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Highly textured ZnO thin films: a novel economical preparation and approachment for optical devices, UV lasers and green LEDs
Materials Chemistry and Physics 85 (2004) 257–262
Highly textured ZnO thin films: a novel economical preparation and approachment for optical devices, UV lasers and green LEDs K. Ramamoorthy a , M. Arivanandhan a , K. Sankaranarayanan b , C. Sanjeeviraja a,∗ a b
Department of Physics, Alagappa University, Karaikudi 630 003, Tamil Nadu, India Crystal Research Centre, Alagappa University, Karaikudi 630 003, Tamil Nadu, India
Received 7 May 2003; received in revised form 1 September 2003; accepted 26 September 2003
Abstract Fabrication of high quality ZnO thin films and analysis on its physical, chemical properties have applications in opto-electronic devices, UV laser, green LEDs, microelectronics and micro-machining technologies. In the present investigation, highly textured zinc oxide (ZnO) thin films with a preferred (1 0 1) orientation were prepared by chemical bath deposition using a sodium zincate bath on glass substrates. The films were characterized by Philips-Xpert MPD-XRD, Hitachi-SEM, EDAX, Optical, FTIR and PL in order to justify the suitability for commercial device quality. The effects of number of dippings on thin film growth and allied properties were investigated. Highly ASTM standard polycrystalline textured thin films contain nano-grains were observed by XRD. The material (ZnO) was confirmed by EDAX and FTIR. SEM reveals the presence of uniform adherent thin film. Increasing in grain size (nm) and decreasing in PL intensity were observed, while increasing the number of dippings. The optimized ZnO thin films have average transmission 85% in the visible range. Optical and PL studies demonstrated the suitability of the prepared films for optical devices, UV lasers and green LEDs. © 2003 Published by Elsevier B.V. Keywords: Chemical deposition; Zinc oxide thin films; Characterizations; Optical transmission; Photoluminescence
1. Introduction One of the important wide band gap II–VI compound semiconductors is zinc oxide. Recently more attention has been paid towards zinc oxide due to its unique property for various valid applications. Fabricating high quality ZnO thin films on substrates and the analysis on its physical and chemical properties will be important for the applications such as energy efficient windows, liquid crystal displays, opto-electronic devices, piezo-electrical devices acoustic-electrical devices, laser diodes and sensors [1–3]. ZnO is a good alternative candidate for indium tin oxide (ITO) and analogous to GaN [4,5]. Further, bulk ZnO is quite expensive and unavailable in large wafers for the time being, ZnO thin films are relatively a good choice. Several techniques such as thermal oxidation [6], electron beam evaporation [7], activated reactive evaporation [8], spray pyrolysis [9], metal organic chemical vapour deposition (MOCVD) [10] electroless bath deposition [11], pulsed laser deposition [12], RF magnetron sput∗ Corresponding author. Tel.: +91-4565-425205; fax: +91-4565-425202. E-mail address: [email protected] (C. Sanjeeviraja).
0254-0584/$ – see front matter © 2003 Published by Elsevier B.V. doi:10.1016/j.matchemphys.2003.09.018
tering [13] and chemical deposition [14] have been used for forming ZnO films. Among these various method, chemical deposition is advantageous on account of its suitability for forming large area thin films. Chemical deposition of ZnO thin films from an aqueous solution is a versatile method due to its simplicity and economy. In this paper, structural, compositional, optical (including infrared study), surface morphological and photoluminescence properties of chemically deposited ZnO thin films were investigated.
2. Experimental Sodium zincate baths having different molar concentrations were prepared by mixing a required volume ratio of zinc sulfate (ZnSO4 ) solution and sodium hydroxide (NaOH) solution. Before deposition, the glass substrates (microscope slides) were cleaned by chromic acid followed by nano-pure distilled water rinse and ultrasonic cleaning with acetone and alcohol. The cleaned substrate was alternatively dipped for a predetermined period in sodium zincate bath and water bath kept at room temperature and near boiling point, respectively. According to the following equation, the complex layer de-
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posited on the substrate during the dipping in sodium zincate bath will be decomposed to ZnO while dipping in hot water. Na2 ZnO2 + H2 O → ZnO + 2NaOH Part of the ZnO so formed was deposited onto the substrate as a strongly adherent film and the remainder formed as a precipitate [14]. After required number of dippings, the substrate with the deposited ZnO film was annealed in air at 250 ◦ C for 30 min. Each pair of immersions in sodium zincate bath and hot water (∼98 ◦ C) is consider as a one complete step of dipping. The thickness of the deposited ZnO films were calculated by weight gain method on knowing the change in weight of the substrate due to film deposition and the area of deposition and using the known density of ZnO, viz., 5.6 g cm−3 as reported in the literature [14]. By keeping the molar concentration (0.167 M) of the sodium zincate bath as constant, the deposition kinetics of ZnO film was analyzed by employing X-ray diffractometer, energy dispersive analysis by X-rays, scanning electron microscope, UV-Vis-NIR spectrophotometer, fluorescence spectrophotometer and Fourier transform infrared spectrophotometer for different number of dippings (10, 25, 50, 75 and 100).
3. Results and discussion The observations made during the experiments revealed that the deposition is feasible only when the molar concentration of the bath is less than 0.25 M. Due to the white precipitate of zinc hydroxide (Zn(OH)2 ) [14] having large size clusters leaving a clear and water-like solution on the top made the deposition unreliable and non-uniform even under stirred condition in baths having molar concentration greater than 0.25 M. Decreasing the concentration of the bath led to size reduction in Zn(OH)2 clusters and enhances its distribution in the solution more uniformly. Deposition experiments conducted under this condition resulted ZnO film having uniform distribution throughout the substrate and bearing strongly adherent nature. The physical verification on the deposited film indicates the near optimized experimental parameters when the molar concentration is 0.167 M. By keeping the concentration of the bath as constant at 0.167 M, ZnO films were prepared for 10, 25, 50, 75 and 100 number of dippings. The determined thickness of the deposited ZnO film over number of dippings were presented in the Fig. 1. The average deposition rate per dipping was calculated from the Fig. 1 is 0.018 m. In order to investigate the structural, compositional and optical properties of the deposited films, samples were subjected to the following studies.
Fig. 1. A plot between film thickness vs. number of dippings.
3.1. X-ray diffraction Fig. 2 depicts the X-ray diffraction spectrums recorded for the deposited samples prepared out of different number of dippings. The obtained d values were compared with the ASTM data (card number 3-0888) and confirmed the hexagonal crystal system and wurtzite structure of ZnO and the polycrystalline nature of the film with the prominent diffraction peaks from crystal planes such as (1 0 0), (0 0 2) and (1 0 1). Also, from the recorded spectrums, one can understand that the degree of crystallinity improves with number of dippings. ZnO film prepared out of 100 number of dippings exhibits a preferential orientation along (1 0 1) crystal plane. The following calculated grain size by Scherrer’s formula for all the recorded spectrums exhibit an increase in grain size with number of dippings as shown in Fig. 3. 3.2. Optical transmission studies Fig. 4 shows a typical optical transmission spectrum recorded for the samples of 10, 25, 50, 75 and 100 number of dippings in the range 200–900 nm. The attained highest percentage of transmission at 550 nm with respect to number of dippings along with the corresponding thickness of the ZnO film were presented as follows. The following observed effect may be the reason for this abnormal behavior, i.e., the periodic and continuous annealing at 250 ◦ C cause better aligned textured films. This effect was enhanced for higher number of dipping and finally leads to reduction in free carrier absorption and low reflection loss, thereby enhance the transmission (Table 1). From the recorded optical transmission spectrums for the samples of 10, 25, 50, 75 and 100 number of dippings, the following informations are derived. The sharp absorption onset and the high transmission values of the ZnO film at wavelengths above 400 nm exhibit the optical quality and low concentration of defects such as pits and voids [15]. The mean band gap measured from the transmission spectrums
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259
Fig. 3. Variation of grain size with number of dippings.
Fig. 2. X-ray diffractograms of ZnO thin films on glass substrates for various number of dippings. (a) 10 dippings, (b) 25 dippings, (c) 50 dippings, (d) 75 dippings, and (e) 100 dippings.
of the samples 25, 50, 75 and 100 number of dippings, using the extrapolation of (αhν)2 vs. hν to zero energy is 3.25 eV. This value agrees with the reported band gap value of bulk undoped ZnO [16]. 3.3. Photoluminescence Photoluminescence characterization on deposited ZnO thin films on glass substrates were carried out at room temTable 1 Variation of optical transmission and thickness for different number of dippings
3.4. Energy dispersive analysis by X-rays (EDAX) In order to study the compositional or elemental property, the sample was subjected to EDAX. The recorded EDAX spectrum presented in Fig. 6 for ZnO thin film on glass substrate shows the presence of Zn, O, Ca, Si and a trace amount of Al. The Si is due to the glass substrate. The presence of elements Ca and Al may be attributed as impurities either from the source material or from the glass substrate. Also, it is evident that the film is Zn rich. 3.5. Scanning electron microscopy (SEM)
No. of dippings
Percentage of transmission (%) Thickness (m)
perature. The recorded PL spectrograms for the samples of 50 and 100 number of dippings were shown in Fig. 5. There are two PL emissions peaks, one is at 390 nm and another one at 510 nm. The UV emission at 390 nm corresponds to exciton transition and the green emission at 510 nm may be attributed to the transition from conduction band bottom to the antisite OZn level formed in the band gap [17,18]. Here, “OZn ” means antisite defects, i.e., it denotes the improper localization (or) placement of oxygen atoms in the place of zinc atoms [17]. From the above results, it was clear that the film obtained from 100 number of dippings has better crystalline property than other samples. Hence, it has been subjected for the following characterization studies.
10
25
50
75
70 0.23
96 0.42
94 0.87
96 1.24
100 96 1.80
Fig. 7 illustrates the surface analysis by SEM made on the sample obtained from 100 number of dippings. The revealed feature confirms the uniformity of the film and the
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Fig. 4. Optical transmission spectra of ZnO thin films for different number of dippings.
adherence nature. Higher magnification of the surface reveals the presence of crystallites. The presence of crystallites evidences the near optimized experimental conditions, viz., the molar concentration, dipping time and number of dipping. 3.6. Fourier transform infrared spectroscopy Infrared spectrum is an important record, which provides more information about the structure of a compound. In this
technique almost all functional group in a molecule absorb characteristically within a definite range of frequency [19]. The transmission of IR radiation causes the various bonds in a molecule to stretch and bend with respect to one another. In the present study, infrared transmission spectrum of the chemically deposited zinc oxide thin film on glass substrate for 100 number of dipping was recorded in the range of 4000–400 cm−1 using Perkin-Elmer double beam infrared spectrophotometer. The result is a transmittance infrared spectrum, which is shown in Fig. 8. The aim of the
Fig. 5. Photoluminescence spectra of ZnO thin films for 50 and 100 number of dippings.
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261
Fig. 8. FTIR spectrum of “100 times dipped” ZnO thin film.
4. Conclusion
Fig. 6. EDAX result of ZnO thin film for 100 number of dippings.
present IR spectral analysis on the deposited ZnO thin film includes, confirming the material formation, in addition to proving the absence of any un-reacted starting materials in the deposited ZnO thin films. The systematic interpretation of the IR spectrum can be of great help in determining, for example, whether a reaction has occurred to give the predicted product or not, since there may be a possibility of other reaction having occurred. The absorption region from 650 to 1500 cm−1 generally represents the finger print region of the materials, which are unique in characteristics. As reported in the literature [19], Zn–H vibrations (both symmetric and asymmetric) are indexed around 1500 cm−1 and O–H stretching is observed around 3500 cm−1 . The presence of Zn–H vibration may be attributed to the adsorption of hydrogen during the hot bath dipping while the presence of O–H vibration may be attributed to the residual Zn(OH)2 in the film.
As a conclusion, zinc oxide (ZnO) thin film deposition experiments carried out from sodium zincate baths having different molar concentrations (0.625–0.127 M) revealed that a novel ZnO thin film deposition with uniform and controllable growth rate is possible only when the molar concentration is less than 0.25 M. Hundred number of dippings has yielded ZnO films with better crystallinity for the bath having molar concentration of 0.167 M. Further, the optical transmission investigation made on the sample of 100 number of dippings shows a sharp absorption onset and high transmission values at wavelengths above 400 nm exhibit the optical quality and low concentration of defects such as pits and voids [2]. SEM analysis confirms the presence of ZnO crystallites and the structural uniformity of the film. Two PL emissions peaks, one at 3.18 eV and another one at 2.44 eV were observed. The UV emission at 3.18 eV corresponds to exciton transition and the green emission at 2.44 eV may be attributed to the transition from conduction band bottom to the OZn level formed in the band gap [4,5]. Thus from the above results, we concluded that the obtained ZnO thin films were optical quality in nature suitable for optical devices, UV lasers and green LEDs. References
Fig. 7. SEM photographs of “100 times dipped” ZnO thin film for different magnifications.
[1] T.L. Yang, D.H. Zhang, H.L. Ma, Y. Chen, Thin Solid Films 60 (1998) 326. [2] B.J. Jin, S.H. Bae, S.Y. Lee, S. Im, Mater. Sci. Eng. B 71 (2000) 301. [3] P. Verardi, N. Nastase, C. Gherasim, C. Ghica, M. Dinescu, R. Dinu, C. Flueraru, J. Cryst. Growth 197 (1999) 523. [4] X.W. Sun, H.S. Kwok, J. Appl. Phys. 86 (1999) 408. [5] J.C. Manifacier, Thin solid Films. 90 (1982) 297. [6] P. Bonaewicz, W. Hirchwald, G. Neumann, Thin Solid Films 142 (1986) 77. [7] A. Kuroyanagi, Jpn. J. Appl. Phys. 28 (1989) 219. [8] H. Gopalaswamy, P.J. Reddy, Semicond. Sci. Technol. 5 (1990) 980. [9] A. Ghosh, S. Basu, Mater. Chem. Phys. 27 (1991) 45.
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[10] J.S. Kim, H.A. Marzouk, P.J. Reocroft Jr., C.E. Hamrin, Thin Solid Films 217 (1992) 133. [11] M. Ristov, G.J. Grozdanov, M. Mitreski, Thin solid Films 149 (1987) 65. [12] K. Ramamoorthy, C. Sanjeeviraja, M. Jayachandran, K. Sankaranarayanan, P. Bhattacharya, L.M. Kukreja, J. Cryst. Growth 226 (2001) 281. [13] H. Sato, T. Minama, S. Takata, J. Vac. Sci. Technol. A 11 (1993) 2975. [14] P. Mitra, A.P. Chatterjee, H. Maiti, J. Mater. Sci. 9 (1998) 441.
[15] T. Yamamoto, T. Hiosaki, A. Kawabata, J. Appl. Phys 51 (1980) 3113. [16] B. Joeph, K.G. Gopchandran, P.K. Manoj, P. Koshy, V.K. Vaidyan, Bull. Mater. Sci. 22 (1999) 921. [17] B. Lin, Z. Fu, Y. Jia, G. Liao, J. Electrochem. Soc. 148 (2001) G110. [18] S.A. Studenikin, N. Golego, M. Cocivera, J. Appl. Phys. 84 (1998) 2287. [19] P. Kalsi, Spectroscopy of Organic Compounds, Wiley Eastern, New Delhi, 1985.
Highly textured ZnO thin films: a novel economical preparation and approachment for optical devices, UV lasers and green LEDs K. Ramamoorthy a , M. Arivanandhan a , K. Sankaranarayanan b , C. Sanjeeviraja a,∗ a b
Department of Physics, Alagappa University, Karaikudi 630 003, Tamil Nadu, India Crystal Research Centre, Alagappa University, Karaikudi 630 003, Tamil Nadu, India
Received 7 May 2003; received in revised form 1 September 2003; accepted 26 September 2003
Abstract Fabrication of high quality ZnO thin films and analysis on its physical, chemical properties have applications in opto-electronic devices, UV laser, green LEDs, microelectronics and micro-machining technologies. In the present investigation, highly textured zinc oxide (ZnO) thin films with a preferred (1 0 1) orientation were prepared by chemical bath deposition using a sodium zincate bath on glass substrates. The films were characterized by Philips-Xpert MPD-XRD, Hitachi-SEM, EDAX, Optical, FTIR and PL in order to justify the suitability for commercial device quality. The effects of number of dippings on thin film growth and allied properties were investigated. Highly ASTM standard polycrystalline textured thin films contain nano-grains were observed by XRD. The material (ZnO) was confirmed by EDAX and FTIR. SEM reveals the presence of uniform adherent thin film. Increasing in grain size (nm) and decreasing in PL intensity were observed, while increasing the number of dippings. The optimized ZnO thin films have average transmission 85% in the visible range. Optical and PL studies demonstrated the suitability of the prepared films for optical devices, UV lasers and green LEDs. © 2003 Published by Elsevier B.V. Keywords: Chemical deposition; Zinc oxide thin films; Characterizations; Optical transmission; Photoluminescence
1. Introduction One of the important wide band gap II–VI compound semiconductors is zinc oxide. Recently more attention has been paid towards zinc oxide due to its unique property for various valid applications. Fabricating high quality ZnO thin films on substrates and the analysis on its physical and chemical properties will be important for the applications such as energy efficient windows, liquid crystal displays, opto-electronic devices, piezo-electrical devices acoustic-electrical devices, laser diodes and sensors [1–3]. ZnO is a good alternative candidate for indium tin oxide (ITO) and analogous to GaN [4,5]. Further, bulk ZnO is quite expensive and unavailable in large wafers for the time being, ZnO thin films are relatively a good choice. Several techniques such as thermal oxidation [6], electron beam evaporation [7], activated reactive evaporation [8], spray pyrolysis [9], metal organic chemical vapour deposition (MOCVD) [10] electroless bath deposition [11], pulsed laser deposition [12], RF magnetron sput∗ Corresponding author. Tel.: +91-4565-425205; fax: +91-4565-425202. E-mail address: [email protected] (C. Sanjeeviraja).
0254-0584/$ – see front matter © 2003 Published by Elsevier B.V. doi:10.1016/j.matchemphys.2003.09.018
tering [13] and chemical deposition [14] have been used for forming ZnO films. Among these various method, chemical deposition is advantageous on account of its suitability for forming large area thin films. Chemical deposition of ZnO thin films from an aqueous solution is a versatile method due to its simplicity and economy. In this paper, structural, compositional, optical (including infrared study), surface morphological and photoluminescence properties of chemically deposited ZnO thin films were investigated.
2. Experimental Sodium zincate baths having different molar concentrations were prepared by mixing a required volume ratio of zinc sulfate (ZnSO4 ) solution and sodium hydroxide (NaOH) solution. Before deposition, the glass substrates (microscope slides) were cleaned by chromic acid followed by nano-pure distilled water rinse and ultrasonic cleaning with acetone and alcohol. The cleaned substrate was alternatively dipped for a predetermined period in sodium zincate bath and water bath kept at room temperature and near boiling point, respectively. According to the following equation, the complex layer de-
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posited on the substrate during the dipping in sodium zincate bath will be decomposed to ZnO while dipping in hot water. Na2 ZnO2 + H2 O → ZnO + 2NaOH Part of the ZnO so formed was deposited onto the substrate as a strongly adherent film and the remainder formed as a precipitate [14]. After required number of dippings, the substrate with the deposited ZnO film was annealed in air at 250 ◦ C for 30 min. Each pair of immersions in sodium zincate bath and hot water (∼98 ◦ C) is consider as a one complete step of dipping. The thickness of the deposited ZnO films were calculated by weight gain method on knowing the change in weight of the substrate due to film deposition and the area of deposition and using the known density of ZnO, viz., 5.6 g cm−3 as reported in the literature [14]. By keeping the molar concentration (0.167 M) of the sodium zincate bath as constant, the deposition kinetics of ZnO film was analyzed by employing X-ray diffractometer, energy dispersive analysis by X-rays, scanning electron microscope, UV-Vis-NIR spectrophotometer, fluorescence spectrophotometer and Fourier transform infrared spectrophotometer for different number of dippings (10, 25, 50, 75 and 100).
3. Results and discussion The observations made during the experiments revealed that the deposition is feasible only when the molar concentration of the bath is less than 0.25 M. Due to the white precipitate of zinc hydroxide (Zn(OH)2 ) [14] having large size clusters leaving a clear and water-like solution on the top made the deposition unreliable and non-uniform even under stirred condition in baths having molar concentration greater than 0.25 M. Decreasing the concentration of the bath led to size reduction in Zn(OH)2 clusters and enhances its distribution in the solution more uniformly. Deposition experiments conducted under this condition resulted ZnO film having uniform distribution throughout the substrate and bearing strongly adherent nature. The physical verification on the deposited film indicates the near optimized experimental parameters when the molar concentration is 0.167 M. By keeping the concentration of the bath as constant at 0.167 M, ZnO films were prepared for 10, 25, 50, 75 and 100 number of dippings. The determined thickness of the deposited ZnO film over number of dippings were presented in the Fig. 1. The average deposition rate per dipping was calculated from the Fig. 1 is 0.018 m. In order to investigate the structural, compositional and optical properties of the deposited films, samples were subjected to the following studies.
Fig. 1. A plot between film thickness vs. number of dippings.
3.1. X-ray diffraction Fig. 2 depicts the X-ray diffraction spectrums recorded for the deposited samples prepared out of different number of dippings. The obtained d values were compared with the ASTM data (card number 3-0888) and confirmed the hexagonal crystal system and wurtzite structure of ZnO and the polycrystalline nature of the film with the prominent diffraction peaks from crystal planes such as (1 0 0), (0 0 2) and (1 0 1). Also, from the recorded spectrums, one can understand that the degree of crystallinity improves with number of dippings. ZnO film prepared out of 100 number of dippings exhibits a preferential orientation along (1 0 1) crystal plane. The following calculated grain size by Scherrer’s formula for all the recorded spectrums exhibit an increase in grain size with number of dippings as shown in Fig. 3. 3.2. Optical transmission studies Fig. 4 shows a typical optical transmission spectrum recorded for the samples of 10, 25, 50, 75 and 100 number of dippings in the range 200–900 nm. The attained highest percentage of transmission at 550 nm with respect to number of dippings along with the corresponding thickness of the ZnO film were presented as follows. The following observed effect may be the reason for this abnormal behavior, i.e., the periodic and continuous annealing at 250 ◦ C cause better aligned textured films. This effect was enhanced for higher number of dipping and finally leads to reduction in free carrier absorption and low reflection loss, thereby enhance the transmission (Table 1). From the recorded optical transmission spectrums for the samples of 10, 25, 50, 75 and 100 number of dippings, the following informations are derived. The sharp absorption onset and the high transmission values of the ZnO film at wavelengths above 400 nm exhibit the optical quality and low concentration of defects such as pits and voids [15]. The mean band gap measured from the transmission spectrums
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259
Fig. 3. Variation of grain size with number of dippings.
Fig. 2. X-ray diffractograms of ZnO thin films on glass substrates for various number of dippings. (a) 10 dippings, (b) 25 dippings, (c) 50 dippings, (d) 75 dippings, and (e) 100 dippings.
of the samples 25, 50, 75 and 100 number of dippings, using the extrapolation of (αhν)2 vs. hν to zero energy is 3.25 eV. This value agrees with the reported band gap value of bulk undoped ZnO [16]. 3.3. Photoluminescence Photoluminescence characterization on deposited ZnO thin films on glass substrates were carried out at room temTable 1 Variation of optical transmission and thickness for different number of dippings
3.4. Energy dispersive analysis by X-rays (EDAX) In order to study the compositional or elemental property, the sample was subjected to EDAX. The recorded EDAX spectrum presented in Fig. 6 for ZnO thin film on glass substrate shows the presence of Zn, O, Ca, Si and a trace amount of Al. The Si is due to the glass substrate. The presence of elements Ca and Al may be attributed as impurities either from the source material or from the glass substrate. Also, it is evident that the film is Zn rich. 3.5. Scanning electron microscopy (SEM)
No. of dippings
Percentage of transmission (%) Thickness (m)
perature. The recorded PL spectrograms for the samples of 50 and 100 number of dippings were shown in Fig. 5. There are two PL emissions peaks, one is at 390 nm and another one at 510 nm. The UV emission at 390 nm corresponds to exciton transition and the green emission at 510 nm may be attributed to the transition from conduction band bottom to the antisite OZn level formed in the band gap [17,18]. Here, “OZn ” means antisite defects, i.e., it denotes the improper localization (or) placement of oxygen atoms in the place of zinc atoms [17]. From the above results, it was clear that the film obtained from 100 number of dippings has better crystalline property than other samples. Hence, it has been subjected for the following characterization studies.
10
25
50
75
70 0.23
96 0.42
94 0.87
96 1.24
100 96 1.80
Fig. 7 illustrates the surface analysis by SEM made on the sample obtained from 100 number of dippings. The revealed feature confirms the uniformity of the film and the
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Fig. 4. Optical transmission spectra of ZnO thin films for different number of dippings.
adherence nature. Higher magnification of the surface reveals the presence of crystallites. The presence of crystallites evidences the near optimized experimental conditions, viz., the molar concentration, dipping time and number of dipping. 3.6. Fourier transform infrared spectroscopy Infrared spectrum is an important record, which provides more information about the structure of a compound. In this
technique almost all functional group in a molecule absorb characteristically within a definite range of frequency [19]. The transmission of IR radiation causes the various bonds in a molecule to stretch and bend with respect to one another. In the present study, infrared transmission spectrum of the chemically deposited zinc oxide thin film on glass substrate for 100 number of dipping was recorded in the range of 4000–400 cm−1 using Perkin-Elmer double beam infrared spectrophotometer. The result is a transmittance infrared spectrum, which is shown in Fig. 8. The aim of the
Fig. 5. Photoluminescence spectra of ZnO thin films for 50 and 100 number of dippings.
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Fig. 8. FTIR spectrum of “100 times dipped” ZnO thin film.
4. Conclusion
Fig. 6. EDAX result of ZnO thin film for 100 number of dippings.
present IR spectral analysis on the deposited ZnO thin film includes, confirming the material formation, in addition to proving the absence of any un-reacted starting materials in the deposited ZnO thin films. The systematic interpretation of the IR spectrum can be of great help in determining, for example, whether a reaction has occurred to give the predicted product or not, since there may be a possibility of other reaction having occurred. The absorption region from 650 to 1500 cm−1 generally represents the finger print region of the materials, which are unique in characteristics. As reported in the literature [19], Zn–H vibrations (both symmetric and asymmetric) are indexed around 1500 cm−1 and O–H stretching is observed around 3500 cm−1 . The presence of Zn–H vibration may be attributed to the adsorption of hydrogen during the hot bath dipping while the presence of O–H vibration may be attributed to the residual Zn(OH)2 in the film.
As a conclusion, zinc oxide (ZnO) thin film deposition experiments carried out from sodium zincate baths having different molar concentrations (0.625–0.127 M) revealed that a novel ZnO thin film deposition with uniform and controllable growth rate is possible only when the molar concentration is less than 0.25 M. Hundred number of dippings has yielded ZnO films with better crystallinity for the bath having molar concentration of 0.167 M. Further, the optical transmission investigation made on the sample of 100 number of dippings shows a sharp absorption onset and high transmission values at wavelengths above 400 nm exhibit the optical quality and low concentration of defects such as pits and voids [2]. SEM analysis confirms the presence of ZnO crystallites and the structural uniformity of the film. Two PL emissions peaks, one at 3.18 eV and another one at 2.44 eV were observed. The UV emission at 3.18 eV corresponds to exciton transition and the green emission at 2.44 eV may be attributed to the transition from conduction band bottom to the OZn level formed in the band gap [4,5]. Thus from the above results, we concluded that the obtained ZnO thin films were optical quality in nature suitable for optical devices, UV lasers and green LEDs. References
Fig. 7. SEM photographs of “100 times dipped” ZnO thin film for different magnifications.
[1] T.L. Yang, D.H. Zhang, H.L. Ma, Y. Chen, Thin Solid Films 60 (1998) 326. [2] B.J. Jin, S.H. Bae, S.Y. Lee, S. Im, Mater. Sci. Eng. B 71 (2000) 301. [3] P. Verardi, N. Nastase, C. Gherasim, C. Ghica, M. Dinescu, R. Dinu, C. Flueraru, J. Cryst. Growth 197 (1999) 523. [4] X.W. Sun, H.S. Kwok, J. Appl. Phys. 86 (1999) 408. [5] J.C. Manifacier, Thin solid Films. 90 (1982) 297. [6] P. Bonaewicz, W. Hirchwald, G. Neumann, Thin Solid Films 142 (1986) 77. [7] A. Kuroyanagi, Jpn. J. Appl. Phys. 28 (1989) 219. [8] H. Gopalaswamy, P.J. Reddy, Semicond. Sci. Technol. 5 (1990) 980. [9] A. Ghosh, S. Basu, Mater. Chem. Phys. 27 (1991) 45.
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[10] J.S. Kim, H.A. Marzouk, P.J. Reocroft Jr., C.E. Hamrin, Thin Solid Films 217 (1992) 133. [11] M. Ristov, G.J. Grozdanov, M. Mitreski, Thin solid Films 149 (1987) 65. [12] K. Ramamoorthy, C. Sanjeeviraja, M. Jayachandran, K. Sankaranarayanan, P. Bhattacharya, L.M. Kukreja, J. Cryst. Growth 226 (2001) 281. [13] H. Sato, T. Minama, S. Takata, J. Vac. Sci. Technol. A 11 (1993) 2975. [14] P. Mitra, A.P. Chatterjee, H. Maiti, J. Mater. Sci. 9 (1998) 441.
[15] T. Yamamoto, T. Hiosaki, A. Kawabata, J. Appl. Phys 51 (1980) 3113. [16] B. Joeph, K.G. Gopchandran, P.K. Manoj, P. Koshy, V.K. Vaidyan, Bull. Mater. Sci. 22 (1999) 921. [17] B. Lin, Z. Fu, Y. Jia, G. Liao, J. Electrochem. Soc. 148 (2001) G110. [18] S.A. Studenikin, N. Golego, M. Cocivera, J. Appl. Phys. 84 (1998) 2287. [19] P. Kalsi, Spectroscopy of Organic Compounds, Wiley Eastern, New Delhi, 1985.