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Structure and magnetism of Zn0.9Co0.1O DMS films prepared by chemical solution deposition method
Applied Surface Science 258 (2011) 64–67
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Structure and magnetism of Zn0.9 Co0.1 O DMS films prepared by chemical solution deposition method Jinghai Yang a,b , Yan Cheng a,b,∗ , Yongjun Zhang a,b , Yaxin Wang a,b , Yang Liu a,b a b
Institute of Condensed State Physics, Jilin Normal University, Siping 136000, PR China Key Laboratory of Functional Materials Physics and Chemistry, Jilin Normal University, Ministry of Education, Siping 136000, PR China
a r t i c l e
i n f o
Article history: Received 10 June 2011 Received in revised form 22 July 2011 Accepted 1 August 2011 Available online 6 August 2011 Keywords: Oxygen Vacancy XANES EFXAS Ferromagnetism
a b s t r a c t The Zn0.9 Co0.1 O films are fabricated by chemical solution deposition method. All the films have the ZnO wurtzite structure with a preferential orientation along the c-axis. The analysis of X-ray near-edge absorption spectroscopy and X-ray photoelectron spectroscopy indicates that the valence of Co is +2, and there are oxygen vacancies in Zn0.9 Co0.1 O films annealed in Ar atmosphere. Extended X-ray absorption fine structure results reveal that Co2+ ions have dissolved into ZnO and substituted for Zn2+ ions. Magnetization measurements show that the film annealed in Ar exhibits ferromagnetism which can be explained by the formation of bound magnetic polarons. © 2011 Elsevier B.V. All rights reserved.
1. Introduction ZnO-based semiconductors are of great interest these years due to their application in the field of spintronics. Recently, theoretical works have predicted that ZnO-based semiconductors could present ferromagnetic behavior at room temperature when doped with transition metals such as Mn [1] and Co [2]. Lately, some experimental results have been reported that room temperature ferromagnetism exists in transition metals doped ZnO diluted magnetic semiconductors (DMS), such as Cu-doped ZnO [3,4], Fe-doped ZnO [5,6], Mn-doped ZnO [7,8], and Co-doped ZnO [9–11]. Many methods are developed to prepare Co-doped ZnO films, including pulsed laser deposition (PLD) [9,10], magnetron sputtering [11,12] and ion-beam sputtering (IBS) [13]. Compared with these methods, chemical solution deposition (CSD) method has its own advantages on fabricating DMS films, because one can fabricate samples with various compositions and increase the solubility easily at low cost. Ueda [9] has reported Co-doped ZnO films with Curie temperature higher than room temperature. In addition, the similar radius with Zn atom makes Co behave similar to Zn, resulting in a high solubility of Co into ZnO. As a result, Co-doped ZnO DMS are another research focus recently. Whether Co ions substitute for Zn2+ or form clusters and secondary phases is an important factor of the origin of ferromagnetism. Coey [14] has proposed that the donor defects
∗ Corresponding author at: Beihang University, School of Physics and Nuclear Energy Engineering, XueYuan Road No.37, Beijing, PR China. E-mail address: [email protected] (Y. Cheng). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.08.006
play a key role in mediating ferromagnetic coupling in oxide based DMS. Generally, the most common donor defects are oxygen vacancies (VO ) and interstitial Zn (Zni ) in ZnO. However, it is difficult to measure VO and Zni directly. In this work, we fabricated Zn0.9 Co0.1 O (ZCO) films by CSD. In order to detect the secondary phases and analyze the type of donor defects, the elements analysis was carried out by X-ray photoelectron spectroscopy (XPS), and the structure of the films were analyzed by X-ray near-edge absorption spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS). 2. Experiment The ZCO films were deposited on Si(1 0 0) substrates by CSD method. The solution preparation was carried out in air with zinc acetate and cobalt acetate according to the desired stoichiometry. 2-methoxyethanol was selected as solvent, and diethanolamine was used as stabilizing agent. A spin-coating speed of 4000 rpm and 30 s were used in the progress of depositing. The deposited films were placed on the preheated hot-plate and dried at 200 ◦ C for 10 min. The above coating was repeated six times. Then the dried films were annealed at 600 ◦ C for 2 h under flowing argon and air atmosphere, respectively. The structural characterization of ZCO films were performed by X-ray diffraction (XRD) on D/max-2500 copper rotating-anode Xray diffractometer with Cu K␣ radiation (40 kV, 200 mA). Magnetic measurement was carried out by Lake Shore M-7407 Vibrating Sample Magnetometer (VSM). The valence state of Co element was analyzed by XPS (VG ESCALAB Mark II). The XANES and EXAFS spectra were performed on beam line U7C at synchrotron radiation
J. Yang et al. / Applied Surface Science 258 (2011) 64–67
65
Intensity(a.u.)
ZCO/Ar ZCO/Air
Zn2p Zn2p
Zn Zn Zn Zn
Co2p OO
Zn3p
Zn3s
C1s
Zn
Zn3d
0
200
400
600
800
1000
1200
Binding energy(eV)
Fig. 1. The XRD patterns for ZCO films and air and ZnO film. The inset is the magnified portion of ZnO (0 0 2) peaks.
Fig. 2. The XPS survey spectra of ZCO films.
laboratory in the University of Science and Technology of China in Hefei. 3. Results and discussion Fig. 1 shows the X-ray diffraction patterns for ZCO films of the wurtzite structure. All the films are textured with a c-axis preferred orientation as evidenced from strong (0 0 2) and weak (1 0 0), (1 0 1) and (1 0 3) peaks. The inset displays the magnified portion of (0 0 2) peak, which shows that the position of (0 0 2) peaks shifts to higher angles. It is because that the radius of Co2+ (0.074 nm) is a little smaller than Zn2+ (0.076 nm). The shift also reveals that there is no octahedral cobalt in our sample. The radius of octahedral cobalt is bigger that zinc, so the peaks of ZCO shift to lower angles, which is opposite to our result. As a result, Co2+ substitutes for Zn2+ without changing the wurtzite structure. Fig. 2 shows the XPS spectra of ZCO films. The binding energy is calibrated by taking carbon C 1s peak (284 eV) as reference. The Co 2p peak is observed besides the C 1s, O 1s, Zn 2p, which indicates that no impurity elements are observed in our sample. Fig. 3 shows the Co 2p XPS spectra of our films. Both spectra show four peaks, Co 2p3/2 and 2p1/2 and the shake-up resonance transitions of these two peaks at higher core level binding energies. The Co 2p3/2 core levels for Co–O bonding is 781.2 eV, showing that Co is divalent according to the handbook [15]. The difference between Co 2p3/2 and 2p1/2 is 15.3 eV. But the literature value of the binding energy of Co 2p3/2 in Co metal is 778.3 eV and the difference
between Co 2p3/2 and 2p1/2 core levels for metallic Co is 14.97 eV [16]. As a result, the formation of Co metal clusters in our sample could be excluded and the valance of Co is +2. Fig. 4 shows the O 1s XPS spectra of our films. For the film annealed in air, there are three binding energy peaks around 530.15, 531.25 and 532.15 eV [17]. The low binding energy peak and middle binding energy peak are attributed to O2− ions at the intrinsic sites and O2− ions in the oxygen-deficient regions, respectively. The high binding energy component located at 532.15 eV is usually attributed to the presence of loosely bound oxygen on the surface of ZnO film, belonging to a specific species, e.g., –CO3 , adsorbed H2 O or adsorbed O2 . However, when the films are annealed in Ar, there are only two peaks at 530.15 and 531.25 eV. It indicates that the majority of oxygen exists in oxygen deficiencies. We consider that the deficiencies are oxygen vacancies due to the annealing conditions. Fig. 5 is the XANES patterns of ZCO film annealed in Ar and the reference samples. There is a pronounce shoulder A1 on the rising absorption edge in Co reference, which is different from our sample. So we believe that there are no Co clusters in our sample. This result is the same to that of XPS. In addition, there is a pre-edge peak A1 in our samples which is assigned to the transition of Co 1s core electron to the hybridized orbital of Co 3d and O 2p states. For comparison, Zn K-edge XANES of ZnO reference spectra was shifted to the position of Co K-edge XANES spectra. The Co K-edge features of our sample of peak A, B, C and D and Zn K-edge features
Air
Ar
Co 2p3/2
Intensity(a.u.)
Intensity(a.u.)
Co 2p3/2 Co 2p1/2
775
780
785
790
795
800
805
Binding Energy(eV)
810
Co 2p1/2
775
780
785
790
795
800
805
Binding Energy(eV)
Fig. 3. XPS spectra and simulated lines of Co 2p for ZCO films.
810
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J. Yang et al. / Applied Surface Science 258 (2011) 64–67
Ar
531.25
Air
530.15
Intensity(a.u.)
Intensity(a.u.)
530.15
526
528
530
532
534
532.15 531.25
536
526
528
530
532
534
536
Binding Energy(eV)
Binding Energy(eV)
Absorption(a.u.)
Co CoO ZnO ZnCoO
7680
A B
C
D
Co
Zn0.9Co0.1O
ZnO 0
1
2
3
4
5
6
7
Interatomic distanace(Å) Fig. 6. Fourier transformed amplitude at Co K edge of ZCO, Co and CoO, and the Zn K-edge of ZnO.
1.2
0.8
ZnO ZCO/Air ZCO/Ar
0.4
0.0
-0.4
-0.8
-1.2 -6000
A2
-4000
-2000
0
2000
4000
6000
Applied Field(Oe) A1
7700
CoO
Magnetization(μΒ/Co)
of ZnO reference are almost the same, implying that the chemical environment of Co in ZCO film is similar to that of Zn in ZnO. Liu [18] have calculated four structural models of Co K edge spectra, Co atom substituting Zn without VO and Zni (M1), with one VO in the nearest coordination shell (M2), with two VO (one is in the nearest coordination shell, the other is far from the Co atoms) (M3), with one Zni in the nearest coordination shell (M4). There is a smooth peak B in our samples which is similar to M2 calculated by Liu. This indicates that oxygen vacancies, which can induce a donor state overlapping with the d states of the Co atoms, exist in our samples. Fig. 6 is the radial distribution function (RDF) results from typical Fourier transformed amplitude of EXAFS spectra. The two main coordination peaks located at around 1.48 A˚ and 2.88 A˚ are attributed to the first Co–O shell and the second Co–Zn (Co) shell, respectively. In general, the RDF of our sample is different from CoO and Co references. As shown in Fig. 6, the two peaks of the Co K-edge RDF resemble those of ZnO as viewed, implying that the interatomic distances of our sample is similar to that of ZnO. This indicates that Co2+ has substituted for Zn2+ . As a result, we can conclude that Co atoms have dissolved into ZnO lattice and substituted Zn in the valence of +2 without forming secondary phases. Fig. 7 shows the M–H (magnetization versus field) curves of ZnO and ZCO films annealed in Ar and air. We can observe ferromagnetism (FM) in room temperature in the film annealed in Ar. While the film annealed in air is paramagnetism. At present, there are
Fourier transform magnitude(a.u.)
Fig. 4. XPS spectra and simulated lines of O 1s for ZCO films.
7720
Fig. 7. The M–H curves of ZnO and ZCO films annealed in Ar and Air.
7740
7760
7780
7800
7820
7840
Energy (eV) Fig. 5. Normalized absorption spectra for ZCO film and reference samples of Co, CoO and Co3 O4 . The Zn K-edge XANES spectra is shifted to the position of Co K-edge XANES spectra of Co-doped ZnO films.
two possible explanations for the origin of FM in Co-doped ZnO system. One possibility of the origin of FM is the Co metal clusters [19,20], the other is the Co substitution on Zn sites [9,21]. As discussed above, there are no Co clusters in our sample, so FM in ZCO film is intrinsic behavior of Co2+ substitution for Zn2+ in ZnO lattice. The ferromagnetic behavior could be explained on the formation of bound magnetic polarons (BMPs) [22]. As shown in Figs. 4 and 5,
J. Yang et al. / Applied Surface Science 258 (2011) 64–67
the oxygen vacancies exit when Co is doped into ZnO. So BMPs are formed by the exchange interaction of localized oxygen vacancies with Co ions. The localized carriers of these polarons act on the impurities surrounding them thus producing an effective magnetic field for these impurities. The maximum of this effective magnetic field is achieved when the spins of the localized carriers are parallel. Therefore, the system should eventually reach the state where the spins of all carriers point in the same direction, and all impurity spins point in the same direction, resulting ferromagnetic behavior in Co-doped ZnO [23,24]. 4. Conclusion In summary, we have synthesized ZCO films on Si(1 0 0) substrates by CSD method. The films are textured with a c-axis preferred orientation. There are no Co clusters or the secondary phases in our sample. Co ions have dissolved into ZnO and substituted for Zn2+ ions in the valence of +2. Annealing in Ar atmosphere introduces oxygen vacancies in the films. Ferromagnetism is the intrinsic behavior which could be explained on the formation of BMPs which are formed by exchange interaction of oxygen vacancies and Co ions. Acknowledgments This work is supported by the National Natural Science Foundation of China (Grant Nos. 60778040), the Eleventh Five-Year Program for Science and Technology of Education Department of Jilin Province (Item Nos. 20080156, 20080150), Program for the development of Science and Technology of Jilin Province (Item Nos. 20090140, 20080514, and 20082112). The author would like to thank NSRL for synchrotron radiation beamtime. References [1] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Zener model description of ferromagnetism in zinc-blende magnetic semiconductors, Science 287 (2000) 1019–1022. [2] Z. Jin, T. Fukumura, M. Kawasaki, K. Ando, H. Satio, T. Sekiguchi, High throughput fabrication of transition-metal-doped epitaxial ZnO thin films: a series of oxide-diluted magnetic semiconductors and their properties, Appl. Phys. Lett. 78 (2001) 3824–3826. [3] D. Chakraborti, J. Narayan, J.T. Prater, Room temperature ferromagnetism in Zn1−x Cux O thin films, Appl. Phys. Lett. 90 (2007) 062504.
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[4] D.L. Hou, X.J. Ye, H.J. Meng, H.J. Zhou, X.L. Li, C.M. Zhen, G.D. Tang, Magnetic properties of n-type Cu-doped ZnO thin films, Appl. Phys. Lett. 90 (2007) 142502. [5] C.Z. Wang, Z. Chen, Y. He, L.Y. Li, D. Zhang, Structure, morphology and properties of Fe-doped ZnO films prepared by facing-target magnetron sputtering system, Appl. Surf. Sci. 255 (2009) 6881–6887. [6] D. Karmakar, S.K. Mandal, R.M. Kadam, P.L. Paulose, A.K. Rajarajan, T.K. Nath, A.K. Das, I. Dasgupta, G.P. Das, Ferromagnetism in Fe-doped ZnO nanocrystals: experiment and theory, Phys. Rev. B 75 (2007) 144404. [7] W.B. Mi, H.L. Bai, H. Liu, C.Q. Sun, Microstructure, magnetic, and optical properties of sputtered Mn-doped ZnO films with high-temperature ferromagnetism, J. Appl. Phys. 101 (2007) 023904. [8] C.J. Cong, L. Liao, J.C. Li, L.X. Fan, K.L. Zhang, Synthesis, structure and ferromagnetic properties of Mn-doped ZnO nanoparticles, Nanotechnology 16 (2005) 981–984. [9] K. Ueda, H. Tabata, T. Kawai, Magnetic and electric properties of transitionmetal-doped ZnO films, Appl. Phys. Lett. 79 (2001) 988–990. [10] C.B. Fitzgerald, M. Venkatesan, J.G. Lunney, L.S. Dorneles, J.M.D. Coey, Cobaltdoped ZnO-a room temperature dilute magnetic semiconductor, Appl. Surf. Sci. 247 (2005) 493–496. [11] C. Song, X.J. Liu, K.W. Geng, F. Zeng, F. Pan, B. He, S.Q. Wei, Transition from diluted magnetic insulator to semiconductor in Co-doped ZnO transparent oxide, J. Appl. Phys. 101 (2007) 103903. [12] J.H. Park, M.G. Kim, H.H. Jang, S. Ryu, Y.M. Kim, Co-metal clustering as the origin of ferromagnetism in Co-doped ZnO thin films, Appl. Phys. Lett. 84 (2004) 1338–1340. [13] H.S. Hsu, J.C. Huang, S.F. Chen, C.P. Liu, Role of grain boundary and grain defects on ferromagnetism in Co:ZnO films, Appl. Phys. Lett. 90 (2007) 102506. [14] J.M.D. Coey, M. Venkatesan, C.V. Fitzgerald, Donor impurity band exchange in dilute ferromagnetic oxides, Nat. Mater. 4 (2005) 173–179. [15] C.D. Wanger, W.M. Riggs, L.E. Davis, J.F. Moulder, Handbook of X-ray Photoelectron Spectroscopy, PerkinElmer Corporation, Physical Electronics Division, 1979, p. 78 (edited by G. E. Mulenberg PerkinElmer Co.). [16] J.F. Moudler, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, PerkinElmer Corporation, Eden Praine, 1992. [17] M. Chen, X. Wang, Y.H. Yu, Z.L. Pei, X.D. Bai, C. Sun, R.F. Huang, L.S. Wen, Xray photoelectron spectroscopy and auger electron spectroscopy studies of Aldoped ZnO films, Appl. Surf. Sci. 158 (2000) 134–140. [18] X.C. Liu, E.R. Shi, Z.Z. Chen, B.Y. Chen, W. Huang, L.X. Song, K.J. Zhou, M.Q. Cui, Z. Xie, B. He, S.Q. Wei, The local structure of Co-doped ZnO films studied by X-ray absorption spectroscopy, J. Alloys Compounds 463 (2008) 435–439. [19] D.P. Norton, M.E. Overberg, S.J. Pearton, K. Pruessner, J.D. Budai, L.A. Boatner, M.F. Chisholm, J.S. Lee, Z.G. Khim, Y.D. Park, R.G. Wilson, Ferromagnetism in cobalt-implanted ZnO, Appl. Phys. Lett. 83 (2003) 5488–5490. [20] K.P. Bhatti, S. Chaudhary, D.K. Pandya, S.C. Kashyap, High temperature investigation of the magnetization behavior in cobalt substituted ZnO, J. Appl. Phys. 101 (2007) 033902. [21] K. Kim, E.K. Kim, Y.S. Kim, Growth and physical properties of sol–gel derived Co doped ZnO thin film, Superlattices Microstruct. 42 (2007) 246–250. [22] T. Dietl, J. Spalek, Effect of fluctuations of magnetization on the bound magnetic polaron: comparison with experiment, Phys. Rev. Lett. 48 (1982) 355–358. [23] A. Kaminski, S. Das Sarma, Polaron percolation in diluted magnetic semiconductors, Phys. Rev. Lett. 88 (2002) 247202. [24] J.M.D. Coey, High-temperature ferromagnetism in dilute magnetic oxides, J. Appl. Phys. 97 (2005) 10D313.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Structure and magnetism of Zn0.9 Co0.1 O DMS films prepared by chemical solution deposition method Jinghai Yang a,b , Yan Cheng a,b,∗ , Yongjun Zhang a,b , Yaxin Wang a,b , Yang Liu a,b a b
Institute of Condensed State Physics, Jilin Normal University, Siping 136000, PR China Key Laboratory of Functional Materials Physics and Chemistry, Jilin Normal University, Ministry of Education, Siping 136000, PR China
a r t i c l e
i n f o
Article history: Received 10 June 2011 Received in revised form 22 July 2011 Accepted 1 August 2011 Available online 6 August 2011 Keywords: Oxygen Vacancy XANES EFXAS Ferromagnetism
a b s t r a c t The Zn0.9 Co0.1 O films are fabricated by chemical solution deposition method. All the films have the ZnO wurtzite structure with a preferential orientation along the c-axis. The analysis of X-ray near-edge absorption spectroscopy and X-ray photoelectron spectroscopy indicates that the valence of Co is +2, and there are oxygen vacancies in Zn0.9 Co0.1 O films annealed in Ar atmosphere. Extended X-ray absorption fine structure results reveal that Co2+ ions have dissolved into ZnO and substituted for Zn2+ ions. Magnetization measurements show that the film annealed in Ar exhibits ferromagnetism which can be explained by the formation of bound magnetic polarons. © 2011 Elsevier B.V. All rights reserved.
1. Introduction ZnO-based semiconductors are of great interest these years due to their application in the field of spintronics. Recently, theoretical works have predicted that ZnO-based semiconductors could present ferromagnetic behavior at room temperature when doped with transition metals such as Mn [1] and Co [2]. Lately, some experimental results have been reported that room temperature ferromagnetism exists in transition metals doped ZnO diluted magnetic semiconductors (DMS), such as Cu-doped ZnO [3,4], Fe-doped ZnO [5,6], Mn-doped ZnO [7,8], and Co-doped ZnO [9–11]. Many methods are developed to prepare Co-doped ZnO films, including pulsed laser deposition (PLD) [9,10], magnetron sputtering [11,12] and ion-beam sputtering (IBS) [13]. Compared with these methods, chemical solution deposition (CSD) method has its own advantages on fabricating DMS films, because one can fabricate samples with various compositions and increase the solubility easily at low cost. Ueda [9] has reported Co-doped ZnO films with Curie temperature higher than room temperature. In addition, the similar radius with Zn atom makes Co behave similar to Zn, resulting in a high solubility of Co into ZnO. As a result, Co-doped ZnO DMS are another research focus recently. Whether Co ions substitute for Zn2+ or form clusters and secondary phases is an important factor of the origin of ferromagnetism. Coey [14] has proposed that the donor defects
∗ Corresponding author at: Beihang University, School of Physics and Nuclear Energy Engineering, XueYuan Road No.37, Beijing, PR China. E-mail address: [email protected] (Y. Cheng). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.08.006
play a key role in mediating ferromagnetic coupling in oxide based DMS. Generally, the most common donor defects are oxygen vacancies (VO ) and interstitial Zn (Zni ) in ZnO. However, it is difficult to measure VO and Zni directly. In this work, we fabricated Zn0.9 Co0.1 O (ZCO) films by CSD. In order to detect the secondary phases and analyze the type of donor defects, the elements analysis was carried out by X-ray photoelectron spectroscopy (XPS), and the structure of the films were analyzed by X-ray near-edge absorption spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS). 2. Experiment The ZCO films were deposited on Si(1 0 0) substrates by CSD method. The solution preparation was carried out in air with zinc acetate and cobalt acetate according to the desired stoichiometry. 2-methoxyethanol was selected as solvent, and diethanolamine was used as stabilizing agent. A spin-coating speed of 4000 rpm and 30 s were used in the progress of depositing. The deposited films were placed on the preheated hot-plate and dried at 200 ◦ C for 10 min. The above coating was repeated six times. Then the dried films were annealed at 600 ◦ C for 2 h under flowing argon and air atmosphere, respectively. The structural characterization of ZCO films were performed by X-ray diffraction (XRD) on D/max-2500 copper rotating-anode Xray diffractometer with Cu K␣ radiation (40 kV, 200 mA). Magnetic measurement was carried out by Lake Shore M-7407 Vibrating Sample Magnetometer (VSM). The valence state of Co element was analyzed by XPS (VG ESCALAB Mark II). The XANES and EXAFS spectra were performed on beam line U7C at synchrotron radiation
J. Yang et al. / Applied Surface Science 258 (2011) 64–67
65
Intensity(a.u.)
ZCO/Ar ZCO/Air
Zn2p Zn2p
Zn Zn Zn Zn
Co2p OO
Zn3p
Zn3s
C1s
Zn
Zn3d
0
200
400
600
800
1000
1200
Binding energy(eV)
Fig. 1. The XRD patterns for ZCO films and air and ZnO film. The inset is the magnified portion of ZnO (0 0 2) peaks.
Fig. 2. The XPS survey spectra of ZCO films.
laboratory in the University of Science and Technology of China in Hefei. 3. Results and discussion Fig. 1 shows the X-ray diffraction patterns for ZCO films of the wurtzite structure. All the films are textured with a c-axis preferred orientation as evidenced from strong (0 0 2) and weak (1 0 0), (1 0 1) and (1 0 3) peaks. The inset displays the magnified portion of (0 0 2) peak, which shows that the position of (0 0 2) peaks shifts to higher angles. It is because that the radius of Co2+ (0.074 nm) is a little smaller than Zn2+ (0.076 nm). The shift also reveals that there is no octahedral cobalt in our sample. The radius of octahedral cobalt is bigger that zinc, so the peaks of ZCO shift to lower angles, which is opposite to our result. As a result, Co2+ substitutes for Zn2+ without changing the wurtzite structure. Fig. 2 shows the XPS spectra of ZCO films. The binding energy is calibrated by taking carbon C 1s peak (284 eV) as reference. The Co 2p peak is observed besides the C 1s, O 1s, Zn 2p, which indicates that no impurity elements are observed in our sample. Fig. 3 shows the Co 2p XPS spectra of our films. Both spectra show four peaks, Co 2p3/2 and 2p1/2 and the shake-up resonance transitions of these two peaks at higher core level binding energies. The Co 2p3/2 core levels for Co–O bonding is 781.2 eV, showing that Co is divalent according to the handbook [15]. The difference between Co 2p3/2 and 2p1/2 is 15.3 eV. But the literature value of the binding energy of Co 2p3/2 in Co metal is 778.3 eV and the difference
between Co 2p3/2 and 2p1/2 core levels for metallic Co is 14.97 eV [16]. As a result, the formation of Co metal clusters in our sample could be excluded and the valance of Co is +2. Fig. 4 shows the O 1s XPS spectra of our films. For the film annealed in air, there are three binding energy peaks around 530.15, 531.25 and 532.15 eV [17]. The low binding energy peak and middle binding energy peak are attributed to O2− ions at the intrinsic sites and O2− ions in the oxygen-deficient regions, respectively. The high binding energy component located at 532.15 eV is usually attributed to the presence of loosely bound oxygen on the surface of ZnO film, belonging to a specific species, e.g., –CO3 , adsorbed H2 O or adsorbed O2 . However, when the films are annealed in Ar, there are only two peaks at 530.15 and 531.25 eV. It indicates that the majority of oxygen exists in oxygen deficiencies. We consider that the deficiencies are oxygen vacancies due to the annealing conditions. Fig. 5 is the XANES patterns of ZCO film annealed in Ar and the reference samples. There is a pronounce shoulder A1 on the rising absorption edge in Co reference, which is different from our sample. So we believe that there are no Co clusters in our sample. This result is the same to that of XPS. In addition, there is a pre-edge peak A1 in our samples which is assigned to the transition of Co 1s core electron to the hybridized orbital of Co 3d and O 2p states. For comparison, Zn K-edge XANES of ZnO reference spectra was shifted to the position of Co K-edge XANES spectra. The Co K-edge features of our sample of peak A, B, C and D and Zn K-edge features
Air
Ar
Co 2p3/2
Intensity(a.u.)
Intensity(a.u.)
Co 2p3/2 Co 2p1/2
775
780
785
790
795
800
805
Binding Energy(eV)
810
Co 2p1/2
775
780
785
790
795
800
805
Binding Energy(eV)
Fig. 3. XPS spectra and simulated lines of Co 2p for ZCO films.
810
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J. Yang et al. / Applied Surface Science 258 (2011) 64–67
Ar
531.25
Air
530.15
Intensity(a.u.)
Intensity(a.u.)
530.15
526
528
530
532
534
532.15 531.25
536
526
528
530
532
534
536
Binding Energy(eV)
Binding Energy(eV)
Absorption(a.u.)
Co CoO ZnO ZnCoO
7680
A B
C
D
Co
Zn0.9Co0.1O
ZnO 0
1
2
3
4
5
6
7
Interatomic distanace(Å) Fig. 6. Fourier transformed amplitude at Co K edge of ZCO, Co and CoO, and the Zn K-edge of ZnO.
1.2
0.8
ZnO ZCO/Air ZCO/Ar
0.4
0.0
-0.4
-0.8
-1.2 -6000
A2
-4000
-2000
0
2000
4000
6000
Applied Field(Oe) A1
7700
CoO
Magnetization(μΒ/Co)
of ZnO reference are almost the same, implying that the chemical environment of Co in ZCO film is similar to that of Zn in ZnO. Liu [18] have calculated four structural models of Co K edge spectra, Co atom substituting Zn without VO and Zni (M1), with one VO in the nearest coordination shell (M2), with two VO (one is in the nearest coordination shell, the other is far from the Co atoms) (M3), with one Zni in the nearest coordination shell (M4). There is a smooth peak B in our samples which is similar to M2 calculated by Liu. This indicates that oxygen vacancies, which can induce a donor state overlapping with the d states of the Co atoms, exist in our samples. Fig. 6 is the radial distribution function (RDF) results from typical Fourier transformed amplitude of EXAFS spectra. The two main coordination peaks located at around 1.48 A˚ and 2.88 A˚ are attributed to the first Co–O shell and the second Co–Zn (Co) shell, respectively. In general, the RDF of our sample is different from CoO and Co references. As shown in Fig. 6, the two peaks of the Co K-edge RDF resemble those of ZnO as viewed, implying that the interatomic distances of our sample is similar to that of ZnO. This indicates that Co2+ has substituted for Zn2+ . As a result, we can conclude that Co atoms have dissolved into ZnO lattice and substituted Zn in the valence of +2 without forming secondary phases. Fig. 7 shows the M–H (magnetization versus field) curves of ZnO and ZCO films annealed in Ar and air. We can observe ferromagnetism (FM) in room temperature in the film annealed in Ar. While the film annealed in air is paramagnetism. At present, there are
Fourier transform magnitude(a.u.)
Fig. 4. XPS spectra and simulated lines of O 1s for ZCO films.
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Fig. 7. The M–H curves of ZnO and ZCO films annealed in Ar and Air.
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7780
7800
7820
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Energy (eV) Fig. 5. Normalized absorption spectra for ZCO film and reference samples of Co, CoO and Co3 O4 . The Zn K-edge XANES spectra is shifted to the position of Co K-edge XANES spectra of Co-doped ZnO films.
two possible explanations for the origin of FM in Co-doped ZnO system. One possibility of the origin of FM is the Co metal clusters [19,20], the other is the Co substitution on Zn sites [9,21]. As discussed above, there are no Co clusters in our sample, so FM in ZCO film is intrinsic behavior of Co2+ substitution for Zn2+ in ZnO lattice. The ferromagnetic behavior could be explained on the formation of bound magnetic polarons (BMPs) [22]. As shown in Figs. 4 and 5,
J. Yang et al. / Applied Surface Science 258 (2011) 64–67
the oxygen vacancies exit when Co is doped into ZnO. So BMPs are formed by the exchange interaction of localized oxygen vacancies with Co ions. The localized carriers of these polarons act on the impurities surrounding them thus producing an effective magnetic field for these impurities. The maximum of this effective magnetic field is achieved when the spins of the localized carriers are parallel. Therefore, the system should eventually reach the state where the spins of all carriers point in the same direction, and all impurity spins point in the same direction, resulting ferromagnetic behavior in Co-doped ZnO [23,24]. 4. Conclusion In summary, we have synthesized ZCO films on Si(1 0 0) substrates by CSD method. The films are textured with a c-axis preferred orientation. There are no Co clusters or the secondary phases in our sample. Co ions have dissolved into ZnO and substituted for Zn2+ ions in the valence of +2. Annealing in Ar atmosphere introduces oxygen vacancies in the films. Ferromagnetism is the intrinsic behavior which could be explained on the formation of BMPs which are formed by exchange interaction of oxygen vacancies and Co ions. Acknowledgments This work is supported by the National Natural Science Foundation of China (Grant Nos. 60778040), the Eleventh Five-Year Program for Science and Technology of Education Department of Jilin Province (Item Nos. 20080156, 20080150), Program for the development of Science and Technology of Jilin Province (Item Nos. 20090140, 20080514, and 20082112). The author would like to thank NSRL for synchrotron radiation beamtime. References [1] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Zener model description of ferromagnetism in zinc-blende magnetic semiconductors, Science 287 (2000) 1019–1022. [2] Z. Jin, T. Fukumura, M. Kawasaki, K. Ando, H. Satio, T. Sekiguchi, High throughput fabrication of transition-metal-doped epitaxial ZnO thin films: a series of oxide-diluted magnetic semiconductors and their properties, Appl. Phys. Lett. 78 (2001) 3824–3826. [3] D. Chakraborti, J. Narayan, J.T. Prater, Room temperature ferromagnetism in Zn1−x Cux O thin films, Appl. Phys. Lett. 90 (2007) 062504.
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