Adv. Studies Theor. Phys., Vol. 5, 2011, no. 4, 171-183 Studies on CdS Nanocrystalline Thin Films with Different S/Cd Ratios Prepared using Chemical Bath Deposition Method C. Gopinathan*, T. Sarveswaran and K. Mahalakshmi PG and Research Department of Physics The Madura College, Madurai 625 011, India K. Saravanakumar PG and Research Department of Physics Kongunadu Arts and Science College Coimbatore -29, India * Corresponding author cgnmc64@gmail.com, dr.k.saravanakumar@gmail.com Abstract Cadmium sulphide (CdS) a wide energy gap semiconductor has emerged as an important material due to its applications in photovoltaic cell as window layers, optical filters and multilayer light emitting diodes, photo detectors, thin film field effect transistors, gas sensors and transparent conducting semiconductor for optoelectronic devices, CdS in naturally an n-type material with an optical band gap of 2.4ev. Cadmium sulphide (CdS) thin films of different S/Cd ratio were prepared by the chemical bath deposition technique (CBD) onto well-cleaned glass substrates at 80 C and constant ph value 10. The structural characterization was carried out by X- ray diffraction (XRD), Scanning electron microscopic (SEM) and atomic force
172 C. Gopinathan et al microscopy (AFM) studies. The composition of the various constituents in CdS thin films has been determined by energy dispersive X-ray analysis (EDAX). Keywords: CdS thin films, chemical bath deposition, AFM 1. Introduction Cadmium sulphide (CdS) a wide energy gap semiconductor has emerged as an important material due to its applications in photovoltaic cell as window layers [1], optical filters and multilayer light emitting diodes, photo detectors, thin film field effect transistors, gas sensors and transparent conducting semiconductor for optoelectronic devices, CdS in naturally an n-type material with an optical band gap of 2.4 ev. This paper deals with the structural analyses of CBD CdS thin films prepared by different S/Cd ratio. 2. Experimental procedure Based on operation, experimental set up is employed for the growth of the films from bulk precipitation of the solution [2]. In this technique substrates and solution are stationary and solution is stirred with the help of magnetic stirrer. Water or paraffin baths with constant stirring are used to heat the chemical bath to the desired temperature. In some cases, stirring is continuous from room temperature, while in some cases, it is started after attaining the desired temperature CBD CdS films are prepared in an aqueous solution that contains NH 4 OH, cadmium acetate, ammonium acetate, and thiourea. The deposition is carried out at a temperature of 85-90 C. The ph of the solution as measured at the start of the process is about 10. During the deposition the solution is being continuously stirred. The deposition conditions are summarized in table I. Slow deposition rates help minimize the formation of CdS precipitates in the solution (homogeneous reaction). After the deposition the substrates are first rinsed with DI water to remove any CdS particulates from the surface, and then rinsed under running DI water prior to being dried.
Studies on CdS nanocrystalline thin films 173 Table 2.1: Compositions of chemical bath for the preparation of CdS thin films Experiment number Cd(CH 3 COO) 2 Cadmium acetate S/Cd Thiourea Deposition Time (min) Molarity Molarity 1 0.500 1.00 0.5 90 2 0.475 1.05 0.5 90 3 0.450 1.11 0.5 90 4 0.425 1.18 0.5 90 5 0.400 1.25 0.5 90 3. Results and discussion 3.1 Structural studies Fig.3.1 shows the XRD pattern of the CdS thin films prepared at different S/Cd ratio such as 1, 1.05, 1.11, 1.18 and 1.25 on corning glass substrates. The diffraction peak around 26.5º indicates the films are crystallize in wurtzite phase with the hexagonal crystal structure [3]. The intense peak along (002) plane in all the films indicates that the films are highly oriented along the c axis. The film prepared at equal molecular ratio of Cd and S shows an amorphous nature. There is some crystallinity appeared in the film prepared at a S/Cd ratio of 1.05 ie decreased cadmium content in the precursor solution. The crystallinity of the films increased further by decreasing the cadmium content in the precursor solution. A good crystalline film is obtained at an S/Cd ratio of 1.18 and 1.25. This can be observed in the XRD peak at a higher intensity than the other films. The increase in the ratio of S/Cd ratio from 1.18 to 1.25, the intensity of the peak is decreased.
174 C. Gopinathan et al Fig. 3.1: X-ray diffraction pattern of CdS film prepared at different ratio of S/Cd From the peak position and the integral width at half maximum of the respective peak, the particle sizes in the S/Cd ratio in films are calculated; the average crystallite size with c-axis orientation is estimated from the Debye-Scherrer formula [4], as follows: 0.9λ D XRD =, (3.1) B cosθ Where D XRD is the mean particle size, λ = 0.15406 nm is the x-ray wavelength, θ is the Bragg diffraction angle and B is the full width at half maximum (FWHM) of the diffraction peak, respectively. The calculated crystallite sizes of the different S/Cd ratio of CdS films are given in Table 4.1. The interplanar distance (d) is calculated for all set of films using the Bragg equation d = nλ/2sinθ. From the d values, lattice constant c is calculated using the following relations, [5],
Studies on CdS nanocrystalline thin films 175 1 d 2 2 2 2 4 h + hk + k l = + 2 2 3 a (3.2) c On the (002) plane, the above equation reduces to c = 2d (3.3) It is found that the lattice constant c for hexagonal close packed CdS is equal to twice the inter-planar spacing (d) of the basal planes. Fig. 3.2: Variations of inter-planar spacing and lattice constant with S/Cd ratio in CdS films Fig. 3.2 shows the variation of d and c values with S/Cd ratio of CdS thin films. From the figure it is observed that the while S/Cd ratio of CdS thin films increases or in other words in decreasing in the Cd Mol % in the film the inter planar distance and lattice constant c are linearly decreases. But the particle size of the film is increased while increasing the S/Cd ratio of CdS thin films (table 3.1). The increase in the particle size of the film indicates that the crystalline property of the film is increased.
176 C. Gopinathan et al Table 3.1: Structural parameters and AFM data s of CdS film deposited at different S/Cd ratios on corning glass substrates S/Cd ratio D XRD (nm) d (Å) c (Å) R q (nm) 1.00 - - - 12.28 1.05-3.353 6.706 21.00 1.11 31.05 3.347 6.694 18.81 1.18 46.58 3.343 6.686 13.90 1.25 46.63 3.337 6.674 9.462 3.2 Surface Morphological Studies 3.2.1 Scanning Electron Microscope Figure 3.2 (a-e) shows the scanning electron micrographs for the sample prepared under five different S/Cd ratios (1, 1.05, 1.11, 1.18 and 1.25). It is observed from the figure that there in no uniform coalescence takes place in the film at an S/Cd ratio of 1 and 1.05. The decrease in the Cd percentage in the film increases the film smoothness. The rms roughness was calculated for all films using AFM and is given in table 4.1. There is a uniform distribution of particles in the film and improved coalescence observed in the film at a ratio of 1.11. This shows that the Cd particles are grouped together and restricting the coalescence of the films. (a) (b)
Studies on CdS nanocrystalline thin films 177 (c) (d) (e) Fig. 3.3: Scanning Electron Micrographs of CdS thin films prepared at S/Cd ratio of a) 1, b) 1.05, c) 1.11, d) 1.18 and e) 1.25
178 C. Gopinathan et al 3.2.2 ATOMIC FORCE MICROSOCOPE Fig. 3.3 (a-e) shows the atomic microscopy images of CdS thin films at different S/Cd ratios. At an S/Cd ratio of 1, the particles are non-uniformly dispersed. The decrease in the Cd content or increase in the S/Cd ratio increases in the coalescence and the crystalline property also increases. This can be observed from the columnar structure of grouped particles. Columnar structure is a typical structure of hexagonal CdS film. Also it is observed from the fig.4.3 that the particle size of the films is increased further by decreasing the Cd content in the film. The AFM results also well correlated with SEM results.
Studies on CdS nanocrystalline thin films 179 2D (a) 3D 2D (b) 3D 2D 3D (c) Fig. 3.4: Atomic Force Micrographs of CdS thin films prepared at S/Cd ratio of a) 1, b) 1.05 and c) 1.11
180 C. Gopinathan et al 2D (d) 3D 2D (e) 3D Fig. 3.4: Atomic Force Micrographs of CdS thin films prepared at S/Cd ratio of d) 1.18 and e) 1.25
Studies on CdS nanocrystalline thin films 181 Root mean square roughness is the often used amplitude parameter equal to the surface heights relative to the least square fitted line of the profile. It is calculated by the following relation, L 1 2 rms roughness = z (x) dx L (4.4) 0 1 2 = Z(x) i (4.5) N L L where, L is the cutoff length, N L is the number of discrete measurement point and Z i (x) is the height deviation of the profile from a least square fitted line that eliminates the slope or shape of the entire profile. The roughness values are given in table 4.1. The values depicts that the roughness of the films decreased from 18 to 9 nm while the ratio S/Cd increases from 1.1 to 1.25, which indicates the film quality is increased while decreasing the Cd content in the solution bath. 3.3 ELEMENTAL ANALYSIS In material characterization, it is important to determine how an element is distributed laterally and to find the inclusions on the surface. This is most conveniently done by using a focused probe of x-rays or ion that is scanned over the surface and the characteristic elemental signals are used to produce an elemental map of the surface. Fig. 4.5 shows the EDAX spectrum of CdS films prepared at different S/Cd ratios. The spectrum confirms the Cd and S atoms are presented in the film. There is no impurity atoms are present on the film which indicates the purity of the films. (a) (b)
182 C. Gopinathan et al (c) (d) (e) Fig. 3.5: EDAX Spectrum of CdS thin films prepared at S/Cd ratio of a) 1, b) 1.05, c) 1.11, d) 1.18 and e) 1.25 4.1 Conclusion The CdS films are prepared by low cost chemical bath deposition technique with different ratio of S and Cd elements. The films are deposited on the corning glass substrates at uniform temperature with a ph value between 9 and 10. The structural properties of the films was analysed using XRD pattern and the surface morphological studies was made using SEM and AFM s micro and nanographs. The elemental study was made using EDAX spectrum.
Studies on CdS nanocrystalline thin films 183 The results indicate that the CdS film crystallized in hexagonal structure and decrease in the Cd content in bath solution increases the crystalline property of the films. The particle size, lattice constant and inter-planar distance was calculated using Sherrer formula and Bragg relation. The surface morphological studies indicate that the decrease in the Cd content improves the surface smoothness. From the elemental studies it is evidenced that the film having only Cd and S atoms with free from impurity atoms from the buffer and other precursors. References [1] McCandless BE, Hegedus SS.(1991), Influence of CdS window layer on Thinfilm CdS/CdTe solar cell performance. Proceedings of the 22th IEEE photovoltaic specialists conference, pp.967-972. [2] R.S. Mane and C.D. Lokhande, Materials Chemistry and physics, 65 (2000) 1-31. [3] S. Prabahar and M. Dhanam, Journal of Crystal Growth 285 (2005) 41. [4] X.B Wang, D.M. Li, F. Zeng and F. Pan, J. Phys. D: Appl. Phys. 38 (2005) 4104. [5] Fumitaka Goto, Katsunori Shirai, Masaya Ichimura, Solar Energy Materials and Solar Cells, 50 (1998) 41. Received: December, 2010