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Structural, optical and magnetic properties of Co-doped ZnO films
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Journal of Crystal Growth 296 (2006) 135–140 www.elsevier.com/locate/jcrysgro
Structural, optical and magnetic properties of Co-doped ZnO films Xue-Chao Liua,b, Er-Wei Shia, Zhi-Zhan Chena,, Hua-Wei Zhanga,b, Li-Xin Songa, Huan Wangc, Shu-De Yaoc a
Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China b Graduate School of the Chinese Academy of Sciences, Beijing 100049, China c Department of Technical Physics, School of Physics, Peking University, Beijing 100871, China Received 9 May 2006; received in revised form 29 July 2006; accepted 22 August 2006 Communicated by S. Uda Available online 10 October 2006
Abstract Zn1xCoxO (x ¼ 0:05, 0.1, 0.15) films were deposited on Si (1 0 0) and quartz glass substrates using inductively coupled plasma enhanced physical vapor deposition system with magnetic confinement. Structural and chemical composition characterizations by X-ray absorption spectroscopy and Rutherford backscattering spectroscopy indicate that Co2+ substitute for Zn2+ in the tetrahedral configuration and form homogeneous films with wurtzite structure when xp0.15. UV–vis transmittance spectra are used to characterize the optical properties. It is found that both transmittance and band gap of Zn1xCoxO films decrease with increasing Co content, owing to the enhancement of sp–d exchange interactions and typical d–d transitions. Magnetic measurements show that all the as-grown films exhibit intrinsic ferromagnetism above room temperature. r 2006 Elsevier B.V. All rights reserved. PACS: 75.50.Pp; 68.55.Jk; 78.20.e Keywords: A1. Optical properties; A1. X-ray absorption spectroscopy; B1. Co-doped ZnO; B2. Diluted magnetic semiconductors
1. Introduction The semiconductor ZnO has gained substantial interests in the research community because of its wide direct band gap (3.3 eV) and large exciton binding energy (60 MeV) which could lead to lasing action based on exciton recombination even above room temperature. It is a wellknown piezoelectric and electro-optic material with potential applications such as opto-electronic and luminescent devices as well as chemical sensors. Even though the research focusing on ZnO goes back to several decades, one of the renewed interests is fueled by the observation of ferromagnetism in transition metals doped ZnO which is called diluted magnetic semiconductor (DMS) [1]. DMS has inspired a great deal of research interest in the field of ‘‘spintronics’’, which could pave the way to exploit spin as well as charge in semiconductor devices. The main Corresponding author. Tel.: +86 21 52411109; fax: +86 21 52413903.
E-mail address: [email protected] (Z.-Z. Chen). 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.08.034
challenge for practical application of the DMS materials is the attainment of intrinsic ferromagnetism with Curie temperature (TC) at or preferably above room temperature [2,3]. Much attention has been paid to the magnetic properties of ZnO-based DMS materials due to the theoretical prediction by Dietl [4] and Sato [5]. For Codoped ZnO system, one of the most important questions is whether Co atoms substitute for Zn2+ in the ZnO lattice or form the secondary clustering phases in the matrix. It is still controversial that whether the observed ferromagnetism and optical properties are intrinsic or comes from secondary phase. The solubility of Co element in ZnO is quite different in the references. Ueda et al. [6] found that the solubility limit of Co ions in ZnO could reach 50 at%. Lee et al. [7] got high-temperature ferromagnetism in Codoped ZnO films by the sol–gel method. No secondary phase was detected until x40.25. Park et al [8]. prepared Zn1xCoxO films by Rf magnetron sputtering method. Cometal clusters, which might be responsible for the ferromagnetism, were found by extended X-ray absorption
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fine structure (EXAFS) measurement when x40.12. Kim found that the secondary phases, such as Co clusters or cobalt oxides, were sensitive to the growth conditions like oxygen pressure, substrate temperature, heat treatment, etc. It was reported that Co can be dissolved in ZnO over 40 at% under optimum growth conditions [9]. It seems that the solubility of Co element in ZnO depends on the fabricating method and preparing conditions. Small precipitates, especially in micro- or nano-scaled magnitude, are difficult to be detected by using general techniques. This increases the difficulty in clarifying intrinsic properties of Co-doped ZnO. In this paper, the structural, optical and magnetic properties of Zn1xCoxO films were characterized in detail. The characterizations revealed an intrinsic nature between properties and structure in Co-doped ZnO films. 2. Experimental details Zn1xCoxO (x ¼ 0:05, 0.1, 0.15) films were deposited on Si (1 0 0) and quartz glass substrates by inductively coupled plasma enhanced physical vapor deposition (ICP-PVD) system with magnetic confinement. The films grown on quartz glass substrates were used to measure the transmittance spectra. The novel ICP-PVD with magnetic confinement system has been introduced in detail in Ref. [10]. The sputtering targets were fabricated by standard solid-state reaction method. In brief, prescribed amounts of ZnO (99.99%) and CoO (99.99%) were mixed and sintered at 1273 K for 24 h in air [11]. Plasma of Ar was used for sputtering and the DC sputtering power was maintained at 100 W with RF coil net power of 25 W. The substrate temperature was kept at 673 K when sputtering. The base and sputtering pressures were 5 104 and 0.5 Pa, respectively. The distance from target to substrate was 100 mm. The structure and crystal orientation of the as-prepared films were determined by X-ray diffraction (XRD, D/ MAX-2550 V) using monochromic Cu Ka radiation ˚ The chemical composition and thickness (l ¼ 1:5406 A). of the films were investigated by the RBS with 4He+ particles of 1.57 MeV at a scattering angle of 167.81. The simulated fit was obtained by the RUMP software. The Xray absorption spectroscopy of near-edge structure (XANES) and extended fine structure measurements were performed on synchrotron radiation from beam line U7c at the National Synchrotron Radiation Laboratory in The University of Science and Technology of China. Room temperature UV–vis transmittance spectra were performed by Cary 500 UV–Vis-NIR spectrophotometer. A quantum design superconducting quantum interference device (SQUID, MPMS XL-7) magnetometer was used to investigate the magnetic properties. Magnetization vs. magnetic field measurement was performed both at low temperature (5 K) and at room temperature (300 K). Temperature dependence of magnetization was measured in an applied field of 500 Oe upon warming a sample which had been cooled from room temperature to 5 K.
3. Results and discussion The XRD patterns of undoped ZnO and Zn1xCoxO (x ¼ 0:05, 0.1, 0.15) samples are shown in Fig. 1. The patterns show only ZnO (0 0 0 2) reflection peak, which indicate that all the films are in c-axis orientation. No evidence for impurity phases is observed in the range of x ¼ 020:15. Compared with the undoped ZnO, the (0 0 0 2) diffraction reflections of Zn1xCoxO trivially move to higher 2y angles with increasing Co contents. Lim [12] and Risbud [13] got similar results in Co-doped ZnO films and polycrystalline powders. This shift is consistent with the fact that the radius of Zn2+ (0.60 A˚) is a little bigger than that of Co2+ (0.58 A˚) in their tetrahedral coordination [13,14]. This result also reveals that there is no divalent octahedral cobalt (i.e. Co2+ in the CoO) that induces a significant moving to the lower angles of (0 0 0 2) diffraction peak due to its larger ionic radius between 0.65 A˚ (low spin) and 0.745 A˚ (high spin) [15]. The XRD patterns indicate that the Co2+ ions systematically substitute for the Zn2+ ions in the films without changing the wurtzite structure. RBS measurement was used to determine the composition and in-depth distribution of the Co element in the ZnCoO layers. Fig. 2 shows the experimental and simulated RBS results for a typical Zn0.85Co0.15O film deposited on Si (1 0 0) substrate. The thickness D of the layer is about 300 nm. The experimental data and simulated fit are shown in Fig. 2(a). Fig. 2(b) shows the separate spectra of Co and Zn from the simulated RBS results. The Co content in the Zn1xCoxO films was estimated to be about 5.5%, 11% and 17% from a quantitative fit for x ¼ 0:05, 0.1 and 0.15, respectively. The percentages of Co in the respective films are close to the nominal values in the ceramic targets.
Fig. 1. X-ray diffraction patterns of ZnO and Zn1xCoxO films on Si (1 0 0) substrate. The (0 0 0 2) diffraction reflections slightly move to higher 2y angles with increasing x.
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Fig. 3. (a) Room temperature optical transmittance spectra of ZnO and Zn1xCoxO films deposited on quartz glass substrates. (b) a2 vs. hu curves measured at room temperature for ZnO and Zn1xCoxO films. The absorption edge shifts toward lower energy with increasing Co contents. The inset shows the relationship between band gap energy and Co contents.
Fig. 2. (a) Random (o) and simulated (solid line) RBS spectra of Zn0.85Co0.15O/Si (1 0 0) sample. (b) Separate spectra of Co and Zn from the simulated RBS results.
Pure ZnO film is transparent and colorless, whereas the as-grown Co-doped ZnO films are less transparent with slight green color. Fig. 3(a) shows the transmittance spectra of ZnO and Zn1xCoxO films deposited on quartz glass substrates. The excitonic nature is clearly apparent in both ZnO and Zn1xCoxO samples. Because the exciton binding energy is almost the same as that of ZnO (E60 MeV) in the Zn1xCoxO films, the exciton peak remains present for all alloy compositions with increasing Co concentrations. The oscillations in the transmittance spectra are caused by multi-reflections at the film–air and film–substrate interfaces. The ZnO film shows an average transmittance above 85% and a sharp absorption edge at 380 nm (3.27 eV). However, the transparency of Zn1xCoxO films fades away
(about 50% for Zn0.85Co0.15O) when the Co contents increase. The darkening of the green color is assigned as typical d–d transitions of high spin states Co2+ 3d7 (4F) in a tetrahedral oxygen coordination. In its neutral charge state, the Co2+ ions has an [Ar]3d7 electron configuration. The atomic 4F ground state splits under the influence of the tetrahedral component of the crystal field into a 4A2 ground state and 4T2+4T1 excited states. The smaller trigonal distortion and spin–orbit interaction split the ground 4A2 state into E1/2+E3/2 [16]. The absorption around 660, 615, and 560 nm in the visible range was derived from separately 4A2(4F)-2E(2G), 4A2(4F)4 T1(4P), and 4A2(4F)-4A1(4G) transitions of tetrahedrally coordinated Co2+ [17,18]. These absorptions are ascribed to the charge-transfer transitions between donor and acceptor ionization levels presumably located within the band gap of the host ZnO as verified by result of ZnCoS and ZnCoSe from both experiments and hypothetical
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calculations [19]. Besides the lower transparency, the absorption edges of Zn1xCoxO films are red shifted with increasing Co content. It is well known that the band gap Eg of ZnO can be tuned over a large energy range by doping Cd2+ and Mg2+ [20,21]. In order to evaluate the Eg of Co-doped ZnO films, the characteristics of a2 vs. hu were plotted as shown in Fig. 3(b), where a is the absorption coefficient and hu the photon energy. We can calculate a from the transmittance T using the equation a ¼ ð1=DÞ ln T. On the other hand, a can be expressed as a ¼ Aðhv E g Þ1=2 for allowed direct transitions at a given photon energy hu, where A is a function of the refractive index and hole/electron effective masses. The inset in Fig. 3(b) shows the relationship between band gap energy and Co content. As can be seen clearly, Eg decreases from 3.27 to 2.88 eV with increasing Co content from x ¼ 0 to x ¼ 0:15. The red shift of Eg with Co doping has already been observed and explained due to sp–d exchange interactions between the band electrons in ZnO and the localized d electrons of the Co2+ [22,23]. Magnetization measurements showed a distinct ferromagnetic behavior. Fig. 4(a) shows the magnetization versus magnetic field (M–H) curve of the Zn0.95Co0.05O film. The M–H curve shows obvious hysteresis loop with the coercive field Hc of 200 Oe at 5 K. When the temperature was increased to 300 K, the M–H curve still exhibits hysteresis loop with the Hc of 100 Oe as shown in the left inset of Fig. 4(a). The temperature dependence of magnetization for Zn0.95Co0.05O film is shown in the right inset of Fig. 4(a). The abrupt increase of magnetization, which corresponds to the TC, appears above 375 K. It is hard to determine the exact value, since the value of TC is rather high, exceeding the range of the measuring equipment. For x ¼ 0:1 and 0.15 samples, ferromagnetic behavior was also observed at room temperature as shown in Fig. 4(b). The HC are 135 and 145 Oe, respectively. The Co content dependence of saturation magnetization Ms is shown in the inset of Fig. 4(b). The Ms slightly increases with increasing Co content. However, it is smaller than that of Co2+ in a tetrahedral crystal field (3.0mB/Co). Whether the observed optical and magnetic properties are intrinsic nature of Co-doped ZnO, we need to know the local structure of Co element in ZnO. Since X-ray absorption spectroscopy is a spectroscopic tool that provides a ‘‘fingerprint’’ of chemical states and local electronic structure of incorporated atoms in the host compounds even in a dilute concentration, this technique was used to investigate the electronic state of Co element and surrounding environment around Co [24]. The absorption edge of XANES spectra moves to lower energy as the effective valency decreases [25]. Fig. 5(a) shows the XANES at the Co K edge of Zn1xCoxO and reference samples of Co, CoO and Co2O3. The XANES spectra of Zn1xCoxO is marked different from those of Co and Co2O3, particularly in the pre-edge feature, which is due to the transition to bound state. However, it has a close proximity to the CoO reference. The XANES reveals that
Fig. 4. (a) Magnetization hysteresis curves of Zn0.95Co0.05O film at 5 and 300 K. The left inset shows magnified portion of the curve at 300 K. The right inset shows magnetization vs. temperature curve measured in an applied field of 500 Oe. (b) Magnetization hysteresis curves of Zn0.90Co0.10O and Zn0.85Co0.15O films at 300 K. The inset shows the Co content dependence of saturation magnetization.
the Co elements dissolve into ZnO and substitute for Zn2+ ions in the valence of +2 state in the Zn1xCoxO films. Fig. 5(b) shows the radial distribution functions (RDFs) results from typical Fourier transformed amplitude of EXAFS spectra. The Co K edge of Zn1xCoxO and reference sample together with the Zn K edge of ZnO are shown. Note that the first and second major peaks at the Co K edge correspond to the nearest oxygen and Zn (or Co), respectively, as viewed from a specific Co atom. For x ¼ 0:05, 0.1 and 0.15, the interatomic distances of the two major peaks of the Co K edge RDF resemble those of ZnO as viewed from a specific Zn atom. This implies that the Co2+ substitute for Zn2+ without forming secondary phases. As discussed above, we can conclude that the lower transmittance and red shift of Eg in Co-doped ZnO films are the intrinsic behaviors of Co2+ substituting for Zn2+ in the tetrahedral configuration. All the Co-doped ZnO films
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not all of the doped Co2+ ions are induced to ferromagnetic couplings. The exchange interactions in Co-doped ZnO might be effected by several factors such as electron concentration, defect and crystal orientation etc. Philip [27] and Hsu [28] found that the electron concentration and oxygen vacancy play an important role in oxide-based DMS. Details of the magnetic properties correlated with residual defect and carrier concentration are in progress. 4. Conclusion Highly c-axis orientated Zn1xCoxO films were fabricated by ICP-PVD method. The RBS analyses reveal that the composition and in-depth distribution of Co content in the doped films are homogenous. XRD and detailed structural characterizations using X-ray absorption spectroscopy indicate that Co2+ substitute for Zn2+ in the tetrahedral configuration without forming secondary phases. Two dominant interactions exist in the case of Co2+ substituting for Zn2+, which were confirmed by room-temperature UV–vis spectra. One is the interactions between the d electrons of Co2+ and the s and p electrons of the host ZnO bands, which leads to the red shift of Eg with increasing Co content. The other is the interactions between the localized d electrons of Co2+, which reduce the transmittance of the doped films in the visible wavelength region. The sp–d and d–d exchanges probably induce intrinsic ferromagnetism in the Co-doped ZnO films. Acknowledgments
Fig. 5. (a) Normalized absorption spectra for the Zn1xCoxO and reference samples of Co, CoO and Co2O3 at the Co K edge. (b) Fourier transformed amplitude at the Co K edge of Zn1xCoxO, Co and CoO, and the Zn K edge of ZnO.
This work was supported by the Shanghai Nanotechnology Promotion Center under Grant no. 0452nm071. The authors are grateful to the X-ray absorption spectroscopy measurements from beam line U7c at National Synchrotron Radiation Laboratory (NSRL) in The University of Science and Technology of China. References
studied here show ferromagnetic behavior well above room temperature. Detailed characterizations on the quality of the as-grown samples indicate that the observed ferromagnetism is intrinsic. The UV–vis results further reveal that sp–d exchange interactions induce intrinsic ferromagnetism in the Co-doped ZnO films. The slight increase in Ms was possibly due to the enhancement of effective ferromagnetic couplings between doped Co2+ which was mediated by free carriers. All the samples studied here were found to be n type with electron concentration above 1019 cm3. This is consistent with what Sato and Lee et al. has found in Codoped ZnO system based on first principle spin-density functional calculation. It was suggested that a high doping level of Co ions was required to achieve ferromagnetism, together with a sufficient supply of electron carriers [5,26]. It should be mentioned that the Ms is smaller than that of Co2+ in a tetrahedral crystal field (3.0mB/Co), which means
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Journal of Crystal Growth 296 (2006) 135–140 www.elsevier.com/locate/jcrysgro
Structural, optical and magnetic properties of Co-doped ZnO films Xue-Chao Liua,b, Er-Wei Shia, Zhi-Zhan Chena,, Hua-Wei Zhanga,b, Li-Xin Songa, Huan Wangc, Shu-De Yaoc a
Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China b Graduate School of the Chinese Academy of Sciences, Beijing 100049, China c Department of Technical Physics, School of Physics, Peking University, Beijing 100871, China Received 9 May 2006; received in revised form 29 July 2006; accepted 22 August 2006 Communicated by S. Uda Available online 10 October 2006
Abstract Zn1xCoxO (x ¼ 0:05, 0.1, 0.15) films were deposited on Si (1 0 0) and quartz glass substrates using inductively coupled plasma enhanced physical vapor deposition system with magnetic confinement. Structural and chemical composition characterizations by X-ray absorption spectroscopy and Rutherford backscattering spectroscopy indicate that Co2+ substitute for Zn2+ in the tetrahedral configuration and form homogeneous films with wurtzite structure when xp0.15. UV–vis transmittance spectra are used to characterize the optical properties. It is found that both transmittance and band gap of Zn1xCoxO films decrease with increasing Co content, owing to the enhancement of sp–d exchange interactions and typical d–d transitions. Magnetic measurements show that all the as-grown films exhibit intrinsic ferromagnetism above room temperature. r 2006 Elsevier B.V. All rights reserved. PACS: 75.50.Pp; 68.55.Jk; 78.20.e Keywords: A1. Optical properties; A1. X-ray absorption spectroscopy; B1. Co-doped ZnO; B2. Diluted magnetic semiconductors
1. Introduction The semiconductor ZnO has gained substantial interests in the research community because of its wide direct band gap (3.3 eV) and large exciton binding energy (60 MeV) which could lead to lasing action based on exciton recombination even above room temperature. It is a wellknown piezoelectric and electro-optic material with potential applications such as opto-electronic and luminescent devices as well as chemical sensors. Even though the research focusing on ZnO goes back to several decades, one of the renewed interests is fueled by the observation of ferromagnetism in transition metals doped ZnO which is called diluted magnetic semiconductor (DMS) [1]. DMS has inspired a great deal of research interest in the field of ‘‘spintronics’’, which could pave the way to exploit spin as well as charge in semiconductor devices. The main Corresponding author. Tel.: +86 21 52411109; fax: +86 21 52413903.
E-mail address: [email protected] (Z.-Z. Chen). 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.08.034
challenge for practical application of the DMS materials is the attainment of intrinsic ferromagnetism with Curie temperature (TC) at or preferably above room temperature [2,3]. Much attention has been paid to the magnetic properties of ZnO-based DMS materials due to the theoretical prediction by Dietl [4] and Sato [5]. For Codoped ZnO system, one of the most important questions is whether Co atoms substitute for Zn2+ in the ZnO lattice or form the secondary clustering phases in the matrix. It is still controversial that whether the observed ferromagnetism and optical properties are intrinsic or comes from secondary phase. The solubility of Co element in ZnO is quite different in the references. Ueda et al. [6] found that the solubility limit of Co ions in ZnO could reach 50 at%. Lee et al. [7] got high-temperature ferromagnetism in Codoped ZnO films by the sol–gel method. No secondary phase was detected until x40.25. Park et al [8]. prepared Zn1xCoxO films by Rf magnetron sputtering method. Cometal clusters, which might be responsible for the ferromagnetism, were found by extended X-ray absorption
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fine structure (EXAFS) measurement when x40.12. Kim found that the secondary phases, such as Co clusters or cobalt oxides, were sensitive to the growth conditions like oxygen pressure, substrate temperature, heat treatment, etc. It was reported that Co can be dissolved in ZnO over 40 at% under optimum growth conditions [9]. It seems that the solubility of Co element in ZnO depends on the fabricating method and preparing conditions. Small precipitates, especially in micro- or nano-scaled magnitude, are difficult to be detected by using general techniques. This increases the difficulty in clarifying intrinsic properties of Co-doped ZnO. In this paper, the structural, optical and magnetic properties of Zn1xCoxO films were characterized in detail. The characterizations revealed an intrinsic nature between properties and structure in Co-doped ZnO films. 2. Experimental details Zn1xCoxO (x ¼ 0:05, 0.1, 0.15) films were deposited on Si (1 0 0) and quartz glass substrates by inductively coupled plasma enhanced physical vapor deposition (ICP-PVD) system with magnetic confinement. The films grown on quartz glass substrates were used to measure the transmittance spectra. The novel ICP-PVD with magnetic confinement system has been introduced in detail in Ref. [10]. The sputtering targets were fabricated by standard solid-state reaction method. In brief, prescribed amounts of ZnO (99.99%) and CoO (99.99%) were mixed and sintered at 1273 K for 24 h in air [11]. Plasma of Ar was used for sputtering and the DC sputtering power was maintained at 100 W with RF coil net power of 25 W. The substrate temperature was kept at 673 K when sputtering. The base and sputtering pressures were 5 104 and 0.5 Pa, respectively. The distance from target to substrate was 100 mm. The structure and crystal orientation of the as-prepared films were determined by X-ray diffraction (XRD, D/ MAX-2550 V) using monochromic Cu Ka radiation ˚ The chemical composition and thickness (l ¼ 1:5406 A). of the films were investigated by the RBS with 4He+ particles of 1.57 MeV at a scattering angle of 167.81. The simulated fit was obtained by the RUMP software. The Xray absorption spectroscopy of near-edge structure (XANES) and extended fine structure measurements were performed on synchrotron radiation from beam line U7c at the National Synchrotron Radiation Laboratory in The University of Science and Technology of China. Room temperature UV–vis transmittance spectra were performed by Cary 500 UV–Vis-NIR spectrophotometer. A quantum design superconducting quantum interference device (SQUID, MPMS XL-7) magnetometer was used to investigate the magnetic properties. Magnetization vs. magnetic field measurement was performed both at low temperature (5 K) and at room temperature (300 K). Temperature dependence of magnetization was measured in an applied field of 500 Oe upon warming a sample which had been cooled from room temperature to 5 K.
3. Results and discussion The XRD patterns of undoped ZnO and Zn1xCoxO (x ¼ 0:05, 0.1, 0.15) samples are shown in Fig. 1. The patterns show only ZnO (0 0 0 2) reflection peak, which indicate that all the films are in c-axis orientation. No evidence for impurity phases is observed in the range of x ¼ 020:15. Compared with the undoped ZnO, the (0 0 0 2) diffraction reflections of Zn1xCoxO trivially move to higher 2y angles with increasing Co contents. Lim [12] and Risbud [13] got similar results in Co-doped ZnO films and polycrystalline powders. This shift is consistent with the fact that the radius of Zn2+ (0.60 A˚) is a little bigger than that of Co2+ (0.58 A˚) in their tetrahedral coordination [13,14]. This result also reveals that there is no divalent octahedral cobalt (i.e. Co2+ in the CoO) that induces a significant moving to the lower angles of (0 0 0 2) diffraction peak due to its larger ionic radius between 0.65 A˚ (low spin) and 0.745 A˚ (high spin) [15]. The XRD patterns indicate that the Co2+ ions systematically substitute for the Zn2+ ions in the films without changing the wurtzite structure. RBS measurement was used to determine the composition and in-depth distribution of the Co element in the ZnCoO layers. Fig. 2 shows the experimental and simulated RBS results for a typical Zn0.85Co0.15O film deposited on Si (1 0 0) substrate. The thickness D of the layer is about 300 nm. The experimental data and simulated fit are shown in Fig. 2(a). Fig. 2(b) shows the separate spectra of Co and Zn from the simulated RBS results. The Co content in the Zn1xCoxO films was estimated to be about 5.5%, 11% and 17% from a quantitative fit for x ¼ 0:05, 0.1 and 0.15, respectively. The percentages of Co in the respective films are close to the nominal values in the ceramic targets.
Fig. 1. X-ray diffraction patterns of ZnO and Zn1xCoxO films on Si (1 0 0) substrate. The (0 0 0 2) diffraction reflections slightly move to higher 2y angles with increasing x.
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Fig. 3. (a) Room temperature optical transmittance spectra of ZnO and Zn1xCoxO films deposited on quartz glass substrates. (b) a2 vs. hu curves measured at room temperature for ZnO and Zn1xCoxO films. The absorption edge shifts toward lower energy with increasing Co contents. The inset shows the relationship between band gap energy and Co contents.
Fig. 2. (a) Random (o) and simulated (solid line) RBS spectra of Zn0.85Co0.15O/Si (1 0 0) sample. (b) Separate spectra of Co and Zn from the simulated RBS results.
Pure ZnO film is transparent and colorless, whereas the as-grown Co-doped ZnO films are less transparent with slight green color. Fig. 3(a) shows the transmittance spectra of ZnO and Zn1xCoxO films deposited on quartz glass substrates. The excitonic nature is clearly apparent in both ZnO and Zn1xCoxO samples. Because the exciton binding energy is almost the same as that of ZnO (E60 MeV) in the Zn1xCoxO films, the exciton peak remains present for all alloy compositions with increasing Co concentrations. The oscillations in the transmittance spectra are caused by multi-reflections at the film–air and film–substrate interfaces. The ZnO film shows an average transmittance above 85% and a sharp absorption edge at 380 nm (3.27 eV). However, the transparency of Zn1xCoxO films fades away
(about 50% for Zn0.85Co0.15O) when the Co contents increase. The darkening of the green color is assigned as typical d–d transitions of high spin states Co2+ 3d7 (4F) in a tetrahedral oxygen coordination. In its neutral charge state, the Co2+ ions has an [Ar]3d7 electron configuration. The atomic 4F ground state splits under the influence of the tetrahedral component of the crystal field into a 4A2 ground state and 4T2+4T1 excited states. The smaller trigonal distortion and spin–orbit interaction split the ground 4A2 state into E1/2+E3/2 [16]. The absorption around 660, 615, and 560 nm in the visible range was derived from separately 4A2(4F)-2E(2G), 4A2(4F)4 T1(4P), and 4A2(4F)-4A1(4G) transitions of tetrahedrally coordinated Co2+ [17,18]. These absorptions are ascribed to the charge-transfer transitions between donor and acceptor ionization levels presumably located within the band gap of the host ZnO as verified by result of ZnCoS and ZnCoSe from both experiments and hypothetical
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calculations [19]. Besides the lower transparency, the absorption edges of Zn1xCoxO films are red shifted with increasing Co content. It is well known that the band gap Eg of ZnO can be tuned over a large energy range by doping Cd2+ and Mg2+ [20,21]. In order to evaluate the Eg of Co-doped ZnO films, the characteristics of a2 vs. hu were plotted as shown in Fig. 3(b), where a is the absorption coefficient and hu the photon energy. We can calculate a from the transmittance T using the equation a ¼ ð1=DÞ ln T. On the other hand, a can be expressed as a ¼ Aðhv E g Þ1=2 for allowed direct transitions at a given photon energy hu, where A is a function of the refractive index and hole/electron effective masses. The inset in Fig. 3(b) shows the relationship between band gap energy and Co content. As can be seen clearly, Eg decreases from 3.27 to 2.88 eV with increasing Co content from x ¼ 0 to x ¼ 0:15. The red shift of Eg with Co doping has already been observed and explained due to sp–d exchange interactions between the band electrons in ZnO and the localized d electrons of the Co2+ [22,23]. Magnetization measurements showed a distinct ferromagnetic behavior. Fig. 4(a) shows the magnetization versus magnetic field (M–H) curve of the Zn0.95Co0.05O film. The M–H curve shows obvious hysteresis loop with the coercive field Hc of 200 Oe at 5 K. When the temperature was increased to 300 K, the M–H curve still exhibits hysteresis loop with the Hc of 100 Oe as shown in the left inset of Fig. 4(a). The temperature dependence of magnetization for Zn0.95Co0.05O film is shown in the right inset of Fig. 4(a). The abrupt increase of magnetization, which corresponds to the TC, appears above 375 K. It is hard to determine the exact value, since the value of TC is rather high, exceeding the range of the measuring equipment. For x ¼ 0:1 and 0.15 samples, ferromagnetic behavior was also observed at room temperature as shown in Fig. 4(b). The HC are 135 and 145 Oe, respectively. The Co content dependence of saturation magnetization Ms is shown in the inset of Fig. 4(b). The Ms slightly increases with increasing Co content. However, it is smaller than that of Co2+ in a tetrahedral crystal field (3.0mB/Co). Whether the observed optical and magnetic properties are intrinsic nature of Co-doped ZnO, we need to know the local structure of Co element in ZnO. Since X-ray absorption spectroscopy is a spectroscopic tool that provides a ‘‘fingerprint’’ of chemical states and local electronic structure of incorporated atoms in the host compounds even in a dilute concentration, this technique was used to investigate the electronic state of Co element and surrounding environment around Co [24]. The absorption edge of XANES spectra moves to lower energy as the effective valency decreases [25]. Fig. 5(a) shows the XANES at the Co K edge of Zn1xCoxO and reference samples of Co, CoO and Co2O3. The XANES spectra of Zn1xCoxO is marked different from those of Co and Co2O3, particularly in the pre-edge feature, which is due to the transition to bound state. However, it has a close proximity to the CoO reference. The XANES reveals that
Fig. 4. (a) Magnetization hysteresis curves of Zn0.95Co0.05O film at 5 and 300 K. The left inset shows magnified portion of the curve at 300 K. The right inset shows magnetization vs. temperature curve measured in an applied field of 500 Oe. (b) Magnetization hysteresis curves of Zn0.90Co0.10O and Zn0.85Co0.15O films at 300 K. The inset shows the Co content dependence of saturation magnetization.
the Co elements dissolve into ZnO and substitute for Zn2+ ions in the valence of +2 state in the Zn1xCoxO films. Fig. 5(b) shows the radial distribution functions (RDFs) results from typical Fourier transformed amplitude of EXAFS spectra. The Co K edge of Zn1xCoxO and reference sample together with the Zn K edge of ZnO are shown. Note that the first and second major peaks at the Co K edge correspond to the nearest oxygen and Zn (or Co), respectively, as viewed from a specific Co atom. For x ¼ 0:05, 0.1 and 0.15, the interatomic distances of the two major peaks of the Co K edge RDF resemble those of ZnO as viewed from a specific Zn atom. This implies that the Co2+ substitute for Zn2+ without forming secondary phases. As discussed above, we can conclude that the lower transmittance and red shift of Eg in Co-doped ZnO films are the intrinsic behaviors of Co2+ substituting for Zn2+ in the tetrahedral configuration. All the Co-doped ZnO films
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not all of the doped Co2+ ions are induced to ferromagnetic couplings. The exchange interactions in Co-doped ZnO might be effected by several factors such as electron concentration, defect and crystal orientation etc. Philip [27] and Hsu [28] found that the electron concentration and oxygen vacancy play an important role in oxide-based DMS. Details of the magnetic properties correlated with residual defect and carrier concentration are in progress. 4. Conclusion Highly c-axis orientated Zn1xCoxO films were fabricated by ICP-PVD method. The RBS analyses reveal that the composition and in-depth distribution of Co content in the doped films are homogenous. XRD and detailed structural characterizations using X-ray absorption spectroscopy indicate that Co2+ substitute for Zn2+ in the tetrahedral configuration without forming secondary phases. Two dominant interactions exist in the case of Co2+ substituting for Zn2+, which were confirmed by room-temperature UV–vis spectra. One is the interactions between the d electrons of Co2+ and the s and p electrons of the host ZnO bands, which leads to the red shift of Eg with increasing Co content. The other is the interactions between the localized d electrons of Co2+, which reduce the transmittance of the doped films in the visible wavelength region. The sp–d and d–d exchanges probably induce intrinsic ferromagnetism in the Co-doped ZnO films. Acknowledgments
Fig. 5. (a) Normalized absorption spectra for the Zn1xCoxO and reference samples of Co, CoO and Co2O3 at the Co K edge. (b) Fourier transformed amplitude at the Co K edge of Zn1xCoxO, Co and CoO, and the Zn K edge of ZnO.
This work was supported by the Shanghai Nanotechnology Promotion Center under Grant no. 0452nm071. The authors are grateful to the X-ray absorption spectroscopy measurements from beam line U7c at National Synchrotron Radiation Laboratory (NSRL) in The University of Science and Technology of China. References
studied here show ferromagnetic behavior well above room temperature. Detailed characterizations on the quality of the as-grown samples indicate that the observed ferromagnetism is intrinsic. The UV–vis results further reveal that sp–d exchange interactions induce intrinsic ferromagnetism in the Co-doped ZnO films. The slight increase in Ms was possibly due to the enhancement of effective ferromagnetic couplings between doped Co2+ which was mediated by free carriers. All the samples studied here were found to be n type with electron concentration above 1019 cm3. This is consistent with what Sato and Lee et al. has found in Codoped ZnO system based on first principle spin-density functional calculation. It was suggested that a high doping level of Co ions was required to achieve ferromagnetism, together with a sufficient supply of electron carriers [5,26]. It should be mentioned that the Ms is smaller than that of Co2+ in a tetrahedral crystal field (3.0mB/Co), which means
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