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Influence of preparation condition and doping concentration of Fe-doped ZnO thin films: Oxygen-vacancy related room temperature ferromagnetism
Thin Solid Films 519 (2011) 6624–6628
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Influence of preparation condition and doping concentration of Fe-doped ZnO thin films: Oxygen-vacancy related room temperature ferromagnetism Wei-Guang Zhang, Bin Lu ⁎, Li-Qiang Zhang, Jian-Guo Lu, Min Fang, Ke-Wei Wu, Bing-Hui Zhao, Zhi-Zhen Ye ⁎ State Key Laboratory of Silicon Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, PR China
a r t i c l e
i n f o
Article history: Received 31 July 2010 Received in revised form 29 April 2011 Accepted 29 April 2011 Available online 7 May 2011 Keywords: Diluted magnetic semiconductors Fe-Ga co-doping Oxygen vacancies Zinc oxide Miscocropy
a b s t r a c t Fe-doped and Fe–Ga co-doped ZnO diluted magnetic semiconductor thin films on quartz substrate were studied. Rapid annealing enhanced the ferromagnetism (FM) of the films grown in Ar/O2. All the films grown in Ar are n-type and the carrier concentration could increase significantly when Ga is doped. The state of Fe in the films was investigated exhibiting Fe 3+. Magnetic measurements revealed that room temperature ferromagnetism in the films were doping concentration dependent and would enhance slightly with Ga doping. The origin of the observed FM is interpreted by the overlapping of polarons mediated through oxygen vacancy based on the bound magnetic polaron model. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Diluted magnetic semiconductors (DMSs) have attracted much attention recently for their potential applications in spintronics such as nonvolatile storage, spin-valve transistor and spin light emitting diode [1–3]. On the other hand, ZnO has many attractive points such as its low cost, abundance, being environmentally-friendly and wide range of applications in optoelectronic devices due to its large band gap (3.37 eV) and high exciton binding energy (60 meV). Therefore, ZnO-based DMSs have been considerably studied since Dietl et al. [4] and Sato et al. [5, 6] theoretically predicted that room temperature ferromagnetism (RTFM) of transition metal (TM) doped ZnO films could be achieved. Consequently, a great number of research groups reported the experimental observation of RTFM in TM-doped ZnO. However, the origin of FM in ZnO-based DMSs is still contradictory although considerable amount of expanding data and mechanisms accumulated. Furthermore, Sato et al. [6] and Tiwari et al. [7] claimed that TM doped n-type ZnO would be FM while Buchholz et al. [8] suggested that p-type doping was key to realize FM. Additionally, some precipitates, secondary phase or defects such as oxygen vacancies also play an important role in obtaining FM in ZnO-based DMSs. As for ZnO-based DMSs, Co and Mn doped ZnO have been extensively studied while there are relatively less reports for Fe-doped
ZnO. Actually, besides both theoretical [9] and experimental [10–12] RTFM being reported for ZnFeO, it has good optical [13] and electrical properties as well [14, 15]. Analogically, the origin of FM in Fe-doped ZnO is controversial and confusing likewise. Ahn et al. [16] suggested that it was carrier mediated mechanism while Wu et al. [17] found that the FM of Fe ions implanted in a-plane ZnO epitaxial films was related with oxygen vacancies. Moreover, Shim et al. [18] claimed that the FM of ZnFeCuO bulks stems from the secondary phase. Besides, the influence of doping polarity on the magnetic properties of ZnFeO is also controversial. It was theoretically and experimentally proved that hole doping could stabilize the FM of Fe-doped ZnO [19] while Sato et al. [6] claimed that n-type Fe doped ZnO would be FM. Thus, to shed more light on the origin of FM in ZnFeO thin films, more investigation is needed. In this paper, we prepared a series of Fe-doped and Fe–Ga co-doped ZnO DMS films by RF magnetron sputtering. We firstly studied the influence of annealing on the FM of Zn0.98Fe0.02O film grown in Ar/O2. After that, we would focus on the samples grown in Ar and investigate the doping concentration influence on the FM ordering. Besides, since Ga is an appropriate electron dopant for ZnO [20], we introduced Ga into the ZnFeO films to study the electron doping effect on the magnetic properties. Additionally, the structural, electrical and magnetic properties of the films are investigated and will be discussed in detail. 2. Experimental details
⁎ Corresponding authors. Tel.: + 86 571 87952186; fax: + 86 571 87952625. E-mail addresses: [email protected] (B. Lu), [email protected] (Z.-Z. Ye). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.04.233
Fe-doped ZnO and Fe–Ga codoped ZnO DMS thin films were deposited on quartz substrate by RF magnetron sputtering. The ceramic
W.-G. Zhang et al. / Thin Solid Films 519 (2011) 6624–6628
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To investigate the annealing effect in an oxygen-absent atmosphere, that is, the effect of oxygen vacancy on FM ordering, we firstly studied the RT magnetic properties of Zn0.98Fe0.02O film grown in Ar/ O2 with and without annealing, as shown in Fig. 1. By comparing the two M-H hysteresis loops, it can be seen that annealing plays significant influence on the RTFM of the Zn0.98Fe0.02O film. The inset in Fig. 1 clearly shows that both saturation magnetization (Ms) and coercive field (Hc) increase when the film was rapidly annealed. The value of Hc calculated from M-H hysteresis loop for samples without and with annealing is 2985 A/m and 4640 A/m, respectively.
Readily, during annealing in absence of oxygen, the defects of oxygen vacancies are easily formed, which was indirectly confirmed by Halleffect measurements showing increased carrier concentration and decreased resistivity of the film after annealing. Previously, it was experimentally suggested that the FM of Fe-doped ZnO was related with oxygen vacancies [18, 21]. This explains why annealing enhanced the FM in our study and oxygen defects could be one of the possible origins of FM in Fe-doped ZnO thin film. Following this path, we further prepared samples with argon as the working gas, and all the films discussed below are grown in Ar atmosphere. Fig. 2 shows the XRD patterns of pristine ZnO, Zn0.99Fe0.01O, Zn0.98Fe0.02O and Zn0.97Fe0.02Ga0.01O as-deposited on quartz substrates. It is obvious that all of them show a preferential (002) orientation and there's no other phase such as Fe2O3, Fe3O4 or Fe clusters within the limitation of XRD. Compared to that of the pristine ZnO film, (002) peaks position of the Zn0.99Fe0.01O, Zn0.98Fe0.02O and the Zn0.97Fe0.02Ga0.01O films all shift to higher angle as seen from the inset in Fig. 2, which may indicate that Fe is incorporated into the host wurtzite lattice. This is because of the smaller radius of Fe 3+ (0.64 Å) and Ga 3+ (0.62 Å) compared with Zn2+ (0.74 Å), according to the Vegard law, the (002) peaks shift to larger angle. What should be pointed out is that, the element contents of Fe and Ga in the films were investigated by EDS, and the results were in close agreement with that of the corresponding targets. The SEM image of the Zn0.99Fe0.01O, Zn0.98Fe0.02O and the Zn0.97 Fe0.02Ga0.01O films are shown in Fig. 3a, b and c, respectively. All the films have relatively smooth surface and tightly packed grains. However, the grain size of the film become smaller after Ga was doped, which is consistent with the results of XRD in Fig. 2. It is thus concluded that with Ga co-doping, the crystallinity of the ZnFeO thin films is decreased. Fig. 4 shows the typical HRTEM image and SAED pattern (inset (a)) of the Zn0.98Fe0.02O film. As can be seen, SAED pattern matches well with wurtzite ZnO. According to the TEM investigation in combination with complementary XRD results, we may carefully infer that there was no (or negligible) observation for the presence of Fe cluster or secondary phase in the overall of films. The spacing between adjacent lattice planes is about 0.257 nm which agrees with the distance of (002) planes but is slightly smaller than that of pristine ZnO, indicating that Fe have doped into the lattice of ZnO. Inset (b) presents the enlarged image of the selected rectangular area, in which, partial dislocations associated with nano-sized stacking faults can be seen as marked. These defects are expected to form via the
Fig. 1. Magnetization hysteresis curves at 300 K of Zn0.98Fe0.02O thin film grown in Ar/ O2 with and without annealing. The inset displays a zoomed part of M-H loops taken in low fields.
Fig. 2. XRD pattern of referenced pristine ZnO, ZnFeO and Zn(Fe, Ga)O films. The Fe and Fe–Ga doped ZnO films are grown in Ar. The inset of the figure shows a comparison of (002) reflections of the Fe-doped ZnO, Fe–Ga co-doped ZnO and the referenced pristine ZnO.
targets were prepared by mixing of high-purity (99.99%) ZnO, Fe2O3 and Ga2O3 powders and agate balls with ethanol milled for 24 h and further dried at 50 °C for 24 h. After that, mold the powder into the pellet with a diameter of 4 cm. In the end, the pellets were sintered at 1200 °C for 4 h. The cooling process was operated in the air. We made two ZnFeO targets (the concentration of Fe are 1 at.% and 2 at.%, respectively), and a Zn (Fe,Ga)O target (2 at.% Fe and 1 at.% Ga). The base pressure of the chamber was 1 × 10− 4 Pa and then ambient gas (Ar/O2 or Ar) with a working pressure of 1 Pa was introduced into it. The substrate was held at 500 °C during deposition. The deposition duration was 0.5 h and a film about 500 nm-thick was obtained. As for sample grown in Ar/O2 (the ratio of Ar to O2 is 10 to 1), a rapid post thermal annealing was performed at 600 °C in N2 for 2 min. The crystallographic structure of the as-grown films was characterized by a Bede D1 X-ray diffraction (XRD) system with a Cu Kα (λ = 0.15406 nm) radiation. The electrical properties were investigated by using a four-point probe van der Pauw configuration (HL5500PC) at room temperature. The surface morphology and microstructure were characterized by field-emission scanning electron microscopy (FE-SEM). Energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) with Mg Kα monochromatic source were employed to study the elements present and the valence state of Fe in the ZnO films. High-resolution transmission electron microscopy (HRTEM) imaging and selected area electron diffraction (SAED) were used to study the structural characteristics. The magnetization studies were carried out using a superconducting quantum interference device (SQUID) magnetometer. Inductively-coupled-plasma (ICP) atomic emission spectra were used to determine the Fe contents in the films after measuring the magnetic properties in order to calculate the average magnetic moment per Fe atom. 3. Results and discussion
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condensation of oxygen vacancies or zinc interstitials as previously suggested in phosphorus doped ZnO by Sun et al. [22]. The contents and chemical bonding states of Fe and Ga cations in the films were examined by XPS measurements, as shown in Fig. 5. The charge shifted spectra were corrected using the adventitious C 1s photoelectron signal at 285 eV. In our study, the Fe related signal is relatively small in the Zn0.99Fe0.01O film (not shown). As can be seen, there is slight change in both the line shape and peak intensity for the Zn0.98Fe0.02O and the Zn0.97Fe0.02Ga0.01O thin films (see Fig. 5a and b). Detailed investigation of the XPS spectra of Fe 2p showed that Fe 2p3/2 peak of the Zn0.98Fe0.02O is centered at 710.99 eV in binding energy (BE) and the spin-orbit split energy difference between Fe 2p1/2 and Fe 2p3/2 is 13.65 eV, while for the Zn0.97Fe0.02Ga0.01O, it is 710.90 eV and 13.62 eV, respectively. This is reasonable and is attributed to chemical environment variation for Fe in the two films. Readily, these results excluded the possibility of existence of Fe2+ or Fe0 in the films because the energy difference should be 13.4 eV for FeO and 13.10 eV for metallic ion (from XPS handbook (Perkin Elmer)). In order to maintain the charge balance, Fe3+ ions should exist in the form of FeZn–O–VZn complex [12]. Moreover, the occasionally observed defects and dislocations formed by VO and VZn in the HRTEM image (Fig. 4) could partially reduce the strain induced by doping. From Fig. 5c, it is clear that Ga has been doped into the ZnFeO thin film. As a result, Fe ions evidenced a +3 state in the films and possibly occupy the Zn site without forming any detectable impurity phase, such as Fe metal cluster, FeO and Fe3O4. The Hall-effect measurement showed that the Zn0.99Fe0.01O, Zn0.98Fe0.02O films are n-type with a carrier concentration of
Fig. 3. SEM images of the Zn0.99Fe0.01O (a), Zn0.98Fe0.02O (b) and the Zn0.97Fe0.02Ga0.01O (c) films grown in Ar atmosphere.
Fig. 4. HRTEM image of the Zn0.98Fe0.02O film deposited on quartz substrate. Inset (a) indicates the SAED pattern of the film with the electron beam along the [0001] direction. Inset (b) is the enlarged image of the selected area, showing defects and partial dislocations as marked. Spacing between adjacent lattice planes perpendicular to the surface is measured as 0.257 nm.
Fig. 5. XPS spectra of Fe 2p for the Zn0.98Fe0.02O (a) and the Zn0.97Fe0.02Ga0.01O films (b). The spin-orbit split difference energy between Fe 2p3/2 and 2p1/2 is given. (c) Ga 3d spectra of the Zn0.97Fe0.02Ga0.01O film (the black dotted line: experimental; the red line: the fitted O 2p; the green line: the fitted Ga 3d).
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vacancy defect constituted BMPs are a promising candidate for the origin of RTFM in this system. Within the BMP model, the higher density of oxygen vacancy yields a greater overall volume occupied by BMPs, thus increasing their probability of overlapping more Fe ions into ferromagnetic domains and enhancing ferromagnetism, as evidenced by increased saturation magnetization for the Zn0.98Fe0.02O film grown in Ar than that in Ar/O2. Furthermore, as for Fe-Ga co-doped ZnO film, this model can be applied as well. Ga as a donor dopant in ZnFeO films, formed shallow donor level and generated electron carriers which will occupy Fe 3d states. These electrons will take part in exchange interaction between two polarons and lower the kinetic energy of the ferromagnetic state as suggested by Sato et al. [6]. This could explain why the RTFM was enhanced when Ga was doped in ZnFeO films. The observed intrinsic ferromagnetism makes these DMSs potential for future spintronic devices. Fig. 6. Magnetization hysteresis curves at RT of as-deposited the Zn0.99Fe0.01O, Zn0.98Fe0.02O and the Zn0.97Fe0.02Ga0.01O film.
1015–1016 cm− 3. When 1 at.% Ga was doped into the Zn0.98Fe0.02O film, the carrier concentration increased rapidly and could reach above 1019 cm− 3. Meanwhile, the resistivity of the Zn0.97Fe0.02Ga0.01O film decreased. Thus, the results indicated that the Ga atoms are substituted into the ZnO lattice as dopants, generating free carriers when the Ga is doped, as previously reported in ZnO [20, 23]. Fig. 6 shows the magnetic hysteresis loops of the Zn0.99Fe0.01O, Zn0.98Fe0.02O and the Zn0.97Fe0.02Ga0.01O films at room temperature (~300 K). The average magnetic moment per Fe atom is gained in terms of SQUID measurement and ICP atomic emission spectra, and the diamagnetic magnetization of the substrate were subtracted. Without considering the error of ICP measurement, the net saturation magnetization of the Zn0.99Fe0.01O, Zn0.98Fe0.02O and the Zn0.97Fe0.02Ga0.01O is calculated as 0:51; 0:90; and 1:06μB = FeionðμB : BohrmagnetonÞ respectively. Compared to the Zn0.98Fe0.02O film grown in Ar/O2 as shown in Fig. 1, it can be readily found that the saturation magnetization increased significantly when Ar was made as the working gas, verifying our speculation that oxygen-absent atmosphere is favorable to increase magnetic moment in Fe-doped ZnO DMS and will be discussed detailedly below. One may note that the ferromagnetic moment of the Zn0.98Fe0.02O is nearly twice that of the Zn0.99Fe0.01O film from Fig. 6. This is because the higher the Fe content, the stronger is the exchange interaction between Fe ions due to less distance with assumption that Fe ions are doped uniformly. However, further increase in Fe content beyond 2 at.% will lead to formation of secondary phase, for instance, ZnFe2O4 as previously reported by Wang et al. [24], and should be avoided. Additionally, it is evidenced that gallium doping in our study enhanced the magnetic moment of the ZnFeO film. The origin of observed ferromagnetism at RT in these films could arise from a number of possibilities. As we have discussed, the existence of secondary phase such as Fe metal, FeO and Fe3O4 in the films have been ruled out. Therefore, ferromagnetism is expected to arise from the intrinsic exchange interaction of magnetic moments in doped films. In addition to the magnetic doping effect, oxygen vacancy (Vo) defects have been suggested to play an important role in high temperature ferromagnetic origin for TM-doped ZnO DMSs [25, 26]. That is, by formation of bound magnetic polarons (BMPs), which include electrons locally trapped by oxygen vacancy, with the trapped electron occupying an orbital overlapping with the d shells of TM neighbors, leading to ferromagnetic ordering. In our study, oxygen vacancies are easily generated in ZnO films during the films growth in Ar owing to an oxygen-absent environment. Therefore, we propose that oxygen-
4. Conclusion In summary, we have investigated the Zn1 − xFexO (×= 0.01, 002) and the Zn(Fe, Ga)O films by XRD, HRTEM and XPS, the results revealed that Fe replaced Zn within ZnO lattice exhibiting Fe3+. Magnetic measurements revealed that room temperature ferromagnetism in the ZnFeO films were doping concentration dependent and could be improved when grown in oxygen-absent atmosphere. Both the electron concentration and the ferromagnetism of the Zn0.98Fe0.02O were enhanced when Ga was doped. The origin of the observed FM is related with oxygen vacancies and the exact mechanism is discussed on the basis of the BMP model with overlapping of polarons. Acknowledgments This work was supported by National Natural Science Foundation of China under grant no. 51002134, Fundamental Research Funds for the Central Universities under grant no. 2010QNA4002, Cultivation Fund of the Key Scientific and Technical Innovation Project, Ministry of Education of China under grant no. 707035, and was also sponsored by SRF for ROCS, SEM under grant no. J20091215. References [1] H. Ohno, Science 281 (1998) 951. [2] S.W. Jung, S.-J. An, G.-C. Yi, C.U. Jung, S.-I. Lee, S. Cho, Appl Phys Lett 80 (2002) 4561. [3] S.J. Pearton, D.P. Norton, K. Ip, Y.W. Heo, T. Steiner, J Vac Sci Technol B 22 (2004) 932. [4] T. Dietal, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) 1019. [5] K. Sato, H. Katayama-Yoshida, Jpn J Appl Phys 39 (2000) L555. [6] K. Sato, H. Katayama-Yoshida, Jpn J Appl Phys 40 (2001) L334. [7] A. Tiwari, M. Snure, D. Kumar, J.T. Abiade, Appl Phys Lett 92 (2008) 062509. [8] D.B. Buchholz, R.P.H. Chang, J.H. Song, J.B. Ketterson, Appl Phys Lett 87 (2005) 082504. [9] A. Debernardi, M. Fanciulli, Appl Phys Lett 90 (2007) 212510. [10] X.X. Wei, C. Song, K.W. Geng, F. Zeng, B. He, F. Pan, J Phys Condens Matter 18 (2006) 1747. [11] S. Karamat, C. Ke, T.L. Tan, W. Zhou, P. Lee, R.S. Rawat, Appl Surf Sci 255 (2009) 4814. [12] G. Weyer, H.P. Gunnlaugsson, R. Mantowan, M. Fanciulli, D. Naidoo, K. BaharuthRam, T. Agne, J Appl Physi 102 (2007) 113925. [13] Z.C. Chen, L.J. Zhuge, X.M. Wu, Y.D. Meng, Thin Solid Films 515 (2007) 5462. [14] K.J. Kim, Y.R. Park, J Appl Physi 96 (2000) 8. [15] L.M. Wang, J.W. Liao, Z.A. Peng, J.H. Lai, J Electrochem Soc 156 (2) (2009) H138. [16] G.Y. Ahn, S.I. Park, C.S. Kim, Phys Status Solidi A 204 (2007) 4037. [17] P. Wu, G. Saraf, Y. Lu, D.H. Hill, R. Gateau, L. Wielunski, R.A. Bartynski, D.A. Arena, J. Dvorak, A. Moodenbaugh, T. Siegrest, J.A. Raley, Y.K. Yeo, Appl Phys Lett 89 (2006) 012508. [18] J.H. Shim, T. Hwang, S. Lee, J.H. Park, S.J. Han, Y.H. Jeong, Appl Phys Lett 86 (2005) 082503. [19] 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, Phys Rev B 75 (2007) 144404. [20] Q.B. Ma, Z.Z. Ye, H.P. He, L.P. Zhu, W.C. Liu, Y.F. Yang, L. Gong, J.Y. Huang, Y.Z. Zhang, B.H. Zhao, J Phys D Appl Phys 41 (2008) 055302. [21] N.H. Hong, J Magn Magn Mater 303 (2006) 338.
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[22] H.P. Sun, X.Q. Pan, X.L. Du, Z.X. Mei, Z.Q. Feng, Q.X. Xue, Appl Phys Lett 85 (2004) 4385. [23] Z.L. Lu, H.S. Hsu, Y.H. Tzeng, F.M. Zhang, Y.W. Du, J.C.A. Huang, Appl Phys Lett 95 (2009) 062509. [24] Y.Q. Wang, S.L. Yuan, L. Liu, P. Li, X.X. Lan, Z.M. Tian, J.H. He, S.Y. Yin, J Magn Magn Mater 320 (2004) 1423.
[25] D.C. Kundaliya, S.B. Ogale, S.E. Lofland, S. Dhar, C.J. Metting, S.R. Shinde, Z. Ma, B. Varughese, K.V. Ramanujachary, L. Salamanca-Riba, T. Venkatesan, Nat Mater 3 (2004) 709. [26] J.M.D. Coey, M. Venkatesan, C.B. Fitzggerald, Nat Mater 4 (2005) 173.
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Influence of preparation condition and doping concentration of Fe-doped ZnO thin films: Oxygen-vacancy related room temperature ferromagnetism Wei-Guang Zhang, Bin Lu ⁎, Li-Qiang Zhang, Jian-Guo Lu, Min Fang, Ke-Wei Wu, Bing-Hui Zhao, Zhi-Zhen Ye ⁎ State Key Laboratory of Silicon Materials, Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, PR China
a r t i c l e
i n f o
Article history: Received 31 July 2010 Received in revised form 29 April 2011 Accepted 29 April 2011 Available online 7 May 2011 Keywords: Diluted magnetic semiconductors Fe-Ga co-doping Oxygen vacancies Zinc oxide Miscocropy
a b s t r a c t Fe-doped and Fe–Ga co-doped ZnO diluted magnetic semiconductor thin films on quartz substrate were studied. Rapid annealing enhanced the ferromagnetism (FM) of the films grown in Ar/O2. All the films grown in Ar are n-type and the carrier concentration could increase significantly when Ga is doped. The state of Fe in the films was investigated exhibiting Fe 3+. Magnetic measurements revealed that room temperature ferromagnetism in the films were doping concentration dependent and would enhance slightly with Ga doping. The origin of the observed FM is interpreted by the overlapping of polarons mediated through oxygen vacancy based on the bound magnetic polaron model. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Diluted magnetic semiconductors (DMSs) have attracted much attention recently for their potential applications in spintronics such as nonvolatile storage, spin-valve transistor and spin light emitting diode [1–3]. On the other hand, ZnO has many attractive points such as its low cost, abundance, being environmentally-friendly and wide range of applications in optoelectronic devices due to its large band gap (3.37 eV) and high exciton binding energy (60 meV). Therefore, ZnO-based DMSs have been considerably studied since Dietl et al. [4] and Sato et al. [5, 6] theoretically predicted that room temperature ferromagnetism (RTFM) of transition metal (TM) doped ZnO films could be achieved. Consequently, a great number of research groups reported the experimental observation of RTFM in TM-doped ZnO. However, the origin of FM in ZnO-based DMSs is still contradictory although considerable amount of expanding data and mechanisms accumulated. Furthermore, Sato et al. [6] and Tiwari et al. [7] claimed that TM doped n-type ZnO would be FM while Buchholz et al. [8] suggested that p-type doping was key to realize FM. Additionally, some precipitates, secondary phase or defects such as oxygen vacancies also play an important role in obtaining FM in ZnO-based DMSs. As for ZnO-based DMSs, Co and Mn doped ZnO have been extensively studied while there are relatively less reports for Fe-doped
ZnO. Actually, besides both theoretical [9] and experimental [10–12] RTFM being reported for ZnFeO, it has good optical [13] and electrical properties as well [14, 15]. Analogically, the origin of FM in Fe-doped ZnO is controversial and confusing likewise. Ahn et al. [16] suggested that it was carrier mediated mechanism while Wu et al. [17] found that the FM of Fe ions implanted in a-plane ZnO epitaxial films was related with oxygen vacancies. Moreover, Shim et al. [18] claimed that the FM of ZnFeCuO bulks stems from the secondary phase. Besides, the influence of doping polarity on the magnetic properties of ZnFeO is also controversial. It was theoretically and experimentally proved that hole doping could stabilize the FM of Fe-doped ZnO [19] while Sato et al. [6] claimed that n-type Fe doped ZnO would be FM. Thus, to shed more light on the origin of FM in ZnFeO thin films, more investigation is needed. In this paper, we prepared a series of Fe-doped and Fe–Ga co-doped ZnO DMS films by RF magnetron sputtering. We firstly studied the influence of annealing on the FM of Zn0.98Fe0.02O film grown in Ar/O2. After that, we would focus on the samples grown in Ar and investigate the doping concentration influence on the FM ordering. Besides, since Ga is an appropriate electron dopant for ZnO [20], we introduced Ga into the ZnFeO films to study the electron doping effect on the magnetic properties. Additionally, the structural, electrical and magnetic properties of the films are investigated and will be discussed in detail. 2. Experimental details
⁎ Corresponding authors. Tel.: + 86 571 87952186; fax: + 86 571 87952625. E-mail addresses: [email protected] (B. Lu), [email protected] (Z.-Z. Ye). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.04.233
Fe-doped ZnO and Fe–Ga codoped ZnO DMS thin films were deposited on quartz substrate by RF magnetron sputtering. The ceramic
W.-G. Zhang et al. / Thin Solid Films 519 (2011) 6624–6628
6625
To investigate the annealing effect in an oxygen-absent atmosphere, that is, the effect of oxygen vacancy on FM ordering, we firstly studied the RT magnetic properties of Zn0.98Fe0.02O film grown in Ar/ O2 with and without annealing, as shown in Fig. 1. By comparing the two M-H hysteresis loops, it can be seen that annealing plays significant influence on the RTFM of the Zn0.98Fe0.02O film. The inset in Fig. 1 clearly shows that both saturation magnetization (Ms) and coercive field (Hc) increase when the film was rapidly annealed. The value of Hc calculated from M-H hysteresis loop for samples without and with annealing is 2985 A/m and 4640 A/m, respectively.
Readily, during annealing in absence of oxygen, the defects of oxygen vacancies are easily formed, which was indirectly confirmed by Halleffect measurements showing increased carrier concentration and decreased resistivity of the film after annealing. Previously, it was experimentally suggested that the FM of Fe-doped ZnO was related with oxygen vacancies [18, 21]. This explains why annealing enhanced the FM in our study and oxygen defects could be one of the possible origins of FM in Fe-doped ZnO thin film. Following this path, we further prepared samples with argon as the working gas, and all the films discussed below are grown in Ar atmosphere. Fig. 2 shows the XRD patterns of pristine ZnO, Zn0.99Fe0.01O, Zn0.98Fe0.02O and Zn0.97Fe0.02Ga0.01O as-deposited on quartz substrates. It is obvious that all of them show a preferential (002) orientation and there's no other phase such as Fe2O3, Fe3O4 or Fe clusters within the limitation of XRD. Compared to that of the pristine ZnO film, (002) peaks position of the Zn0.99Fe0.01O, Zn0.98Fe0.02O and the Zn0.97Fe0.02Ga0.01O films all shift to higher angle as seen from the inset in Fig. 2, which may indicate that Fe is incorporated into the host wurtzite lattice. This is because of the smaller radius of Fe 3+ (0.64 Å) and Ga 3+ (0.62 Å) compared with Zn2+ (0.74 Å), according to the Vegard law, the (002) peaks shift to larger angle. What should be pointed out is that, the element contents of Fe and Ga in the films were investigated by EDS, and the results were in close agreement with that of the corresponding targets. The SEM image of the Zn0.99Fe0.01O, Zn0.98Fe0.02O and the Zn0.97 Fe0.02Ga0.01O films are shown in Fig. 3a, b and c, respectively. All the films have relatively smooth surface and tightly packed grains. However, the grain size of the film become smaller after Ga was doped, which is consistent with the results of XRD in Fig. 2. It is thus concluded that with Ga co-doping, the crystallinity of the ZnFeO thin films is decreased. Fig. 4 shows the typical HRTEM image and SAED pattern (inset (a)) of the Zn0.98Fe0.02O film. As can be seen, SAED pattern matches well with wurtzite ZnO. According to the TEM investigation in combination with complementary XRD results, we may carefully infer that there was no (or negligible) observation for the presence of Fe cluster or secondary phase in the overall of films. The spacing between adjacent lattice planes is about 0.257 nm which agrees with the distance of (002) planes but is slightly smaller than that of pristine ZnO, indicating that Fe have doped into the lattice of ZnO. Inset (b) presents the enlarged image of the selected rectangular area, in which, partial dislocations associated with nano-sized stacking faults can be seen as marked. These defects are expected to form via the
Fig. 1. Magnetization hysteresis curves at 300 K of Zn0.98Fe0.02O thin film grown in Ar/ O2 with and without annealing. The inset displays a zoomed part of M-H loops taken in low fields.
Fig. 2. XRD pattern of referenced pristine ZnO, ZnFeO and Zn(Fe, Ga)O films. The Fe and Fe–Ga doped ZnO films are grown in Ar. The inset of the figure shows a comparison of (002) reflections of the Fe-doped ZnO, Fe–Ga co-doped ZnO and the referenced pristine ZnO.
targets were prepared by mixing of high-purity (99.99%) ZnO, Fe2O3 and Ga2O3 powders and agate balls with ethanol milled for 24 h and further dried at 50 °C for 24 h. After that, mold the powder into the pellet with a diameter of 4 cm. In the end, the pellets were sintered at 1200 °C for 4 h. The cooling process was operated in the air. We made two ZnFeO targets (the concentration of Fe are 1 at.% and 2 at.%, respectively), and a Zn (Fe,Ga)O target (2 at.% Fe and 1 at.% Ga). The base pressure of the chamber was 1 × 10− 4 Pa and then ambient gas (Ar/O2 or Ar) with a working pressure of 1 Pa was introduced into it. The substrate was held at 500 °C during deposition. The deposition duration was 0.5 h and a film about 500 nm-thick was obtained. As for sample grown in Ar/O2 (the ratio of Ar to O2 is 10 to 1), a rapid post thermal annealing was performed at 600 °C in N2 for 2 min. The crystallographic structure of the as-grown films was characterized by a Bede D1 X-ray diffraction (XRD) system with a Cu Kα (λ = 0.15406 nm) radiation. The electrical properties were investigated by using a four-point probe van der Pauw configuration (HL5500PC) at room temperature. The surface morphology and microstructure were characterized by field-emission scanning electron microscopy (FE-SEM). Energy-dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) with Mg Kα monochromatic source were employed to study the elements present and the valence state of Fe in the ZnO films. High-resolution transmission electron microscopy (HRTEM) imaging and selected area electron diffraction (SAED) were used to study the structural characteristics. The magnetization studies were carried out using a superconducting quantum interference device (SQUID) magnetometer. Inductively-coupled-plasma (ICP) atomic emission spectra were used to determine the Fe contents in the films after measuring the magnetic properties in order to calculate the average magnetic moment per Fe atom. 3. Results and discussion
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condensation of oxygen vacancies or zinc interstitials as previously suggested in phosphorus doped ZnO by Sun et al. [22]. The contents and chemical bonding states of Fe and Ga cations in the films were examined by XPS measurements, as shown in Fig. 5. The charge shifted spectra were corrected using the adventitious C 1s photoelectron signal at 285 eV. In our study, the Fe related signal is relatively small in the Zn0.99Fe0.01O film (not shown). As can be seen, there is slight change in both the line shape and peak intensity for the Zn0.98Fe0.02O and the Zn0.97Fe0.02Ga0.01O thin films (see Fig. 5a and b). Detailed investigation of the XPS spectra of Fe 2p showed that Fe 2p3/2 peak of the Zn0.98Fe0.02O is centered at 710.99 eV in binding energy (BE) and the spin-orbit split energy difference between Fe 2p1/2 and Fe 2p3/2 is 13.65 eV, while for the Zn0.97Fe0.02Ga0.01O, it is 710.90 eV and 13.62 eV, respectively. This is reasonable and is attributed to chemical environment variation for Fe in the two films. Readily, these results excluded the possibility of existence of Fe2+ or Fe0 in the films because the energy difference should be 13.4 eV for FeO and 13.10 eV for metallic ion (from XPS handbook (Perkin Elmer)). In order to maintain the charge balance, Fe3+ ions should exist in the form of FeZn–O–VZn complex [12]. Moreover, the occasionally observed defects and dislocations formed by VO and VZn in the HRTEM image (Fig. 4) could partially reduce the strain induced by doping. From Fig. 5c, it is clear that Ga has been doped into the ZnFeO thin film. As a result, Fe ions evidenced a +3 state in the films and possibly occupy the Zn site without forming any detectable impurity phase, such as Fe metal cluster, FeO and Fe3O4. The Hall-effect measurement showed that the Zn0.99Fe0.01O, Zn0.98Fe0.02O films are n-type with a carrier concentration of
Fig. 3. SEM images of the Zn0.99Fe0.01O (a), Zn0.98Fe0.02O (b) and the Zn0.97Fe0.02Ga0.01O (c) films grown in Ar atmosphere.
Fig. 4. HRTEM image of the Zn0.98Fe0.02O film deposited on quartz substrate. Inset (a) indicates the SAED pattern of the film with the electron beam along the [0001] direction. Inset (b) is the enlarged image of the selected area, showing defects and partial dislocations as marked. Spacing between adjacent lattice planes perpendicular to the surface is measured as 0.257 nm.
Fig. 5. XPS spectra of Fe 2p for the Zn0.98Fe0.02O (a) and the Zn0.97Fe0.02Ga0.01O films (b). The spin-orbit split difference energy between Fe 2p3/2 and 2p1/2 is given. (c) Ga 3d spectra of the Zn0.97Fe0.02Ga0.01O film (the black dotted line: experimental; the red line: the fitted O 2p; the green line: the fitted Ga 3d).
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vacancy defect constituted BMPs are a promising candidate for the origin of RTFM in this system. Within the BMP model, the higher density of oxygen vacancy yields a greater overall volume occupied by BMPs, thus increasing their probability of overlapping more Fe ions into ferromagnetic domains and enhancing ferromagnetism, as evidenced by increased saturation magnetization for the Zn0.98Fe0.02O film grown in Ar than that in Ar/O2. Furthermore, as for Fe-Ga co-doped ZnO film, this model can be applied as well. Ga as a donor dopant in ZnFeO films, formed shallow donor level and generated electron carriers which will occupy Fe 3d states. These electrons will take part in exchange interaction between two polarons and lower the kinetic energy of the ferromagnetic state as suggested by Sato et al. [6]. This could explain why the RTFM was enhanced when Ga was doped in ZnFeO films. The observed intrinsic ferromagnetism makes these DMSs potential for future spintronic devices. Fig. 6. Magnetization hysteresis curves at RT of as-deposited the Zn0.99Fe0.01O, Zn0.98Fe0.02O and the Zn0.97Fe0.02Ga0.01O film.
1015–1016 cm− 3. When 1 at.% Ga was doped into the Zn0.98Fe0.02O film, the carrier concentration increased rapidly and could reach above 1019 cm− 3. Meanwhile, the resistivity of the Zn0.97Fe0.02Ga0.01O film decreased. Thus, the results indicated that the Ga atoms are substituted into the ZnO lattice as dopants, generating free carriers when the Ga is doped, as previously reported in ZnO [20, 23]. Fig. 6 shows the magnetic hysteresis loops of the Zn0.99Fe0.01O, Zn0.98Fe0.02O and the Zn0.97Fe0.02Ga0.01O films at room temperature (~300 K). The average magnetic moment per Fe atom is gained in terms of SQUID measurement and ICP atomic emission spectra, and the diamagnetic magnetization of the substrate were subtracted. Without considering the error of ICP measurement, the net saturation magnetization of the Zn0.99Fe0.01O, Zn0.98Fe0.02O and the Zn0.97Fe0.02Ga0.01O is calculated as 0:51; 0:90; and 1:06μB = FeionðμB : BohrmagnetonÞ respectively. Compared to the Zn0.98Fe0.02O film grown in Ar/O2 as shown in Fig. 1, it can be readily found that the saturation magnetization increased significantly when Ar was made as the working gas, verifying our speculation that oxygen-absent atmosphere is favorable to increase magnetic moment in Fe-doped ZnO DMS and will be discussed detailedly below. One may note that the ferromagnetic moment of the Zn0.98Fe0.02O is nearly twice that of the Zn0.99Fe0.01O film from Fig. 6. This is because the higher the Fe content, the stronger is the exchange interaction between Fe ions due to less distance with assumption that Fe ions are doped uniformly. However, further increase in Fe content beyond 2 at.% will lead to formation of secondary phase, for instance, ZnFe2O4 as previously reported by Wang et al. [24], and should be avoided. Additionally, it is evidenced that gallium doping in our study enhanced the magnetic moment of the ZnFeO film. The origin of observed ferromagnetism at RT in these films could arise from a number of possibilities. As we have discussed, the existence of secondary phase such as Fe metal, FeO and Fe3O4 in the films have been ruled out. Therefore, ferromagnetism is expected to arise from the intrinsic exchange interaction of magnetic moments in doped films. In addition to the magnetic doping effect, oxygen vacancy (Vo) defects have been suggested to play an important role in high temperature ferromagnetic origin for TM-doped ZnO DMSs [25, 26]. That is, by formation of bound magnetic polarons (BMPs), which include electrons locally trapped by oxygen vacancy, with the trapped electron occupying an orbital overlapping with the d shells of TM neighbors, leading to ferromagnetic ordering. In our study, oxygen vacancies are easily generated in ZnO films during the films growth in Ar owing to an oxygen-absent environment. Therefore, we propose that oxygen-
4. Conclusion In summary, we have investigated the Zn1 − xFexO (×= 0.01, 002) and the Zn(Fe, Ga)O films by XRD, HRTEM and XPS, the results revealed that Fe replaced Zn within ZnO lattice exhibiting Fe3+. Magnetic measurements revealed that room temperature ferromagnetism in the ZnFeO films were doping concentration dependent and could be improved when grown in oxygen-absent atmosphere. Both the electron concentration and the ferromagnetism of the Zn0.98Fe0.02O were enhanced when Ga was doped. The origin of the observed FM is related with oxygen vacancies and the exact mechanism is discussed on the basis of the BMP model with overlapping of polarons. Acknowledgments This work was supported by National Natural Science Foundation of China under grant no. 51002134, Fundamental Research Funds for the Central Universities under grant no. 2010QNA4002, Cultivation Fund of the Key Scientific and Technical Innovation Project, Ministry of Education of China under grant no. 707035, and was also sponsored by SRF for ROCS, SEM under grant no. J20091215. References [1] H. Ohno, Science 281 (1998) 951. [2] S.W. Jung, S.-J. An, G.-C. Yi, C.U. Jung, S.-I. Lee, S. Cho, Appl Phys Lett 80 (2002) 4561. [3] S.J. Pearton, D.P. Norton, K. Ip, Y.W. Heo, T. Steiner, J Vac Sci Technol B 22 (2004) 932. [4] T. Dietal, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) 1019. [5] K. Sato, H. Katayama-Yoshida, Jpn J Appl Phys 39 (2000) L555. [6] K. Sato, H. Katayama-Yoshida, Jpn J Appl Phys 40 (2001) L334. [7] A. Tiwari, M. Snure, D. Kumar, J.T. Abiade, Appl Phys Lett 92 (2008) 062509. [8] D.B. Buchholz, R.P.H. Chang, J.H. Song, J.B. Ketterson, Appl Phys Lett 87 (2005) 082504. [9] A. Debernardi, M. Fanciulli, Appl Phys Lett 90 (2007) 212510. [10] X.X. Wei, C. Song, K.W. Geng, F. Zeng, B. He, F. Pan, J Phys Condens Matter 18 (2006) 1747. [11] S. Karamat, C. Ke, T.L. Tan, W. Zhou, P. Lee, R.S. Rawat, Appl Surf Sci 255 (2009) 4814. [12] G. Weyer, H.P. Gunnlaugsson, R. Mantowan, M. Fanciulli, D. Naidoo, K. BaharuthRam, T. Agne, J Appl Physi 102 (2007) 113925. [13] Z.C. Chen, L.J. Zhuge, X.M. Wu, Y.D. Meng, Thin Solid Films 515 (2007) 5462. [14] K.J. Kim, Y.R. Park, J Appl Physi 96 (2000) 8. [15] L.M. Wang, J.W. Liao, Z.A. Peng, J.H. Lai, J Electrochem Soc 156 (2) (2009) H138. [16] G.Y. Ahn, S.I. Park, C.S. Kim, Phys Status Solidi A 204 (2007) 4037. [17] P. Wu, G. Saraf, Y. Lu, D.H. Hill, R. Gateau, L. Wielunski, R.A. Bartynski, D.A. Arena, J. Dvorak, A. Moodenbaugh, T. Siegrest, J.A. Raley, Y.K. Yeo, Appl Phys Lett 89 (2006) 012508. [18] J.H. Shim, T. Hwang, S. Lee, J.H. Park, S.J. Han, Y.H. Jeong, Appl Phys Lett 86 (2005) 082503. [19] 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, Phys Rev B 75 (2007) 144404. [20] Q.B. Ma, Z.Z. Ye, H.P. He, L.P. Zhu, W.C. Liu, Y.F. Yang, L. Gong, J.Y. Huang, Y.Z. Zhang, B.H. Zhao, J Phys D Appl Phys 41 (2008) 055302. [21] N.H. Hong, J Magn Magn Mater 303 (2006) 338.
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