Effect of ambient hydrogen sulphide on the opticalproperties of evaporated cadmium sulphide films

Virendra Singh, Beer Pal Singh, T.P. Sharma *, R.C. Tyagi

Thin Film Laboratory, Department of Physics, Ch. Charan Singh University, Meerut 250004, India

Received 5 April 2002; accepted 23 April 2002

Abstract

The optical constants ðn; kÞ of evaporated cadmium sulphide (CdS) thin films deposited in an ambient atmosphere ofhydrogen sulphide (H2S) were determined over the wavelength range 350–800 nm from the measurement of reflection

and transmission spectra. Low ambient atmosphere of H2S was obtained by thermal decomposition of thiourea inside

the vacuum chamber. Cadmium sulphide thin films deposited with H2S ambient are more uniform, more adherent, pin

hole free and have better crystallinity in comparison with the films without H2S atmosphere as inferred by the sharpness

of the absorption spectra. Such films are inherently better suited for device fabrication.

� 2002 Published by Elsevier Science B.V.

PACS: 78.20.Ci; 78.50.Ge; 78.66.)w; 78.66.HfKeywords: Cadmium sulphide; Evaporation; Transmission spectra; Reflection spectra; Optical properties

1. Introduction

Cadmium sulphide (CdS) is a very useful opto-electronic [1,2], piezo-electric [3,4] and semi-conducting material. Thin films of CdS are ofconsiderable interest for their efficient use in thefabrication of solar cells [5,6]. The optical andstructural properties of vacuum evaporated thinfilms of CdS are very sensitive to the depositionconditions as substrate temperature [7] and sub-sequent heat treatment. CdS thin films have beenprepared by different techniques including sput-tering [8] and laser deposition [9]. The vacuum

evaporated thin films of CdS are usually poly-crystalline and have excess of cadmium owing tothe dissociation of CdS during evaporation. Thestoichiometry can be restored by codeposition ofsulphur [10] together with CdS or by annealing thefilm in CdS powder [11]. However, compensationfor sulphur deficiency can also be accomplishedby exposing the film to a hydrogen sulphide at-mosphere during growth. The higher reactivity ofhydrogen sulphide will ensure a better conversionof the dissociated Cd ions into CdS and also willnot produce any excess of sulphur at the substrate.From the point of view of vacuum evaporation,the thermal decomposition of thiourea is a con-venient source of H2S which can be controlled byregulating the temperature of the electrically he-ated borosil test tube in the evaporation chamber.

Optical Materials 20 (2002) 171–175

www.elsevier.com/locate/optmat

*Corresponding author. Fax: +91-0121-760554.

E-mail address: upal@sirio.ifuap.buap.mx (T.P. Sharma).

0925-3467/02/$ - see front matter � 2002 Published by Elsevier Science B.V.

PII: S0925-3467 (02 )00043-5

This communication describes the effect of H2S onthe optical properties of vacuum evaporated CdSfilms.

2. Experimental

Cadmium sulphide powder of 5 N purity wasevaporated at about 850 �C from a deep narrowmouthed molybdenum boat. Deposition was madeonto glass substrates held at 200 �C in a vacuumof the order of 10�5 Torr. The substrates werecleaned in aquaregia, washed in distilled water andisopropyl alcohol (IPA). A borocil test tube wasused for the thermal decomposition of thiourea at150 �C for ambient atmosphere of hydrogen sul-phide, it was separated from the CdS molybdenumboat by a stainless steel heat shielder for compar-ison of the films were deposited both with andwithout hydrogen sulphide atmosphere. Absorp-

tion and transmission spectra of vacuum evapo-rated cadmium sulphide films both with andwithout hydrogen sulphide atmosphere were takenat room temperature with the help of ‘‘Hitachispectrometer model U-3400’’ and shown in Figs. 1and 2 respectively. In this model all the lenses havebeen replaced with mirrors. So the image deviationdue to chromatic aberration is eliminated in thewavelength range is 187–2600 nm. The PbS de-tector is used for the detection of infrared rays.The visible wavelength light source was a long lifeWL lamp.

3. Results and discussion

The energy band gap of these films has beencalculated with the help of absorption spectrashown Fig. 1. To measure the energy band gap

Fig. 1. Absorption spectra of vacuum evaporated cadmium

sulphide films with and without H2S atmosphere.

Fig. 2. Transmission spectra of vacuum evaporated cadmium

sulphide films with and without H2S atmosphere.

172 V. Singh et al. / Optical Materials 20 (2002) 171–175

from absorption spectra, the Tauc relation [12] isused

ahm ¼ Aðhm � EgÞn

where hm is the photon energy, a the absorptioncoefficient, Eg the energy band gap, A the constant,n ¼ 1=2 for direct band gap.To measure the energy band gap from absorp-

tion spectra a graph ðahmÞ2 versus hm is plotted.The extrapolation of the straight line to ðahmÞ2 ¼ 0axis gives the value of the energy band gap. Figs. 3and 4 show the plot of ðahmÞ2 versus hm for thevacuum evaporated CdS thin film with and with-out H2S atmosphere respectively. From thesegraphs, the value of the energy band gap of CdSwith and without H2S atmosphere comes out as2.47 and 2.39 eV respectively. The band gap offilms prepared with H2S atmosphere shows goodresults than that without H2S atmosphere.Elemental analyses of these films for both with

and without H2S atmosphere have been performed

and shown in Figs. 5 and 6 respectively. The CdSfilms prepared without H2S atmosphere have alarge difference in the atomic percentage of Cd(52.79%) and S (47.21%), whereas the CdS filmsprepared in H2S atmosphere have almost the sameatomic percentage of Cd (50.11%) and S (49.89%)which is indicative of the better stoichiometry ofthe films prepared in H2S atmosphere.The thickness of these films was measured by

a Hitachi spectrophotometer model U-3400. Thevalues of thickness of CdS films both with andwithout H2S atmosphere are 0.42 and 0.39 lmrespectively.The optical constants (especially refractive index

and extinction coefficient) of vacuum evaporatedthin films of cadmium sulphide have been deter-mined from transmission spectra of these films byusing Manifacier�s envelope method [13] and thesefilms showed, in general, good transparency ex-hibiting an interference patterns. The transmissionspectra of the CdS films both with and without H2Satmosphere in the range of 350–800 nm were used

Fig. 3. Plot of ðahmÞ2 versus hm for a CdS film prepared in H2S

atmosphere.

Fig. 4. Plot of ðahmÞ2 versus hm for a CdS film prepared withoutH2S atmosphere.

V. Singh et al. / Optical Materials 20 (2002) 171–175 173

to determine the refractive index (n) and extinctioncoefficient (k) and shown in Fig. 2.The refractive index (n) and extinction coeffi-

cient (k) were calculated using the formula [13].

n ¼ ½N þ ðN 2 þ n20n21Þ1=2�1=2

where n0 is the refractive index of air, n1 the re-fractive index of substrate (glass),

N ¼ ðn20 þ n21Þ=2þ 2n0n1ðTmax � TminÞ=TmaxTminwhere Tmax is the upper extreme transmission pointand Tmin is the lower extreme transmission pointfor particular wavelength and

K ¼ ð�k=4ptÞ lnðP Þwhere t is the thickness of the film.

P ¼ C1=C2½1� Tmax=Tmin�=½1þ Tmax=Tmin�

c1 ¼ ðnþ n0Þðnþ n1Þ

c2 ¼ ðn� n0Þðn1 � nÞ

Fig. 5. Elemental analysis of a CdS film prepared without H2S

atmosphere.Fig. 6. Elemental analysis of a CdS film prepared in H2S at-

mosphere.

Table 1

Variation of refractive index (n) and extinction coefficient (k)

for CdS film prepared in H2S atmosphere

S.

no.

Wave-

length (nm)

Tmax(%)

Tmin(%)

N n k

1 550 72.22 55.55 2.87 2.30 0.0462

2 600 80.55 63.88 2.59 2.17 0.0608

3 650 88.19 70.83 2.45 2.09 0.0757

4 700 93.05 76.38 2.32 2.02 0.0879

5 750 94.44 81.25 2.14 1.91 0.0961

6 800 94.44 84.02 2.01 1.83 0.1026

Table 2

Variation of refractive index (n) and extinction coefficient (k)

for CdS film without H2S atmosphere

S.

no.

Wave-

length (nm)

Tmax(%)

Tmin(%)

N n k

1 550 85.41 50.69 4.03 2.78 0.0612

2 600 90.97 61.80 3.18 2.44 0.0727

3 650 93.05 67.36 2.85 2.29 0.0813

4 700 94.44 69.44 2.76 2.25 0.0893

5 750 95.13 70.13 2.74 2.24 0.0966

6 800 95.13 70.13 2.74 2.24 0.0966

174 V. Singh et al. / Optical Materials 20 (2002) 171–175

The values of the n and k of vacuum evaporatedcadmium sulphide films both with and without

hydrogen sulphide atmosphere are given in Tables1 and 2. Plots of n and k versus k for vacuumevaporated cadmium sulphide films both with andwithout H2S atmosphere are shown in Figs. 7 and8 respectively. It is seen that the CdS films de-posited in H2S ambient atmosphere are dark col-our, uniform, pin hole free and have betteradhesion to the substrates. Their transparency isalso better than that of the films deposited withoutH2S atmosphere.

4. Conclusion

Transmission spectra of cadmium sulphide filmwith and without H2S atmosphere are sufficient tocalculate the refractive index (n) and extinctioncoefficient (k) of such films also show better re-sults. It is found that this method for the deter-mination of optical constants is better forsemiconducting thin films. It is also found that it isaccurate and fast, besides being one of the simplestmethods.

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Fig. 7. Variation of refractive index with wavelength.

Fig. 8. Variation of extinction coefficient with wavelength.

V. Singh et al. / Optical Materials 20 (2002) 171–175 175