Room temperature ferromagnetism in Ni-doped ZnO films

Current Applied Physics 10 (2010) 124–128

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Room temperature ferromagnetism in Ni-doped ZnO films Deng-Lu Hou a, Rui-Bin Zhao a,b,*, Yan-Yan Wei a, Cong-Mian Zhen a, Cheng-Fu Pan a, Gui-De Tang a a b

Department of Physics, Hebei Normal University, Shijiazhuang 050016, China Department of Medical Physics, Hebei Medical University, Shijiazhuang, 050017, China

a r t i c l e

i n f o

Article history: Received 25 March 2009 Received in revised form 9 May 2009 Accepted 13 May 2009 Available online 21 May 2009 PACS: 75.50.Pp 75.70. i 74.25.Ha

a b s t r a c t Zn1 xNixO (x = 0.02, 0.03, 0.04, 0.05, 0.07) films were prepared using magnetron sputtering. X-ray diffraction indicates that all samples have a wurtzite structure with c-axis orientation. X-ray photoelectron spectroscopy results reveal that the Ni ion is in a +2 charge state in these films. Magnetization measurements indicate that all samples have room temperature ferromagnetism. In order to elucidate the origin of the ferromagnetism, Zn0.97Ni0.03O films were grown under different atmospheric ratios of argon to oxygen. The results show that as the fraction of oxygen in the atmosphere decreases, both the saturation magnetization and the number of oxygen vacancies increase, confirming that the ferromagnetism is correlated with the oxygen vacancy level. Ó 2009 Elsevier B.V. All rights reserved.

Keywords: ZnO Room temperature ferromagnetism Oxygen vacancy Photoluminescence

1. Introduction Dilute magnetic semiconductors (DMS) have recently attracted a great deal of attention due to their potential applications in spintronics devices [1,2]. In particular, since Dietl et al. [3] predicted that room temperature ferromagnetism might exist in p-type Mn-doped ZnO, intense interest has been focused on ZnO doped with a variety of transition metals (TM) [4,5]. Several groups have prepared Ni-doped ZnO and demonstrated room temperature ferromagnetism [6,7]. However, others have reported paramagnetism in Ni-doped ZnO [8]. Discussion concerning the origin of ferromagnetism in DMS is therefore still inconsistent [9–11]. Recently, theoretical calculations have shown that oxygen vacancies play an important role in altering the band structure of a host oxide and make a significant contribution to ferromagnetism in oxide semiconductors [12,13]. Hsu et al. reported that the enhancement of ferromagnetism was strongly correlated with an increase in the oxygen vacancies in Co-doped ZnO prepared using ion beam sputtering [14]. Similar results were reported by Hong et al. [15]. In order to elucidate the origin of ferromagnetism and to delineate the effects of oxygen vacancies on ferromagnetism in

* Corresponding author. Address: Department of Physics, Hebei Normal University, Shijiazhuang 050016, China. E-mail addresses: [email protected] (D.-L. Hou), zhaoruibin8103@ yahoo.cn (R.-B. Zhao). 1567-1739/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2009.05.007

Zn1 xNixO films, we have prepared films for a variety of Ni doping levels and have investigated, in detail, Zn0.97Ni0.03O films prepared under different atmospheric ratios of argon to oxygen. Oxygen vacancy concentrations were estimated using photoluminescence. This has enabled us to study the relationship between ferromagnetism and oxygen vacancy concentrations in Zn0.97Ni0.03O films. 2. Experimental details Zn1 xNixO (x = 0.02, 0.03, 0.04, 0.05, 0.07) thin films were grown on n-type Si (1 0 0) substrates using magnetron sputtering. Metallic Zn (99.999%) and Ni (99.999%) were used as the sputtering targets. The sputtering was performed in a mixed atmosphere of argon (99.999%) and oxygen (99.999%) with a flow-rate ratio of 8:1, and total pressure of 0.5 Pa. The base pressure was 3  10 5 Pa and the substrate temperature was kept at 673 K. Subsequently, the films were annealed at 873 K for 10 min in vacuum. The structural forms of the Zn1 xNixO thin films were characterized by X-ray diffraction (XRD) with Cu Ka radiation. The valence states of the films were analyzed using X-ray photoelectron spectroscopy (XPS), the surface of sample was cleaned by Ar sputtering for 0.5 min. The energy spectra were analyzed with a hemispherical mirror analyzer with an energy resolution of approximately 0.2– 0.3 eV. The magnetic properties of the Zn1 xNixO thin films were measured using a vibrating sample magnetometer (VSM) at room temperature. Photoluminescence (PL) measurements were per-

D.-L. Hou et al. / Current Applied Physics 10 (2010) 124–128

formed at room temperature with an excitation wavelength of 340 nm. Electrical properties were determined by Hall effect measurements in a van der Pauw four-point configuration. 3. Results and discussion Fig. 1 shows XRD patterns for Zn1 xNixO (x = 0.02, 0.03, 0.04, 0.05, 0.07) thin films. Diffraction peaks from wurtzite ZnO (0 0 2) planes are observed, which indicates a preferential (0 0 2) oriented growth of the films as well as a weaker peaks which are observed at 36.2° from ZnO (1 0 1) planes. No secondary phases or metal clusters are found within the detection sensitivity of the instrument. With increasing Ni doping concentration, the lattice constant c decreases, as can be seen from Fig. 2. Since the radius of Ni2+ (0.69 Å) is smaller than that of Zn2+ (0.74 Å), the variation of the c-axis lattice suggests that Ni substitutionally replaces Zn in the films [16,17]. Fig. 3 shows XPS spectra for a Zn0.95Ni0.05O film. The Ni 2p and Zn 2p peaks are observed. The binding energy of Zn 2p3/2 is 1021.72 eV which indicates a single component of Zn2+ ions. The binding energies of the Ni 2p3/2 and Ni 2p1/2 orbitals are situated at 855.26 and 873.01 eV, respectively, showing that the valence state of the Ni ion is +2 in the films [18]. Meanwhile, the Ni 2p3/2 main peak has a satellite peak at 861.83 eV, which is typical for the Ni2+ cation [18]. In addition, the energy difference between Ni 2p3/2 and Ni 2p1/2 is 17.75 eV, which suggests that NiO (energy

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2 (deg.) Fig. 1. XRD patterns for Zn1 xNixO (x = 0.02, 0.03, 0.04, 0.05, 0.07) films.

c-axis lattice constant

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difference 18.4 eV) is not present in the films. This is consistent with results reported by Yin et al. [8]. Fig. 4 shows the hysteresis curves for the Zn1 xNixO (x = 0.02, 0.03, 0.04, 0.05, 0.07) thin films. All the curves are characteristic of ferromagnetic behavior with small coercive field and small remanence. The inset in Fig. 4 shows that the saturation magnetization first increases and then decreases as the Ni concentration increases. When the Ni concentration is 0.04, the saturation magnetization reaches a maximum value of 0.43 lB/Ni. The dependence of the moment per Ni ion on the Ni concentration can be understood as resulting from the longer Ni2+–Ni2+ distance at low Ni concentration and a correspondingly weaker ferromagnetic interaction. At a Ni concentration of 0.04, the ferromagnetic interaction between Ni ions is strongest so that the moment per Ni ion is the largest. With further increase in the Ni concentration, the average distance between adjacent Ni ions decreases, and the antiferromagnetic energy of Ni ions is lower than the ferromagnetic energy. This results in an antiferromagnetic arrangement of the Ni moments, so that the average magnetic moment per Ni ion decreases. These results are consistent with Mi et al.’s report concerning Mn-doped ZnO films [19]. Moreover, in comparison with a saturation magnetization of 0.37 lB/Ni obtained by Liu et al. [20] who used pulsed laser deposition for sample preparation, and 0.14 lB/Ni found by Wang et al. [18] using a wet chemical reaction, our methods result in a larger magnetic moment per ion. Temperature dependence of magnetization Zn0.93Ni0.07O film is shown in Fig. 5. Note that the film is ferromagnetic with the Curie temperature above 340 K. Recently, many groups have reported that oxygen vacancies play a dominant role in ferromagnetic Mn-doped ZnO and Codoped ZnO films [21,22]. Considering our preparation conditions, we infer that the ferromagnetism of the films is related to oxygen vacancies. In order to confirm this hypothesis, we prepared three samples (designated as samples 1, 2 and 3) in an argon–oxygen atmosphere with flow-ratios of 4:1, 8:1 and 12:1 all with a fixed Ni doping concentration of x = 0.03. All other preparation conditions were the same as described above. Fig. 6 shows the magnetization curves for the Zn0.97Ni0.03O films grown in atmospheres with different ratios of Ar to O2. The distinct hysteresis loops shown in Fig. 6, all at room temperature indicate that Zn0.97Ni0.03O films grown in Ar and O2 with different Ar:O2 ratios are all ferromagnetic. From the inset of Fig. 6, we find that the saturation magnetization increases as the fraction of oxygen in the argon–oxygen atmosphere decreases. Because oxygen vacancies are expected to be compensated by increasing oxygen in atmosphere, the oxygen vacancy concentration will be reduced as the fraction of oxygen in the atmosphere increases. Correspondingly, the increase in the magnetic moment per ion with decreasing oxygen in the atmosphere indicates that the ferromagnetism in the films may well be related to oxygen vacancies. Several groups have likewise reported that structural defects and oxygen vacancies appear to influence the magnetism of dilute magnetic oxide films [15,23,24]. Photoluminescence measurements were carried out in order to confirm the relationship between oxygen vacancies and oxygen concentration in the sputtering atmosphere. Fig. 7a–c compare room temperature PL spectra for Ni-doped ZnO samples grown with the above Ar:O2 ratios and with a fixed Ni content of 3 at.%. It is seen that there is an ultraviolet (UV) emission peak centered at 390 nm, which is ascribed to the near-band-edge (NBE) recombination transitions of ZnO-like band structures of Ni-doped ZnO films [25]. Moreover, a broad band PL emission ranging from 400 to 500 nm is found. When the spectrum is extracted by Gauss fitting, two peaks at wavelengths of 430 and 470 nm are observed for samples 1 and 2. One might consider assigning the peak at 430 nm to Zn interstitials, because the energy interval from the

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48000 78000

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Fig. 3. XPS spectra for the Zn0.95Ni0.05O films.

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H(Oe) Fig. 4. Room temperature magnetic hysteresis hoops for Zn1 xNixO (x = 0.02, 0.03, 0.04, 0.05, 0.07) films. The inset shows the changes of the magnetization with the Ni concentration.

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T(K) Fig. 5. Temperature dependence of magnetization of 7 at.% Ni-doped ZnO film.

top of the valence band to the interstitial Zn level is 2.9 eV, which coincides with the energy of the deep level emission at 430 nm in our experiments. However, this peak disappears when the Ar:O2 ratio is 12:1 (sample 3) and a peak at 408 nm appears which

Fig. 6. The M–H curves of samples 1, 2, 3, grown at argon to oxygen ratios of 4:1, 8:1 and 12:1, respectively. The inset shows the magnetic moment per ion as a function of the Ar:O2 ratio.

may be due to Zn vacancies [26]. The blue emission peak (around 470 nm) which is mainly attributed to surface defect levels associated with oxygen vacancies [27,28]. The blue emission peak strengthened with decreasing oxygen which also indicates that this peak is ascribed to oxygen vacancy. From Fig. 7d, we can infer that the increased ratio of blue emission intensity to UV emission intensity with decreasing oxygen in the atmosphere is evidence for an increase of oxygen vacancy concentrations in the impurity band. The concentrations of oxygen vacancies increase steadily with decreasing atmospheric oxygen, which resembles the increasing trend of the magnetization. This further suggests that the ferromagnetism in these films is related mainly to the oxygen vacancy. Resistivity vs. temperature curves for the samples are shown in Fig. 8. It can be clearly seen that all samples exhibit similar temperature dependences on the resistivity, which shows typical semiconductor behavior. Hall effect measurements indicate that all the films are n-type semiconductors. The resistivity shows an abrupt decrease from 112.0 X cm for sample 1 (Ar:O2 = 4:1) to 2.9 X cm for sample 3 (Ar:O2 = 12:1) at 270 K, which is mainly caused by the increase of oxygen vacancies when the amount of oxygen in the argon–oxygen atmosphere decreases, in agreement with the PL measurements described above. Therefore, the fewer the oxygen vacancies, the higher the resistivity [29]. Recently, fer-

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Fig. 7. (a–c) PL spectra obtained from samples grown under argon to oxygen ratios of 4:1, 8:1 and 12:1 at room temperature. (d) Shows the dependence of the ratios of blue emission intensity to UV emission intensity at 470 nm as a function of the flow-ratios of Ar:O2.

120 90 60 30 0 30 20 10 0 3

4. Conclusions 4:1

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In summary, Zn1 xNixO films with c-axis oriented wurtzite structure were grown using magnetron sputtering. XRD measurements found no metal clusters or secondary phases. With increasing doping concentration, the (0 0 2) peaks shifted to larger angles which suggests that Ni substitutionally replaces the Zn sites in the films. XPS shows that the valence state of the Ni ion is +2 in Nidoped ZnO films. PL measurements indicate that the blue emission at about 470 nm is related to oxygen vacancies, which contribute to the ferromagnetism in all the films.

260 280 300 320 340 360 380 400 420 440 460

T(K) Fig. 8. Resistivity vs. temperature curves for samples grown under argon to oxygen ratios of 4:1, 8:1 and 12:1.

romagnetic oxides and nitrides with magnetic cation doped can be understood by the donor impurity band exchange model [30]. As for n-type Zn1 xNixO thin films, the oxygen vacancies act as shallow donors which form bound magnetic polarons coupling the 3d moments of the Ni ions within their orbits. The bound magnetic polarons overlap to create a spin-split impurity band. The charge transfer from the spin-split impurity band to an unoccupied 3d state of Ni ions at the Fermi level stabilizes the ferromagnetism of Zn1 xNixO thin films.

Acknowledgments This work has been supported by Natural Science Foundation of China (10774037&10804026) and Natural Science Foundation of Hebei Province (E2007000280). References [1] [2] [3] [4] [5]

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