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Comparative study of room temperature ferromagnetism in Cu, Co codoped ZnO film enhanced by hybridization
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CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 3613–3617 www.elsevier.com/locate/ceramint
Comparative study of room temperature ferromagnetism in Cu, Co codoped ZnO film enhanced by hybridization Huilian Liu, Weijun Li, Xu Zhang, Yunfei Sun, Junlin Song, Jinghai Yangn, Ming Gao, Xiaoyan Liu Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University, Siping 136000, Jilin, China. Received 22 September 2014; received in revised form 4 November 2014; accepted 4 November 2014 Available online 13 November 2014
Abstract A series of Cu and Co codoped ZnO thin films with different Cu content were prepared using the RF magnetron sputtering method. The XRD results indicated that the crystal quality of the ZnO films were influenced by the level of Cu-doping during the sputtering process. X-ray photoelectron spectroscopy was used to detect the elemental valence states of Zn, Co, Cu and O. Magnetic measurements showed that the Cu and Co codoped ZnO thin films displayed room temperature ferromagnetism. The enhanced FM in the Cu, Co codoped ZnO can be attributed to the 3d orbitals split of Cu2 þ . The results of the absorption spectra showed that the bandgap of the ZnO thin films narrow down caused by the Cu doping. It also can be attributed to 3d orbitals split which further supported our interpretation of magnetic. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: C. Magnetic properties; Diluted magnetic semiconductors; Cu and Co codoped ZnO; RF magnetron sputtering
1. Introduction Zinc oxide is an important and promising material as a wide band gap (3.37 eV) semiconductor. It is also an n-type semiconductor in nature and its electrical conductivity is mainly due to the zinc excess at interstitial sites [1]. Its electrical and optical properties can be modified by doping with either cationic or anionic into the ZnO lattice [2–4]. Recently, the transition metal doped ZnO [5–9] thin films have aroused a lot of interest due to their importance in the basic science and technological applications. The transition metal doped ZnO is one of the most promising diluted magnetic semiconductor (DMS) candidate as it is predicted to be ferromagnetic above room temperature [10–12]. Among the reported DMS materials, Co-doped ZnO is rather interesting for its ferromagnetis and ferromagnetic transport properties, even though the origin of the ferromagnetic (FM) behavior are still controversial [13,14]. It has been reported that codoping with another magnetic species is a possible approach for n
Corresponding author.
http://dx.doi.org/10.1016/j.ceramint.2014.11.023 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
mediating the ferromagnetic response of Co-doped ZnO[15– 17]. Moreover, a computational study by Spaldin [18] showed that co-doping with Cu to be one possible route for realizing room temperature ferromagnetism in Co-doped ZnO. It would be helpful for understanding the important mechanism of how to mediate intrinsic FM in Co:ZnO DMSs through codoping with Cu ions. The ZnO thin films have been prepared by many methods, such as thermal evaporation [19], sol–gel process [20–22], RF magnetron sputtering [23], pulsed laser deposition [24,25] and electrodeposition method [26]. Among these methods, RF magnetron sputtering method has advantages of relatively simplicity and large area deposition and uniformity of the films. Since that the structure and properties of sputtered films are strongly influenced by the gas composition [27,28]. In this paper,.We report our investigation on the structural and magnetic properties of Cu, Co codoped ZnO films, which synthesized by the RF magnetron sputtering using the high purity argon as the sputtering gas. The effects of doping level on the physical properties of ZnO are analyzed and the possible mechanism is discussed.
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Fig. 1. Abridged general view of Cu, Co codoped ZnO films for one sputter cycle.
2. Experiment The Cu, Co codoped ZnO thin films have been grown on the c plane of Si single-crystal substrates using RF magnetron sputtering technique. The substrates were ultrasonically cleaned in ammonia and peroxide solution, then rinsed in deionized water. The deposition chamber was evacuated by a turbo molecular pump to a base pressure of 2.0 10-4 Pa. The sputtering was performed in an argon atmosphere, and working pressure during deposition was fixed at 0.6 Pa, RF power was 50 W. The substrate temperature was maintained at room temperature. Our samples were prepared by sputtering multilayer films on substrates, as shown in Fig. 1. In order to label samples with different Cu concent, the films prepared with the CuO target sputtering about 0 s, 9 s, 12 s, 15 s in ten cycles are denoted as samples A, B, C and D, respectively. Sequential post-annealing of the as grown thin films was performed by placing the films in a quartz tube with 600 1C and air atmosphere for 120 min. The crystal structures of the thin films were determined by using X-ray diffraction (XRD). The surface morphology was characterized by atomic force microscopy (AFM, D3100CAP). The electronic states of zinc, oxygen, cobalt and copper in the films were investigated by X-ray photoelectron spectroscopy (XPS) on ESCA LAB 220-XL photoelectron spectrometer (VG Scientific, USA). The magnetic properties at room temperature were measured by a vibrating sample magnetometer (VSM). Absorption spectra were taken by Shimadzu UV-2401PC spectrophotometer. 3. Results and discussions Fig. 2 shows the XRD patterns of the Cu, Co codoped ZnO films with different Cu concent deposited on the Si (1 0 0) substrates, the samples were annealed at 600 1C for 120 min. The diffraction peaks located at 34.41 could be observed for all samples which is attributed to the (002) plane of the ZnO. No diffraction peak for any secondary phase could be observed which indicates that there is no other impurity phase existing in the samples. In the XRD pattens, no other peaks of ZnO can be observed except for (002), which indicated that the thin films had a high preferential c-axis orientation. The existence of characteristic diffraction peaks reveal the wurtzite structural phase in all compositions. Meanwhile, we can see that the
Fig. 2. XRD patterns for Zn0.982 xCo0.018CuxO thin films with various Cu concentrations: x¼ 0, 0.011, 0.023 and 0.037.
(0 0 2) diffraction peaks of all the films have no obvious change compared to the undoped ZnO (34.41). This is consistent with the Vegard's law since that the ionic radius of the Cu (0.073 nm) ions and the Co ionic (0.0745 nm) is comparable to that of the Zn ionic (0.074 nm), and the concentration of the Cu and Co in our films is much smaller than the solubility limit. The average crystallite size is also estimated by measuring the full-width at half maximum of the intense diffraction peaks in all the compositions using Scherer's formula. The full-width at halfmaximum (FWHM) of the (0 0 2) peak is changed with the variation of the Cu concentration. In other word, the nanocrystal size of ZnO changes with the variation of the Cu concentration. Particularly, when the Cu concent is more than 2.3 at%, the size of sample is degenerated obviously. The surface roughness of the ZnO film was investigated by atomic force microscopy. The AFM images of Cu, Co codoped ZnO thin films are shown in Fig. 3(a), (b), (c) and (d), respectively. All images are obtained in contacting mode taken over a scale of 1 mm 1 mm. The root mean squares of the average surface roughnesses of Cu, Co codoped ZnO thin film at 0%, 1.1%, 2.3%, 3.7% are 5.376, 6.971, 7.193 and 7.430 nm, respectively. X-ray photoelectron spectroscopy is well known to be sensitive to the chemical environment, which might provide information for the change of the chemical states. Fig. 4(a) presents the full- spectrum of the as-grown sample. The selected data has been corrected with C1s (284.6 eV). Only the characteristic peaks of Zn, O, Co, Cu can be observed in the spectrum. Fig. 4(b) show two peaks centered at 779.2– 795 eV, corresponding to the Co 2p3/2 and Co2p1/2. The peak position of the Co 2p3/2 (779.2 eV) peak is found to be similar to CoO, specific to Co–O bonding (780.1 eV) and much different from the Co metal (778 eV) and Co2O3 (779 eV). The energy gap between the Co2p3/2 and Co2p1/2 is about 15.8 eV, which confirm that the Co ions stay bivalent in the ZnO lattice having a high-spin d state, rather than forming
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Fig. 3. AFM images of ZnO fims with the following Cu concentrations: (a) 0%, (b) 1.1%, (c) 2.3% and (d) 3.7%.
impurity phases like metallic Co or Co2O3. The XPS peaks of the Cu in the doped ZnO films are shown in Fig. 4(c). The Cu 2p3/2 and 2p1/2 peaks of the Cu, Co codoped ZnO films are located at 933.5 eV and 953.1 eV, respectively. Literatures have reported the following ranges for the binding energys of Cu2p3/2 in metallic and cationic Cu species: 932.6 7 0.2 eV for CuO, 932.5 7 0.3 eV for Cu þ , and 933.6 7 0.3 eV for Cu2 þ . It can be concluded that the most likely oxidation state of our dopant is Cu2 þ . We do not observe other typical peaks such as CuO2 (normally at 942 eV and 963 eV). Fig. 5 shows the room-temperature magnetization vs. magnetic field (M–H) curves for Cu, Co codoped ZnO thin films. All the samples show ferromagnetic ordering at room temperature. A considerable difference is present in the magnetic parameters measured for the films with different Cu concentration at the same temperature. In order to exclude the effect of any potential contamination of the substrate that may induce to the observed magnetism, we measured the bare silicon substrate following the same procedures, which showed the diamagnetic property. The shape of the magnetization curves is a result of the raw data signal with the subtracted diamagnetic background fraction originating from the silicon substrate. The second reason for the observed magnetism may arise from the existence of the Co clusters or impurity phase. However, no impurity
phase was found in the Cu, Co codoped ZnO thin films from the XRD measurements, and we could not find any signals of the Co clusters and other impurity phase. In the Co doped ZnO films, the Co2 þ ions are in tetrahedral coordination, the fivefold degenerate 3d orbitals split into t2 and e1 bands, the isolated Co2 þ ions thus have levels e-spin-down, t2-spin-down, e-spinup, and t2-spin-up from low to high energy. When the Cu ions doped into the ZnCoO films, the tetrahedral ligand field around Cu would also cause the Cu 3d states split. The partial DOSs of Co t2-spin-down are strongly hybridized with the Co e-spindown states at the Fermi level, promoting the electron transfer from the Cu donor to the Co ions [29]. The codoped system's Fermi level is pushed upwards, illustrating the donor role of the Cu ions. The Cu-to-Co charge transfer increases the electron occupation of the Co 3d states. Based on the bound magnetic polaron model, the room temperature ferromagnetism (RTFM) in Cu, Co codoped ZnO thin films is caused by the hybridization between the unoccupied 3d states of the magnetic ions and the donor-derived impurity band at the Fermi level. Fig. 5 shows that the Ms is significantly increased with the concentration of Cu increasing in the B and C, which proves that Cu doping promotes the hybridization in Cu,Co codoped ZnO thin films. Compared to C and D, the saturation magnetization is decreased with the Cu contents increased in the films. Since that the
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Fig. 5. Room-temperature magnetization hysteresis curves of Cu, Co codoped ZnO films.
Fig. 4. XPS spectra of Cu,Co codoped ZnO films: the full-spectrum (a) and high-resolution scan of Co2p (b) and Cu2p (c).
increased Cu contents would increase the probability for more dopant–dopant associations with predominant direct antiferromagnetic coupling, which in turn leads to a decrease in the magnetic moment. Fig. 6(a) depicts the absorption spectra of the Cu, Co codoped ZnO thin films with different Cu contents. In order to measure the absorption spectra of these codoped ZnO films, the samples were sputtered on quartz substrates. All the films exhibit an average optical absorption of higher than 80% in the visible region from 400 to 500 nm and have a sharp fundamental absorption edge in the ultraviolet region. The optical band edge shifts to the longer wavelength with the increasing contents of the Cu ions. The optical absorption at the absorption edge corresponds to the transition from the valence band to the conduction band, so the absorption edge shifting to the lower energy relates to some local energy levels caused by some intrinsic defects [30]. It has been theoretically shown
Fig. 6. (a) UV–vis absorption spectra of Cu,Co codoped ZnO prepared with different Cu doping concentrations. (b) (αhν)2 plots as a function of photon energy (hν) for the sputtered Cu, Co codoped ZnO films.
[31] that Cu doping induced bandgap narrowing and also correlated with room temperature ferromagnetism RTFM, which is due to strong d–p mixing between Cu and O. We have measured the bandgap of the Zn0.982 xCo0.018CuxO with
H. Liu et al. / Ceramics International 41 (2015) 3613–3617
different Cu concentration using the fundamental absorption edge, the optical bandgap was calculated by the following relationship: m hν ¼ A hν E g where A is an energy-independent constant, Eg is the optical bandgap and m is the constant which determine type of optical transition, as shown in Fig. 6(b). The evaluated optical band-gap values are 3.03, 3.15, 3.20 and 3.25 eV. It is well known that the band gap of pure ZnO is 3.37 eV at room temperature. The possible reason for the decreased band gap is inferred as follows: the impurity states of d-electrons of Cu rive under the influence of tetrahedral field of ZnO, which cause lower energy eg doublet and higher energy t2g triplet states. The triplet states hybridizing with valence p-states forms t{bonding} and t{antibonding} states. These states within the range of the band gap such that bonding states are near the valence band and antibonding states occur near the conduction band edge [32]. The optical absorption takes place among these states and manifests in the red shift of the cut-off wavelength, reducing the band gap. 4. Conclusions The influences of the Cu contents on the structural and magnetic properties of the Cu, Co codoped ZnO thin films were discussed in this paper. No impurity phase have been observed from structural analysis. Ferromagnetic behavior was clearly observed at room temperature for all the films. The result of the magnetic measurements show that the concentration of the Cu ions have significant impact on the ferromagnetic property of the Co doped ZnO thin films, which can be interpretated by the 3d orbitals split. Acknowledgments This work was supported by the financial support of the National Youth Program Foundation of China (Grant nos. 10904050 and 61008051), Program for the development of Science and Technology of Jilin province (Item no. 201115218 and 20100113), and the Natural Science Foundation Project of Jilin Province (Item no. 201215223). References [1] [2] [3] [4]
G. Neumann, Status Solidi B 105 (1981) 605. S. Major, A. Banerjee, K.L. Chopra, Thin Solid Films 125 (1985) 179. R. Elilarassi, G. Chandrasekaran, Mater. Chem. Phys. 123 (2010) 450. A. Sa΄aedi, R. Yousefi, F. Jamali-Sheini, M. Cheraghizade, A.K. Zak, N. M. Huang, Ceram. Int. 40 (2014) 4327.
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[5] H. Pan, J.B. Yi, L. Shen, R.Q. Wu, J.H. Yang, Y.P. Feng, J. Ding, Phys. Rev. Lett. 99127201 (2007). [6] L. Shen, R.Q. Wu, H. Pan, G.W. Peng, M. Yang, Z.D. Sha, Y.P. Feng, Phys. Rev. B 78 (2008) 073306. [7] Y.K. Lakshmi, K. Srinivas, B. Sreedhar, M.M. Raja, M. Vithal, P. V. Reddy, Mater. Chem. Phys. 113 (2009) 749. [8] V.R. Shinde, T.P. Gujar, C.D. Lokhande, R.S. Mane, Sung-Hwan Han, Mater. Chem. Phys. 96 (2006) 326. [9] S. Guo, Z. Du, J. Magn. Magn. Mater. 324 (2012) 782. [10] K.R. Kittilstved, D.R. Gamelin, J. Am. Chem. Soc. 127 (2005) 5292. [11] K.R. Kittilstved, N.S. Norberg, D.R. Gamelin, Phys. Rev. Lett. 94 (2005) 14209. [12] R. Saleh, S.P. Prakoso, A. Fishli, J. Magn. Magn. Mater. 324 (2012) 665. [13] S.G. Yang, A.B. Pakhomov, S.T. Hung, C.Y. Wong, IEEE Trans. Magn. 38 (2002) 2877. [14] Y.M. Cho, W.K. Choo, H. Kim, D. Kim, Y. Ihm, Appl. Phys. Lett. 80 (2002) 3358. [15] N.N. Lathiotakis, A.N. Andriotis, M. Menon, Phys. Rev. B 78 (2008) 193311. [16] Z.C. Quan, D. Li, B. Sebo, W. Liu, S.S. Guo, S. Xu, H.M. Huang, G. J. Fang, M.Y. Li, X.Z. Zhao, Appl. Surf. Sci. 256 (2010) 3669. [17] L.H. Hoang, T.M.H. Nuyen, N.H. Hai, P.V. Hai, N.T. Khoi, I.S. Yang, J. Raman Spectrosc. 40 (2009) 1535. [18] N.A. Spaldin, Phys. Rev. B 69 (2004) 125201. [19] M. El-Hagary, M. Emam-Ismail, E.R. Shaaband, A. Al-Rashidi, S. Althoyaib, Mater. Chem. Phys. 132 (2012) 581. [20] M. Azarang, A. Shuhaimi, R. Yousefi, M. Sookhakian, J. Appl. Phys. 116 (2014) 084307. [21] M.a. Azarang, A. Shuhaimi, R. Yousefi, A.M. Golsheikh, M. Sookhakian, Ceram. Int. 40 (2014) 10217. [22] R. Elilarassi, G. Chandrasekaran, Mater. Chem. Phys. 123 (2010) 450. [23] C.G. Jin, T. Yu, Y. Yang, Z.F. Wu, L.J. Zhuge, X.M. Wu, Z.C. Feng, Mater. Chem. Phys. 139 (2013) 506. [24] L.H. Van, M.H. Hong, J. Ding, J. Alloys Compd. 449 (2008) 207. [25] X.Q. Wei, B.Y. Man, M. Liu, C.S. Xue, H.Z. Zhuang, C. Yang, Phys. B: Condens. Matter 388 (2007) 145. [26] F. Jamali-Sheini, R. Yousefi, M.A. Morec, D.S. Joag, Mater. Lett. 111 (2013) 181. [27] S. Yang, B.Y. Man, M. Liu, C.S. Chen, X.G. Gao, C.C. Wang, B. Hu, Physica B 405 (2010) 4027. [28] S. Wang, P. Li, H. Liu, J.B. Li, Y. Wei, J. Alloys Compd. 505 (2010) 362. [29] W.S. Yan, Z.H. Sun, Q.H. Liu, Z.R. Li, Z.Y. Pan, J. Wang, S.Q. Wei, D. Wang, Y.X. Zhou, X.Y. Zhang, Appl. Phys. Lett. 91 (2007) 062113. [30] L. Zhao, J. Lian, Y. Liu, Q. Jiang, Appl. Surf. Sci. 252 (2006) 8451. [31] M. Ferhat, A. Zaoui, R. Ahuja, Appl. Phys. Lett. 94 (2009) 142502. [32] S. Singh, M.S.R. Rao, Phys. Rev. B 80 (2009) 045210.
CERAMICS INTERNATIONAL
Ceramics International 41 (2015) 3613–3617 www.elsevier.com/locate/ceramint
Comparative study of room temperature ferromagnetism in Cu, Co codoped ZnO film enhanced by hybridization Huilian Liu, Weijun Li, Xu Zhang, Yunfei Sun, Junlin Song, Jinghai Yangn, Ming Gao, Xiaoyan Liu Key Laboratory of Functional Materials Physics and Chemistry of the Ministry of Education, Jilin Normal University, Siping 136000, Jilin, China. Received 22 September 2014; received in revised form 4 November 2014; accepted 4 November 2014 Available online 13 November 2014
Abstract A series of Cu and Co codoped ZnO thin films with different Cu content were prepared using the RF magnetron sputtering method. The XRD results indicated that the crystal quality of the ZnO films were influenced by the level of Cu-doping during the sputtering process. X-ray photoelectron spectroscopy was used to detect the elemental valence states of Zn, Co, Cu and O. Magnetic measurements showed that the Cu and Co codoped ZnO thin films displayed room temperature ferromagnetism. The enhanced FM in the Cu, Co codoped ZnO can be attributed to the 3d orbitals split of Cu2 þ . The results of the absorption spectra showed that the bandgap of the ZnO thin films narrow down caused by the Cu doping. It also can be attributed to 3d orbitals split which further supported our interpretation of magnetic. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: C. Magnetic properties; Diluted magnetic semiconductors; Cu and Co codoped ZnO; RF magnetron sputtering
1. Introduction Zinc oxide is an important and promising material as a wide band gap (3.37 eV) semiconductor. It is also an n-type semiconductor in nature and its electrical conductivity is mainly due to the zinc excess at interstitial sites [1]. Its electrical and optical properties can be modified by doping with either cationic or anionic into the ZnO lattice [2–4]. Recently, the transition metal doped ZnO [5–9] thin films have aroused a lot of interest due to their importance in the basic science and technological applications. The transition metal doped ZnO is one of the most promising diluted magnetic semiconductor (DMS) candidate as it is predicted to be ferromagnetic above room temperature [10–12]. Among the reported DMS materials, Co-doped ZnO is rather interesting for its ferromagnetis and ferromagnetic transport properties, even though the origin of the ferromagnetic (FM) behavior are still controversial [13,14]. It has been reported that codoping with another magnetic species is a possible approach for n
Corresponding author.
http://dx.doi.org/10.1016/j.ceramint.2014.11.023 0272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
mediating the ferromagnetic response of Co-doped ZnO[15– 17]. Moreover, a computational study by Spaldin [18] showed that co-doping with Cu to be one possible route for realizing room temperature ferromagnetism in Co-doped ZnO. It would be helpful for understanding the important mechanism of how to mediate intrinsic FM in Co:ZnO DMSs through codoping with Cu ions. The ZnO thin films have been prepared by many methods, such as thermal evaporation [19], sol–gel process [20–22], RF magnetron sputtering [23], pulsed laser deposition [24,25] and electrodeposition method [26]. Among these methods, RF magnetron sputtering method has advantages of relatively simplicity and large area deposition and uniformity of the films. Since that the structure and properties of sputtered films are strongly influenced by the gas composition [27,28]. In this paper,.We report our investigation on the structural and magnetic properties of Cu, Co codoped ZnO films, which synthesized by the RF magnetron sputtering using the high purity argon as the sputtering gas. The effects of doping level on the physical properties of ZnO are analyzed and the possible mechanism is discussed.
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Fig. 1. Abridged general view of Cu, Co codoped ZnO films for one sputter cycle.
2. Experiment The Cu, Co codoped ZnO thin films have been grown on the c plane of Si single-crystal substrates using RF magnetron sputtering technique. The substrates were ultrasonically cleaned in ammonia and peroxide solution, then rinsed in deionized water. The deposition chamber was evacuated by a turbo molecular pump to a base pressure of 2.0 10-4 Pa. The sputtering was performed in an argon atmosphere, and working pressure during deposition was fixed at 0.6 Pa, RF power was 50 W. The substrate temperature was maintained at room temperature. Our samples were prepared by sputtering multilayer films on substrates, as shown in Fig. 1. In order to label samples with different Cu concent, the films prepared with the CuO target sputtering about 0 s, 9 s, 12 s, 15 s in ten cycles are denoted as samples A, B, C and D, respectively. Sequential post-annealing of the as grown thin films was performed by placing the films in a quartz tube with 600 1C and air atmosphere for 120 min. The crystal structures of the thin films were determined by using X-ray diffraction (XRD). The surface morphology was characterized by atomic force microscopy (AFM, D3100CAP). The electronic states of zinc, oxygen, cobalt and copper in the films were investigated by X-ray photoelectron spectroscopy (XPS) on ESCA LAB 220-XL photoelectron spectrometer (VG Scientific, USA). The magnetic properties at room temperature were measured by a vibrating sample magnetometer (VSM). Absorption spectra were taken by Shimadzu UV-2401PC spectrophotometer. 3. Results and discussions Fig. 2 shows the XRD patterns of the Cu, Co codoped ZnO films with different Cu concent deposited on the Si (1 0 0) substrates, the samples were annealed at 600 1C for 120 min. The diffraction peaks located at 34.41 could be observed for all samples which is attributed to the (002) plane of the ZnO. No diffraction peak for any secondary phase could be observed which indicates that there is no other impurity phase existing in the samples. In the XRD pattens, no other peaks of ZnO can be observed except for (002), which indicated that the thin films had a high preferential c-axis orientation. The existence of characteristic diffraction peaks reveal the wurtzite structural phase in all compositions. Meanwhile, we can see that the
Fig. 2. XRD patterns for Zn0.982 xCo0.018CuxO thin films with various Cu concentrations: x¼ 0, 0.011, 0.023 and 0.037.
(0 0 2) diffraction peaks of all the films have no obvious change compared to the undoped ZnO (34.41). This is consistent with the Vegard's law since that the ionic radius of the Cu (0.073 nm) ions and the Co ionic (0.0745 nm) is comparable to that of the Zn ionic (0.074 nm), and the concentration of the Cu and Co in our films is much smaller than the solubility limit. The average crystallite size is also estimated by measuring the full-width at half maximum of the intense diffraction peaks in all the compositions using Scherer's formula. The full-width at halfmaximum (FWHM) of the (0 0 2) peak is changed with the variation of the Cu concentration. In other word, the nanocrystal size of ZnO changes with the variation of the Cu concentration. Particularly, when the Cu concent is more than 2.3 at%, the size of sample is degenerated obviously. The surface roughness of the ZnO film was investigated by atomic force microscopy. The AFM images of Cu, Co codoped ZnO thin films are shown in Fig. 3(a), (b), (c) and (d), respectively. All images are obtained in contacting mode taken over a scale of 1 mm 1 mm. The root mean squares of the average surface roughnesses of Cu, Co codoped ZnO thin film at 0%, 1.1%, 2.3%, 3.7% are 5.376, 6.971, 7.193 and 7.430 nm, respectively. X-ray photoelectron spectroscopy is well known to be sensitive to the chemical environment, which might provide information for the change of the chemical states. Fig. 4(a) presents the full- spectrum of the as-grown sample. The selected data has been corrected with C1s (284.6 eV). Only the characteristic peaks of Zn, O, Co, Cu can be observed in the spectrum. Fig. 4(b) show two peaks centered at 779.2– 795 eV, corresponding to the Co 2p3/2 and Co2p1/2. The peak position of the Co 2p3/2 (779.2 eV) peak is found to be similar to CoO, specific to Co–O bonding (780.1 eV) and much different from the Co metal (778 eV) and Co2O3 (779 eV). The energy gap between the Co2p3/2 and Co2p1/2 is about 15.8 eV, which confirm that the Co ions stay bivalent in the ZnO lattice having a high-spin d state, rather than forming
H. Liu et al. / Ceramics International 41 (2015) 3613–3617
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Fig. 3. AFM images of ZnO fims with the following Cu concentrations: (a) 0%, (b) 1.1%, (c) 2.3% and (d) 3.7%.
impurity phases like metallic Co or Co2O3. The XPS peaks of the Cu in the doped ZnO films are shown in Fig. 4(c). The Cu 2p3/2 and 2p1/2 peaks of the Cu, Co codoped ZnO films are located at 933.5 eV and 953.1 eV, respectively. Literatures have reported the following ranges for the binding energys of Cu2p3/2 in metallic and cationic Cu species: 932.6 7 0.2 eV for CuO, 932.5 7 0.3 eV for Cu þ , and 933.6 7 0.3 eV for Cu2 þ . It can be concluded that the most likely oxidation state of our dopant is Cu2 þ . We do not observe other typical peaks such as CuO2 (normally at 942 eV and 963 eV). Fig. 5 shows the room-temperature magnetization vs. magnetic field (M–H) curves for Cu, Co codoped ZnO thin films. All the samples show ferromagnetic ordering at room temperature. A considerable difference is present in the magnetic parameters measured for the films with different Cu concentration at the same temperature. In order to exclude the effect of any potential contamination of the substrate that may induce to the observed magnetism, we measured the bare silicon substrate following the same procedures, which showed the diamagnetic property. The shape of the magnetization curves is a result of the raw data signal with the subtracted diamagnetic background fraction originating from the silicon substrate. The second reason for the observed magnetism may arise from the existence of the Co clusters or impurity phase. However, no impurity
phase was found in the Cu, Co codoped ZnO thin films from the XRD measurements, and we could not find any signals of the Co clusters and other impurity phase. In the Co doped ZnO films, the Co2 þ ions are in tetrahedral coordination, the fivefold degenerate 3d orbitals split into t2 and e1 bands, the isolated Co2 þ ions thus have levels e-spin-down, t2-spin-down, e-spinup, and t2-spin-up from low to high energy. When the Cu ions doped into the ZnCoO films, the tetrahedral ligand field around Cu would also cause the Cu 3d states split. The partial DOSs of Co t2-spin-down are strongly hybridized with the Co e-spindown states at the Fermi level, promoting the electron transfer from the Cu donor to the Co ions [29]. The codoped system's Fermi level is pushed upwards, illustrating the donor role of the Cu ions. The Cu-to-Co charge transfer increases the electron occupation of the Co 3d states. Based on the bound magnetic polaron model, the room temperature ferromagnetism (RTFM) in Cu, Co codoped ZnO thin films is caused by the hybridization between the unoccupied 3d states of the magnetic ions and the donor-derived impurity band at the Fermi level. Fig. 5 shows that the Ms is significantly increased with the concentration of Cu increasing in the B and C, which proves that Cu doping promotes the hybridization in Cu,Co codoped ZnO thin films. Compared to C and D, the saturation magnetization is decreased with the Cu contents increased in the films. Since that the
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Fig. 5. Room-temperature magnetization hysteresis curves of Cu, Co codoped ZnO films.
Fig. 4. XPS spectra of Cu,Co codoped ZnO films: the full-spectrum (a) and high-resolution scan of Co2p (b) and Cu2p (c).
increased Cu contents would increase the probability for more dopant–dopant associations with predominant direct antiferromagnetic coupling, which in turn leads to a decrease in the magnetic moment. Fig. 6(a) depicts the absorption spectra of the Cu, Co codoped ZnO thin films with different Cu contents. In order to measure the absorption spectra of these codoped ZnO films, the samples were sputtered on quartz substrates. All the films exhibit an average optical absorption of higher than 80% in the visible region from 400 to 500 nm and have a sharp fundamental absorption edge in the ultraviolet region. The optical band edge shifts to the longer wavelength with the increasing contents of the Cu ions. The optical absorption at the absorption edge corresponds to the transition from the valence band to the conduction band, so the absorption edge shifting to the lower energy relates to some local energy levels caused by some intrinsic defects [30]. It has been theoretically shown
Fig. 6. (a) UV–vis absorption spectra of Cu,Co codoped ZnO prepared with different Cu doping concentrations. (b) (αhν)2 plots as a function of photon energy (hν) for the sputtered Cu, Co codoped ZnO films.
[31] that Cu doping induced bandgap narrowing and also correlated with room temperature ferromagnetism RTFM, which is due to strong d–p mixing between Cu and O. We have measured the bandgap of the Zn0.982 xCo0.018CuxO with
H. Liu et al. / Ceramics International 41 (2015) 3613–3617
different Cu concentration using the fundamental absorption edge, the optical bandgap was calculated by the following relationship: m hν ¼ A hν E g where A is an energy-independent constant, Eg is the optical bandgap and m is the constant which determine type of optical transition, as shown in Fig. 6(b). The evaluated optical band-gap values are 3.03, 3.15, 3.20 and 3.25 eV. It is well known that the band gap of pure ZnO is 3.37 eV at room temperature. The possible reason for the decreased band gap is inferred as follows: the impurity states of d-electrons of Cu rive under the influence of tetrahedral field of ZnO, which cause lower energy eg doublet and higher energy t2g triplet states. The triplet states hybridizing with valence p-states forms t{bonding} and t{antibonding} states. These states within the range of the band gap such that bonding states are near the valence band and antibonding states occur near the conduction band edge [32]. The optical absorption takes place among these states and manifests in the red shift of the cut-off wavelength, reducing the band gap. 4. Conclusions The influences of the Cu contents on the structural and magnetic properties of the Cu, Co codoped ZnO thin films were discussed in this paper. No impurity phase have been observed from structural analysis. Ferromagnetic behavior was clearly observed at room temperature for all the films. The result of the magnetic measurements show that the concentration of the Cu ions have significant impact on the ferromagnetic property of the Co doped ZnO thin films, which can be interpretated by the 3d orbitals split. Acknowledgments This work was supported by the financial support of the National Youth Program Foundation of China (Grant nos. 10904050 and 61008051), Program for the development of Science and Technology of Jilin province (Item no. 201115218 and 20100113), and the Natural Science Foundation Project of Jilin Province (Item no. 201215223). References [1] [2] [3] [4]
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