Thin Solid Films 263 (1995) 145-149

Structural and optical properties of electrodeposited B&S,, Sb,S, and A@, thin films

N.S. Yesugade, C.D. Lokhande, C.H. Bhosale

Thin Film Physics Laboratory, Department of Physics, Shivaji University, Kolhapur-416 004. India

Received 13 June 1994; accepted 28 February 1995

-.

Abstract

Thin films of Bi,S,, Sb2S, and As,S, have been prepared by the electrodeposition technique from aqueous acidic baths using Na,S,O, as a sulphide source. The films deposited from an EDTA-complexed bath are thin, uniform and adherent to the substrate. The electrodeposition potentials were estimated by polarization curves. The structural and optical properties of the films have been studied. The X-ray diffraction pattern of the films shows that they are polycrystalline. The estimated optical bandgap energies for Bi,S,, Sb,S and As,& are 1.58 eV, 1.74eV and 2.35 eV respectively. - 3

Keywords: Deposition process; Optical properties; Structural properties; Sulphides

1. Introduction

Group V-VI compound thin films have potential applications in optoelectronic devices, photoelectroch- emical devices, thermoelectric coolers, solar selective and decorative coatings, etc. A number of methods have been employed for the preparation of group V-VI compound films. Pramanik and Bhattacharya [l] have obtained Bi,S, films by a solution growth tech- nique. Lokhande and Bhosale [2] have prepared B&S, films by the electrodeposition technique. Krishnamur- thy and Shivkumar [3] deposited B&S, films by a hot-wall chemical vapour deposition technique. Pawar et al. [4] obtained Bi,S,, Sb,S,, As& and Sb,_,Bi,S films by a solution-gas interface technique. Lokhande [5] and Desai and Lokhande [6-81 have prepared Sb,S, and As& films by a solution growth technique. Bhosale et al. [9] prepared Sb,S, films by spray pyrolysis.

Amongst various deposition methods, electrodepo- sition is a simple and attractive method for the preparation of thin films. The growth rate can easily be controlled through electrical quantities such as current density and deposition potential. In this paper we report the electrodeposition of B&S,, Sb,S, and As,& thin films on a variety of substrates using aqueous EDTA complexed baths. The polarisation curves are plotted to determine the deposition po-

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tentials. The films are characterised by optical absorp- tion and X-ray diffraction (XRD).

2. Experimental

Bi,S, thin films were prepared on conducting sub- strates like copper, brass, stainless steel, titanium and fluorine-doped tin oxide (FTO) coated glass (area, 4 x 2 cm’) by .electrodeposition technique. The metal- lic substrates were mirror polished by polish paper, detergent powder, brasso and finally by an ultrasonic cleaner, while conducting glass substrates were cleaned by 50% diluted HCl and then by double- distilled water. The economical and inert polished graphite plate (4 x 2 x 0.2 cm’) was used as a counter electrode. The equimolar (0.1 M) solutions of Bi(NO,), (98% pure) and Na,S,O, (99% pure) were prepared in double-distilled water. The depositions were carried out from an unstirred solution at room temperature with potentiostatic and galvanostatic con- ditions using a saturated calomel electrode (SCE) as a reference. The polarisation curves were recorded from bath compositions of Bi(NO,), and Na,S,O, in the proportion O:lO, 1:9, 2:8, 3:7, 4:6, 5:5, 6:4, 7:3, 8:2, 9: 1, 10:O. The complexing agent EDTA was used to complex the BI ‘3+ ions. The polarisation curves

146 N.S. Yesugade et al. I Thin Solid Films 263 (1995) 145-149

were recorded for various concentrations of the EDTA ranging from 0.025 M to 0.2 M in the bath at pH2.

Sb,S, and As& films were deposited respectively from 0.05M SbCl, (99% pure) and 0.1 M As,O, (99% pure) aqueous solutions with the same sulphur source. After deposition the films were washed with double-distilled water and preserved in a desiccator. The film thickness was measured by the weight differ- ence method. XRD patterns of the as-deposited and annealed films deposited potentiostatically were taken with the help of a Philips X-ray machine (PW-1710) using Cu Ka target. Optical absorption studies of the films deposited potentiostatically on FTO-coated glass substrates in the wavelength range of 400-1200nm was carried out using a UV-VIS-NIR spectrophotom- eter (Hitachi Model 330).

3. Results and discussion

The polarization curves were plotted to determine the deposition potentials of bismuth, antimony, arse- nic, sulphur, bismuth trisulphide, antimony trisulphide and arsenic trisulphide from their respective baths containing a complexing agent with different sub- strates. It has been known that the electrodeposition potential could be reduced to its standard reduction potential using a complexing agent and different substrates [lo]. The complexing agent forms a com- plex with the metal ions and acts as a reservoir of metal ions. It slowly releases the metal ions into the solution causing a change in the deposition potential.

The estimated deposition potentials of Bi, Sb, As, S on stainless steel substrates are shown in Table 1. Similarly deposition potentials of B&S,, Sb,S, and As,& onto brass, copper, stainless steel, Titanium and FTO-coated glass substrates have been deter- mined and are reported in Table 1. It is found that sulphur reduction occurs relatively at more cathodic potentials than Bi, Sb, As potentials. Fig. 1 shows polarization curves for bismuth, sulphur and Bi,S, deposited on a stainless steel substrate. The polariza- tion curves for Bi,S, deposited onto brass, stainless steel copper, titanium and IT0 are shown in Figs. 2 and 3. A similar type of polarization curves are drawn for Sb,S, and As,& and deposition potentials are estimated which are given in Table 1.

Volt (SCE 1

Fig. 1. Polarisation curves for: 0, bismuth; 0, bismuth trisulphide; and X, sulphur on stainless steel substrates.

Table 1 Estimated deposition potentials of B&S,, Sb,S, and As,& from polarisation curves for different substrates

Deposited Bath Potential V vs. (SCE) film composition

Brass Copper Stainless Titanium FTO- steel coated

glass

Bi Sb As S

Bi,S,

Sb,S,

As,%

0.1 M Bi(NO,),(pH = 2) 0.05 M SbCI, (pH = 2) 0.1 M As,O, (pH = 2) 0.1 M Na,S,O,

0.1 M Bi(NO,),+ 0.1 M Na,S,O,+ 0.1 M EDTA (pH = 2) composition (2:8) 0.05 M SbCI,+ 0.05 M Na,S,O,+ 0.05 M EDTA (pH = 2)(Composition 7: 3) 0.1 M As203+ 0.1 M Na,S,O,+ 0.15 M EDTA (pH = 2)( Composition 2 : 8)

_ - -0.1 _ _ - -0.425 _ _

- -0.25 _ _ - -1.2 _ _

-0.27 -0.20 -0.25 -0.26 -0.9

-0.25 -0.125 -0.45 -0.325 -0.5

-0.25 -0.3 -0.52 -0.425 -0.6

N.S. Yesugade et al. I Thin Solid Films 263 (1995) 145-149 147

.8 -

.6-

L

Voll (SCE)

I5 -0.20 -0.35

Fig. 2. Polarisation curves of BiZS3 for: 0, brass; 0, stainless steel; and X. copper substrates.

Volt (SCE)

o#C)+0;8 -I;6 ,

Fig. 3. Polarisation curves of B&S, for: 0, titanium; and n , FTO- coated glass substrates.

The reaction mechanisms of B&S,, Sb,S, and As& compounds can be explained as follows. Jacobsen and Sawyer [ll] reported that the S,O:- ion is not stable in acidic medium and decomposes into S and SO, in the reaction

S,O; - + 2H++ Sads + SO, -t H,O

The electrons react with sulphur as

3 S,,, + 6e -+ 3S2-

(I)

(2)

The polarization curves for bismuth, sulphur and Bi,S, were recorded using the baths containing solu- tions of 0.1 M Bi(NO,),, 0.1 M Na,S,O, and mixture of 0.1 M Bi(NO,), and 0.1 M Na,S,O, respectively and are shown in Fig. 1. The reduction of Bismuth was observed at about -0.1 Vvs. SCE. A complex of Bi3+ reacts to give

2 Bi’+ + 3SzP + Bi,S, (3)

Similarly, the formation of Sb,S, and As,& can be written as

2Sb’+ f 3S’- -+ Sb$,

and

(4)

2As”’ + 3S’- -+ As,S,

The variation of film thickness with de

(5)

at constant current density 2 mA cm -Y

osition time for Bi,S,,

Sb?S, and As2S, onto stainless steel substrate is studied and is shown in Fig. 4. The film thickness increases upto 10 min of deposition time, beyond which it remains nearly constant (OS-o.6 pm), which may be attributed to the same values of dissolution and deposition rates. Normally it is observed that films deposited from the aqueous bath result in high resis- tivity (102-10’ am). It is therefore quite possible that such semiconductors behave as insulators above thres- hold thickness [6, 121. Variation of film thickness with current density at a fixed deposition time of 5 min for Bi,S,, Sb,S, and As,S, films deposited on a stainless steel substrate is shown in Fig. 5. The thickness almost remains constant after 3 mA cm-’ current density. At high current densities the saturation of film thickness occurs at a relatively shorter time. This is also con- firmd frnm Fias 4 and 5

Figs. 6, 7 and 8 show the XRD patterns of as- deposited and annealed Bi,S,, Sb,S, and As,S, thin films respectively. The as-deposited and annealed films are polycrystalline in nature. The peak intensity has been increased significantly after annealing the films. The XRD pattern of as-deposited films of Bi,S, shows the orientations in the (430) and (610) planes. For Sb2S, thin films the observed planes are (520) and (351), and for As,S, the calculated d values are in

1 0+ I

I I

10 20 Time ( min 1

Fig. 4. Variation of film thickness with time at constant current density of 2 mA cm-’ for: l . Bi$,; X, Sb?S,; and 0, AsZS2.

148 N.S. Yesugade et al. I Thin Solid Films 263 (1995) 145-149

r

.- u. I

0.0; I I 1

Current Znsitv (rnA/cn-~~)~

Fig. 5. Variation of film thickness with current density at a constant deposition time of 5 min for: 0, B&S,; X, Sb,S,; and 0, ASS,.

ss 4 ‘5( c S.S.-st.stee1 S.S. 5500 S.-Sulptlur s (260) --II -4oo cy @4 DZl)

3%1- I 20 40 60 80 100 1

Angle, 20 (deg)

Fig. 6. XRD patterns of as-deposited and annealed B&S, thin films.

400 Unannealed

sI h.1 5.5

40 60 80 100

Angle.29 (deg)

Fig. 7. XRD patterns of as-deposited and annealed Sb,S, thin films.

good agreement with standard d values. The elec- trodeposited films were annealed .for one hour in a nitrogen atmosphere. The annealing temperature for B&S, and Sb,S, was 250 “C and 175 “C for As&. The XRD patterns after annealing in a nitrogen atmos- phere shown in Figs. 6-8 show that the crystallinity of the films has been increased significantly. The XRD pattern of annealed B&S, films shows that the (260) peak is more intense. Most of the peaks disappear in the annealing process. In the case of Sb,S, films the

700 Undnnealed A%

300; lJ ’ I 20 40 b0 80 100

Angle, 29 fdeg)

Fig. 8. XRD patterns of as-deposited and annealed As,S, thin films.

intensity of the (520) plane is increased. Though the intensity of some peaks is low there is quite a low possibility of impurity-compound deposition, as in electrodeposition, the deposition of a particular com- pound taking place at a specific deposition potential. Similar results have been reported by Savadogo and Mandal [12] in the case of Sb,S, and Lokhande [13] in the case of CuInSe,.

Optical absorption of Bi,S,, Sb,S, and As,& films deposited onto FTO-coated glass substrates has been studied in the wavelength range of 400-800 nm at room temperature and is shown in Fig. 9. The value of the absorption coefficient for Bi,S, and As,& is of the order of lO’m_‘. In order to estimate the bandgap energy E, of B&S, and As,S, the plots of (ahz~)~ vs. hv are plotted and are shown in Fig. 10. The direct bandgap energies for Bi,S, and As,& were deter- mined by extrapolating the straight portion to the energy axis and are found to be 1.58 eV and 2.35 eV respectively. The values are in agreement with the values reported by Desai [S]. The indirect bandgap of Sb,S, was determined by plotting (&zv)“~ vs. hv and

400 800 I I

1200 I r

0.2 IO.0 400 500 600 700

Wavelength A ( nm )

Fig. 9. Plot of optical density (ar) versus wavelength (A): 0, Bi,S,;

0, As,&; X, Sb,S,.’

N.S. Yesugade et al. I Thin Solid Films 263 (1995) 145- 149 149

hY’, (eV)

Fig. 10. Plot of (ahv)’ against hv for: 0, B&S,; and 0, As& films;

and ((rhv)“’ against hv for X, Sb$, thin films.

it is observed to be 1.74 eV, in good agreement with the value reported by Bhosale et al. [9].

4. Conclusions

Electrodeposition of B&S,, Sb,S, and As,S, thin films can be carried out at room temperature. The films are polycrystalline and showed improvement in crystallinity after annealing.

Acknowledgements

The authors are grateful to K.M. Gadave and A.B. Kulkarni for their experimental help. One of the authors (NSY) is indebted to the U.G.C. New Delhi for the award of a Teacher Fellowship and also to the secretary, Mahatma Phule Shikshan Sanstha, Islam- pur.

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