- Email: [email protected]
Preparation and characterization of ZnTe thin films by SILAR method
Applied Surface Science 253 (2007) 4335–4337 www.elsevier.com/locate/apsusc
Preparation and characterization of ZnTe thin films by SILAR method S.S. Kale b, R.S. Mane b, H.M. Pathan a, A.V. Shaikh c, Oh-Shim Joo a, Sung-Hwan Han b,* a b
Eco-Nano Research Centre, Korea Institute of Science and Technology, P.O. Box 131, Chongryang, Seoul 130-650, Republic of Korea Inorganic Nano-Materials Laboratory, Department of Chemistry, Hanyang University, Sungdong-Ku, Seoul 133-791, Republic of Korea c AKI’s Poona College of Arts, Science & Commerce, Pune, India Received 24 July 2006; received in revised form 21 September 2006; accepted 21 September 2006 Available online 27 October 2006
Abstract Nanocrystalline zinc telluride (ZnTe) thin films were prepared by using successive ionic layer adsorption and reaction (SILAR) method from aqueous solutions of zinc sulfate and sodium telluride. The films were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis and optical absorption measurement techniques. The synthesized ZnTe thin films were nanocrystalline with densely aggregated particles in nanometer scale and were free from the voids or cracks. The optical band gap energy of the film was found to be thickness dependent. The elemental chemical compositional stoichiometric analysis revealed good Zn:Te elemental ratio of 53:47. # 2006 Elsevier B.V. All rights reserved. Keywords: ZnTe; Thin films; Solution chemistry; XRD; SEM; Optical studies
1. Introduction Zinc telluride is one of the important semiconductor materials of II–VI group due to its extensive potential application in different opto-electronic devices [1,2]. Recent study on ZnTe thin films reveals that it can be used in optoelectronic detection of THz radiation [3]. The ZnTe thin films grown at room temperature and high temperature are found to be polycrystalline in nature [4]. Techniques for the synthesis of thin films from aqueous solutions at low temperatures are emerging as possible alternatives to vapor-phase and chemical-precursor techniques. Lower temperatures allow films to be deposited on the substrates that might not be chemically or mechanically stable at high temperatures. Unlike vapor-phase processes, techniques that use liquids as the deposition medium do not rely on line-ofsight deposition, so that non-planar substrates can be easily coated. The equipment for liquid-based techniques is simple and much less costly than vacuum systems. Aqueous deposition techniques for films have not yet reached this level of development. Nevertheless, their potential to produce films over large areas at comparatively low cost has continued to
* Corresponding author. Tel.: +822 2292 5212; fax: +822 2290 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.2006.09.043
make these techniques attractive and has stimulated a resurgence of interest in them. Successive ionic layer adsorption and reaction (SILAR) method is one of the solution methods. SILAR method does not require any sophisticated instruments like physical techniques. More details of SILAR method are well documented in literature [5]. Due to the scarcity of p-type thin films used for p– n junction solar cell devices, we have reported synthesis of ZnTe thin films using SILAR method and characterized for its structural, optical, surface morphological properties. 2. Experimental details The substrates used for the deposition of ZnTe thin films were commercial microscope glass slides of dimensions, 15 mm 25 mm 25 mm. Before deposition, the substrates were degreased with ethanol, cleaned with de-ionized water and finally dried in air. Aqueous solutions of 0.1 M zinc sulfate (Zn(CH3COO)2), 0.1 M sodium telluride were used to prepare ZnTe thin films. First, 40 mL zinc sulfate solution was placed in a 50 mL beaker. After stirring for several minutes, the solution became clear and homogenous. Then under continuous stirring, 10 mL ammonia solution was introduced. The pH value of the result solution was adjusted to 10. 0.1 M sodium telluride solution was placed in another 50 mL beaker. One SILAR cycle of ZnTe consisted four steps: (a) substrate immersed into cationic
4336
S.S. Kale et al. / Applied Surface Science 253 (2007) 4335–4337
precursor (0.1 M zinc sulfate) for 10 s so that Zn2+ ions were adsorbed on the substrate surface, (b) the substrate was then rinsed with distilled water for 20 s to remove loosely bounded Zn2+ ions from the substrates, (c) further, the substrate was immersed into 0.1 M sodium telluride anionic precursor solution for 10 s where Te2 ions were adsorbed and reacted with Zn2+ to form ZnTe thin film, and (d) finally, again substrate was rinsed with distilled water for 20 s to remove un-adsorbed and unreacted Te2 ions from the substrate. Thus, 10 s adsorption of cations, 20 s rinsing of cations, 10 s adsorption and reaction of anions and 20 s rinsing anions and loosely bounded ions, presents one SILAR cycle. The deposited ZnTe films were white, homogeneous with a good adherence to the substrate. The films were deposited for 10, 20 and 30 deposition cycles. For the structural elucidation of the films of different thicknesses, X-ray diffraction (XRD) analysis was performed on a Philips (PW-3710) diffractometer with copper target ˚ ). The surface morphology of ZnTe films was (l = 1.54 A confirmed from scanning electron microscopy (SEM) images for different magnifications under the same operating conditions. The SEM unit was operated at 20 kV. For this, the films were coated with gold–palladium (Au–Pd) using polaron SEM sputter coating with E-2500 due to the high electrical resistivity (107 to 108 V cm). The optical absorbance was measured within the wavelength range of 350–850 nm using a Systronic spectrophotometer-119. Energy dispersive X-ray analysis (EDX) was employed for the compositional analysis. Thickness of ZnTe film was measured using gravimetric weight difference method and was 270 nm for 30 deposition cycles. 3. Results and discussion 3.1. Structural studies Fig. 1 shows XRD pattern of ZnTe thin film deposited on glass substrate subjected to above conditions. The presence of broad hump is due to the glass substrate. Nanocrystalline nature of deposited ZnTe films is confirmed from XRD pattern as observed diffraction peaks are weak and are of low intensity. Comparison of observed interplanar spacing ‘d’ values with standard ‘d’ values shows that the deposited in thin film form material is ZnTe. The two observed diffraction peaks at an angle 2u 278 and 30.058 are correspond to (1 0 1) and (2 0 0)
Fig. 1. XRD pattern of ZnTe thin films deposited CBD.
Fig. 2. Plot of (absorbance)2 against energy for ZnTe thin films [inset: plot of absorbance against wavelength for ZnTe thin films].
planes of ZnTe. For standard ‘d’ values 19-1482 and 15-0746 JCPDF files were preferred. 3.2. Optical studies The absorption spectra of samples with different thicknesses were studied in the wavelength range of 300–850 nm. The optical absorbance of thin film is closely related to its morphology. Fig. 2 (inset) shows the variation of optical absorption with wavelength. This data was further used for analyzing optical direct band gap energy, a = a(hn Eg)n/2/hn classical relation for near edge optical absorption in semiconductor where ‘a’ is a constant, Eg is the semiconductor band gap energy and ‘n’ is a number equal to 1 for the direct band gap and 4 for the indirect gap semiconductors. It is seen from the graph that optical absorption increases with increase in
Fig. 3. SEM images of ZnTe thin films at 60,000 magnifications.
S.S. Kale et al. / Applied Surface Science 253 (2007) 4335–4337
the film thickness. The plots of (absorbance)2 versus hn are plotted (Fig. 2) for estimating the value of direct band gap energy of ZnTe films by extrapolating curves to zero absorption coefficient. The estimated optical band gap energies are 2.75, 3.00 and 3.15 eV for ZnTe film thicknesses of 75, 140 and 270 nm, respectively. All these values of band gap are above the normal values of ZnTe (1.44 eV). We can infer that the band gap energy is partially dependent on the crystallinity of the film, as the band gap energies of highly crystalline thin films are similar to those of crystalline bulk materials, whereas amorphous or poorly crystallized films show band gap energies higher than those of the corresponding bulk materials.
4. Conclusion
3.3. Surface morphological and composition studies
References
Scanning electron micrographs of zinc telluride thin film of 270 nm film thickness on glass substrate is shown in Fig. 3. The film surfaces were free from the pinholes and cracks. Zinc telluride film with un-even grains mostly falling in nanometer regime is clearly seen. EDX study of ZnTe film showed the presence of Zn and Te in the film. The composition of the Zn:Te was observed to be 53:47.
4337
The nanocrystalline ZnTe thin films were deposited on commercial glass substrates by successive ionic layer adsorption and reaction (SILAR) method. The XRD study reveals that formed material is nanocrystalline. From SEM micrograph, well coverage of substrate surface and nanometer scale spherical grains of ZnTe were confirmed. The optical absorption studies showed that the zinc telluride is direct band gap material and band gap energy increases from 2.75 to 3.15 eV with increase in film thickness from 75 to 270 nm.
[1] R.C. Weast, CRC Handbook of Chemistry and Physics, 58th ed., CRC Press Inc, Cleveland, 1977–78, p. 101. [2] P.C. Kalita, K.C. Sarma, H.L. Das, J. Assam Sci. Soc. 39 (1998) 117. [3] C. Winnerwisser, P.U. Jepsen, M. Schall, V. Schiya, H. Helm, Appl. Phys. Lett. 70 (1997) 3069. [4] P.K. Kalita, B.K. Sarma, H.L. Das, Indian J. Pure Appl. Phys. 37 (1999) 885. [5] H.M. Pathan, C.D. Lokhande, Bull. Mater. Sci. 27 (2004) 85.
Preparation and characterization of ZnTe thin films by SILAR method S.S. Kale b, R.S. Mane b, H.M. Pathan a, A.V. Shaikh c, Oh-Shim Joo a, Sung-Hwan Han b,* a b
Eco-Nano Research Centre, Korea Institute of Science and Technology, P.O. Box 131, Chongryang, Seoul 130-650, Republic of Korea Inorganic Nano-Materials Laboratory, Department of Chemistry, Hanyang University, Sungdong-Ku, Seoul 133-791, Republic of Korea c AKI’s Poona College of Arts, Science & Commerce, Pune, India Received 24 July 2006; received in revised form 21 September 2006; accepted 21 September 2006 Available online 27 October 2006
Abstract Nanocrystalline zinc telluride (ZnTe) thin films were prepared by using successive ionic layer adsorption and reaction (SILAR) method from aqueous solutions of zinc sulfate and sodium telluride. The films were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis and optical absorption measurement techniques. The synthesized ZnTe thin films were nanocrystalline with densely aggregated particles in nanometer scale and were free from the voids or cracks. The optical band gap energy of the film was found to be thickness dependent. The elemental chemical compositional stoichiometric analysis revealed good Zn:Te elemental ratio of 53:47. # 2006 Elsevier B.V. All rights reserved. Keywords: ZnTe; Thin films; Solution chemistry; XRD; SEM; Optical studies
1. Introduction Zinc telluride is one of the important semiconductor materials of II–VI group due to its extensive potential application in different opto-electronic devices [1,2]. Recent study on ZnTe thin films reveals that it can be used in optoelectronic detection of THz radiation [3]. The ZnTe thin films grown at room temperature and high temperature are found to be polycrystalline in nature [4]. Techniques for the synthesis of thin films from aqueous solutions at low temperatures are emerging as possible alternatives to vapor-phase and chemical-precursor techniques. Lower temperatures allow films to be deposited on the substrates that might not be chemically or mechanically stable at high temperatures. Unlike vapor-phase processes, techniques that use liquids as the deposition medium do not rely on line-ofsight deposition, so that non-planar substrates can be easily coated. The equipment for liquid-based techniques is simple and much less costly than vacuum systems. Aqueous deposition techniques for films have not yet reached this level of development. Nevertheless, their potential to produce films over large areas at comparatively low cost has continued to
* Corresponding author. Tel.: +822 2292 5212; fax: +822 2290 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.2006.09.043
make these techniques attractive and has stimulated a resurgence of interest in them. Successive ionic layer adsorption and reaction (SILAR) method is one of the solution methods. SILAR method does not require any sophisticated instruments like physical techniques. More details of SILAR method are well documented in literature [5]. Due to the scarcity of p-type thin films used for p– n junction solar cell devices, we have reported synthesis of ZnTe thin films using SILAR method and characterized for its structural, optical, surface morphological properties. 2. Experimental details The substrates used for the deposition of ZnTe thin films were commercial microscope glass slides of dimensions, 15 mm 25 mm 25 mm. Before deposition, the substrates were degreased with ethanol, cleaned with de-ionized water and finally dried in air. Aqueous solutions of 0.1 M zinc sulfate (Zn(CH3COO)2), 0.1 M sodium telluride were used to prepare ZnTe thin films. First, 40 mL zinc sulfate solution was placed in a 50 mL beaker. After stirring for several minutes, the solution became clear and homogenous. Then under continuous stirring, 10 mL ammonia solution was introduced. The pH value of the result solution was adjusted to 10. 0.1 M sodium telluride solution was placed in another 50 mL beaker. One SILAR cycle of ZnTe consisted four steps: (a) substrate immersed into cationic
4336
S.S. Kale et al. / Applied Surface Science 253 (2007) 4335–4337
precursor (0.1 M zinc sulfate) for 10 s so that Zn2+ ions were adsorbed on the substrate surface, (b) the substrate was then rinsed with distilled water for 20 s to remove loosely bounded Zn2+ ions from the substrates, (c) further, the substrate was immersed into 0.1 M sodium telluride anionic precursor solution for 10 s where Te2 ions were adsorbed and reacted with Zn2+ to form ZnTe thin film, and (d) finally, again substrate was rinsed with distilled water for 20 s to remove un-adsorbed and unreacted Te2 ions from the substrate. Thus, 10 s adsorption of cations, 20 s rinsing of cations, 10 s adsorption and reaction of anions and 20 s rinsing anions and loosely bounded ions, presents one SILAR cycle. The deposited ZnTe films were white, homogeneous with a good adherence to the substrate. The films were deposited for 10, 20 and 30 deposition cycles. For the structural elucidation of the films of different thicknesses, X-ray diffraction (XRD) analysis was performed on a Philips (PW-3710) diffractometer with copper target ˚ ). The surface morphology of ZnTe films was (l = 1.54 A confirmed from scanning electron microscopy (SEM) images for different magnifications under the same operating conditions. The SEM unit was operated at 20 kV. For this, the films were coated with gold–palladium (Au–Pd) using polaron SEM sputter coating with E-2500 due to the high electrical resistivity (107 to 108 V cm). The optical absorbance was measured within the wavelength range of 350–850 nm using a Systronic spectrophotometer-119. Energy dispersive X-ray analysis (EDX) was employed for the compositional analysis. Thickness of ZnTe film was measured using gravimetric weight difference method and was 270 nm for 30 deposition cycles. 3. Results and discussion 3.1. Structural studies Fig. 1 shows XRD pattern of ZnTe thin film deposited on glass substrate subjected to above conditions. The presence of broad hump is due to the glass substrate. Nanocrystalline nature of deposited ZnTe films is confirmed from XRD pattern as observed diffraction peaks are weak and are of low intensity. Comparison of observed interplanar spacing ‘d’ values with standard ‘d’ values shows that the deposited in thin film form material is ZnTe. The two observed diffraction peaks at an angle 2u 278 and 30.058 are correspond to (1 0 1) and (2 0 0)
Fig. 1. XRD pattern of ZnTe thin films deposited CBD.
Fig. 2. Plot of (absorbance)2 against energy for ZnTe thin films [inset: plot of absorbance against wavelength for ZnTe thin films].
planes of ZnTe. For standard ‘d’ values 19-1482 and 15-0746 JCPDF files were preferred. 3.2. Optical studies The absorption spectra of samples with different thicknesses were studied in the wavelength range of 300–850 nm. The optical absorbance of thin film is closely related to its morphology. Fig. 2 (inset) shows the variation of optical absorption with wavelength. This data was further used for analyzing optical direct band gap energy, a = a(hn Eg)n/2/hn classical relation for near edge optical absorption in semiconductor where ‘a’ is a constant, Eg is the semiconductor band gap energy and ‘n’ is a number equal to 1 for the direct band gap and 4 for the indirect gap semiconductors. It is seen from the graph that optical absorption increases with increase in
Fig. 3. SEM images of ZnTe thin films at 60,000 magnifications.
S.S. Kale et al. / Applied Surface Science 253 (2007) 4335–4337
the film thickness. The plots of (absorbance)2 versus hn are plotted (Fig. 2) for estimating the value of direct band gap energy of ZnTe films by extrapolating curves to zero absorption coefficient. The estimated optical band gap energies are 2.75, 3.00 and 3.15 eV for ZnTe film thicknesses of 75, 140 and 270 nm, respectively. All these values of band gap are above the normal values of ZnTe (1.44 eV). We can infer that the band gap energy is partially dependent on the crystallinity of the film, as the band gap energies of highly crystalline thin films are similar to those of crystalline bulk materials, whereas amorphous or poorly crystallized films show band gap energies higher than those of the corresponding bulk materials.
4. Conclusion
3.3. Surface morphological and composition studies
References
Scanning electron micrographs of zinc telluride thin film of 270 nm film thickness on glass substrate is shown in Fig. 3. The film surfaces were free from the pinholes and cracks. Zinc telluride film with un-even grains mostly falling in nanometer regime is clearly seen. EDX study of ZnTe film showed the presence of Zn and Te in the film. The composition of the Zn:Te was observed to be 53:47.
4337
The nanocrystalline ZnTe thin films were deposited on commercial glass substrates by successive ionic layer adsorption and reaction (SILAR) method. The XRD study reveals that formed material is nanocrystalline. From SEM micrograph, well coverage of substrate surface and nanometer scale spherical grains of ZnTe were confirmed. The optical absorption studies showed that the zinc telluride is direct band gap material and band gap energy increases from 2.75 to 3.15 eV with increase in film thickness from 75 to 270 nm.
[1] R.C. Weast, CRC Handbook of Chemistry and Physics, 58th ed., CRC Press Inc, Cleveland, 1977–78, p. 101. [2] P.C. Kalita, K.C. Sarma, H.L. Das, J. Assam Sci. Soc. 39 (1998) 117. [3] C. Winnerwisser, P.U. Jepsen, M. Schall, V. Schiya, H. Helm, Appl. Phys. Lett. 70 (1997) 3069. [4] P.K. Kalita, B.K. Sarma, H.L. Das, Indian J. Pure Appl. Phys. 37 (1999) 885. [5] H.M. Pathan, C.D. Lokhande, Bull. Mater. Sci. 27 (2004) 85.