Contemporary Engineering Sciences, Vol. 5, 2012, no. 1, 1 - 8
Optical Characterization of Cadmium Sulphide Films Grown by Newly Modified Hot Substrate Chemical Bath Colloidal Route Y. C. Goswami Department of Engg Physics, Institute of Technology and Management Sithouli Gwalior M P India 474001
[email protected] Rajeev Singh Department of Engg Chemistry Institute of Information Technology and Management Sithouli Gwalior M P India 474001 Ranjana Sharma Department of Engg Chemistry, Institute of Technology and Management Sithouli Gwalior M P India 474001 D Kumar PG Department of Chemistry, SMS Govt Science College Gwalior M P India 474001
Abstract A newly modified hot substrates colloidal route was developed for the growth of CdS films on glass and SnO2 coated substrates. The reaction chamber was made up of glass and consists of substrate holder arrangement such that the samples kept at relatively higher temperature than the solution. Glass and SnO2 coated glass used as substrate. The colloidal mixture made in deionized water consists of CdCl2 and Thiourea, which is complexed by a mixture of Ammonia and EDTA.
2
Y. C. Goswami et al
Results show that good quality CdS films can be obtained at a S/Cd ratio greater than 4, at bath temperatures between 40-45ºC, substrate temperature 60-70 ºC and various pH values higher than 10. Shift in absorption edge towards lower wavelength on increasing the pH of the solution is observed. This has also improved the structure of the CdS layers as evidenced by the sharpening of the optical absorption edge. This paper includes deposition details and various optical studies. Keywords: CdS, Chemical bath deposition, thin films
Introduction II VI materials attracted considerable attention from researchers due to their wide range of applications in the field of solar cells and other optoelectronic devices. [1]. CdS thin films are very important semiconductor and can be used as the window layer for Solar cell purpose. Presently CdS gain much attention as material for preparation of quantum dots whose properties are much different then bulk due to their large surface to volume ratio and their reduced size. In quantum dots as a result of geometrical constraints the electron feel the particle boundaries and responds to particle size by adjusting its energy, resulting in increase in bandgap [2]. CdS films are prepared by many methods like Vacuum evaporation, sputtering, electrodeposition Metal organic Vapour phase epitaxy, Spray pyrolysis, CBD (Chemical Bath Deposition), Flowing liquid method and Laser ablation [3-11] etc. Chemical bath deposition (CBD) is a process to achieve high quality films having tunable band gaps. Stiochiometry of the films depends upon deposition conditions like pH of the solution, temperature, stirring and deposition time. The CBD has recently drawn much attention due to its very simple instrumentation and low cost. This method requires low temperature as compared to other methods and since no fumes are given out, it is less hazardous then other methods. At the same time it is also realized that substrate temperature is also play a crucial role to get CdS films. Generally the substrates in CBD are at lower temperature then container. In the solution the substrate gains very small amount of heat from the solution. On the other hand the container receive more heat from the heater in addition to heat by solution. Which results in more growth on walls of the container then the substrates. In this paper we presented the newly modified reaction chamber and optimized the parameters like bath temperature and pH for the growth of CdS films.
Experimental Films are grown by newly modified hot substrate method. The reaction chamber shown in Figure 1 made up of glass and consists of substrate holder glass jacket
Optical characterization of cadmium sulphide films
3
such that the samples kept at relatively higher temperature than the solution by using heater in the jacket. Glass and SnO2 coated glass used as substrate. The substrate were fixed on the outer side of the jacket. The colloidal mixture made in deionized water consists of CdCl2 and Thiourea, which was complexed by a mixture of Ammonia and acetonitrile. The pH of the solution was maintained between 9 to 11 by using NH4OH solution. The bath temperature was in the range 40-450C. The bath was continuously stirred during the growth of the films, using a Magnetic Stirrer. To improve quality the stirring was kept at a very slow rate. The film was deposited within 10-15 minutes, after which the coated substrate was pulled out from the bath and dried after thorough cleaning.
Fig. 1. Hot substrate Chemical bath Setup The growth conditions were optimized using Optical Transmission spectra, which were obtained using Shimadzu Model Spectrophotometer. The thickness of some of the films were obtained by using the Tolansky method.
Results and Discussions Good yellow thin film is observed within 20-30 minutes. Since the substrates were kept at relatively higher temperature than the solution, the maximum deposition occurs on substrate than anywhere else. It has also been observed that
4
Y. C. Goswami et al
the thickness is limited by the growth powdery non sticky growth. Interference pattern suggests that the films are adherent and have uniform thickness. The film growth on substrate is dominant than the reaction chamber. The preffered growth on hot substrates reduces the unnecessary unwanted growth on anywhere then the substrates. At the same time it is also cost effective since material is fully utilized. Transmission spectra were obtained for these CdS films at various pH values and S/Cd ratios. Results show that good quality CdS films can be obtained at a S/Cd ratio greater than 4, at bath temperatures between 40-45ºC and at various pH values higher than 10. Figure 2 shows the transmission curves for CdS films prepared at various pH values (Sample 1: t%(14at) at pH 8; Sample 2: tm at pH 9; Sample t% (17at) at pH 10 and Sample 4: at pH 11). Increase in pH improves the structure of the CdS layers as evidenced by the sharpening of the optical absorption edge. Shift in bsorption edge towards lower wavelength on increasing pH of the solution is observed.
100 90 80 70 60
t%(17a)
50
t%(17at)
40
tm
30
t%(14at)
20 10 0 400
450
500
550
600
650
700
750
800
Fig.2. Transmission spectra for CdS films at various pH values. The optical band gap of the films were obtained by using following equation [10]. / Where is absorbance, K is constant m is equal to 1 for direct and 2 for indirect transitions. In order to obtain the band gap, graph between (αhν)2 and hν in is plotted and shown in in Fig. 3. Linearity of the plot indicates that the materials is of direct
5
Optical characterization of cadmium sulphide films
band gap in structure. The extrapolation of the straight line to the energy band gap of the material.
=0 axis give
160 140 120 100 80 60 40 20 0 1
1,2
1,4
1,6
1,8
2
2,2
2,4
2,6
2,8
Fig. 3. (αhν)2 and hν graph for CdS films at various pH values. The intercept on the hν axis gives a direct optical band gap around 2.4 eV, which agrees closely with the band gap of bulk CdS. The extrapolation of these graph give the energy band gap increment on increasing pH. This clearly shows that the films are nanocrystalline as bulk materials doesn’t exhibit this change in gap with grain size. Curves also show step like nature in the region where absorption occurs. This could be due to discrete energy levels and step like density of states for nano materials instead of quasicontinuous levels in conduction and valance band for bulk materials. Fig. 4. Shows the variation of coefficient of absorption (α) with photon energy hν for CdS films . The magnitude of coefficient of absorption with the range is suitable for polycrystalline thin films solar cells. Shift in spectra for pH 8 to 11 show that the absorption are blue shifted with respect to bulk CdS, indicates quantum confinement in nanoparticles . The refractive index of thin films can be calculated form transmission data using method proposed by swanpoil [12]. Fig. 5 is showing the graph between refractive index and wavelength the basis of calculation Fig 6 is showing refractive index
6
Y. C. Goswami et al
variation with respect to wavelength. Refractive index is stabilize in the range 500-900 nm.
alpha
5
alpha
4,5
alpha
4
alpha
3,5 3 2,5 2 1,5 1 0,5 0 0
0,5
1
1,5
2
2,5
3
Fig. 4. Graph between Coefficient of absorption ( α) with photon energy hν for CdS films 14 12 10 Series1
8
Series2
6
Series3
4
Series4
2 0 400
500
600
700
800
900
1000
Fig. 5. Graph between refractive index and wavelength
7
Optical characterization of cadmium sulphide films
The graph between extinction coefficient and the energy hν is shown in the Fig. 6. Fall in absorption is due to the variation in absorbance with decrease in energy due to existence of absorption edge located at λ500nm. 180 160 140 120 100 80 60 40 20 0 1
1,2
1,4
1,6
1,8
2
2,2
2,4
2,6
2,8
Fig. 6 Graph between extinction coefficient and the energy hν
Conclusions A newly modified hot substrates colloidal route has been developed for growth of CdS films. Keeping the substrates at high temperature than the bath results in more growth on wall of the container than the substrates. Good yellow thin film is observed within 20-30 minutes. Since the substrates were kept at relatively high temperature than the solution the maximum deposition occurs on substrate than anywhere else. It has also been observed that the thickness is limited by the growth powdery non sticky growth.
Acknowledgements. Corresponding author is thankful to funding agencies All India council for technical education (AICTE) and MP Council of Science and Technology (MPCST) for funding this project. Authors are also thankful to Shri Daulat Singh Chauhan, Executive Director ITM Universe, for his continuous encouragement and motivation to perform the research work.
8
Y. C. Goswami et al
References 1. T Ohashi, K Inaakoshi Y Hashimoto, K Ito, Sol. Energy Mater. And Solar cells, 50,37,(1998). 2. N. Revaprasadu and S. N. Miondo, Pure Appl. Chem. , 78, 9, 1691, (2006). 3..J.Santamaria, Solar cells, 28, (1990), 31 4. .B.E. McCandless, A.Mondal, R.W.Birkmire, Solar energy materials and solar cells, 36, (1995), 369 5. Steffen Preusser and Michael Cocivera, Solar Energy Materials, 20 (1990), 1 6. S.Yamaga,A. Yoshikawa, J.Cryst Growth 117(1992) 353 7. K.T.R. Reddy, P.J.Reddy. J.Physics D. Appl. Phys.,25, (1992), 1345 8. M.E. Ozsan, D.R. Johnson,M.Sadeghi and D.Sivapethasundarm,IEEE, (1994) 9. S. Kuranouchi, T.Nakazawa, A.Ashida, N. Yamamoto, Solar Energy Materials and Solar cells 35, (1994),185 10. S Keitoku, H Ezumi, H Osono, N Ohto, Jpn J App Phys 34, 138, (1995). 11. B. Subrmanian, C Sanjeevviraju, M Jaya chandran, J Cryst Growth, 234,421,(2002). 12. M V Kurik, Phys Status Solidi, A 8, 9 (1971). 13. J Sanchez-Gonzalez, A. Diaz Parralego, A L Ortiz , F Guibeartean, Applied Surface Science, 252, (2006) 6013. 14. D Sankia, P K Gogoi, P K Sankia, Chalognide letters, VOl 7 No 5 May 2010, 317. Received: August, 2011
