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Structural and electrical properties of spray deposited thin films of CuInS2 nanocrystals
Materials Letters 68 (2012) 497–500
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Structural and electrical properties of spray deposited thin films of CuInS2 nanocrystals M.A. Majeed Khan a,⁎, Sushil Kumar b, Maqusood Ahamed a, Mohamad S. AlSalhi a, c a b c
King Abdullah Institute for Nanotechnology, King Saud University, Riyadh-11451, Saudi Arabia Department of Physics, Chaudhary Devi Lal University, Sirsa-125 055, Haryana, India Department of Physics and Astronomy, King Saud University, Riyadh-11451, Saudi Arabia
a r t i c l e
i n f o
Article history: Received 13 October 2011 Accepted 9 November 2011 Available online 15 November 2011 Keywords: CuInS2 thin films Solar energy materials Spray pyrolysis Electrical properties
a b s t r a c t Nanocrystalline thin films of CuInS2 were grown onto ultra clean glass substrates using the spray pyrolysis technique. The films were characterized by X-ray diffraction, field emission scanning electron microscopy, field emission transmission electron microscopy, high resolution transmission electron microscopy, energy dispersive X-ray spectroscopy and resistivity measurement. XRD pattern, SEM and TEM micrographs show that the films grown at about 300 °C are made up of single phase nano-sized (12–15 nm) particles of CuInS2. Using Williamson–Hall equation, crystallite size and lattice strain of the film were estimated with the broadening of XRD peaks. EDX of nanoparticles dispersion confirmed the presence of elemental CuInS2 and no peaks of impurity have been detected. The temperature dependence of resistivity of CuInS2 thin films, determined in the temperature range of 100–300 K, exhibits their semiconducting behavior. The films showed the activated variable range hopping (VRH) in the localized states near the Fermi level. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Thin films of semiconductor nanocrystals have attracted significant attention in research and applications in areas including energy conversion, sensing, electronics, photonics, and biomedicine. Crystal parameters such as size, shape, and surface characteristics can be modified to control their properties for different applications of interest. CuInS2, a quality absorber material for solar cells [1] having high chemical and thermal stability; also has perspective application in biomedical labeling [2]. Many studies have been focused on finding the correlation between its preparation methods and the resulting properties regarding structure, morphology and electrical conductivity. A variety of techniques have been applied to deposit CuInS2 thin films including reactive radio frequency magnetron sputtering [3], co-evaporation [4], ion layer gas reaction [5], wet chemical process [6] and spray pyrolysis [7]. Earlier much less attention has been paid to sprayed CuInS2 thin films. For economic reasons, spray pyrolysis is a viable method because large area films with good uniformity can be grown at low cost. The properties of films could be controlled effectively by varying the deposition parameters. The composition of films could be controlled by changing the concentrations of constituents in spray solution. In this work, we report the results of structural, morphological and electrical
⁎ Corresponding author. Tel.: + 966 14676188; fax: + 966 14670662. E-mail address: [email protected] (M.A.M. Khan). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.11.033
investigations on CuInS2 thin films prepared by spray pyrolysis technique. 2. Experimental details The nanocrystalline CuInS2 films were grown onto ultra-clean pre-heated glass substrates by spraying a thoroughly stirred mixture of aqueous solutions of CuCl2, InCl3 and (NH2)2CS precursors. We started with a molar concentration of 0.1 M for all of them and the molar ratio of Cu:In:S was kept at 1:1:3 by varying the relative volume of each reactant in the solution. The good quality films were grown at optimum deposition parameters such as substrate temperature 300 °C, heat treatment after deposition 30 min, spray solution volume 50 ml, solution concentration ([Cu 2 + 1] = 2 mmol/l), solution flow rate 5 ml/min and the carrier gas (nitrogen) pressure 4.5 × 10 3 Pa, nozzle to substrate distance 20 cm which were kept constant for all samples. Spray nozzle and heater with substrate were placed in a chamber having an exhaust fan for removing gaseous products and vapors of solvent. As deposited CuInS2 films were characterized by various techniques like X-ray diffraction, field emission scanning electron microscopy, field emission transmission electron microscopy, high resolution transmission electron microscopy, and energy dispersive X-ray spectroscopy. For electrical resistivity measurements, the planer geometry of films (length~ 1.5 cm, electrode gap~ 5.0 mm and thickness ~0.5 μ) was used. Indium contacts were deposited on the films to act as electrodes. The measurements were carried out in a specially designed metallic sample holder where a vacuum of 10− 3 Torr was maintained. The dc voltage (1.5 V) was
498
M.A.M. Khan et al. / Materials Letters 68 (2012) 497–500
Fig. 1. X-ray diffraction pattern of thin film of CuInS2 nanocrystals and inset shows the Hall and Williamson plot for the same.
applied across the sample and a digital electrometer (Keithley-617) was used to measure the current; and for low temperature measurement, liquid nitrogen was used to maintain the desired temperature in the cryostat. 3. Results and discussion 3.1. Structural analysis X-ray diffraction technique has been employed to identify the structure of film material. Fig. 1 shows the X-ray diffraction pattern of thin films of CuInS2 nanocrystals. All diffraction peaks can be indexed to the chalcopyrite (tetragonal) structured CuInS2. After intensive observation, lattice constants, a = 5.524 Å and c = 11.137 Å, have been obtained which matched well to the reported values for CuInS2 crystal (JCPDS card No. 85-1575). The broadening of XRD peaks suggests that the grain size of sample is on a nanometer scale. The relationship between grain (crystallite) size and X-ray line broadening can be described by Scherrer's equation: D ¼ 0:9λ=β Cos θ
ð1Þ
3.2. Morphological analysis The surface morphology and microstructure of CuInS2 thin films were analyzed using electron microscopy techniques. Fig. 2 (and inset) shows the SEM images, which reveals that the films are observed to be almost uniform with only few very small pinholes or cracks and are composed of nano-sized cubic-like grains. The TEM image of CuInS2 nanocrystals is shown in inset of Fig. 3, which confirms that the crystals have an average size of 15 nm and their surface is smooth. The HRTEM image Fig. 4(a) shows that as prepared nanocrystals consist of small nanocrystalline domains, displaying their polycrystalline structure. The lattice plane spacing was calculated to be ~ 0.31 nm corresponding to (112) planes of CuInS2 nanocrystals in tetragonal form. The EDX spectrum shows intense peaks of Cu, In and S, displaying the composition as containing Cu, In and S only (Fig. 3). 3.3. Electrical studies The electrical resistivity as a function of temperature for CuInS2 thin films is shown in Fig. 4(b). A decrease in resistivity has been
where λ, β, and θ represent the wavelength of X-ray source, the full width at half maximum (FWHM) of a peak, and the Bragg angle, respectively. The crystallite size of CuInS2 nanocrystals was determined to be 12.1 nm corresponding to the prominent peak (112). In order to distinguish the effect of crystallite-size-induced broadening and strain-induced broadening at the FWHM of XRD profile, Williamson–Hall plot [8] was performed as β cos θ ¼ 2η sin θ þ c λ=D
ð2Þ
where D, η and C are the grain size, strain and correlation factor. β is the FWHM in radians and λ is the X-ray wavelength in nanometers. A plot of β cosθ vs. sinθ produced a straight line of slope equal to 2η and intercept equal to Cλ/D as shown in the inset of Fig. 1. The grain size and strain of CuInS2 nanocrystals were found to be 13.83 nm and 7.9 × 10 − 3 respectively. The obtained value of grain size reveals nanostructured CuInS2.
Fig. 2. FESEM images of CuInS2 nanocrystals.
M.A.M. Khan et al. / Materials Letters 68 (2012) 497–500
499
Fig. 3. EDX profile of CuInS2 nanocrystals and inset shows the FETEM image for the same.
observed in the temperature range 100–300 K with increase in temperature showing the semiconducting behavior of the films. In these semiconductors, there are additional energy levels in the band gap, which are localized and close to either the conduction or the valence band. Since the energy difference between these levels and band edges is very small, a slight thermal excitation is sufficient to accept or donate electrons; thereby the electrical resistivity decreases with increase in temperature. For electron transport in nanocrystalline materials, due to small size of grains and large grain boundaries, the electronic states close to Fermi level are localized. When the states are localized the conduction occurs by hopping of carriers between occupied to unoccupied localized state which depends on the density of these states and the position of Fermi level. Mott [9] established the dependence of resistivity on temperature for such systems. According to Mott's variable range hopping (VRH) model, the electrical resistivity can be written as s ρðTÞ ¼ ρo exp ðTo =TÞ
ð3Þ
where T0 is a characteristic temperature coefficient and ρ0 is the high temperature limit of resistivity. The value of exponent s depends critically on the nature of hopping process. In Mott's VRH model, if the density of states at the Fermi level is constant, s = 1/4 for a three dimensional (3D) system and s = 1/3 for a two dimensional (2D) system. In this case, for a 3D system, the resistivity can be expressed as [10] h i 1=4 ρðTÞ ¼ ρo exp ðTo =TÞ
ð4Þ
where T0 is given by the following expression [11] 3
To ¼ Co =kB NðEF Þα
ð5Þ
where kB is the Boltzmann constant, N(EF) is the unit energy density of states and α is the localized length which typically varies in the range 3–30 Å, C0 is a constant which has a typical value in the range 16–310 [12]. The linear fit of electrical data, shown in the
Fig. 4. (a) HRTEM image of CuInS2 nanocrystals (b) Resistivity as function of temperature for thin film of CuInS2 nanocrystals and inset shows the plot of ln ρ vs T− 1/4.
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M.A.M. Khan et al. / Materials Letters 68 (2012) 497–500
inset of Fig. 4(b), confirms that the temperature dependence of resistivity in this temperature range satisfies Mott's formula. From the fitted value of To, we have found the value of density of states at the Fermi level approximately 2.09 × 1027 eV− 1 m − 3 for CuInS2 nanocrystals (values of C0 = 16 and 310 and decay length of electronic wave function = 10 Å have been used). The value of density of states at the Fermi level for CuInS2 has also been reported by different authors [13]. 4. Conclusions In this paper, nanocrystalline CuInS2 thin films were prepared at deposition temperature of 300 °C by employing spray pyrolysis technique. XRD results exhibit that the crystallite size of polycrystalline film material ranges from 10 to 13 nm. SEM and TEM micrographs show that the particle size of the films is in the range of 12–15 nm. HRTEM image shows that as prepared nanocrystals consist of small nanocrystalline domains with lattice plane spacing ~ 0.31 nm. The temperature dependent resistivity of CuInS2 thin films shows the activated variable range hopping (VRH) in the localized states near the Fermi level.
Acknowledgments Thanks are due to National plan for Science and Technology (NPST), KACST, Riyadh, Saudi Arabia (Grant No.: 10-NAN1001-02) for providing financial assistant in the form of major research project. References [1] Peng S, Cheng F, Liang J, Tao Z, Chen J. J Alloys Compd 2009;481:786. [2] Nakamura H, Kato W, Uehara M, Nose K, Omata T, Otsuka-Yao-Matsuo S, et al. Chem Mater 2006;18:3330. [3] Liu XP, Shao LX. Surf CoatTechnol 2007;201:5340. [4] Gossla M, Metzner H, Mahnke HE. Thin Solid Films 2001;387:77. [5] Qiu J, Jin Z, Qian J, Shi Y, Wu W. Mater Lett 2005;59:2735. [6] Guha P, Gorai S, Ganguli D, Chaudhuri S. Mater Lett 2003;57:1786. [7] Sebastian T, Gopinath M, Sudha kartha C, Vijaya kumar KP, Abe T, Kashiwaba Y. Sol Energy 2009;83:1683. [8] Williamson GK, Hall WH. Acta Metall 1953;1:22. [9] Mott NF, Davis EA. Electronic processes in non-crystalline materials. 2nd Edition. New York: Oxford University Press; 1979. [10] Mott NF. J Non-Cryst Solids 1968;1:1. [11] Paasch G, Lindner T, Scheinert S. Synth Met 2002;132:97. [12] Godet CJ. J Non-Cryst Solids 2002;299–302:333. [13] Amara A, Rezaiki W, Ferdi A, Hendaoui A, Drici A, Guerioune M, et al. Sol Energy Mater Sol Cells 2007;91:1916.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Structural and electrical properties of spray deposited thin films of CuInS2 nanocrystals M.A. Majeed Khan a,⁎, Sushil Kumar b, Maqusood Ahamed a, Mohamad S. AlSalhi a, c a b c
King Abdullah Institute for Nanotechnology, King Saud University, Riyadh-11451, Saudi Arabia Department of Physics, Chaudhary Devi Lal University, Sirsa-125 055, Haryana, India Department of Physics and Astronomy, King Saud University, Riyadh-11451, Saudi Arabia
a r t i c l e
i n f o
Article history: Received 13 October 2011 Accepted 9 November 2011 Available online 15 November 2011 Keywords: CuInS2 thin films Solar energy materials Spray pyrolysis Electrical properties
a b s t r a c t Nanocrystalline thin films of CuInS2 were grown onto ultra clean glass substrates using the spray pyrolysis technique. The films were characterized by X-ray diffraction, field emission scanning electron microscopy, field emission transmission electron microscopy, high resolution transmission electron microscopy, energy dispersive X-ray spectroscopy and resistivity measurement. XRD pattern, SEM and TEM micrographs show that the films grown at about 300 °C are made up of single phase nano-sized (12–15 nm) particles of CuInS2. Using Williamson–Hall equation, crystallite size and lattice strain of the film were estimated with the broadening of XRD peaks. EDX of nanoparticles dispersion confirmed the presence of elemental CuInS2 and no peaks of impurity have been detected. The temperature dependence of resistivity of CuInS2 thin films, determined in the temperature range of 100–300 K, exhibits their semiconducting behavior. The films showed the activated variable range hopping (VRH) in the localized states near the Fermi level. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Thin films of semiconductor nanocrystals have attracted significant attention in research and applications in areas including energy conversion, sensing, electronics, photonics, and biomedicine. Crystal parameters such as size, shape, and surface characteristics can be modified to control their properties for different applications of interest. CuInS2, a quality absorber material for solar cells [1] having high chemical and thermal stability; also has perspective application in biomedical labeling [2]. Many studies have been focused on finding the correlation between its preparation methods and the resulting properties regarding structure, morphology and electrical conductivity. A variety of techniques have been applied to deposit CuInS2 thin films including reactive radio frequency magnetron sputtering [3], co-evaporation [4], ion layer gas reaction [5], wet chemical process [6] and spray pyrolysis [7]. Earlier much less attention has been paid to sprayed CuInS2 thin films. For economic reasons, spray pyrolysis is a viable method because large area films with good uniformity can be grown at low cost. The properties of films could be controlled effectively by varying the deposition parameters. The composition of films could be controlled by changing the concentrations of constituents in spray solution. In this work, we report the results of structural, morphological and electrical
⁎ Corresponding author. Tel.: + 966 14676188; fax: + 966 14670662. E-mail address: [email protected] (M.A.M. Khan). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.11.033
investigations on CuInS2 thin films prepared by spray pyrolysis technique. 2. Experimental details The nanocrystalline CuInS2 films were grown onto ultra-clean pre-heated glass substrates by spraying a thoroughly stirred mixture of aqueous solutions of CuCl2, InCl3 and (NH2)2CS precursors. We started with a molar concentration of 0.1 M for all of them and the molar ratio of Cu:In:S was kept at 1:1:3 by varying the relative volume of each reactant in the solution. The good quality films were grown at optimum deposition parameters such as substrate temperature 300 °C, heat treatment after deposition 30 min, spray solution volume 50 ml, solution concentration ([Cu 2 + 1] = 2 mmol/l), solution flow rate 5 ml/min and the carrier gas (nitrogen) pressure 4.5 × 10 3 Pa, nozzle to substrate distance 20 cm which were kept constant for all samples. Spray nozzle and heater with substrate were placed in a chamber having an exhaust fan for removing gaseous products and vapors of solvent. As deposited CuInS2 films were characterized by various techniques like X-ray diffraction, field emission scanning electron microscopy, field emission transmission electron microscopy, high resolution transmission electron microscopy, and energy dispersive X-ray spectroscopy. For electrical resistivity measurements, the planer geometry of films (length~ 1.5 cm, electrode gap~ 5.0 mm and thickness ~0.5 μ) was used. Indium contacts were deposited on the films to act as electrodes. The measurements were carried out in a specially designed metallic sample holder where a vacuum of 10− 3 Torr was maintained. The dc voltage (1.5 V) was
498
M.A.M. Khan et al. / Materials Letters 68 (2012) 497–500
Fig. 1. X-ray diffraction pattern of thin film of CuInS2 nanocrystals and inset shows the Hall and Williamson plot for the same.
applied across the sample and a digital electrometer (Keithley-617) was used to measure the current; and for low temperature measurement, liquid nitrogen was used to maintain the desired temperature in the cryostat. 3. Results and discussion 3.1. Structural analysis X-ray diffraction technique has been employed to identify the structure of film material. Fig. 1 shows the X-ray diffraction pattern of thin films of CuInS2 nanocrystals. All diffraction peaks can be indexed to the chalcopyrite (tetragonal) structured CuInS2. After intensive observation, lattice constants, a = 5.524 Å and c = 11.137 Å, have been obtained which matched well to the reported values for CuInS2 crystal (JCPDS card No. 85-1575). The broadening of XRD peaks suggests that the grain size of sample is on a nanometer scale. The relationship between grain (crystallite) size and X-ray line broadening can be described by Scherrer's equation: D ¼ 0:9λ=β Cos θ
ð1Þ
3.2. Morphological analysis The surface morphology and microstructure of CuInS2 thin films were analyzed using electron microscopy techniques. Fig. 2 (and inset) shows the SEM images, which reveals that the films are observed to be almost uniform with only few very small pinholes or cracks and are composed of nano-sized cubic-like grains. The TEM image of CuInS2 nanocrystals is shown in inset of Fig. 3, which confirms that the crystals have an average size of 15 nm and their surface is smooth. The HRTEM image Fig. 4(a) shows that as prepared nanocrystals consist of small nanocrystalline domains, displaying their polycrystalline structure. The lattice plane spacing was calculated to be ~ 0.31 nm corresponding to (112) planes of CuInS2 nanocrystals in tetragonal form. The EDX spectrum shows intense peaks of Cu, In and S, displaying the composition as containing Cu, In and S only (Fig. 3). 3.3. Electrical studies The electrical resistivity as a function of temperature for CuInS2 thin films is shown in Fig. 4(b). A decrease in resistivity has been
where λ, β, and θ represent the wavelength of X-ray source, the full width at half maximum (FWHM) of a peak, and the Bragg angle, respectively. The crystallite size of CuInS2 nanocrystals was determined to be 12.1 nm corresponding to the prominent peak (112). In order to distinguish the effect of crystallite-size-induced broadening and strain-induced broadening at the FWHM of XRD profile, Williamson–Hall plot [8] was performed as β cos θ ¼ 2η sin θ þ c λ=D
ð2Þ
where D, η and C are the grain size, strain and correlation factor. β is the FWHM in radians and λ is the X-ray wavelength in nanometers. A plot of β cosθ vs. sinθ produced a straight line of slope equal to 2η and intercept equal to Cλ/D as shown in the inset of Fig. 1. The grain size and strain of CuInS2 nanocrystals were found to be 13.83 nm and 7.9 × 10 − 3 respectively. The obtained value of grain size reveals nanostructured CuInS2.
Fig. 2. FESEM images of CuInS2 nanocrystals.
M.A.M. Khan et al. / Materials Letters 68 (2012) 497–500
499
Fig. 3. EDX profile of CuInS2 nanocrystals and inset shows the FETEM image for the same.
observed in the temperature range 100–300 K with increase in temperature showing the semiconducting behavior of the films. In these semiconductors, there are additional energy levels in the band gap, which are localized and close to either the conduction or the valence band. Since the energy difference between these levels and band edges is very small, a slight thermal excitation is sufficient to accept or donate electrons; thereby the electrical resistivity decreases with increase in temperature. For electron transport in nanocrystalline materials, due to small size of grains and large grain boundaries, the electronic states close to Fermi level are localized. When the states are localized the conduction occurs by hopping of carriers between occupied to unoccupied localized state which depends on the density of these states and the position of Fermi level. Mott [9] established the dependence of resistivity on temperature for such systems. According to Mott's variable range hopping (VRH) model, the electrical resistivity can be written as s ρðTÞ ¼ ρo exp ðTo =TÞ
ð3Þ
where T0 is a characteristic temperature coefficient and ρ0 is the high temperature limit of resistivity. The value of exponent s depends critically on the nature of hopping process. In Mott's VRH model, if the density of states at the Fermi level is constant, s = 1/4 for a three dimensional (3D) system and s = 1/3 for a two dimensional (2D) system. In this case, for a 3D system, the resistivity can be expressed as [10] h i 1=4 ρðTÞ ¼ ρo exp ðTo =TÞ
ð4Þ
where T0 is given by the following expression [11] 3
To ¼ Co =kB NðEF Þα
ð5Þ
where kB is the Boltzmann constant, N(EF) is the unit energy density of states and α is the localized length which typically varies in the range 3–30 Å, C0 is a constant which has a typical value in the range 16–310 [12]. The linear fit of electrical data, shown in the
Fig. 4. (a) HRTEM image of CuInS2 nanocrystals (b) Resistivity as function of temperature for thin film of CuInS2 nanocrystals and inset shows the plot of ln ρ vs T− 1/4.
500
M.A.M. Khan et al. / Materials Letters 68 (2012) 497–500
inset of Fig. 4(b), confirms that the temperature dependence of resistivity in this temperature range satisfies Mott's formula. From the fitted value of To, we have found the value of density of states at the Fermi level approximately 2.09 × 1027 eV− 1 m − 3 for CuInS2 nanocrystals (values of C0 = 16 and 310 and decay length of electronic wave function = 10 Å have been used). The value of density of states at the Fermi level for CuInS2 has also been reported by different authors [13]. 4. Conclusions In this paper, nanocrystalline CuInS2 thin films were prepared at deposition temperature of 300 °C by employing spray pyrolysis technique. XRD results exhibit that the crystallite size of polycrystalline film material ranges from 10 to 13 nm. SEM and TEM micrographs show that the particle size of the films is in the range of 12–15 nm. HRTEM image shows that as prepared nanocrystals consist of small nanocrystalline domains with lattice plane spacing ~ 0.31 nm. The temperature dependent resistivity of CuInS2 thin films shows the activated variable range hopping (VRH) in the localized states near the Fermi level.
Acknowledgments Thanks are due to National plan for Science and Technology (NPST), KACST, Riyadh, Saudi Arabia (Grant No.: 10-NAN1001-02) for providing financial assistant in the form of major research project. References [1] Peng S, Cheng F, Liang J, Tao Z, Chen J. J Alloys Compd 2009;481:786. [2] Nakamura H, Kato W, Uehara M, Nose K, Omata T, Otsuka-Yao-Matsuo S, et al. Chem Mater 2006;18:3330. [3] Liu XP, Shao LX. Surf CoatTechnol 2007;201:5340. [4] Gossla M, Metzner H, Mahnke HE. Thin Solid Films 2001;387:77. [5] Qiu J, Jin Z, Qian J, Shi Y, Wu W. Mater Lett 2005;59:2735. [6] Guha P, Gorai S, Ganguli D, Chaudhuri S. Mater Lett 2003;57:1786. [7] Sebastian T, Gopinath M, Sudha kartha C, Vijaya kumar KP, Abe T, Kashiwaba Y. Sol Energy 2009;83:1683. [8] Williamson GK, Hall WH. Acta Metall 1953;1:22. [9] Mott NF, Davis EA. Electronic processes in non-crystalline materials. 2nd Edition. New York: Oxford University Press; 1979. [10] Mott NF. J Non-Cryst Solids 1968;1:1. [11] Paasch G, Lindner T, Scheinert S. Synth Met 2002;132:97. [12] Godet CJ. J Non-Cryst Solids 2002;299–302:333. [13] Amara A, Rezaiki W, Ferdi A, Hendaoui A, Drici A, Guerioune M, et al. Sol Energy Mater Sol Cells 2007;91:1916.