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Excimer laser processing of ZnO thin films prepared by the sol–gel process
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Applied Surface Science 254 (2007) 855–858 www.elsevier.com/locate/apsusc
Excimer laser processing of ZnO thin films prepared by the sol–gel process R.J. Winfield a,*, L.H.K. Koh a,b, Shane O’Brien a, Gabriel M. Crean a,b b
a Tyndall National Institute, Cork, Ireland Department of Microelectronic Engineering, University College Cork, Ireland
Available online 26 August 2007
Abstract ZnO thin films were prepared on soda-lime glass from a single spin-coating deposition of a sol–gel prepared with anhydrous zinc acetate [Zn(C2H3O2)2], monoethanolamine [H2NC2H4OH] and isopropanol. The deposited films were dried at 50 and 300 8C. X-ray analysis showed that the films were amorphous. Laser annealing was performed using an excimer laser. The laser pulse repetition rate was 25 Hz with a pulse energy of 5.9 mJ, giving a fluence of 225 mJ cm 2 on the ZnO film. Typically, five laser pulses per unit area of the film were used. After laser processing, the hexagonal wurtzite phase of zinc oxide was observed from X-ray diffraction pattern analysis. The thin films had a transparency of greater than 70% in the visible region. The optical band-gap energy was 3.454 eV. Scanning electron microscopy and profilometry analysis highlighted the change in morphology that occurred as a result of laser processing. This comparative study shows that our sol–gel processing route differs significantly from ZnO sol–gel films prepared by conventional furnace annealing which requires temperatures above 450 8C for the formation of crystalline ZnO. # 2007 Elsevier B.V. All rights reserved. Keywords: Zinc oxide; Laser annealing; Sol–gel
1. Introduction ZnO thin films now attract significant attention due to their wide range of electrical and optical properties. They have potential application in electronics, optoelectronics and information technology devices including displays, solar cells and sensors [1,2]. Several thin-film deposition techniques have been used to produce pure ZnO films, including sputtering [3], molecular beam epitaxy [4], metal-organic chemical vapour deposition [5], pulsed laser deposition [6], spray pyrolysis [7] and the sol–gel process [8,9]. The sol–gel method has distinct potential advantages over these other techniques due to its ability to tune microstructure via sol–gel chemistry, a conformal deposition ability, compositional control and large surface area coating capability [10–12]. However, an annealing step is required for film crystallization. Laser annealing of thin films is a technique of rapid crystallization that has the benefits of being area selective, requiring no high temperature bulk heating and results in
* Corresponding author. Tel.: +353 21 4904377. E-mail address: [email protected] (R.J. Winfield). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.08.072
minimal disturbance to the surrounding area [13]. This allows additional processing on devices that cannot be post processed at elevated temperatures. The aim of this work is to investigate a low-temperature process for area selective crystalline ZnO, using hybrid sol–gel materials science and excimer laser annealing. Excimer laser annealing of ZnO thin films prepared using the sol–gel process has been demonstrated [14]. However, the sol–gel used was prepared with the solvent 2-methoxyethanol, which has a boiling point of 125 8C. In order to enable low-temperature drying and removal of organics the sol–gel materials used in this study were prepared with isopropyl alcohol (IPA) which has a lower boiling point of 82 8C. In addition, in this work, the use of anhydrous zinc acetate (instead of the commonly used zinc acetate dehydrate) in sol–gel formulation avoids the introduction of large amounts of water to the sol–gel, enabling control over reactions taking place within the sol–gel and also contributing to a reduction of the drying temperature. The crystallinity of the thin film is the property of most interest in this work, as this significantly influences the desired optical and electrical material properties. In particular, a wurtzite phase of ZnO is required for the thinfilm material to have adequate electrical properties for the intended transparent conductive oxide application. It is
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R.J. Winfield et al. / Applied Surface Science 254 (2007) 855–858
demonstrated in this study that using the above materials, the temperature at which crystalline X-ray diffraction features is observed can be reduced to 300 8C. Typically, temperatures of 450 8C and above are required in conventional furnace annealing of ZnO sol–gel films to produce crystalline ZnO. The effect of laser annealing on ZnO film microstructure, morphology and optical transparency was examined and compared with data obtained from films prepared using a conventional thermal annealing process. 2. Experimental The ZnO sol–gel was prepared was as follows: zinc acetate (M = 183.46, Zn(C2H3O2)2, 99.99% chemical purity) was first dissolved in isopropanol at room temperature. Then monoethanolammine (M = 61.08, H2NCH2CH2OH, MEA, AR) was added as a sol stabilizer. The molar ratio of MEA to zinc acetate was maintained at 1.0 and the final concentration of the zinc acetate was 0.6 mol L 1. The resulting mixture was then stirred at 50 8C for 1 h to form a clear and transparent homogeneous mixture and upon cooling was filtered using a 0.45 mm, Acrodisc Versapor1 filter and aged for 24 h at room temperature. Amorphous thin films were prepared by spin coating the aged solution onto soda-lime glass at rotation speeds of 2000 rpm for a duration of 30 s. The substrates were then dried on a hotplate at 60 8C for 30 min and then at 300 8C for a further 30 min [15]. The samples were irradiated with an ATLEX 300 SI KrF excimer laser (wavelength 248 nm and pulse duration 5 ns). Typically, five laser pulses per unit area of the film were used. The pulse repetition rate was 25 Hz and the laser pulse energy 5.9 mJ resulting in a laser fluence on the ZnO film of 225 mJ cm 2. The beam was apertured to give a 3 5 mm2 area of quasi uniform illumination. This was demagnified to concentrate the beam with, the degree of demagnification used to control the pulse energy per unit area. The laser was fired at 25 pulses per second. An XY cnc controlled stage system moved the film to build up an exposed area of 15 15 mm2. The degree of crystallinity and crystalline orientation of the ZnO thin films was measured using a Phillips (PW3719) X’pert materials research X-ray diffractometer (XRD) with a Cu Ka radiation, and a scanning range of 2u set between 258 and 508. Top-view and cross-sectional SEM images of the films were collected using a Hitachi S-4000 Field Effect SEM instrument, whose operation voltage was 20 keV. Optical transmittance spectra were recorded using a Shimadzu (UV-2401 PC) UV–vis recording spectrophotometer over the wavelength range between 200 and 800 nm. The surface morphology of the films was evaluated using a Philips (XL30TMP) scanning electron microscope (SEM). The optical band-gap energy was estimated from optical transmittance and wavelength data, using an extrapolation of the linear portion of a2 versus hn where a is the absorption coefficient and hn the photon energy. The optical absorption edge was determined using a first derivative method to acquire the wavelength value required for calculation of the optical bandgap [16].
Fig. 1. X-ray diffraction patterns obtained from a film which was unprocessed after drying at 300 8C and also from an identically produced film which was subsequently processed by laser irradiation.
3. Results and discussion The laser annealing of the ZnO thin films was studied as a function of laser fluence. Initially, the ZnO films were irradiated at a range of fluences from 54 to 318 mJ cm 2. X-ray diffraction measurements showed that at the drying conditions
Fig. 2. (a) UV–vis % transmittance spectra and (b) first derivative of ZnO thin films recorded before and after laser processing.
R.J. Winfield et al. / Applied Surface Science 254 (2007) 855–858
used, the optimum production of 0 0 2 plane orientation occurred at 225 mJ cm 2. This is consistent with previous reported work, where using a different precursor and drying conditions, an optimum fluence of 170 mJ cm 2 was found [14]. The influence of laser annealing at this optimum laser fluence on the formation of a crystalline phase ZnO thin film is clearly illustrated in Fig. 1. The representative X-ray diffraction patterns are of two ZnO thin films deposited on soda-lime glass by spin-coating at 2000 rpm, followed by hotplate drying at 60 and 300 8C with one film subsequently laser irradiated. The laser irradiated film exhibited three X-ray diffraction peaks, assigned to the (1 0 0), (0 0 2) and (1 0 1) crystal planes of the hexagonal wurtzite structure [17], consistent with crystalline zinc oxide formed as a result of the laser annealing process employed. Representative optical transmittance spectra of sol–gel deposited ZnO thin films in the UV–vis wavelength range (200–800 nm) before and after laser annealing are presented in Fig. 2(a). It is observed that laser-processed films have lower optical transparency. This is consistent with light scattering arising from the presence of grain boundaries in the crystalline material [12]. The bandgap of the fabricated ZnO thin films, calculated from the maximum of the 1st derivative of the UV– vis spectra (shown in Fig. 2(b)), was 3.454 eV and 3.36 eV, respectively, for the laser-processed and non-laser-processed samples, consistent with published values of the ZnO electronic
857
Table 1 UV–vis % transmittance and calculated bandgap of laser-processed and nonlaser-processed materials
Laser annealed film Dried film
Transmittance range (%)
Band-gap (eV)
46.47–80.49 75.24–88.46
3.360 3.454
transition band-gap data for amorphous and crystalline materials [18]. These data are summarised in Table 1. Representative SEM micrographs are presented in Fig. 3. It is observed in Fig. 3(a) that the morphology of the amorphous film (drying at 300 8C without laser processing) is flat and generally featureless. In contrast, Fig. 3(b) shows the ZnO grain structure in an area where the C axis crystalization is maximised. The measured surface roughness (Ra) of the ZnO thin films increased from 2.43 to 11.03 nm following laser irradiation. This is again consistent with the formation of crystalline ZnO grains in the laser-annealed thin film. 4. Conclusion ZnO thin films were prepared on soda-lime glass from a single spin-coating deposition of a novel sol–gel material system prepared with anhydrous zinc acetate [Zn(C2H3O2)2], monoethanolamine [H2NC2H4OH] and isopropanol. The deposited films were dried at 50 and 300 8C. X-ray analysis showed that these films were amorphous. Samples were then irradiated using an excimer laser employing a pulse repetition rate of 25 Hz, a laser pulse energy of 5.9 mJ with a fluence of 225 mJ cm 2 on the ZnO film. After laser processing, the hexagonal wurtzite phase of zinc oxide was observed from X-ray diffraction pattern analysis. This differs significantly from ZnO sol–gel films prepared by conventional furnace annealing which requires temperatures of 450 8C and above for the formation of crystalline ZnO. The long-term goal of this work is to produce selective crystalline ZnO using the sol–gel route at the lowest possible thermal budget. This will enable crystalline coatings to be formed on temperature sensitive substrates using hybrid processing routes which offer the capacity for deposition and subsequent processing over large areas. References
Fig. 3. SEM micrographs of ZnO films recorded (a) before laser processing and (b) after laser processing.
[1] S. Bandyopadhyay, G.K. Paul, S.K. Sen, Sol. Energy Mater. Sol. Cells 71 (2002) 103. [2] Y. Natsume, H. Sakata, Thin Solid Films 372 (2000) 30. [3] P. Nunes, D. Costa, E. Fortunato, R. Martins, Vacuum 64 (2002) 293. [4] D.C. Look, D.C. Reynolds, C.W. Litton, R.L. Jones, D.B. Eason, G. Gantwell, Appl. Phys. Lett. 81 (10) (2002) 1830. [5] K. Tominaga, T. Takao, A. Fukushima, T. Moriga, I. Nakabayashi, Vacuum 66 (2002) 505. [6] N. Naghavi, C. Marcel, L. Dupont, A. Rougier, J.B. Leriche, C. Guery, J. Mater. Chem. 10 (2000) 2315. [7] M. Krunks, E. Mellikov, Thin Solid Films 270 (1995) 33. [8] T. Shuler, M.A. Aegerter, Thin Solid Films 351 (1999) 125. [9] Y. Natsume, H. Sakata, Mater. Chem. Phys. 78 (2002) 170.
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R.J. Winfield et al. / Applied Surface Science 254 (2007) 855–858
[10] E.J. Luna-Arredondo, A. Maldonado, R. Asomoza, D.R. Acosta, M.A. Melendez-Lira, M. de la L. Olvera, Thin Solid Films 490 (2005) 132. [11] N.R.S. Farley, C.R. Staddon, L.X. Zhao, K.W. Edmunds, B.L. Gallagher, D.H. Gregory, J. Mater. Chem. 14 (2004) 1087. [12] M. Ohyama, H. Kozuka, T. Yoko, Thin Solid Films 306 (1997) 78. [13] T. Sameshima, S. Usui, M. Sekiya, IEEE Electron Device Lett. EDL-7 (1986) 276. [14] T. Nagase, T. Ooie, J. Sakakibara, Thin Solid Films 357 (1997) 151.
[15] S. O’Brien, L.H.K. Koh, G.M. Crean, ZnO thin films prepared by a single step sol–gel process, Thin Solid Films (2007) doi:10.1016/j.tsf.2007.03.160. [16] H. Li, J. Wang, H. Liu, H. Zhang, X. Li, J. Crystal Growth 275 (2005) e943. [17] Joint Committee on Powder Diffraction Standards, Powder diffraction file, International Center for Diffraction Data, Swarthmore, PA, Card, 1988, 36. [18] J.-H. Lee, K.-H. Ko, B.-O. Park, J. Crystal Growth 247 (2003) 119.
Applied Surface Science 254 (2007) 855–858 www.elsevier.com/locate/apsusc
Excimer laser processing of ZnO thin films prepared by the sol–gel process R.J. Winfield a,*, L.H.K. Koh a,b, Shane O’Brien a, Gabriel M. Crean a,b b
a Tyndall National Institute, Cork, Ireland Department of Microelectronic Engineering, University College Cork, Ireland
Available online 26 August 2007
Abstract ZnO thin films were prepared on soda-lime glass from a single spin-coating deposition of a sol–gel prepared with anhydrous zinc acetate [Zn(C2H3O2)2], monoethanolamine [H2NC2H4OH] and isopropanol. The deposited films were dried at 50 and 300 8C. X-ray analysis showed that the films were amorphous. Laser annealing was performed using an excimer laser. The laser pulse repetition rate was 25 Hz with a pulse energy of 5.9 mJ, giving a fluence of 225 mJ cm 2 on the ZnO film. Typically, five laser pulses per unit area of the film were used. After laser processing, the hexagonal wurtzite phase of zinc oxide was observed from X-ray diffraction pattern analysis. The thin films had a transparency of greater than 70% in the visible region. The optical band-gap energy was 3.454 eV. Scanning electron microscopy and profilometry analysis highlighted the change in morphology that occurred as a result of laser processing. This comparative study shows that our sol–gel processing route differs significantly from ZnO sol–gel films prepared by conventional furnace annealing which requires temperatures above 450 8C for the formation of crystalline ZnO. # 2007 Elsevier B.V. All rights reserved. Keywords: Zinc oxide; Laser annealing; Sol–gel
1. Introduction ZnO thin films now attract significant attention due to their wide range of electrical and optical properties. They have potential application in electronics, optoelectronics and information technology devices including displays, solar cells and sensors [1,2]. Several thin-film deposition techniques have been used to produce pure ZnO films, including sputtering [3], molecular beam epitaxy [4], metal-organic chemical vapour deposition [5], pulsed laser deposition [6], spray pyrolysis [7] and the sol–gel process [8,9]. The sol–gel method has distinct potential advantages over these other techniques due to its ability to tune microstructure via sol–gel chemistry, a conformal deposition ability, compositional control and large surface area coating capability [10–12]. However, an annealing step is required for film crystallization. Laser annealing of thin films is a technique of rapid crystallization that has the benefits of being area selective, requiring no high temperature bulk heating and results in
* Corresponding author. Tel.: +353 21 4904377. E-mail address: [email protected] (R.J. Winfield). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.08.072
minimal disturbance to the surrounding area [13]. This allows additional processing on devices that cannot be post processed at elevated temperatures. The aim of this work is to investigate a low-temperature process for area selective crystalline ZnO, using hybrid sol–gel materials science and excimer laser annealing. Excimer laser annealing of ZnO thin films prepared using the sol–gel process has been demonstrated [14]. However, the sol–gel used was prepared with the solvent 2-methoxyethanol, which has a boiling point of 125 8C. In order to enable low-temperature drying and removal of organics the sol–gel materials used in this study were prepared with isopropyl alcohol (IPA) which has a lower boiling point of 82 8C. In addition, in this work, the use of anhydrous zinc acetate (instead of the commonly used zinc acetate dehydrate) in sol–gel formulation avoids the introduction of large amounts of water to the sol–gel, enabling control over reactions taking place within the sol–gel and also contributing to a reduction of the drying temperature. The crystallinity of the thin film is the property of most interest in this work, as this significantly influences the desired optical and electrical material properties. In particular, a wurtzite phase of ZnO is required for the thinfilm material to have adequate electrical properties for the intended transparent conductive oxide application. It is
856
R.J. Winfield et al. / Applied Surface Science 254 (2007) 855–858
demonstrated in this study that using the above materials, the temperature at which crystalline X-ray diffraction features is observed can be reduced to 300 8C. Typically, temperatures of 450 8C and above are required in conventional furnace annealing of ZnO sol–gel films to produce crystalline ZnO. The effect of laser annealing on ZnO film microstructure, morphology and optical transparency was examined and compared with data obtained from films prepared using a conventional thermal annealing process. 2. Experimental The ZnO sol–gel was prepared was as follows: zinc acetate (M = 183.46, Zn(C2H3O2)2, 99.99% chemical purity) was first dissolved in isopropanol at room temperature. Then monoethanolammine (M = 61.08, H2NCH2CH2OH, MEA, AR) was added as a sol stabilizer. The molar ratio of MEA to zinc acetate was maintained at 1.0 and the final concentration of the zinc acetate was 0.6 mol L 1. The resulting mixture was then stirred at 50 8C for 1 h to form a clear and transparent homogeneous mixture and upon cooling was filtered using a 0.45 mm, Acrodisc Versapor1 filter and aged for 24 h at room temperature. Amorphous thin films were prepared by spin coating the aged solution onto soda-lime glass at rotation speeds of 2000 rpm for a duration of 30 s. The substrates were then dried on a hotplate at 60 8C for 30 min and then at 300 8C for a further 30 min [15]. The samples were irradiated with an ATLEX 300 SI KrF excimer laser (wavelength 248 nm and pulse duration 5 ns). Typically, five laser pulses per unit area of the film were used. The pulse repetition rate was 25 Hz and the laser pulse energy 5.9 mJ resulting in a laser fluence on the ZnO film of 225 mJ cm 2. The beam was apertured to give a 3 5 mm2 area of quasi uniform illumination. This was demagnified to concentrate the beam with, the degree of demagnification used to control the pulse energy per unit area. The laser was fired at 25 pulses per second. An XY cnc controlled stage system moved the film to build up an exposed area of 15 15 mm2. The degree of crystallinity and crystalline orientation of the ZnO thin films was measured using a Phillips (PW3719) X’pert materials research X-ray diffractometer (XRD) with a Cu Ka radiation, and a scanning range of 2u set between 258 and 508. Top-view and cross-sectional SEM images of the films were collected using a Hitachi S-4000 Field Effect SEM instrument, whose operation voltage was 20 keV. Optical transmittance spectra were recorded using a Shimadzu (UV-2401 PC) UV–vis recording spectrophotometer over the wavelength range between 200 and 800 nm. The surface morphology of the films was evaluated using a Philips (XL30TMP) scanning electron microscope (SEM). The optical band-gap energy was estimated from optical transmittance and wavelength data, using an extrapolation of the linear portion of a2 versus hn where a is the absorption coefficient and hn the photon energy. The optical absorption edge was determined using a first derivative method to acquire the wavelength value required for calculation of the optical bandgap [16].
Fig. 1. X-ray diffraction patterns obtained from a film which was unprocessed after drying at 300 8C and also from an identically produced film which was subsequently processed by laser irradiation.
3. Results and discussion The laser annealing of the ZnO thin films was studied as a function of laser fluence. Initially, the ZnO films were irradiated at a range of fluences from 54 to 318 mJ cm 2. X-ray diffraction measurements showed that at the drying conditions
Fig. 2. (a) UV–vis % transmittance spectra and (b) first derivative of ZnO thin films recorded before and after laser processing.
R.J. Winfield et al. / Applied Surface Science 254 (2007) 855–858
used, the optimum production of 0 0 2 plane orientation occurred at 225 mJ cm 2. This is consistent with previous reported work, where using a different precursor and drying conditions, an optimum fluence of 170 mJ cm 2 was found [14]. The influence of laser annealing at this optimum laser fluence on the formation of a crystalline phase ZnO thin film is clearly illustrated in Fig. 1. The representative X-ray diffraction patterns are of two ZnO thin films deposited on soda-lime glass by spin-coating at 2000 rpm, followed by hotplate drying at 60 and 300 8C with one film subsequently laser irradiated. The laser irradiated film exhibited three X-ray diffraction peaks, assigned to the (1 0 0), (0 0 2) and (1 0 1) crystal planes of the hexagonal wurtzite structure [17], consistent with crystalline zinc oxide formed as a result of the laser annealing process employed. Representative optical transmittance spectra of sol–gel deposited ZnO thin films in the UV–vis wavelength range (200–800 nm) before and after laser annealing are presented in Fig. 2(a). It is observed that laser-processed films have lower optical transparency. This is consistent with light scattering arising from the presence of grain boundaries in the crystalline material [12]. The bandgap of the fabricated ZnO thin films, calculated from the maximum of the 1st derivative of the UV– vis spectra (shown in Fig. 2(b)), was 3.454 eV and 3.36 eV, respectively, for the laser-processed and non-laser-processed samples, consistent with published values of the ZnO electronic
857
Table 1 UV–vis % transmittance and calculated bandgap of laser-processed and nonlaser-processed materials
Laser annealed film Dried film
Transmittance range (%)
Band-gap (eV)
46.47–80.49 75.24–88.46
3.360 3.454
transition band-gap data for amorphous and crystalline materials [18]. These data are summarised in Table 1. Representative SEM micrographs are presented in Fig. 3. It is observed in Fig. 3(a) that the morphology of the amorphous film (drying at 300 8C without laser processing) is flat and generally featureless. In contrast, Fig. 3(b) shows the ZnO grain structure in an area where the C axis crystalization is maximised. The measured surface roughness (Ra) of the ZnO thin films increased from 2.43 to 11.03 nm following laser irradiation. This is again consistent with the formation of crystalline ZnO grains in the laser-annealed thin film. 4. Conclusion ZnO thin films were prepared on soda-lime glass from a single spin-coating deposition of a novel sol–gel material system prepared with anhydrous zinc acetate [Zn(C2H3O2)2], monoethanolamine [H2NC2H4OH] and isopropanol. The deposited films were dried at 50 and 300 8C. X-ray analysis showed that these films were amorphous. Samples were then irradiated using an excimer laser employing a pulse repetition rate of 25 Hz, a laser pulse energy of 5.9 mJ with a fluence of 225 mJ cm 2 on the ZnO film. After laser processing, the hexagonal wurtzite phase of zinc oxide was observed from X-ray diffraction pattern analysis. This differs significantly from ZnO sol–gel films prepared by conventional furnace annealing which requires temperatures of 450 8C and above for the formation of crystalline ZnO. The long-term goal of this work is to produce selective crystalline ZnO using the sol–gel route at the lowest possible thermal budget. This will enable crystalline coatings to be formed on temperature sensitive substrates using hybrid processing routes which offer the capacity for deposition and subsequent processing over large areas. References
Fig. 3. SEM micrographs of ZnO films recorded (a) before laser processing and (b) after laser processing.
[1] S. Bandyopadhyay, G.K. Paul, S.K. Sen, Sol. Energy Mater. Sol. Cells 71 (2002) 103. [2] Y. Natsume, H. Sakata, Thin Solid Films 372 (2000) 30. [3] P. Nunes, D. Costa, E. Fortunato, R. Martins, Vacuum 64 (2002) 293. [4] D.C. Look, D.C. Reynolds, C.W. Litton, R.L. Jones, D.B. Eason, G. Gantwell, Appl. Phys. Lett. 81 (10) (2002) 1830. [5] K. Tominaga, T. Takao, A. Fukushima, T. Moriga, I. Nakabayashi, Vacuum 66 (2002) 505. [6] N. Naghavi, C. Marcel, L. Dupont, A. Rougier, J.B. Leriche, C. Guery, J. Mater. Chem. 10 (2000) 2315. [7] M. Krunks, E. Mellikov, Thin Solid Films 270 (1995) 33. [8] T. Shuler, M.A. Aegerter, Thin Solid Films 351 (1999) 125. [9] Y. Natsume, H. Sakata, Mater. Chem. Phys. 78 (2002) 170.
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R.J. Winfield et al. / Applied Surface Science 254 (2007) 855–858
[10] E.J. Luna-Arredondo, A. Maldonado, R. Asomoza, D.R. Acosta, M.A. Melendez-Lira, M. de la L. Olvera, Thin Solid Films 490 (2005) 132. [11] N.R.S. Farley, C.R. Staddon, L.X. Zhao, K.W. Edmunds, B.L. Gallagher, D.H. Gregory, J. Mater. Chem. 14 (2004) 1087. [12] M. Ohyama, H. Kozuka, T. Yoko, Thin Solid Films 306 (1997) 78. [13] T. Sameshima, S. Usui, M. Sekiya, IEEE Electron Device Lett. EDL-7 (1986) 276. [14] T. Nagase, T. Ooie, J. Sakakibara, Thin Solid Films 357 (1997) 151.
[15] S. O’Brien, L.H.K. Koh, G.M. Crean, ZnO thin films prepared by a single step sol–gel process, Thin Solid Films (2007) doi:10.1016/j.tsf.2007.03.160. [16] H. Li, J. Wang, H. Liu, H. Zhang, X. Li, J. Crystal Growth 275 (2005) e943. [17] Joint Committee on Powder Diffraction Standards, Powder diffraction file, International Center for Diffraction Data, Swarthmore, PA, Card, 1988, 36. [18] J.-H. Lee, K.-H. Ko, B.-O. Park, J. Crystal Growth 247 (2003) 119.