This content has been downloaded from IOPscience. Please scroll down to see the full text.

Download details:

IP Address: 136.159.235.223

This content was downloaded on 22/10/2014 at 09:07

Please note that terms and conditions apply.

In situ deposition of Sn-doped CdS thin films by chemical bath deposition and their

characterization

View the table of contents for this issue, or go to the journal homepage for more

2006 J. Phys. D: Appl. Phys. 39 4771

(http://iopscience.iop.org/0022-3727/39/22/006)

Home Search Collections Journals About Contact us My IOPscience

INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS

J. Phys. D: Appl. Phys. 39 (2006) 4771–4776 doi:10.1088/0022-3727/39/22/006

In situ deposition of Sn-doped CdS thinfilms by chemical bath deposition andtheir characterizationPoulomi Roy and Suneel Kumar Srivastava

Inorganic Materials and Nanocomposite Laboratory, Department of Chemistry,Indian Institute of Technology, Kharagpur 721302, India

E-mail: sunit@chem.iitkgp.ernet.in

Received 27 July 2006, in final form 21 September 2006Published 3 November 2006Online at stacks.iop.org/JPhysD/39/4771

AbstractIn-situ Sn-doped CdS thin films have been deposited successfully by thechemical bath deposition method using tartaric acid as a complexing agent.The films have been characterized by x-ray diffraction for structuredetermination, and microstructural parameters like crystallite size, rmsstrain and dislocation density have been calculated using x-ray line profileanalysis. The composition of the films has been determined by x-rayphotoelectron spectroscopy and energy dispersive x-ray analysis. Opticalstudies show that the band gap decreases significantly for pure CdS(2.39 eV) to 3.8 mol% of Sn-doped CdS (1.84 eV). A detailed PL study ofCdS when doped with varying Sn concentrations has also been discussed.Temperature variation of the electrical resistivity of Sn-doped CdS thin filmsconfirmed their semiconducting behaviour similar to the pure CdS. It alsoshows that doping of Sn in CdS makes a pronounced drop in the roomtemperature resistivity value from 1010–103 � cm for pure CdS to 3.8 mol%of Sn-doped CdS, respectively.

1. Introduction

Nowadays, II–VI semiconductors in the form of thin filmshave attracted ever increasing attention due to their potential,experimental and of course interesting applications in manyindustries. Among them, cadmium sulfide (CdS) owing to itsgood optical transmittance and wide band-gap (2.43 eV) is themost commonly used material. It is used in a variety of thin filmdevices, such as laser screen materials, projection colour TVs,nuclear radiation detectors, light emitting diodes, etc [1–3].

In solar cell and other optoelectronic devices twobasic requirements are needed, namely moderate electricalresistivity and high optical transmittance [4]. The CdSbuffer layer being very thin, it should be quite resistive toavoid a short circuit effect between the electrodes throughthe grain boundaries of a semiconductor. However, theelectrical resistivity of the undoped CdS thin films at roomtemperature is high enough [5], which limits its use as abuffer layer in solar cells. To make the undoped CdSthin films useful in optoelectronic applications, the darkresistivity must be reduced from 107–109 � cm to at most

10 � cm [6]. Although, it strongly depends upon itspreparative methods, it is not possible to prepare thin filmsup to the requirements. Alternatively, some post-depositiontreatments are also reported to reduce the electrical resistivity[7, 8]. Another effective way of achieving the requiredproperties is by introducing a foreign atom having less orextra electrons in its valence orbital than the pure metalatom. The impurities with less electrons act as an acceptorimpurity forming a p-type conductivity and impurities withextra electrons act as a donor thereby forming a n-typeconductivity. However, the choice of metal as an impurityis a very difficult task where the size of foreign metal atomsshould also be within a 15% size of pure metal atoms. Thechemical bath deposition as well as other methods have beenreported in the literature for the deposition of CdS films usingvarious metals as dopant. Lee et al [9] studied the electricaland optical properties of B-doped CdS thin films prepared bychemical bath deposition. In situ deposition of p-type Cu-doped CdS thin films in a chemical bath have also been reportedby Sebastian [10]. Megahid and his group [4] prepared In-doped CdS thin films having n-type conductivity by electron

0022-3727/06/224771+06$30.00 © 2006 IOP Publishing Ltd Printed in the UK 4771

P Roy and S K Srivastava

beam evaporation. Warad et al [11] studied the effect of Bi3+

doping on the properties of chemically deposited CdS. Theoptical and structural properties of Sb-doped CdS by chemicalbath deposition method have been investigated by Desmukhand his co-workers [12]. However, no such studies on thedeposition of Sn-doped CdS films are reported in the existingliterature.

It should also be noted that physical properties of thesecompounds are largely influenced by the atomic arrangementsin them as well as by the nature and extent of the defects(in terms of strain, dislocations, etc) present in their atomicarrangements. These are developed partly during the growthof the crystallites or due to the incorporation of impurity.The strength of materials is found to be dependent on size,lattice distortion and dislocations etc. Hence, understandingthe lattice defects in materials is important in order to explainthe electrical properties of these materials.

Earlier, we have reported about the chemical bathdeposition of pure CdS thin films on a glass substrate usingfor the first time tartaric acid as a complexing agent alongwith CdCl2 and thiourea precursors making pH range between10 and 11 at 60 ◦C [5]. This has led to depositing a muchbetter quality thin film in terms of thickness, uniformityand adherence to the surface compared with the same filmobtained by using ammonia as a complexing agent. Thedeposited film when annealed at 300 ◦C has a band gap of2.34 eV and resisitivity of very high order (1010 � cm), whichrestricts their applications. Therefore, in this communication,we report for the first time a successful deposition of Sn-dopedCdS thin films in situ by the CBD method using the samecomplexing agent.

2. Experimental details

A detailed preparation procedure of pure CdS thin filmsdeposition using tartaric acid as a complexing agent has beengiven in our earlier work [5]. In the present work, a similarreaction has been carried out followed by the addition ofdifferent concentrations of SnCl2 · 2H2O as dopant. Variouscalculated quantities of Sn-dopant have been added into thebulk solution to make them 0.025, 0.050 and 0.075 mol%in dopant concentration. At further high concentration(>0.1 mol%), thin films obtained are not of good quality asquick precipitation of SnS occurs. The bath temperature wasmaintained at 60 ◦C. After 2 h high quality yellow colouredthin films were obtained having good adherence property andwere washed with de-ionized water ultrasonically until theheterogeneities were removed. The films were then dried ina N2 atmosphere and the annealing was carried out at 300 ◦Cfor half an hour in a N2 atmosphere. After annealing, the filmswere left as it is in the N2 atmosphere until the temperaturecame down to room temperature to avoid any possibility ofaerial oxidation.

An x-ray diffraction (XRD) pattern of samples wererecorded using a Miniflex diffractometer (30 kV, 10 Maq;Rigaku Corp., Tokyo, Japan) with a CuK α source. Thediffractograms were recorded between angles 20◦ and 60◦

with a scan rate of 2◦ min−1 at room temperature. Since(0 0 2) reflection is of maximum intensity in all these dopedsamples, the calculation of microstructural parameters have

been confined to only this reflection using the correspondingintensity profile of the samples. Assuming that the broadeningof the x-ray reflection is due to the presence of size broadeningand strain broadening [13], the variance can be given by

W(2θ) = λ�(2θ)

2π2PV cos θ+ 4 tan2 θ〈e2〉, (1)

where �(2θ) is the angular range, PV the apparent crystallitesize, 〈e2〉 the mean squared strain, λ the wavelength of thex-rays and θ the Bragg angle [14]. Thus the plot of W(2θ)

versus �(2θ) will be linear (not shown here) and the slope willgive the apparent crystallite size PV and from the intercept rmsstrain 〈e2〉1/2 can be calculated. The apparent crystallite sizePV from variance and true crystallite size PF, which is the sizeof the coherently diffracting domain or grain which constitutesa crystal from Fourier analysis are related as

1

PV= 1

PF+

βd

d, (2)

where βd is the integral width of the defect profile [15].Dislocation densities have been calculated from the

particle size and rms strain values by using the relations givenby Williamson and Smallman [16] as

ρ = 2(3〈e2〉)1/2

bPV, (3)

where b is the Burger’s vector associated with Burger’s circuit.If it is assumed that b = a, the lattice parameter of the sample,ρ can be easily calculated.

X-ray photoelectron spectroscopy (XPS) was recorded ona Rigaku XPS 7000 spectrometer using non-monochromaticMg Kα (1253.6 eV) at a source of 200 W. Chamber pressureduring the measurement was about 10−7 Pa. The bindingenergies were referred to the adventitious C 1s peak at 284.6 eV.Energy dispersive x-ray (EDX) analysis (OXFORD ISIS-300model) was used to determine the chemical composition ofthe films. The deviations between nominal and measuredcompositions were within the errors of the analysis used.The thickness of the deposited film was measured using aDektak3 ST thickness profiler with an accuracy of ±10 Å. APerkin–Elmer Lambda 20 UV–vis spectrometer ranging from400 to 800 nm using a blank substrate as the reference positionexamined the optical properties of the deposited thin film. Thephotoluminescence (PL) measurements were performed ona Perkin-Elmer LS55 luminescence spectrophotometer. Theresistivity measurements were carried out using a two-probemethod in the temperature range 300–500 K. Ohmic contactswere provided by conducting silver paste on the samples(∼1 cm2 area).

3. Result and discussion

Deposition of undoped and doped CdS thin films bythe chemical bath deposition process occurs by controlledcondensation of Cd2+, Sn2+ and S2− ions. In the presentcommunication, tartaric acid has been used as the complexingagent to control the release of Cd2+ as well as Sn2+ by

4772

In situ Sn-doped CdS thin films by chemical bath deposition

00.20.40.60.8

11.21.4

0 1 2 3 4Dopant concentration in mol%

Th

ickn

ess

in m

icro

n

Figure 1. Variation of film thickness with deposition time.

making their respective tartarate complex. This may be shownaccording to the following reaction:

Cd[Tartarate] + Sn[Tartarate] + (NH2)2C + 2OH−

→ Cd(Sn)S + tartaric acid + (NH2)2CO + H2O.

A small quantity of ammonia was used to alkaline the medium(pH = 10–11) as well as for the hydrolysis of thiourea. Thesolubility products of CdS and SnS are very close (Ksp ofCdS = 10−27, Ksp of SnS = 10−25) [17], thereby precipitationof CdS and SnS occurs simultaneously, which results inthe formation of very high quality Sn-doped CdS thin films.The film deposition takes place initially via nucleation andthe growth process continues through ion-by-ion condensation.On increasing the deposition time, adsorption of more Cd2+ andS2− along with the incorporation of Sn2+ leads to the formationof Sn-doped CdS thin films. The growth also depends uponthe deposition temperature. Initially, low temperature and thena slow increase in temperature controls the concentration ofprecursor ions and thereby restricts quick precipitation. Ithas been found that on increasing the dopant concentration(0 mol% to 0.075 mol%) the thicknesses of the films decreaseand the variation is shown in figure 1. Beyond 0.075 mol%of dopant, the films formed are very thin. This may be dueto the increase in the number of ions with increasing dopantconcentration, which do not get sufficient time and space tocombine on the substrate and leads to quick precipitation [18].The incorporation of Sn into the CdS lattice can also beexplained by comparing their sizes. The ionic radii of Sn2+ andCd2+ are quite similar (Cd2+ = 0.97 Å, Sn2+ = 1.02 Å) [17]and both exhibit the same coordination number and as a resultSn can easily substitute Cd in the lattice.

EDX analysis has been carried out to examine thecomposition of doped as well as undoped CdS thin films. Itconfirms that on increasing the dopant quantity in the reactionmixtures the atomic percentage of Sn in the resulting filmsincreases from 0% to 2.2 ± 0.2%, 3.6 ± 0.3% and 3.8 ± 0.2%for pure films and films doped with 0.025 mol%, 0.050 mol%,0.075 mol% of Sn, respectively.

XPS analysis of undoped as well as Sn-doped CdS hasalso been carried out to confirm the purity and composition ofthe films. The survey scan of surfaces of doped films revealsthe presence of peaks corresponding to Cd 3d, S 2p, C 1s andO 1s and a broad, low intensity peak around 486 eV in the Sn3d region. No energy shift in the binding energy value of Snconfirms that there is no contamination of any other phasesof Sn, e.g. the presence of SnS2 and SnO2 in the depositedfilms can be ruled out. With increasing dopant concentration,

the peak intensity of Sn 3d increases and on the contrary theintensity of the Cd 3d peak decreases. However, the peakintensity of S 2p remains more or less constant (not shownhere). The composition of films has been calculated from thepeak area and has been given in table 1 along with the peakpositions of elements. It can be noted that the compositionof films determined from the peak areas of elements in XPSanalysis are well consistent with the EDX values.

CdS thin films exist in two crystalline modifications: thehexagonal (wurtzite) phase and the cubic (zincblende) phase.It is believed that the hexagonal phase is stable in between 25◦

and 900 ◦C. Figure 2 shows the XRD pattern of pure CdS aswell as Sn-doped CdS thin films when annealed at 300 ◦C innitrogen atmosphere confirming that the films are crystallinein nature. All the XRD patterns are thoroughly scanned forthe presence of all possible phases, e.g. CdS, SnS, CdO, Sn,SnS2, etc. The presence of two peaks at 26.18◦ and 28.01◦

in figures 2(a)–(d) corresponds to the mixture of the (0 0 2)hexagonal/(1 1 1) cubic plane and the (1 0 1) hexagonal plane ofCdS, respectively [JCPDS No. 80-0006, 80-0019]. Moreover,two other peaks of relatively lower intensity tends to appear inthe diffractogram at 44.21◦ and 52.02◦ for (2 2 0) cubic and(3 1 1) cubic reflections for CdS. Interestingly, with increasingdopant concentration, the intensity of the peak correspondingto ≈26◦ increases dramatically while the (2 2 0) and (3 1 1)peaks are not developed significantly and may be attributed tothe preferential growth of films along the (1 1 1) plane with thecubic phase. This increase in peak intensity with increasingdopant concentration indicates that Sn in all probability issubstituting Cd in the CdS lattice rather than occupying theinterstitial sites, and is also supported by their comparableatomic sizes as discussed earlier [19,20]. A considerable shiftin the peak position of the (1 1 1) plane from 26.18◦ for pureCdS to 26.4◦ for 2.2 mol% Sn-doped CdS films is noted andremains more or less unaltered with a further increase in dopantconcentration of 3.8 mol % in CdS. This may be due to the factthat the introduction of such a low amount of Sn into the CdSfilms results in no significant change in the lattice parameters(d-values). Moreover, an additional peak appears at 31.56◦

for 3.8 mol% Sn-doped CdS films, due to the presence of anorthorhombic (111 plane) SnS [JCPDS No. 83-1758]. Thissuggests that the films are no longer single phase at a higherconcentration of dopant. Furthermore, the XRD of Sn-dopedCdS shows that the diffraction peaks are asymmetric as theyrise more steeply on the low-angle side than on the high-angleside. It is well known that such asymmetric behaviour is dueto the presence of defects.

The intensity data in XRD profiles for the (0 0 2) reflectionof the hexagonal undoped CdS and Sn-doped CdS films havebeen used in calculating the microstructural parameters usingthe aforesaid equations (equations (1)–(3)). Variance andFourier crystallite sizes are calculated using these equations.Figure 3 shows the variation of apparent as well as truecrystallite size with dopant concentrations of Sn in CdS.It is observed that the variance crystallite size (apparent),in general, is always lower than that calculated by Fourier(true), as this technique is not influenced by defect broadeningunlike the variance technique. It is also noted that the CdS,when doped with 2.2 mol% of Sn, has a lower crystallitesize. Thereafter, it gradually increases with increasing Sn2+

4773

P Roy and S K Srivastava

Table 1. XPS and EDX parameters of pure and Sn-doped CdS thin films.

Binding energy (eV) Composition (Cd : S: Sn)

Sample Cd 3d S 2p Sn 3d XPS EDX

Undoped CdS 404.8 161.2 — 45.7 : 54.3 : 0 45.2 ± 0.2 : 54.8 ± 0.4 : 0CdS doped with 0.025 mol% 404.9 161.1 486.1 47.3 : 50.6 : 2.1 47.7 ± 0.3 : 50.1 ± 0.2 : 2.2 ± 0.2of SnCl2

CdS doped with 0.050 mol% 404.7 161.5 486.4 46.1 : 50.5 : 3.4 46.3 ± 0.2 : 50.1 ± 0.4 : 3.6 ± 0.3of SnCl2

CdS doped with 0.075 mol% 404.9 161.3 486.1 45.4 : 50.7 : 3.9 45.9 ± 0.3 : 50.4 ± 0.4 : 3.8 ± 0.2of SnCl2

Figure 2. X-ray diffractograms of CdS thin films after annealing at300 ◦C for half an hour doped with (a) 0.0 mol% (b) 2.2 mol% (c)3.6 mol% and (d) 3.8 mol% of Sn.

concentration and becomes even higher for 3.8 mol% Sn-dopedCdS in agreement with other workers [18, 19]. The variationof rms strain value (〈e2〉1/2) and dislocation density (ρ) withSn2+ concentration is displayed in figure 4. It shows thatfor 3.6 mol% Sn-doped CdS, 〈e2〉1/2 goes up by about 40%and in particular ρ reaches its maximum value due to thebreaking of the domains causing smaller crystallite sizes. Fordoping concentration of 3.8 mol% Sn in CdS, ρ and 〈e2〉1/2

both become considerably low which in all probability is dueto the relaxation of strain within the CdS lattice.

The optical properties of undoped as well as doped CdSthin films have been studied after annealing at 300 ◦C. Thetransmission spectrum of samples at room temperature in thewavelength range of 350–850 nm (not shown here) shows thatthe undoped CdS exhibits the highest transmittance in thevisible range and on increasing the dopant concentration the

0 1 2 3 4

2

4

6

Dopant concentration in mol%

App

eran

t cry

stal

lite

size

(nm

)

10

12

14

True crystallite size (nm)

Figure 3. Variations of apparent and true crystallite size ofSn-doped CdS thin films with doping concentration.

0 1 2 3 40

2

4

6

8

10

12

14

16

18

20

22

24

Dopant concentration in mol%

r.m

.s. s

trai

n (1

0-3)

10

20

30

40

50

Dislocation density (10

15) lines/m2

Figure 4. Variation of rms strain and dislocation density ofSn-doped CdS thin films with doping concentration.

transmittance of films decreases. The absorption coefficient(α), calculated from the transmission spectrum, is found to behigh, of the order of 105 cm−1 for both undoped and doped CdS,indicating a direct type transition [21]. The energy band gapis estimated by plotting (αhν)2 versus hν as shown in figure 5.The band gap for undoped CdS thin films is found to be 2.39 eV,which is lowered considerably (2.21 eV) on introducing an Sn-solution as a dopant and becomes minimum (1.81 eV) at highdopant concentration (3.8 mol% of Sn) (table 2). This decrease

4774

In situ Sn-doped CdS thin films by chemical bath deposition

1.5 2.0 2.50

5

10

15

20

25

30(α

hν)2 x1

0-8 (

eV/c

m)2

hν (eV)

doped with 3.8 mol% of Sn2+

doped with 3.6 mol% of Sn2+

doped with 2.2 mol% of Sn2+

pure CdS

Figure 5. Plot of (αhν)2 versus hν for undoped as well as Sn-dopedCdS thin films.

Table 2. Variation of optical band gap (eV) and activation energy(eV) for pure and Sn-doped CdS thin films.

Activationenergy (eV)

Band gapSample (eV) 303–363 K 363–473 K

Undoped CdS 2.39 0.059 0.223CdS doped with 2.2 mol% 2.23 0.032 0.128

of Sn-doped CdSCdS doped with 3.6 mol% 1.95 0.025 0.098

of Sn-doped CdSCdS doped with 3.8 mol% 1.84 0.022 0.092

of Sn-doped CdS

1.8 2.0 2.2 2.4 2.6 2.8

(d)

(c)

(b)

(a)

PL

inte

nsity

(ar

b. u

nit)

Energy (eV)

Figure 6. PL spectrum of CdS thin films doped with (a) 0.0 mol%(b) 2.2 mol% (c) 3.6 mol% and (d) 3.8 mol% of Sn.

in band gap value with increasing dopant concentration is dueto the creation of donor levels in the forbidden zone.

Doping of CdS by Sn also has a significant effect on the PLproperties of CdS. Figure 6 represents the room temperaturePL spectrum of pure CdS as well as Sn-doped CdS thin films

Figure 7. Temperature dependence of electrical conductivity forundoped as well as Sn-doped CdS thin films.

after annealing at 300 ◦C. PL spectra of CdS consist of a broadband at ≈2.3 eV and a small band at 1.8 eV, which are known asgreen emission (GE) and red emission (RE) bands, respectively[22, 23]. On adding impurity, Cd as well as some S-atoms arelikely to be displaced to the interstitial position of the CdSlattice. As the binding energy of Cd-atoms is smaller thanthat of S-atoms, Cd-atoms can be displaced more easily thanS-atoms [24] and their presence in the interstitial position maycause an important level of structural disorder and broadeningof the emission band [25]. In addition, it is also observed thatas the dopant concentration increases, the intensity of the PLpeaks also decreases. This can be attributed to the increasein non-radiative traps and defect centres due to the structuraldisorder on doping [25]. The small shifting of the peak maximais also observable towards longer wavelength regions whendopant concentration is gradually increased. This shift may bedue to the indirect recombination of the free electrons movingfrom the trap levels formed by the Sn-atoms to the holes in thevalence band. These trap levels are in the energy gap and shifttowards the valence band as the concentration of the Sn-atomsincreases [26]. The RE band has been ascribed to the surfacestates, in particular, Cd-vacancies [22].

The electrical resistivity of doped and undoped CdS thinfilms was measured using a dc two-probe method in air inthe temperature region of 300–500 K. It has been found thatthe room temperature electrical conductivity of pure CdS thinfilms enhances from 2.96 × 10−9 to 3.70 × 10−3 �−1 cm−1 ondoping by 3.8 mol% of Sn2+ concentration. Figure 7 representsthe variation of the logarithm of conductivity (log σ) with thereciprocal of temperature (1000/T ) for pure and doped films.It is observed that the conductivity of the films increases withincreasing temperature, indicating the semiconducting natureof the films. It is also seen from the figure 7 that there existtwo linear regions, at low temperature (303–363 K) and at hightemperature (363–473 K) zone, thereby indicating the presenceof two conduction mechanisms. The activation energy can becalculated using the following equation:

σ = σ0 exp

[−Ea

kT

], (4)

4775

P Roy and S K Srivastava

where σ is the specific conductivity, Ea the activation energy,k the Boltzmann constant, T the absolute temperature and σ0

is a constant. The above equation can be rearranged as

log σ = −2.303

[−Ea

kT

]+ C, (5)

which represents an equation of a straight line and from theslope of log σ versus 1000/T the activation energy can becalculated.

The activation energy calculated from the slope of theinitial part (303–363 K) are very small, e.g. 0.059 eV for pureCdS and 0.022 eV for 3.8 mol% Sn-doped CdS thin films,indicating the predominance of extrinsic (impurity) conductionprocesses [27]. In the case of pure CdS thin films, the extrinsicconduction process is observable due to the presence of Cd2+

vacancies formed during the deposition of the films [28]. Atfurther higher temperatures, activation energy for the pure CdSthin films is found to be 0.223 eV. This decreases further withincreasing dopant concentration and is 0.092 eV for 3.8 mol%Sn-doped CdS thin films. This may be attributed to thedecrease in the band gap due to the incorporation of Sn, whichacts as a donor level below the conduction band (forbiddengap) and decreases the electrical resistivity of the CdS [18].

The electrical conduction mechanism in polycrystallinesemiconductors is vastly influenced by the inherentintercrystalline boundaries [13]. These grain boundaryregions are dominated by a network of dislocations which areaccompanied by large strain fields. Like grain boundaries,these dislocations also act as scattering centres for the chargecarriers. The situation is quite complicated in compounds.Although accurate modelling is a very difficult task, severalmodels [29, 30] exist where, inside the crystallite, a series ofdistinct high conductivity (bulk) and low conductivity (grainboundary) regions are considered. Some of these modelspredict a dependence of the mobility on the crystallite size.Hence, small crystallites possess low mobility and lowerconductivity as compared with larger crystallites. As canbe seen from the room temperature conductivities of CdSand its Sn-doped films (figure 7) and crystal sizes (figure 3),there is no significant correspondence between the size andthe conductivity of pure CdS as suggested by these models.However, when the carrier concentration also changes for thesecompound semiconductors, the overall change in conductivityhas to be attributed to both these causes. Therefore, a lineardependence between size and conductivity cannot be expectedas the generation of charge carriers is affected by the latticedefects which vary strongly within this group of samples.

4. Conclusion

In this paper, we have successfully deposited in situ Sn-dopedCdS thin films by the chemical bath deposition method usingtartaric acid as a complexing agent. The composition ofsamples has been studied from XPS as well as EDX analysis.Although, the XRD pattern of pure CdS films shows theformation of a mixture of hexagonal and cubic structures,film growth occurs with the cubic phase along the (1 1 1)plane on increasing the dopant concentration. At high dopantconcentration, the films are no longer single phase. Opticalproperties show a significant change in band gap value on

doping. The PL spectra show two peaks located at 2.3 eV(GE) and 1.8 eV (RE), respectively. The electrical propertyof CdS films shows a pronounced effect on doping and theelectrical resistivity value decreases from 1010 to 103 � cmand the temperature dependence of conductivity of Sn-dopedCdS films confirms the semiconductor nature of the films.

Acknowledgments

The author gratefully acknowledges the financial support givenby our Institute and CSIR for this work.

References

[1] Pande P C, Russell G J and Woods J 1984 Thin Solid Films121 85

[2] Basam A A, Brinkman A W, Russel G J and Woods K J 1988J. Cryst. Growth 86 667

[3] Hankare P P, Bhuse V M, Garadhar K M, Delekar S D andMulla I S 2004 Semicond. Sci. Technol. 19 70

[4] Megahid N M, Wakkad M M, Shokr E K H and Abass N M2004 Physica B 353 150

[5] Roy P and Srivastava S K 2006 Mater. Chem. Phys. 95 235[6] Petre D, Pintilie I, Pentia E, Pintilie I and Botila T 1999 Mater.

Sci. Eng. B 58 238[7] Holhe S, Kulkarni S K , Nigavekar A S and Bhide V G 1987

J. Mater. Sci. 22 1067[8] Jayakrishnan R, Kumar S R and Pandey R K 1994 Semicond.

Sci. Technol. 9 97[9] Lee J-H, Yi J-S, Yang K-J, Park J-H and Oh R-D 2003 Thin

Solid Films 431–432 344[10] Sebastian P J 1993 Appl. Phys. Lett. 62 2956[11] Warad A U, Uplane M D and Pawar S H 1985 Mater. Chem.

Phys. 13 91[12] Deshmukh L P, Holikatti S G and More B M 1995 Mater.

Chem. Phys. 39 278[13] Srivastava S K, Pramanik M, Palit D, Mathur B K, Kar A,

Samantaray B K, Haeuseler H and Codres W 2001 Chem.Mater. 13 4342

[14] Klug P H and Alexander L E 1974 X-Ray DiffractionProcedure 2nd edn (New York: Wiley)

[15] Palit D, Srivastava S K, Chakravorty M C and Samantaray B K2000 Mater. Chem. Phys. 49 22

[16] Williamson G B and Smallman R C 1956 Phil. Mag. 1 34[17] Dean J A 1987 Lange’s Handbook of Chemistry 13th ed (New

York: McGraw-Hill) pp 5–12[18] Bhuse V M 2005 Mater. Chem. Phys. 91 60[19] Hankare P P, Yadhav A D, Bhuse V M, Khomane A S and

Garadhar K M 2003 Mater. Chem. Phys. 80 102[20] Wang X B, Li D M, Zeng F and Pan F 2005 J. Phys. D: Appl.

Phys. 38 4104[21] Shanthi E, Dutta V, Banerjee A and Chopra K L 1980 J. Appl.

Phys. 51 6243[22] Lee J 2004 Thin Solid Films 451–452 170[23] Vigil O, Riech I, Garcia-Rocha M and Zelaya-Angel O 1997

J. Vac. Sci. Technol. A 15 2282[24] Nanda K K, Sarangi S N, Mohanty S and Sahu S N 1998 Thin

Solid Films 322 21[25] Perna G, Capozzi V, Minafra A, Pallara M and Ambrico M

2003 Eur. Phys. J. B 32 339[26] Battisha I K, Afify H H, El Fattah G A and Badr Y 2002 Fizika

A 11 31[27] Hankare P P, Delekar S D, Chate P A, Sabane S D, Garadkar,

K M and Bhuse V M 2005 Semicond. Sci. Technol. 20 257[28] Ristov M, Sinadinovski G, Grozdanov I and Mitreski M 1989

Thin Solid Films 173 53[29] Endo S and Irie T 1976 J. Phys. Chem. Solids 37 201[30] Pankove J I 1971 Optical processes in Semiconductors

(Englewood Cliffs, NJ: Prentice Hall)

4776