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Defects enhanced ferromagnetism in Cu-doped ZnO thin films
Solid State Communications 152 (2012) 257–260
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Defects enhanced ferromagnetism in Cu-doped ZnO thin films Shi-Yi Zhuo a,b , Xue-Chao Liu a,∗ , Ze Xiong a,b , Jian-Hua Yang a , Er-Wei Shi a a
Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China
b
Graduate School of the Chinese Academy of Sciences, Beijing 100049, China
article
info
Article history: Received 18 November 2011 Accepted 26 November 2011 by A.H. MacDonald Available online 3 December 2011 Keywords: A. Thin films C. Point defects D. Ferromagnetism
abstract We present here new evidences of point defects enhanced ferromagnetism in Cu-doped ZnO thin films by different characterization methods. Cu-doped ZnO thin films with Cu concentrations ranging from 0.05 to 5 at.% were prepared by an inductively coupled plasma enhanced physical vapor deposition system. Room-temperature ferromagnetism is observed in all the films. The saturation magnetization shows an increasing trend with the increase of Cu concentration except a slight decrease for the 1 at.% Cu-doping. Further study performed by Raman spectra, X-ray absorption spectra and extended X-ray absorption fine structure indicate the existence of Cu2+ ions and point defects in all the films. The local structural characterization and magnetic properties reveal that the sample with larger saturation magnetization has a higher concentration of point defects. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Non-magnetic element doped ZnO-based diluted magnetic semiconductors (DMSs) such as ZnO:Al [1], ZnO:C [2] and ZnO:Pt [3] have attracted extensive attention in recent years for their potential to overcome the problems encountered by magnetic transition metal dopants (Mn [4], Fe [5] and Co [6], Ni [7], etc.), in which possible precipitates or clusters will make the origin of ferromagnetism indistinct. Cu is a typical non-magnetic transition metal dopant, because metallic Cu and Cu-related oxides are non-ferromagnetic materials. If room-temperature ferromagnetism (RTFM) is observed in Cu-doped ZnO, then it is the intrinsic property of this system. For this reason, Cu-doped ZnO is considered as an ideal candidate to study the mechanism of ferromagnetism in ZnO-based DMSs. Chien et al. [8] studied the ferromagnetism in Cu-doped ZnO by ab initio electronic structure calculations based on generalized gradient approximation (GGA) and GGA +U. Herng et al. [9] prepared Cu-doped films by filtered cathodic vacuum arc technique and found that the ferromagnetism was associated with the substitution of Zn2+ by Cu2+ . Recently, researchers have reported that the ferromagnetism may be mediated by point defects in Cu-doped ZnO, but it is still a highly controversial issue. Li et al. [10] annealed Cu-doped ZnO thin films by using two series of sequential annealing processes and found that the zinc interstitial (Zni ) and/or oxygen vacancy (VO ) defects would activate the ferromagnetism. Ye et al. [11] calculated the ferromagnetic properties of Cu-doped ZnO by using
∗
Corresponding author. Tel.: +86 21 69987663; fax: +86 21 69987661. E-mail address: [email protected] (X.-C. Liu).
0038-1098/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2011.11.038
the full-potential linearized augmented plane-wave method by introducing VO defects in the system and found that the VO defects tended to destroy the ferromagnetic exchange. Hence, it is quite necessary to clarify whether the point defects are essential to mediate the ferromagnetic exchange in Cu-doped ZnO system. X-ray absorption fine structure (XAFS) [12] and X-ray absorption spectroscopy (XAS) are powerful methods to analyze the local structure, and can give the information of chemical bonding or coordination structure environment around specific atoms. In addition, point defects such as Zni and VO are easily formed in ZnO thin films for their low formation energies than that of other defects such as zinc vacancies and oxygen interstitials. In this paper, a series of Cu-doped ZnO thin films were prepared, and the relationship between point defects (VO ) and ferromagnetism was studied. 2. Experimental details Cu-doped ZnO thin films with nominal composition of 0.5 at.%, 1 at.%, 3 at.% and 5 at.% (which is marked as S1, S2, S3 and S4 in the following context and figures, respectively) were grown on Si(100) substrates by an inductively coupled plasma enhanced physical vapor deposition (ICP-PVD) system. The sputtering targets were prepared by the standard solid-state reaction method. The base pressure of sputtering chamber was <1.0 × 10−3 Pa and the depositing pressure was fixed at 1.0 Pa with the sputtering power of 150 W. The targets were presputtered for 10 min to remove the surface contamination before deposition. The thicknesses of above samples were found to be about 600 nm, which were measured by a cross-sectional scanning electron microscope. The electrical properties were characterized by a Hall effect measurement system (Lakeshore, 7704 A) using
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Fig. 1. The magnetization as a function of applied field curves of S1–S4. The inset shows the expanded view around zero field.
Fig. 2. Raman spectra of S1–S4.
a four terminal van der Pauw configuration. The magnetization measurements were performed by a superconducting quantum interference device (SQUID) magnetometer (Quantum Design, MPMS XL-7) at room temperature. The magnetic field was applied parallel to the film surface. The Raman spectra were recorded on a confocal Raman spectroscope using a 325 nm excitation laser (Renishaw Invia) at room temperature. The O K -edge XAS were measured at the National Synchrotron Radiation Laboratory (NSRL, U19 station) using the total electron yield (TEY) mode at a vacuum environment of 1.5 × 10−9 mbar. The Zn K -edge and Cu K -edge XAFS were measured at Shanghai Synchrotron Radiation Facility (SSRF, BL14W1 station) using the fluorescence mode. 3. Results and discussion The magnetization as a function of applied field (M–H) curves of S1, –S4 are shown in Fig. 1. The diamagnetic signals from the Si substrate have been subtracted. The magnetization approaches a saturation value when the applied field is increased to ∼5000 Oe. Clear hysteresis loops are observed, which indicates the obvious RTFM behavior in all the samples. With the increase of Cu dopants, the saturation magnetization increases from 0.07 to 0.14 emu/cm3 except a slight decrease in S2. An expanded view around zero field is shown in the inset of Fig. 1. It can be seen that the coercive field (Hc ) of all the samples is about 100 Oe and slightly decreases with the increasing of Cu dopants. The Raman spectra of S1–S4 are shown in Fig. 2. We can clearly see four Raman peaks at 433, 574, 1153, and 1729 cm−1 . They are corresponding to the E2 H mode and the first-order to thirdorder A1 (LO) modes of ZnO [13]. Besides the E2 H and A1 (LO), no other phonon mode is observed. This indicates that Cu ions are dissolved into the ZnO matrix without forming secondary phase or clusters. Owing to the resonance Raman scattering effect (the energy of excitation laser is 3.8 eV, which is close to the band gap of ZnO), the intensity of E2 H is very low in contrast with A1 (LO). With the increase in Cu dopants, the intensity of E2 H mode shows a decreasing trend, while the 1-A1 (LO) mode shows a broaden trend. These Raman results present a structural disorder in Cu-doped ZnO thin films, which may be caused by Cu doping [14] and/or the Vo defects [15]. The chemical bonding structure around oxygen was measured by XAS in TEY mode at room temperature. The photoelectrons excited by X-ray will transfer from O 1s core level states to O 2p unoccupied states in the conduction band or its hybridization with different Zn and Cu orbitals, which gives the local structural characteristics around oxygen. Fig. 3 shows the normalized O
Fig. 3. The O K -edge XAS spectra of Cu-doped ZnO thin films and reference samples of ZnO, Cu2 O and CuO. The curves are divided into lower energy region (LER) and higher energy region (HER).
K -edge XAS spectra of S1–S4 and the reference samples of ZnO, Cu2 O and CuO. According to the photon energy, the spectral features are assigned to two main regions: the lower energy region (LER) and the higher energy region (HER). The LER ranges from 525 to 537 eV. Peaks in this region are mainly attributed to O 2p hybridization with highly dispersive Zn 3d4s/Cu 3d states which form the bottom of the conduction band, while the peak at ∼536 eV is attributed to nondispersive O 2p states. The HER ranges from 537 to 550 eV. Peaks in this region are attributed to O 2p hybridized with Zn 4p/Cu 4sp states, and above 550 eV the spectra arise due to the O 2p states that extend to Zn/Cu higher orbital. It can be seen that the features of S1–S4 are similar to ZnO, but quite different from Cu, Cu2 O and CuO. This indicates that no Cu-related clusters exist in Cu-doped ZnO thin films. It should be mentioned that there is an obvious shoulder peak around 533 eV as marked by an arrow in Fig. 3. The description of this shoulder peak is still controversial. Thakur et al. [16,17] attributed it to oxygen defects, while Pan et al. [18] attributed it to Zni or VO . It is interesting to notice that a slight broadening of this shoulder peak is observed in S1 and S4, which indicates a higher concentration of point defects in these samples. A shift to higher energy is also observed after Cu doping, which further proves the dispersive character in our samples [19]. The X-ray absorption fine structure can provide the local coordination information around specific element, and is very sensitive to the existence of clusters even in nanoscale. Fig. 4(a) illustrates the Zn K -edge and Cu K -edge XAFS of S3 and the
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b
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and Zn ions, but also with the defects in Cu doped ZnO thin films. Oxygen vacancies usually cause a decrease in lattice constants, which are consistent with XRD data reported in Refs. [20,21]. The mechanism of ferromagnetism in Cu-doped ZnO thin films is discussed. Since undoped ZnO [22], metallic Cu and its oxides are non-ferromagnetic materials, the observed room-temperature ferromagnetism should be the intrinsic property of Cu-doped ZnO thin films. Carrier-mediated double exchange theory is often used to explain the ferromagnetism in ZnO-based DMSs. However, the resistivity and the carrier density of all the samples are >1.0 × 101 cm and <1.0 × 1016 cm−3 , respectively. Therefore, the Cu-doped ZnO samples are typical semi-insulating materials and the ferromagnetic exchange cannot be mediated by free carriers. On the other hand, bound magnetic polarons (BMPs) [23] model is usually applied to explain the ferromagnetism in the defectrich insulating oxides. For Cu ions, the outer shell electronic configuration of Cu2+ is 3d9 with one unpaired electron available for ferromagnetic exchange. For the point defects (VO ) in doped ZnO thin films, carriers will be loosely bound to them and occupy an extended orbital forming the BMPs. The interactions between Cu2+ ions through BMPs make the Cu2+ ions parallel with each other and lead to effective ferromagnetic coupling among Cu2+ magnetic moments. As discussed above, the ferromagnetism of Cu-doped ZnO thin films show an increasing trend with the increase of Cu concentration. It is interesting that there is a slight decrease in S2. The XAFS results indicate that the chemical valence state of Cu ions is +2. Raman and XAS results also imply that there are a great number of point defects in Cu doped ZnO thin films. More importantly, the number of point defects in S1 is larger than that in S2, which shows a similar phenomenon when S4 is compared to S3. It can be concluded that both Cu2+ ions and point defects are essential for the ferromagnetic exchange in Cu-doped ZnO thin films, and the ferromagnetism can be enhanced by point defects. 4. Conclusions
Fig. 4. (a) XAFS and (b) RDF spectra at Zn K -edge and Cu K -edge of S3 ZnO:Cu thin film and reference samples of ZnO, Cu, Cu2 O and CuO. The Zn K and Cu K absorption edges at 9656.5 and 8976.9 eV are set as zero energy.
reference samples of ZnO, Cu, Cu2 O and CuO in the range −150–500 eV (the Zn K and Cu K absorption edges at 9656.5 and 8976.9 eV are set as zero energy). For the Zn K -edge absorption spectra, the features in S3 are quite similar to ZnO, which indicates that S3 has a typical hexagonal wurtzite structure. Concerning the Cu K -edge absorption spectra, the features in S3 are different from reference materials, which reveals that the coordination environment of Cu ions in S3 is not consistent with that of Cu metal and oxides such as Cu2 O and CuO. It also indicates that there are no Cu-related clusters in S3, and Cu ions are substituted into the Zn sites. The XAFS spectra were further analyzed using IFEFFIT software, and the radial distribution functions (RDF) were obtained by Fourier transformation as shown in Fig. 4(b). For the Zn RDF, the first and second main peaks at about 1.6 Å and 2.9 Å are identified as Zn–O and Zn–Zn bonding, respectively. For the Cu RDF, the first main peak at about 1.5 Å is identified as Cu–O bonding. The other RDF features at larger radius are related to further neighbor bonding of Cu or Zn ions, which will not be discussed here. The bond length of Cu–O is slightly smaller than that of Zn–O, which may be caused by the difference between the radius of Zn and Cu ions (the radius of Cu1+ , Cu2+ and Zn2+ are 0.096 nm, 0.072 nm and 0.074 nm, respectively). Additionally, the first neighbor bonding distance of Zn or Cu are slightly smaller than that of theoretical values (Zn–O ∼ 1.9 Å, Zn–Zn ∼ 3.2 Å). This lattice distortion may be associated not only with the radius difference between Cu ions
Cu-doped ZnO thin films have been prepared by an ICP-PVD system with different concentrations ranging from 0.5 to 5 at.% in Ar atmosphere. The magnetic measurements show that all the films exhibit room-temperature ferromagnetism. The saturation magnetization increases with the increase of Cu dopants. Raman scattering and O K -edge XAS results indicate lots of defects (VO ) and a strong hybridization between O-2p and Cu-3d in the Cu-doped ZnO thin films. The Zn K -edge and Cu K -edge XAFS results show that Cu ions are dissolved into ZnO lattice with a chemical valence state of +2. The Hall effect measurements imply that all the films show high resistivity and low electron concentration. BMPs theory is applied to explain the ferromagnetism observed in Cu-doped ZnO thin films. The ferromagnetic exchanges are induced by Cu2+ ions and bound magnetic polarons, and enhanced by point defects. Acknowledgments This work was supported by the Young Scientists Fund of the National Natural Science Foundation of China (Grant no. 51002176), the Innovation Programs of the Chinese Academy of Sciences (Grant no. KJCX2-EW-W10) and the Innovation Programs of the Shanghai Institute of Ceramics. References [1] S.J. Chen, K. Suzuki, J.S. Garitaonandia, Appl. Phys. Lett. 95 (2009) 172507. [2] S. Akbar, S.K. Hasanain, M. Abbas, S. Ozcan, B. Ali, S.I. Shah, Solid State Commun. 151 (2011) 17–20. [3] Y.W. Ma, J.B. Yi, J. Ding, L.H. Van, H.T. Zhang, C.M. Ng, Appl. Phys. Lett. 93 (2008) 042514. [4] X.C. Liu, H.W. Zhang, T. Zhang, B.Y. Chen, Z.Z. Chen, L.X. Song, E.W. Shi, Chin. Phys. B 17 (2008) 1371–1376.
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[5] S.Q. Zhou, K. Potzger, G. Talut, H. Reuther, J. von Borany, R. Grotzschel, W. Skorupa, M. Helm, J. Fassbender, N. Volbers, M. Lorenz, T. Herrmannsdorfer, J. Appl. Phys. 103 (2008) 023902. [6] J.H. Yang, Y. Cheng, Y. Liu, X. Ding, Y.X. Wang, Y.J. Zhang, H.L. Liu, Solid State Commun. 149 (2009) 1164–1167. [7] K. Ando, H. Saito, Z.W. Jin, T. Fukumura, M. Kawasaki, Y. Matsumoto, H. Koinuma, J. Appl. Phys. 89 (2001) 7284–7286. [8] C.H. Chien, S.H. Chiou, G.Y. Guo, Y.D. Yao, J. Magn. Magn. Mater. 282 (2004) 275–278. [9] T.S. Herng, S.P. Lau, S.F. Yu, H.Y. Yang, X.H. Ji, J.S. Chen, N. Yasui, H. Inaba, J. Appl. Phys. 99 (2006) 086101. [10] X.L. Li, X.H. Xu, Z.Y. Quan, J.F. Guo, H.S. Wu, G.A. Gehring, J. Appl. Phys. 105 (2009) 103914. [11] L.H. Ye, A.J. Freeman, B. Delley, Phys. Rev. B 73 (2006) 033203. [12] Q. Ma, D.B. Buchholz, R.P.H. Chang, Phys. Rev. B 78 (2008) 214429. [13] R. Cusco, E. Alarcon-Llado, J. Ibanez, L. Artus, J. Jimenez, B.G. Wang, M.J. Callahan, Phys. Rev. B 75 (2007) 165202.
[14] K. Samanta, P. Bhattacharya, R.S. Katiyar, J. Appl. Phys. 105 (2009) 113929. [15] S.F. Zhao, Q. Lu, C.H. Xu, Y.W. Mu, J.G. Wan, F.Q. Song, G.H. Wang, Lasers & Electro-Optics & the Pacific Rim Conference on Lasers and Electro-Optics Vols 1 and 2, 2009, pp. 790–791. [16] P. Thakur, K.H. Chae, J.Y. Kim, M. Subramanian, R. Jayavel, K. Asokan, Appl. Phys. Lett. 91 (2007) 162503. [17] P. Thakur, V. Bisogni, J.C. Cezar, N.B. Brookes, G. Ghiringhelli, S. Gautam, K.H. Chae, J. Appl. Phys. 107 (2010) 103915. [18] F. Pan, C. Song, X.J. Liu, Y.C. Yang, F. Zeng, Mat. Sci. Eng. R 62 (2008) 1–35. [19] T. Tietze, M. Gacic, G. Schutz, G. Jakob, S. Bruck, E. Goering, New J. Phys. 10 (2008) 055009. [20] C.W. Zou, H.J. Wang, M.L. Yin, M. Li, C.S. Liu, L.P. Guo, D.J. Fu, T.W. Kang, J. Cryst. Growth 312 (2010) 906–909. [21] T. Nagase, T. Ooie, J. Sakakibara, Thin Solid Films 357 (1999) 151–158. [22] A. Sundaresan, R. Bhargavi, N. Rangarajan, U. Siddesh, C.N.R. Rao, Phys. Rev. B 74 (2006) 161306. [23] J.M.D. Coey, M. Venkatesan, C.B. Fitzgerald, Nat. Mater. 4 (2005) 173–179.
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Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Defects enhanced ferromagnetism in Cu-doped ZnO thin films Shi-Yi Zhuo a,b , Xue-Chao Liu a,∗ , Ze Xiong a,b , Jian-Hua Yang a , Er-Wei Shi a a
Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China
b
Graduate School of the Chinese Academy of Sciences, Beijing 100049, China
article
info
Article history: Received 18 November 2011 Accepted 26 November 2011 by A.H. MacDonald Available online 3 December 2011 Keywords: A. Thin films C. Point defects D. Ferromagnetism
abstract We present here new evidences of point defects enhanced ferromagnetism in Cu-doped ZnO thin films by different characterization methods. Cu-doped ZnO thin films with Cu concentrations ranging from 0.05 to 5 at.% were prepared by an inductively coupled plasma enhanced physical vapor deposition system. Room-temperature ferromagnetism is observed in all the films. The saturation magnetization shows an increasing trend with the increase of Cu concentration except a slight decrease for the 1 at.% Cu-doping. Further study performed by Raman spectra, X-ray absorption spectra and extended X-ray absorption fine structure indicate the existence of Cu2+ ions and point defects in all the films. The local structural characterization and magnetic properties reveal that the sample with larger saturation magnetization has a higher concentration of point defects. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Non-magnetic element doped ZnO-based diluted magnetic semiconductors (DMSs) such as ZnO:Al [1], ZnO:C [2] and ZnO:Pt [3] have attracted extensive attention in recent years for their potential to overcome the problems encountered by magnetic transition metal dopants (Mn [4], Fe [5] and Co [6], Ni [7], etc.), in which possible precipitates or clusters will make the origin of ferromagnetism indistinct. Cu is a typical non-magnetic transition metal dopant, because metallic Cu and Cu-related oxides are non-ferromagnetic materials. If room-temperature ferromagnetism (RTFM) is observed in Cu-doped ZnO, then it is the intrinsic property of this system. For this reason, Cu-doped ZnO is considered as an ideal candidate to study the mechanism of ferromagnetism in ZnO-based DMSs. Chien et al. [8] studied the ferromagnetism in Cu-doped ZnO by ab initio electronic structure calculations based on generalized gradient approximation (GGA) and GGA +U. Herng et al. [9] prepared Cu-doped films by filtered cathodic vacuum arc technique and found that the ferromagnetism was associated with the substitution of Zn2+ by Cu2+ . Recently, researchers have reported that the ferromagnetism may be mediated by point defects in Cu-doped ZnO, but it is still a highly controversial issue. Li et al. [10] annealed Cu-doped ZnO thin films by using two series of sequential annealing processes and found that the zinc interstitial (Zni ) and/or oxygen vacancy (VO ) defects would activate the ferromagnetism. Ye et al. [11] calculated the ferromagnetic properties of Cu-doped ZnO by using
∗
Corresponding author. Tel.: +86 21 69987663; fax: +86 21 69987661. E-mail address: [email protected] (X.-C. Liu).
0038-1098/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2011.11.038
the full-potential linearized augmented plane-wave method by introducing VO defects in the system and found that the VO defects tended to destroy the ferromagnetic exchange. Hence, it is quite necessary to clarify whether the point defects are essential to mediate the ferromagnetic exchange in Cu-doped ZnO system. X-ray absorption fine structure (XAFS) [12] and X-ray absorption spectroscopy (XAS) are powerful methods to analyze the local structure, and can give the information of chemical bonding or coordination structure environment around specific atoms. In addition, point defects such as Zni and VO are easily formed in ZnO thin films for their low formation energies than that of other defects such as zinc vacancies and oxygen interstitials. In this paper, a series of Cu-doped ZnO thin films were prepared, and the relationship between point defects (VO ) and ferromagnetism was studied. 2. Experimental details Cu-doped ZnO thin films with nominal composition of 0.5 at.%, 1 at.%, 3 at.% and 5 at.% (which is marked as S1, S2, S3 and S4 in the following context and figures, respectively) were grown on Si(100) substrates by an inductively coupled plasma enhanced physical vapor deposition (ICP-PVD) system. The sputtering targets were prepared by the standard solid-state reaction method. The base pressure of sputtering chamber was <1.0 × 10−3 Pa and the depositing pressure was fixed at 1.0 Pa with the sputtering power of 150 W. The targets were presputtered for 10 min to remove the surface contamination before deposition. The thicknesses of above samples were found to be about 600 nm, which were measured by a cross-sectional scanning electron microscope. The electrical properties were characterized by a Hall effect measurement system (Lakeshore, 7704 A) using
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Fig. 1. The magnetization as a function of applied field curves of S1–S4. The inset shows the expanded view around zero field.
Fig. 2. Raman spectra of S1–S4.
a four terminal van der Pauw configuration. The magnetization measurements were performed by a superconducting quantum interference device (SQUID) magnetometer (Quantum Design, MPMS XL-7) at room temperature. The magnetic field was applied parallel to the film surface. The Raman spectra were recorded on a confocal Raman spectroscope using a 325 nm excitation laser (Renishaw Invia) at room temperature. The O K -edge XAS were measured at the National Synchrotron Radiation Laboratory (NSRL, U19 station) using the total electron yield (TEY) mode at a vacuum environment of 1.5 × 10−9 mbar. The Zn K -edge and Cu K -edge XAFS were measured at Shanghai Synchrotron Radiation Facility (SSRF, BL14W1 station) using the fluorescence mode. 3. Results and discussion The magnetization as a function of applied field (M–H) curves of S1, –S4 are shown in Fig. 1. The diamagnetic signals from the Si substrate have been subtracted. The magnetization approaches a saturation value when the applied field is increased to ∼5000 Oe. Clear hysteresis loops are observed, which indicates the obvious RTFM behavior in all the samples. With the increase of Cu dopants, the saturation magnetization increases from 0.07 to 0.14 emu/cm3 except a slight decrease in S2. An expanded view around zero field is shown in the inset of Fig. 1. It can be seen that the coercive field (Hc ) of all the samples is about 100 Oe and slightly decreases with the increasing of Cu dopants. The Raman spectra of S1–S4 are shown in Fig. 2. We can clearly see four Raman peaks at 433, 574, 1153, and 1729 cm−1 . They are corresponding to the E2 H mode and the first-order to thirdorder A1 (LO) modes of ZnO [13]. Besides the E2 H and A1 (LO), no other phonon mode is observed. This indicates that Cu ions are dissolved into the ZnO matrix without forming secondary phase or clusters. Owing to the resonance Raman scattering effect (the energy of excitation laser is 3.8 eV, which is close to the band gap of ZnO), the intensity of E2 H is very low in contrast with A1 (LO). With the increase in Cu dopants, the intensity of E2 H mode shows a decreasing trend, while the 1-A1 (LO) mode shows a broaden trend. These Raman results present a structural disorder in Cu-doped ZnO thin films, which may be caused by Cu doping [14] and/or the Vo defects [15]. The chemical bonding structure around oxygen was measured by XAS in TEY mode at room temperature. The photoelectrons excited by X-ray will transfer from O 1s core level states to O 2p unoccupied states in the conduction band or its hybridization with different Zn and Cu orbitals, which gives the local structural characteristics around oxygen. Fig. 3 shows the normalized O
Fig. 3. The O K -edge XAS spectra of Cu-doped ZnO thin films and reference samples of ZnO, Cu2 O and CuO. The curves are divided into lower energy region (LER) and higher energy region (HER).
K -edge XAS spectra of S1–S4 and the reference samples of ZnO, Cu2 O and CuO. According to the photon energy, the spectral features are assigned to two main regions: the lower energy region (LER) and the higher energy region (HER). The LER ranges from 525 to 537 eV. Peaks in this region are mainly attributed to O 2p hybridization with highly dispersive Zn 3d4s/Cu 3d states which form the bottom of the conduction band, while the peak at ∼536 eV is attributed to nondispersive O 2p states. The HER ranges from 537 to 550 eV. Peaks in this region are attributed to O 2p hybridized with Zn 4p/Cu 4sp states, and above 550 eV the spectra arise due to the O 2p states that extend to Zn/Cu higher orbital. It can be seen that the features of S1–S4 are similar to ZnO, but quite different from Cu, Cu2 O and CuO. This indicates that no Cu-related clusters exist in Cu-doped ZnO thin films. It should be mentioned that there is an obvious shoulder peak around 533 eV as marked by an arrow in Fig. 3. The description of this shoulder peak is still controversial. Thakur et al. [16,17] attributed it to oxygen defects, while Pan et al. [18] attributed it to Zni or VO . It is interesting to notice that a slight broadening of this shoulder peak is observed in S1 and S4, which indicates a higher concentration of point defects in these samples. A shift to higher energy is also observed after Cu doping, which further proves the dispersive character in our samples [19]. The X-ray absorption fine structure can provide the local coordination information around specific element, and is very sensitive to the existence of clusters even in nanoscale. Fig. 4(a) illustrates the Zn K -edge and Cu K -edge XAFS of S3 and the
S.-Y. Zhuo et al. / Solid State Communications 152 (2012) 257–260
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b
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and Zn ions, but also with the defects in Cu doped ZnO thin films. Oxygen vacancies usually cause a decrease in lattice constants, which are consistent with XRD data reported in Refs. [20,21]. The mechanism of ferromagnetism in Cu-doped ZnO thin films is discussed. Since undoped ZnO [22], metallic Cu and its oxides are non-ferromagnetic materials, the observed room-temperature ferromagnetism should be the intrinsic property of Cu-doped ZnO thin films. Carrier-mediated double exchange theory is often used to explain the ferromagnetism in ZnO-based DMSs. However, the resistivity and the carrier density of all the samples are >1.0 × 101 cm and <1.0 × 1016 cm−3 , respectively. Therefore, the Cu-doped ZnO samples are typical semi-insulating materials and the ferromagnetic exchange cannot be mediated by free carriers. On the other hand, bound magnetic polarons (BMPs) [23] model is usually applied to explain the ferromagnetism in the defectrich insulating oxides. For Cu ions, the outer shell electronic configuration of Cu2+ is 3d9 with one unpaired electron available for ferromagnetic exchange. For the point defects (VO ) in doped ZnO thin films, carriers will be loosely bound to them and occupy an extended orbital forming the BMPs. The interactions between Cu2+ ions through BMPs make the Cu2+ ions parallel with each other and lead to effective ferromagnetic coupling among Cu2+ magnetic moments. As discussed above, the ferromagnetism of Cu-doped ZnO thin films show an increasing trend with the increase of Cu concentration. It is interesting that there is a slight decrease in S2. The XAFS results indicate that the chemical valence state of Cu ions is +2. Raman and XAS results also imply that there are a great number of point defects in Cu doped ZnO thin films. More importantly, the number of point defects in S1 is larger than that in S2, which shows a similar phenomenon when S4 is compared to S3. It can be concluded that both Cu2+ ions and point defects are essential for the ferromagnetic exchange in Cu-doped ZnO thin films, and the ferromagnetism can be enhanced by point defects. 4. Conclusions
Fig. 4. (a) XAFS and (b) RDF spectra at Zn K -edge and Cu K -edge of S3 ZnO:Cu thin film and reference samples of ZnO, Cu, Cu2 O and CuO. The Zn K and Cu K absorption edges at 9656.5 and 8976.9 eV are set as zero energy.
reference samples of ZnO, Cu, Cu2 O and CuO in the range −150–500 eV (the Zn K and Cu K absorption edges at 9656.5 and 8976.9 eV are set as zero energy). For the Zn K -edge absorption spectra, the features in S3 are quite similar to ZnO, which indicates that S3 has a typical hexagonal wurtzite structure. Concerning the Cu K -edge absorption spectra, the features in S3 are different from reference materials, which reveals that the coordination environment of Cu ions in S3 is not consistent with that of Cu metal and oxides such as Cu2 O and CuO. It also indicates that there are no Cu-related clusters in S3, and Cu ions are substituted into the Zn sites. The XAFS spectra were further analyzed using IFEFFIT software, and the radial distribution functions (RDF) were obtained by Fourier transformation as shown in Fig. 4(b). For the Zn RDF, the first and second main peaks at about 1.6 Å and 2.9 Å are identified as Zn–O and Zn–Zn bonding, respectively. For the Cu RDF, the first main peak at about 1.5 Å is identified as Cu–O bonding. The other RDF features at larger radius are related to further neighbor bonding of Cu or Zn ions, which will not be discussed here. The bond length of Cu–O is slightly smaller than that of Zn–O, which may be caused by the difference between the radius of Zn and Cu ions (the radius of Cu1+ , Cu2+ and Zn2+ are 0.096 nm, 0.072 nm and 0.074 nm, respectively). Additionally, the first neighbor bonding distance of Zn or Cu are slightly smaller than that of theoretical values (Zn–O ∼ 1.9 Å, Zn–Zn ∼ 3.2 Å). This lattice distortion may be associated not only with the radius difference between Cu ions
Cu-doped ZnO thin films have been prepared by an ICP-PVD system with different concentrations ranging from 0.5 to 5 at.% in Ar atmosphere. The magnetic measurements show that all the films exhibit room-temperature ferromagnetism. The saturation magnetization increases with the increase of Cu dopants. Raman scattering and O K -edge XAS results indicate lots of defects (VO ) and a strong hybridization between O-2p and Cu-3d in the Cu-doped ZnO thin films. The Zn K -edge and Cu K -edge XAFS results show that Cu ions are dissolved into ZnO lattice with a chemical valence state of +2. The Hall effect measurements imply that all the films show high resistivity and low electron concentration. BMPs theory is applied to explain the ferromagnetism observed in Cu-doped ZnO thin films. The ferromagnetic exchanges are induced by Cu2+ ions and bound magnetic polarons, and enhanced by point defects. Acknowledgments This work was supported by the Young Scientists Fund of the National Natural Science Foundation of China (Grant no. 51002176), the Innovation Programs of the Chinese Academy of Sciences (Grant no. KJCX2-EW-W10) and the Innovation Programs of the Shanghai Institute of Ceramics. References [1] S.J. Chen, K. Suzuki, J.S. Garitaonandia, Appl. Phys. Lett. 95 (2009) 172507. [2] S. Akbar, S.K. Hasanain, M. Abbas, S. Ozcan, B. Ali, S.I. Shah, Solid State Commun. 151 (2011) 17–20. [3] Y.W. Ma, J.B. Yi, J. Ding, L.H. Van, H.T. Zhang, C.M. Ng, Appl. Phys. Lett. 93 (2008) 042514. [4] X.C. Liu, H.W. Zhang, T. Zhang, B.Y. Chen, Z.Z. Chen, L.X. Song, E.W. Shi, Chin. Phys. B 17 (2008) 1371–1376.
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