OPTICAL PROPERTIES OF CdS THIN FILMS OBTAINED BY CHEMICAL BATH DEPOSITION

Baku, Azerbaijan | 368

INTERNATIONAL JOURNAL Of ACADEMIC RESEARCH Vol. 3. No. 6. November, 2011, II Part

OPTICAL PROPERTIES OF CdS THIN FILMS OBTAINED BY CHEMICAL BATH DEPOSITION

Ndubuisi I. Achuko1, Chima C. Ugwuegbu2

1Department of Physics, Federal University of Technology, Owerri, 2Department of Materials and

Metallurgical Engineering, Federal University of Technology, Owerri (NIGERIA) E-mails: [email protected], [email protected]

ABSTRACT This study investigates the optical properties of Cadmium Sulphide (CdS) thin films obtained by chemical

bath deposition technique. CdS thin films were prepared by chemical bath deposition using cleaned glass substrates. The reagents concentration and other deposition conditions were varied. The films were characterized by optical absorption measurement and corresponding theoretical calculations. The optical properties showed that the films can be used for solar cell and control coating applications. Direct band gap values of 2.45eV and 2.44eV were obtained.

Key words: CdS, thin films, Chemical bath deposition, Optical properties 1. INTRODUCTION In recent years, there has been an increasing interest in the study of compound semiconductors e.g. CdS,

CdTe, SnS, CuInSe2 for solar cells [1]. Among the wide band gap II-VI semiconductors cadmium sulphide (CdS) with its direct band gap of 2.42eV

at room temperature is a promising material and is applied in wide variety of fields such as solar cells [2], thin film FET transistors [3], Light emitting diodes [4] and photonic devices [5]. Polycrystalline CdS thin films are widely used as window material [6] in several heterojunction solar cells such as CdS/CdTe, CdS/CuS, CdS/Cu(In)Se [7-8] for their favourable optical properties.

CdS thin films have been deposited by several techniques including chemical bath deposition (CBD) [9], thermal evaporation [10], spray pyrolysis [11], laser ablation [12], close space sublimation [13], molecular beam epitaxy (MBE) [14], electrochemical deposition technique [15] and SILAR deposition [16]. However, among all, CBD is a very simple and low cost technique and is suitable for large area deposition. CBD is a process to achieve high quality films by controlled chemical reaction which allows fabrication of solar cells with suitable efficiencies. The chemical mechanism of CdS CBD usually involves the thermal decomposition of thiourea [CS(NH2 )2] in an alkaline solution containing cadmium salt.

There are a number of reports [9-18] on the different structural, optical, electrical, photoelectric and physical properties of CdS thin films deposited by different methods including CBD. It is worthy to note that all points to the fact that the deposition conditions (bath composition, reagents concentrations, temperature, PH etc) have significant effect on the quality of the films. The choice of parameter for any given condition is usually found by trial and error, and we have no means of guaranteeing that any combination of parameters leads to the best possible properties [19].

Thus, in this work CdS thin films were successfully deposited by modified CBD technique and their optical properties investigated in search of optimal deposition condition.

2. EXPERIMENTAL 2.1. Chemicals All reagents were of analytical grade and were used as obtained from manufacturers without further

purification. Distilled water was used throughout the experiments. 2.2. Synthesis of CdS Thin Films The glass substrates were first soaked in aqua regia composed of ratio 3:1 concentrations of HCl and HNO3

solutions to dissolve and remove oil and any unwanted substances on their surfaces, for about 2hrs. They were then washed with detergent, rinsed with distilled water and dried in air. The cleaned and dried substrates were weighed using SLAUGTER electronic weighing balance in order to determine their initial masses.

The chemical bath constitute cadmium chloride (CdCl2) as Cd2+ ion source, TEA as complexing agent, NH3 as alkaline solution to adjust the PH of the bath and Thiourea [CS(NH2)2] as S-2 ion source, added in that order respectively. In a typical reaction, equal volumes of the 1M solutions of the reactants were added into a 50ml beaker and then stirred for about 5 minutes for stabilization. This served as a reference deposition condition and other conditions were brought about by variation of the reactants volumes which in turn varies the reactants concentrations, the PH of the bath etc, and the time of growth.

For the deposition of CdS thin films, the cleaned glass substrates were introduced vertically into the chemical baths with the help of synthetic substrate holders. Each bath was kept at room temperature and the set up left for growth to occur. Several bath compositions were employed but optimum deposition was achieved for reactants volume ratio of 2:2:2:2 and 3:3:3:2 for CdCl2, TEA, NH3 and Thiourea respectively. Thus a set of thin

369 | www.ijar.lit.az


films (C15 and C23) were obtained in a deposition time of 18hrs. After deposition, the substrates were removed from the chemical baths and thoroughly rinsed in distilled water. The deposited samples were allowed to dry and then reweighed to obtain the masses of the deposited thin films.

2.3. Characterization and Theoretical Procedures The thickness of the films in this work was obtained by gravimetric method. For optical studies, uncoated glass substrate was used as reference and optical absorbance spectra of the

samples in the spectral range of 300nm to 1000nm were measured using ALPHA-072519 Spectrophotometer. The corresponding Transmittance (T) values were calculated from the Absorbance (A) values using the relation, T = 10-

A [20] and then, the Reflectance (R) deduced from both parameters. The absorption coefficient α, of the investigated samples, which is the rate of decrease (or attenuation) of radiation intensity when it passes through a thin film sample of thickness t, was equally deduced from the transmittance values, using the diffusion (penetration) equation [21].

Ix = Io exp (-αt) (1) Where Ix is the transmitted radiation and Io is the incidence radiation. But T = I

Io So that, α (2)

The refractive index, n and the extinction (attenuation) coefficient, k of the deposited thin film samples were

also calculated. At normal incidence, the reflectance in terms of the refractive index and extinction coefficient is given by [22],

R = (( ) )(( ) )

(3) Where multiple reflection inside the thin film is ignored. In the case of semiconductors and for materials

within range of frequencies in which absorption is weak k2 << (n-1)2, the relation (3) the reduces to [23],

R = ( )( ) and n= (1+√R)

(1 - √R) (4)

The relation between the extinction coefficient and the absorption coefficient is given by [24]

α = π λ

So that 푘 = α λ π

(5) The dielectric constant is a characteristic property of a given dielectric material which varies not only from

one substance to another but also with the physical state of substances [25], was also calculated. In general the dielectric constant is a complex quantity that is related to complex refractive index through the equation

Є = Є1 + iЄ2 = (n + ik)2 (6)

Where Є1 and Є2 are the real and imaginary parts, respectively. Therefore, expanding (6) and collecting like

terms together gives Є1 = n2 – k2 and Є2 = 2nk (7)

Where the real part, Є1 relates to the refractive surface properties (Fresnel reflection coefficients) and the

imaginary part, Є2 gives the radio absorption coefficient [26]. The optical conductivity, σo (the frequency response of a material when irradiated by light) of the films was

also calculated using the relation 휎 = (8) Where c is the speed of light in vacuum. The optical band gap Eg was determined by analysing the optical data with the expression for the optical

absorbance α and the photon energy hν [11] using the relation

α = ( ν ) /

ν (9)

Where k is a constant and n is equal to one for a direct-gap material, and four for an indirect-gap material. 3. RESULTS AND DISCUSSION 3.1. Film Thickness The gravimetric method involves the use of mass of deposited CdS, density of bulk CdS material and

volume to calculated film thickness. Recall



Density, D = Mass, MVolume, V

(10) But Volume, V = tA. Where t = film thickness and A = Area of the deposited thin film given by length x width

i.e. A = L x W. Density, D is the density of bulk CdS. Thus, 푡 = (11) For samples C15 and C23, the area of deposition was found to be A15 = 12.6546 cm2 and A23 = 11.0166 cm2

respectively. Also the mass of the deposited films was M15 = 0.0036g and M23 = 0.0073g respectively. The bulk density of CdS is 4.82 g/cm3.

Therefore, t15 = .. × .

=59.02µm.

t23 = .. × .

=137.59µm. Since there was deposition on both sides of the glass substrate, the considered thickness assuming uniform

deposition on both sides of the substrate becomes t/2 so that t15 = 29.51µm and t23 = 68.80 µm. 3.2. Optical Studies The optical absorption spectra of the CdS thin films deposited at different deposition condition as depicted

on samples C15 and C23 are shown in Figure 1.

The film C15 has a low absorbance and C23 has a higher absorbance. This may be attributed to the higher

thickness of C23. It can be observed that in general, both films have decreasing absorbance from 흀 = 324nm to 흀=990nm.

The transmittance spectrum shown in Figure 2(a) indicates that the films have poor transparency in the

Ultraviolet (UV), good transmittance (45% - 85%) in the Visible and high transmittance (85% - 94%) in the Near-Infrared (NIR) regions of the electromagnetic spectrum thus can be used as UV filter. Also the higher the thickness of the film the lower the transparency as observed in Figure 2(b).

Fig. 2(a). Transmittance spectrum for thin film sample C15 Fig. 2(b). Transmittance spectrum for thin film sample C23

Fig. 1(a). Absorbance spectrum for thin film sample C15 Fig. 1(b). Absorbance spectrum for thin film sample C23



The film has a low reflectance of 20% in the UV region to 3% in the NIR region as reported in Figure 3. C23

gave almost the same reflectance as C15 in the Visible to NIR region. This property makes CdS useful in the area of solar control coating.

The absorption coefficient α, of the thin films, as illustrated in Figure 4, has a steep onset at about 500nm

for both films. It can be observed that the value of α for C23 is higher than of C15 at onset but lower at shorter wavelength region (<450nm).

The refractive index of film C15 rises in the UV region and then falls as the wavelength increases in the

Visible region as shown in Figure 5(a). However for C23, the refractive index was completely imaginary in the UV region, then rises sharply to 2.64 in the Visible region before dropping gradually to 1.87 at 흀 = 1000nm. This feature makes the film a useful material in solar control coating applications.

Fig. 5(a). Refractive Index of thin film sample C15 Fig. 5(b). Refractive Index of thin film sample C23

Fig. 4(a). Absorption coefficient of thin film sample C15 Fig. 4(b). Absorption coefficient of thin film sample C23

Fig. 3(a). Reflectance spectrum for thin film sample C15 Fig. 3(b). Reflectance spectrum for thin film sample C23



The complex dielectric constant is described by Figures 7 and 8 respectively. From Figure 7(a) the real

dielectric constant rises from 5.98 at 350nm to the maximum value of 6.98 at 400nm (i.e. in the UV region) and then decreases as the wavelength increases in the Visible region to 3.24 at 700nm. This is as a result of long deposition time, which modifies the dielectric features of mainly dielectric thin films [27].

Fig. 9(a). Optical conductivity of thin film sample C15 Fig. 9(b). Optical Conductivity of thin film sample C23

Fig. 8(a). Imaginary Dielectric Constant of thin film sample C15

Fig. 8(b). Imaginary Dielectric Constant of thin film sample C23

Fig. 7(a). Real Dielectric Constant of thin film sample C15 Fig. 7(b). Real Dielectric Constant of thin film sample C23

Fig. 6(a). Extinction coefficient of thin film sample C15 Fig. 6(b). Extinction Coefficient of thin film sample C23



Figure 9(a) shows the decrease of optical conductivity from the peak value of 2.54x1012 Ω-1m-1 at 324nm to 6.73x1011 Ω-1m-1 at 500nm and then to 7.19x1010 Ω-1m-1 at 992nm. This behaviour is caused by the dependency of optical conductivity on the nature of refractive index and absorption coefficient because at the UV region, α and n are greatest and diminishes as the wavelength increases.

As shown in Figure 10(a), the plot of (αhν)2 versus photon energy hν for film C15 was analysed using equation (9). Extrapolation of the linear portion of the plot to the (αhν)2= 0 energy axis yielded the direct band gap value of 2.45eV, which is in close agreement with reported value by Ashour [11] and the bulk CdS band gap of 2.42eV [28]. Also, similar analysis for thin film C23 yielded a direct band gap of 2.44eV as reported in Figure 10(b).

4. CONCLUSION CdS thin films were successfully deposited by modified CBD technique using cadmium chloride and

Thiourea. The films were deposited onto substrates at different deposition conditions determined by concentration of reagents. It was noticed that film thickness increased with relative reduction of Thiourea concentration. The CdS thin films have high transmittance of about 85% to 94% in the UV-Vis-NIR regions; hence they could be used as thermal control window coatings for cold weather and antireflection coatings. The investigated films exhibited a direct transition of 2.44eV and 2.45eV thus, are well-suited for solar cell applications. The results obtained suggest further investigation of the method employed.

REFERENCES

1. A. Adachi, A. Kudo, T. Sakata, Bull. Chem. Soc. Jpn, 68, 3283-3288(1995). 2. J. Britt, C. Ferekids, Appl. Phys. Lett.. 62, 285(1993). 3. R. Frerichs, J. Appl. Phys., 21, 312(1959). 4. H. Murai, T. Abe, J. Matsuda, H. Sato, S. Chiba, Y. Kashiwaba. Appl. Surf. Sci. 244, 351(2005). 5. B. Ullrich, D.M. Bangall, H. Sakai, Y. Segawa, Solide State Commu.,109, 757(1999). 6. B.M. Basol, V.K Kapur, A. Halani, Conf. Rec. 22nd IEEE, Photovoltaic Specialists Conf. Las Vigas,

NV, USA, pp-893. 7. I. Oladeji, L. Chow, C. Ferekides, V. Viswanathan, Z. Zhao, Sol. Energy Mater. Sol. Cells, 61, 203

(2000). 8. K. D. Dobson, I. Visoly-Fisher, G.Hodes and D. Cahen, Solar Energy Materials & Solar Cells, 62, 295

(2000).

Fig. 10(b). Variation of (αhν)2 with photon energy for thin film sample C23

Fig. 10(a). Variation of (αhν)2 with photon energy hν for thin film sample C15



9. A.I. Oliva, O. Solis-Canto, R. Castro-Rodriguez, P. Quintana, Thin Solid Films 391, 28(2001). 10. A. Ashour, N. El-Kadry, S.A. Raid, Thin Solid Films, 269, 117(1995). 11. A. Ashour, Turk. J. Phys.27, 551(2003). 12. S. Keitoku, H. Ezumi, H. Osono, N. Ohto, Jpn. J. Appl. Phys., 34, 138(1995). 13. D. Albin, D. Rose, R. Dhere, D. Levi, L.Woods, A. Swartzlander, P. Sheldon, 26th IEEE Photovoltaic

Specialists Conf. 1997, Anaheim, Callifornia. 14. J.T. Mullis, T. Tagushi, J. Crys. Growth, 117, 432(1992). 15. U. Madhu, N. Mukherjee, N. R. Bandyopadhyay, A. Mondal, Indian Journal of Pure & Appllied

Physics, 45, 226-230(2007). 16. H. Sun, J. Mu, Journal of dispersion science and Technology, 26, 719-722(2005). 17. X. Li, Y. Yin and X. Dong, Proc. Int. Conf. Solid Dielectrics, 270-273(2007). 18. CUI Yan, JIE Wan-qi, GAO Jun-ning, ZHA Gang-giang, He Jian-bo, Journal of Functional Materials

2,197-200(2009). 19. F. C. Eze, J. Phys. D, 31, 3(1998). 20. G. F. Lothian, Absorption spectrophotometry, Hilger and Watts Ltd., London (1958). 21. J. Thewlis, (ed) Encyclopedia Dictionary of Physics, 5, Pergamon Press, Oxford (1962). 22. N. W. Ashoroft, N. D. Mermin, Solid State Physics, Holt, Rinehart and Wilson, New York (1976). 23. A. B. Meinel, M. P. Meinel, Applied solar energy-An Introduction, Addison-Wesley, Massachussetts

(1976). 24. J. I. Pankove, Optical processes in semiconductors, Prentice-Hall, New York (1971). 25. A. O. E. Animalu, Intermediate Quantum Theory of Crystalline Solids, Prentice-Hall, New Jersey

(1977). 26. G. Bekefi, A. H. Barret, Electromagnetic Vibrations, Waves and Radiations, MIT Press, Cambridge,

P.418-420 (1987). 27. I. C. Ndukwe, Solar Energy Materials and Solar Cells, P.130 (1996). 28. B. G. Streetman, S. Banerjee, Solid State Electronic Devices, (5th ed.). New Jersey: Prentice Hall.

p. 524 (2000).

Documents

OPTICAL PROPERTIES OF CdS THIN FILMS OBTAINED BY CHEMICAL BATH DEPOSITION