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Cd0.5Zn0.5Se wide range composite thin films for solar cell buffer layer application
Applied Surface Science 253 (2007) 3109–3112 www.elsevier.com/locate/apsusc
Cd0.5Zn0.5Se wide range composite thin films for solar cell buffer layer application R.B. Kale a, C.D. Lokhande a,b, R.S. Mane b, Sung-Hwan Han b,* b
a Department of Physics, Rajaram College, Kolhapur 416004, Maharashtra, India Department of Chemistry, Hanyang University, Sungdong-Ku, Haengdang-dong 17, Seoul 133-791, Republic of Korea
Received 13 June 2006; received in revised form 29 June 2006; accepted 30 June 2006 Available online 22 August 2006
Abstract Cd0.5Zn0.5Se composite thin films were obtained on glass substrate using aqueous alkaline solution at low temperature using cadmium acetate and zinc acetate as Cd2+ and Zn2+ and Se2 ion sources. Different phases of individuals i.e. CdSe and ZnSe, spherical and needle shape surface morphology and good elemental chemical stoichiometric ratio were observed from X-ray diffraction, scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX) studies, respectively. The band gap and electrical resistivity of the composite film were 2.35 eV and about 107 V cm, respectively. # 2006 Elsevier B.V. All rights reserved. Keywords: Composite thin films; XRD; SEM; AFM; Electrical resistivity
1. Introduction Composite semiconductor thin films have attracted considerable attention during last 2 decades due to their excellent properties for catalysis, gas sensor and working electrode in photoelectrochemical solar cells and the three-dimensional confinement of the electrons [1]. The interesting characteristics and related applications were dominated by several factors, such as surface morphology, grain size, crystallinity, and so on. Altering the size of the particles, alters the degree of the confinement of the electrons and affects the electronic structure of the semiconductor, especially the band gap edges, which are tunable with particle size. Metal selenide composite thin films have attracted considerable attention because of their interesting size dependent properties and wide range of applications in thermoelectric cooling, optical fibers, light emitting diodes, sensors, solar cells, laser material, thin film transistors, gamma ray detectors, etc. [2–10]. In the literature, the reports are available on the formation of separate phases of chemically deposited ternary thin film such as CdS–CdSe [11], CdS–CuXS [12], CdS–Bi2S3 [13], CdS–HgS
* Corresponding author. Tel.: +82 22292 5212; fax: +82 22290 0762. E-mail address: [email protected] (S.-H. Han). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.06.064
[14,15], CdS–ZnS [16–18], etc. In this report, emphasis was made to synthesize low temperature Cd0.5Zn0.5Se composite thin films as there are earlier reports [19,20] with conclusion that the photoelectrochemical (PEC) conversion efficiency and stability of composite films are higher than individuals and characterized for structural, optical and electric properties. 2. Experimental details 2.1. Cd0.5Zn0.5Se composite thin film formation Cadmium zinc selenide thin films have been deposited onto glass substrates using CBD method. The reagents used were analytical grade cadmium acetate, zinc acetate, selenium, liquor ammonia and sodium sulphite supplied by s.d. fine chem. Ltd. In the present study, the deposition of Cd0.5Zn0.5Se wide band gap composite thin films was carried out at 343 K. 0.25 M freshly prepared sodium selenosulfate solution [21] was used as Se2 ion source. The concentrations of cadmium acetate and zinc acetate were optimized to deposit good quality and stoichiometric Cd0.5Zn0.5Se composite thin films. At same concentrations of Cd2+ and Zn2+ in stock solution the CdSe: ZnSe ratio was found to vary largely (CdSe/ZnSe 1.7) due to the differences in solubility of products of CdSe and ZnSe
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(KSP, CdSe 4 10 35, KSP, ZnSe 10 27). Also, Zn forms very strong amine complex with ammonium hydroxide to that of Cd, results into Zn rich composite thin films. For the good elemental stoichiometry, the concentration of Zn must be lower than Cd, which was achieved by mixing 0.1 and 0.5 M concentrations of Cd2+ and Zn2+ from stock solutions, respectively. Cadmium and zinc acetate solutions were mixed in equal proportional to make 20 mL stock solution and then in that 20 mL sodium selenosulphate solution was added for Cd0.5Zn0.5Se formation. Due to the strong complex formation of Zn(NH3)4+ that does not releases free Zn2+ ions at lower temperatures (<313 K), Zn2+ cannot be co-deposited with Cd2+ and Se2 for Cd0.5Zn0.5Se composite thin film. Fig. 1 shows time dependent composite film thickness variation during the deposition. The graph reveals that initial linear film thickness growth up to 4 h and then a decrease due to the film dissolution mechanism, as motioned in the literature [22]. The optimized deposition parameters are; 0.1 M Cd2+, 0.5 M Zn2+ in equal proportional and same quantity of 0.25 M Na2SeSO3 at deposition temperature 343 K for 4 h. For thickness measurement, mass different method was used by scrubbing the film from precise area and using density thickness relation due to the compact nature of the film within the limit of relative experimental error (50 nm). For structural studies, Phillips PW-1710, X-ray diffractometer using Cu Ka radiation in the 2u range 20–1008 was used. For surface morphological study of Cd0.5Zn0.5Se composite thin film, a scanning electron microscopy (Cambridge Stereoscan 250 MK-III) attached with energy dispersive X-ray analysis, (EDX) analyzer, was ˚ gold palladium preferred. The film was coated with 100 A (Au–Pd) layer using polaron SEM sputter coating unit E-2500, before taking SEM and EDX. The atomic force microscopy (AFM) (digital unit) was used to study the surface morphology of Cd0.5Zn0.5Se composite thin film. An optical absorption spectrum was recorded in the wavelength range of 300–800 nm using UV–vis–NIR spectrophotometer (Hitachi model-330, Japan). The dc electrical resistivity of Cd0.5Zn0.5Se composite
Fig. 1. Composite Cd0.5Zn0.5Se film thickness variation as a function of deposition time synthesized from 0.1 M cadmium acetate, 0.5 M zinc acetate and 0.25 M sodium selenosulphate stock solution.
Fig. 2. The XRD pattern of Cd0.5Zn0.5Se composite thin film (H, hexagonal and C, cubic).
thin film was measured using a two-probe method in the temperature range of 300–500 K by defining area of 0.5 cm2 on film and for good ohmic contacts, silver paste was used. For accurate measurement of temperature, digital temperature controller based on PT-100 sensor of accuracy 1 8C was used.
Fig. 3. The SEM image of Cd0.5Zn0.5Se composite thin film (a) and to that of EDX (b) showing stoichiometric elemental percentage analysis.
R.B. Kale et al. / Applied Surface Science 253 (2007) 3109–3112
3111
Similar results are reported by our and other groups earlier for Bi2Se3–Sb2Se3 [23], CdS–Bi2S3 [24] and CdS–HgS [25] composite films. 3.2. Morphological studies
Fig. 4. The 2D AFM of Cd0.5Zn0.5Se composite thin film.
3. Results and discussion 3.1. Structural studies We believe, ion-by-ion mechanism of film formation is more applicable for Cd0.5Zn0.5Se composite thin film, as it is well suited and applied for CdSe and ZnSe individual thin films [19,20] in which there is reaction between free metal ions Mn+ (M = Cd, Zn) present in the low concentration by complex ions M salt, and slowly released selenide ions Se2 or more probable HSe , (designed here as X2 ) typically formed by hydrolysis of selenosulfate. This results in formation of MX, when the product of the concentrations of M2+ and X2 is greater than the solubility product (KSP) of MX. The XRD pattern of Cd0.5Zn0.5Se composite thin film is shown in Fig. 2. It shows that the Cd0.5Zn0.5Se composite thin film is polycrystalline, with well defined diffraction peaks that corresponds to mixed cubic and hexagonal phases of CdSe and cubic phase of ZnSe, confirming formation of Cd0.5Zn0.5Se. The subscript 0.5 of composite used here is due to the initially taken solution quantity of Cd and Zn.
Fig. 3(a) shows SEM image of Cd0.5Zn0.5Se composite thin film. From microstructure point of view Cd0.5Zn0.5Se composite thin film is well covered to the glass substrate, with some cracks. The grains are irregular in size and shape, with spherical and elongated needle-like architecture which may be the overgrowth on the film surface. The spherical nanosized grains are due to cubic phase of CdSe and ZnSe, while needle-like bigger size grains are due to hexagonal phase of CdSe whose c-axis is parallel to surface of the film. The elemental EDX analysis (Fig. 3(b)) was carried out only for Cd, Zn and Se. The average atomic percentage ratios of Cd:Zn:Se were 26.5:24.9:48.6, showing that the film was slightly Cd rich, as mentioned earlier. This result reveals that the as taken solution composition and corresponding elemental composition in thin film are almost same, indicating real merit of this method. Fig. 4 shows two-dimensional (2D) AFM image of Cd0.5Zn0.5Se composite thin film. The contrast in the image indicates roughness of the surface. It is seen from the intensity distribution that the film consists of smaller and larger nanoparticles due to the mixture of cubic and hexagonal CdSe and ZnSe phases. 3.3. Optical absorption and electrical resistivity studies Optical absorption of Cd0.5Zn0.5Se composite thin film was carried out in the wavelength range of 350–800 nm. The plot of optical absorption versus wavelength is as shown in Fig. 5(a). The band gap, ‘Eg’ was calculated using the standard relation, a = A (hn Eg)n/hn, where A is constant, ‘Eg’ is the band gap, n is a constant, 1/2 for direct band gap semiconductors. The estimated band gap from the plot of (ahn)2 versus hn is as shown in Fig. 5(b). The linear nature of the plot indicates the direct band gap of thin film. The band gap ‘Eg’, was obtained by extrapolating the straight portion to energy axis at a = 0. It is
Fig. 5. Variation of optical absorption with wavelength (a) and the graph of (ahn)2 vs. hn of Cd0.5Zn0.5Se composite thin film.
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R.B. Kale et al. / Applied Surface Science 253 (2007) 3109–3112
Acknowledgements R.B. Kale is grateful to Western Regional Centre, UGC, New Delhi, for its financial assistance under the minor research project no. F: 47-185/2002. This work is partially supported by Hanyang University Next Generation Development Program. References
Fig. 6. Variation of log r vs. 1000/T for Cd0.5Zn0.5Se composite thin film.
found that the band gap ‘Eg’ for Cd0.5Zn0.5Se composite thin film was 2.35 eV, that falls between the band gap value of CdSe (1.7 eV) and ZnSe film (2.7 eV) [26–28]. Due to higher band gap energy, this finds an application as a buffer layer for CuInSe2 based solid state solar cells in addition to higher resistance (next section), good surface coverage etc. The dark electrical conductivity of Cd0.5Zn0.5Se composite thin film was determined using a ‘dc’ two-probe method, in the temperature range of 300–525 K. At room temperature the resistivity was found to be in the order of 107 V cm. The plot of the log r versus inverse of absolute temperature at the cooling time is shown in Fig. 6. A plot shows that electrical resistivity has two linear regions, an intrinsic region setting at low temperature, characterized by small slope (300–350 K and that of high temperature region that is associated with extrinsic conduction due to the presence of donor states produced by excess cadmium. The foreseen reasons for high resistivity are; nanocrystalline nature, crystallite boundary discontinuities, presence of defect states, adsorbed and absorbed gases, and presence of different phases of CdSe and ZnSe [21]. 4. Conclusions Cd0.5Zn0.5Se composite thin films composed of mixed cubic and hexagonal phases of CdSe and ZnSe are synthesized by CBD process at low temperature and characterized by XRD, SEM and AFM techniques which revealed the nanocrystalline nature of film with different grain sizes and shapes. Optical band gap of Cd0.5Zn0.5Se composite thin film was found to be in between individual band-gaps of CdSe and ZnSe and the dark electrical resistivity was about 107 V cm. Due to above properties, these films find applications as a buffer layer in solar cells.
[1] W. Wang, I. Germanenko, M.S. El-Shall, Chem. Mater. 14 (2002) 3028, and references cited therein. [2] F. Mongellaz, A. Fillot, R. Griot, L.J. Lallee, Proc. SPIE-Int. Soc. Opt. Eng. 156 (1994) 2227. [3] W.Z. Wang, Y. Geng, P. Van, F.Y. Liu, Y. Xie, Y.T. Qian, J. Am. Chem. Soc. 121 (1999) 4062. [4] S.T. Lakshmikvar, A.C. Rastogi, Sol. Energy Mater. Sol. Cells 32 (1994) 7. [5] M. Mukherjee, N. Deshmukh, S.K. Kulkarni, Appl. Surf. Sci. 218 (2003) 324. [6] A. Van Claster, A. Vervaet, I. De Rycke, J.D. Bates, J. Cryst. Growth 86 (1988) 924. [7] M. Roth, Nucl. Instrum. Methods 283 (1989) 291. [8] P.A. Krishna Murthy, P.A. Shivkumar, Thin Solid Films 121 (1984) 151. [9] P. Sreekumari Nair, K.P. Fritz, G.D. Scholes, Chem. Commun. 24 (2005) 2084. [10] G. Hodes, Chemical Solution Deposition of Semiconductors Thin Films, Marcel Dekker, Inc., New York, 2003, p. 280. [11] G.B. Reddy, D.K. Pandya, K.L. Chopra, Sol. Energy Mater. 15 (1987) 383. [12] P.K. Nair, M.T.S. Nair, J. Compos., Proc. SPIE 823 (1987) 256. [13] R.R. Ahire, B.R. Sankapal, C.D. Lokhande, Mater. Chem. Phys. 72 (2001) 48. [14] K.M. Garadkar, P.P. Hankare, R.K. Patil, Mater. Chem. Phys. 58 (1999) 64. [15] L.P. Deshmukh, K.M. Garadkar, D.S. Sutrave, Mater. Chem. Phys. 55 (1998) 30. [16] J.M. Dona, J. Herrero, Thin Solid Films 268 (1995) 5. [17] N.C. Sharma, R.C. Kainthla, D.K. Pandya, K.L. Chopra, Thin Solid Films 60 (1979) 55. [18] T. Nakazawa, S. Kuranouchi, A. Ashida, N. Yamamoto, 12th ECPV Solar Energy Conf., Amsterdam, Netherland, 1994, p. 601. [19] G. Hodes, D. Cahen, J. Manassen, M. David, J. Electrochem. Soc. 127 (1980) 2252. [20] K. Singh, A.K. Shukla, Sol. Energy Mater. Sol. Cells 30 (1993) 169. [21] R.B. Kale, C.D. Lokhande, Appl. Surf. Sci. 223 (2004) 343. [22] P.P. Hankare, V.M. Bhuse, K.M. Garadkar, A.D. Jadhav, Mater. Chem. Phys. 71 (2001) 53. [23] C.D. Lokhande, B.R. Sankapal, S.D. Sartale, H.M. Patahan, M. Giersig, V. Ganesan, Appl. Surf. Sci. 182 (2001) 413. [24] L.P. Deshmukh, B.M. More, S.G. Holikatti, P.P. Hankare, Bull. Mater. Sci. 17 (1994) 455. [25] K.M. Gadave, C.D. Lokhande, P.P. Hankare, Mater. Chem. Phys. 38 (1994) 393. [26] M. Skyllas-Kazcos, J.F. McCann, R. Arruzza, Appl. Surf. Sci. 22 (1985) 1091. [27] R.B. Kale, C.D. Lokhande, Semicond. Sci. Technol. 20 (2005) 1. [28] R.B. Kale, C.D. Lokhande, Appl. Surf. Sci. 252 (2005) 929.
Cd0.5Zn0.5Se wide range composite thin films for solar cell buffer layer application R.B. Kale a, C.D. Lokhande a,b, R.S. Mane b, Sung-Hwan Han b,* b
a Department of Physics, Rajaram College, Kolhapur 416004, Maharashtra, India Department of Chemistry, Hanyang University, Sungdong-Ku, Haengdang-dong 17, Seoul 133-791, Republic of Korea
Received 13 June 2006; received in revised form 29 June 2006; accepted 30 June 2006 Available online 22 August 2006
Abstract Cd0.5Zn0.5Se composite thin films were obtained on glass substrate using aqueous alkaline solution at low temperature using cadmium acetate and zinc acetate as Cd2+ and Zn2+ and Se2 ion sources. Different phases of individuals i.e. CdSe and ZnSe, spherical and needle shape surface morphology and good elemental chemical stoichiometric ratio were observed from X-ray diffraction, scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX) studies, respectively. The band gap and electrical resistivity of the composite film were 2.35 eV and about 107 V cm, respectively. # 2006 Elsevier B.V. All rights reserved. Keywords: Composite thin films; XRD; SEM; AFM; Electrical resistivity
1. Introduction Composite semiconductor thin films have attracted considerable attention during last 2 decades due to their excellent properties for catalysis, gas sensor and working electrode in photoelectrochemical solar cells and the three-dimensional confinement of the electrons [1]. The interesting characteristics and related applications were dominated by several factors, such as surface morphology, grain size, crystallinity, and so on. Altering the size of the particles, alters the degree of the confinement of the electrons and affects the electronic structure of the semiconductor, especially the band gap edges, which are tunable with particle size. Metal selenide composite thin films have attracted considerable attention because of their interesting size dependent properties and wide range of applications in thermoelectric cooling, optical fibers, light emitting diodes, sensors, solar cells, laser material, thin film transistors, gamma ray detectors, etc. [2–10]. In the literature, the reports are available on the formation of separate phases of chemically deposited ternary thin film such as CdS–CdSe [11], CdS–CuXS [12], CdS–Bi2S3 [13], CdS–HgS
* Corresponding author. Tel.: +82 22292 5212; fax: +82 22290 0762. E-mail address: [email protected] (S.-H. Han). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.06.064
[14,15], CdS–ZnS [16–18], etc. In this report, emphasis was made to synthesize low temperature Cd0.5Zn0.5Se composite thin films as there are earlier reports [19,20] with conclusion that the photoelectrochemical (PEC) conversion efficiency and stability of composite films are higher than individuals and characterized for structural, optical and electric properties. 2. Experimental details 2.1. Cd0.5Zn0.5Se composite thin film formation Cadmium zinc selenide thin films have been deposited onto glass substrates using CBD method. The reagents used were analytical grade cadmium acetate, zinc acetate, selenium, liquor ammonia and sodium sulphite supplied by s.d. fine chem. Ltd. In the present study, the deposition of Cd0.5Zn0.5Se wide band gap composite thin films was carried out at 343 K. 0.25 M freshly prepared sodium selenosulfate solution [21] was used as Se2 ion source. The concentrations of cadmium acetate and zinc acetate were optimized to deposit good quality and stoichiometric Cd0.5Zn0.5Se composite thin films. At same concentrations of Cd2+ and Zn2+ in stock solution the CdSe: ZnSe ratio was found to vary largely (CdSe/ZnSe 1.7) due to the differences in solubility of products of CdSe and ZnSe
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(KSP, CdSe 4 10 35, KSP, ZnSe 10 27). Also, Zn forms very strong amine complex with ammonium hydroxide to that of Cd, results into Zn rich composite thin films. For the good elemental stoichiometry, the concentration of Zn must be lower than Cd, which was achieved by mixing 0.1 and 0.5 M concentrations of Cd2+ and Zn2+ from stock solutions, respectively. Cadmium and zinc acetate solutions were mixed in equal proportional to make 20 mL stock solution and then in that 20 mL sodium selenosulphate solution was added for Cd0.5Zn0.5Se formation. Due to the strong complex formation of Zn(NH3)4+ that does not releases free Zn2+ ions at lower temperatures (<313 K), Zn2+ cannot be co-deposited with Cd2+ and Se2 for Cd0.5Zn0.5Se composite thin film. Fig. 1 shows time dependent composite film thickness variation during the deposition. The graph reveals that initial linear film thickness growth up to 4 h and then a decrease due to the film dissolution mechanism, as motioned in the literature [22]. The optimized deposition parameters are; 0.1 M Cd2+, 0.5 M Zn2+ in equal proportional and same quantity of 0.25 M Na2SeSO3 at deposition temperature 343 K for 4 h. For thickness measurement, mass different method was used by scrubbing the film from precise area and using density thickness relation due to the compact nature of the film within the limit of relative experimental error (50 nm). For structural studies, Phillips PW-1710, X-ray diffractometer using Cu Ka radiation in the 2u range 20–1008 was used. For surface morphological study of Cd0.5Zn0.5Se composite thin film, a scanning electron microscopy (Cambridge Stereoscan 250 MK-III) attached with energy dispersive X-ray analysis, (EDX) analyzer, was ˚ gold palladium preferred. The film was coated with 100 A (Au–Pd) layer using polaron SEM sputter coating unit E-2500, before taking SEM and EDX. The atomic force microscopy (AFM) (digital unit) was used to study the surface morphology of Cd0.5Zn0.5Se composite thin film. An optical absorption spectrum was recorded in the wavelength range of 300–800 nm using UV–vis–NIR spectrophotometer (Hitachi model-330, Japan). The dc electrical resistivity of Cd0.5Zn0.5Se composite
Fig. 1. Composite Cd0.5Zn0.5Se film thickness variation as a function of deposition time synthesized from 0.1 M cadmium acetate, 0.5 M zinc acetate and 0.25 M sodium selenosulphate stock solution.
Fig. 2. The XRD pattern of Cd0.5Zn0.5Se composite thin film (H, hexagonal and C, cubic).
thin film was measured using a two-probe method in the temperature range of 300–500 K by defining area of 0.5 cm2 on film and for good ohmic contacts, silver paste was used. For accurate measurement of temperature, digital temperature controller based on PT-100 sensor of accuracy 1 8C was used.
Fig. 3. The SEM image of Cd0.5Zn0.5Se composite thin film (a) and to that of EDX (b) showing stoichiometric elemental percentage analysis.
R.B. Kale et al. / Applied Surface Science 253 (2007) 3109–3112
3111
Similar results are reported by our and other groups earlier for Bi2Se3–Sb2Se3 [23], CdS–Bi2S3 [24] and CdS–HgS [25] composite films. 3.2. Morphological studies
Fig. 4. The 2D AFM of Cd0.5Zn0.5Se composite thin film.
3. Results and discussion 3.1. Structural studies We believe, ion-by-ion mechanism of film formation is more applicable for Cd0.5Zn0.5Se composite thin film, as it is well suited and applied for CdSe and ZnSe individual thin films [19,20] in which there is reaction between free metal ions Mn+ (M = Cd, Zn) present in the low concentration by complex ions M salt, and slowly released selenide ions Se2 or more probable HSe , (designed here as X2 ) typically formed by hydrolysis of selenosulfate. This results in formation of MX, when the product of the concentrations of M2+ and X2 is greater than the solubility product (KSP) of MX. The XRD pattern of Cd0.5Zn0.5Se composite thin film is shown in Fig. 2. It shows that the Cd0.5Zn0.5Se composite thin film is polycrystalline, with well defined diffraction peaks that corresponds to mixed cubic and hexagonal phases of CdSe and cubic phase of ZnSe, confirming formation of Cd0.5Zn0.5Se. The subscript 0.5 of composite used here is due to the initially taken solution quantity of Cd and Zn.
Fig. 3(a) shows SEM image of Cd0.5Zn0.5Se composite thin film. From microstructure point of view Cd0.5Zn0.5Se composite thin film is well covered to the glass substrate, with some cracks. The grains are irregular in size and shape, with spherical and elongated needle-like architecture which may be the overgrowth on the film surface. The spherical nanosized grains are due to cubic phase of CdSe and ZnSe, while needle-like bigger size grains are due to hexagonal phase of CdSe whose c-axis is parallel to surface of the film. The elemental EDX analysis (Fig. 3(b)) was carried out only for Cd, Zn and Se. The average atomic percentage ratios of Cd:Zn:Se were 26.5:24.9:48.6, showing that the film was slightly Cd rich, as mentioned earlier. This result reveals that the as taken solution composition and corresponding elemental composition in thin film are almost same, indicating real merit of this method. Fig. 4 shows two-dimensional (2D) AFM image of Cd0.5Zn0.5Se composite thin film. The contrast in the image indicates roughness of the surface. It is seen from the intensity distribution that the film consists of smaller and larger nanoparticles due to the mixture of cubic and hexagonal CdSe and ZnSe phases. 3.3. Optical absorption and electrical resistivity studies Optical absorption of Cd0.5Zn0.5Se composite thin film was carried out in the wavelength range of 350–800 nm. The plot of optical absorption versus wavelength is as shown in Fig. 5(a). The band gap, ‘Eg’ was calculated using the standard relation, a = A (hn Eg)n/hn, where A is constant, ‘Eg’ is the band gap, n is a constant, 1/2 for direct band gap semiconductors. The estimated band gap from the plot of (ahn)2 versus hn is as shown in Fig. 5(b). The linear nature of the plot indicates the direct band gap of thin film. The band gap ‘Eg’, was obtained by extrapolating the straight portion to energy axis at a = 0. It is
Fig. 5. Variation of optical absorption with wavelength (a) and the graph of (ahn)2 vs. hn of Cd0.5Zn0.5Se composite thin film.
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R.B. Kale et al. / Applied Surface Science 253 (2007) 3109–3112
Acknowledgements R.B. Kale is grateful to Western Regional Centre, UGC, New Delhi, for its financial assistance under the minor research project no. F: 47-185/2002. This work is partially supported by Hanyang University Next Generation Development Program. References
Fig. 6. Variation of log r vs. 1000/T for Cd0.5Zn0.5Se composite thin film.
found that the band gap ‘Eg’ for Cd0.5Zn0.5Se composite thin film was 2.35 eV, that falls between the band gap value of CdSe (1.7 eV) and ZnSe film (2.7 eV) [26–28]. Due to higher band gap energy, this finds an application as a buffer layer for CuInSe2 based solid state solar cells in addition to higher resistance (next section), good surface coverage etc. The dark electrical conductivity of Cd0.5Zn0.5Se composite thin film was determined using a ‘dc’ two-probe method, in the temperature range of 300–525 K. At room temperature the resistivity was found to be in the order of 107 V cm. The plot of the log r versus inverse of absolute temperature at the cooling time is shown in Fig. 6. A plot shows that electrical resistivity has two linear regions, an intrinsic region setting at low temperature, characterized by small slope (300–350 K and that of high temperature region that is associated with extrinsic conduction due to the presence of donor states produced by excess cadmium. The foreseen reasons for high resistivity are; nanocrystalline nature, crystallite boundary discontinuities, presence of defect states, adsorbed and absorbed gases, and presence of different phases of CdSe and ZnSe [21]. 4. Conclusions Cd0.5Zn0.5Se composite thin films composed of mixed cubic and hexagonal phases of CdSe and ZnSe are synthesized by CBD process at low temperature and characterized by XRD, SEM and AFM techniques which revealed the nanocrystalline nature of film with different grain sizes and shapes. Optical band gap of Cd0.5Zn0.5Se composite thin film was found to be in between individual band-gaps of CdSe and ZnSe and the dark electrical resistivity was about 107 V cm. Due to above properties, these films find applications as a buffer layer in solar cells.
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