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Properties of pulsed electrodeposited CdIn2S4 thin film
Materials Science and Engineering B 133 (2006) 37–41
Properties of pulsed electrodeposited CdIn2S4 thin film P.P. Hankare a,∗ , A.V. Kokate b,∗ , M.R. Asabe a , S.D. Delekar a , B.K. Chougule b a
Department of Chemistry, Shivaji University, Kolhapur 416 004, India Department of Physics, Shivaji University, Kolhapur 416 004, India
b
Received 23 February 2006; received in revised form 23 April 2006; accepted 29 April 2006
Abstract CdIn2 S4 thin films are prepared by pulsed electrodeposition technique over F:SnO2 glass and stainless steel substrates in galvanostatic mode from an aqueous acidic bath containing CdSO4 , InCl3 and Na2 S2 O3 . The growth kinetics of the film was studied and the deposition parameters such as electrolyte bath concentration, bath temperature, time of deposition, deposition current, and pH of the electrolyte bath are optimized. X-ray diffraction (XRD) analysis of the as deposited and annealed films showed the presence of polycrystalline nature. Energy dispersive analysis (EDAX) spectrum of the surface composition confirms the nearly stoichiometric CdIn2 S4 nature of the film. Surface morphology studies by scanning electron microscope (SEM) shows that, the deposited films are well adherent and grains are uniformly distributed over the surface of substrate. The optical transmission spectra show a direct band gap value of 2.16 eV. © 2006 Elsevier B.V. All rights reserved. Keywords: Semiconductor; Electrodeposition; EDAX; SEM
1. Introduction For the last couple of decades interest in the use of photoelectrochemical solar cells lead to large amount of research in the search for thin film polycrystalline materials with acceptable efficiency, some times approaching that of single crystals. In recent years, thin films have attracted much interest because of their varied applications such as semiconducting devices, photovoltaics, optoelectronic devices, radiation detectors, laser materials, thermoelectric devices, solar energy converters, etc. [1–4]. Interest in the use of photoelectrochemical (PEC) solar cells for low-cost energy conversion has lead to an extensive research in the field for novel and suitable thin film semiconductor materials [5–8]. Recent investigations have shown that the layered type semiconducting cadmium chalcogenide group (CdSe, CdS, CdTe), which absorb visible and near-IR light are particularly promising materials for photoelectrochemical solar energy conversion. These applications include intercalation compounds
∗
Corresponding authors. Tel.: +91 231 2609381; fax: +91 231 2692333. E-mail addresses: p [email protected] (P.P. Hankare), [email protected] (A.V. Kokate). 0921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2006.04.037
and long-life PEC solar cells. Some studies on PEC with CdIn2 Se4 have been carried out on single crystals. However, polycrystalline electrodes are economically desirable for solar cell applications, where large area semiconductor substrates are necessary. Hence, this study has been directed towards obtaining CdIn2 S4 in polycrystalline thin film form. Ternary chalcogenides have potential application in solar energy conversion [9–12]. The photoelectrochemical properties of CdIn2 Se4 single crystals were investigated by many workers [13,14]. Structural and optical properties of electrodeposited CdIn2 Se4 thin films have been reported [16]. Many workers have been succeeded in depositing thin films of CdIn2 Se4 and CdIn2 S4 by vacuum evaporation [17,18]. The optical properties, photoluminescence study, Raman scattering of CdIn2 S4 thin films have been reported by several workers [19–21]. The electrodeposition of ternary and other higher multinary compounds containing more than two constituent elements is rather complex owing to a large difference in the deposition potential of the constituent elements, and due to the possibility of the formation of intermediate phases during electrodeposition. No attempt has been made so far to electrodeposit stoichiometric ternary CdIn2 S4 thin films. In this report, an attempt is made to prepare CdIn2 S4 films through electrodeposition technique on conducting glass and
38
P.P. Hankare et al. / Materials Science and Engineering B 133 (2006) 37–41
stainless steel substrates which enables the film to be used for characterization studies like structural, surface composition, surface morphology and optical properties. 2. Experimental Thin films of CdIn2 S4 were cathodically pulsed electrodeposited on F:SnO2 glass substrates and on stainless steel substrates under galvanostatic mode. The pulse duty cycle is defined as the ratio of ‘ON’ time to ‘ON + OFF’ time. The ‘ON’ times were 1–5 s with corresponding ‘OFF’ time fixed as 1, 3, 7, 16 and 45 s which correspond to a duty cycle of 50, 40, 30, 20, and 10%, respectively. The F:SnO2 glass or stainless steel plates were used as the cathode in the three-electrode cell with graphite as the counter electrode and saturated calomel electrode (SCE) was the reference electrode. The electrolyte was prepared by mixing solution of CdSO4 (0.1 M), InCl3 (0.1 M) and Na2 S2 O3 (0.01 M) in the ratio of 1:2:4, respectively. The pH of the electrolytic solution was varied by dilute H2 SO4 . Double distilled water was used for the preparation of aqueous solution of the above precursor chemicals. Before deposition, the substrates were thoroughly cleaned with double distilled water. The distance between the working electrode and the counter electrode was kept constant as 1 cm during deposition. From the visual observation during pulse plating it was observed that a formation of dark yellow thin film of CdIn2 S4 takes place. These pulse plated CdIn2 S4 films were found to be well adherent and uniform. Detailed growth kinetics was studied by changing the dominant deposition parameters, the pH of the solution, bath temperature and current density. The deposited films of CdIn2 S4 were used for further characterization by XRD, SEM, EDAX and optical absorption to study the structural, morphological compositional and optical properties. The X-ray diffraction patterns for CdIn2 S4 thin films deposited onto stainless steel and FTO coated substrates were recorded by Philips X-ray diffractometer model 1710 with Cr K␣ radiation in the span of angle between 10◦ and 100◦ . The surface morphology was studied by using JEOL, JXA840 reflection scanning electron microscope using magnification of 5000× at potential 15 kV. The optical absorption studies were carried using UV–vis–IR spectrophotometer model Hitachi 119 in the wavelength range 380–1250 nm. 3. Results and discussion A detailed kinetics study was carried out by changing the pH of the electrolyte and bath temperatures. The film thickness was slowly built up at the initial stages linearly and finally saturated to a maximum of about 1–2 m for about 15–60 min keeping the bath temperatures as 30, 40 and 50 ◦ C, respectively, for different pH of the electrolyte. Fig. 1 shows the growth kinetics of CdIn2 S4 thin film deposited at 40 ◦ C, for electrolytic bath of different pH values 2, 2.5 and 3. For, pH 2 of the electrolytic bath, the growth of the film is very slow as time of deposition increases and the film thickness increases marginally up to 40 min. For 2.5 pH of the electrolytic bath, the film thickness is linear up to 60 min and reaches a value of 0.6 m after which the
Fig. 1. Growth kinetics of CdIn2 S4 thin films at: (a) pH 2; (b) pH 2.5; (c) pH 3.
thickness built up is nearly constant. For 3 pH of the electrolytic bath, the film growth is found to be fast and linear up to about 40 min and the thickness reached was about 1.4 m. The growth rate is highly reduced in the deposition range of 60–80 min. The films deposited at 30 and 50 ◦ C also showed almost the same growth trend and for 3 pH of the electrolytic bath only a maximum thickness of about 1.2 and 1.62 m was obtained, respectively. From these observations, it can be concluded that CdIn2 S4 thin films of uniform and maximum thickness were formed under the following optimized conditions: 40 ◦ C bath temperature, 40 min of deposition duration, and 3 pH of the electrolytic bath. After 40 min, the growth of thickness of the film became slow and attained saturation. For other pH, the thickness was not sufficiently built on the substrate. For other higher and lower temperatures, the thickness of the film was less as compared to 40 ◦ C as the bath temperature. By keeping 40 ◦ C bath temperature, 40 min of deposition duration, 3 pH of the electrolytic bath, the duty cycle was varied as 10, 20, 30, 40, and 50% and growth kinetics was studied and the thickness variation is shown in Fig. 2. Thickness of the film is found increasing for all duty cycles up to 60 min. The maximum thickness is obtained for the duty cycle of 30%. The current density was varied from 1.2 to 2.5 mA/cm2 during the deposition. The film deposited at current density 2.1 mA/cm2 was found to be uniform, thick and well adherent to the substrate. For other higher and lower values of current density, the thickness of the film was less as compared to 2.1 mA/cm2 as the current density. The optimized electrodeposition parameters are tabulated in Table 1. Table 1 Optimized preparative parameters for electrodeposited CdIn2 S4 Parameter
Optimized value
Bath pH Bath temperature (◦ C) Deposition time (min) Current density (mA/cm2 ) Duty cycle (%)
3 40 40 2.1 30
P.P. Hankare et al. / Materials Science and Engineering B 133 (2006) 37–41
Fig. 2. Growth kinetics of CdIn2 S4 thin films for different duty cycles.
The X-ray diffraction patterns (XRD) of the pulsed electrodeposited CdIn2 S4 films with pH values as 2, 2.5 and 3 at duty cycle 30% were recorded. The Bragg peak heights for (h k l) are found continuously increasing with increase in pH from 2 to 3. By varying the duty cycle of the pulsed electrodeposited films under optimized conditions for various duty cycles of 10, 20, 30, 40, and 50%. Films deposited with duty cycle 30% show very sharp and intense peaks indicating the highly crystalline nature with nearly stoichiometric composition of CdIn2 S4 film. The observed d values are compared with ASTM values to determine the crystal structure. The d values obtained are tabulated along with ASTM data in Table 2. The sharp peak reveals the polycrystalline nature of the as deposited CdIn2 S4 films. The structural features fit into spinel ˚ The crystalcubic one with lattice parameter value a = 10.83 A. lite size was calculated from the full-width at half-maximum (FWHM) measurements for the prominent X-ray diffraction ˚ Annealpeaks, and the values are in the range of 210–340 A. ing of the 30% duty cycle films was done in vacuum at four different temperatures 100, 150, 200, 250 ◦ C for 60 min. The XRD patterns for the as prepared CdIn2 S4 film and the annealed films obtained are shown in Fig. 3a–e, respectively. Annealing at 200 ◦ C shows sharp intense peaks, which reveals that the films annealed at this temperature are more suitable for device application. A significant increase in the XRD peak intensities is
Fig. 3. XRD pattern of CdIn2 S4 : (a) as deposited; (b) annealed at 100 ◦ C; (c) annealed at 150 ◦ C; (d) annealed at 200 ◦ C; (e) annealed at 250 ◦ C.
observed for the CdIn2 S4 films at 250 ◦ C when compared to the peaks at 100 ◦ C. This reveals that the as deposited films were well crystallized and heating in vacuum produces more realignment in orientation leading to improved crystallinity. But while heating above 250 ◦ C the films start peeling off from the substrate. The EDAX recorded in the binding energy region of 0–10 keV for the film deposited at optimized preparative parameters is shown in Fig. 4. The spectrum peak reveals the presence of Cd, In, and S in the film. The ratio of the weight percentage of In to Cd is nearly 2 and S to Cd is nearly 4, which is required stoichiometric ratio of CdIn2 S4 film deposited at optimized deposition condition as given in Table 1. The SEM micrograph shows well adherent, smooth film surface without cracks. Fig. 5a shows the surface morphology of
Table 2 Comparison of experimental d-values with ASTM data Sr. no.
˚ Standard d value (A)
˚ Observed d value (A)
(h k l) planes
1 2 3 4
3.270 2.712 2.087 1.916
3.2659 2.7045 2.085 1.913
(3 1 1) (4 0 0) (5 1 1) (4 4 0)
39
Fig. 4. EDAX spectrum of CdIn2 S4 thin film.
40
P.P. Hankare et al. / Materials Science and Engineering B 133 (2006) 37–41
Fig. 6. Plot of (αhν)2 vs. hν for CdIn2 S4 thin film.
For allowed direct transition, n = 1/2, the value of absorption coefficient is of the order of 104 cm−1 supporting the direct band gap nature of the semiconductor. The plot of (αhν)2 versus hν for a typical sample deposited at optimized preparative parameters is depicted in Fig. 6. It is linear indicating the presence of direct transition. The linear portion is extrapolated to α = 0, at energy axis giving the band gap energy of CdIn2 S4 to be 2.16 eV. 5. Conclusions
Fig. 5. SEM photograph of CdIn2 S4 thin film: (a) as deposited; (b) annealed at 200 ◦ C.
the CdIn2 S4 film prepared under optimized conditions, which ˚ spread exhibits spherical grains of uniform size of about 320 A all over the surface. SEM photograph of vacuum annealed sample prepared as above is shown in Fig. 5b. It also shows a smooth surface with grains spread over the surface. One can note that after annealing the compactness of film increases.
References
4. Optical studies The optical absorption of the films has been studied in the range 350–1250 nm. The variation of optical density with wavelength is analyzed to find out the nature of transition involved and the optical bandgap. The nature of the transition is determined by using the relation [7]: α = A(hν − Eg )n / hν where the symbols have their usual meanings.
Polycrystalline CdIn2 S4 thin films are prepared by pulsed electrodeposition technique on conducting glass and stainless steel substrates. Films prepared using the optimized deposition parameters show preferential orientation along (4 0 0) plane. Annealing treatment of the film in vacuum shows an improvement in the polycrystalline nature of the film with a smoothened surface. EDAX result confirms the presence of Cd, In, and S and the formed material is CdIn2 S4 . The SEM micrograph shows the deposited films are well adherent, uniform and smooth without cracks. From optical studies, the direct band gap value was found to 2.16 eV.
(1)
[1] K.Y. Rajpure, C.D. Lokhande, C.H. Bhosale, Mater. Chem. Phys. 252 (1997) 51. [2] V.L. Mathe, K.Y. Rajpure, C.H. Bhosale, Bull. Mater. Sci. 22 (1999) 927. [3] N.G. Patel, P.G. Patel, Solid State Electrochem. 35 (1992) 1269. [4] H.J. Goldsmid, J.E. Giutronich, M.M. Kalia, Solar Energy 24 (1980) 435. [5] X. Mathew, P.J. Sebastian, Solar Energy Mater. Solar Cells 59 (1999) 85. [6] X. Mathew, J. Phys. D: Appl. Phys. 33 (2000) 1565. [7] A.V. Kokate, U.B. Suryavanshi, C.H. Bhosale, J. Solar Energy 80 (2006) 156. [8] P.P. Hankare, S.D. Delekar, V.M. Bhuse, P.A. Chate, K.M. Garadkar, Semicond. Sci. Technol. 20 (2005) 257.
P.P. Hankare et al. / Materials Science and Engineering B 133 (2006) 37–41 [9] Y. Wada, S. Nishimatsu, J. Electrochem. Sco. 125 (1978) 1499. [10] R. Chandramohan, T. Mahalingam, J.P. Chu, P.J. Sebastian, Solar Energy Mater. Solar Cells 81 (2004) 371. [11] S.K. Deshmukh, A.V. Kokate, D.J. Sathe, Mater. Sci. Eng. B 122 (2005) 206. [12] L. Fornarini, F. Strippe, E. Chardarelli, B.S. Scrosati, Solar Cells 11 (1984) 389. [13] V.M. Nikale, C.H. Bhosale, Solar Energy Mater Solar Cells 82 (2004) 3. [14] V.M. Nikale, N.S. Gaikwad, K.Y. Rajpure, C.H. Bhosale, Mater. Chem. Phys. 78 (2003) 363.
41
[16] H. Hahn, G. Frank, W. Klinger, A.D. Storger, G. Storger, Z. Anorg. Allg. Chem. 279 (1955) 241. [17] M.M. El-Nahass, Appl. Phys. A 52 (1991) 353. [18] H. Naknishi, S. Eudo, T. Trie, Jpn. J. Appl. Phys. 12 (1973) 1646. [19] G. Mengoli, M. Musiani, F. Paolucci, Chim. Ind. (Milan) 73 (1991) 477. [20] K. Rajeshwar, Adv. Mater. 4 (1992) 23. [21] E.A. Dalchiele, S. Cattarin, M.M. Musiani, J. Appl. Electrochem. 28 (1998) 1005.
Properties of pulsed electrodeposited CdIn2S4 thin film P.P. Hankare a,∗ , A.V. Kokate b,∗ , M.R. Asabe a , S.D. Delekar a , B.K. Chougule b a
Department of Chemistry, Shivaji University, Kolhapur 416 004, India Department of Physics, Shivaji University, Kolhapur 416 004, India
b
Received 23 February 2006; received in revised form 23 April 2006; accepted 29 April 2006
Abstract CdIn2 S4 thin films are prepared by pulsed electrodeposition technique over F:SnO2 glass and stainless steel substrates in galvanostatic mode from an aqueous acidic bath containing CdSO4 , InCl3 and Na2 S2 O3 . The growth kinetics of the film was studied and the deposition parameters such as electrolyte bath concentration, bath temperature, time of deposition, deposition current, and pH of the electrolyte bath are optimized. X-ray diffraction (XRD) analysis of the as deposited and annealed films showed the presence of polycrystalline nature. Energy dispersive analysis (EDAX) spectrum of the surface composition confirms the nearly stoichiometric CdIn2 S4 nature of the film. Surface morphology studies by scanning electron microscope (SEM) shows that, the deposited films are well adherent and grains are uniformly distributed over the surface of substrate. The optical transmission spectra show a direct band gap value of 2.16 eV. © 2006 Elsevier B.V. All rights reserved. Keywords: Semiconductor; Electrodeposition; EDAX; SEM
1. Introduction For the last couple of decades interest in the use of photoelectrochemical solar cells lead to large amount of research in the search for thin film polycrystalline materials with acceptable efficiency, some times approaching that of single crystals. In recent years, thin films have attracted much interest because of their varied applications such as semiconducting devices, photovoltaics, optoelectronic devices, radiation detectors, laser materials, thermoelectric devices, solar energy converters, etc. [1–4]. Interest in the use of photoelectrochemical (PEC) solar cells for low-cost energy conversion has lead to an extensive research in the field for novel and suitable thin film semiconductor materials [5–8]. Recent investigations have shown that the layered type semiconducting cadmium chalcogenide group (CdSe, CdS, CdTe), which absorb visible and near-IR light are particularly promising materials for photoelectrochemical solar energy conversion. These applications include intercalation compounds
∗
Corresponding authors. Tel.: +91 231 2609381; fax: +91 231 2692333. E-mail addresses: p [email protected] (P.P. Hankare), [email protected] (A.V. Kokate). 0921-5107/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2006.04.037
and long-life PEC solar cells. Some studies on PEC with CdIn2 Se4 have been carried out on single crystals. However, polycrystalline electrodes are economically desirable for solar cell applications, where large area semiconductor substrates are necessary. Hence, this study has been directed towards obtaining CdIn2 S4 in polycrystalline thin film form. Ternary chalcogenides have potential application in solar energy conversion [9–12]. The photoelectrochemical properties of CdIn2 Se4 single crystals were investigated by many workers [13,14]. Structural and optical properties of electrodeposited CdIn2 Se4 thin films have been reported [16]. Many workers have been succeeded in depositing thin films of CdIn2 Se4 and CdIn2 S4 by vacuum evaporation [17,18]. The optical properties, photoluminescence study, Raman scattering of CdIn2 S4 thin films have been reported by several workers [19–21]. The electrodeposition of ternary and other higher multinary compounds containing more than two constituent elements is rather complex owing to a large difference in the deposition potential of the constituent elements, and due to the possibility of the formation of intermediate phases during electrodeposition. No attempt has been made so far to electrodeposit stoichiometric ternary CdIn2 S4 thin films. In this report, an attempt is made to prepare CdIn2 S4 films through electrodeposition technique on conducting glass and
38
P.P. Hankare et al. / Materials Science and Engineering B 133 (2006) 37–41
stainless steel substrates which enables the film to be used for characterization studies like structural, surface composition, surface morphology and optical properties. 2. Experimental Thin films of CdIn2 S4 were cathodically pulsed electrodeposited on F:SnO2 glass substrates and on stainless steel substrates under galvanostatic mode. The pulse duty cycle is defined as the ratio of ‘ON’ time to ‘ON + OFF’ time. The ‘ON’ times were 1–5 s with corresponding ‘OFF’ time fixed as 1, 3, 7, 16 and 45 s which correspond to a duty cycle of 50, 40, 30, 20, and 10%, respectively. The F:SnO2 glass or stainless steel plates were used as the cathode in the three-electrode cell with graphite as the counter electrode and saturated calomel electrode (SCE) was the reference electrode. The electrolyte was prepared by mixing solution of CdSO4 (0.1 M), InCl3 (0.1 M) and Na2 S2 O3 (0.01 M) in the ratio of 1:2:4, respectively. The pH of the electrolytic solution was varied by dilute H2 SO4 . Double distilled water was used for the preparation of aqueous solution of the above precursor chemicals. Before deposition, the substrates were thoroughly cleaned with double distilled water. The distance between the working electrode and the counter electrode was kept constant as 1 cm during deposition. From the visual observation during pulse plating it was observed that a formation of dark yellow thin film of CdIn2 S4 takes place. These pulse plated CdIn2 S4 films were found to be well adherent and uniform. Detailed growth kinetics was studied by changing the dominant deposition parameters, the pH of the solution, bath temperature and current density. The deposited films of CdIn2 S4 were used for further characterization by XRD, SEM, EDAX and optical absorption to study the structural, morphological compositional and optical properties. The X-ray diffraction patterns for CdIn2 S4 thin films deposited onto stainless steel and FTO coated substrates were recorded by Philips X-ray diffractometer model 1710 with Cr K␣ radiation in the span of angle between 10◦ and 100◦ . The surface morphology was studied by using JEOL, JXA840 reflection scanning electron microscope using magnification of 5000× at potential 15 kV. The optical absorption studies were carried using UV–vis–IR spectrophotometer model Hitachi 119 in the wavelength range 380–1250 nm. 3. Results and discussion A detailed kinetics study was carried out by changing the pH of the electrolyte and bath temperatures. The film thickness was slowly built up at the initial stages linearly and finally saturated to a maximum of about 1–2 m for about 15–60 min keeping the bath temperatures as 30, 40 and 50 ◦ C, respectively, for different pH of the electrolyte. Fig. 1 shows the growth kinetics of CdIn2 S4 thin film deposited at 40 ◦ C, for electrolytic bath of different pH values 2, 2.5 and 3. For, pH 2 of the electrolytic bath, the growth of the film is very slow as time of deposition increases and the film thickness increases marginally up to 40 min. For 2.5 pH of the electrolytic bath, the film thickness is linear up to 60 min and reaches a value of 0.6 m after which the
Fig. 1. Growth kinetics of CdIn2 S4 thin films at: (a) pH 2; (b) pH 2.5; (c) pH 3.
thickness built up is nearly constant. For 3 pH of the electrolytic bath, the film growth is found to be fast and linear up to about 40 min and the thickness reached was about 1.4 m. The growth rate is highly reduced in the deposition range of 60–80 min. The films deposited at 30 and 50 ◦ C also showed almost the same growth trend and for 3 pH of the electrolytic bath only a maximum thickness of about 1.2 and 1.62 m was obtained, respectively. From these observations, it can be concluded that CdIn2 S4 thin films of uniform and maximum thickness were formed under the following optimized conditions: 40 ◦ C bath temperature, 40 min of deposition duration, and 3 pH of the electrolytic bath. After 40 min, the growth of thickness of the film became slow and attained saturation. For other pH, the thickness was not sufficiently built on the substrate. For other higher and lower temperatures, the thickness of the film was less as compared to 40 ◦ C as the bath temperature. By keeping 40 ◦ C bath temperature, 40 min of deposition duration, 3 pH of the electrolytic bath, the duty cycle was varied as 10, 20, 30, 40, and 50% and growth kinetics was studied and the thickness variation is shown in Fig. 2. Thickness of the film is found increasing for all duty cycles up to 60 min. The maximum thickness is obtained for the duty cycle of 30%. The current density was varied from 1.2 to 2.5 mA/cm2 during the deposition. The film deposited at current density 2.1 mA/cm2 was found to be uniform, thick and well adherent to the substrate. For other higher and lower values of current density, the thickness of the film was less as compared to 2.1 mA/cm2 as the current density. The optimized electrodeposition parameters are tabulated in Table 1. Table 1 Optimized preparative parameters for electrodeposited CdIn2 S4 Parameter
Optimized value
Bath pH Bath temperature (◦ C) Deposition time (min) Current density (mA/cm2 ) Duty cycle (%)
3 40 40 2.1 30
P.P. Hankare et al. / Materials Science and Engineering B 133 (2006) 37–41
Fig. 2. Growth kinetics of CdIn2 S4 thin films for different duty cycles.
The X-ray diffraction patterns (XRD) of the pulsed electrodeposited CdIn2 S4 films with pH values as 2, 2.5 and 3 at duty cycle 30% were recorded. The Bragg peak heights for (h k l) are found continuously increasing with increase in pH from 2 to 3. By varying the duty cycle of the pulsed electrodeposited films under optimized conditions for various duty cycles of 10, 20, 30, 40, and 50%. Films deposited with duty cycle 30% show very sharp and intense peaks indicating the highly crystalline nature with nearly stoichiometric composition of CdIn2 S4 film. The observed d values are compared with ASTM values to determine the crystal structure. The d values obtained are tabulated along with ASTM data in Table 2. The sharp peak reveals the polycrystalline nature of the as deposited CdIn2 S4 films. The structural features fit into spinel ˚ The crystalcubic one with lattice parameter value a = 10.83 A. lite size was calculated from the full-width at half-maximum (FWHM) measurements for the prominent X-ray diffraction ˚ Annealpeaks, and the values are in the range of 210–340 A. ing of the 30% duty cycle films was done in vacuum at four different temperatures 100, 150, 200, 250 ◦ C for 60 min. The XRD patterns for the as prepared CdIn2 S4 film and the annealed films obtained are shown in Fig. 3a–e, respectively. Annealing at 200 ◦ C shows sharp intense peaks, which reveals that the films annealed at this temperature are more suitable for device application. A significant increase in the XRD peak intensities is
Fig. 3. XRD pattern of CdIn2 S4 : (a) as deposited; (b) annealed at 100 ◦ C; (c) annealed at 150 ◦ C; (d) annealed at 200 ◦ C; (e) annealed at 250 ◦ C.
observed for the CdIn2 S4 films at 250 ◦ C when compared to the peaks at 100 ◦ C. This reveals that the as deposited films were well crystallized and heating in vacuum produces more realignment in orientation leading to improved crystallinity. But while heating above 250 ◦ C the films start peeling off from the substrate. The EDAX recorded in the binding energy region of 0–10 keV for the film deposited at optimized preparative parameters is shown in Fig. 4. The spectrum peak reveals the presence of Cd, In, and S in the film. The ratio of the weight percentage of In to Cd is nearly 2 and S to Cd is nearly 4, which is required stoichiometric ratio of CdIn2 S4 film deposited at optimized deposition condition as given in Table 1. The SEM micrograph shows well adherent, smooth film surface without cracks. Fig. 5a shows the surface morphology of
Table 2 Comparison of experimental d-values with ASTM data Sr. no.
˚ Standard d value (A)
˚ Observed d value (A)
(h k l) planes
1 2 3 4
3.270 2.712 2.087 1.916
3.2659 2.7045 2.085 1.913
(3 1 1) (4 0 0) (5 1 1) (4 4 0)
39
Fig. 4. EDAX spectrum of CdIn2 S4 thin film.
40
P.P. Hankare et al. / Materials Science and Engineering B 133 (2006) 37–41
Fig. 6. Plot of (αhν)2 vs. hν for CdIn2 S4 thin film.
For allowed direct transition, n = 1/2, the value of absorption coefficient is of the order of 104 cm−1 supporting the direct band gap nature of the semiconductor. The plot of (αhν)2 versus hν for a typical sample deposited at optimized preparative parameters is depicted in Fig. 6. It is linear indicating the presence of direct transition. The linear portion is extrapolated to α = 0, at energy axis giving the band gap energy of CdIn2 S4 to be 2.16 eV. 5. Conclusions
Fig. 5. SEM photograph of CdIn2 S4 thin film: (a) as deposited; (b) annealed at 200 ◦ C.
the CdIn2 S4 film prepared under optimized conditions, which ˚ spread exhibits spherical grains of uniform size of about 320 A all over the surface. SEM photograph of vacuum annealed sample prepared as above is shown in Fig. 5b. It also shows a smooth surface with grains spread over the surface. One can note that after annealing the compactness of film increases.
References
4. Optical studies The optical absorption of the films has been studied in the range 350–1250 nm. The variation of optical density with wavelength is analyzed to find out the nature of transition involved and the optical bandgap. The nature of the transition is determined by using the relation [7]: α = A(hν − Eg )n / hν where the symbols have their usual meanings.
Polycrystalline CdIn2 S4 thin films are prepared by pulsed electrodeposition technique on conducting glass and stainless steel substrates. Films prepared using the optimized deposition parameters show preferential orientation along (4 0 0) plane. Annealing treatment of the film in vacuum shows an improvement in the polycrystalline nature of the film with a smoothened surface. EDAX result confirms the presence of Cd, In, and S and the formed material is CdIn2 S4 . The SEM micrograph shows the deposited films are well adherent, uniform and smooth without cracks. From optical studies, the direct band gap value was found to 2.16 eV.
(1)
[1] K.Y. Rajpure, C.D. Lokhande, C.H. Bhosale, Mater. Chem. Phys. 252 (1997) 51. [2] V.L. Mathe, K.Y. Rajpure, C.H. Bhosale, Bull. Mater. Sci. 22 (1999) 927. [3] N.G. Patel, P.G. Patel, Solid State Electrochem. 35 (1992) 1269. [4] H.J. Goldsmid, J.E. Giutronich, M.M. Kalia, Solar Energy 24 (1980) 435. [5] X. Mathew, P.J. Sebastian, Solar Energy Mater. Solar Cells 59 (1999) 85. [6] X. Mathew, J. Phys. D: Appl. Phys. 33 (2000) 1565. [7] A.V. Kokate, U.B. Suryavanshi, C.H. Bhosale, J. Solar Energy 80 (2006) 156. [8] P.P. Hankare, S.D. Delekar, V.M. Bhuse, P.A. Chate, K.M. Garadkar, Semicond. Sci. Technol. 20 (2005) 257.
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