Oxygen vacancy induced ferromagnetism in.doc
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1、精品论文Oxygen vacancy induced ferromagnetism inNi doped ZnO films1Deng-Lu Hou, Rui-Bin Zhao, Qian Zhang, Cong-Mian Zhen, Cheng-Fu Pan, Gui-DeTangDepartment of Physics Hebei Normal University Shijiazhuang (050016)E-mail: houdengluAbstractZn1-xNixO ( x =0.02, 0.03, 0.04, 0.05, 0.07) films were prepared u
2、sing 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
3、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 vacanc
4、ies increase, confirming that the ferromagnetism is correlated with the oxygenvacancy level.Keywords: ZnO; room temperature ferromagnetism; Oxygen vacancy; Photoluminescence1. IntroductionDilute magnetic semiconductors (DMS) have recently attracted a great deal of attention due to their potential ap
5、plications 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 de
6、monstrated 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, 10, 11.Recently, theoretical calculations have shown that oxygen vacancies play an important ro
7、le 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
8、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 Zn1-xNixO films, we have prepared films for a variety of Ni doping levels and have investigated, in detail, Zn0.97Ni
9、0.03O 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 detailsZn1-x
10、NixO (x=0.02, 0.03, 0.04, 0.05, 0.07) thin films were grown on n-type Si (100) 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-rat
11、e ratio of 8:1, and total pressure of 0.5 Pa. The base pressure was 310-5Pa and the substrate temperature was kept at 400C.Subsequently, the films were annealed at 600C for 10 minutes in vacuum. The structural1 本课题得到国家自然科学基金(10774037),国家自然科学基金(10804026)的资助。- 7 -forms of the Zn1-xNixO thin films were
12、 characterized by x-ray diffraction (XRD) with Cu Kradiation. The valence states of the films were analyzed using x-ray photoelectron spectroscopy (XPS). The magnetic properties of the Zn1-xNixO thin films were measured using a vibrating sample magnetometer (VSM) at room temperature. Photoluminescen
13、ce (PL) measurements were performed at room temperature with an excitation wavelength of 340 nm. Electrical properties were determined by Hall measurements in a van der Pauw four-point configuration.3. Results and discussionFIG. 1. XRD patterns for Zn1-xNixOFIG. 2.The c-axis lattice constant( x =0.0
14、2, 0.03, 0.04, 0.05, 0.07) films.dependence on the Ni concentration.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 (002) planes are observed, which indicates a preferential (002) oriented growth of the films as well as a weake
15、r peaks which are observed at 36.2from ZnO (101) planes. No secondary phases or metal clusters are found. With increasing Ni doping concentration, the lattice constant c decreases, as can be seen from Fig.2. Since the radius of Ni2+( 0.69) is smaller than that of Zn2+ (0.74 ), the variation of thec-
16、axis lattice suggests that Ni substitutionally replaces Zn in the films 16, 17.42000400003600030000C 1sc/s36000O 1sc/s24000180001200032000300 295 290 285 280275Binding Energy (eV)5 45 540 53 5 53 0525 520B ind ing E n ergy (eV )4800078 00044000c/s40000Ni 2p72 000c/s66 00060 000Zn 2p 3600054 00048 00
17、0890 880 870 860 850 840Binding Energy (eV)1060 1050 1040 10 30 10 20 101 0Bin din g E nergy (eV )FIG. 3. XPS spectra for the Zn0.95Ni0.05O films.Fig. 3 shows XPS spectra for a Zn0.95Ni0.05O film. The C 1s, O 1s, Ni 2p and Zn 2p peaks are observed. The binding energy of Zn 2p3/2 is 1021.72 eV which
18、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 eV and 873.01 eV respectively, showing that the valence state of the Ni ion is +2 in the films. Meanwhile, the Ni 2p3/2 main peak has a satellite peak at 861.83eV, which is ty
19、pical for the Ni2+ cation 18. In addition, the energy difference between Ni 2p3/2 and Ni2p1/2 is 17.75 eV, which suggests that NiO (energy difference 18.4 eV) is not present in the films. This is consistent with results reported by Yin et al. 8.Fig 4(a) shows that the room temperature M-H cures of Z
20、n0.97Ni0.03O films annealed at500 and 600 in vacuum, respectively, where the applied magnetic field is parallel to thesurface plane. A distinct magnetic hysteresis loop is observed for the film annealed at600,while the film annealed at 500 is only paramagnetism. That is to say, the magnetic ordering
21、 in Zn0.97Ni0.03O film is very sensitivity to the annealing temperature. Fig. 4(b)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(
22、b) 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 B /Ni. The dependence of the moment per Ni ion on the Ni concentration can be understood as
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