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Fabrication and post-anneal activation of p-type ZnMgO:Li film using dc reactive magnetron sputtering
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Materials Letters 62 (2008) 2554 – 2556 www.elsevier.com/locate/matlet
Fabrication and post-anneal activation of p-type ZnMgO:Li film using dc reactive magnetron sputtering Lingxiang Chen, Zhizhen Ye ⁎, Shisheng Lin, Haiping He, Yujia Zeng, Binghui Zhao, Liping Zhu State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, China Received 12 September 2007; accepted 20 December 2007 Available online 31 December 2007
Abstract Monodoping Li as acceptor, p-type ZnMgO thin films have been realized via dc reactive magnetron sputtering followed by a thermal anneal process. The conductivity of Li-doped films are transformed from highly resistant to p-type via anneal. This phenomenon may be associated with eliminating of LiZn–Lii complexes and Lii. The optimized p-type Zn0.96Mg0.04O:Li thin film possesses a resistivity of 72.3Ω cm and a carrier concentration of 1.49 × 1017cm− 3, while the electrical properties of Zn0.84Mg0.16O:Li thin film degrade in comparison to Zn0.96Mg0.04O:Li thin film. Moreover, the absorption spectra confirm Mg can be a good candidate for band gap engineering in p-type ZnO film. © 2007 Elsevier B.V. All rights reserved. Keywords: Semiconductors; Thin films; P-type; ZnMgO; Li-doped
1. Introduction ZnO, with a direct gap of 3.37eV and exciton binding energy of 60meV at room temperature, is attractive for optoelectronic applications in the blue and UV regions, e.g., light emitting diodes and laser diodes. However, in order to realize the highperformance ZnO-based optoelectronic devices, one of the important requirements is the band gap engineering [1]. For this, a large number of reports indicate that incorporation of magnesium into ZnO lattice is an effective way to increase the fundamental band gap energy of ZnO [2,3]. Another critical issue is to achieve stable and reproducible p-type ZnMgO. On the other hand, although a number of candidates have been received particular attentions, such as N [4–6], P [7], As [8], Sb [9], the optimal choice of acceptor specie remains to be determined. Group-I species substituting for Zn, such as LiZn, theoretically possess shallow acceptor levels [10]. In our previous work, we have reported stable Li-doped p-type ZnO films ⁎ Corresponding author. Tel./fax: +86 571 87952625. E-mail addresses: [email protected] (L. Chen), [email protected] (Z. Ye), [email protected] (S. Lin), [email protected] (H. He), [email protected] (Y. Zeng), [email protected] (L. Zhu). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.12.046
[11,12]. In this letter, we investigated p-type behavior in ZnMgO:Li thin films grown by dc reactive magnetron sputtering. 2. Experiments ZnMgO films were prepared on glass substrates by dc reactive magnetron sputtering. The glass substrates were cleaned in ultrasonic bath with alcohol for 30min. Ternary ZnxMg1 − x:Li alloys (x = 0.04, 0.16), doped with 0.1at.% Li, (99.99% purity), were used as the targets for fabricating the Li monodoped ZnMgO films. The deposition chamber was evacuated to a base pressure of 10− 3Pa. Ar (99.99%) and O2 (99.99%) with the ratio of 1:1 (in volume) were introduced as the sputtering gases at a total pressure of 4.5Pa. The growth temperature was kept at 550°C. Before deposition, the target was pre-sputtered for 10min to remove any contaminants on the target surface. The sputtering current and voltage were 300mA, 180V, respectively. All the films were deposited for 30min with a typical thickness of about 200nm obtained from cross-sectional scanning electron microscopy (SEM) measurements. After growth, the films were rapidly annealed at various temperatures ranging from 430°C to 560°C in O2 ambient for 5min.
L. Chen et al. / Materials Letters 62 (2008) 2554–2556
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Table 1 Electrical properties of the as-grown Zn1 − xMgxO:Li (x = 0.04, 0.16) films at 550 °C and the films annealed at various temperatures from 450 °C to 560 °C Sample
Anneal temperature
Carrier type
(°C) Zn0.96Mg0.04O:Li
Zn0.84Mg0.16O:Li
As-grown 450 470 500 500 (after a month) 530 560 500 500 (after a month)
? p p p p p ? p p
Resistivity
Carrier concentration
Hall mobility
(Ω cm)
(cm− 3)
(cm2V− 1s− 1)
9.37 × 10 4 602 68.1 72.3 92.4 99.4 8.04 × 103 157 194
? 1.37 × 1016 5.73 × 1017 1.49 × 1017 8.37 × 1016 1.79 × 1017 ? 2.05 × 1017 9.82 × 1016
? 0.76 0.16 0.58 0.81 0.35 ? 0.19 0.33
Electrical properties of the annealed Zn1 − xMgxO:Li films at 500 °C were reexamined a month later.
Room temperature Hall-effect measurements were carried out in the van der Pauw configuration on a BIO-RAD HL5500PC system. X-ray diffraction (XRD) patterns were measured via a Philips X'Pert system with Cu Kα radiation (λ = 0.1542nm). The morphology of the films was examined using HITACHI S-4800 field emission scanning electronic microscopy (FESEM). The optical absorption spectra through the films were measured using a Lambda20 spectrometer. Moreover, the chemical states and
Fig. 1. XRD patterns of Zn1 − xMgxO:Li films (x = 0.04, 0.16) and ZnO film. (a) Diffraction angle of the (0002) peak depending on Mg content. Inset shows the magnification pattern of (0002) peaks. (b) XRD patterns of as-grown Zn0.96Mg0.04O:Li film and it annealed at 500 °C.
content of elements in the film were analyzed by XPS using an Omicron EAC2000-125 hemispherical analyzer. 3. Results and discussions Table 1 summarizes the electrical properties of the as-grown 0.1at.% Li-doped Zn1 − xMgxO (x = 0.04, 0.16) films and the films post-annealed at various temperatures. The as-grown Zn0.96Mg0.04O:Li films have high resistivity and ambiguous signal of carrier type. It is seen that when the anneal temperature is relatively low, the films also possess high resistivity and the carrier type is ambiguous. As the annealing temperature increases, p-type ZnMgO films are obtained and the electrical properties are
Fig. 2. XPS spectra of Mg 1s for Zn0.96Mg0.04O:Li films: (a) As-deposited. (b) Post-annealed at 500 °C.
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L. Chen et al. / Materials Letters 62 (2008) 2554–2556
to be 6at.% while Mg 1s peak energy slightly shifts, which means that postanneal has no obvious effect on the chemical state and content of Mg in the films. On the other hand, the signal of Li is near the noise level due to the detection limit of XPS, in accordance with our previous work [11]. Furthermore, room-temperature absorption spectra of the three annealed films (pure ZnO, Zn0.96Mg0.04O:Li and Zn0.84Mg0.16O:Li) were measured, as shown in Fig. 3. Absorption edges of the spectra shift to higher energy as Mg content increases. Moreover, the difference in absorption spectra between as-grown and post-annealed films is tiny (not shown in the graph), in accordance with the XPS results, revealing that post-anneal has no obvious effect on the content and chemical states of Mg element. The optical band gap (Eg) of pure ZnO and Zn1 − xMgxO:Li films (x = 0.04, 0.16) can be determined to be 3.18, 3.31 and 3.61 eV, respectively. It is seen that the optical band gap of the films blue shift as Mg content increases, which means that Mg can be a good candidate for band gap engineering in p-type ZnO film. Fig. 3. The absorption spectra of the pure ZnO, Zn1 − xMgxO:Li (x = 0.04, 0.16) films.
4. Conclusions improved. The optimal result is achieved at the annealing temperature of 500°C, with a resistivity of 72.3Ω cm, a carrier concentration of 1.49 × 1017cm−3, and a Hall mobility of 0.58cm2V−1s−1. The realization of p-type films indicates the formation or activation of Li acceptor after anneal. As the annealing temperature further increases, the electrical properties of the p-type Zn0.96Mg0.04O:Li thin films degrade, which may result from the re-evaporation of Li in the films [11]. On the other hand, we achieved p-type Zn0.84Mg0.16O:Li thin films via postanneal at 500°C, with a resistivity of 157Ω cm, a carrier concentration of 2.05 × 1017cm−3, and a Hall mobility of 0.19cm2V−1s−1. With Mg content increasing, the electrical properties of the films degrade, which may be due to the deeper Li acceptor level in heavily Mg alloyed films, in well accordance with our previous work [13]. Furthermore, it is found that after a month, all films post-annealed at 500°C keep p-type conduction without obvious degradation of electrical properties, demonstrating the stability of the p-type behavior in Li-doped ZnMgO films. The XRD patterns of as-deposited Zn1 − xMgxO:Li films (x = 0.04, 0.16) and pure ZnO film are shown in Fig. 1(a). Only two peaks indexed to (0002) and (0004) of ZnO are observed even when the Mg content increases to 16at. %, indicating that the Zn1 − xMgxO:Li films possess a single wurtzite phase with preferential c-axis orientation. The magnified patterns of (0002) peak are shown in the inset of Fig. 1(a). As Mg content increases, the (0002) peak shifts from 34.38° to 34.43°, then to 34.58°, which is due to the substitution of larger Zn2+ ions ´ ) by smaller Mg2+ ions (0.57Å ´ ). (0.60Å Fig. 1(b) shows the XRD patterns of the as-grown Zn0.96Mg0.04O:Li thin film and that film annealed at 500°C. Compared to the as-grown thin film, the (0002) peak of the annealed film shifts to higher angle, from 34.43° to 34.51°, which means decreasing of the lattice parameter. The lattice contraction could be attributed to defects evolution, since doping with electrically active atoms into semiconductor could result in change of lattice constant due to size effect [14]. In addition, the XRD intensity of postannealed film is greatly enhanced, comparing with the as-deposited film. Considering the better p-type conduction, decreasing lattice parameter, enhanced XRD intensity, upgrading crystal morphology observed in SEM (not shown here) of post-annealed sample, we suggest that the as-grown thin film may have more “hole killer” associated with Li, such as LiZn–Lii complex or Lii predicted by Wardle et al. [15], which would result in lattice expansion and compensate p-type doping [12,16]. The Li acceptor may be activated after post-anneal by eliminating of “hole killer”. Fig. 2 shows the XPS spectra of Mg 1s peak of the as-grown and annealed Zn0.96Mg0.04O:Li thin film. The content of Mg are both calculated
In summary, we have realized p-type Zn1 − xMgxO:Li thin films by using dc reactive magnetron sputtering, with post-anneal in pure O2 ambient. The optimal result is achieved at the annealing temperature of 500 °C for the Zn0.96Mg0.04O:Li film. The optical band gap can be controlled by tuning Mg content in the films. Acknowledgements The authors thank Professor J.X. Wu for the assistance in the XPS measurements. This work was supported by the National Natural Science Foundation of China under grant no 2006CB 604906 and the National Natural Science Foundation of China under contract nos. 50532060 and 90601003. References [1] Y.R. Ryu, J.A. Lubguban, T.S. Lee, H.W. White, T.S. Jeong, C.J. Youn, et al., Appl. Phys. Lett. 90 (2007) 131115-1-3. [2] S. Choonpun, R.D. Vispute, W. Yang, R.P. Sharma, T. Venkatesan, Appl. Phys. Lett. 80 (2002) 1529–1531. [3] A. Ohtomo, M. Kawasaki, T. Koida, K. Masubuchi, H. Koinuma, Appl. Phys. Lett. 72 (1998) 2466–2468. [4] D.C. Look, D.C. Reynolds, C.W. Litton, R.L. Jones, D.B. Eason, G. Cantwell, Appl. Phys. Lett. 81 (2002) 1830–1832. [5] T.M. Barnes, K. Olson, C.A. Wolden, Appl. Phys. Lett. 86 (2005) 112112-1-3. [6] G.D. Yuan, Z.Z. Ye, L.P. Zhu, Q. Qian, B.H. Zhao, et al., Appl. Phys. Lett. 86 (2005) 202106-1-3. [7] Y.W. Heo, S.J. Park, K. Ip, S.J. Pearton, D.P. Noton, Appl. Phys. Lett. 83 (2003) 1128–1130. [8] Y.R. Ryu, T.S. Lee, J.H. Leem, H.W. White, Appl. Phys. Lett. 83 (2003) 4032–4034. [9] F.X. Xiu, Z. Yang, L.J. Mandalapu, D.T. Zhao, J.L. Liu, Appl. Phys. Lett. 87 (2005) 252102-1-3. [10] C.H. Park, S.B. Zhang, S.-H. Wei, Phys. Rev., B 66 (2002) 073202-1-3. [11] Y.J. Zeng, Z.Z. Ye, W.Z. Xu, D.Y. Li, J.G. Lu, L.P. Zhu, et al., Appl. Phys. Lett. 88 (2006) 062107-1-3. [12] Y.J. Zeng, Z.Z. Ye, J.G. Lu, W.Z. Xu, L.P. Zhu, B.H. Zhao, Appl. Phys. Lett. 89 (2006) 042106-1-3. [13] M.X. Qiu, Z.Z. Ye, H.P. He, Y.Z. Zhang, X.Q. Gu, L.P. Zhu, et al., Appl. Phys. Lett. 90 (2007) 182116-1-3. [14] W. Li, M. Pessa, Phys. Rev., B 57 (1998) 14327–14329. [15] M.G. Wardle, J.P. Goss, P.R. Briddon, Phys. Rev., B 71 (2005) 155205-1-10. [16] S.B. Zhang, S.H. Wei, A. Zunger, Phys. Rev., B 63 (2001) 075205-1-7.
Materials Letters 62 (2008) 2554 – 2556 www.elsevier.com/locate/matlet
Fabrication and post-anneal activation of p-type ZnMgO:Li film using dc reactive magnetron sputtering Lingxiang Chen, Zhizhen Ye ⁎, Shisheng Lin, Haiping He, Yujia Zeng, Binghui Zhao, Liping Zhu State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, China Received 12 September 2007; accepted 20 December 2007 Available online 31 December 2007
Abstract Monodoping Li as acceptor, p-type ZnMgO thin films have been realized via dc reactive magnetron sputtering followed by a thermal anneal process. The conductivity of Li-doped films are transformed from highly resistant to p-type via anneal. This phenomenon may be associated with eliminating of LiZn–Lii complexes and Lii. The optimized p-type Zn0.96Mg0.04O:Li thin film possesses a resistivity of 72.3Ω cm and a carrier concentration of 1.49 × 1017cm− 3, while the electrical properties of Zn0.84Mg0.16O:Li thin film degrade in comparison to Zn0.96Mg0.04O:Li thin film. Moreover, the absorption spectra confirm Mg can be a good candidate for band gap engineering in p-type ZnO film. © 2007 Elsevier B.V. All rights reserved. Keywords: Semiconductors; Thin films; P-type; ZnMgO; Li-doped
1. Introduction ZnO, with a direct gap of 3.37eV and exciton binding energy of 60meV at room temperature, is attractive for optoelectronic applications in the blue and UV regions, e.g., light emitting diodes and laser diodes. However, in order to realize the highperformance ZnO-based optoelectronic devices, one of the important requirements is the band gap engineering [1]. For this, a large number of reports indicate that incorporation of magnesium into ZnO lattice is an effective way to increase the fundamental band gap energy of ZnO [2,3]. Another critical issue is to achieve stable and reproducible p-type ZnMgO. On the other hand, although a number of candidates have been received particular attentions, such as N [4–6], P [7], As [8], Sb [9], the optimal choice of acceptor specie remains to be determined. Group-I species substituting for Zn, such as LiZn, theoretically possess shallow acceptor levels [10]. In our previous work, we have reported stable Li-doped p-type ZnO films ⁎ Corresponding author. Tel./fax: +86 571 87952625. E-mail addresses: [email protected] (L. Chen), [email protected] (Z. Ye), [email protected] (S. Lin), [email protected] (H. He), [email protected] (Y. Zeng), [email protected] (L. Zhu). 0167-577X/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2007.12.046
[11,12]. In this letter, we investigated p-type behavior in ZnMgO:Li thin films grown by dc reactive magnetron sputtering. 2. Experiments ZnMgO films were prepared on glass substrates by dc reactive magnetron sputtering. The glass substrates were cleaned in ultrasonic bath with alcohol for 30min. Ternary ZnxMg1 − x:Li alloys (x = 0.04, 0.16), doped with 0.1at.% Li, (99.99% purity), were used as the targets for fabricating the Li monodoped ZnMgO films. The deposition chamber was evacuated to a base pressure of 10− 3Pa. Ar (99.99%) and O2 (99.99%) with the ratio of 1:1 (in volume) were introduced as the sputtering gases at a total pressure of 4.5Pa. The growth temperature was kept at 550°C. Before deposition, the target was pre-sputtered for 10min to remove any contaminants on the target surface. The sputtering current and voltage were 300mA, 180V, respectively. All the films were deposited for 30min with a typical thickness of about 200nm obtained from cross-sectional scanning electron microscopy (SEM) measurements. After growth, the films were rapidly annealed at various temperatures ranging from 430°C to 560°C in O2 ambient for 5min.
L. Chen et al. / Materials Letters 62 (2008) 2554–2556
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Table 1 Electrical properties of the as-grown Zn1 − xMgxO:Li (x = 0.04, 0.16) films at 550 °C and the films annealed at various temperatures from 450 °C to 560 °C Sample
Anneal temperature
Carrier type
(°C) Zn0.96Mg0.04O:Li
Zn0.84Mg0.16O:Li
As-grown 450 470 500 500 (after a month) 530 560 500 500 (after a month)
? p p p p p ? p p
Resistivity
Carrier concentration
Hall mobility
(Ω cm)
(cm− 3)
(cm2V− 1s− 1)
9.37 × 10 4 602 68.1 72.3 92.4 99.4 8.04 × 103 157 194
? 1.37 × 1016 5.73 × 1017 1.49 × 1017 8.37 × 1016 1.79 × 1017 ? 2.05 × 1017 9.82 × 1016
? 0.76 0.16 0.58 0.81 0.35 ? 0.19 0.33
Electrical properties of the annealed Zn1 − xMgxO:Li films at 500 °C were reexamined a month later.
Room temperature Hall-effect measurements were carried out in the van der Pauw configuration on a BIO-RAD HL5500PC system. X-ray diffraction (XRD) patterns were measured via a Philips X'Pert system with Cu Kα radiation (λ = 0.1542nm). The morphology of the films was examined using HITACHI S-4800 field emission scanning electronic microscopy (FESEM). The optical absorption spectra through the films were measured using a Lambda20 spectrometer. Moreover, the chemical states and
Fig. 1. XRD patterns of Zn1 − xMgxO:Li films (x = 0.04, 0.16) and ZnO film. (a) Diffraction angle of the (0002) peak depending on Mg content. Inset shows the magnification pattern of (0002) peaks. (b) XRD patterns of as-grown Zn0.96Mg0.04O:Li film and it annealed at 500 °C.
content of elements in the film were analyzed by XPS using an Omicron EAC2000-125 hemispherical analyzer. 3. Results and discussions Table 1 summarizes the electrical properties of the as-grown 0.1at.% Li-doped Zn1 − xMgxO (x = 0.04, 0.16) films and the films post-annealed at various temperatures. The as-grown Zn0.96Mg0.04O:Li films have high resistivity and ambiguous signal of carrier type. It is seen that when the anneal temperature is relatively low, the films also possess high resistivity and the carrier type is ambiguous. As the annealing temperature increases, p-type ZnMgO films are obtained and the electrical properties are
Fig. 2. XPS spectra of Mg 1s for Zn0.96Mg0.04O:Li films: (a) As-deposited. (b) Post-annealed at 500 °C.
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L. Chen et al. / Materials Letters 62 (2008) 2554–2556
to be 6at.% while Mg 1s peak energy slightly shifts, which means that postanneal has no obvious effect on the chemical state and content of Mg in the films. On the other hand, the signal of Li is near the noise level due to the detection limit of XPS, in accordance with our previous work [11]. Furthermore, room-temperature absorption spectra of the three annealed films (pure ZnO, Zn0.96Mg0.04O:Li and Zn0.84Mg0.16O:Li) were measured, as shown in Fig. 3. Absorption edges of the spectra shift to higher energy as Mg content increases. Moreover, the difference in absorption spectra between as-grown and post-annealed films is tiny (not shown in the graph), in accordance with the XPS results, revealing that post-anneal has no obvious effect on the content and chemical states of Mg element. The optical band gap (Eg) of pure ZnO and Zn1 − xMgxO:Li films (x = 0.04, 0.16) can be determined to be 3.18, 3.31 and 3.61 eV, respectively. It is seen that the optical band gap of the films blue shift as Mg content increases, which means that Mg can be a good candidate for band gap engineering in p-type ZnO film. Fig. 3. The absorption spectra of the pure ZnO, Zn1 − xMgxO:Li (x = 0.04, 0.16) films.
4. Conclusions improved. The optimal result is achieved at the annealing temperature of 500°C, with a resistivity of 72.3Ω cm, a carrier concentration of 1.49 × 1017cm−3, and a Hall mobility of 0.58cm2V−1s−1. The realization of p-type films indicates the formation or activation of Li acceptor after anneal. As the annealing temperature further increases, the electrical properties of the p-type Zn0.96Mg0.04O:Li thin films degrade, which may result from the re-evaporation of Li in the films [11]. On the other hand, we achieved p-type Zn0.84Mg0.16O:Li thin films via postanneal at 500°C, with a resistivity of 157Ω cm, a carrier concentration of 2.05 × 1017cm−3, and a Hall mobility of 0.19cm2V−1s−1. With Mg content increasing, the electrical properties of the films degrade, which may be due to the deeper Li acceptor level in heavily Mg alloyed films, in well accordance with our previous work [13]. Furthermore, it is found that after a month, all films post-annealed at 500°C keep p-type conduction without obvious degradation of electrical properties, demonstrating the stability of the p-type behavior in Li-doped ZnMgO films. The XRD patterns of as-deposited Zn1 − xMgxO:Li films (x = 0.04, 0.16) and pure ZnO film are shown in Fig. 1(a). Only two peaks indexed to (0002) and (0004) of ZnO are observed even when the Mg content increases to 16at. %, indicating that the Zn1 − xMgxO:Li films possess a single wurtzite phase with preferential c-axis orientation. The magnified patterns of (0002) peak are shown in the inset of Fig. 1(a). As Mg content increases, the (0002) peak shifts from 34.38° to 34.43°, then to 34.58°, which is due to the substitution of larger Zn2+ ions ´ ) by smaller Mg2+ ions (0.57Å ´ ). (0.60Å Fig. 1(b) shows the XRD patterns of the as-grown Zn0.96Mg0.04O:Li thin film and that film annealed at 500°C. Compared to the as-grown thin film, the (0002) peak of the annealed film shifts to higher angle, from 34.43° to 34.51°, which means decreasing of the lattice parameter. The lattice contraction could be attributed to defects evolution, since doping with electrically active atoms into semiconductor could result in change of lattice constant due to size effect [14]. In addition, the XRD intensity of postannealed film is greatly enhanced, comparing with the as-deposited film. Considering the better p-type conduction, decreasing lattice parameter, enhanced XRD intensity, upgrading crystal morphology observed in SEM (not shown here) of post-annealed sample, we suggest that the as-grown thin film may have more “hole killer” associated with Li, such as LiZn–Lii complex or Lii predicted by Wardle et al. [15], which would result in lattice expansion and compensate p-type doping [12,16]. The Li acceptor may be activated after post-anneal by eliminating of “hole killer”. Fig. 2 shows the XPS spectra of Mg 1s peak of the as-grown and annealed Zn0.96Mg0.04O:Li thin film. The content of Mg are both calculated
In summary, we have realized p-type Zn1 − xMgxO:Li thin films by using dc reactive magnetron sputtering, with post-anneal in pure O2 ambient. The optimal result is achieved at the annealing temperature of 500 °C for the Zn0.96Mg0.04O:Li film. The optical band gap can be controlled by tuning Mg content in the films. Acknowledgements The authors thank Professor J.X. Wu for the assistance in the XPS measurements. This work was supported by the National Natural Science Foundation of China under grant no 2006CB 604906 and the National Natural Science Foundation of China under contract nos. 50532060 and 90601003. References [1] Y.R. Ryu, J.A. Lubguban, T.S. Lee, H.W. White, T.S. Jeong, C.J. Youn, et al., Appl. Phys. Lett. 90 (2007) 131115-1-3. [2] S. Choonpun, R.D. Vispute, W. Yang, R.P. Sharma, T. Venkatesan, Appl. Phys. Lett. 80 (2002) 1529–1531. [3] A. Ohtomo, M. Kawasaki, T. Koida, K. Masubuchi, H. Koinuma, Appl. Phys. Lett. 72 (1998) 2466–2468. [4] D.C. Look, D.C. Reynolds, C.W. Litton, R.L. Jones, D.B. Eason, G. Cantwell, Appl. Phys. Lett. 81 (2002) 1830–1832. [5] T.M. Barnes, K. Olson, C.A. Wolden, Appl. Phys. Lett. 86 (2005) 112112-1-3. [6] G.D. Yuan, Z.Z. Ye, L.P. Zhu, Q. Qian, B.H. Zhao, et al., Appl. Phys. Lett. 86 (2005) 202106-1-3. [7] Y.W. Heo, S.J. Park, K. Ip, S.J. Pearton, D.P. Noton, Appl. Phys. Lett. 83 (2003) 1128–1130. [8] Y.R. Ryu, T.S. Lee, J.H. Leem, H.W. White, Appl. Phys. Lett. 83 (2003) 4032–4034. [9] F.X. Xiu, Z. Yang, L.J. Mandalapu, D.T. Zhao, J.L. Liu, Appl. Phys. Lett. 87 (2005) 252102-1-3. [10] C.H. Park, S.B. Zhang, S.-H. Wei, Phys. Rev., B 66 (2002) 073202-1-3. [11] Y.J. Zeng, Z.Z. Ye, W.Z. Xu, D.Y. Li, J.G. Lu, L.P. Zhu, et al., Appl. Phys. Lett. 88 (2006) 062107-1-3. [12] Y.J. Zeng, Z.Z. Ye, J.G. Lu, W.Z. Xu, L.P. Zhu, B.H. Zhao, Appl. Phys. Lett. 89 (2006) 042106-1-3. [13] M.X. Qiu, Z.Z. Ye, H.P. He, Y.Z. Zhang, X.Q. Gu, L.P. Zhu, et al., Appl. Phys. Lett. 90 (2007) 182116-1-3. [14] W. Li, M. Pessa, Phys. Rev., B 57 (1998) 14327–14329. [15] M.G. Wardle, J.P. Goss, P.R. Briddon, Phys. Rev., B 71 (2005) 155205-1-10. [16] S.B. Zhang, S.H. Wei, A. Zunger, Phys. Rev., B 63 (2001) 075205-1-7.