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Influence of post-annealing conditions on properties of ZnO:Ag films
Solid State Communications 145 (2008) 479–481 www.elsevier.com/locate/ssc
Influence of post-annealing conditions on properties of ZnO:Ag films Li Duan a , Wei Gao a,∗ , Ruiqun Chen a , Zhuxi Fu b a Department of Chemical and Materials Engineering, The University of Auckland, Private Bag 92019, Auckland, New Zealand b Department of Physics, University of Science and Technology China, Hefei, Anhui 230026, People’s Republic of China
Received 11 December 2007; accepted 17 December 2007 by A.H. MacDonald Available online 23 December 2007
Abstract Silver-doped ZnO films were grown on glass substrates by RF reactive magnetron sputtering. The as-grown ZnO:Ag film is insulating but behaves as p-type conduction with a resistivity of 152 cm, a carrier concentration of 2.24 × 1016 cm−3 and a Hall mobility of 1.83 cm2 /Vs after annealing in O2 atmosphere at 600 ◦ C for 1 h. The influence of post-annealing temperature and ambience on the electrical, structural and optical properties of the films was investigated. c 2007 Elsevier Ltd. All rights reserved.
PACS: 73.40.Lq Keywords: A. Semiconductors; C. p–n junction; E. Luminescence
ZnO, a wide-band-gap semiconductor material with a direct band gap of 3.36 eV and high exciton binding energy of 60 meV, has been receiving extensive attention for shortwavelength optoelectronic applications [1–5]. To realize the light-emitting devices, various dopants were attempted for fabricating high quality p-type ZnO [6–11]. Group Ia elements (such as Li and Na) are possible p-type doping candidates by substituting Zn in ZnO, but their smaller ionic radii cause the formation of interstitial ion as donors [12,13]. Ag, as a group Ib element with larger ionic radius, can be a potential candidate for p-type doping of ZnO theoretically if it substitutes Zn [14]. Very recently, p-type ZnO:Ag films were fabricated by pulsed laser deposition [15]. Strong ultraviolet emission from Agdoped ZnO films due to Ag acceptor was also reported [16]. In this paper, the influence of post-annealing temperature on the electrical, structural and optical properties of ZnO:Ag films was investigated. The results show that ZnO:Ag films were converted to p-type conduction after annealing in O2 at 550 ◦ C or 600 ◦ C. ZnO:Ag films and pure ZnO films were grown on glass substrates by RF reactive magnetron sputtering. The sputtering target for ZnO:Ag films was a disk of ceramic ZnO (99.99% ∗ Corresponding author.
E-mail address: [email protected] (W. Gao). c 2007 Elsevier Ltd. All rights reserved. 0038-1098/$ - see front matter doi:10.1016/j.ssc.2007.12.013
purity) dispersed with silver dots of ∼0.1% sputtering area. For ZnO film, a disk of pure ceramic ZnO target was used. Ar and O2 with the volume ratio of 1:2 were used as the working gas at a total pressure of 2 × 10−2 Torr. The film thickness of samples was about 1000 nm measured by scanning electron microscopy (SEM). The sputtering power was 250 W. The process has been described in more detail in our previous papers [17,18]. The sputtered films were annealed on various conditions for 1 hour. The structure of the films was investigated using a Druker D8 Advanced X-Ray Diffractometer (XRD) and a Philips XL30S SEM with a field emission gun. Electrical properties of films were measured by a Hall automatic measuring system using the Van Der Pauw technique. The electrodes of Ag-doped ZnO and pure ZnO were silver and indium, respectively [19]. Photoluminescence (PL) was measured in a He–Cd laser (λ = 325 nm) with a power density of 1 W/cm2 . Table 1 summarizes the electrical properties of the asgrown ZnO:Ag film and ZnO:Ag films annealed under various conditions. The ZnO:Ag films, annealed in O2 ambience at 450 and 500 ◦ C, show n-type conductivity. As the annealing temperature is increased to 550 ◦ C, the ZnO:Ag film shows ptype conduction. The optimal result is achieved at the annealing temperature of 600 ◦ C with a resistivity of 152 cm, a carrier concentration of 1.83 cm2 /Vs, and a Hall mobility of 2.24 × 1016 cm−3 . These results indicated two crucial post-
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Fig. 1. X-ray diffraction patterns of ZnO and ZnO:Ag films: (a) pure ZnO annealed at 600 ◦ C in O2 , (b) as-grown ZnO:Ag, (c) ZnO:Ag annealed at 600 ◦ C in N2 , (d) ZnO:Ag annealed at 600 ◦ C in O2 . The inset shows the diffraction angle of ZnO (002) peak of ZnO:Ag films versus annealing temperature (in O2 ).
Fig. 2. I –V curve and the schematic structure of ZnO:Ag/ZnO p–n homojunction.
Table 1 Electrical properties of the as-grown and annealed ZnO:Ag films Annealing Annealing temperatmosphere ature (◦ C)
Resistivity ( cm)
Hall mobility (cm2 /Vs)
Carrier con- Carriercentration type (cm−3 )
Asgrown 450 500 550 600 600
Not conductive 97 16 692 152 Not conductive
–
–
–
0.72 1.34 1.65 1.83 –
8.95×1016 2.83 × 1017 5.47 × 1015 2.24×1016 –
n n p p –
– O2 O2 O2 O2 N2
annealing conditions for obtaining p-type ZnO:Ag films: the proper annealing temperature and oxygen-rich atmosphere. It was reported that the conversion of ZnO:Li films from ntype to p-type depends on the post-annealing temperature [20]. This similarity may imply that Ag and Li in ZnO has the same doping mechanism. Annealing could not be performed > 600 ◦ C due to the glass substrates. The as-grown ZnO:Ag and nitrogen annealed ZnO:Ag films do not show any conductivity. This may be caused by the ‘self-compensation’ effect that the acceptors from silver are compensated by the native donor defects such as Zni , V0 and ZnO [21]. Since the formation enthalpies of Zni , V0 and ZnO in the O2 -rich environment are higher than that in the Zn-rich environment [22], annealing in O2 is necessary for p-type ZnO film formation. Fig. 1 shows XRD patterns of samples. Only one peak corresponding to ZnO (002) is observed in each film, which indicates highly c-axis oriented ZnO and ZnO:Ag films. The ZnO (002) peaks of pure ZnO film and as-grown ZnO:Ag film are very close, at 34.40◦ and 34.38◦ , respectively. However, ZnO (002) peak of p-type ZnO:Ag film annealed at 600 ◦ C in O2 shows an obvious shift to a lower angle of 34.20◦ . The shift
Fig. 3. SEM images of ZnO and ZnO:Ag films: (a) as-grown ZnO, (b) as-grown ZnO:Ag, (c) ZnO:Ag annealed at 600 ◦ C in N2 , and (d) ZnO:Ag annealed at 600 ◦ C in O2 .
of ZnO (002) peak indicates an increase of its lattice constant due to the substitution of Zn2+ ions (radius of 0.056 nm) by Ag+ ions (radius of 0.102 nm), implying AgZn defects in ZnO:Ag film annealed in O2 at 600 ◦ C. However, the as-grown ZnO:Ag film does not show AgZn defects. The ZnO (002) peak of ZnO:Ag film annealed in N2 at 600 ◦ C is located at 34.32◦ , which means that AgZn defects may also exist. The inset of Fig. 1 shows the position of ZnO (002) peak varying with the annealing temperature (in O2 ). It can be seen that the two p-type ZnO:Ag films annealed at 600 ◦ C and 550 ◦ C have the lowest 2θ of (002), implying more acceptors in ZnO. ZnO:Ag/ZnO p–n homojunction was also fabricated. The schematic structure of the p–n junction and its I –V curve are shown in Fig. 2. ZnO:Ag film and ZnO film were annealed at 600 ◦ C for 1 hour in O2 and N2 respectively. The I –V curve shows the characteristics of a typical p–n junction. Fig. 3 shows SEM images of ZnO and ZnO:Ag films. It is obvious that the as-grown ZnO film is compact but the asgrown ZnO:Ag film is porous, indicating that Ag doping caused changes in the growth of ZnO crystals. It also indicates that the non-compact microstructure of the as-grown ZnO:Ag may contribute to the non-conductive behaviour, together with the
L. Duan et al. / Solid State Communications 145 (2008) 479–481
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In summary, the influence of post-annealing temperature on the electrical, structural, and photonic properties of ZnO:Ag films is investigated. Conversion from n-type conduction to ptype at annealing temperature above 550 ◦ C has been observed. The optimal p-type conduction of ZnO:Ag film is achieved at the post-annealing temperature of 600 ◦ C with a resistivity of 152 cm, a carrier concentration of 2.24 × 1016 cm−3 and a Hall mobility of 1.83 cm2 /Vs. Acknowledgments This work was partially supported by a Marsden Grant from the Royal Society of New Zealand. The authors would like to thank the technical supports from the Advanced and Nano Materials Research Cluster and the Research Centre of Surface and Materials Science. References
Fig. 4. PL spectra for ZnO samples measured at 7 K: (a) undoped ZnO, (b) p-type ZnO:Ag annealed at 600 ◦ C in O2 .
lack of carriers due to the ‘self-compensation’. The ZnO:Ag film annealed in N2 at 600 ◦ C showed a quite different feature: long, irregular grains mixed with small round grains. As for the ZnO:Ag film annealed in O2 at 600 ◦ C, the grain size is obviously larger than that of as-grown ZnO:Ag; and the porosity seems reduced to a certain extent, which again may help in the conductivity of the film. PL of ZnO films was studied at 7K using He–Cd laser with an excitation wavelength of 325 nm. For undoped ZnO film, as shown in Fig. 4(a), a peak at 3.360 eV is attributed to the donorbound exciton emission (D 0 X ) [23]. For the p-type ZnO:Ag films, the D 0 X emission completely vanishes and two new Agrelated peaks located at 3.319 and 3.280 eV appeared, as shown in Fig. 4(b). This PL property is similar to those observed for some other p-type ZnO films in comparison with undoped ZnO [24,25]. The dominant peak at 3.319 eV of ZnO:Ag is attributed to the acceptor-bound exciton emissions (A0 X ) [15]. Besides, it was observed that only p-type ZnO:Ag films have the A0 X peak. This confirms that ZnO:Ag films were converted to p-type through proper annealing process. Nevertheless, to understand the origin and mechanism of PL emissions from ZnO:Ag, further study is needed.
[1] D.M. Bagnall, Y.F. Chen, Z. Zhu, T. Yao, S. Koyama, M.Y. Shen, T. Goto, Appl. Phys. Lett. 70 (1997) 2230. [2] P. Yu, Z.K. Tang, G.K.L. Wong, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Solid State Commun. 103 (1997) 459. [3] D.C. Look, D.C. Reynolds, J.R. Sizelove, R.L. Jones, C.W. Litton, G. Cantwell, W.C. Harsch, Solid State Commun. 150 (1998) 399. [4] D.C. Look, Mater. Sci. Eng. B 80 (2001) 383. [5] S.J. Pearton, D.P. Norton, K. Ip, Y.W. Heo, T. Steiner, Prog. Mater. Sci. 50 (2005) 293. [6] A. Tsukazaki, A. Ohtomo, T. Onuma, M. Ohtani, T. Makino, M. Sumiya, K. Ohtani, S.F. Chichibu, S. Fuke, Y. Segawa, H. Ohno, H. Koinuma, M. Kawasaki, Nat. Mater. 4 (2005) 42. [7] T. Aoki, Y. Hatanaka, D.C. Look, Appl. Phys. Lett. 76 (2000) 3257. [8] M. Joseph, H. Tabata, H. Saeki, K. Ueda, T. Kawai, Physica B 3 (02) (2001) 140. [9] D.C. Look, G.M. Renlund, R.H. Burgener II, J.R. Sizelove, Appl. Phys. Lett. 85 (2004) 5269. [10] G.D. Yuan, Z.Z. Ye, L.P. Zhu, Q. Qian, B.H. Zhao, R.X. Fan, Appl. Phys. Lett. 86 (2005) 202106. [11] Y.J. Zeng, Z.Z. Ye, W.Z. Xu, D.Y. Li, J.G. Lu, L.P. Zhu, B.H. Zhao, Appl. Phys. Lett. 88 (2006) 062107. [12] C.H. Park, S.B. Zhang, Su-Huai Wei, Phys. Rev. B 66 (2002) 073202. [13] D.C. Look, R.L. Jones, J.R. Sizelove, N.Y. Garces, N.C. Giles, L.E. Halliburton, Phys. Status Solidi A 195 (2003) 171. [14] J. Fan, R. Freer, J. Appl. Phys. 77 (1995) 9. [15] H.S. Kang, B.D. Ahn, J.H. Kim, G.H. Kim, S.H. Lim, H.W. Chang, S.Y. Lee, Appl. Phys. Lett. 88 (2006) 202108. [16] L. Duan, B. Lin, W. Zhang, S. Zhong, Z. Fu, Appl. Phys. Lett. 88 (2006) 232110. [17] Wei Gao, Zhengwei Li, Ceramics International 30 (2004) 1155–1159. [18] Z.W. Li, W. Gao, R.J. Reeves, Surface & Coating Technology 319–323 (2005) 198. [19] T. Akane, K. Sugioka, K. Midorikawa, J. Vac. Sci. Technol. B 18 (2000) 1406. [20] L.L. Chen, H.P. He, Z.Z. Ye, Y.J. Zeng, J.G. Lu, B.H. Zhao, L.P. Zhu, Chem. Phys. Lett. 420 (2006) 358. [21] G. Mandel, Phys. Rev. A 134 (1964) 1073. [22] S.B. Zhang, S.-H. Wei, Alex Zunger, Phys. Rev. B 63 (2001) 075205. ¨ ur, Ya.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Doˇgan, ¨ Ozg¨ [23] U. V. Avrutin, S.-J. Cho, H. Morkoc¸, J. Appl. Phys. 98 (2005) 041301. [24] Y.R. Ryu, T.S. Lee, Appl. Phys. Lett. 83 (2003) 87. [25] F.X. Xiu, Z. Yang, L.J. Mandalapu, D.T. Zhao, J.L. Liu, Appl. Phys. Lett. 87 (2005) 152101.
Influence of post-annealing conditions on properties of ZnO:Ag films Li Duan a , Wei Gao a,∗ , Ruiqun Chen a , Zhuxi Fu b a Department of Chemical and Materials Engineering, The University of Auckland, Private Bag 92019, Auckland, New Zealand b Department of Physics, University of Science and Technology China, Hefei, Anhui 230026, People’s Republic of China
Received 11 December 2007; accepted 17 December 2007 by A.H. MacDonald Available online 23 December 2007
Abstract Silver-doped ZnO films were grown on glass substrates by RF reactive magnetron sputtering. The as-grown ZnO:Ag film is insulating but behaves as p-type conduction with a resistivity of 152 cm, a carrier concentration of 2.24 × 1016 cm−3 and a Hall mobility of 1.83 cm2 /Vs after annealing in O2 atmosphere at 600 ◦ C for 1 h. The influence of post-annealing temperature and ambience on the electrical, structural and optical properties of the films was investigated. c 2007 Elsevier Ltd. All rights reserved.
PACS: 73.40.Lq Keywords: A. Semiconductors; C. p–n junction; E. Luminescence
ZnO, a wide-band-gap semiconductor material with a direct band gap of 3.36 eV and high exciton binding energy of 60 meV, has been receiving extensive attention for shortwavelength optoelectronic applications [1–5]. To realize the light-emitting devices, various dopants were attempted for fabricating high quality p-type ZnO [6–11]. Group Ia elements (such as Li and Na) are possible p-type doping candidates by substituting Zn in ZnO, but their smaller ionic radii cause the formation of interstitial ion as donors [12,13]. Ag, as a group Ib element with larger ionic radius, can be a potential candidate for p-type doping of ZnO theoretically if it substitutes Zn [14]. Very recently, p-type ZnO:Ag films were fabricated by pulsed laser deposition [15]. Strong ultraviolet emission from Agdoped ZnO films due to Ag acceptor was also reported [16]. In this paper, the influence of post-annealing temperature on the electrical, structural and optical properties of ZnO:Ag films was investigated. The results show that ZnO:Ag films were converted to p-type conduction after annealing in O2 at 550 ◦ C or 600 ◦ C. ZnO:Ag films and pure ZnO films were grown on glass substrates by RF reactive magnetron sputtering. The sputtering target for ZnO:Ag films was a disk of ceramic ZnO (99.99% ∗ Corresponding author.
E-mail address: [email protected] (W. Gao). c 2007 Elsevier Ltd. All rights reserved. 0038-1098/$ - see front matter doi:10.1016/j.ssc.2007.12.013
purity) dispersed with silver dots of ∼0.1% sputtering area. For ZnO film, a disk of pure ceramic ZnO target was used. Ar and O2 with the volume ratio of 1:2 were used as the working gas at a total pressure of 2 × 10−2 Torr. The film thickness of samples was about 1000 nm measured by scanning electron microscopy (SEM). The sputtering power was 250 W. The process has been described in more detail in our previous papers [17,18]. The sputtered films were annealed on various conditions for 1 hour. The structure of the films was investigated using a Druker D8 Advanced X-Ray Diffractometer (XRD) and a Philips XL30S SEM with a field emission gun. Electrical properties of films were measured by a Hall automatic measuring system using the Van Der Pauw technique. The electrodes of Ag-doped ZnO and pure ZnO were silver and indium, respectively [19]. Photoluminescence (PL) was measured in a He–Cd laser (λ = 325 nm) with a power density of 1 W/cm2 . Table 1 summarizes the electrical properties of the asgrown ZnO:Ag film and ZnO:Ag films annealed under various conditions. The ZnO:Ag films, annealed in O2 ambience at 450 and 500 ◦ C, show n-type conductivity. As the annealing temperature is increased to 550 ◦ C, the ZnO:Ag film shows ptype conduction. The optimal result is achieved at the annealing temperature of 600 ◦ C with a resistivity of 152 cm, a carrier concentration of 1.83 cm2 /Vs, and a Hall mobility of 2.24 × 1016 cm−3 . These results indicated two crucial post-
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Fig. 1. X-ray diffraction patterns of ZnO and ZnO:Ag films: (a) pure ZnO annealed at 600 ◦ C in O2 , (b) as-grown ZnO:Ag, (c) ZnO:Ag annealed at 600 ◦ C in N2 , (d) ZnO:Ag annealed at 600 ◦ C in O2 . The inset shows the diffraction angle of ZnO (002) peak of ZnO:Ag films versus annealing temperature (in O2 ).
Fig. 2. I –V curve and the schematic structure of ZnO:Ag/ZnO p–n homojunction.
Table 1 Electrical properties of the as-grown and annealed ZnO:Ag films Annealing Annealing temperatmosphere ature (◦ C)
Resistivity ( cm)
Hall mobility (cm2 /Vs)
Carrier con- Carriercentration type (cm−3 )
Asgrown 450 500 550 600 600
Not conductive 97 16 692 152 Not conductive
–
–
–
0.72 1.34 1.65 1.83 –
8.95×1016 2.83 × 1017 5.47 × 1015 2.24×1016 –
n n p p –
– O2 O2 O2 O2 N2
annealing conditions for obtaining p-type ZnO:Ag films: the proper annealing temperature and oxygen-rich atmosphere. It was reported that the conversion of ZnO:Li films from ntype to p-type depends on the post-annealing temperature [20]. This similarity may imply that Ag and Li in ZnO has the same doping mechanism. Annealing could not be performed > 600 ◦ C due to the glass substrates. The as-grown ZnO:Ag and nitrogen annealed ZnO:Ag films do not show any conductivity. This may be caused by the ‘self-compensation’ effect that the acceptors from silver are compensated by the native donor defects such as Zni , V0 and ZnO [21]. Since the formation enthalpies of Zni , V0 and ZnO in the O2 -rich environment are higher than that in the Zn-rich environment [22], annealing in O2 is necessary for p-type ZnO film formation. Fig. 1 shows XRD patterns of samples. Only one peak corresponding to ZnO (002) is observed in each film, which indicates highly c-axis oriented ZnO and ZnO:Ag films. The ZnO (002) peaks of pure ZnO film and as-grown ZnO:Ag film are very close, at 34.40◦ and 34.38◦ , respectively. However, ZnO (002) peak of p-type ZnO:Ag film annealed at 600 ◦ C in O2 shows an obvious shift to a lower angle of 34.20◦ . The shift
Fig. 3. SEM images of ZnO and ZnO:Ag films: (a) as-grown ZnO, (b) as-grown ZnO:Ag, (c) ZnO:Ag annealed at 600 ◦ C in N2 , and (d) ZnO:Ag annealed at 600 ◦ C in O2 .
of ZnO (002) peak indicates an increase of its lattice constant due to the substitution of Zn2+ ions (radius of 0.056 nm) by Ag+ ions (radius of 0.102 nm), implying AgZn defects in ZnO:Ag film annealed in O2 at 600 ◦ C. However, the as-grown ZnO:Ag film does not show AgZn defects. The ZnO (002) peak of ZnO:Ag film annealed in N2 at 600 ◦ C is located at 34.32◦ , which means that AgZn defects may also exist. The inset of Fig. 1 shows the position of ZnO (002) peak varying with the annealing temperature (in O2 ). It can be seen that the two p-type ZnO:Ag films annealed at 600 ◦ C and 550 ◦ C have the lowest 2θ of (002), implying more acceptors in ZnO. ZnO:Ag/ZnO p–n homojunction was also fabricated. The schematic structure of the p–n junction and its I –V curve are shown in Fig. 2. ZnO:Ag film and ZnO film were annealed at 600 ◦ C for 1 hour in O2 and N2 respectively. The I –V curve shows the characteristics of a typical p–n junction. Fig. 3 shows SEM images of ZnO and ZnO:Ag films. It is obvious that the as-grown ZnO film is compact but the asgrown ZnO:Ag film is porous, indicating that Ag doping caused changes in the growth of ZnO crystals. It also indicates that the non-compact microstructure of the as-grown ZnO:Ag may contribute to the non-conductive behaviour, together with the
L. Duan et al. / Solid State Communications 145 (2008) 479–481
481
In summary, the influence of post-annealing temperature on the electrical, structural, and photonic properties of ZnO:Ag films is investigated. Conversion from n-type conduction to ptype at annealing temperature above 550 ◦ C has been observed. The optimal p-type conduction of ZnO:Ag film is achieved at the post-annealing temperature of 600 ◦ C with a resistivity of 152 cm, a carrier concentration of 2.24 × 1016 cm−3 and a Hall mobility of 1.83 cm2 /Vs. Acknowledgments This work was partially supported by a Marsden Grant from the Royal Society of New Zealand. The authors would like to thank the technical supports from the Advanced and Nano Materials Research Cluster and the Research Centre of Surface and Materials Science. References
Fig. 4. PL spectra for ZnO samples measured at 7 K: (a) undoped ZnO, (b) p-type ZnO:Ag annealed at 600 ◦ C in O2 .
lack of carriers due to the ‘self-compensation’. The ZnO:Ag film annealed in N2 at 600 ◦ C showed a quite different feature: long, irregular grains mixed with small round grains. As for the ZnO:Ag film annealed in O2 at 600 ◦ C, the grain size is obviously larger than that of as-grown ZnO:Ag; and the porosity seems reduced to a certain extent, which again may help in the conductivity of the film. PL of ZnO films was studied at 7K using He–Cd laser with an excitation wavelength of 325 nm. For undoped ZnO film, as shown in Fig. 4(a), a peak at 3.360 eV is attributed to the donorbound exciton emission (D 0 X ) [23]. For the p-type ZnO:Ag films, the D 0 X emission completely vanishes and two new Agrelated peaks located at 3.319 and 3.280 eV appeared, as shown in Fig. 4(b). This PL property is similar to those observed for some other p-type ZnO films in comparison with undoped ZnO [24,25]. The dominant peak at 3.319 eV of ZnO:Ag is attributed to the acceptor-bound exciton emissions (A0 X ) [15]. Besides, it was observed that only p-type ZnO:Ag films have the A0 X peak. This confirms that ZnO:Ag films were converted to p-type through proper annealing process. Nevertheless, to understand the origin and mechanism of PL emissions from ZnO:Ag, further study is needed.
[1] D.M. Bagnall, Y.F. Chen, Z. Zhu, T. Yao, S. Koyama, M.Y. Shen, T. Goto, Appl. Phys. Lett. 70 (1997) 2230. [2] P. Yu, Z.K. Tang, G.K.L. Wong, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Solid State Commun. 103 (1997) 459. [3] D.C. Look, D.C. Reynolds, J.R. Sizelove, R.L. Jones, C.W. Litton, G. Cantwell, W.C. Harsch, Solid State Commun. 150 (1998) 399. [4] D.C. Look, Mater. Sci. Eng. B 80 (2001) 383. [5] S.J. Pearton, D.P. Norton, K. Ip, Y.W. Heo, T. Steiner, Prog. Mater. Sci. 50 (2005) 293. [6] A. Tsukazaki, A. Ohtomo, T. Onuma, M. Ohtani, T. Makino, M. Sumiya, K. Ohtani, S.F. Chichibu, S. Fuke, Y. Segawa, H. Ohno, H. Koinuma, M. Kawasaki, Nat. Mater. 4 (2005) 42. [7] T. Aoki, Y. Hatanaka, D.C. Look, Appl. Phys. Lett. 76 (2000) 3257. [8] M. Joseph, H. Tabata, H. Saeki, K. Ueda, T. Kawai, Physica B 3 (02) (2001) 140. [9] D.C. Look, G.M. Renlund, R.H. Burgener II, J.R. Sizelove, Appl. Phys. Lett. 85 (2004) 5269. [10] G.D. Yuan, Z.Z. Ye, L.P. Zhu, Q. Qian, B.H. Zhao, R.X. Fan, Appl. Phys. Lett. 86 (2005) 202106. [11] Y.J. Zeng, Z.Z. Ye, W.Z. Xu, D.Y. Li, J.G. Lu, L.P. Zhu, B.H. Zhao, Appl. Phys. Lett. 88 (2006) 062107. [12] C.H. Park, S.B. Zhang, Su-Huai Wei, Phys. Rev. B 66 (2002) 073202. [13] D.C. Look, R.L. Jones, J.R. Sizelove, N.Y. Garces, N.C. Giles, L.E. Halliburton, Phys. Status Solidi A 195 (2003) 171. [14] J. Fan, R. Freer, J. Appl. Phys. 77 (1995) 9. [15] H.S. Kang, B.D. Ahn, J.H. Kim, G.H. Kim, S.H. Lim, H.W. Chang, S.Y. Lee, Appl. Phys. Lett. 88 (2006) 202108. [16] L. Duan, B. Lin, W. Zhang, S. Zhong, Z. Fu, Appl. Phys. Lett. 88 (2006) 232110. [17] Wei Gao, Zhengwei Li, Ceramics International 30 (2004) 1155–1159. [18] Z.W. Li, W. Gao, R.J. Reeves, Surface & Coating Technology 319–323 (2005) 198. [19] T. Akane, K. Sugioka, K. Midorikawa, J. Vac. Sci. Technol. B 18 (2000) 1406. [20] L.L. Chen, H.P. He, Z.Z. Ye, Y.J. Zeng, J.G. Lu, B.H. Zhao, L.P. Zhu, Chem. Phys. Lett. 420 (2006) 358. [21] G. Mandel, Phys. Rev. A 134 (1964) 1073. [22] S.B. Zhang, S.-H. Wei, Alex Zunger, Phys. Rev. B 63 (2001) 075205. ¨ ur, Ya.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Doˇgan, ¨ Ozg¨ [23] U. V. Avrutin, S.-J. Cho, H. Morkoc¸, J. Appl. Phys. 98 (2005) 041301. [24] Y.R. Ryu, T.S. Lee, Appl. Phys. Lett. 83 (2003) 87. [25] F.X. Xiu, Z. Yang, L.J. Mandalapu, D.T. Zhao, J.L. Liu, Appl. Phys. Lett. 87 (2005) 152101.