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Characterization of compositional variation and luminescence of ZnGa2O4:Mn thin film phosphor
Materials Letters 59 (2005) 786 – 789 www.elsevier.com/locate/matlet
Characterization of compositional variation and luminescence of ZnGa2O4:Mn thin film phosphor Sung Mook Chunga, Sang Hyuk Hanb, Young Jin Kimb,* b
a Organic EL team, ETRI, Daejeon 305-350, Korea Dept. of Materials Science and Engineering, Kyonggi University, Suwon 442-760, Korea
Received 3 May 2004; received in revised form 5 November 2004; accepted 14 November 2004 Available online 2 December 2004
Abstract ZnGa2O4:Mn thin film phosphors were prepared on ITO/glass by rf magnetron reactive sputtering. The effects of the oxygen partial pressure in sputtering and annealing atmospheres on crystallinity, compositional variations, and luminescent properties of thin films were investigated. Atomic ratio of Ga/Zn in films strongly depended on the postannealing conditions, as well as the oxygen partial pressure in sputtering gas during sputtering, and accordingly affected the luminous properties. D 2004 Elsevier B.V. All rights reserved. Keywords: ZnGa2O4:Mn; Thin film; Phosphors; Luminescence
1. Introduction ZnGa2O4:Mn thin film phosphor is one of representative green-emitting phosphors due to excellent luminescent properties [1,2]. It is known that Ga acts as a sensitizer, while Mn is an activator [3]. So Ga/Zn atomic ratios are very important for the luminescence. According to Shea et al. [4], the minor absorption band at 290 nm is due to the direct absorption of photons by the Mn2+ centers, and the primary absorption at 245 nm is attributed to the Ga3+ ions. The latter’s excitation energy could be transferred in a nonradiative way to the doped Mn2+ ions. The loss of the Zn in the host lattice may enhance the substitution of Mn2+ ions into tetrahedral Zn sites, resulting in enhanced luminescent intensity. Yu and Lin [5] also found that Zn-deficient ZnGa2O4 films have excellent cathodoluminescence characteristics and suggested that the excess Ga content creates activators for the luminescence. According to Hsieh et al. [6], the improve* Corresponding author. Dept. of Materials Science and Engineering, Kyonggi University, Suwon 443-760, Kyonggi-Do, Korea. Tel.: +82 31 249 9766; fax: +82 31 249 9775. E-mail address: [email protected] (Y.J. Kim). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.11.021
ment in emission properties after annealing is a consequence of changes to the growth orientation of thin film phosphors and an increase in the Ga/Zn atomic ratio. Hsu et al. [7] proposed that the luminescent properties improve after annealing in vacuum rather than air. However, in ZnGa2O4 films, the compositional changes and their influence to the luminescent properties have not been investigated properly. In this experiment, the effects of oxygen partial pressures in sputtering gas and postannealing atmospheres on the compositional variation of Ga/Zn in films and the luminescent properties were investigated.
2. Experiment ZnGa2O4:Mn thin film phosphors were deposited on indium tin oxide (ITO)-coated glass substrates by rf magnetron sputtering method using an oxide target, which was fabricated by a conventional ceramic process as follows. Ga2O3, ZnO, and MnO powders were weighed and mixed together for the composition of Zn1x MnxGa2O4 (x=0.006) and pressed for a circular disk type, 3-in. diameter. And then it was sintered at 1350 8C in air for 5 h. O2/(Ar+O2) ratios were varied maintaining the working
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pressure at 5 mTorr. The substrate temperature and rf power were 550 8C and 150 W, respectively. Heat treatment of thin films was carried out in the tube furnace for 3 h at 700 8C in air, vacuum, and N2+vacuum(N2 purging), respectively. The structural changes were monitored by using X-ray diffractometer (XRD). Scanning electron microscope (SEM) with energy dispersive spectrometer (EDS) was used to determine the Ga/Zn atomic ratio and film thickness. Photoluminescence (PL) and cathodoluminescence (CL) were measured by DASA 5000 system (PSI, Korea). The excitation source of PL was a xenon lamp with a monochrometer. Accelerating voltages of the electron gun of CL ranged between 50 and 950 V.
3. Result and discussion Fig. 1 shows the XRD patterns of ZnGa2O4:Mn thin film phosphors deposited by rf sputtering at different sputtering gas ratio, O2/(Ar+O2). Because the deposition rate depended on the gas ratio, the deposition time was controlled to make all films to have a same thickness of about 2 Am. As shown, at the gas ratio of 5%, well-developed polycrystalline ZnGa2O4:Mn film could be obtained, while increasing oxygen partial pressure contributed to the deterioration of the peak intensity due to the negative oxygen ions bombardment to the growing films. It is known that the added oxygen causes the sputtering rate reduction because it decreases the
Fig. 1. XRD patterns of ZnGa2O4:Mn as a function of O2/(Ar+O2).
Fig. 2. Ga/Zn atomic ratio of ZnGa2O4:Mn films as a function of O2/ (Ar+O2).
amount of ionization and therefore reduces the sputtering ion impingement on the target [8]. In addition, it contributes to the resputtering of the deposited films. Negative oxygen ions that are generated at the oxide target and from an oxidant, O2, added in sputtering gas gain enough energy and are accelerated passing thorough the cathode dark space [9,10]. With increasing oxygen partial pressure, more negative oxygen ions could reach the growing films with high energy, which caused the deterioration of the films, as shown in Fig. 1. The values of the full width of half maximum (FWHM) of (311) main peaks in XRD are 0.6, 0.9, 1.0, 1.1, 1.1, and 1.2 for the films deposited at the oxygen partial pressure of 5%, 10%, 20%, 30%, 40%, and 50%, respectively. The broader FWHM of the films deposited at higher oxygen partial pressure indicated that they had inferior crystallinity due to the bombardment of the negative oxygen ions. Generally, the bombarding damages on the growing films are mainly attributed to the secondary electrons emitted from the cathode, the high-energy sputtered particles, and negative ions, such as oxygen ions, added in sputtering gas. The secondary electrons effects are minimized in a magnetron sputtering because they are captured around the cathode due to the magnetic field. The high-energy-sputtered particles can impact and deteriorate the surface of growing films when rf power increases over the optimum level. In Fig. 1, rf power was constant, so we could ignore its effect. Finally, only the negative oxygen ions could be the source of the bombarding.
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In addition, they affected the compositional ratio of the deposited films, as shown in Fig. 2. Target composition was slightly higher than 2, a stoichiometric Ga/Zn ratio in ZnGa2O4. Because the vapor pressure of zinc was high, Zn evaporated more than Ga during sintering process for a sputtering target. The Ga/Zn atomic ratios increase with increasing the oxygen partial pressure, O2/(Ar+O2). During sputtering, the compositions of the films are varied by some physical and chemical reasons: (I) scattering angle differences of sputtered atoms [11], (II) sputtering yield at the target, (III) reevaporation, and (IV) resputtering at the growing films. (III) and (IV) strongly depend on the oxygen partial pressure but not for (I) and (II). Previous works reported that resputtering effect was significant by adding O2 in the sputtering gas, especially in the planar magnetron sputtering system employed in this experiment [9]. Furthermore, this effect is more accelerated by heating the substrate during sputtering [10]. Fig. 2 exhibits the linear increase of Ga/Zn ratio as a function of oxygen partial pressure, which means that Zn atoms selectively diminish due to resputtering that is exacerbated by in situ substrate heating at 550 8C. Figs. 3 and 4 show PL intensity (k emission: 510 nm) of ZnGa2O4:Mn as a function of oxygen partial pressure and PL spectra, respectively. Each sample was postannealed at different atmospheres: air, vacuum (~103 Torr), and vacuum+N2 (high purity, 99.999%, N2 was purged maintaining the vacuum). In all cases, PL intensity decreased
Fig. 4. PL spectra of ZnGa2O4:Mn films annealed at different atmospheres.
significantly with increasing oxygen partial pressure. It is closely related to the Ga/Zn ratio that increases with Po2 in sputtering gas, as shown in Fig. 2. Some previous works [4– 6] report that excess Ga can improve the luminescent properties, on the contrary, our results show the deteriorated luminescence. In this experiment, an excess of Ga3+ ions and vacancy defects that originated by increasing Po2 create a perturbation in the crystal field surrounding the Mn2+ ions, which is identical with Yang and Yokoyama’s report [12]. Excess Ga3+ ions mean that the atomic ratio of Ga to Zn is larger than the stoichiometric composition, 2, which contributed to the vacancies creation of Zn. Zinc ions are reevaporated more easily than gallium ions. Thus, some zinc sites remained as vacancies. These vacancies do not affect the emission wavelength but only the emission intensity. Finally,
Table 1 Ga/Zn atomic ratio of zinc gallate films
Fig. 3. PL intensity of ZnGa2O4:Mn films annealed at different atmospheres.
Annealing ambient
Ga/Zn
Air Vacuum (~103 Torr) Vacuum (~103 Torr)+Nitrogen* As-deposited
2.98 2.68 2.51 2.44
Sputtering conditions—substrate temperature of 550 8C, sputtering pressure of 5 mTorr, working gas Ar/(Ar+O2)=5%, and then annealed 700 8C for 3 h. * N2 was purged into ~103 Torr vacuum chamber.
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With increasing oxygen partial pressure in annealing atmosphere, Zn atoms reevaporated more than Ga, according to Eqs. (1) and (2); therefore, Ga/Zn increased. Annealing atmosphere could also cause the oxidation of Mn2+ ions, which was severe at air atmosphere, while it was almost protected at vacuum and N2+vacuum. Thus, the oxidation of Mn2+ at air atmosphere partially contributed to the decrease of PL intensity. Fig. 5 shows CL of annealed ZnGa2O4:Mn thin films in various atmosphere. CL intensity severely depended on annealing conditions because of the same reason, as explained in Fig. 3.
4. Conclusion
Fig. 5. CL intensity of ZnGa2 O4:Mn films annealed at different atmospheres.
ZnGa2O4:Mn thin films were deposited by rf magnetron sputtering. The effects of the oxygen partial pressure and the annealing atmosphere on the compositional change and the luminescent properties were investigated. Ga/Zn atomic ratio strongly depended on the oxygen partial pressure both in sputtering gas and annealing atmosphere. With increasing the oxygen partial pressure, Ga/Zn increased, accordingly, the luminescent intensities of PL and CL decreased due to excess Ga and Zn deficiency.
Acknowledgement both excess Ga3+ and zinc vacancies disturbed the atomic configuration and the crystal field surrounding the Mn2+ ions in the films. They caused the perturbation of the ground level and the excited level of the activated electrons in the Mn2+ ions, and the PL intensities decreased, as shown in Fig. 4. In addition to the effects of Po2 in sputtering gas, annealing atmospheres affect Ga/Zn ratio, accordingly the luminescence. The PL intensity of annealed ZnGa2O4:Mn thin films was in sequence of annealing atmosphere of vacuum+N2, vacuum alone, and air. Table 1 shows Ga/Zn ratio of ZnGa2O4:Mn thin film annealed at air, vacuum, and vacuum+N2. Exposure to oxygen during air annealing increases the Ga/Zn ratio dramatically, while a little increases after vacuum annealing. The Ga/Zn ratios of the as-deposited and the N2+vacuum annealed thin films are nearly identical because oxidization was minimized under these conditions. The inclusion of N2 purge during vacuum annealing was able to protect the samples from oxygen more effectively than a vacuum alone. Finally, the diffusing out amounts of Ga and Zn depended on the oxygen pressure surrounding the samples during the annealing process. By the defect chemistry [13], the following equations can be considered. 1 þ1 O2 YOxo þ VZn W þ 2h˙Z½VZnW ˜ PO26 2
ð1Þ
1 þ1 O2YOxo þ VGa j þ 3h˙Z½VGa j ˜ PO28 2
ð2Þ
This work was supported by grant No. R05-2003-00010772-0 from Korea Science and Engineering Foundation.
References [1] T. Minami, T. Maeno, Y. Kuroi, S. Takata, Jpn. J. Appl. Phys. 34 (1995) L684. [2] Y.J. Kim, S.M. Chung, Y.H. Jeong, Y.E. Lee, J. Vac. Sci. Technol., A, Vac. Surf. Films 19 (4) (2001) 1095. [3] C.F. Yu, P. Lin, Jpn. J. Appl. Phys. 35 (11) (1996) 5726. [4] L.E. Shea, R.K. Datta, J.J. Brown Jr., J. Electrochem. Soc. 141 (8) (1994) 2198. [5] C.F. Yu, P. Lin, J. Appl. Phys. 79 (1996) 7191. [6] I.J. Hsieh, M.S. Feng, K.T. Juo, P. Lin, J. Electrochem. Soc. 141 (6) (1994) 1617. [7] K.H. Hsu, M.R. Yang, K.S. Chen, J. Mater. Sci., Mater. Electron. 9 (1998) 283. [8] J.B. Lounsbury, J. Vac. Sci. Technol. 6 (5) (1969) 838. [9] A. Hamerich, R. Wunderlich, J. Muller, J. Vac. Technol. A 12 (5) (1994) 2873. [10] J.M. Grace, D.B. McDonald, M.T. Reiten, J. Olson, R.T. Kampwirth, K.E. Gray, J. Vac. Technol. A 10 (4) (1992) 1600. [11] M. Hill, Solid State Technol. (1980) 53. [12] Su-Hua Yang, Meiso Yokoyama, Jpn. J. Appl. Phys. 36 (8) (1997) 5145. [13] Y.M. Chiang, Dunbar Birnielll, W.D. Kingery, Physical Ceramics, Wiley, New York, 1997, p. 110.
Characterization of compositional variation and luminescence of ZnGa2O4:Mn thin film phosphor Sung Mook Chunga, Sang Hyuk Hanb, Young Jin Kimb,* b
a Organic EL team, ETRI, Daejeon 305-350, Korea Dept. of Materials Science and Engineering, Kyonggi University, Suwon 442-760, Korea
Received 3 May 2004; received in revised form 5 November 2004; accepted 14 November 2004 Available online 2 December 2004
Abstract ZnGa2O4:Mn thin film phosphors were prepared on ITO/glass by rf magnetron reactive sputtering. The effects of the oxygen partial pressure in sputtering and annealing atmospheres on crystallinity, compositional variations, and luminescent properties of thin films were investigated. Atomic ratio of Ga/Zn in films strongly depended on the postannealing conditions, as well as the oxygen partial pressure in sputtering gas during sputtering, and accordingly affected the luminous properties. D 2004 Elsevier B.V. All rights reserved. Keywords: ZnGa2O4:Mn; Thin film; Phosphors; Luminescence
1. Introduction ZnGa2O4:Mn thin film phosphor is one of representative green-emitting phosphors due to excellent luminescent properties [1,2]. It is known that Ga acts as a sensitizer, while Mn is an activator [3]. So Ga/Zn atomic ratios are very important for the luminescence. According to Shea et al. [4], the minor absorption band at 290 nm is due to the direct absorption of photons by the Mn2+ centers, and the primary absorption at 245 nm is attributed to the Ga3+ ions. The latter’s excitation energy could be transferred in a nonradiative way to the doped Mn2+ ions. The loss of the Zn in the host lattice may enhance the substitution of Mn2+ ions into tetrahedral Zn sites, resulting in enhanced luminescent intensity. Yu and Lin [5] also found that Zn-deficient ZnGa2O4 films have excellent cathodoluminescence characteristics and suggested that the excess Ga content creates activators for the luminescence. According to Hsieh et al. [6], the improve* Corresponding author. Dept. of Materials Science and Engineering, Kyonggi University, Suwon 443-760, Kyonggi-Do, Korea. Tel.: +82 31 249 9766; fax: +82 31 249 9775. E-mail address: [email protected] (Y.J. Kim). 0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2004.11.021
ment in emission properties after annealing is a consequence of changes to the growth orientation of thin film phosphors and an increase in the Ga/Zn atomic ratio. Hsu et al. [7] proposed that the luminescent properties improve after annealing in vacuum rather than air. However, in ZnGa2O4 films, the compositional changes and their influence to the luminescent properties have not been investigated properly. In this experiment, the effects of oxygen partial pressures in sputtering gas and postannealing atmospheres on the compositional variation of Ga/Zn in films and the luminescent properties were investigated.
2. Experiment ZnGa2O4:Mn thin film phosphors were deposited on indium tin oxide (ITO)-coated glass substrates by rf magnetron sputtering method using an oxide target, which was fabricated by a conventional ceramic process as follows. Ga2O3, ZnO, and MnO powders were weighed and mixed together for the composition of Zn1x MnxGa2O4 (x=0.006) and pressed for a circular disk type, 3-in. diameter. And then it was sintered at 1350 8C in air for 5 h. O2/(Ar+O2) ratios were varied maintaining the working
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pressure at 5 mTorr. The substrate temperature and rf power were 550 8C and 150 W, respectively. Heat treatment of thin films was carried out in the tube furnace for 3 h at 700 8C in air, vacuum, and N2+vacuum(N2 purging), respectively. The structural changes were monitored by using X-ray diffractometer (XRD). Scanning electron microscope (SEM) with energy dispersive spectrometer (EDS) was used to determine the Ga/Zn atomic ratio and film thickness. Photoluminescence (PL) and cathodoluminescence (CL) were measured by DASA 5000 system (PSI, Korea). The excitation source of PL was a xenon lamp with a monochrometer. Accelerating voltages of the electron gun of CL ranged between 50 and 950 V.
3. Result and discussion Fig. 1 shows the XRD patterns of ZnGa2O4:Mn thin film phosphors deposited by rf sputtering at different sputtering gas ratio, O2/(Ar+O2). Because the deposition rate depended on the gas ratio, the deposition time was controlled to make all films to have a same thickness of about 2 Am. As shown, at the gas ratio of 5%, well-developed polycrystalline ZnGa2O4:Mn film could be obtained, while increasing oxygen partial pressure contributed to the deterioration of the peak intensity due to the negative oxygen ions bombardment to the growing films. It is known that the added oxygen causes the sputtering rate reduction because it decreases the
Fig. 1. XRD patterns of ZnGa2O4:Mn as a function of O2/(Ar+O2).
Fig. 2. Ga/Zn atomic ratio of ZnGa2O4:Mn films as a function of O2/ (Ar+O2).
amount of ionization and therefore reduces the sputtering ion impingement on the target [8]. In addition, it contributes to the resputtering of the deposited films. Negative oxygen ions that are generated at the oxide target and from an oxidant, O2, added in sputtering gas gain enough energy and are accelerated passing thorough the cathode dark space [9,10]. With increasing oxygen partial pressure, more negative oxygen ions could reach the growing films with high energy, which caused the deterioration of the films, as shown in Fig. 1. The values of the full width of half maximum (FWHM) of (311) main peaks in XRD are 0.6, 0.9, 1.0, 1.1, 1.1, and 1.2 for the films deposited at the oxygen partial pressure of 5%, 10%, 20%, 30%, 40%, and 50%, respectively. The broader FWHM of the films deposited at higher oxygen partial pressure indicated that they had inferior crystallinity due to the bombardment of the negative oxygen ions. Generally, the bombarding damages on the growing films are mainly attributed to the secondary electrons emitted from the cathode, the high-energy sputtered particles, and negative ions, such as oxygen ions, added in sputtering gas. The secondary electrons effects are minimized in a magnetron sputtering because they are captured around the cathode due to the magnetic field. The high-energy-sputtered particles can impact and deteriorate the surface of growing films when rf power increases over the optimum level. In Fig. 1, rf power was constant, so we could ignore its effect. Finally, only the negative oxygen ions could be the source of the bombarding.
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In addition, they affected the compositional ratio of the deposited films, as shown in Fig. 2. Target composition was slightly higher than 2, a stoichiometric Ga/Zn ratio in ZnGa2O4. Because the vapor pressure of zinc was high, Zn evaporated more than Ga during sintering process for a sputtering target. The Ga/Zn atomic ratios increase with increasing the oxygen partial pressure, O2/(Ar+O2). During sputtering, the compositions of the films are varied by some physical and chemical reasons: (I) scattering angle differences of sputtered atoms [11], (II) sputtering yield at the target, (III) reevaporation, and (IV) resputtering at the growing films. (III) and (IV) strongly depend on the oxygen partial pressure but not for (I) and (II). Previous works reported that resputtering effect was significant by adding O2 in the sputtering gas, especially in the planar magnetron sputtering system employed in this experiment [9]. Furthermore, this effect is more accelerated by heating the substrate during sputtering [10]. Fig. 2 exhibits the linear increase of Ga/Zn ratio as a function of oxygen partial pressure, which means that Zn atoms selectively diminish due to resputtering that is exacerbated by in situ substrate heating at 550 8C. Figs. 3 and 4 show PL intensity (k emission: 510 nm) of ZnGa2O4:Mn as a function of oxygen partial pressure and PL spectra, respectively. Each sample was postannealed at different atmospheres: air, vacuum (~103 Torr), and vacuum+N2 (high purity, 99.999%, N2 was purged maintaining the vacuum). In all cases, PL intensity decreased
Fig. 4. PL spectra of ZnGa2O4:Mn films annealed at different atmospheres.
significantly with increasing oxygen partial pressure. It is closely related to the Ga/Zn ratio that increases with Po2 in sputtering gas, as shown in Fig. 2. Some previous works [4– 6] report that excess Ga can improve the luminescent properties, on the contrary, our results show the deteriorated luminescence. In this experiment, an excess of Ga3+ ions and vacancy defects that originated by increasing Po2 create a perturbation in the crystal field surrounding the Mn2+ ions, which is identical with Yang and Yokoyama’s report [12]. Excess Ga3+ ions mean that the atomic ratio of Ga to Zn is larger than the stoichiometric composition, 2, which contributed to the vacancies creation of Zn. Zinc ions are reevaporated more easily than gallium ions. Thus, some zinc sites remained as vacancies. These vacancies do not affect the emission wavelength but only the emission intensity. Finally,
Table 1 Ga/Zn atomic ratio of zinc gallate films
Fig. 3. PL intensity of ZnGa2O4:Mn films annealed at different atmospheres.
Annealing ambient
Ga/Zn
Air Vacuum (~103 Torr) Vacuum (~103 Torr)+Nitrogen* As-deposited
2.98 2.68 2.51 2.44
Sputtering conditions—substrate temperature of 550 8C, sputtering pressure of 5 mTorr, working gas Ar/(Ar+O2)=5%, and then annealed 700 8C for 3 h. * N2 was purged into ~103 Torr vacuum chamber.
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With increasing oxygen partial pressure in annealing atmosphere, Zn atoms reevaporated more than Ga, according to Eqs. (1) and (2); therefore, Ga/Zn increased. Annealing atmosphere could also cause the oxidation of Mn2+ ions, which was severe at air atmosphere, while it was almost protected at vacuum and N2+vacuum. Thus, the oxidation of Mn2+ at air atmosphere partially contributed to the decrease of PL intensity. Fig. 5 shows CL of annealed ZnGa2O4:Mn thin films in various atmosphere. CL intensity severely depended on annealing conditions because of the same reason, as explained in Fig. 3.
4. Conclusion
Fig. 5. CL intensity of ZnGa2 O4:Mn films annealed at different atmospheres.
ZnGa2O4:Mn thin films were deposited by rf magnetron sputtering. The effects of the oxygen partial pressure and the annealing atmosphere on the compositional change and the luminescent properties were investigated. Ga/Zn atomic ratio strongly depended on the oxygen partial pressure both in sputtering gas and annealing atmosphere. With increasing the oxygen partial pressure, Ga/Zn increased, accordingly, the luminescent intensities of PL and CL decreased due to excess Ga and Zn deficiency.
Acknowledgement both excess Ga3+ and zinc vacancies disturbed the atomic configuration and the crystal field surrounding the Mn2+ ions in the films. They caused the perturbation of the ground level and the excited level of the activated electrons in the Mn2+ ions, and the PL intensities decreased, as shown in Fig. 4. In addition to the effects of Po2 in sputtering gas, annealing atmospheres affect Ga/Zn ratio, accordingly the luminescence. The PL intensity of annealed ZnGa2O4:Mn thin films was in sequence of annealing atmosphere of vacuum+N2, vacuum alone, and air. Table 1 shows Ga/Zn ratio of ZnGa2O4:Mn thin film annealed at air, vacuum, and vacuum+N2. Exposure to oxygen during air annealing increases the Ga/Zn ratio dramatically, while a little increases after vacuum annealing. The Ga/Zn ratios of the as-deposited and the N2+vacuum annealed thin films are nearly identical because oxidization was minimized under these conditions. The inclusion of N2 purge during vacuum annealing was able to protect the samples from oxygen more effectively than a vacuum alone. Finally, the diffusing out amounts of Ga and Zn depended on the oxygen pressure surrounding the samples during the annealing process. By the defect chemistry [13], the following equations can be considered. 1 þ1 O2 YOxo þ VZn W þ 2h˙Z½VZnW ˜ PO26 2
ð1Þ
1 þ1 O2YOxo þ VGa j þ 3h˙Z½VGa j ˜ PO28 2
ð2Þ
This work was supported by grant No. R05-2003-00010772-0 from Korea Science and Engineering Foundation.
References [1] T. Minami, T. Maeno, Y. Kuroi, S. Takata, Jpn. J. Appl. Phys. 34 (1995) L684. [2] Y.J. Kim, S.M. Chung, Y.H. Jeong, Y.E. Lee, J. Vac. Sci. Technol., A, Vac. Surf. Films 19 (4) (2001) 1095. [3] C.F. Yu, P. Lin, Jpn. J. Appl. Phys. 35 (11) (1996) 5726. [4] L.E. Shea, R.K. Datta, J.J. Brown Jr., J. Electrochem. Soc. 141 (8) (1994) 2198. [5] C.F. Yu, P. Lin, J. Appl. Phys. 79 (1996) 7191. [6] I.J. Hsieh, M.S. Feng, K.T. Juo, P. Lin, J. Electrochem. Soc. 141 (6) (1994) 1617. [7] K.H. Hsu, M.R. Yang, K.S. Chen, J. Mater. Sci., Mater. Electron. 9 (1998) 283. [8] J.B. Lounsbury, J. Vac. Sci. Technol. 6 (5) (1969) 838. [9] A. Hamerich, R. Wunderlich, J. Muller, J. Vac. Technol. A 12 (5) (1994) 2873. [10] J.M. Grace, D.B. McDonald, M.T. Reiten, J. Olson, R.T. Kampwirth, K.E. Gray, J. Vac. Technol. A 10 (4) (1992) 1600. [11] M. Hill, Solid State Technol. (1980) 53. [12] Su-Hua Yang, Meiso Yokoyama, Jpn. J. Appl. Phys. 36 (8) (1997) 5145. [13] Y.M. Chiang, Dunbar Birnielll, W.D. Kingery, Physical Ceramics, Wiley, New York, 1997, p. 110.