Monitoring the Cadmium Sulfide Thin Films by Double-Exposure Holographic Interferometry Technique

 

S.A. Gangawane*, S.D. Kamat, V.P. Malekar and V.J. Fulari

Holography and Materials Research Laboratory, Dept. of Physics, Shivaji University, Kolhapur-416 004, India.

*Corresponding Author E-mail: g_satish2007@rediffmail.com

 

ABSTRACT:

Here, the Double Exposure Holographic Interferometry (DEHI) technique is used to study the surface deformation on stainless steel substrate, when CdS is deposited on it. In the electrodeposition method at different concentrations, CdSO4 is used as cadmium source, while Na2S2O3 is used as a sulphur source and ethylene diamine tetra acetic acid (EDTA) is used as a complexing agent. The electro deposition of CdS thin films is carried out by varying the time of deposition. The deposition potential of the compound was studied by cyclic voltammetry. The structural, surface morphological and optical properties of the deposited films have been studied using X-ray diffraction (XRD), scanning electron microscope (SEM) and optical absorption technique respectively. The DEHI technique is used to determine, thickness of thin film and stress to substrate of electrodeposited CdS thin films for various deposition time and at various concentrations.

 

KEYWORDS: DEHI technique; Thin Films; Electrodeposition; CdS; XRD; SEM

 


 

1. INTRODUCTION:

Cadmium sulfide (CdS) is the members of the family of group II and VI compounds and it is one of the best photoconducting materials. It is widely used in solar cells as well as opto-electric and photoconductive devices. CdS primarily is photoconductive only in the short wavelength visible region. Specimens prepared by various ways have shown sensitivity to ultra-violet (UV), infra-red (IR), X-ray radiation, gamma rays and alpha rays1,2.

 

Several of the deposition techniques have been reported in the literature for the preparation of CdS films, including anodic oxidation3, cathodic reduction4. The cathodic electrodeposition of CdS from aqueous and non-aqueous solution has been developed5-7. Extenive literature is available on preparation and characterization of CdS thin films by various techniques. The aim of the present paper is to study the electrodeposited CdS thin films using double exposure holographic interferometry (DEHI) technique.

 

2. EXPERIMENTAL:

CdS thin films are cathodically electrodeposited from aqueous solution containing the following in a) 0.08 M CdSO4 + 0.8 M Na2S2O3 + 0.1 M EDTA, b) 0.06 M CdSO4 + 0.6 M Na2S2O3 + 0.08 M EDTA and c) 0.04 M CdSO4 + 0.4 M Na2S2O3 + 0.06 M EDTA. CdSO4 and Na2S2O3 (A.R grade) are used without further purification. EDTA was used as a complexing agent. The stainless steel and fluorine doped tin oxide (FTO) were used as substrates. First at all stainless steel substrate are mirror polished by zero grade polish paper and FTO coated glass substrate are cleaned with 10% HCl for 15 minutes and then both the substrates were ultrasonically cleaned with double distilled water. Thin film deposition is carried out using a three electrode system in which a saturated calomel electrode (SCE) is used as the reference electrode. The well cleaned, mirror polished, stainless steel substrate acts as a working electrode with graphite as a counter electrode. Applied potentials are measured with respect to SCE. The deposition is carried out for different concentrations, which results the yellow coloured, uniform and adherent CdS thin films.

 

The experimental set up for recording hologram of the thin film is as shown in Fig. 1. Light from the laser source is passed through beam splitter (B.S.). The 50% of light is transmitted and incident on the mirror, it illuminates the stainless steel substrate. Scattered light from the stainless steel substrate is incident on holographic plate, which is called as object beam. Another 50% reflected beam is incident on holographic plate, which is called as reference beam. Initially, steel substrate as an object was recorded on holographic film without deposition of the film. Secondly, the holographic film was exposed by depositing CdS thin films for few second8,9. The CdS solution of three different concentrations i.e., 0.08 M, 0.06 M, 0.04 M CdSO4 as cadmium source, 0.8 M, 0.6 M, 0.4 M Na2S2O3 as a sulphur source and 0.1 M, 0.08 M, 0.06 M EDTA as a complexing agent are used for deposition at room temperature. The holograms were recorded on holographic plates (Kodak 8E75 HD) at different time intervals10,11. Exposed holographic plate is immersed into Kodak D-19 developer for 3 min and then kept in rapid fixer for about 10 min to fix the image of the substrate. The holographic film is processed and replaced only in the reference wave path. The reconstructed image of substrate was observed with the reference beam, which shows the fringes localized on its surface, where the thin film is deposited.

 

Fig.1. Experimental set up for recording hologram.

 

(A)

 

(B)

 

(C)

Fig.2. Cyclic voltammogram on stainless steel substrate in the solution containing

A)                0.08 M CdSO4 ,  B) 0.8 M Na2S2O3 and C) 0.08 M CdSO4  +  0.8 M Na2S2O3 + 0.1 M EDTA.

 

The phase formation is characterized by X-ray diffraction (XRD,) which is performed on a PW-3710 diffractometer using Cu Kα radiations. Surface morphology of these films was studied using a JEOL-JSM 6360 Japan, scanning electron microscope (SEM). The optical band gap of the material was determined by UV-VIS-NIR spectrophotometer in the wavelength rang of 350-950 nm.

 

3. RESULT AND DISCUSSION:

3.1. Cyclic voltammetry:

Cyclic voltammetry is employed to study the kinetics of the electrochemical reactions in electrolytic bath. Fig. 2 (a-c) shows the cyclic voltamograms recorded on stainless steel substrate from electrolytic solutions containing 0.08M CdSO4, 0.8M Na2S2O3 and 0.08M CdSO4 + 0.8M Na2S2O3 +0.1M EDTA, in order to find the suitable reduction potentials of CdSO4, Na2S2O3 and CdS are found to be -0.4 V/ SCE, -0.68 V/SCE and -0.8 V/ SCE  respectively. The electrodeposition of CdS thin films were carried out at the deposition potential -0.8 V/ SCE.

 

3.2. Double exposure holographic interferometry (DEHI) technique :

Fig. 3 (A-C) shows the recording holograms of CdS films developed on the holographic film. From the hologram study, it is observed that the time of deposition increases, the number of fringes localized on the surface of stainless steel substrate increases and consequently the fringe width decreases. While recording the hologram, the object was illuminated with a beam of light making an angle of θ1 with the normal and viewed at an angle θ2 during reconstruction. The reconstructed image has a superimposed fringe pattern corresponding to displacement of the surface12-15.

 

The displacement d of the surface, in normal direction is given by

                                                     (1)

Where, n is the number of fringes and λ is the wavelength of light used.

 

 

 

(A)

 

(B)

 

(C)

Fig.3. Holograms of  CdS thin films from a bath containing

(A) 0.04 M CdSO4 + 0.4 M Na2S2O3 + 0.06 M EDTA,

(B) 0.06 M CdSO4 + 0.6 M Na2S2O3 +0.08 M EDTA,

(C) 0.08 M CdSO4 + 0.8 M Na2S2O3 + 0.1 M EDTA for deposition time, (a) 10 Sec, (b)  20 Sec, and (c) 30 Sec)

 

In general, the angles θ1 and θ2 are sufficiently small so that,

 

After counting the relevant number of fringes, one can quantitatively deternmine the displacement of a point on the surface of object i.e., deformation of the object surface was determined. Thus, using Eq. (2), the thickness of the films is calculated and listed in Table1.

The mass of the film is calculated using the relation,

Mass = density x

 

3.3. SEM Studies:

Fig.5. (a-c) shows the SEM pictures of CdS thin film on to stainless steel substrate for 1.58 μm, 2.53 μm and 2.84 μm thickness with different concentrations. It is observed that the increase in deposition concentration shows a substantial granular growth.

 

Fig.4. [A] The Variation of mass of CdS deposited on stainless steel substrate versus time of deposition

 

Fig.4. [B] The Variation of CdS films stress to the substrate with deposition times.


 

Table. 1. Thickness and stress of CdS thin films to the substrate for different deposition  time. for bath concentrations,

(a) 0.04 M CdSO4 + 0.4 M Na2S2O3 + 0.06 M EDTA,

(b) 0.06 M  CdSO4 + 0.6 M Na2S2O3 +0.08 M EDTA,

(c) 0.08 M CdSO4 + 0.8 M Na2S2O3  + 0.1 M EDTA.

Deposition

Time (Sec.)

Number of Fringes ‘n’

Thickness of film(μm)

Mass deposited

(mg)

Stress, x1011 (dyne/cm2)

Fringe Width (cm)

Bath concentration (a) 0.04 M CdSO4 + 0.4 M Na2S2O3 + 0.06 M EDTA

10

20

30

3

4

5

0.94

1.26

1.58

0.019

0.025

0.031

0.0116

0.0087

0.0069

0.312

0.287

0.272

Bath concentration (b) 0.06 M CdSO4 + 0.6 M Na2S2O3 + 0.08 M EDTA

10

20

30

4

5

8

1.26

1.58

2.53

0.025

0.031

0.051

0.0087

0.0069

0.0043

0.283

0.275

0.269

Bath concentration   (c) 0.08 M CdSO4  + 0.8 M Na2S2O3 + 0.1 M EDTA

10

20

30

6

7

9

1.89

2.21

2.84

0.038

0.044

0.057

0.0058

0.0049

0.0038

0.242

0.165

0.106

 

Table. 2. Comparison of observed and standard ‘d’ values of CdS thin films for

(a) 0.04 M CdSO4 + 0.4 M Na2S2O3 + 0.06 M EDTA,

(b) 0.06 M  CdSO4 + 0.6 M Na2S2O3 +0.08 M EDTA,

(c) 0.08 M CdSO4 + 0.8 M Na2S2O3  + 0.1 M EDTA.

 

Obs. No.

Standard ‘d’ values ( A0 )

Observed ‘d’ values  ( A0 ) bath concentration

Refracting plane (hkl)

(a)

(b)

(c)

1

2.068

2.075

2.073

2.073

1 1 0

2

1.790

1.801

1.8545

1.804

2 0 0

3

1.257

1.269

1.247

1.247

1 0 5

 

Obs. No

Standard ‘d’ values ( A0 )

Observed ‘d’ values  ( A0 )

Refracting plane (hkl)

1

2.067

2.073

(1  1  0)

2

1.800

1.862

(2  0  1)

 


 

Fig.4. [C] The Variation of Fringe width versus time of deposition. (10 Sec, 20 Sec, and 30 Sec) for bath concentrations,

(a) 0.04 M CdSO4 + 0.4 M Na2S2O3 + 0.06 M EDTA,

(b) 0.06 M  CdSO4 + 0.6 M Na2S2O3 +0.08 M EDTA,

(c) 0.08 M CdSO4 + 0.8 M Na2S2O3  + 0.1 M EDTA.

 

3.4. X-ray diffraction studies:

The structural properties of CdS thin films are studied by X-ray diffraction pattern. Fig. 6 (a-c) shows the XRD of CdS films deposited on stainless steel substrate for the different concentrations. It reveals that the CdS films are polycrystalline with hexagonal crystal structure.

The XRD spectra of the CdS films deposited at room temperature shows a mixed phase similar to those reported by other workers18,19. The d-values of XRD reflection were compared with standard d-values taken from the JCPDS data and reported in Table 2.

 

Fig.5. SEM micrograph of CdS thin films on to glass substrate For different concentrations,

(a) 0.04 M CdSO4 + 0.4 M Na2S2O3 + 0.06 M EDTA,

(b) 0.06 M CdSO4 0.6 M Na2S2O3 +0.08 M EDTA,

(c) 0.08 M, CdSO4 + 0.8 M Na2S2O3 + 0.1 M EDTA.

 

3.5. Optical absorption studies20-22:

The optical absorption studies of CdS films deposited on FTO coated glass substrates was carried out in the wavelength range from 350-850 nm using a UV-VIS-NIR spectrophotometer. In order to estimate the bandgap energy  of the CdS film for different concentrations. Fig. 7 (a-c) shows the variation of  with. The bandgap for CdS is determined by extrapolating the straight line portion to the energy axis and it is found to be different concentrations, a) 0.04 M CdSO4 + 0.4 M Na2S2O3 + 0.06 M EDTA, b) 0.06 M CdSO4 + 0.6 M Na2S2O3 + 0.08 M EDTA . c) 0.08 M CdSO4 + 0.8 M Na2S2O3 + 0.1 M EDTA. The band gap was found to be 2.40eV, 2.24eV & 2.13eV. for different concentration. The optical absorption studies show that the as deposited CdS thin films have decreasing band gap energy with increasing bath concentration.

 

Fig.6. X-ray diffraction pattern for CdS film deposited on stainless steel substrate for different concentrations,

(a) 0.04 M CdSO4+ 0.4 M Na2S2O3 + 0.06 M EDTA, (b) 0.06 M CdSO4 + 0.6 M Na2S2O3 +0.08 M EDTA, (c) 0.08 M CdSO4 + 0.8 M Na2S2O3 + 0.1 M EDTA,

 

Fig.7. Plot of (αhν)2  versus (hν) for determination of band gaps of CdS films as deposited for different concentrations,

(a) 0.04 M CdSO4+ 0.4 M Na2S2O3 + 0.06 M EDTA,

(b) 0.06 M CdSO4 + 0.6 M Na2S2O3 +0.08 M EDTA,

(c) 0.08 M CdSO4 + 0.8 M Na2S2O3 + 0.1 M EDTA,

 

4. CONCLUSION:

In conclusion, DEHI technique is useful for determination of certain thin film parameters such as film thickness, film mass deposition and the stress to substrates. This is the easiest technique and gives better information as compared to other conventional technique. It is observed that as the time of deposition increases, the number of fringes localized on the surface of stainless steel substrate increases with increase in deposited mass and decrease in stress to substrate consequently decreasing the fringes width.

 

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Received on 12.05.2010        Modified on 20.05.2010

Accepted on 24.05.2010        © AJRC All right reserved

Asian J. Research Chem. 3(4): Oct. - Dec. 2010; Page 950-954