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Preparation and characterization of ZnO thin films prepared by thermal oxidation of evaporated Zn thin films
Superlattices and Microstructures 42 (2007) 116–122 www.elsevier.com/locate/superlattices
Preparation and characterization of ZnO thin films prepared by thermal oxidation of evaporated Zn thin films G.G. Rusu ∗ , Mihaela Gˆırtan, M. Rusu Al.I.Cuza University, Faculty of Physics, 11 Carol I Bd., 700506 Iasi, Romania Available online 23 May 2007
Abstract In this paper, the experimental results regarding some structural, electrical and optical properties of ZnO thin films prepared by thermal oxidation of metallic Zn thin films are presented. Zn thin films (d = 200–400 nm) were deposited by thermal evaporation under vacuum, onto unheated glass substrates, using the quasi-closed volume technique. In order to obtain ZnO films, zinc-coated glass substrates were isochronally heated in air in the 300–660 K temperature range, for thermal oxidation. X-ray diffraction (XRD) studies revealed that the ZnO films obtained present a randomly oriented hexagonal nanocrystalline structure. Depending on the heating temperature of the Zn films, the optical transmittance of the ZnO films in the visible wavelength range varied from 85% to 95%. The optical band gap of the ZnO films was found to be about 3.2 eV. By in situ studying of the temperature dependence of the electrical conductivity during the oxidation process, the value of about 2 × 10−2 −1 m−1 was found for the conductivity of completely oxidized ZnO films. c 2007 Elsevier Ltd. All rights reserved.
Keywords: Zinc oxide; Oxidation process; Electrical and optical properties; X-ray diffraction; Thin films
1. Introduction Zinc oxide (ZnO) exhibits many interesting properties, such as a wide energy band gap (3.37 eV), high excitonic binding energy (60 meV), high photoconductivity, and important piezoelectric and pyroelectric properties. These behaviors make this compound useful for ∗ Corresponding author.
E-mail address: [email protected] (G.G. Rusu). c 2007 Elsevier Ltd. All rights reserved. 0749-6036/$ - see front matter doi:10.1016/j.spmi.2007.04.021
G.G. Rusu et al. / Superlattices and Microstructures 42 (2007) 116–122
117
numerous technological applications, such as transparent conducting electrodes, ultraviolet light detectors, surface acoustic waves filters, gas sensors and recently in nanoelectronics and photonics. A variety of techniques such as pulse laser deposition, molecular beam epitaxy, vapor phase transport, chemical vapor deposition, r.f. sputtering, sol–gel, spray pyrolysis, etc. have been used to prepare ZnO thin films on different substrates such as sapphire, silicon, stainless steel, alumina plate, glass, etc. [1–7]. However, not much attention has been paid to the preparation of ZnO films by thermal oxidation of metallic Zn thin films. Such a method is a relatively simple and low-cost procedure that does not require any catalyst or higher-temperature growth. In the work reported in this paper, ZnO thin films obtained by thermal oxidation of Zn films evaporated in vacuum onto glass substrates were studied. The structural, electrical and optical properties of as-obtained ZnO films were investigated. 2. Experimental The metallic Zn films were deposited by thermal evaporation under vacuum onto clean glass substrates, using the quasi-closed volume technique. A special Pyrex cylinder chamber of height 10 cm and radius 3 cm, closed at the top by the substrate holder, was used to limit the deposition space. The residual pressure in the standard vacuum system used was about 10−5 Torr. No heating system for the substrate was used. The temperature of the evaporation source, maintained constant during film deposition, was 720 K. Other details regarding the deposition set-up used are described in [8]. After preparation, the as-deposited Zn films were heated under ambient conditions at a rate of 5 K/min from room temperature to a final temperature of 653 K and then cooled at the same rate. During the heating process the color of the Zn films changes from silver–grey at room temperature to black–brown at a temperature of about 500 K. Then, beginning at a temperature of 550 K, the films have a white-like transparent aspect that remains practically unchanged up to a temperature of 653 K, at which the film heating was stopped. The film thickness, d, ranging between 200 and 400 nm, was determined by Fizeau’s method for fringes of equal thickness [9] using an interferential microscope. The crystalline structure of the studied films was investigated by X-ray diffraction (XRD) analysis using Cu-Kα radiation ˚ in the range 2θ = 20◦ –80◦ . (λ = 1.5418 A) The electrical resistivity was measured using surface-type cells [10]. Gold vacuum-evaporated electrodes (d ∼ = 1 µm) separated by a gap of 3 mm have been used. The film transmittance was measured using a UV–VIS spectrometer in the wavelength range 350–1400 nm. 3. Results and discussion 3.1. Structural characteristics In Fig. 1, the representative XRD patterns for as-deposited and heat-treated Zn films are shown. Important changes of the structural characteristics of the films during their heating may be observed. As can be seen from Fig. 1(a), the as-deposited Zn films are polycrystalline and have a hexagonal structure [11]. The intense (002) reflection at 2θ ∼ = 36.3◦ indicates that in respective films the Zn microcrystallites grow preponderantly with the (002) plane parallel to the substrate surface. The relative intensity I002 /I101 ∼ = 1.83, calculated for respective Zn film, greater than 0.53 for standard Zn powder samples [11], confirms the above observation. The heating of the Zn films up to the temperature of 500 K at which they become dark-brown colored determines a
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Fig. 1. The XRD patterns for evaporated Zn film, annealed at different temperatures: (a) as-deposited; (b) 500 K; (c) 573 K; (d) 633 K.
significant increase of respective (002) preferred orientation (Fig. 1(b)). The XRD pattern for the same sample after annealing up to 573 K, when it starts to whiten, is presented in Fig. 1(c). The well-defined diffraction peaks from this pattern, characteristic for the polycrystalline hexagonal structure of bulk ZnO [12], indicate the formation of the respective compound with randomly oriented microcrystallites. As the heating temperature increases, the crystallinity of the ZnO films is improved, revealed by the decreasing of the full-width at half-maximum (FWHM) of the principal diffraction peaks from Fig. 1(d). The average grain size estimated using the XRD data [8] from Fig. 1(d) is 23.3 nm. This value indicates a nanocrystalline structure of the ZnO films obtained. The improvement of the quality of the ZnO films with increase of the annealing temperature has been reported by other researchers [12,13]. 3.2. Electronic transport properties It is well known that there is a strong correlation between structural characteristics of the thin films and their electronic transport properties. On the other hand, a heat treatment of the films may modify these structural characteristics [15,16]. Consequently, in situ measurement of some electrical properties such as the electrical resistivity of thin films during their heat treatment may offer very useful information about possible changes in the film structure determined by the heating process [10]. Keeping in mind this assumption, the temperature dependence of the electrical resistivity, ρ, during heating of the Zn films was measured in the temperature range
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Fig. 2. Typical variation of electrical resistivity during Zn film annealing.
Fig. 3. The temperature dependence of electrical conductivity during the heating–cooling cycle for ZnO films.
290–660 K to emphasize the oxidation process. In Fig. 2, the typical temperature dependence ρ = f (T ) obtained for the films studied is plotted. It is seen that, up to a temperature of 550 K, the film resistivity varied in a relatively narrow range. Then, the electrical resistivity begins to increase quickly. This fact reveals that, beginning with increasing of the temperature, some important structural changes take place in the film due to the Zn oxidation. These results are in good agreement with those revealed by the XRD patterns presented in Fig. 1(a)–(d). For as-oxidized Zn films, the temperature dependence of electrical conductivity, σ , during a successive heating and cooling at a rate of 5 K/min was also studied. In Fig. 3, such dependence, for ZnO film (the XRD pattern is illustrated in Fig. 1(d)) is plotted. It may be observed that up to a temperature of 420 K, the film heating determines an exponential increase of the σ . Such dependence is characteristic for ZnO polycrystalline thin films [17] and reveals semiconducting
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Fig. 4. Transmission spectra for the samples studied in Fig. 1(b)–(d).
properties of respective films. At higher temperature, σ remains constant and then decreases. This behavior can be attributed to the decrease of oxygen vacancies that act as electron donors in ZnO films [3,18]. The decrease of the concentration of such carriers determines the decrease of σ . Also, the oxidation process, not yet finished in the previous heat treatment of the sample can determine the decrease of σ at higher temperatures. After two more heating–cooling cycles, the dependence ln σ = f (1/T ) becomes reversible, indicating a stabilized structure of the film for the temperature range studied. The value of the electrical conductivity for as-stabilized ZnO films is about 2 × 10−2 −1 m−1 . Similar results have been reported for ZnO films prepared by other methods [19–21]. 3.3. Optical properties In Fig. 4, the transmission spectra (relative to uncovered substrate) for the samples from Fig. 1(b)–(d) are plotted. As may be observed, the increase of the heating temperature determines an important increase of the film transmittance. Even after Zn film oxidation, the heat treatment determines the increase of film transmittance (Fig. 4, curves 2, 3). Such an increase in transmittance of ZnO with higher annealing temperature was previously reported for ZnO films [14] and was attributed mainly to the improvement of the crystalline structures of the films. The XRD patterns shown in Fig. 1(c), (d), confirm this assumption. From the transmission spectra, the absorption coefficient was calculated. Assuming allowed direct transitions, the dependence of (αhν)2 on hν is plotted in Fig. 5. By extrapolating the linear portion of the plot to (αhν)2 = 0, the value of the optical band gap energy, E g , was calculated. The value of 3.19 eV obtained for the optical band gap is relatively lower than those of ZnO single crystals (3.37 eV). Such a band edge lower by 0.1 eV than the usual 3.3 eV has also been reported by Srikant and Clarke [22] for ZnO thin films grown by laser ablation on fused silica, and it was attributed to the small grain size in respective films and to the effect of electrostatic potential that exists in the grain boundaries. In the case of our ZnO films, the lower value of E g may be also due to the greater density of donor states near the conduction band, determined by the oxygen vacancies.
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Fig. 5. Typical (αhν)2 = f (hν) dependence for ZnO films.
4. Conclusions Polycrystalline Zn thin films were evaporated onto glass substrates by the quasi-closed volume technique. By post-deposition heating, ZnO films were obtained. These present a polycrystalline hexagonal structure, an electrical conductivity at room temperature of 2 × 10−2 −1 m−1 , an optical transmittance higher than 85% and a value of about 3.2 eV for the optical band gap. These behaviors are comparable with those of ZnO films grown by other preparation techniques. The temperature of about 660 K for oxidation of the evaporated zinc film is moderate enough to be easily applied in device technology based on ZnO films. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
P. Wu, J. Zhong, N.W. Emanetoglu, S. Muthukumar, Y. Lu, J. Electron. Mater. 33 (6) (2004) 596. S. Kumar, V. Gupta, K. Sreenivas, Nanotechnology 16 (2005) 1167. Z.L. Wang, J. Phys. Condens. Matter 16 (2004) R829. M.N. Kamalasanan, S. Chandra, Thin Solid Films 288 (1996) 112. F. Xiu, Z. Yang, D. Zhao, J. Liu, K.A. Alim, A.A. Baladin, M.E. Itkis, R.C. Haddon, J. Cryst. Growth 286 (2006) 61. K. Ramamoorthy, C. Sanjeeviraja, K. Sankaranarayanan, P. Misra, L.M. Kukreja, Curr. Appl. Phys. 6 (2006) 103. Y.J. Kim, Y.T. Kim, H.K. Yang, J.C. Park, J.I. Han, Y.E. Lee, H.J. Kim, J. Vac. Sci. Technol. A 15 (3) (1997) 1103. M. Rusu, I.I. Nicolaescu, G.G. Rusu, Appl. Phys. A 70 (2000) 565. K.L. Chopra, Thin Film Phenomena, McGraw-Hill, New York, 1969. G.G. Rusu, M. Rusu, Solid State Commun. 116 (2000) 363. ASTM X-ray powder data file, card no. 4-0831. ASTM X-ray powder data file, card no. 5-0664. H. Li, J. Wang, H. Liu, C. Yang, H. Xu, X. Li, H. Cui, Vacuum 77 (2004) 57. S.Y. Kuo, W.C. Chen, C.P. Cheng, Superlatt. Microstruct. 39 (2006) 162. K.L. Chopra, Thin Film Phenomena, Mc. Graw-Hill, New York, 1996. L.L. Kazmerski (Ed.), Polycrystalline and Amorphous Thin Films and Devices, Academic Press, New York, 1980. W. Tang, D.C. Cameron, Thin Solid Films 238 (1994) 83. G. Harbeke (Ed.), Polycrystalline Semiconductors. Physical Properties and Applications, Spring-Verlag, Berlin, 1985. S. Kohiki, M. Nishitani, T. Wada, J. Appl. Phys. 75 (4) (1994) 2069.
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[20] M. Jin, L.S. Ying, Thin Solid Films 237 (1994) 16. [21] V. Musat, B. Teixeira, E. Fortinato, R.C.C. Monteiro, Thin Solid Films 502 (1–2) (2006) 219. [22] V. Srikant, D.R. Clarke, J. Appl. Phys. 81 (9) (1997) 6357.
Preparation and characterization of ZnO thin films prepared by thermal oxidation of evaporated Zn thin films G.G. Rusu ∗ , Mihaela Gˆırtan, M. Rusu Al.I.Cuza University, Faculty of Physics, 11 Carol I Bd., 700506 Iasi, Romania Available online 23 May 2007
Abstract In this paper, the experimental results regarding some structural, electrical and optical properties of ZnO thin films prepared by thermal oxidation of metallic Zn thin films are presented. Zn thin films (d = 200–400 nm) were deposited by thermal evaporation under vacuum, onto unheated glass substrates, using the quasi-closed volume technique. In order to obtain ZnO films, zinc-coated glass substrates were isochronally heated in air in the 300–660 K temperature range, for thermal oxidation. X-ray diffraction (XRD) studies revealed that the ZnO films obtained present a randomly oriented hexagonal nanocrystalline structure. Depending on the heating temperature of the Zn films, the optical transmittance of the ZnO films in the visible wavelength range varied from 85% to 95%. The optical band gap of the ZnO films was found to be about 3.2 eV. By in situ studying of the temperature dependence of the electrical conductivity during the oxidation process, the value of about 2 × 10−2 −1 m−1 was found for the conductivity of completely oxidized ZnO films. c 2007 Elsevier Ltd. All rights reserved.
Keywords: Zinc oxide; Oxidation process; Electrical and optical properties; X-ray diffraction; Thin films
1. Introduction Zinc oxide (ZnO) exhibits many interesting properties, such as a wide energy band gap (3.37 eV), high excitonic binding energy (60 meV), high photoconductivity, and important piezoelectric and pyroelectric properties. These behaviors make this compound useful for ∗ Corresponding author.
E-mail address: [email protected] (G.G. Rusu). c 2007 Elsevier Ltd. All rights reserved. 0749-6036/$ - see front matter doi:10.1016/j.spmi.2007.04.021
G.G. Rusu et al. / Superlattices and Microstructures 42 (2007) 116–122
117
numerous technological applications, such as transparent conducting electrodes, ultraviolet light detectors, surface acoustic waves filters, gas sensors and recently in nanoelectronics and photonics. A variety of techniques such as pulse laser deposition, molecular beam epitaxy, vapor phase transport, chemical vapor deposition, r.f. sputtering, sol–gel, spray pyrolysis, etc. have been used to prepare ZnO thin films on different substrates such as sapphire, silicon, stainless steel, alumina plate, glass, etc. [1–7]. However, not much attention has been paid to the preparation of ZnO films by thermal oxidation of metallic Zn thin films. Such a method is a relatively simple and low-cost procedure that does not require any catalyst or higher-temperature growth. In the work reported in this paper, ZnO thin films obtained by thermal oxidation of Zn films evaporated in vacuum onto glass substrates were studied. The structural, electrical and optical properties of as-obtained ZnO films were investigated. 2. Experimental The metallic Zn films were deposited by thermal evaporation under vacuum onto clean glass substrates, using the quasi-closed volume technique. A special Pyrex cylinder chamber of height 10 cm and radius 3 cm, closed at the top by the substrate holder, was used to limit the deposition space. The residual pressure in the standard vacuum system used was about 10−5 Torr. No heating system for the substrate was used. The temperature of the evaporation source, maintained constant during film deposition, was 720 K. Other details regarding the deposition set-up used are described in [8]. After preparation, the as-deposited Zn films were heated under ambient conditions at a rate of 5 K/min from room temperature to a final temperature of 653 K and then cooled at the same rate. During the heating process the color of the Zn films changes from silver–grey at room temperature to black–brown at a temperature of about 500 K. Then, beginning at a temperature of 550 K, the films have a white-like transparent aspect that remains practically unchanged up to a temperature of 653 K, at which the film heating was stopped. The film thickness, d, ranging between 200 and 400 nm, was determined by Fizeau’s method for fringes of equal thickness [9] using an interferential microscope. The crystalline structure of the studied films was investigated by X-ray diffraction (XRD) analysis using Cu-Kα radiation ˚ in the range 2θ = 20◦ –80◦ . (λ = 1.5418 A) The electrical resistivity was measured using surface-type cells [10]. Gold vacuum-evaporated electrodes (d ∼ = 1 µm) separated by a gap of 3 mm have been used. The film transmittance was measured using a UV–VIS spectrometer in the wavelength range 350–1400 nm. 3. Results and discussion 3.1. Structural characteristics In Fig. 1, the representative XRD patterns for as-deposited and heat-treated Zn films are shown. Important changes of the structural characteristics of the films during their heating may be observed. As can be seen from Fig. 1(a), the as-deposited Zn films are polycrystalline and have a hexagonal structure [11]. The intense (002) reflection at 2θ ∼ = 36.3◦ indicates that in respective films the Zn microcrystallites grow preponderantly with the (002) plane parallel to the substrate surface. The relative intensity I002 /I101 ∼ = 1.83, calculated for respective Zn film, greater than 0.53 for standard Zn powder samples [11], confirms the above observation. The heating of the Zn films up to the temperature of 500 K at which they become dark-brown colored determines a
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Fig. 1. The XRD patterns for evaporated Zn film, annealed at different temperatures: (a) as-deposited; (b) 500 K; (c) 573 K; (d) 633 K.
significant increase of respective (002) preferred orientation (Fig. 1(b)). The XRD pattern for the same sample after annealing up to 573 K, when it starts to whiten, is presented in Fig. 1(c). The well-defined diffraction peaks from this pattern, characteristic for the polycrystalline hexagonal structure of bulk ZnO [12], indicate the formation of the respective compound with randomly oriented microcrystallites. As the heating temperature increases, the crystallinity of the ZnO films is improved, revealed by the decreasing of the full-width at half-maximum (FWHM) of the principal diffraction peaks from Fig. 1(d). The average grain size estimated using the XRD data [8] from Fig. 1(d) is 23.3 nm. This value indicates a nanocrystalline structure of the ZnO films obtained. The improvement of the quality of the ZnO films with increase of the annealing temperature has been reported by other researchers [12,13]. 3.2. Electronic transport properties It is well known that there is a strong correlation between structural characteristics of the thin films and their electronic transport properties. On the other hand, a heat treatment of the films may modify these structural characteristics [15,16]. Consequently, in situ measurement of some electrical properties such as the electrical resistivity of thin films during their heat treatment may offer very useful information about possible changes in the film structure determined by the heating process [10]. Keeping in mind this assumption, the temperature dependence of the electrical resistivity, ρ, during heating of the Zn films was measured in the temperature range
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119
Fig. 2. Typical variation of electrical resistivity during Zn film annealing.
Fig. 3. The temperature dependence of electrical conductivity during the heating–cooling cycle for ZnO films.
290–660 K to emphasize the oxidation process. In Fig. 2, the typical temperature dependence ρ = f (T ) obtained for the films studied is plotted. It is seen that, up to a temperature of 550 K, the film resistivity varied in a relatively narrow range. Then, the electrical resistivity begins to increase quickly. This fact reveals that, beginning with increasing of the temperature, some important structural changes take place in the film due to the Zn oxidation. These results are in good agreement with those revealed by the XRD patterns presented in Fig. 1(a)–(d). For as-oxidized Zn films, the temperature dependence of electrical conductivity, σ , during a successive heating and cooling at a rate of 5 K/min was also studied. In Fig. 3, such dependence, for ZnO film (the XRD pattern is illustrated in Fig. 1(d)) is plotted. It may be observed that up to a temperature of 420 K, the film heating determines an exponential increase of the σ . Such dependence is characteristic for ZnO polycrystalline thin films [17] and reveals semiconducting
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Fig. 4. Transmission spectra for the samples studied in Fig. 1(b)–(d).
properties of respective films. At higher temperature, σ remains constant and then decreases. This behavior can be attributed to the decrease of oxygen vacancies that act as electron donors in ZnO films [3,18]. The decrease of the concentration of such carriers determines the decrease of σ . Also, the oxidation process, not yet finished in the previous heat treatment of the sample can determine the decrease of σ at higher temperatures. After two more heating–cooling cycles, the dependence ln σ = f (1/T ) becomes reversible, indicating a stabilized structure of the film for the temperature range studied. The value of the electrical conductivity for as-stabilized ZnO films is about 2 × 10−2 −1 m−1 . Similar results have been reported for ZnO films prepared by other methods [19–21]. 3.3. Optical properties In Fig. 4, the transmission spectra (relative to uncovered substrate) for the samples from Fig. 1(b)–(d) are plotted. As may be observed, the increase of the heating temperature determines an important increase of the film transmittance. Even after Zn film oxidation, the heat treatment determines the increase of film transmittance (Fig. 4, curves 2, 3). Such an increase in transmittance of ZnO with higher annealing temperature was previously reported for ZnO films [14] and was attributed mainly to the improvement of the crystalline structures of the films. The XRD patterns shown in Fig. 1(c), (d), confirm this assumption. From the transmission spectra, the absorption coefficient was calculated. Assuming allowed direct transitions, the dependence of (αhν)2 on hν is plotted in Fig. 5. By extrapolating the linear portion of the plot to (αhν)2 = 0, the value of the optical band gap energy, E g , was calculated. The value of 3.19 eV obtained for the optical band gap is relatively lower than those of ZnO single crystals (3.37 eV). Such a band edge lower by 0.1 eV than the usual 3.3 eV has also been reported by Srikant and Clarke [22] for ZnO thin films grown by laser ablation on fused silica, and it was attributed to the small grain size in respective films and to the effect of electrostatic potential that exists in the grain boundaries. In the case of our ZnO films, the lower value of E g may be also due to the greater density of donor states near the conduction band, determined by the oxygen vacancies.
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Fig. 5. Typical (αhν)2 = f (hν) dependence for ZnO films.
4. Conclusions Polycrystalline Zn thin films were evaporated onto glass substrates by the quasi-closed volume technique. By post-deposition heating, ZnO films were obtained. These present a polycrystalline hexagonal structure, an electrical conductivity at room temperature of 2 × 10−2 −1 m−1 , an optical transmittance higher than 85% and a value of about 3.2 eV for the optical band gap. These behaviors are comparable with those of ZnO films grown by other preparation techniques. The temperature of about 660 K for oxidation of the evaporated zinc film is moderate enough to be easily applied in device technology based on ZnO films. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
P. Wu, J. Zhong, N.W. Emanetoglu, S. Muthukumar, Y. Lu, J. Electron. Mater. 33 (6) (2004) 596. S. Kumar, V. Gupta, K. Sreenivas, Nanotechnology 16 (2005) 1167. Z.L. Wang, J. Phys. Condens. Matter 16 (2004) R829. M.N. Kamalasanan, S. Chandra, Thin Solid Films 288 (1996) 112. F. Xiu, Z. Yang, D. Zhao, J. Liu, K.A. Alim, A.A. Baladin, M.E. Itkis, R.C. Haddon, J. Cryst. Growth 286 (2006) 61. K. Ramamoorthy, C. Sanjeeviraja, K. Sankaranarayanan, P. Misra, L.M. Kukreja, Curr. Appl. Phys. 6 (2006) 103. Y.J. Kim, Y.T. Kim, H.K. Yang, J.C. Park, J.I. Han, Y.E. Lee, H.J. Kim, J. Vac. Sci. Technol. A 15 (3) (1997) 1103. M. Rusu, I.I. Nicolaescu, G.G. Rusu, Appl. Phys. A 70 (2000) 565. K.L. Chopra, Thin Film Phenomena, McGraw-Hill, New York, 1969. G.G. Rusu, M. Rusu, Solid State Commun. 116 (2000) 363. ASTM X-ray powder data file, card no. 4-0831. ASTM X-ray powder data file, card no. 5-0664. H. Li, J. Wang, H. Liu, C. Yang, H. Xu, X. Li, H. Cui, Vacuum 77 (2004) 57. S.Y. Kuo, W.C. Chen, C.P. Cheng, Superlatt. Microstruct. 39 (2006) 162. K.L. Chopra, Thin Film Phenomena, Mc. Graw-Hill, New York, 1996. L.L. Kazmerski (Ed.), Polycrystalline and Amorphous Thin Films and Devices, Academic Press, New York, 1980. W. Tang, D.C. Cameron, Thin Solid Films 238 (1994) 83. G. Harbeke (Ed.), Polycrystalline Semiconductors. Physical Properties and Applications, Spring-Verlag, Berlin, 1985. S. Kohiki, M. Nishitani, T. Wada, J. Appl. Phys. 75 (4) (1994) 2069.
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[20] M. Jin, L.S. Ying, Thin Solid Films 237 (1994) 16. [21] V. Musat, B. Teixeira, E. Fortinato, R.C.C. Monteiro, Thin Solid Films 502 (1–2) (2006) 219. [22] V. Srikant, D.R. Clarke, J. Appl. Phys. 81 (9) (1997) 6357.