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Some structural studies on successive ionic layer adsorption and reaction (SILAR)-deposited CdS thin films
Applied Surface Science 181 (2001) 277±282
Some structural studies on successive ionic layer adsorption and reaction (SILAR)-deposited CdS thin ®lms C.D. Lokhandea,*, B.R. Sankapala, H.M. Pathana, M. Mullerb, M. Giersigb, H. Tributschb a
Thin Film Physics Laboratory, Department of Physics, Shivaji University, Kolhapur 416 004, India b Hahn Meitner Institute, Glienicker Strasse-100, D 14109 Berlin, Germany Received 3 April 2001; accepted 4 July 2001
Abstract Cadmium sul®de thin ®lms have been deposited by a simple and inexpensive successive ionic layer adsorption and reaction (SILAR) method from aqueous as well as non-aqueous media. The CdS ®lms have been characterized by X-ray diffraction (XRD), scanning electron microcopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray analyses (EDAX) and Rutherford back scattering (RBS). X-ray studies showed the hexagonal crystal structure of CdS ®lms. The surface morphology is found to be smooth and dense from SEM images for both the ®lms. High resolution TEM (HRTEM) showed that the ®lms consist of nanoparticles. The EDAX and RBS studies showed stoichiometric formation of CdS from both the media. Inclusion of oxygen is observed from RBS studies. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Thin ®lms; CdS; SILAR; Film characterization
1. Introduction Nanocrystalline materials are novel materials, which are not only scienti®cally interesting but also hold great potential for varied applications. Their properties are different and often superior to those of conventional coarse-grained polycrystalline materials and also amorphous alloys of the same composition. Nanocrystalline materials are polycrystalline material with grain sizes of up to 100 nm. Because of the extremely small dimensions, a large fraction of the atoms in these materials is located in the grain boundaries and this con®rms special attributes. Nanocrystalline materials can be prepared by inert gas *
Corresponding author. Tel.: 91-231-690571; fax: 91-231-690533. E-mail address: [email protected] (C.D. Lokhande).
condensation, mechanical alloying, plasma deposition, spray conversion processing and many other methods. Nanocrystalline materials exhibit increased strength, hardness, enhance diffusivity, improved quality, roughness, reduced elastic modulus, higher electrical resistivity, increased speci®c heat, higher thermal expansion coef®cient, lower thermal conductivity and superior soft magnetic properties in comparison to conventional coarse-grained materials [1]. Thin ®lms of CdS have received considerable attention during the recent years because of their actual and potential applications in variety of semiconductor devices such as photoconductors, photoresisters, transistors image magni®cation and recently in solar cells and light activated valves for large screen liquid crystal display. CdS ®lms have been prepared using various chemical methods, including electorodepostion [2], chemical bath deposition [3,4], spray pyrolysis [5]
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 3 9 2 - 0
278
C.D. Lokhande et al. / Applied Surface Science 181 (2001) 277±282
and screen printing [6]. Chemically deposited thin ®lms of CdS have been used as buffer layers in CuInS2-, CuInSe2- and CuInGaSe2-based solar cells. The requirements of a buffer layer such as high resistivity, high bandgap and total coverage of the absorber layer with a small thickness of about 20± 30 nm are satis®ed by chemically deposited CdS ®lm. Successive ionic layer adsorption and reaction (SILAR) method belongs to chemical methods, may be termed as step wise chemical deposition. In chemical bath deposition method, deposition of metal chalcogenide semiconductor thin ®lm occurs due to substrates maintained in contact with dilute chemical baths containing the metal and chalcogen ions. The ®lm formation takes place when ionic product exceeds solubility product. This also results into precipitate formation into the solution and control over the process is lost. In order to avoid such dif®culties, SILAR method has been developed. It is based on the immersion of the substrate into separately placed cationic and anionic precursors and rinsing between every emission with ion exchanged solution (water) to avoid homogeneous precipitation. Some reports are available for the deposition of CdS ®lms by SILAR method [7,8] from aqueous medium but no report is available for the deposition of CdS ®lms from non-aqueous medium. In our previous report, we have deposited CdS ®lms from aqueous medium by SILAR method [9]. By modifying some deposition conditions, here attempt is made to deposit nanocrystalline CdS ®lms from aqueous and non-aqueous media. The optimized ®lms are characterised by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDAX) and Rutherford back scattering (RBS) techniques.
2. Experimental 2.1. Substrate cleaning Glass microslides 26 mm 76 mm 1 mm were boiled in chromic acid for 15 min washed with laboline, rinsed in acetone and ®nally ultrasonically cleaned with double distilled water before use. 2.2. Sample preparation from aqueous Loba analytical reagent grade cadmium acetate and sodium sul®de were used in the deposition of cadmium sul®de thin ®lms. The cationic precursor for CdS was 0.0125 M cadmium acetate with pH 5. The source for sul®de ions was 0.05 M sodium sul®de with pH 12. For the deposition of CdS thin ®lms, a well-cleaned glass substrate was immersed in the cationic precursor solution (cadmium acetate) for 20 s, causing cadmium ions to be absorbed on the surface of glass substrate. This substrate was immersed in an ample amount of double distilled water for 20 s to prevent homogenous precipitation. The substrate was then immersed in anionic precursor solution (sodium sul®de) for 20 s. Sul®de ions reacted with the adsorbed cadmium ions on the glass substrate. The substrate was then immersed in double distilled water for 20 s. Thus, one SILAR cycle of CdS ®lm deposition was completed. Such 160 SILAR cycles were repeated. The deposition was carried out at 308C, the preparative parameters used for the deposition of CdS ®lms from aqueous medium are summarized in Table 1. Thickness of CdS ®lm from aqueous medium is 0.28 mm.
Table 1 Optimized preparative parameters for the deposition of CdS thin ®lms Parameters
Sources Concentration (M) Immersion time (s) Immersion cycles Temperature (8C)
Aqueous medium
Non-aqueous medium
Cationic
Anionic
Cationic
Anionic
Cadmium acetate 0.0125 20 160 30
Sodium sulfide 0.05 20 160 30
Cadmium acetate 1.0 10 40 95
Thiourea 1.0 10 40 95
C.D. Lokhande et al. / Applied Surface Science 181 (2001) 277±282
279
2.3. Sample preparation from non-aqueous media For the deposition of CdS ®lms from non-aqueous medium, ethylene glycol was used as a solvent. By repeating same procedure as discussed in Section 2.2, CdS ®lms were prepared from non-aqueous medium.
Fig. 1. X-ray diffraction patterns of CdS ®lms on glass substrate from; (a) aqueous and (b) non-aqueous media.
Fig. 2. Scanning electron micrographs of CdS ®lm from; (a) aqueous and (b) non-aqueous media at magni®cation 10,000 status bar length is 2 mm.
Fig. 3. High-resolution TEMs for CdS ®lms from; (a) aqueous and (b) non-aqueous media.
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C.D. Lokhande et al. / Applied Surface Science 181 (2001) 277±282
Here, ion exchanged solvent was ethylene glycol instead of double distilled water. The optimized preparative parameters are listed in Table 1. Thickness of CdS ®lm from non-aqueous medium is 0.37 mm 3. Characterization The structure of CdS ®lms deposited by SILAR method was examined by Philips X-ray diffractometer PW-1710 in a y±2y coupled geometry using a copper Ê ). Scanning electron microanode (with l 1:5418 A graphs were taken with a Cambridge Stereoscan 350 MK after coating with gold palladium (thickness Ê ) via a polaron SEM sputter coating unit E-2500. 100 A Transmission electron microscope analyses were
performed with a Philips CM±12 electron microscope Ê ) equipped with EDAX-DX-4 (point resolution, 2.8 A analyzer to measure qualitatively the sample stoichiometry. For the preparation of samples for TEM, the thin ®lm of CdS deposited on glass was scratched and Ê) placed on to an amorphous carbon coated (ca. 50 A grid. The grid was then transferred to an electron microscope. A number of such grids were prepared from each sample in order to check the reproducibility of the preparation procedure. The RBS analyses were carried out using 2.0 MeV 4He ions. The CdS ®lm was deposited on glassy carbon substrates. The incident beam was normal to the ®lm surface and semiconductor detector (implanted Si detector, solid angle 3:4 10 3 sr) was mounted at the scattering angle of 1708. The energy resolution was better than
Fig. 4. The EDAX spectra of CdS ®lms (on glass substrate) from; (a) aqueous and (b) non-aqueous media.
C.D. Lokhande et al. / Applied Surface Science 181 (2001) 277±282
20 keV. About (1±3 105 counts per peak were sampled to ensure a statistical error of 1%. 4. Results and discussion Fig. 1(a) and (b) shows XRD patterns of CdS ®lms deposited from aqueous and non-aqueous media, respectively. It is evident that the CdS ®lms from both the mediums are amorphous or consists of small grains. The broad hump is due to the amorphous glass substrate. Both the ®lms formed are hexagonal crystal structure [10]. Scanning electron microscopy technique is well known to study the surface morphology of the ®lms. Fig. 2(a) and (b) shows SEM photographs of CdS ®lms from aqueous and non-aqueous media at magni®cation 10,000. It can be seen that all ®lms are dense, smooth and homogenous without visible pores. The ®lm surfaces are seen to be continuous with some overgrowth of particles. Lincot and Borges [11] have proposed a duplex structure of CdS ®lms and reported that the longer deposition times diverge in the nucleation. This phenomenon has been attributed to the transition between the growth of a compact adherent and homogenous ®lm and that of a porous layer containing a large amount of trapping electrolyte. It is also reported that once the porous grown mechanism occurs, growth of the inner compact layer stops almost completely. At long reaction times, it is clear from SEM photographs obtained on relatively thick layers that grains are of larger surface size. This means that during the reaction there is probably a progressive decrease in repulsive strength between the colloids. Due to the poor sample conductivity, however, it was impossible to use intense electron beams on the bare sample. Fig. 3(a) and (b) shows transmission electron micrograph of CdS ®lms deposited on glass substrate from aqueous and non-aqueous media. In this bright ®eld images were taken under the condition of minimum phase contrast. By high resolution, random orientation of nanoparticles can be clearly seen for both the samples. The Fourier transformation for CdS ®lm from aqueous medium showed the typical lattice spacing of hexagonal CdS (for example h k l 1 0 0, Ê and h k l 1 0 1, d 3:17 A Ê ). Small d 3:58 A nanoparticles with diameters of 7±8 and 6±7 nm are
281
clearly recognized, for CdS ®lms from aqueous and non-aqueous media, respectively. The quantitative analyses by EDAX were performed for CdS ®lms on various samples at different points. The average atomic percentage of CdS was 51.9:48.1 and 52.3:47.7 from aqueous and non-aqueous media, respectively, showing that both the samples are in good stoichiometric form. Typical EDAX spectra for CdS from aqueous and non-aqueous media are presented in Fig. 4(a) and (b), respectively. The RBS spectra in Fig. 5(a) and (b) display the number of detected He ions as a function of their energy which is plotted together with a simulated spectrum for the calculation of the simulated spectrum, homogenously distributed CdS in two different layers has been assumed. Clearly the presence of oxygen and silicon could be identi®ed by the two small extra peaks with the edge energy of 0.66 and 1.02 MeV for CdS ®lms from both the media. The composition of CdS is found in well stoichiometric form for both the samples. The presence of oxygen in chemically deposited CdS thin ®lms has been
Fig. 5. Rutherford back scattered spectra for CdS ®lm (on glassy carbon substrate) from; (a) aqueous and (b) non-aqueous media.
282
C.D. Lokhande et al. / Applied Surface Science 181 (2001) 277±282
detected by Kylner et al. [12] using the RBS technique. Further, long-range tails are observable near the low energy part of the peaks. This could be due to the following: 1. Diffusion of substrate carbon into the CdS layer. 2. Film is not closed, i.e. there are some regions on the glassy carbon substrate which are not covered by CdS ®lm. The diffusion of carbon into CdS is very unlikely because this would be possible only at very high temperatures. Therefore, the longrange tails near the low energy parts of the peaks are attributed to non-coverage of the substrate of the CdS ®lms. The introduction of silicon in the CdS ®lm might take place due to presence of carbon glassy substrate. 5. Conclusion Simple and inexpensive, SILAR method was employed to deposit CdS ®lms from aqueous and non-aquous media, at relatively low temperature. Both the ®lms are of hexagonal crystal structures and uniform surface morphology. Nanocrystalline nature is also con®rmed for both ®lms using HRTEM studies. Good stoichiometric form is observed for EDAX and RBS analyses. Inclusion of oxygen is detected from RBS studies, which may be due to uncoverage of the substrate surface. It is concluded that CdS deposition
from aqueous and non-aqueous media yield to similar morphological and structural results. Acknowledgements One of the author (CDL) is thankful to Alexander Von Humboldt Foundation, Bonn, Germany for the award of Humboldt fellowship. References [1] C. Suryanarayana, Bull. Mater. Sci. 17 (1994) 307. [2] B.M. Basol, E.S. Tseng, D.S. Lo, US Patent 4,548,681 (1985). [3] S.S. Kale, U.S. Jadhav, C.D. Lokhande, Indian J. Pure Appl. Phys. 34 (1996) 324. [4] A. Mondal, T.K. Chaudhari, P. Pramanik, Sol. Energy Mater. 7 (1983) 431. [5] R.R. Chamberlain, J.S. Skarman, J. Electrochem. Soc. 113 (1996) 86. [6] H. Matsumoto, A. Nakayama, S. Ikegami, Y. Hiroi, Jpn. J. Appl. Phys. 15 (1980) 129. [7] Y.F. Nicolau, M. Oupuy, J. Electrochem. Soc. 137 (1990) 2915. [8] Y.F. Nicolau, Appl. Surf. Sci. 22/23 (1985) 1061. [9] B.R. Sankapal, R.S. Mane, C.D. Lokhande, Mater. Res. Bull. 35 (2000) 177. [10] Diffraction Data File Card 06-0314. [11] D. Lincot, R.O. Borges, J. Electrochem. Soc. 139 (1992) 1880. [12] A. Kylner, J. Lindgreh, L. Stolt, J. Electrochem. Soc. 143 (1996) 2662.
Some structural studies on successive ionic layer adsorption and reaction (SILAR)-deposited CdS thin ®lms C.D. Lokhandea,*, B.R. Sankapala, H.M. Pathana, M. Mullerb, M. Giersigb, H. Tributschb a
Thin Film Physics Laboratory, Department of Physics, Shivaji University, Kolhapur 416 004, India b Hahn Meitner Institute, Glienicker Strasse-100, D 14109 Berlin, Germany Received 3 April 2001; accepted 4 July 2001
Abstract Cadmium sul®de thin ®lms have been deposited by a simple and inexpensive successive ionic layer adsorption and reaction (SILAR) method from aqueous as well as non-aqueous media. The CdS ®lms have been characterized by X-ray diffraction (XRD), scanning electron microcopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray analyses (EDAX) and Rutherford back scattering (RBS). X-ray studies showed the hexagonal crystal structure of CdS ®lms. The surface morphology is found to be smooth and dense from SEM images for both the ®lms. High resolution TEM (HRTEM) showed that the ®lms consist of nanoparticles. The EDAX and RBS studies showed stoichiometric formation of CdS from both the media. Inclusion of oxygen is observed from RBS studies. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Thin ®lms; CdS; SILAR; Film characterization
1. Introduction Nanocrystalline materials are novel materials, which are not only scienti®cally interesting but also hold great potential for varied applications. Their properties are different and often superior to those of conventional coarse-grained polycrystalline materials and also amorphous alloys of the same composition. Nanocrystalline materials are polycrystalline material with grain sizes of up to 100 nm. Because of the extremely small dimensions, a large fraction of the atoms in these materials is located in the grain boundaries and this con®rms special attributes. Nanocrystalline materials can be prepared by inert gas *
Corresponding author. Tel.: 91-231-690571; fax: 91-231-690533. E-mail address: [email protected] (C.D. Lokhande).
condensation, mechanical alloying, plasma deposition, spray conversion processing and many other methods. Nanocrystalline materials exhibit increased strength, hardness, enhance diffusivity, improved quality, roughness, reduced elastic modulus, higher electrical resistivity, increased speci®c heat, higher thermal expansion coef®cient, lower thermal conductivity and superior soft magnetic properties in comparison to conventional coarse-grained materials [1]. Thin ®lms of CdS have received considerable attention during the recent years because of their actual and potential applications in variety of semiconductor devices such as photoconductors, photoresisters, transistors image magni®cation and recently in solar cells and light activated valves for large screen liquid crystal display. CdS ®lms have been prepared using various chemical methods, including electorodepostion [2], chemical bath deposition [3,4], spray pyrolysis [5]
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 3 9 2 - 0
278
C.D. Lokhande et al. / Applied Surface Science 181 (2001) 277±282
and screen printing [6]. Chemically deposited thin ®lms of CdS have been used as buffer layers in CuInS2-, CuInSe2- and CuInGaSe2-based solar cells. The requirements of a buffer layer such as high resistivity, high bandgap and total coverage of the absorber layer with a small thickness of about 20± 30 nm are satis®ed by chemically deposited CdS ®lm. Successive ionic layer adsorption and reaction (SILAR) method belongs to chemical methods, may be termed as step wise chemical deposition. In chemical bath deposition method, deposition of metal chalcogenide semiconductor thin ®lm occurs due to substrates maintained in contact with dilute chemical baths containing the metal and chalcogen ions. The ®lm formation takes place when ionic product exceeds solubility product. This also results into precipitate formation into the solution and control over the process is lost. In order to avoid such dif®culties, SILAR method has been developed. It is based on the immersion of the substrate into separately placed cationic and anionic precursors and rinsing between every emission with ion exchanged solution (water) to avoid homogeneous precipitation. Some reports are available for the deposition of CdS ®lms by SILAR method [7,8] from aqueous medium but no report is available for the deposition of CdS ®lms from non-aqueous medium. In our previous report, we have deposited CdS ®lms from aqueous medium by SILAR method [9]. By modifying some deposition conditions, here attempt is made to deposit nanocrystalline CdS ®lms from aqueous and non-aqueous media. The optimized ®lms are characterised by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDAX) and Rutherford back scattering (RBS) techniques.
2. Experimental 2.1. Substrate cleaning Glass microslides 26 mm 76 mm 1 mm were boiled in chromic acid for 15 min washed with laboline, rinsed in acetone and ®nally ultrasonically cleaned with double distilled water before use. 2.2. Sample preparation from aqueous Loba analytical reagent grade cadmium acetate and sodium sul®de were used in the deposition of cadmium sul®de thin ®lms. The cationic precursor for CdS was 0.0125 M cadmium acetate with pH 5. The source for sul®de ions was 0.05 M sodium sul®de with pH 12. For the deposition of CdS thin ®lms, a well-cleaned glass substrate was immersed in the cationic precursor solution (cadmium acetate) for 20 s, causing cadmium ions to be absorbed on the surface of glass substrate. This substrate was immersed in an ample amount of double distilled water for 20 s to prevent homogenous precipitation. The substrate was then immersed in anionic precursor solution (sodium sul®de) for 20 s. Sul®de ions reacted with the adsorbed cadmium ions on the glass substrate. The substrate was then immersed in double distilled water for 20 s. Thus, one SILAR cycle of CdS ®lm deposition was completed. Such 160 SILAR cycles were repeated. The deposition was carried out at 308C, the preparative parameters used for the deposition of CdS ®lms from aqueous medium are summarized in Table 1. Thickness of CdS ®lm from aqueous medium is 0.28 mm.
Table 1 Optimized preparative parameters for the deposition of CdS thin ®lms Parameters
Sources Concentration (M) Immersion time (s) Immersion cycles Temperature (8C)
Aqueous medium
Non-aqueous medium
Cationic
Anionic
Cationic
Anionic
Cadmium acetate 0.0125 20 160 30
Sodium sulfide 0.05 20 160 30
Cadmium acetate 1.0 10 40 95
Thiourea 1.0 10 40 95
C.D. Lokhande et al. / Applied Surface Science 181 (2001) 277±282
279
2.3. Sample preparation from non-aqueous media For the deposition of CdS ®lms from non-aqueous medium, ethylene glycol was used as a solvent. By repeating same procedure as discussed in Section 2.2, CdS ®lms were prepared from non-aqueous medium.
Fig. 1. X-ray diffraction patterns of CdS ®lms on glass substrate from; (a) aqueous and (b) non-aqueous media.
Fig. 2. Scanning electron micrographs of CdS ®lm from; (a) aqueous and (b) non-aqueous media at magni®cation 10,000 status bar length is 2 mm.
Fig. 3. High-resolution TEMs for CdS ®lms from; (a) aqueous and (b) non-aqueous media.
280
C.D. Lokhande et al. / Applied Surface Science 181 (2001) 277±282
Here, ion exchanged solvent was ethylene glycol instead of double distilled water. The optimized preparative parameters are listed in Table 1. Thickness of CdS ®lm from non-aqueous medium is 0.37 mm 3. Characterization The structure of CdS ®lms deposited by SILAR method was examined by Philips X-ray diffractometer PW-1710 in a y±2y coupled geometry using a copper Ê ). Scanning electron microanode (with l 1:5418 A graphs were taken with a Cambridge Stereoscan 350 MK after coating with gold palladium (thickness Ê ) via a polaron SEM sputter coating unit E-2500. 100 A Transmission electron microscope analyses were
performed with a Philips CM±12 electron microscope Ê ) equipped with EDAX-DX-4 (point resolution, 2.8 A analyzer to measure qualitatively the sample stoichiometry. For the preparation of samples for TEM, the thin ®lm of CdS deposited on glass was scratched and Ê) placed on to an amorphous carbon coated (ca. 50 A grid. The grid was then transferred to an electron microscope. A number of such grids were prepared from each sample in order to check the reproducibility of the preparation procedure. The RBS analyses were carried out using 2.0 MeV 4He ions. The CdS ®lm was deposited on glassy carbon substrates. The incident beam was normal to the ®lm surface and semiconductor detector (implanted Si detector, solid angle 3:4 10 3 sr) was mounted at the scattering angle of 1708. The energy resolution was better than
Fig. 4. The EDAX spectra of CdS ®lms (on glass substrate) from; (a) aqueous and (b) non-aqueous media.
C.D. Lokhande et al. / Applied Surface Science 181 (2001) 277±282
20 keV. About (1±3 105 counts per peak were sampled to ensure a statistical error of 1%. 4. Results and discussion Fig. 1(a) and (b) shows XRD patterns of CdS ®lms deposited from aqueous and non-aqueous media, respectively. It is evident that the CdS ®lms from both the mediums are amorphous or consists of small grains. The broad hump is due to the amorphous glass substrate. Both the ®lms formed are hexagonal crystal structure [10]. Scanning electron microscopy technique is well known to study the surface morphology of the ®lms. Fig. 2(a) and (b) shows SEM photographs of CdS ®lms from aqueous and non-aqueous media at magni®cation 10,000. It can be seen that all ®lms are dense, smooth and homogenous without visible pores. The ®lm surfaces are seen to be continuous with some overgrowth of particles. Lincot and Borges [11] have proposed a duplex structure of CdS ®lms and reported that the longer deposition times diverge in the nucleation. This phenomenon has been attributed to the transition between the growth of a compact adherent and homogenous ®lm and that of a porous layer containing a large amount of trapping electrolyte. It is also reported that once the porous grown mechanism occurs, growth of the inner compact layer stops almost completely. At long reaction times, it is clear from SEM photographs obtained on relatively thick layers that grains are of larger surface size. This means that during the reaction there is probably a progressive decrease in repulsive strength between the colloids. Due to the poor sample conductivity, however, it was impossible to use intense electron beams on the bare sample. Fig. 3(a) and (b) shows transmission electron micrograph of CdS ®lms deposited on glass substrate from aqueous and non-aqueous media. In this bright ®eld images were taken under the condition of minimum phase contrast. By high resolution, random orientation of nanoparticles can be clearly seen for both the samples. The Fourier transformation for CdS ®lm from aqueous medium showed the typical lattice spacing of hexagonal CdS (for example h k l 1 0 0, Ê and h k l 1 0 1, d 3:17 A Ê ). Small d 3:58 A nanoparticles with diameters of 7±8 and 6±7 nm are
281
clearly recognized, for CdS ®lms from aqueous and non-aqueous media, respectively. The quantitative analyses by EDAX were performed for CdS ®lms on various samples at different points. The average atomic percentage of CdS was 51.9:48.1 and 52.3:47.7 from aqueous and non-aqueous media, respectively, showing that both the samples are in good stoichiometric form. Typical EDAX spectra for CdS from aqueous and non-aqueous media are presented in Fig. 4(a) and (b), respectively. The RBS spectra in Fig. 5(a) and (b) display the number of detected He ions as a function of their energy which is plotted together with a simulated spectrum for the calculation of the simulated spectrum, homogenously distributed CdS in two different layers has been assumed. Clearly the presence of oxygen and silicon could be identi®ed by the two small extra peaks with the edge energy of 0.66 and 1.02 MeV for CdS ®lms from both the media. The composition of CdS is found in well stoichiometric form for both the samples. The presence of oxygen in chemically deposited CdS thin ®lms has been
Fig. 5. Rutherford back scattered spectra for CdS ®lm (on glassy carbon substrate) from; (a) aqueous and (b) non-aqueous media.
282
C.D. Lokhande et al. / Applied Surface Science 181 (2001) 277±282
detected by Kylner et al. [12] using the RBS technique. Further, long-range tails are observable near the low energy part of the peaks. This could be due to the following: 1. Diffusion of substrate carbon into the CdS layer. 2. Film is not closed, i.e. there are some regions on the glassy carbon substrate which are not covered by CdS ®lm. The diffusion of carbon into CdS is very unlikely because this would be possible only at very high temperatures. Therefore, the longrange tails near the low energy parts of the peaks are attributed to non-coverage of the substrate of the CdS ®lms. The introduction of silicon in the CdS ®lm might take place due to presence of carbon glassy substrate. 5. Conclusion Simple and inexpensive, SILAR method was employed to deposit CdS ®lms from aqueous and non-aquous media, at relatively low temperature. Both the ®lms are of hexagonal crystal structures and uniform surface morphology. Nanocrystalline nature is also con®rmed for both ®lms using HRTEM studies. Good stoichiometric form is observed for EDAX and RBS analyses. Inclusion of oxygen is detected from RBS studies, which may be due to uncoverage of the substrate surface. It is concluded that CdS deposition
from aqueous and non-aqueous media yield to similar morphological and structural results. Acknowledgements One of the author (CDL) is thankful to Alexander Von Humboldt Foundation, Bonn, Germany for the award of Humboldt fellowship. References [1] C. Suryanarayana, Bull. Mater. Sci. 17 (1994) 307. [2] B.M. Basol, E.S. Tseng, D.S. Lo, US Patent 4,548,681 (1985). [3] S.S. Kale, U.S. Jadhav, C.D. Lokhande, Indian J. Pure Appl. Phys. 34 (1996) 324. [4] A. Mondal, T.K. Chaudhari, P. Pramanik, Sol. Energy Mater. 7 (1983) 431. [5] R.R. Chamberlain, J.S. Skarman, J. Electrochem. Soc. 113 (1996) 86. [6] H. Matsumoto, A. Nakayama, S. Ikegami, Y. Hiroi, Jpn. J. Appl. Phys. 15 (1980) 129. [7] Y.F. Nicolau, M. Oupuy, J. Electrochem. Soc. 137 (1990) 2915. [8] Y.F. Nicolau, Appl. Surf. Sci. 22/23 (1985) 1061. [9] B.R. Sankapal, R.S. Mane, C.D. Lokhande, Mater. Res. Bull. 35 (2000) 177. [10] Diffraction Data File Card 06-0314. [11] D. Lincot, R.O. Borges, J. Electrochem. Soc. 139 (1992) 1880. [12] A. Kylner, J. Lindgreh, L. Stolt, J. Electrochem. Soc. 143 (1996) 2662.