optical & electrical characterization of chemical bath deposited cd-pb-s thin films

7/28/2019 Optical & Electrical Characterization of Chemical Bath Deposited CD-Pb-S Thin Films

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Optical and electrical characterization of chemical bath deposited Cd–Pb–S thin films

M.A. Barote a,⁎, S.S. Kamble b, A.A. Yadav c, E.U. Masumdar c

a Department of Physics, Azad college, Ausa, (M.S.), 413520, Indiab Bharat Ratna Indira Gandhi College of Engineering, Kegaon, Solapur, (M.S.), 413255, Indiac Thin Film Research Laboratory, Department of Physics, Rajarshi Shahu Mahavidyalaya, Latur, (M.S.), 413512, India

a b s t r a c ta r t i c l e i n f o

Article history:

Received 18 May 2011

Received in revised form 10 November 2012Accepted 13 November 2012Available online 28 November 2012

Keywords:

Thin filmsChemical bath depositionOptical propertiesElectrical conductivityCadmium lead sulfide

CdzPbyS thin films have been deposited on glass substrates using inexpensive chemical bath deposition tech-nique. The aqueous solution containing precursors of Cd2+ and Pb2+ has been used to obtain good qualitydeposits at optimized preparative parameters. The thin film samples have been characterized through opticalabsorption, electrical conductivity and thermoelectric power measurement techniques. From optical studies,the absorption coef ficient ‘α,’ is found to be of the order of 10 4 cm−1. The optical absorption studies revealeddirect band to band transition. The band gap energy is found to vary nonlinearly from 2.47 eV (CdS) to0.49 eV (PbS) as the composition parameter ‘x’ was increased from 0 to 1. The electrical characterization re-vealed increased conductivity (σ ) with the increased composition parameter up to x=0.175. The electricalconductivity measurements indicate two types of conduction mechanism, namely grain boundary scatteringlimited and variable range hopping conductions. The activation energies of the films of different composi-tions were determined at low and high temperature regions. The activation energies were observed to bein the range of 0.168–0.240 eV and 0.514–0.711 eV respectively. Thermoelectric power measurementshighlighted n-type behavior of the as-grown thin film samples.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

The ternary derivative materials have boosted a lot of researchinterest in the field of optoelectronic devices due to the potential of tailoring both the lattice parameters and the band gap by controllingdeposition conditions [1,2]. One such prominent ternary candidate iscadmium lead sulfide having wide band gap for photoconducting andphotovoltaic devices, IR detectors, photodiodes and laser applications[3]. Thin films of cadmium and lead sulfide are promising photovolta-ic materials as their variable band gap can be engineered to match theideal band gap (≈1.5 eV) required for most ef ficient solar cell [4].These materials are usedin optoelectronicdevices, solar control coatings,gas and humidity sensors and photoelectrochemical solar cells [4–6].

Skyllas-Kazacus et al. [7] are the one who gave detailed data onhigh cadmium mole fraction in solution. Roger and Crocker carriedout an extensive study of the electrical properties of Pb1−xCdxTeusing bulk material [8]. Harman and co-workers reported that, thebulk crystals of Pb1−xCdxS could be prepared beyond the stablephase boundary limits by quenching [9].

Thin metal chalcogenide films can be obtained by chemical bathdeposition (CBD) method [10,11], spray pyrolysis [12], and physicaland electrochemical techniques [13,14]. Many researchers havedeposited ternary derivative materials in thin film form Cd1−xZnxS

[15], PbS–

CuxS [6], Bi2S3–

CuxS [16], Cd1−

xCuxS [17] and Bi2Se3–

Sb2Se3[18], using simple and inexpensive chemical bath deposition. The over-riding advantage of CBD over the other methods is that films can bedeposited on different kinds of substrates with variable size and shape[19]. The present study deals with the optical, electrical and thermo-electric analysis of cadmium lead sulfide ternary compounds which isessential imply their active properties in various optoelectronic devices.

2. Experimental details

CdzPbyS thin films with the nominal composition Cd1−xPbxS(0≤x≤1) were deposited onto glass substrates by chemical bath de-position method reported earlier [20]. For the deposition, solutions of cadmium sulfate, lead sulfate and thiourea (all A. R. grade) weremixed in stoichiometric proportion to obtain ‘x’ value from 0 to 1.Triethanolamine was used as complexing agent and pH of the reac-tion mixture was adjusted to 10.5±0.1. The ultrasonically cleanedglass substrates were mounted on a specially designed substrateholder and were rotated with a constant speed in the reactionmixture. To obtain good quality thin films deposition time, tempera-ture and speed of substrate rotation were optimized. These optimizedparameters were 60 min, 80 °C and 65 rpm respectively.

The thickness of as-deposited thin films was determined usinggravimetric weight difference method with sensitive microbalance.The X-ray diffractograms were obtained for these samples withPhilips-PW 1710 X-ray diffractogram using CuKα line (λ=1.5406 Å)within the 2θ range from 20° to 80°. The microscopic features were

Thin Solid Films 526 (2012) 97–102

⁎ Corresponding author. Tel.: +91 9422658959.E-mail address: [email protected] (M.A. Barote).

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observed through a scanning electron microscope JEOL-JSM-5600(Japan) with operating voltage 25 kV. The optical absorption spectrawere recorded at room temperature using UV –VIS-IR spectrophotome-ter (Carry-5000Japan). The absorption coef ficient, energy band gap andmode of transition were determined. A two probe press contact methodwas employed to measure electrical conductivity and thermo-emf of the as-obtained samples in the temperature range of 300–500 K.

3. Results and discussion

The X-ray diffraction (XRD) pattern for few representative sam-ples is shown in Fig. 1. These diffractograms reveal the polycrystallinenature of as-deposited thin films, irrespective of the composition pa-rameter ‘x’ over entire range (0≤x≤1). It is observed that, the CdS(x=0) exhibit cubic and hexagonal crystal structure. It is reportedthat chemically deposited CdSfilms depending upon preparative con-ditions, show cubic, hexagonal or mixed (cubic+hexagonal) crystalstructures [21]. For Cd1.35S prepared without Pb, the prominentpeaks corresponding to (111), (200), (220) and (311) planes of thematerial with cubic phases, while peaks corresponding to (002),(111), and (110) planes are of CdS hexagonal structure. For Pb1.56Sprepared without Cd, the preferential orientation is along (111),(200), (220), (311), (222), (420) and (422) planes. The XRD patternfairly matches with the peak positions (2θ) of standard X-ray powderdiffraction data (JCPDS) file [22]. No shift in peak position withincreased Pb was observed, which indicates that, the Cd1−xPbxS(0≤x≤1) thin films are of composite type. The average latticeparameters ‘a’ and ‘c’ for hexagonal phase were found to have anon-linear variation with the composition (a=4.1035 Å to 4.1720 Åand c=6.6061 Å to 6.7288 Å). The similar trend was observed incase of cubic phase (a=5.8015 Å to 5.9171 Å). The determined aver-age grain size was found to lie in between 7 and 17 nm.

The compositional analysis of as-grown samples was determinedby energy dispersive X-ray spectroscopy (EDAX) using JEOL JSM5600 at the operating voltage of 25 kV for three different locations.EDAX study confirmed that as-obtained thin films are sulfur deficient

nature [23]. The elemental composition of Cd1−xPbxS thin films depos-ited by chemical bath method is given in Table 1. Fig. 2 shows a typicalEDAX pattern of chemical bath deposited Cd1−xPbxS (x=0.175) thinfilm sample. The possible composition and structure of chemicallydeposited thin films can vary widely due to the variation in pHvalue and due to the presence of reducing agent in the reactionmixture [24].

3.1. Optical properties

The optical absorption study of the material provides a simplemethod for explaining some features concerning the band structureand energy gap of non-metallic materials. These studies constitutemost important means of determining the band structures of semi-conductors. The optical absorption spectra of as-deposited CdzPbySthin films have been used to determine the absorption coef ficient

(α), energy band gap (Eg) and the nature of transition involved. Theaction spectra were taken in the range of 350–3300 nm. The absorp-tion coef ficient is higher for all the film compositions (104 cm−1).The wavelength dependence of absorption coef ficient for five repre-sentative compositions is shown in Fig. 3. It is observed that, theabsorption edge of the films varies with the composition parameter‘x’. The relation between the absorption coef ficient ‘α’ and theincident photon energy ‘ ν’ for allowed direct type of transitions canbe written as [25]

αhυ ¼ A hυ−E g

1=2

: ð1Þ

To understand the onset of high photon energy corresponding tothe direct band gap energy we plotted (α ν)2 versus ν as shown in

Fig. 4. The straight line nature of the plots over a wide range of photonenergy suggested allowed direct type of transition. It is well knownthat direct transition across the band gap is feasible between thevalence and conduction band edges in k space [26]. The opticalband gaps have been then determined by extrapolation of the linearregions on ν axis. The variation of band gap with composition param-eter ‘x’ is as shown in Fig. 5. The non-linearity of the band gap energyvariation with composition has already reported for Cd1−xZnxS[27,28] and Cd–S–Se [29,26] thin films. Such a kind of broad andfine tunable band gap properties of ternary compounds havepotential applications in gas sensors, solar cells, detectors and opto-electronic devices.

20 30 40 50 60 70 80

Pb1.56S

Cd0.48Pb0.89S

Cd0.68Pb0.61S

Cd1.13Pb0.45S

Cd1.77Pb0.15S

Cd1.77Pb0.15S

Cd1.35S

( 2 2 2 )

( 3 1 1 )

( 2 2 0 )

S ( 2 0 0 )

( 1 1 1 )

( 2 0 0 )

I n t e n s i t y

( a r b . u n i t s )

2θ (deg.)

Fig. 1. XRD pattern of chemical bath deposited Cd1−xPbxS (0≤x≤1) thin films.

Table 1

Elemental composition of Cd–Pb–S thin films deposited by chemical bath method.

Nominal Concentration, x as“Cd1−xPbxS”

Cd1−xPbxS (0≤x≤1)concentration

As observed atomic% in film by EDAXanalysis

Cd Pb S

0 Cd1.35S 57.45 00.00 42.550.1 Cd1.77Pb0.15S 60.51 05.25 34.240.175 Cd1.77Pb0.15S 52.03 05.37 42.600.3 Cd1.13Pb0.45S 43.68 17.56 38.760.5 Cd0.68Pb0.61S 29.68 26.56 43.760.7 Cd0.48Pb0.89S 20.15 37.56 42.291 Pb1.56S 00.00 60.95 39.05

Fig. 2. Typical EDAX spectrum of chemical bath deposited Cd1.77Pb0.15S thin film.

98 M.A. Barote et al. / Thin Solid Films 526 (2012) 97 –102



The nature of the optical transition in these films has also beendetermined as

ln αhυð Þ ¼ ln Að Þ þ mln hυ−E g

: ð2Þ

Straight line plot (Fig. 6) of ln (αhν) vs. ln (hν−Eg) with slope≈0.5 has also confirmed the direct type of transition. The values of slope for various film compositions are listed in Table 2.

3.2. Electrical transport studies

The electrical transport properties of the materials are of great im-portance in determining whether the material is suitable for our ne-cessities or not. The electrical properties are mainly dependent onthe preparative parameters such as film composition, film thickness,deposition temperature, deposition time and basic ingredients in

the reaction solution. The electrical conductivity of the material isprime characterization for various device applications.

The dc electrical conductivity of a semiconductor at temperature Tis given by [30]

σ ¼ σ 0exp−Ea

kT

ð3Þ

where, σ 0 is the pre-exponential factor, Ea is the activation energy forthe generation process and k is Boltzmann constant. We may write,

lnσ ¼ lnσ 0−Ea

kT

or

lnσ ¼− Ea

1000k

1000

T

þ lnσ 0:

ð4Þ

The dc electrical conductivities of the as-deposited thin films indark were carried out in the range of temperature from 300 to500 K. The temperature dependence of the dark conductivity is asshown in Fig. 7. The conductivity of all these samples increases withincrease in temperature.

The electrical conductivity is found to be composition dependent.It is observed that electrical conductivity is increased as the actual Cd

content of as-deposited film layers decreases up to compositionparameter ‘x’=0.175 (7.94×10−6Ω−1 cm−1) and thereafter it isdecreased. The room temperature electrical conductivities for CdSand PbS are 6.13× 10−7(Ω cm)−1 and 1.70×10−6(Ω cm)−1 respec-tively. The lower magnitudes of electrical conductivity of the films areattributed to the deposition method itself i.e. generally chemicalmethods result into lower magnitude of electrical conductivity [21].The presence of defects viz. structural disorders, dislocations and sur-face imperfections are also play a vital role in decreasing the conduc-tivity [31]. From Fig. 7 the variation shows an Arrhenius behavior

500 1000 1500 2000 2500 3000 3500

0

1

2

3

4

5

6

C o e f f i c i e n t o f

a b s o r p t i o n

x 1 0 4 ( c m - 1 )

Wavelength (nm)

Cd1.35S

Cd1.77Pb0.15S

Cd1.77Pb0.15S

Cd1.13Pb0.45S

Cd0.68Pb0.61S

Cd0.48Pb0.89S

Pb1.56S

Fig. 3. The action spectra of five typical compositions of CdzPbyS thin films.

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

0

1

2

3

4

5

6

P b 1 . 5 6 S

C d 0

. 4 8 P b 0

. 8 9 S

C d 0

. 6 8 P b 0

. 6 1 S

C d 1 . 1 3 P b 0 . 4 5 S

C d 1 . 7

7 P b 0 . 1 5 S

C d 1

. 7 7 P b 0

. 1 5 S

C d 1

. 3 5 S

(α h

) 2 × ×

1 0 1 0 ( e V / c m ) 2

hν (eV)

Fig. 4. Plot of (α ν)2

versus ν for Cd1−xPbxS (0≤x≤1) thin films.

Fig. 5. Variation of energy band gap (Eg) versus actual concentration of Pb in as-grownthin films.

Fig. 6. Variation of ln (α ν) versus ln ( ν−Eg) for Cd1−

xPbxS (0≤x≤1) thin films.

99M.A. Barote et al. / Thin Solid Films 526 (2012) 97 –102

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consisting of high and low temperature regions. The activation ener-gies of electrical conduction have been determined from these plotsin high and low temperature regions and listed in Table 2, for eachof the composition parameter ‘x’. The linearity of lnσ against 1/T inthe high temperature region indicates that, the conductivity in thisregion exhibit activated behavior while in the low temperature regionconductivity exhibits non-activated behavior. In low temperature re-gions, between 300 K and 380 K the temperature dependence conduc-tivity for all compositions increases slightly with the small activationenergies. In this region the increase in conductivity with temperatureis due to the intrinsic nature of as-grown films. Also at low tempera-tures, the charge carriers in conduction band may too few to give riseto an appreciable conduction, which suggests that, the conduction isdue to Mott's variable range hoping in localized states near the Fermilevel. This variable range hoping mechanism is characterized by Mott'sexpression [32] as,

σ ¼

σ 0

exp −T

T0 1=4 ð

5Þ

where, T0 ¼ λα3=kN Ef ð Þ

here, λ is a dimensionless constant, k is Boltzman's constant, N(Ef ) isthe density of localized states at Ef , α is the degree of localization andT0 the degree of disorder.

The value σ 0 is obtained by Touraine [33] is,

σ 0 ¼ 3e2ν

N Ef ð Þ8παkT

1=2

where, ‘e’ is the electron charge and ‘ν’ is the Debye frequency.Fig. 8 represents the plot of ln (σ T1/2) versus 1/T1/4 for the

as-deposited thin films. From these curves it is clear that ln (σ T1/2)versus 1/T1/4 is a linear relation. This is in good consonance with theMott's variable range hoping process. At low temperature chargecarriers do not have suf ficient energy for excitation to the adjacentband and hence they move from one impurity to another with thehelp of phonon.

In high temperature region (380 K and 500 K) the conductivity of the film sample increases sharply. At these temperatures the low mo-bility of charge carriers is easily compensated by creation of largenumber of charge carriers and this result in increased conductivityof the film [34].

A polycrystalline film material contains a large number of micro-crystallites with grain boundaries between them. At the grain bound-ary the incomplete atomic bonding can act as trap centers. These trapcenters trap the charge carriers at the grain boundaries, and hence alocal space charge region can be built up. The grain boundary poten-tial model proposed by Seto [35] is given by,

σ ffiffiffi

Tp

¼ σ 0 exp−Ea

kT

ð6Þ

Table 2

Optical and electrical parameters of chemical bath deposited Cd–Pb–S thin films.

Nominal concentration, x as “Cd1−xPbxS” Eg (eV) Power factor Activatio n energy Conductiv ity × 10−6(Ω cm)−1 Mobility×10−3(cm2/V s)

LT (e V) HT (eV)

0 2.47 0.51 0.240 0.711 0.613 1.4140.05 2.38 0.48 0.213 0.645 1.885 2.6590.1 2.28 0.52 0.193 0.578 3.732 4.3360.15 2.17 0.54 0.176 0.523 6.761 5.7610.175 2.13 0.51 0.168 0.514 7.944 6.2130.2 2.05 0.53 0.173 0.545 4.262 5.3830.3 1.86 0.52 0.181 0.566 2.671 3.1380.4 1.64 0.46 0.185 0.578 2.611 3.1320.5 1.47 0.57 0.188 0.622 2.450 3.0950.6 1.22 0.48 0.193 0.631 2.323 3.0630.7 1.03 0.55 0.201 0.638 2.074 2.9330.8 0.87 0.58 0.205 0.647 1.843 2.8800.9 0.62 0.47 0.208 0.668 1.751 2.8651 0.49 0.53 0.212 0.676 1.702 2.814

Fig. 7. Plot of log σ versus (1000/T) of CdzPbyS thin films.

Fig. 8. Variation of log (σ T1/2) vs. (T−1/4) for chemical bath deposited Cd1−xPbxS

(0≤x≤1) thin films.






where, σ 0 is the pre-exponential factor, Ea is the activation energy, kis Boltzman's constant and T is absolute temperature.

The plot of ln (σ T1/2) versus 1000//T have been plotted in Fig. 9.Obviously in the high-temperature region, grain boundary limitedconduction is dominant conduction mechanism.

The thermoelectric power is the ratio of thermally generated volt-age to the temperature difference in the semiconductor, which givesthe information about charge carriers in the deposited material. In ther-moelectric power measurements, the open circuit thermo-voltage gen-erated by the film samples when a temperature gradient is appliedacross a length of a sample is measured. The thermoelectric power in-creases with increase in temperature. The polarity of thermallygenerat-ed voltage for CdzPbyS thin films at hot end is positive indicating that,the as-deposited thin films are of n-type [36]. The variation of thermo-emf as a function of temperature difference is shown inFig. 10. TEPwas also used to evaluate the carrier mobility (μ ) and carrierconcentration (n) using the relation [37],

TEP ¼−Ke

A þ ln 22πmc

ÃkT3=2

nh3

!( )" #ð7Þ

where, k is thermal conductivity, A is a thermoelectric factor, n is elec-tron density, h is Plank's constant, mc* is the effective mass of the

electron and T is the absolute temperature. After substitution of variousconstants Eq. (4) simplifies to

logn ¼ 32

logT−0:005TEP þ 15:719: ð8Þ

The carrier density (n) at room temperature was determined forall samples. The variation of carrier density with composition forchemically deposited CdzPbyS thin films is as shown in Fig. 11. Thecarrier density is found to be of the order of 1015 cm−3. The carrierden-sity is increased with decrement in actual Cd content of as-depositedthin film layers up to the composition parameter ‘x’=0.175. The small-er carrier density is characteristic of the compensation type semicon-ductor involving deep donor or deep acceptors [38]. Moreover,

structural defects and grain boundaries, whose number are generallylarger in deposited materials may also be responsible for reducing car-rier density, since they are capableof trapping carriers [35]. The electronmobility has been calculated using the standard relation,

μ ¼ σ

neð9Þ

where μ is the electron mobility, σ is the electrical conductivity and n isthe carrier density.

The electron mobility is found to be a function of composition. Theelectron mobility for chemical bath deposited Cd–Pb–S thin films isfound to be in the range of 1.414–6.213×10−3 cm2/V s. The smallervalues of electron mobility are due to presence of the grain bound-aries and structural defects which cause scattering of charge carriers.

The calculated values of electron mobility at room temperature arelisted in Table 2.

4. Conclusion

CdzPbyS thin films with nominal composition as Cd1−xPbxS(0≤x≤1) have been deposited by simple and inexpensive chemicaldeposition route. The optical studies indicated direct energy bandgap which strongly depends on the composition parameter ‘x’. Theband gap energy was tailored in the range 2.47 eV (CdS) to 0.49 eV (PbS) which is a prime need for various device application viz. solarcell, photoelectrochemical cell, IR detectors and lasers. The electricalconductivity of as-deposited thin films was found to be increasedwith the composition parameter ‘x’ up to 0.175. The room tempera-

ture electrical conductivity for optimized sample was measured to

2.0 2.1 2.2 2.3 2.4 2.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5Cd1.35S

Cd1.77Pb0.15S

Cd1.77Pb0.15S

Cd1.13Pb0.45S

Cd0.68Pb0.61S

Cd0.48Pb0.89S

Pb1.56S

l o g (σ T

1 / 2 ) (Ω

- 1 c m - 1 K 1 / 2 )

1000/T (K-1)

Fig. 9. Variation of log (σ T1/2) vs. (1000/T) for chemical bath deposited Cd1−xPbxS(0≤x≤1) thin films.

20 40 60 80 100 1 20 1 40 1 60 1 80 2 00

0

50

100

150

200

250

300

350

400 Cd1.35S

Cd1.77Pb0.15S

Cd1.77Pb0.15S

Cd1.13Pb0.45SCd0.68Pb0.61S

Cd0.48Pb0.89S

Pb1.56S

T h e r m o - e m f (μ V )

Temperature difference (oC)

Fig. 10. Thermo-emf variation as a functionof temperaturedifference for CdzPbyS thinfilms.

Fig. 11. Carrier density variations with actual concentration of Pb in as-obtained thin films.

101M.A. Barote et al. / Thin Solid Films 526 (2012) 97 –102














































be 7.94×10−6Ω−1 cm−1. The conductivity measurements revealedtwo types of conduction mechanism, namely grain boundary scatter-ing limited and variable range hoping conductions. n-Type conduc-tion of as-grown Cd–Pb–S thin film layers was highlighted by thethermoelectric power measurements. The wide and fine tunabilityof the energy band gap as well as the uneven changes in the conduc-tivity of ternary Cd–Pb–S thin film layers have potential applicationsin a variety of optoelectronic devices.

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optical & electrical characterization of chemical bath deposited cd-pb-s thin films

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