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Optics & Laser Technology 42 (2010) 1181–1186

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Optics & Laser Technology

0030-39

doi:10.1

n Corr

E-m1 Pe

UP 2730

journal homepage: www.elsevier.com/locate/optlastec

Synthesis and optical characterization of nanocrystalline CdTe thin films

A.A. Al-Ghamdi a,n, Shamshad A. Khan a,1, A. Nagat b, M.S. Abd El-Sadek c

a Department of Physics, Faculty of Science, King Abdul Aziz University, Jeddah, Saudi Arabiab Department of Physics, Faculty of Girls Education, King Abdul Aziz University, Jeddah, Saudi Arabiac Department of Physics, Faculty of Science, South Valley University, Qena, Egypt

a r t i c l e i n f o

Article history:

Received 23 January 2010

Received in revised form

11 March 2010

Accepted 23 March 2010Available online 15 April 2010

Keywords:

CdTe thin films

Optical band gap

Absorption coefficient

92/$ - see front matter & 2010 Elsevier Ltd. A

016/j.optlastec.2010.03.007

esponding author. Tel.: +966 02 6952286; fa

ail address: aghamdi90@hotmail.com (A.A. Al

rmanent address: Department of Physics, St. A

01, India.

a b s t r a c t

From several years the study of binary compounds has been intensified in order to find new materials

for solar photocells. The development of thin film solar cells is an active area of research at this time.

Much attention has been paid to the development of low cost, high efficiency thin film solar cells. CdTe

is one of the suitable candidates for the production of thin film solar cells due to its ideal band gap, high

absorption coefficient. The present work deals with thickness dependent study of CdTe thin films.

Nanocrystalline CdTe bulk powder was synthesized by wet chemical route at pHE11.2 using cadmium

chloride and potassium telluride as starting materials. The product sample was characterized by

transmission electron microscope, X-ray diffraction and scanning electron microscope. The structural

characteristics studied by X-ray diffraction showed that the films are polycrystalline in nature. CdTe

thin films with thickness 40, 60, 80 and 100 nm were prepared on glass substrates by using thermal

evaporation onto glass substrate under a vacuum of 10�6 Torr. The optical constants (absorption

coefficient, optical band gap, refractive index, extinction coefficient, real and imaginary part of dielectric

constant) of CdTe thin films was studied as a function of photon energy in the wavelength region 400–

2000 nm. Analysis of the optical absorption data shows that the rule of direct transitions predominates.

It has been found that the absorption coefficient, refractive index (n) and extinction coefficient (k)

decreases while the values of optical band gap increase with an increase in thickness from 40 to

100 nm, which can be explained qualitatively by a thickness dependence of the grain size through

decrease in grain boundary barrier height with grain size.

& 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Polycrystalline thin films of CdTe continue to be a leadingmaterial for the development of cost effective and reliable photo-voltaics. The growth and crystallization of CdTe nanocrystalline thinfilms are important because of their potential applicationsin semiconducting devices, photovoltaics, optoelectronic devices,radiation detectors, laser materials, thermoelectric devices, solarenergy converters, solar cells, videocon devices, sensors andnano-devices [1–7]. Although the research is progressing well,achieving conversion efficiencies of 19.2% [8] for CIGS-based devices,the complexity and the lack of understanding of underpinning solidstate physics principles hindered rapid progress. Recent reports[9] on CdS/CdTe solar cell structures are good examples ofthis complexity, and the proposed new model is controversial,disagreeing with the model that has been accepted for the past

ll rights reserved.

x: +966 02 6951106.

-Ghamdi).

ndrew’s College, Gorakhpur,

two decades for this device. Polycrystalline CdTe materials areconsidered very suitable for the fabrication of solar cells because oftheir direct band gap. As a consequence of the direct energy gap,the absorption edge is very sharp and thus, more than 90% of theincident light is absorbed in a few micrometers of the material.The maximum photocurrent available from a CdTe cell underthe standard global spectrum normalized to 100 mW/cm2 is30.5 mA/cm2 and the theoretical maximum efficiency of CdTe isover 27%. Recently an energy conversion efficiency record for CdTeof 16.5% has been reported [10]. This record, despite its achievementon a laboratory scale, demonstrates that CdTe thin film technologyhas arrived at a level comparable with the more sophisticatedtechnologies typical of single crystal materials. One of the bestcharacteristics of this semiconductor is that it is possible to fabricatea complete photovoltaic device using only thin film technology.Most surprisingly CdTe solar cells fabricated using thin filmtechnology exhibit higher efficiencies than those fabricated fromsingle crystal materials. In fact, solar cells with an efficiency around10% or higher have been made as heterojunctions, homojunctions,buried homojunctions and MIS junctions, using CdTe single crystal.The higher 13.4% efficiency concerns an n-ITO/p-CdTe single crystalburied homojunction [11]. All photovoltaic devices involving CdTe

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A.A. Al-Ghamdi et al. / Optics & Laser Technology 42 (2010) 1181–11861182

as an absorber material contain a highly transparent andn-conducting partner, which promotes the creation of a depletedregion in the p-conducting CdTe film. Since any mature solar celltechnology tends to the stage where costs are determined by thoseof constituent materials, this means that highly efficient processingoperations that produce solar cells with high-energy conversionefficiency are favored. Despite the good performance and efficiency,the preparation of these thin film solar cells based on CdTe/CdSheterojunction still exhibits quite a few open problems and it istherefore subject to a margin of uncertainty in its progress. One ofthe major open questions is certainly the back contact, which iscrucial for the time stability of the solar cell. A big challenge for thinfilm photovoltaics is the development of large area semiconductortechnology. In fact, one of the advantages of the thin film technologyis the potential increase in the manufacturing unit from a siliconwafer (E100–200 cm2) to a glass sheet (E104 cm2) that is about50–100 times larger. In order to achieve this goal, high qualitymaterials and high throughput on large areas have to be obtained. InCdTe cells, much research has been devoted to two features,particular to this type of cell: the first is the treatment of the CdTeabsorber in chlorine containing environment, usually referred to asthe CdCl2 treatment or the activation treatment [12] and the secondis the technology of the back contact [13].

Cadmium telluride thin films can be fabricated by a variety ofmethods, such as vacuum deposition, metal organic chemicalvapor deposition, liquid phase deposition, molecular beamepitaxy, etc. Among these methods vacuum deposition is anattractive method which has successfully been employed for thepreparation of binary, inter metallic and ternary compounds. CdTethin films would exhibit unusual charge carrier dynamics,improved collection of the photo-generated carriers and theenhanced solar conversion efficiency. It is because, first, due tomultiple reflections, the effective optical path for absorption ismuch larger than the actual film thickness. Second, lightgenerated electron and holes need to travel over a much shorterpath and thus recombination losses are greatly reduced. As aresult, the absorber layer thickness in nanostructured solar cellscan be as thin as 150 nm instead of several micrometers in thetraditional thin film solar cells [14]. Third, the energy band gap ofvarious layers can be tailored to the desired design value byvarying the size of nanoparticles. This allows for more designflexibility in the absorber and window layers in the solar cells[15]. Several researchers [16–22] have carried out research onpreparation and characterization CdTe thin films for photovoltaicapplication. In recent years, effects have been made to preparenanocrystalline CdTe materials for various applications. Still thenanoparticles of polycrystalline CdTe materials are not wellstudied and only few papers are published [23–27], therefore,there is a lot of scope for the studying these materials innanometric scale. It is also understood that the reduction in size(nanoparticles) will change the properties of these materialsdramatically. The aim of the present research work is toinvestigate the effect of thickness on optical properties ofnanocrystalline CdTe thin films.

0-20

0

20

40

60

(4 2

0)

(3 3

1)

(2 2

2)

(4 0

0)

(3 1

1)

(2 2

(1 1

1)

Int

2θ (Degrees)20 40 60 80 100 120

Fig. 1. X-ray diffractrogram of CdTe powder.

2. Experimental

Nanocrystalline CdTe powder was synthesized by wetchemical route at pHE11.2 using cadmium chloride andpotassium telluride as starting materials. The reaction was carriedout by the refluxing the mixture of starting materials at 90 1C for5 h under stirring. The aqueous solution of CdTe nanocrystallinewas formed at this stage. The particles were extracted byparticipation with the addition of 2-propanol to the solution.The resulting nanopowder of CdTe was separated by centrifuging

and then dried at room temperature. Thin films with thickness 40,60, 80 and 100 nm were prepared by using an Edward CoatingUnit E-306, onto glass substrates at room temperature on a basepressure of 10�6 Torr using a molybdenum boat. The substrateswere thoroughly cleaned in a detergent solution and then inchromic acid and finally, cleaned using trichloroethylene. Doubledistilled water was used throughout in different stagesof cleaning. To avoid the fractionation of the alloy duringevaporation and, thereby, to ensure the correct average composi-tion of the films formed, a high deposition rate was used toprepare the studied films. The thickness of the films wasmeasured by using a quartz crystal monitor (Edward modelFTM 7). The earthed face of the crystal monitor was facing thesource and was placed at the same height as the substrate. Theevaporation was controlled by using the same FTM 7 quartzcrystal monitor. A JASCO, V-500, UV/VIS/NIR computerizedspectrophotometer is used for measuring optical absorption andreflection has been changed to optical absorptance andreflectance. The optical absorption was measured as a functionof wavelength of the incidence photon energy.

3. Results and discussion

3.1. Structural studies

The X-ray diffraction techniques are employed for studyingstructural details of the materials. The phenomenon of X-raydiffraction can be pictured as a reflection of the incident beamfrom the lattice plane. The X-ray diffraction patterns ofnanocrystalline CdTe powder was performed by using X-raydiffractometer (Philps Model- PW 1710). Copper target was usedas source of X-rays and l¼1.5406 A (CuKa1). The scanning anglewas in the range of 10–1001. A scan speed of 21/min and a chartspeed of 1 cm/min were maintained. The X-ray diffraction tracesof nanocrystalline CdTe powder were taken at room temperatureand are shown in Fig. 1. The presence of sharp structural peaksconfirms the polycrystalline nature of the sample.

The average particle size of CdTe powder was calculated byusing Scherrer’s equation

Dffið0:9lÞ=ðbcosyÞ ð1Þ

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where b is the full width at half maximum intensity of the peak inradian, l is the X-ray wavelength and y is Bragg angle.

The diffraction pattern shown in Fig. 1 gives an averageparticle size of 76 nm.

A transmission electron microscope (JEOL, JEM-1011, Japan) wasused for morphological characterization of CdTe powder, shown inFig. 2. For TEM observations, we have dispersed CdTe powder in de-ionized water. This solution is kept in ultrasonic bath for 2 h forgetting the well dispersed nanostructures. This solution is finallydispersed on a holy carbon grid for TEM observations. It is clear fromthe TEM measurements that the nanoparticles with typical diameterof about 50–80 nm are present in CdTe powder, which is consistentwith the XRD analysis. The surface microstructure of CdTe powderwas examined by means of JEOL JSM-6360LV, Japan, scanningelectron microscope (SEM), shown in Fig. 3.

3.2. Optical studies

A Jasco spectrophotometer is used for measuring opticalabsorption of the thin films. In fact the ‘‘absorbance’’ reading (i.e.photometric value) is a measure of the amount of light absorbed bythe sample under specified conditions. Optical behavior of materialis generally utilized to determine its optical constants, i.e. refractiveindex (n), extinction coefficient (k), real part (e0) and imaginary part

Fig. 2. Transmission electron microscope (TEM) measurement of CdTe powder.

Fig. 3. Scanning electron microscope (SEM) measurement of CdTe powder.

(e00) of the dielectric constant. The absorption has been measured interms of optical density. Figs. 4 and 5 show the variation ofabsorbance and reflectance (R%) against wavelength.

The absorption coefficient (a) has been obtained directlyfrom the absorbance against wavelength curves using the relation[28–30]

a¼OD=t ð2Þ

where OD is the optical density measured for the given layerthickness (t).

The variation of the absorption coefficient (a) as a function ofincident photon energy (hn) for nanocrystalline CdTe thin films atdifferent thickness are shown in Fig. 6 and the values of a atdifferent thickness are given in Table 1. It has been observed thatthe value of absorption coefficient increases with the increase inphoton energy. The values of the absorption coefficient fornanocrystalline CdTe thin films are in the range �104 cm�1,which is consistent with the other workers [31,32]. In theabsorption process, a photon of known energy excites an electronfrom a lower to a higher energy state, corresponding to anabsorption edge. In CdTe, a typical absorption edge can be broadlyascribed to one of the three processes, first residual below-gapabsorption, second Urbach tails and third interband absorption.CdTe thin films have been found to exhibit highly reproducibleoptical edges which are relatively insensitive to preparationconditions and only the observable absorption [33] with a gapunder equilibrium conditions account for the first process. In thesecond process the absorption edge depends exponentially on the

0

0.2

0.4

0.6

0.8

1

1.2

1.4

300Wavelength (nm)

Abs

orba

nce

40 nm60 nm80 nm100 nm

600 900 1200 1500 1800 2100

Fig. 4. Absorbance against wavelength in CdTe thin films at different thickness.

0

10

20

30

40

50

60

70

400Wavelength (nm)

Ref

lect

ance

(R %

)

40 nm 60 nm80 nm100 nm

600 800 1000 1200 1400 1600 1800 2000

Fig. 5. Reflectance (R%) against wavelength in CdTe thin films at different

thickness.

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photon energy according to the Urbach relation [34]. In CdTe thinfilms, a increases exponentially with the photon energy near theenergy gap. This type of behavior has also been observed in manyother works [31]. It is observed from Table 1 that absorptioncoefficient decreases with increase in the thickness of films.

The fundamental absorption edge in most semiconductor thinfilms follows an exponential law. Above the exponential tail, theabsorption coefficient has been reported to obey the followingequation [28–30]:

ðahvÞ1=n¼ Bðhv�EgÞ ð3Þ

where v is the frequency of the incident beam (o¼2pn), B is aconstant, Eg is optical band gap and n is an exponent, which can beassumed to have values of 1/2, 3/2, 2 and 3 depending on thenature of electronic transition responsible for the absorption:n¼1/2 for allowed direct transition, n¼3/2 for forbidden directtransition, n¼2 for allowed indirect transition and n¼3 forforbidden indirect transition. The electronic transition betweenthe valence and conduction bands starts at the absorption edgecorresponding to the minimum energy difference between thelowest energy of the conduction band and the highest energy ofthe valence band in crystalline materials. In the case when theextremum lie at the same point of k space the transitions arecalled direct, otherwise the transitions are possible only when theassisted and labeled phonons are called indirect [35].

0.5

0

5

10

15

20

25

30

abso

rptio

n co

effic

ient

(α) (

104 )

(cm

-1)

Energy (eV)

40 nm60 nm80 nm100 nm

1.0 1.5 2.0 2.5 3.0 3.5

Fig. 6. Absorption coefficient (a) against photon energy in CdTe thin films at

different thickness.

Table 1

(a) Optical band gap, absorption coefficient (a) and refractive index (n) in CdTe thin film

Thickness (nm) Absorption coefficient (a) (104) (cm�1) Optical band gap (Eg) (eV)

40 18.08 1.22

60 6.14 1.57

80 2.93 2.07

100 1.81 2.19

(b) Extinction coefficient (k), reflectance (R %), real part of dielectric constant (e0) and ima

wavelength¼980 nm

Thickness (nm) Extinction coefficient (k) R (%)

40 1.411 57.22

60 0.479 45.77

80 0.229 26.51

100 0.141 30.91

The present system of CdTe thin films obeys the role of directtransition and the relation between the optical gap, opticalabsorption coefficient (a) and the energy (hn) of the incidentphoton is given by

ðahvÞ2pðhv�EgÞ ð4Þ

The variation curve of (ahn)2 with photon energy (hn) for CdTethin films at different thickness are shown in Fig. 7(a) and (b). Thevalue of direct optical band gap (Eg) has been calculated by takingthe intercept with x-axis and the value of Eg for CdTe thin films atdifferent thickness is given in Table 1. It is evident from this tablethat the value of optical band gap (Eg) increases with increase inthickness from 40 to 100 nm. The increase in the optical band gapwith increase in the thickness may be due to the increase in grainsize, decrease in density of defect states (which results in thereduction of tailing of bands) and due to the shift in Fermi level byincrease in the thickness. The optical band gap increases with theincrease in the film thickness because the crystallinity of the filmincreases also due to the increase in crystallite size. In general,thickness dependence of energy gap can arise due to one orcombined effect of the following causes: (a) change in barrierheight due to the change in grain size in polycrystalline film; (b)large density of dislocations and (c) quantum size effect. However,the first one looks reasonable cause in the present case with smallcontributions from dislocation density as well.

The obtained values of Eg show that CdTe thin films areinfluenced by thickness. The gap increases from 1.22 to 2.19 eV,when the thickness increases from 40 to 100 nm. These resultsindicate that a tendency to a certain expansion in the gap against thethickness. The increase in energy band gap may be attributed todecrease in particle size and an increase in strain and dislocationdensity. This can be further explained from three-dimensionalquantum size effect, leading to an increase of band gap with increaseof particle size, which is well known for colloidal semiconductor solswhere the individual colloidal particles are dispersed in a liquid orglass [36]. The increase in optical band gap with increase in filmthickness has also been observed by other workers [37].

The theory of reflectivity of light has been used to calculate thevalues of refractive index (n) and extinction coefficient (k). Thevalues of n and k have been calculated by using the followingequations [28–30,38]:

k¼ ðalÞ=ð4pÞ ð5Þ

and

n¼ ð1þRÞþfð1þRÞ2-ð1�RÞ2ð1þk2Þg1=2

h i=ð1�RÞ ð6Þ

where l is the wavelength.

s with different thicknesses at wavelength¼980 nm

Refractive index (n)

6.92

5.14

4.04

3.49

ginary part of dielectric constant (e00) in CdTe thin films with different thicknesses at

Real part of dielectric constant (e0) Imaginary part of dielectric constant (e00)

19.52 45.88

4.92 26.16

1.85 12.27

0.99 12.22

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0

2000

4000

6000

8000

10000

40 nm60 nm

(α h

ν)2

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(cm

-1 e

V)2

Energy (eV)

0

200

400

600

800

1000

1200

Energy (eV)

(α h

ν)2 (1

08 ) (c

m-1

eV

)2 80 nm100 nm

0.8 1.2 1.6 2.0 2.4 2.8 3.2

0.4 0.8 1.2 1.6 2.0 2.4 2.8 3.2

Fig. 7. (a) and (b): (ahn)1/2 against photon energy (hn) in CdTe thin films at

different thickness.

0.51

2

3

4

5

6

7

8

9

10

11

Ref

ract

ive

inde

x (n

)

Energy (eV)

40 nm60 nm80 nm100 nm

1.0 1.5 2.0 2.5 3.0 3.5

Fig. 8. Variation of refractive index (n) with incident photon energy (hn) in CdTe

thin films at different thickness.

0.50.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Ext

inct

ion

coef

ficie

nt (k

)

Energy (eV)

40 nm60 nm80 nm100 nm

1.0 1.5 2.0 2.5 3.0 3.5

Fig. 9. Variation of extinction coefficient (k) with incident photon energy (hn) in

CdTe thin films at different thickness.

0.50

10

20

30

40

50

60

70

80

Rea

l par

t of d

iele

ctric

con

stan

t (ε')

Energy (eV)

40 nm60 nm80 nm100 nm

1.0 1.5 2.0 2.5 3.0 3.5

Fig. 10. Variation of real part of dielectric constant (e0) with incident photon

energy (hn) in CdTe thin films at different thickness.

A.A. Al-Ghamdi et al. / Optics & Laser Technology 42 (2010) 1181–1186 1185

The calculated values of n and k for CdTe thin films at differentthickness are shown in Figs. 8 and 9 and the values of n and k atdifferent thickness are given in Table 1. It is clear from this tablethat n and k both decrease with the increase in thickness. We havecompared the thickness dependence of refractive index for fourdifferent thicknesses. The physical interpretation of the variationof refractive index can be ascribed to the variation of both densityand electronic structure. This indicates the dominance of densityeffect in the thickness dependence of refractive index.

For further analysis of the optical data a number of useful,associated relations can be derived to link the real and imaginaryparts of the dielectric function and the optical constants (n, k). Thefollowing relations have been used to calculate the values of thereal part (e0) and imaginary part (e00) of the dielectric constant forCdTe films [28]

e0 ¼ n2�k2 and e00 ¼ 2nk ð7Þ

The values of real part (e0) and imaginary part (e00) of thedielectric constants of CdTe thin films at different thickness areshown as a function of photon energy in Figs. 10 and 11. Thevalues of these two parameters with fixed photon energy aregiven in Table 1. It can be seen that both, the real part andimaginary part of dielectric constant increases with increase inphoton energy at all thickness. In III–V semiconductors, chemicaldisorders produce large change in potential through theCoulombian interaction because of the large ionic contributionto the bonding. In CdTe, the bonding is basically covalent and

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4

8

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16

20

24

Imag

inar

y pa

rt of

die

lect

ric c

onst

ant (

ε'')

Energy (eV)

40 nm60 nm80 nm100 nm

1.0 1.5 2.0 2.5 3.0 3.5

Fig. 11. Variation of imaginary part of dielectric constant (e00) with incident

photon energy (hn) in CdTe thin films at different thickness.

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chemical disorder leads only to small change in the localpotential.

4. Conclusion

CdTe thin films with different thickness have been depositedon glass substrate at room temperature by vacuum evaporationtechnique. From XRD studies it was found that CdTe ispolycrystalline in nature with the CdTe crystallites. The opticalconstants of CdTe thin films are found to be dependent on the filmthickness. The optical absorption measurements indicate that theabsorption occurs due to direct transition. The optical band gapincreases from 1.22 to 2.19 eV with increase in film thicknessfrom 40 to 100 nm. This may be due to the different effects, butthe most relevant are grain size, lattice strain or defect states. Therefractive index and extinction coefficient decrease with theincrease in film thickness, while increases with the increase inwavelength, which may be attributed to the decrease of crystallitesize. The high refractive index and low extinction coefficient makethese films suitable for antireflection multilayers.

Acknowledgement

Thanks are due to Deanship of Scientific Research, King AbdulAziz University, Jeddah, Saudi Arabia (Ref. no. 429/071-3), forproviding financial assistance in the form of research project.

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