangalhara et al. [29]

studied CdSe–CdTe ternary solid solutions in thin film

form and observed the presence of only the cubic phase

over the entire range of concentration x of CdSexTe1�xsystem. However, there are a number of investigations

reporting that these films do not consist of single phase

over the entire range of concentration [27,30,36,37,46].These investigations revealed the presence of cubic or

hexagonal or mixed phase depending on the concen-

tration x of the CdSexTe1�x system.

4. Conclusion

Sintering is an extremely simple and viable techniquecompared to other costly methods. The optical band gap

ðEgÞ of the films of CdSexTe1�x varies from 1.46 to 1.71eV as x varies from 0 to 1, which is suitable for efficientabsorption in the visible region of the solar spectrum.

Fig. 5. X-ray diffraction patterns of CdSe, CdTe and CdSe0:5Te0:5 films

sintered at 550 �C for 10 min.

Table 1

Structural parameters of CdSe0:5Te0:5 sintered film

2h (degree) sin h d ¼ k=2 sin h(�A)

d (�A) Plane

(hkl)

24.50 0.2122 3.6298 3.6300 0 0 2

33.85 0.2911 2.6460 2.6445 1 0 2

40.42 0.3455 2.2290 2.2230 1 1 0

49.05 0.4151 1.8556 1.8595 2 0 1

62.40 0.5180 1.4870 1.4800 2 1 0

d values calculated from XRD pattern.

d values obtained from Vegard’s law.Sample: CdSe0:5Te0:5, sintering temperature¼ 550 �C.Wavelength: 1.5450 �A, sintering time¼ 10 min.

424 M. Sharma et al. / Current Applied Physics 4 (2004) 419–425

The barrier heights ð/BÞ of Al/CdTe and Al/CdSe

Schottky junctions are found to be 0.61 and 0.47 V,

respectively. The ideality factors ðnÞ of Al/CdTe and Al/CdSe Schottky junctions are found to be 1.17 and 1.21,respectively. CdSexTe1�x films prepared by sintering

technique are found to be polycrystalline in nature and

having cubic zinc blende structure.

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