- Email: [email protected]
Influence of CH3COO– on the room temperature photoluminescence of ZnO films prepared by CVD
ARTICLE IN PRESS
Journal of Luminescence 126 (2007) 203–206 www.elsevier.com/locate/jlumin
Influence of CH3COO– on the room temperature photoluminescence of ZnO films prepared by CVD Xiangdong Menga,b, Bixia Linb, Zhuxi Fub, a
School of Physics Science and Technology, Yangzhou University, Yangzhou, Jiangsu 225002, PR China Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, PR China
b
Received 29 August 2005; received in revised form 29 June 2006; accepted 30 June 2006 Available online 4 August 2006
Abstract ZnO films with strong c-axis-preferred orientation have been prepared by a single source chemical vapor deposition technique using zinc acetate as source material at the growth temperature of 230 1C. The strong UV and blue emissions were observed in the photoluminescence spectra of as-grown films. A small quantity of residual zinc acetate was reserved on the surface of as-grown ZnO films and the emission mechanism of blue luminescence was nearly related to the CH3COO– of unidentate type. The blue emission disappeared and the green emission appeared after annealing treatment. The green emission is related to the singly ionized oxygen vacancies. r 2006 Elsevier B.V. All rights reserved. PACS: 78.55.Et; 78.66.Hf; 81.15.Gh Keywords: ZnO films; CH3COO–; Photoluminescence; Emission mechanism
1. Introduction Since the ultraviolet (UV)-stimulated emission from ZnO thin films was realized [1], the wide band gap semiconductor, ZnO, has been studied widely due to its potential commercial applications such as short-wavelength UV laser diodes, light emission diodes, transparent electrodes, gas sensors, surface acoustic wave devices and so on [2,3]. ZnO has wide band gap of 3.37 eV and large exciton binding energy of 60 meV, which could lead to lasing action based on exciton recombination even above room temperature. The photoluminescence (PL) properties of ZnO films are sensitive to the preparation techniques and growth conditions. The high-quality ZnO thin films have been deposited by various techniques such as magnetron sputtering, chemical vapor deposition (CVD), pulsed laser deposition, molecular beam epitaxy (MBE), and sol–gel method [4–8]. The PL spectra of ZnO usually show UV and visible emission peaks. In general, the near band edge (NBE) excitonic emission is concluded as an origin of UV Corresponding author. Tel.: +86 551 3606004; fax: +86 551 3606004.
E-mail address: [email protected] (Z. Fu). 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.06.015
emission and the defect-related deep level emission is responsible for the visible emission peaks. The exact origins of visible emissions including green and blue emissions are still in dispute, although the defects such as oxygen vacancies (VO), oxygen interstitials (Oi), zinc vacancies (VZn), zinc interstitials (Zni) and oxide antisites (OZn) are suggested as an origin of visible emissions by many researchers [9–13]. In this work, ZnO thin films were deposited on Si (1 0 0) substrates using an atmospheric pressure single source chemical vapor deposition (SSCVD) technique. The influence of CH3COO– on the room temperature photoluminescence of as-grown ZnO films was discussed. 2. Experimental ZnO thin films were grown on (1 0 0) oriented p-Si substrates by an atmospheric pressure SSCVD technique using anhydrous zinc acetate [Zn(CH3COO)2, Zn(Ac)2] as a source material. The schematic diagram of the SSCVD apparatus is shown in Fig. 1. A major advantage of the SSCVD process is that the precursor molecules from low kinetic energy vapor deposit evenly on the substrate [14].
ARTICLE IN PRESS X. Meng et al. / Journal of Luminescence 126 (2007) 203–206
(004)
Si (400) (103)
(102)
(110)
(101)
(100)
Intensity (a.u.)
(002)
204
Annealed
As-grown 30
40
50
60 2θ/degree
70
80
Fig. 1. Schematic diagram of the CVD apparatus.
Fig. 2. XRD patterns of the ZnO films prepared by SSCVD.
The deposition took place in a vertical reaction chamber under O2 ambiance. During the deposition process, the source and substrate were maintained at 230 and 350 1C, respectively. The process conditions were optimized to improve the film stoichiometry. The as-grown films were annealed at 800 1C for 1 h in a flowing O2 ambient under atmospheric pressure. The structural characterization of samples was carried out by X-ray diffraction (XRD) performed on a D/MAXRA X-ray diffractometer with CuKa radiation. The morphology of thin films was measured using a JSM6700F field-emission scanning electron microscope (FE-SEM). The PL spectra were obtained at room temperature using a He–Cd laser of wavelength 325 nm. The X-ray photoelectron spectra (XPS) were recorded on a Thermo-VG Escalab 250 X-ray photoelectron spectrometer and the IR spectra were obtained on a MAGNA-IR 750 spectrometer.
substrates have average grain size of 100 nm before annealing and 300 nm after annealing. The film thickness is about 350 nm. FT-IR spectra (Fig. 4) of the as-grown and annealed ZnO films were studied in the transmission mode. The infrared absorption of Si substrate was subtracted. It can be seen from Fig. 4 that besides the absorption band of Zn–O at about 414 cm1, two bands at 1563 and 1398 cm1 are observed before annealing treatment, which are coincident with those observed typically for acetate group complexed with a metal such as zinc and correspond to CQO and C–O stretching, respectively [16]. It indicated that a small quantity of zinc acetate which was not decomposed in time was reserved on the surface of asgrown ZnO films. The two bands disappeared after annealing treatment at 800 1C in O2 ambient for 1 h, which indicated that the residual zinc acetate was decomposed. The conclusion mentioned above can be further verified by the results of XPS. The XPS of C1s for the as-grown and annealed ZnO films are shown in Fig. 5. The peak at binding energy 284.6 eV was ascribed to carbonaceous surface contaminant and the binding energy at 288.4 eV in the spectrum of as-grown ZnO film corresponded to carboxylate group characteristic of the residual zinc acetate [17,18]. The peak at 288.4 eV almost disappeared after annealing treatment, which implied the residual zinc acetate was decomposed. Fig. 6 shows the room temperature PL spectra of the asgrown and annealed ZnO films. The blue emission at 440 nm was observed besides the UV emission at 383 nm before annealing. The blue emission peak disappeared and the strong and wide green band emission centered at around 510 nm emerged after annealing. The UV peaks at room temperature were attributed to NBE free exciton transition [19]. However, the blue emission mechanism can be nearly related to the existence of CH3COO–. In general, three bonding structures are well known for the acetate group complexed with a metal such as zinc, i.e. the unidentate, bidentate, and bridging types [20]. According to CQO and C–O peak positions and frequency difference between them, it can be estimated that the bonding
3. Results and discussion Anhydrous zinc acetate was volatilized at 230 1C and decomposed near Si substrate to form ZnO following the process: ZnðCH3 COOÞ2 ! ZnO þ CO2 þ CH3 COCH3 XRD measurements were performed to identify films quality and crystallite orientation. Fig. 2 shows the typical XRD patterns of the as-grown ZnO film and the film annealed at 800 1C in O2 ambient for 1 h. A dominant (0 0 2) diffraction peak at 2y 34.51 indicates that the asgrown thin film has strong c-axis preferred orientation due to its lowest surface free energy [15]. Besides (0 0 2) peak, the (1 0 0), (1 0 1), (1 0 2), (1 1 0), and (1 0 3) peaks are observed, which implies the as-grown film is polycrystalline. The (0 0 2) peak and its secondary diffraction (0 0 4) peak are enhanced in the XRD pattern of the annealed film, which indicates the film crystallite quality is improved after annealing. From the FE-SEM micrographs shown in Fig. 3, it can be seen that the ZnO thin films with strong (0 0 2) preferred orientation deposited on Si (1 0 0)
ARTICLE IN PRESS X. Meng et al. / Journal of Luminescence 126 (2007) 203–206
205
Fig. 3. FE-SEM images of the ZnO films: (a) as-grown; (b) annealed; (c) cross-section.
As-grown
Intensity (a.u.)
410
284.6
414
As-grown 292
3000
2000
Annealed 288.4
1398
1563
Transmittance (a.u.)
Annealed
1000
290
288
286 284 Binding energy (eV)
282
280
278
Fig. 5. XP spectra of C1s for the ZnO films prepared by SSCVD.
Wavenumber/cm-1 Fig. 4. FT-IR spectra of the ZnO films prepared by SSCVD.
structure for the residual CH3COO– complexed with metal zinc on the surface of as-grown ZnO films is unidentate. Compared with other types, the unidentate type of CH3COO– was reported to be more capable of trapping photogenerated holes near the valence band [21]. The blue emission was produced by the recombination of the electrons from the conduction band and the trapped holes and disappeared with the decomposition of the residual
zinc acetate. The blue bands were also observed by other workers in the nanostructured ZnO samples grown without using zinc acetate as source material [22,23]. This is probably caused by the adsorption of a small quantity of CH3COO– in air on the surface of the nanostructured samples which exposed to air for a long time. The green band emission after annealing is mainly related to the point defects, such as oxygen vacancies.
ARTICLE IN PRESS X. Meng et al. / Journal of Luminescence 126 (2007) 203–206
206
the surface of as-grown ZnO films. The unidentate type of CH3COO– was considered to be responsible for the blue emission.
440 383 As-grown Relative intensity
Acknowledgments 510
This work is supported by the National Natural Science Foundation of China (Grant nos. 90201038, 50472009 and 10474091) and Chinese Academy of Sciences.
Annealed
References
300
350
400
450 500 550 Wavelength (nm)
600
650
700
Fig. 6. Room temperature PL spectra of the ZnO films prepared by SSCVD.
Oxygen vacancies occur in three different charge states: the neutral oxygen vacancy (V0O ), the singly ionized oxygen vacancy (Vþ O ), and the doubly ionized oxygen vacancy þ (V2þ O ) and only VO can act as the so-called luminescence centers [12]. In the polycrystalline ZnO films, the band bending can occur at grain boundaries and will create an electron depletion region at the particle surfaces [24,25]. In the depletion region at the particle surfaces, all of the oxygen vacancies will be in the V2þ O state. However, in the particle interiors, the most oxygen vacancies will be in the Vþ O state under flateband conditions. The percentage volume of the grain which is depleted (compared to the overall grain volume) reduces with the increasing grain size, and thus the green band emission will increase in the larger grain samples, even though the depletion layer width may stay the same, since a higher proportion of the oxygen vacancies will be in the singly ionized state. The ZnO films annealed at 800 1C have average grain size of 300 nm. It can be considered that the depleted percentage volume of the grain is very small and the most oxygen vacancies in the annealed films are in the Vþ O state. The emission mechanism of the green band centered at 510 nm after annealing is consistent with the model of the green band proposed by Vanheusden et al. [24,25], i.e. it is related to the singly ionized oxygen vacancies. 4. Conclusions Highly orientated ZnO thin films were deposited on Si (1 0 0) substrates by an atmospheric pressure SSCVD technique. According to the results of FT-IR and XPS, the residual undecomposed zinc acetate was reserved on
[1] P. Zu, Z.K. Tang, G.K.L. Wong, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Solid State Commun. 103 (1997) 459. [2] H. Ohta, K. Kawamura, M. Orita, M. Hirano, N. Sarukura, H. Hosono, Appl. Phys. Lett. 77 (2000) 475. [3] W.W. Wenas, A. Yamada, K. Takahashi, J. Appl. Phys. 70 (1991) 7119. [4] R. Hong, H. Qi, J. Huang, H. He, Z. Fan, J. Shao, Thin Solid Films 473 (2005) 58. [5] X.D. Meng, B.X. Lin, B.J. Gu, J.J. Zhu, Z.X. Fu, Solid State Commun. 135 (2005) 411. [6] A. Tsukazaki, A. Ohtomo, S. Yoshida, M. Kawasaki, C.H. Chia, T. Makino, Y. Segawa, T. Koida, S.F. Chichibu, H. Koinuma, Appl. Phys. Lett. 83 (2003) 2784. [7] L. Znaidi, G.J.A.A.S. Illia, S. Benyahia, C. Sanchez, A.V. Kanaev, Thin Solid Films 428 (2003) 257. [8] A. Ohtomo, M. Kawasaki, Y. Sakurai, Y. Yoshida, H. Koinuma, P. Yu, Z.K. Tang, G.K.L. Wong, Y. Segawa, Mater. Sci. Eng. B 54 (1998) 24. [9] J.S. Kang, H.S. Kang, S.S. Pang, E.S. Shim, S.Y. Lee, Thin Solid Films 443 (2003) 5. [10] B.X. Lin, Z.X. Fu, Y.B. Jia, Appl. Phys. Lett. 79 (2001) 943. [11] D.H. Zhang, Z.Y. Xue, Q.P. Wang, J. Phys. D 35 (2002) 2837. [12] H.S. Kang, J.S. Kang, J.W. Kim, S.Y. Lee, J. Appl. Phys. 95 (2004) 1246. [13] T.B. Hur, G.S. Jeen, Y.H. Hwang, H.K. Kim, J. Appl. Phys. 94 (2003) 5787. [14] M. Purica, E. Budianu, E. Rusu, M. Danila, R. Gavrila, Thin Solid Films 403–404 (2002) 485. [15] K.H. Bang, D.K. Hwang, M.C. Park, Y.D. Ko, I. Yun, J.M. Myoung, Appl. Surf. Sci. 210 (2003) 177. [16] G.B. Deacon, R. Phillips, J. Coord. Chem. Rev. 33 (1980) 227. [17] L.G. Mar, P.Y. Timbrell, R.N. Lamb, Thin Solid Films 223 (1993) 341. [18] J.C.D. Jesus, I. Gonzalez, A. Quevedo, T. Puerta, J. Mol. Catal. A 228 (2005) 283. [19] S. Cho, J. Ma, Y. Kim, Y. Sun, G.K.L. Wong, J.B. Ketterson, Appl. Phys. Lett. 75 (1999) 2761. [20] S. Sakohara, M. Ishida, M.A. Anderson, J. Phys. Chem. B 102 (1998) 10169. [21] S. Sakohara, L.D. Tickanen, M.A. Anderson, J. Phys. Chem. 96 (1992) 11086. [22] D.H. Liu, L. Liao, J.C. Li, H.X. Guo, Q. Fu, Mater. Sci. Eng. B 121 (2005) 77. [23] S.C. Liu, J.J. Wu, J. Mater. Chem. 12 (2002) 3125. [24] K. Vanheusden, W.L. Warren, C.H. Seager, D.R. Tallant, J.A. Voigt, B.E. Gnade, J. Appl. Phys. 79 (1996) 7983. [25] K. Vanheusden, C.H. Seager, W.L. Warren, D.R. Tallant, J.A. Voigt, Appl. Phys. Lett. 68 (1996) 403.
Journal of Luminescence 126 (2007) 203–206 www.elsevier.com/locate/jlumin
Influence of CH3COO– on the room temperature photoluminescence of ZnO films prepared by CVD Xiangdong Menga,b, Bixia Linb, Zhuxi Fub, a
School of Physics Science and Technology, Yangzhou University, Yangzhou, Jiangsu 225002, PR China Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, PR China
b
Received 29 August 2005; received in revised form 29 June 2006; accepted 30 June 2006 Available online 4 August 2006
Abstract ZnO films with strong c-axis-preferred orientation have been prepared by a single source chemical vapor deposition technique using zinc acetate as source material at the growth temperature of 230 1C. The strong UV and blue emissions were observed in the photoluminescence spectra of as-grown films. A small quantity of residual zinc acetate was reserved on the surface of as-grown ZnO films and the emission mechanism of blue luminescence was nearly related to the CH3COO– of unidentate type. The blue emission disappeared and the green emission appeared after annealing treatment. The green emission is related to the singly ionized oxygen vacancies. r 2006 Elsevier B.V. All rights reserved. PACS: 78.55.Et; 78.66.Hf; 81.15.Gh Keywords: ZnO films; CH3COO–; Photoluminescence; Emission mechanism
1. Introduction Since the ultraviolet (UV)-stimulated emission from ZnO thin films was realized [1], the wide band gap semiconductor, ZnO, has been studied widely due to its potential commercial applications such as short-wavelength UV laser diodes, light emission diodes, transparent electrodes, gas sensors, surface acoustic wave devices and so on [2,3]. ZnO has wide band gap of 3.37 eV and large exciton binding energy of 60 meV, which could lead to lasing action based on exciton recombination even above room temperature. The photoluminescence (PL) properties of ZnO films are sensitive to the preparation techniques and growth conditions. The high-quality ZnO thin films have been deposited by various techniques such as magnetron sputtering, chemical vapor deposition (CVD), pulsed laser deposition, molecular beam epitaxy (MBE), and sol–gel method [4–8]. The PL spectra of ZnO usually show UV and visible emission peaks. In general, the near band edge (NBE) excitonic emission is concluded as an origin of UV Corresponding author. Tel.: +86 551 3606004; fax: +86 551 3606004.
E-mail address: [email protected] (Z. Fu). 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.06.015
emission and the defect-related deep level emission is responsible for the visible emission peaks. The exact origins of visible emissions including green and blue emissions are still in dispute, although the defects such as oxygen vacancies (VO), oxygen interstitials (Oi), zinc vacancies (VZn), zinc interstitials (Zni) and oxide antisites (OZn) are suggested as an origin of visible emissions by many researchers [9–13]. In this work, ZnO thin films were deposited on Si (1 0 0) substrates using an atmospheric pressure single source chemical vapor deposition (SSCVD) technique. The influence of CH3COO– on the room temperature photoluminescence of as-grown ZnO films was discussed. 2. Experimental ZnO thin films were grown on (1 0 0) oriented p-Si substrates by an atmospheric pressure SSCVD technique using anhydrous zinc acetate [Zn(CH3COO)2, Zn(Ac)2] as a source material. The schematic diagram of the SSCVD apparatus is shown in Fig. 1. A major advantage of the SSCVD process is that the precursor molecules from low kinetic energy vapor deposit evenly on the substrate [14].
ARTICLE IN PRESS X. Meng et al. / Journal of Luminescence 126 (2007) 203–206
(004)
Si (400) (103)
(102)
(110)
(101)
(100)
Intensity (a.u.)
(002)
204
Annealed
As-grown 30
40
50
60 2θ/degree
70
80
Fig. 1. Schematic diagram of the CVD apparatus.
Fig. 2. XRD patterns of the ZnO films prepared by SSCVD.
The deposition took place in a vertical reaction chamber under O2 ambiance. During the deposition process, the source and substrate were maintained at 230 and 350 1C, respectively. The process conditions were optimized to improve the film stoichiometry. The as-grown films were annealed at 800 1C for 1 h in a flowing O2 ambient under atmospheric pressure. The structural characterization of samples was carried out by X-ray diffraction (XRD) performed on a D/MAXRA X-ray diffractometer with CuKa radiation. The morphology of thin films was measured using a JSM6700F field-emission scanning electron microscope (FE-SEM). The PL spectra were obtained at room temperature using a He–Cd laser of wavelength 325 nm. The X-ray photoelectron spectra (XPS) were recorded on a Thermo-VG Escalab 250 X-ray photoelectron spectrometer and the IR spectra were obtained on a MAGNA-IR 750 spectrometer.
substrates have average grain size of 100 nm before annealing and 300 nm after annealing. The film thickness is about 350 nm. FT-IR spectra (Fig. 4) of the as-grown and annealed ZnO films were studied in the transmission mode. The infrared absorption of Si substrate was subtracted. It can be seen from Fig. 4 that besides the absorption band of Zn–O at about 414 cm1, two bands at 1563 and 1398 cm1 are observed before annealing treatment, which are coincident with those observed typically for acetate group complexed with a metal such as zinc and correspond to CQO and C–O stretching, respectively [16]. It indicated that a small quantity of zinc acetate which was not decomposed in time was reserved on the surface of asgrown ZnO films. The two bands disappeared after annealing treatment at 800 1C in O2 ambient for 1 h, which indicated that the residual zinc acetate was decomposed. The conclusion mentioned above can be further verified by the results of XPS. The XPS of C1s for the as-grown and annealed ZnO films are shown in Fig. 5. The peak at binding energy 284.6 eV was ascribed to carbonaceous surface contaminant and the binding energy at 288.4 eV in the spectrum of as-grown ZnO film corresponded to carboxylate group characteristic of the residual zinc acetate [17,18]. The peak at 288.4 eV almost disappeared after annealing treatment, which implied the residual zinc acetate was decomposed. Fig. 6 shows the room temperature PL spectra of the asgrown and annealed ZnO films. The blue emission at 440 nm was observed besides the UV emission at 383 nm before annealing. The blue emission peak disappeared and the strong and wide green band emission centered at around 510 nm emerged after annealing. The UV peaks at room temperature were attributed to NBE free exciton transition [19]. However, the blue emission mechanism can be nearly related to the existence of CH3COO–. In general, three bonding structures are well known for the acetate group complexed with a metal such as zinc, i.e. the unidentate, bidentate, and bridging types [20]. According to CQO and C–O peak positions and frequency difference between them, it can be estimated that the bonding
3. Results and discussion Anhydrous zinc acetate was volatilized at 230 1C and decomposed near Si substrate to form ZnO following the process: ZnðCH3 COOÞ2 ! ZnO þ CO2 þ CH3 COCH3 XRD measurements were performed to identify films quality and crystallite orientation. Fig. 2 shows the typical XRD patterns of the as-grown ZnO film and the film annealed at 800 1C in O2 ambient for 1 h. A dominant (0 0 2) diffraction peak at 2y 34.51 indicates that the asgrown thin film has strong c-axis preferred orientation due to its lowest surface free energy [15]. Besides (0 0 2) peak, the (1 0 0), (1 0 1), (1 0 2), (1 1 0), and (1 0 3) peaks are observed, which implies the as-grown film is polycrystalline. The (0 0 2) peak and its secondary diffraction (0 0 4) peak are enhanced in the XRD pattern of the annealed film, which indicates the film crystallite quality is improved after annealing. From the FE-SEM micrographs shown in Fig. 3, it can be seen that the ZnO thin films with strong (0 0 2) preferred orientation deposited on Si (1 0 0)
ARTICLE IN PRESS X. Meng et al. / Journal of Luminescence 126 (2007) 203–206
205
Fig. 3. FE-SEM images of the ZnO films: (a) as-grown; (b) annealed; (c) cross-section.
As-grown
Intensity (a.u.)
410
284.6
414
As-grown 292
3000
2000
Annealed 288.4
1398
1563
Transmittance (a.u.)
Annealed
1000
290
288
286 284 Binding energy (eV)
282
280
278
Fig. 5. XP spectra of C1s for the ZnO films prepared by SSCVD.
Wavenumber/cm-1 Fig. 4. FT-IR spectra of the ZnO films prepared by SSCVD.
structure for the residual CH3COO– complexed with metal zinc on the surface of as-grown ZnO films is unidentate. Compared with other types, the unidentate type of CH3COO– was reported to be more capable of trapping photogenerated holes near the valence band [21]. The blue emission was produced by the recombination of the electrons from the conduction band and the trapped holes and disappeared with the decomposition of the residual
zinc acetate. The blue bands were also observed by other workers in the nanostructured ZnO samples grown without using zinc acetate as source material [22,23]. This is probably caused by the adsorption of a small quantity of CH3COO– in air on the surface of the nanostructured samples which exposed to air for a long time. The green band emission after annealing is mainly related to the point defects, such as oxygen vacancies.
ARTICLE IN PRESS X. Meng et al. / Journal of Luminescence 126 (2007) 203–206
206
the surface of as-grown ZnO films. The unidentate type of CH3COO– was considered to be responsible for the blue emission.
440 383 As-grown Relative intensity
Acknowledgments 510
This work is supported by the National Natural Science Foundation of China (Grant nos. 90201038, 50472009 and 10474091) and Chinese Academy of Sciences.
Annealed
References
300
350
400
450 500 550 Wavelength (nm)
600
650
700
Fig. 6. Room temperature PL spectra of the ZnO films prepared by SSCVD.
Oxygen vacancies occur in three different charge states: the neutral oxygen vacancy (V0O ), the singly ionized oxygen vacancy (Vþ O ), and the doubly ionized oxygen vacancy þ (V2þ O ) and only VO can act as the so-called luminescence centers [12]. In the polycrystalline ZnO films, the band bending can occur at grain boundaries and will create an electron depletion region at the particle surfaces [24,25]. In the depletion region at the particle surfaces, all of the oxygen vacancies will be in the V2þ O state. However, in the particle interiors, the most oxygen vacancies will be in the Vþ O state under flateband conditions. The percentage volume of the grain which is depleted (compared to the overall grain volume) reduces with the increasing grain size, and thus the green band emission will increase in the larger grain samples, even though the depletion layer width may stay the same, since a higher proportion of the oxygen vacancies will be in the singly ionized state. The ZnO films annealed at 800 1C have average grain size of 300 nm. It can be considered that the depleted percentage volume of the grain is very small and the most oxygen vacancies in the annealed films are in the Vþ O state. The emission mechanism of the green band centered at 510 nm after annealing is consistent with the model of the green band proposed by Vanheusden et al. [24,25], i.e. it is related to the singly ionized oxygen vacancies. 4. Conclusions Highly orientated ZnO thin films were deposited on Si (1 0 0) substrates by an atmospheric pressure SSCVD technique. According to the results of FT-IR and XPS, the residual undecomposed zinc acetate was reserved on
[1] P. Zu, Z.K. Tang, G.K.L. Wong, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Solid State Commun. 103 (1997) 459. [2] H. Ohta, K. Kawamura, M. Orita, M. Hirano, N. Sarukura, H. Hosono, Appl. Phys. Lett. 77 (2000) 475. [3] W.W. Wenas, A. Yamada, K. Takahashi, J. Appl. Phys. 70 (1991) 7119. [4] R. Hong, H. Qi, J. Huang, H. He, Z. Fan, J. Shao, Thin Solid Films 473 (2005) 58. [5] X.D. Meng, B.X. Lin, B.J. Gu, J.J. Zhu, Z.X. Fu, Solid State Commun. 135 (2005) 411. [6] A. Tsukazaki, A. Ohtomo, S. Yoshida, M. Kawasaki, C.H. Chia, T. Makino, Y. Segawa, T. Koida, S.F. Chichibu, H. Koinuma, Appl. Phys. Lett. 83 (2003) 2784. [7] L. Znaidi, G.J.A.A.S. Illia, S. Benyahia, C. Sanchez, A.V. Kanaev, Thin Solid Films 428 (2003) 257. [8] A. Ohtomo, M. Kawasaki, Y. Sakurai, Y. Yoshida, H. Koinuma, P. Yu, Z.K. Tang, G.K.L. Wong, Y. Segawa, Mater. Sci. Eng. B 54 (1998) 24. [9] J.S. Kang, H.S. Kang, S.S. Pang, E.S. Shim, S.Y. Lee, Thin Solid Films 443 (2003) 5. [10] B.X. Lin, Z.X. Fu, Y.B. Jia, Appl. Phys. Lett. 79 (2001) 943. [11] D.H. Zhang, Z.Y. Xue, Q.P. Wang, J. Phys. D 35 (2002) 2837. [12] H.S. Kang, J.S. Kang, J.W. Kim, S.Y. Lee, J. Appl. Phys. 95 (2004) 1246. [13] T.B. Hur, G.S. Jeen, Y.H. Hwang, H.K. Kim, J. Appl. Phys. 94 (2003) 5787. [14] M. Purica, E. Budianu, E. Rusu, M. Danila, R. Gavrila, Thin Solid Films 403–404 (2002) 485. [15] K.H. Bang, D.K. Hwang, M.C. Park, Y.D. Ko, I. Yun, J.M. Myoung, Appl. Surf. Sci. 210 (2003) 177. [16] G.B. Deacon, R. Phillips, J. Coord. Chem. Rev. 33 (1980) 227. [17] L.G. Mar, P.Y. Timbrell, R.N. Lamb, Thin Solid Films 223 (1993) 341. [18] J.C.D. Jesus, I. Gonzalez, A. Quevedo, T. Puerta, J. Mol. Catal. A 228 (2005) 283. [19] S. Cho, J. Ma, Y. Kim, Y. Sun, G.K.L. Wong, J.B. Ketterson, Appl. Phys. Lett. 75 (1999) 2761. [20] S. Sakohara, M. Ishida, M.A. Anderson, J. Phys. Chem. B 102 (1998) 10169. [21] S. Sakohara, L.D. Tickanen, M.A. Anderson, J. Phys. Chem. 96 (1992) 11086. [22] D.H. Liu, L. Liao, J.C. Li, H.X. Guo, Q. Fu, Mater. Sci. Eng. B 121 (2005) 77. [23] S.C. Liu, J.J. Wu, J. Mater. Chem. 12 (2002) 3125. [24] K. Vanheusden, W.L. Warren, C.H. Seager, D.R. Tallant, J.A. Voigt, B.E. Gnade, J. Appl. Phys. 79 (1996) 7983. [25] K. Vanheusden, C.H. Seager, W.L. Warren, D.R. Tallant, J.A. Voigt, Appl. Phys. Lett. 68 (1996) 403.