- Email: [email protected]
Annealing effects on properties of Ag–N dual-doped ZnO films
Applied Surface Science 258 (2012) 10064–10067
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Annealing effects on properties of Ag–N dual-doped ZnO films Li Duan ∗ , Wenxue Zhang, Xiaochen Yu, Ziqiang Jiang, Lijun Luan, Yongnan Chen, Donglin Li School of Materials Science and Engineering, Chang’an University, Xi’an 710064, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 13 April 2012 Received in revised form 18 June 2012 Accepted 19 June 2012 Available online 27 June 2012 Keywords: ZnO Dual-doping Post-annealing Sol–gel method
a b s t r a c t Ag–N dual-doped p-type ZnO (ZnO:(Ag,N)) thin films have been prepared using the sol–gel method. The modifications of the structural, electrical and optical properties of ZnO:(Ag,N) films after annealing in various atmosphere in the temperature range of 300–600 ◦ C are discussed. Results show the oxygen-rich environment is benefit to the p-type samples. Transition from n-type to p-type conduction occurred at the annealing temperature of 400 ◦ C. The optimum annealing temperature is about 500 ◦ C. © 2012 Elsevier B.V. All rights reserved.
1. Introduction ZnO is a novel semiconductor material and has many potential applications such as light emitting diodes, laser diodes, photodetectors, and thin film transistors [1–5]. To realize the device applications, high quality p-type ZnO is necessary. For fabricating p-type ZnO, three kinds of doping methods were investigated. They were mono-doping, co-doping with acceptor and donor elements, and dual-doping with group-V and I acceptor elements [6–9]. In recent years, dual-doping attracted attention due to the advantages of stability and reproducibility [9–12]. Most of the dualdoped ZnO samples were doped by Li and N. N is a good group-V acceptor element for ZnO. However, the radius of Li+ is smaller than that of Zn2+ . Li interstitial donors may be easily formed in ZnO and depress the properties of ZnO films [13,14]. Ag is a group-Ib acceptor element for ZnO. It is noteworthy the formation energy for AgZn is lower than that for Agi , which can reduce the formation of interstitial donors and alleviate the self-compensation issue [15–17]. It indicates Ag–N dual-doping is a promising method for preparing p-type ZnO. However, there have been few reports on ZnO:(Ag,N) films [18]. Post-annealing process is important for modifying the properties of ZnO films. However, the effect of ambient gas and annealing temperature on the properties of ZnO:(Ag,N) is not well understood. In this paper, p-type ZnO:(Ag,N) films were grown on glass substrates using the sol–gel method. The influence of
∗ Corresponding author. E-mail address: [email protected] (L. Duan). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.06.075
post-annealing conditions on the structural, electrical and optical properties of ZnO:Ag films was investigated in detail. 2. Experimental details ZnO:(Ag,N) thin films were prepared via a sol–gel dip-coating method. The aqueous sol was prepared by zinc acetate 2-hydrate and polyvinyl alcohol. The concentration of zinc acetate 2-hydrate was 0.5 mol/L. Silver nitrate and ammonium acetate were used as source materials for silver and nitrogen dopants, respectively. The atomic ratio of Zn/Ag/N in the sol was adjusted to 1:0.01:1. The solution was stirred at room temperature for 1 h and spin-coated onto glass substrates at 2000 rpm for 20 s. The ZnO:(Ag,N) films were then dried at 120 ◦ C for 10 min. The aforementioned procedure was repeated 10 times. After deposition, the samples were annealed under various conditions for 1 h. The structure of the samples was investigated using Xray Diffractometer (XRD) and field emission scanning electron microscopy (FE-SEM). Hall measurements were taken by four-point probe in Van der Pauw configuration to determine carrier concentration and mobility. Photoluminescence (PL) spectra of the films were performed by the excitation from 325 nm He-Cd laser at room temperature. 3. Results and discussion Fig. 1 shows the XRD patterns of ZnO:(Ag,N) and pure ZnO films. Both of the films were annealed in O2 at 500 ◦ C for 1 h. A strong peak corresponding to ZnO (0 0 2) is observed in each film. The fullwidth at half maximum (FWHM) of the (0 0 2) peak for pure ZnO and ZnO:(Ag,N) are 0.23◦ and 0.34◦ , respectively. It indicates the
L. Duan et al. / Applied Surface Science 258 (2012) 10064–10067
10065
Fig. 1. XRD patterns of (a) ZnO:(Ag,N) and (b) pure ZnO films.
Fig. 3. The mobility and hole concentration of ZnO:(Ag,N) films depending on oxygen partial pressures in the annealing atmospheres.
film crystallinity is slightly degenerated by doped impurities. The (0 0 2) peak of pure ZnO film is at 34.40◦ . However, the (0 0 2) peak of ZnO:(Ag,N) film shows an obvious shift to a lower angle of 34.20◦ . The angle shift indicates an increase of the lattice constant. Similar phenomena were observed in mono-doped ZnO:Ag or ZnO:N films [19–21]. Considering the ion radius of O2− is similar with that of N3− , we believe the angle shift is mainly caused by the substitution of Zn2+ ions (radius of 0.056 nm) by Ag+ ions (radius of 0.102 nm). For investigating the influence of annealing gas on properties of ZnO:(Ag,N) films, three samples were annealed at 500 ◦ C in pure N2 , the mixed gas of 50% N2 and 50% O2 , and pure O2 respectively. The total pressure in each annealing process was 1 kPa. Fig. 2 shows the FWHM and position of (0 0 2) peak of ZnO:(Ag,N) films depending on oxygen partial pressures in the annealing atmospheres. The ZnO:(Ag,N) film annealed in pure O2 has the smallest FHWM value, indicating the best crystallinity and the largest grain size. The peak position of (0 0 2) decreases with increasing of oxygen partial pressure. It indicates the oxygen-rich environment in annealing process benefits the formation of AgZn acceptors impurities. It is in accordance with the results of first-principles calculations [15,16]. Fig. 3 shows the mobility and hole concentration of ZnO:(Ag,N) films depending on oxygen partial pressures in the annealing atmospheres. The film annealed in pure N2 does not show any conductivity. The hole concentration of film annealed in pure O2 is higher than that of the film annealed in the mixed gas. It is attributed to two reasons. Firstly, the formation energy of AgZn is lower at the O-rich conditions [15,16]. Secondly, the formation enthalpies of intrinsic donor defects in ZnO are higher at the Orich conditions [22]. The film annealed in pure O2 has larger grain
size, as shown in Fig. 2. As the grain size increases, grain boundary scattering is reduced, and the mobility is improved. Thus, the oxygen-rich environment is benefit to the p-type conductivity of films. Annealing temperature can also modify the properties of samples. Here four ZnO:(Ag,N) films were annealed in pure O2 in the temperature range of 300–600 ◦ C. Fig. 4 shows the FWHM and position of (0 0 2) peak of ZnO:(Ag,N) films depending on annealing temperatures. With increasing of annealing temperature, the FWHM of (0 0 2) peak decreases, indicating the grain size was enlarged. It can be attributed to the interface merging processes induced by the thermal annealing [23]. Furthermore, there are dangling bonds related to the zinc of oxygen defects at the grain boundaries. These defects are favorable to the merging process to form larger grains while increasing the annealing temperature [24]. The ZnO (0 0 2) peak position first decreases and then increases with increasing annealing temperature. It indicates the concentration of AgZn acceptor impurities is influenced by the annealing temperature. When the annealing temperature increased from 300 to 500 ◦ C, more AgZn acceptors impurities were formed, so the peak position of ZnO (0 0 2) decreased. It is known the formation energy of Agi donor is higher than that of AgZn acceptor [15]. The Agi can be hardly formed at a lower temperature. However, Ag+ ions may be out-diffused from Zn+ site to the interstitial site at a higher temperature. Thus, the sample annealed at 600 ◦ C has a higher (0 0 2) peak position. Similar phenomena were also observed in ZnO:Ag films [25,26]. Fig. 5 shows the electrical properties of ZnO:(Ag,N) films annealed in O2 at various temperatures. The ZnO:(Ag,N) film
Fig. 2. The FWHM and position of (0 0 2) peak of ZnO:(Ag,N) films depending on oxygen partial pressures in the annealing atmospheres.
Fig. 4. The FWHM and position of (0 0 2) peak of ZnO:(Ag,N) films depending on annealing temperatures.
10066
L. Duan et al. / Applied Surface Science 258 (2012) 10064–10067
Fig. 6. SEM images of (a) as-grown ZnO:(Ag,N) and (b) ZnO:(Ag,N) film annealed in O2 at 500 ◦ C.
Fig. 5. Electrical properties of ZnO:(Ag,N) films annealed at various temperatures: (a) the carrier concentration (ne or nh ) and (b) the mobility.
annealed at 300 ◦ C exhibits n-type conductivity, while the other three ZnO:(Ag,N) films annealed in the temperature range of 400–600 ◦ C, exhibit p-type conductivity. It may be caused by the ‘self-compensation’ effect that the acceptors in ZnO are compensated by the native donor defects such as Zni , Vo and ZnO [27]. Since the formation enthalpies of Zni , Vo and ZnO in the O-rich environment are higher than that in the Zn-rich environment [22], annealing in O2 at higher temperature can reduce the concentration of native donor defects. So the effective hole concentration in ZnO:(Ag,N) film was increased. On the other hand, more AgZn acceptors were formed after the annealing temperature increased to 400 ◦ C. Thus, the samples were inverted from n-type to p-type. However, the hole concentration decreased after the ZnO:(Ag,N) film was annealed at 600 ◦ C. It is due to the reduced concentration of the AgZn acceptor and the compensation effect by the Agi donors, which is in accordance with the XRD results. Fig. 5(b) shows the ZnO:(Ag,N) thin film annealed at higher temperature has higher carrier mobility, implying better crystallinity. Fig. 6 shows the FE-SEM images of as-grown ZnO:(Ag,N) and ZnO:(Ag,N) film annealed in O2 at 500 ◦ C. Both of the films have uniform thickness and smooth surface. The grain sizes of as-grown and annealed ZnO:(Ag,N) films are approximately 15 and 30 nm, respectively. It is obvious the grain size was increased after annealing due to the interface merging processes. Fig. 7 shows room temperature PL spectra of the ZnO:(Ag,N) films annealed in various ambient gases. Two emission bands are apparently observed. One is the UV emission at about 380 nm ascribed to the exciton transition. Another is the green emission at about 520 nm ascribed to the intrinsic deep-level defects in ZnO, such as oxygen vacancy or zinc interstitial, which is under
debate [28–30]. The ZnO:(Ag,N) film annealed in pure O2 has a high UV peak, indicating its potential for short wavelength lightemitting applications. The intensity of green emission decreases with increasing of oxygen partial pressure. It indicates the sample annealed in pure O2 is well close to stoichiometry. When the ZnO:(Ag,N) films were annealed in O2 , the variation in defects can be expressed as follows: VO +
1 O2 = OO 2
(1)
Zni +
1 O2 = ZnZn + OO 2
(2)
Eqs. (1) and (2) indicate that the concentrations of VO and Zni were reduced after oxygen annealing process. Considering VO and Zni are donor defects in ZnO film, the oxygen annealing process benefits the p-type conductivity of ZnO:(Ag,N) films. It is in accordance with the Hall results.
Fig. 7. PL spectra of ZnO:(Ag,N) films annealed in (a) O2 , (b) the mixed gas of 50% N2 and 50% O2 , and (c) N2 .
L. Duan et al. / Applied Surface Science 258 (2012) 10064–10067
4. Conclusions The p-type ZnO:(Ag,N) films were fabricated by a Ag–N dualdoping method. The influence of post-annealing conditions on the structural, electrical, and optical properties of ZnO:(Ag,N) films was investigated. Annealing in oxygen is benefit to the properties of ZnO:(Ag,N) films. Transition from n-type to p-type conduction at annealing temperature above 400 ◦ C has been observed. The optimum temperature is about 500 ◦ C. Results show the Ag–N dualdoping is a potential method for fabricating p-type ZnO films. Acknowledgements This work was supported by the National Natural Science Foundation of China under Grant No. 51102023 and the Special Fund for Basic Scientific Research of Central Colleges (CHD2011ZD012, CHD2012ZD003). References [1] 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. Kawasak, Nature Materials 4 (2005) 42. [2] T. Aoki, Y. Hatanaka, D.C. Look, Applied Physics Letters 76 (2000) 3257. [3] D.-H. Lee, K.-H. Park, S. Kim, S.Y. Lee, Thin Solid Films 520 (2011) 1160. [4] F. Sun, C.X. Shana, S.P. Wang, B.H. Li, Z.Z. Zhang, C.L. Yang, D.Z. Shen, Materials Chemistry and Physics 129 (2011) 27. [5] M.A. Zimmler, T. Voss, C. Ronning, F. Capasso, Applied Physics Letters 94 (2009) 241120. [6] V. Avrutin, D.J. Silversmith, H. Morkoc¸, Proceedings of the IEEE 98 (2010) 1269. [7] U. Ozgur, D. Hofstetter, H. Morkoc¸, Proceedings of the IEEE 98 (2010) 1255.
10067
[8] T. Yamamoto, H.K. Yoshida, Journal of Crystal Growth 214 (2000) 552. [9] A. Krtschil, A. Dadgar, N. Oleynik, J. Bläsing, A. Diez, A. Krost, Applied Physics Letters 87 (2005) 262105. [10] Y.C. Cheng, Y.S. Kuo, Y.H. Li, J.J. Shyue, M.J. Chen, Thin Solid Films 519 (2011) 5558. [11] J.G. Lu, Y.Z. Zhang, Z.Z. Ye, L.P. Zhu, L. Wang, B.H. Zhao, Q.L. Liang, Applied Physics Letters 88 (2006) 222114. [12] X.H. Wang, B. Yao, Z.P. Wei, D.Z. Sheng, Z.Z. Zhang, B.H. Li, Y.M. Lu, D.X. Zhao, J.Y. Zhang, X.W. Fan, L.X. Guan, C.X. Cong, Journal of Physics D: Applied Physics 39 (2006) 4568. [13] C.H. Park, S.B. Zhang, S. Wei, Physical Review B 66 (2002) 073202. [14] D.C. Look, R.L. Jones, J.R. Sizelove, N.Y. Garces, N.C. Giles, L.E. Halliburton, Physica Status Solidi A 195 (2003) 171. [15] Y. Yan, M.M. Al-Jassim, S. Wei, Applied Physics Letters 89 (2006) 181912. [16] Q. Wan, Z. Xiong, J. Dai, J. Rao, F. Jiang, Optical Materials 30 (2008) 817. [17] J. Hu, B.C. Pan, Journal of Chemical Physics 129 (2008) 154706. [18] C.Y. Zuo, J. Wen, Y.L. Bai, Chinese Physics B 19 (2010) 047101. [19] H.S. Kang, B.D. Ahn, J.H. Kim, G.H. Kim, S.H. Lim, H.W. Chang, S.Y. Lee, Applied Physics Letters 88 (2006) 202108. [20] R. Deng, Y. Zou, H. Tang, Physica B 403 (2008) 2004. [21] L. Tang, B. Wang, Y. Zhang, Journal of Alloys and Compounds 509 (2011) 384. [22] S.B. Zhang, S.-H. Wei, A. Zunger, Physical Review B 63 (2001) 075205. [23] E. C¸etinörgü, S. Goldsmith, R.L. Boxman, Surface and Coatings Technology 201 (2007) 7266. [24] S.-Y. Kuo, W.-C. Chen, F.-I. Lai, C.-P. Cheng, H.-C. Kuo, S.-C. Wang, W.-F. Hsieh, Journal of Crystal Growth 287 (2006) 78. [25] B.D. Ahn, H.S. Kang, J.H. Kim, G.H. Kim, H.W. Chang, S.Y. Lee, Journal of Applied Physics 100 (2006) 093701. [26] L. Duan, B.X. Lin, W.Y. Zhang, S. Zhong, Z.X. Fu, Applied Physics Letters 88 (2006) 232110. [27] G. Mandel, Physical Review A 134 (1964) 1073. [28] B. Lin, Z. Fu, Y. Jia, Applied Physics Letters 79 (2001) 943. [29] K. Vanheusdena, C.H. Seagera, W.L. Warrena, D.R. Tallanta, J. Carusob, M.J. Hampden-Smith, T.T. Kodas, Applied Physics Letters 75 (1997) 11. [30] Q. Zhao, X.Y. Xu, X.F. Song, X.Z. Zhang, D.P. Yua, Applied Physics Letters 88 (2006) 033102.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Annealing effects on properties of Ag–N dual-doped ZnO films Li Duan ∗ , Wenxue Zhang, Xiaochen Yu, Ziqiang Jiang, Lijun Luan, Yongnan Chen, Donglin Li School of Materials Science and Engineering, Chang’an University, Xi’an 710064, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 13 April 2012 Received in revised form 18 June 2012 Accepted 19 June 2012 Available online 27 June 2012 Keywords: ZnO Dual-doping Post-annealing Sol–gel method
a b s t r a c t Ag–N dual-doped p-type ZnO (ZnO:(Ag,N)) thin films have been prepared using the sol–gel method. The modifications of the structural, electrical and optical properties of ZnO:(Ag,N) films after annealing in various atmosphere in the temperature range of 300–600 ◦ C are discussed. Results show the oxygen-rich environment is benefit to the p-type samples. Transition from n-type to p-type conduction occurred at the annealing temperature of 400 ◦ C. The optimum annealing temperature is about 500 ◦ C. © 2012 Elsevier B.V. All rights reserved.
1. Introduction ZnO is a novel semiconductor material and has many potential applications such as light emitting diodes, laser diodes, photodetectors, and thin film transistors [1–5]. To realize the device applications, high quality p-type ZnO is necessary. For fabricating p-type ZnO, three kinds of doping methods were investigated. They were mono-doping, co-doping with acceptor and donor elements, and dual-doping with group-V and I acceptor elements [6–9]. In recent years, dual-doping attracted attention due to the advantages of stability and reproducibility [9–12]. Most of the dualdoped ZnO samples were doped by Li and N. N is a good group-V acceptor element for ZnO. However, the radius of Li+ is smaller than that of Zn2+ . Li interstitial donors may be easily formed in ZnO and depress the properties of ZnO films [13,14]. Ag is a group-Ib acceptor element for ZnO. It is noteworthy the formation energy for AgZn is lower than that for Agi , which can reduce the formation of interstitial donors and alleviate the self-compensation issue [15–17]. It indicates Ag–N dual-doping is a promising method for preparing p-type ZnO. However, there have been few reports on ZnO:(Ag,N) films [18]. Post-annealing process is important for modifying the properties of ZnO films. However, the effect of ambient gas and annealing temperature on the properties of ZnO:(Ag,N) is not well understood. In this paper, p-type ZnO:(Ag,N) films were grown on glass substrates using the sol–gel method. The influence of
∗ Corresponding author. E-mail address: [email protected] (L. Duan). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.06.075
post-annealing conditions on the structural, electrical and optical properties of ZnO:Ag films was investigated in detail. 2. Experimental details ZnO:(Ag,N) thin films were prepared via a sol–gel dip-coating method. The aqueous sol was prepared by zinc acetate 2-hydrate and polyvinyl alcohol. The concentration of zinc acetate 2-hydrate was 0.5 mol/L. Silver nitrate and ammonium acetate were used as source materials for silver and nitrogen dopants, respectively. The atomic ratio of Zn/Ag/N in the sol was adjusted to 1:0.01:1. The solution was stirred at room temperature for 1 h and spin-coated onto glass substrates at 2000 rpm for 20 s. The ZnO:(Ag,N) films were then dried at 120 ◦ C for 10 min. The aforementioned procedure was repeated 10 times. After deposition, the samples were annealed under various conditions for 1 h. The structure of the samples was investigated using Xray Diffractometer (XRD) and field emission scanning electron microscopy (FE-SEM). Hall measurements were taken by four-point probe in Van der Pauw configuration to determine carrier concentration and mobility. Photoluminescence (PL) spectra of the films were performed by the excitation from 325 nm He-Cd laser at room temperature. 3. Results and discussion Fig. 1 shows the XRD patterns of ZnO:(Ag,N) and pure ZnO films. Both of the films were annealed in O2 at 500 ◦ C for 1 h. A strong peak corresponding to ZnO (0 0 2) is observed in each film. The fullwidth at half maximum (FWHM) of the (0 0 2) peak for pure ZnO and ZnO:(Ag,N) are 0.23◦ and 0.34◦ , respectively. It indicates the
L. Duan et al. / Applied Surface Science 258 (2012) 10064–10067
10065
Fig. 1. XRD patterns of (a) ZnO:(Ag,N) and (b) pure ZnO films.
Fig. 3. The mobility and hole concentration of ZnO:(Ag,N) films depending on oxygen partial pressures in the annealing atmospheres.
film crystallinity is slightly degenerated by doped impurities. The (0 0 2) peak of pure ZnO film is at 34.40◦ . However, the (0 0 2) peak of ZnO:(Ag,N) film shows an obvious shift to a lower angle of 34.20◦ . The angle shift indicates an increase of the lattice constant. Similar phenomena were observed in mono-doped ZnO:Ag or ZnO:N films [19–21]. Considering the ion radius of O2− is similar with that of N3− , we believe the angle shift is mainly caused by the substitution of Zn2+ ions (radius of 0.056 nm) by Ag+ ions (radius of 0.102 nm). For investigating the influence of annealing gas on properties of ZnO:(Ag,N) films, three samples were annealed at 500 ◦ C in pure N2 , the mixed gas of 50% N2 and 50% O2 , and pure O2 respectively. The total pressure in each annealing process was 1 kPa. Fig. 2 shows the FWHM and position of (0 0 2) peak of ZnO:(Ag,N) films depending on oxygen partial pressures in the annealing atmospheres. The ZnO:(Ag,N) film annealed in pure O2 has the smallest FHWM value, indicating the best crystallinity and the largest grain size. The peak position of (0 0 2) decreases with increasing of oxygen partial pressure. It indicates the oxygen-rich environment in annealing process benefits the formation of AgZn acceptors impurities. It is in accordance with the results of first-principles calculations [15,16]. Fig. 3 shows the mobility and hole concentration of ZnO:(Ag,N) films depending on oxygen partial pressures in the annealing atmospheres. The film annealed in pure N2 does not show any conductivity. The hole concentration of film annealed in pure O2 is higher than that of the film annealed in the mixed gas. It is attributed to two reasons. Firstly, the formation energy of AgZn is lower at the O-rich conditions [15,16]. Secondly, the formation enthalpies of intrinsic donor defects in ZnO are higher at the Orich conditions [22]. The film annealed in pure O2 has larger grain
size, as shown in Fig. 2. As the grain size increases, grain boundary scattering is reduced, and the mobility is improved. Thus, the oxygen-rich environment is benefit to the p-type conductivity of films. Annealing temperature can also modify the properties of samples. Here four ZnO:(Ag,N) films were annealed in pure O2 in the temperature range of 300–600 ◦ C. Fig. 4 shows the FWHM and position of (0 0 2) peak of ZnO:(Ag,N) films depending on annealing temperatures. With increasing of annealing temperature, the FWHM of (0 0 2) peak decreases, indicating the grain size was enlarged. It can be attributed to the interface merging processes induced by the thermal annealing [23]. Furthermore, there are dangling bonds related to the zinc of oxygen defects at the grain boundaries. These defects are favorable to the merging process to form larger grains while increasing the annealing temperature [24]. The ZnO (0 0 2) peak position first decreases and then increases with increasing annealing temperature. It indicates the concentration of AgZn acceptor impurities is influenced by the annealing temperature. When the annealing temperature increased from 300 to 500 ◦ C, more AgZn acceptors impurities were formed, so the peak position of ZnO (0 0 2) decreased. It is known the formation energy of Agi donor is higher than that of AgZn acceptor [15]. The Agi can be hardly formed at a lower temperature. However, Ag+ ions may be out-diffused from Zn+ site to the interstitial site at a higher temperature. Thus, the sample annealed at 600 ◦ C has a higher (0 0 2) peak position. Similar phenomena were also observed in ZnO:Ag films [25,26]. Fig. 5 shows the electrical properties of ZnO:(Ag,N) films annealed in O2 at various temperatures. The ZnO:(Ag,N) film
Fig. 2. The FWHM and position of (0 0 2) peak of ZnO:(Ag,N) films depending on oxygen partial pressures in the annealing atmospheres.
Fig. 4. The FWHM and position of (0 0 2) peak of ZnO:(Ag,N) films depending on annealing temperatures.
10066
L. Duan et al. / Applied Surface Science 258 (2012) 10064–10067
Fig. 6. SEM images of (a) as-grown ZnO:(Ag,N) and (b) ZnO:(Ag,N) film annealed in O2 at 500 ◦ C.
Fig. 5. Electrical properties of ZnO:(Ag,N) films annealed at various temperatures: (a) the carrier concentration (ne or nh ) and (b) the mobility.
annealed at 300 ◦ C exhibits n-type conductivity, while the other three ZnO:(Ag,N) films annealed in the temperature range of 400–600 ◦ C, exhibit p-type conductivity. It may be caused by the ‘self-compensation’ effect that the acceptors in ZnO are compensated by the native donor defects such as Zni , Vo and ZnO [27]. Since the formation enthalpies of Zni , Vo and ZnO in the O-rich environment are higher than that in the Zn-rich environment [22], annealing in O2 at higher temperature can reduce the concentration of native donor defects. So the effective hole concentration in ZnO:(Ag,N) film was increased. On the other hand, more AgZn acceptors were formed after the annealing temperature increased to 400 ◦ C. Thus, the samples were inverted from n-type to p-type. However, the hole concentration decreased after the ZnO:(Ag,N) film was annealed at 600 ◦ C. It is due to the reduced concentration of the AgZn acceptor and the compensation effect by the Agi donors, which is in accordance with the XRD results. Fig. 5(b) shows the ZnO:(Ag,N) thin film annealed at higher temperature has higher carrier mobility, implying better crystallinity. Fig. 6 shows the FE-SEM images of as-grown ZnO:(Ag,N) and ZnO:(Ag,N) film annealed in O2 at 500 ◦ C. Both of the films have uniform thickness and smooth surface. The grain sizes of as-grown and annealed ZnO:(Ag,N) films are approximately 15 and 30 nm, respectively. It is obvious the grain size was increased after annealing due to the interface merging processes. Fig. 7 shows room temperature PL spectra of the ZnO:(Ag,N) films annealed in various ambient gases. Two emission bands are apparently observed. One is the UV emission at about 380 nm ascribed to the exciton transition. Another is the green emission at about 520 nm ascribed to the intrinsic deep-level defects in ZnO, such as oxygen vacancy or zinc interstitial, which is under
debate [28–30]. The ZnO:(Ag,N) film annealed in pure O2 has a high UV peak, indicating its potential for short wavelength lightemitting applications. The intensity of green emission decreases with increasing of oxygen partial pressure. It indicates the sample annealed in pure O2 is well close to stoichiometry. When the ZnO:(Ag,N) films were annealed in O2 , the variation in defects can be expressed as follows: VO +
1 O2 = OO 2
(1)
Zni +
1 O2 = ZnZn + OO 2
(2)
Eqs. (1) and (2) indicate that the concentrations of VO and Zni were reduced after oxygen annealing process. Considering VO and Zni are donor defects in ZnO film, the oxygen annealing process benefits the p-type conductivity of ZnO:(Ag,N) films. It is in accordance with the Hall results.
Fig. 7. PL spectra of ZnO:(Ag,N) films annealed in (a) O2 , (b) the mixed gas of 50% N2 and 50% O2 , and (c) N2 .
L. Duan et al. / Applied Surface Science 258 (2012) 10064–10067
4. Conclusions The p-type ZnO:(Ag,N) films were fabricated by a Ag–N dualdoping method. The influence of post-annealing conditions on the structural, electrical, and optical properties of ZnO:(Ag,N) films was investigated. Annealing in oxygen is benefit to the properties of ZnO:(Ag,N) films. Transition from n-type to p-type conduction at annealing temperature above 400 ◦ C has been observed. The optimum temperature is about 500 ◦ C. Results show the Ag–N dualdoping is a potential method for fabricating p-type ZnO films. Acknowledgements This work was supported by the National Natural Science Foundation of China under Grant No. 51102023 and the Special Fund for Basic Scientific Research of Central Colleges (CHD2011ZD012, CHD2012ZD003). References [1] 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. Kawasak, Nature Materials 4 (2005) 42. [2] T. Aoki, Y. Hatanaka, D.C. Look, Applied Physics Letters 76 (2000) 3257. [3] D.-H. Lee, K.-H. Park, S. Kim, S.Y. Lee, Thin Solid Films 520 (2011) 1160. [4] F. Sun, C.X. Shana, S.P. Wang, B.H. Li, Z.Z. Zhang, C.L. Yang, D.Z. Shen, Materials Chemistry and Physics 129 (2011) 27. [5] M.A. Zimmler, T. Voss, C. Ronning, F. Capasso, Applied Physics Letters 94 (2009) 241120. [6] V. Avrutin, D.J. Silversmith, H. Morkoc¸, Proceedings of the IEEE 98 (2010) 1269. [7] U. Ozgur, D. Hofstetter, H. Morkoc¸, Proceedings of the IEEE 98 (2010) 1255.
10067
[8] T. Yamamoto, H.K. Yoshida, Journal of Crystal Growth 214 (2000) 552. [9] A. Krtschil, A. Dadgar, N. Oleynik, J. Bläsing, A. Diez, A. Krost, Applied Physics Letters 87 (2005) 262105. [10] Y.C. Cheng, Y.S. Kuo, Y.H. Li, J.J. Shyue, M.J. Chen, Thin Solid Films 519 (2011) 5558. [11] J.G. Lu, Y.Z. Zhang, Z.Z. Ye, L.P. Zhu, L. Wang, B.H. Zhao, Q.L. Liang, Applied Physics Letters 88 (2006) 222114. [12] X.H. Wang, B. Yao, Z.P. Wei, D.Z. Sheng, Z.Z. Zhang, B.H. Li, Y.M. Lu, D.X. Zhao, J.Y. Zhang, X.W. Fan, L.X. Guan, C.X. Cong, Journal of Physics D: Applied Physics 39 (2006) 4568. [13] C.H. Park, S.B. Zhang, S. Wei, Physical Review B 66 (2002) 073202. [14] D.C. Look, R.L. Jones, J.R. Sizelove, N.Y. Garces, N.C. Giles, L.E. Halliburton, Physica Status Solidi A 195 (2003) 171. [15] Y. Yan, M.M. Al-Jassim, S. Wei, Applied Physics Letters 89 (2006) 181912. [16] Q. Wan, Z. Xiong, J. Dai, J. Rao, F. Jiang, Optical Materials 30 (2008) 817. [17] J. Hu, B.C. Pan, Journal of Chemical Physics 129 (2008) 154706. [18] C.Y. Zuo, J. Wen, Y.L. Bai, Chinese Physics B 19 (2010) 047101. [19] H.S. Kang, B.D. Ahn, J.H. Kim, G.H. Kim, S.H. Lim, H.W. Chang, S.Y. Lee, Applied Physics Letters 88 (2006) 202108. [20] R. Deng, Y. Zou, H. Tang, Physica B 403 (2008) 2004. [21] L. Tang, B. Wang, Y. Zhang, Journal of Alloys and Compounds 509 (2011) 384. [22] S.B. Zhang, S.-H. Wei, A. Zunger, Physical Review B 63 (2001) 075205. [23] E. C¸etinörgü, S. Goldsmith, R.L. Boxman, Surface and Coatings Technology 201 (2007) 7266. [24] S.-Y. Kuo, W.-C. Chen, F.-I. Lai, C.-P. Cheng, H.-C. Kuo, S.-C. Wang, W.-F. Hsieh, Journal of Crystal Growth 287 (2006) 78. [25] B.D. Ahn, H.S. Kang, J.H. Kim, G.H. Kim, H.W. Chang, S.Y. Lee, Journal of Applied Physics 100 (2006) 093701. [26] L. Duan, B.X. Lin, W.Y. Zhang, S. Zhong, Z.X. Fu, Applied Physics Letters 88 (2006) 232110. [27] G. Mandel, Physical Review A 134 (1964) 1073. [28] B. Lin, Z. Fu, Y. Jia, Applied Physics Letters 79 (2001) 943. [29] K. Vanheusdena, C.H. Seagera, W.L. Warrena, D.R. Tallanta, J. Carusob, M.J. Hampden-Smith, T.T. Kodas, Applied Physics Letters 75 (1997) 11. [30] Q. Zhao, X.Y. Xu, X.F. Song, X.Z. Zhang, D.P. Yua, Applied Physics Letters 88 (2006) 033102.