Oxygenvacancyinducedferromagnetismin.doc

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: abstractzn1-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 temperatureferromagnetism. 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 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 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, 10, 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 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 detailszn1-xnixo (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-rate 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 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. photoluminescence (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 =0.02, 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 weaker 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-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 000890 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 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 typical 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 zn0.97ni0.03o 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 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(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 resulting from the longerni2+ni2+ distance at low ni concentration and a correspondingly weaker ferromagneticinteraction. 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 als report concerning mn-doped zno films 19. moreover, in comparison with a saturation magnetization of 0.37 b /ni obtained by liu et al. 20 who usedpulsed laser deposition for sample preparation, and0.14 b /ni found by wang et al. 18 using a wet chemical reaction, our methods result in alarger magnetic moment per ion.0.40.2 /nib0.0-0.2-0.46004000.2 /ni0.10.0b-0.1-0.2-0.3-0.4-0.52%3%4%5%7%0.450.4 /ni0.35b0.02 0.03 0.04 0.05 0.06 0.07-6000 -4000 -200002000 4000 6000h(oe)ni concentration-6000 -4000 -2000 02000 4000 6000h(oe)fig. 4(a). the m-h cures of zn0.97ni0.03o filmsfig. 4(b). (color online) room temperature annealed at 500 and 600. 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.recently, many groups have reported that oxygen vacancies play a dominant role in ferromagnetic mn-doped zno and co-doped 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 and3 ) 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 describedabove.0.0b-0.1 /ni-0.2-0.3-0.4-0.5-0.6sample 1 sample 2 sample 30.50.4 /nib0.30.24 6 8 10 12ar:o2-6000 -4000 -200002000 4000 6000h(oe)fig. 5. (color online) the m-h curves of samples 1, 2, 3, grown at argon tooxygen 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.fig. 5 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. 5, 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 figure 5, 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 filmspl intensity(a.u)15,23,24.pl intensity(a.u)4:1a8:1b350400450500550600wavelength(nm)350400450500550600wave lengthen(nm)pl intensity(a.u)ratio of pl intensity(a.u)12:1 c d350400450500550600wavelength(nm)4 6 8 1012ar:o2fig. 6. (color online) (a,b,c) pl spectra obtained from samples grown under argon to oxygen ratios of 4:1, 8:1 and12:1 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.photoluminescence measurements were carried out in order to confirm the relationship between oxygen vacancies and oxygen concentration in the sputtering atmosphere. fig. 6 (a, b, 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 nm to 500 nm is found. when the spectrum is extracted by gauss fitting, two peaks at wavelengths of 430nm and 470nm are observed forsamples 1 and 2. one might consider assigning the peak at 430 nm to zn interstitials, because the energy interval from the top of the valence band to the interstitial zn level is 2.9ev，which coincides with the energy of the deep level emission at 430nm 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 may be due to zn vacancies 26. the blue emission peak (around 470nm) which is mainly attributed to surface defect levels associated with oxygen vacancies27,28. the blue emission peak strengthened with decreasing oxygen which also indicates that this peak is ascribed to oxygen vacancy. from fig. 6 (d), 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 theferromagnetism in these films is related mainly to the oxygen vacancy.1209060300cm (ho)30201003214:18:112:1260 280 300 320 340 360 380 400 420 440 460t(k)fig 7. resistivity vs temperature curves for samples grown under argon to oxygen ratios of 4:1, 8:1 and 12:1.resistivity versus temperature curves for the samples are shown in fig.7. 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 cm for sample 1 (ar:o2 = 4:1) to 2.9 cm for sample 3 (ar:o2 =12:1) at 270k, 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, ferromagnetic oxides and nitrides with magnetic cation doped can be understood by the donor impurity band exchange model30. as for n-type zn1-xnixo thin films, the oxygen vacancies act asshallow 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.4. conclusionsin 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 (002) 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 ni-doped zno films. pl measurements indicate that the blue emission at about 470nm is related to oxygen vacancies, which contribute to the ferromagnetism in all the films.5. 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