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Effects of the oxygen partial pressure during deposition on the material characteristics and magnetic properties of BaM thin films
Journal of Alloys and Compounds 538 (2012) 11–15
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Effects of the oxygen partial pressure during deposition on the material characteristics and magnetic properties of BaM thin films Zhiyong Xu ⇑, Zhongwen Lan, Guangwei Zhu, Ke Sun, Zhong Yu State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, PR China
a r t i c l e
i n f o
Article history: Received 18 April 2012 Received in revised form 17 May 2012 Accepted 27 May 2012 Available online 7 June 2012 Keywords: BaM thin films Oxygen partial pressure Material characteristics Magnetic properties
a b s t r a c t Barium hexaferrite thin films were deposited on Si (1 0 0) substrates using a radio frequency magnetron sputtering system. The effects of oxygen partial pressure during deposition process on the material characteristics and magnetic properties of BaM thin films were investigated. X-ray diffraction data indicated that with the oxygen partial pressure ratio increasing from 0% to 8%, the diffraction peaks shift slightly towards the larger angles and the corresponding lattice parameters (a and c) decrease. The saturation magnetization (Ms) ranges from 200 to 358 kA/m as the oxygen partial pressure ratio is varied from 0% to 8%. These changes are attributed to the different oxidation states of Fe ions in the films with oxygen excess or vacancies. When deposited with 1% oxygen partial pressure, the film possesses the highest saturation magnetization (358 kA/m) and exhibits better perpendicular c-axis orientation in comparison with those deposited without oxygen partial pressure or with higher oxygen partial pressure. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Owing to their excellent chemical stability, large uniaxial anisotropy and the unique feature of self-biasing arising from the high magnetic anisotropy field, barium hexaferrite (BaM) thin films are of great interest for both magnetic recordings and microwave/millimeter wave devices. Especially for the next generation of magnetic microwave devices, such as isolators, filters, phase shifters, circulators and related components, which are desired to be planar, self-biased and low loss, BaM thin films are the potential candidates [1–5]. However, integration of the magnetic components with semiconductor devices continues to be a desirable property that requires ferrite fabrication techniques to be compatible with complementary metal-oxide semiconductor (CMOS) processing. This gives a great challenge to the thin film fabrication techniques, and various vacuum methods, such as pulsed laser deposition, direct current and radio frequency magnetron sputtering, electron-beam evaporation and molecular beam epitaxy have been adopted for the deposition of BaM films. Among these methods, radio frequency magnetron sputtering is extensively studied for its advantages, such as low cost, large area deposition, common in industrial processing and highly compatible with semiconductors technology. Bayard et al. [6] and Capraro et al. [7] deposited BaM thick films by radio frequency sputtering and investigated the effects of the substrate, annealing temperature, film thickness and post-deposition annealing method which are referred to as classic thermal annealing (CTA) and rapid thermal annealing (RTA), on the crystal⇑ Corresponding author. Tel./fax: +86 28 83201673. E-mail address: [email protected] (Z. Xu). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.05.101
lographic and magnetic properties of the BaM films. Morisako et al. [8,9] studied the influences of oxygen partial pressure and total discharge gas pressure on the characteristics of BaM films, finding that the surface smoothness, small c-axis dispersion angle and composition of BaM films depend strongly on the oxygen partial pressure during sputtering. But the Ba content in the target was increased to 1.7 times (BaO3.5Fe2O3) as much as that of stoichiometric composition of BaM. The effects of processing parameters such as oxygen partial pressure, chamber pressure, and substrate bias on the magnetic properties of Ba-rich films were investigated by Zhuang et al. [10]. They reported that the c-axis orientation of 10 wt.% Barich barium ferrite thin films is the most sensitive to the oxygen partial pressure during deposition and the Pt interlayer is very effective in improving c-axis orientation of Ba-rich films. However, detailed microstructural studies of the effects of oxygen partial pressure during deposition on the material characteristics and magnetic properties of stoichiometric BaM thin films have been scarcely attempted. Therefore, in the present study, efforts have been made to correlate the film crystallographic and magnetic properties to the different compositions and oxidation states of Fe ions in the films with oxygen excess or vacancies, which are caused by the different oxygen partial pressures during deposition. 2. Experimental procedures 2.1. Sample preparation BaM thin films were deposited using a radio frequency magnetron sputtering system. The target for the deposition was a sintered ferrite disk (100 mm in diameter) with the stoichiometric composition of BaM (BaFe12O19). After evacuating the chamber to a base pressure below 4.0 104 Pa, a mixture of argon gas (Ar, 99.999%
12
Z. Xu et al. / Journal of Alloys and Compounds 538 (2012) 11–15
in purity) and oxygen gas (O2, 99.999% in purity) was introduced into the chamber as a working gas. During sputtering, the total pressure ðP Ar þ P O2 Þ in the chamber was maintained at 1.4 Pa by adjusting the argon and oxygen partial pressures. The oxygen partial pressure ratio defined as R ¼ P O2 =ðP Ar þ P O2 Þ 100% was varied from 0% to 8% to modify the films’ properties. The planar magnetron cathode was operated with an rf power of 140 W. Pre-sputtering of the target was carried out prior to each deposition. Then BaM films were deposited onto polished 10 10 0.5 mm Si (1 0 0) substrates at a substrate temperature of 300 °C. After deposition, it was found that the as-deposited BaM films were amorphous and nonmagnetic. Therefore, post-deposition thermal annealing was performed at 800 °C for 2 h to obtain the magnetic properties desired. 2.2. Sample analysis and measurement The crystallographic properties of the films were characterized by DX 1000 Xray diffractometry (XRD) (Cu target, Ka radiation, 35 kV, 25 mA) at room temperature. The surface morphologies, thicknesses and stoichiometric compositions of the films were observed by field emission scanning electron microscopy (FESEM) with an energy disperse spectroscopy (EDS) probe. The magnetic properties were measured by Model IBHV-525 vibrating sample magnetometer (VSM) at room temperature. The molar ratios of elements and oxidation states of Fe ions in the films were analyzed by X-ray photoelectron spectroscopy (XPS) (XSAM800). The linear source of X-rays was Al Ka (which has an energy level of 1486.6 eV). The spectrometer was calibrated by the reference binding energy of C1s (284.8 eV), then the peaks were fitted by an XPSPEAK 41 program and the straight-line method was used for background correction.
3. Results and discussion 3.1. Composition and crystallographic characteristics Fig. 1 shows the deposition rates of BaM films deposited with different oxygen partial pressure ratios. It can be seen that the deposition rate declines drastically when the oxygen partial pressure ratio increases from R = 0% to R = 1%, and then decreases slowly with the oxygen partial pressure ratio further increasing to R = 8%. As is known, oxygen is a reactive gas. By introducing oxygen during deposition, high energetic atomic ions ejected from the target may react with the oxygen ions due to collisions and transform into oxides on the target surface [11]. With the oxygen partial pressure increasing, the collision probability between the reactive gas and the atomic ions increases. Hence the sputtering and deposition rates decline, as is shown in Fig. 1. In this experiment, all of the BaM films were with the same thickness of 1 lm, which was realized by adjusting the deposition time. The compositions of the as-deposited films have been analyzed by EDS. It is found that all the films are in lack of Ba content despite of different oxygen partial pressure ratios, and a lower oxygen partial pressure ratio is more beneficial to obtain a stoichiometric BaM
film. The change in the composition of the as-deposited films in comparison to the target composition has been reported in the case of radio frequency sputtered BaM films [12–14]. And Ba-rich nonstoichiometric targets were always adopted to compensate for the lack of Ba content in the films and to deposit the films with stoichiometric composition [8,10,15]. This change has been attributed to the preferential re-sputtering of Ba atoms due to high mass ratio of Ba to Fe [6], thus the numbers of Ba atoms and Fe atoms arriving at the substrate are not in stoichiometry. The typical XRD patterns of BaM films deposited with different oxygen partial pressure ratios are presented in Fig. 2, clearly showing the drastic change of diffraction peak intensities with the oxygen partial pressure ratios. All the diffraction peaks are well indexed on the hexagonal P63/mmc symmetry and labeled by the Miller indices for BaFe12O19 [16], but their relative intensities exhibit strong dependency on the oxygen partial pressure ratios. For the sample deposited without oxygen gas (R = 0%), there are not only (0 0 l) diffraction peaks but also other diffraction peaks, such as (1 0 7), (1 1 4), and (2 0 3). A comparison of the peak relative intensity of the film and that of the BaM powders indicates that the film does not show any preferential orientation. When the oxygen partial pressure ratio increases to R = 1%, the relative intensities of (0 0 l) diffraction peaks become stronger and dominant, whereas relative intensities of other diffraction peaks are diminished. This implicates that preferential perpendicular c-axis orientation growth occurs when the sample is deposited with R = 1%. As the oxygen partial pressure further increases, the (0 0 l) diffraction peaks become weaker. At the same time, the (1 0 7), (1 1 4) and (2 0 3) diffraction peaks gradually become stronger with respect to (0 0 l) peaks. Especially when the oxygen partial pressure increases to R = 6% and R = 8%, the diffraction peaks of (0 0 6) and (0 0 8) almost disappear, whereas the (1 0 7), (1 1 4) and (2 0 3) diffraction peaks become dominant. Meanwhile, it is seen from Fig. 2 that with the increase of oxygen partial pressure ratios, the diffraction peaks of films slightly shift towards larger angles. Corresponding lattice parameters (a and c) for the films calculated by using Bragg’s law are listed in Table 1, which demonstrates that the lattice parameters decrease with the increase of oxygen partial pressure ratios. In particular, the film deposited with 1% oxygen partial pressure has the values of a and c comparable to those of bulk BaM, namely, a = 5.892 Å and c = 23.183 Å, respectively [16]. It is well known that in stoichiometric BaM, there is no possibility in its closed-packed O2 structure for the interstitial sites to accommodate excesses of Ba, Fe, or O without drastic lattice
4
(0014) (2011)
(d) R=4% (c) R=2%
3
0
2 4 6 Oxygen partial pressure ratio R=PO2/PAr+O2 (%)
8
Fig. 1. Deposition rates of BaM films deposited with different oxygen partial pressure ratios.
20
30
40
(a) R=0% 50
(0014) (2011)
(205) (206)
(114)
(203)
(008) (107)
(b) R=1% (006)
2
(f) R=8% (e) R=6%
Intensity (a.u.)
5
(205) (206)
(101) (102)
6 Deposition rate (nm/min)
(110) (107) (114) (108) (203)
7
60
2 Theta (deg) Fig. 2. XRD patterns of BaM films deposited with different oxygen partial pressure ratios.
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Z. Xu et al. / Journal of Alloys and Compounds 538 (2012) 11–15 Table 1 Lattice parameters for the films deposited with different oxygen partial pressure ratios. Oxygen partial pressure ratio R (%)
0
1
2
4
6
8
Lattice parameter a (Å) Lattice parameter c (Å)
5.9088 23.2130
5.8969 23.1818
5.8879 23.1773
5.8797 23.1648
5.8707 23.1594
5.8604 23.1289
(b) R=1%
(a) R=0%
Raw spectrum
Raw spectrum
Fe3+2P3/2
Fe3+2P1/2 Fe2+2P1/2 2+ Fitting spectrum Fe 2P3/2
700
710
720
730
Intensity (a. u.)
Intensity (a. u.)
Fe3+2P3/2
Fe3+2P1/2
Fe2+2P1/2
Fitting spectrum Fe2+2P3/2
700
710
Bingding Energy (eV)
720 Bingding Energy (eV)
730
Fig. 3. XPS spectra of the Fe 2P in the BaM films deposited with different oxygen partial pressure ratios: (a) R = 0%; (b) R = 1%.
0.65
360
S⊥
0.60
S//
320
Squareness
Ms (kA/m)
0.55 280
0.50 0.45
240
0.40 200
0.35 0
2
4
6
8
Oxygen partial pressure ratio R=PO2/PAr+O2 (%)
0
2
4
6
8
Oxygen partial pressure ratio R=PO2 /PAr+O2 (%)
Fig. 4. Saturation magnetization (Ms) of the BaM films as a function of the oxygen partial pressure ratio (R).
Fig. 5. The dependence of out-of-plane squareness (S\) and in-plane squareness (Sk) on the oxygen partial pressure ratios.
distortions. Previous studies reported that when deposited with low oxygen partial pressure, the films become oxygen deficient [8,17–19] and have oxygen vacancies in them, which then induce randomly localized perturbations to the oxygen lattice. Considering the global charge distribution, oxygen vacancies can act as donors which then change, in the ionic model, the Fe valence to 2+ state as the Ba is stable in the 2+ state. Formation of single ionized oxygen in BaM films appears less probable since the ground state of Fe3+ in iron oxides lies generally above the oxygen valence band, thus favoring a localization of the hole near the Fe3+ ion. Therefore random Fe2+ ions are formed in the case of low deposition oxygen partial pressure. While with the increase of oxygen partial pressure, the oxygen vacancies gradually getting compensated and the ratio of Fe2+ ions reduces. In addition, EDS analysis indicates that with the increase of oxygen partial pressure, the molar ration of Ba ions in the films decreases. The decrease in numbers of Fe2+
ions and Ba2+ ions produces localized deformations of the crystal lattice in the films, whose overall effect then results in the shrinkage of lattice parameters as seen in Table 1, since the ionic radius of Fe2+ ion (0.74 Å) is larger than that of Fe3+ ion (0.67 Å) and the ionic radius of Ba2+ ion (1.43 Å) is larger than that of O2 ion (1.32 Å). In order to confirm the existence of Fe2+ ions, the oxidation states of Fe ions in the films were analyzed by XPS. Fig. 3 presents the typical XPS spectra of the Fe element in the BaM films deposited with oxygen partial pressure ratios R = 0% and R = 1%. Fitting was carried out on the Fe2p peaks using an XPSPEAK 41 program. By calculating the ratio of photoelectron counting areas of Fe2+ and Fe3+ ions, the molar ratios of Fe2+/(Fe2++Fe3+) were obtained, which are 12.86% and 5.37% for R = 0% and R = 1% respectively. Thus it can be concluded that when deposited with low oxygen partial pressure, some of the Fe3+ ions change to Fe2+ ions, otherwise the ratio of Fe2+ ions reduces with the increase of oxygen partial pressure.
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Z. Xu et al. / Journal of Alloys and Compounds 538 (2012) 11–15
3.2. Magnetic properties Magnetic hysteresis loops (M vs H) for the films were measured in both the out-of-plane (field applied normal to the film plane) and the in-plane (field applied along the film plane) geometry by VSM at room temperature with a maximum available applied magnetic field of 20 kOe. The saturation magnetization (Ms) was deduced by dividing the EMU as measured by the VSM in the out-of-plane and multiplying it by 1/v, where v is the volume of the sample. In Fig. 4 the saturation magnetizations of the BaM films as a function of the oxygen partial pressure ratio are illustrated. Here, seen from Fig. 4, it is found that with the oxygen partial pressure ratio increasing from 0% to 8%, the saturation magnetization initially increases to a maximum (Ms = 358 kA/m) at R = 1% and then for higher oxygen partial pressure ratios, the saturation magnetization decreases and seems to saturate at a value of around 200 kA/m for R > 4%. The initial increase of saturation magnetization can be related to the decrease of oxygen vacancies. As discussed above, when deposited 4000
Hc⊥ Hc//
Coercivity (Oe)
3500
3000
2500
2000
1500 0
2
4
6
8
Oxygen partial pressure ratio R=PO2 /PAr+O2 (%) Fig. 6. The dependence of out-of-plane coercivity (HC\) and in-plane coercivity (HCk) on the oxygen partial pressure ratios.
with low oxygen partial pressure, the films become oxygen deficient and have oxygen vacancies in them, which then modify globally the charge balance in per unit cell. Considering the number of cations remains constant, in particular all iron sub-lattices remain unaltered for their occupation numbers, on average certain Fe ions will assume a valence state of Fe2+. The Fe2+ ion has a lower magnetic moment (4 lB) compared to Fe3+ ion (5 lB) and prefers to occupy the 2a sub-lattices in the S-blocks of Ba ferrite crystallite [20,21]. Thus the saturation magnetization is lower when the film deposited without oxygen partial pressure. While with the oxygen partial pressure increasing to 1%, the oxygen vacancies gradually get compensated and the ratio of Fe2+ ions reduces, leading to the initial increase of saturation magnetization. However, for higher oxygen pressure ratios (R P 2%), the films might have iron and barium deficiencies, like the situation in the Y3Fe5O12 system [22,23]. In the ionic model, since the barium valence is stable, this implies the formation of Fe4+ in the vicinity of Fe3+ or Ba2+ vacancy. The magnetic moments of both iron vacancy (0 lB) and Fe4+ ion (4 lB) are smaller than that of Fe3+ ion (5 lB), resulting in the decrease of saturation magnetization. Fig. 5 demonstrates variation of the loop squareness (SQ = Mr/ Ms), where Mr is the remanent magnetization and Ms is the saturation magnetization, obtained from the hysteresis loops taken in both the out-of-plane (S\) and in-plane (Sk) of the films as a function of the oxygen partial pressure ratio. Also, the dependence of out-of-plane coercivity (HC\) and in-plane coercivity (HCk) on the oxygen partial pressure ratio is illustrated in Fig. 6. It is found that when the film deposited with 1% oxygen partial pressure, the values of S\ and HC\ are a little bigger than those of Sk and HCk in the film. While for the film deposited without oxygen partial pressure (R = 0%) or higher oxygen partial pressure ratio (R > 1%), the values of squareness and coercivity for the out-of-plane are almost the same with those for the in-plane. This implies that the film deposited at the oxygen partial pressure ratio R = 1% shows better perpendicular c-axis orientation in comparison with those deposited at R = 0% or R > 1%, which show random c-axis orientation. To understand the cause of the different c-axis orientations, microstructures of the films were studied by FESEM. Fig. 7 presents the surface morphologies of BaM films deposited with different
Fig. 7. Surface morphologies of BaM films deposited with different oxygen partial pressure ratios: (a) R = 0%; (b) R = 1%; (c) R = 4%; (d) R = 8%.
Z. Xu et al. / Journal of Alloys and Compounds 538 (2012) 11–15
oxygen partial pressure ratios. It shows that the grains can be classified into two major categories: those with acicular shape and those which are platelets. Previous studies have concluded that the acicular grains have their c-axis in the film plane and perpendicular to their long axis, while platelets have their c-axis perpendicular to the film plane [1,10]. And the large difference between the crystal growth rate in the basal plane and along the c-axis causes a large volume fraction difference between the two kinds of grains. In the film deposited without oxygen partial pressure, the acicular grains occupy a lager area than the platelet grains (see Fig. 7(a)), resulting in the in-plane c-axis orientation of the film. While for the film deposited with 1% oxygen partial pressure ratio, the platelet grains are dominated (see Fig. 7(b)), so the film shows perpendicular c-axis orientation as confirmed in the XRD data (see Fig. 2(b)). Further increase of oxygen partial pressure ratio (R > 1%) seems to gradually enhance the growth of the in-plane and randomly oriented acicular grains, which then fuse each other and limit the platelet grains growth (see Fig. 7(c) and (d)), thus gradually worsening the perpendicular c-axis orientation. 4. Conclusions With the oxygen partial pressure ratio increasing from 0% to 8% during deposition, the films change from a state of oxygen deficiency to oxygen excess, which then results in the different oxidation states of Fe ions in the films. In addition, oxygen partial pressure induces difference between the crystal growth rate in the basal plane and along the c-axis and causes a large volume fraction difference between the acicular grains and platelet grains. Thus it significantly affects the material characteristics and magnetic properties of BaM thin films. The X-ray diffraction peaks shift slightly towards the larger angles and the corresponding lattice parameters (a and c) decrease. The saturation magnetization (Ms) ranges from 200 to 358 kA/m as the oxygen partial pressure ratio is varied from 0% to 8%. When deposited with 1% oxygen partial pressure, the film possesses the highest saturation magnetization (358 kA/m) and exhibits slight perpendicular c-axis orientation in comparison with those deposited without oxygen partial pressure or with higher oxygen partial pressure.
15
Acknowledgements This work is financially supported by the National Natural Science Foundation of China under Grant No. 51101028, the Fundamental Research Funds for the Central Universities under Grant No. E022050205, and the State Key Laboratory of Electronic Thin Films and Integrated Devices of China under Grant No. ZZKT200901 & No. KFJJ2010006. The authors are grateful for the discussion with Prof. Xin Chuan about the XPS analysis. References [1] X.Y. Sui, M.H. Kryder, B.Y. Wong, et al., IEEE Trans. Magn. 29 (6) (1993) 3751– 3753. [2] X.Y. Sui, M. Schergea, M.H. Kryder, et al., J. Magn. Magn. Mater. 155 (1–3) (1996) 132–139. [3] V.G. Harris, Z.H. Chen, Y.J. Chen, et al., J. Appl. Phys. 99 (08M911) (2006). 08M911-1-08M911-5. [4] V.G. Harris, A. Geiler, Y.J. Chen, et al., J. Magn. Magn. Mater. 321 (2009) 2035– 2047. [5] V.G. Harris, IEEE Trans. Magn. 48 (3) (2012) 1075–1104. [6] B. Bayard, J.P. Chatelon, M.L. Berre, et al., Sens. Actuators, A 99 (2002) 207–212. [7] S. Capraro, J.P. Chatelon, H. Joisten, et al., J. Appl. Phys. 93 (12) (2003) 9898– 9901. [8] A. Morisako, M. Matsuioto, M. Naoe, IEEE Trans. Magn. MAG33 (1) (1987) 56– 58. [9] A. Morisako, M. Matsuioto, M. Naoe, IEEE Trans. Magn. 24 (6) (1988) 3024– 3026. [10] Z.L. Zhuang, M. Rao, R.M. White, et al., J. Appl. Phys. 87 (9) (2000) 6370–6372. [11] A. Lisfia, J.C. Lodder, E.G. Keim, et al., Appl. Phys. Lett. 82 (1) (2003) 76–78. [12] Y.J. Chen, D.E. Laughlin, X.D. Ma, et al., J. Appl. Phys. 81 (8) (1997) 4380–4382. [13] K. Noma, N. Matsushita, S. Nakagawa, et al., J. Appl. Phys. 81 (8) (1997) 4377– 4379. [14] M. Naoe, S. Hasunuma, Y. Hoshi, et al., IEEE Trans. Magn. 17 (6) (1981) 3184– 3186. [15] N. Matsushita, K. Noma, S. Nakagawa, et al., J. Magn. Magn. Mater. 176 (1) (1997) 41–45. [16] Joint Committee Powder Diffraction Fiels (JCPDF) Card No. 840757. [17] S.R. Shinde, S.E. Lofland, C.S. Ganpule, et al., J. Appl. Phys. 85 (10) (1999) 7459– 7466. [18] S.D. Yoon, S.A. Oliver, C. Vittoria, J. Appl. Phys. 91 (10) (2002) 7379–7381. [19] S.D. Yoon, C. Vittoria, S.A. Oliver, J. Appl. Phys. 92 (11) (2002) 6733–6738. [20] F.K. Lotgering, J. Phys. Chem. Solids 35 (12) (1974) 1633–1639. [21] A.M.V. Diepen, F.K. Lotgering, J. Phys. Chem. Solids 35 (12) (1974) 1641–1643. [22] Y. Dumont, N. Keller, E. Popova, et al., J. Magn. Magn. Mater. 272–276 (Supplement) (2004) E869–E871. [23] Y. Dumont, N. Keller, E. Popova, et al., Phys. Rev. B 76 (2007). 104413-1104413-6.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Effects of the oxygen partial pressure during deposition on the material characteristics and magnetic properties of BaM thin films Zhiyong Xu ⇑, Zhongwen Lan, Guangwei Zhu, Ke Sun, Zhong Yu State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, PR China
a r t i c l e
i n f o
Article history: Received 18 April 2012 Received in revised form 17 May 2012 Accepted 27 May 2012 Available online 7 June 2012 Keywords: BaM thin films Oxygen partial pressure Material characteristics Magnetic properties
a b s t r a c t Barium hexaferrite thin films were deposited on Si (1 0 0) substrates using a radio frequency magnetron sputtering system. The effects of oxygen partial pressure during deposition process on the material characteristics and magnetic properties of BaM thin films were investigated. X-ray diffraction data indicated that with the oxygen partial pressure ratio increasing from 0% to 8%, the diffraction peaks shift slightly towards the larger angles and the corresponding lattice parameters (a and c) decrease. The saturation magnetization (Ms) ranges from 200 to 358 kA/m as the oxygen partial pressure ratio is varied from 0% to 8%. These changes are attributed to the different oxidation states of Fe ions in the films with oxygen excess or vacancies. When deposited with 1% oxygen partial pressure, the film possesses the highest saturation magnetization (358 kA/m) and exhibits better perpendicular c-axis orientation in comparison with those deposited without oxygen partial pressure or with higher oxygen partial pressure. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Owing to their excellent chemical stability, large uniaxial anisotropy and the unique feature of self-biasing arising from the high magnetic anisotropy field, barium hexaferrite (BaM) thin films are of great interest for both magnetic recordings and microwave/millimeter wave devices. Especially for the next generation of magnetic microwave devices, such as isolators, filters, phase shifters, circulators and related components, which are desired to be planar, self-biased and low loss, BaM thin films are the potential candidates [1–5]. However, integration of the magnetic components with semiconductor devices continues to be a desirable property that requires ferrite fabrication techniques to be compatible with complementary metal-oxide semiconductor (CMOS) processing. This gives a great challenge to the thin film fabrication techniques, and various vacuum methods, such as pulsed laser deposition, direct current and radio frequency magnetron sputtering, electron-beam evaporation and molecular beam epitaxy have been adopted for the deposition of BaM films. Among these methods, radio frequency magnetron sputtering is extensively studied for its advantages, such as low cost, large area deposition, common in industrial processing and highly compatible with semiconductors technology. Bayard et al. [6] and Capraro et al. [7] deposited BaM thick films by radio frequency sputtering and investigated the effects of the substrate, annealing temperature, film thickness and post-deposition annealing method which are referred to as classic thermal annealing (CTA) and rapid thermal annealing (RTA), on the crystal⇑ Corresponding author. Tel./fax: +86 28 83201673. E-mail address: [email protected] (Z. Xu). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.05.101
lographic and magnetic properties of the BaM films. Morisako et al. [8,9] studied the influences of oxygen partial pressure and total discharge gas pressure on the characteristics of BaM films, finding that the surface smoothness, small c-axis dispersion angle and composition of BaM films depend strongly on the oxygen partial pressure during sputtering. But the Ba content in the target was increased to 1.7 times (BaO3.5Fe2O3) as much as that of stoichiometric composition of BaM. The effects of processing parameters such as oxygen partial pressure, chamber pressure, and substrate bias on the magnetic properties of Ba-rich films were investigated by Zhuang et al. [10]. They reported that the c-axis orientation of 10 wt.% Barich barium ferrite thin films is the most sensitive to the oxygen partial pressure during deposition and the Pt interlayer is very effective in improving c-axis orientation of Ba-rich films. However, detailed microstructural studies of the effects of oxygen partial pressure during deposition on the material characteristics and magnetic properties of stoichiometric BaM thin films have been scarcely attempted. Therefore, in the present study, efforts have been made to correlate the film crystallographic and magnetic properties to the different compositions and oxidation states of Fe ions in the films with oxygen excess or vacancies, which are caused by the different oxygen partial pressures during deposition. 2. Experimental procedures 2.1. Sample preparation BaM thin films were deposited using a radio frequency magnetron sputtering system. The target for the deposition was a sintered ferrite disk (100 mm in diameter) with the stoichiometric composition of BaM (BaFe12O19). After evacuating the chamber to a base pressure below 4.0 104 Pa, a mixture of argon gas (Ar, 99.999%
12
Z. Xu et al. / Journal of Alloys and Compounds 538 (2012) 11–15
in purity) and oxygen gas (O2, 99.999% in purity) was introduced into the chamber as a working gas. During sputtering, the total pressure ðP Ar þ P O2 Þ in the chamber was maintained at 1.4 Pa by adjusting the argon and oxygen partial pressures. The oxygen partial pressure ratio defined as R ¼ P O2 =ðP Ar þ P O2 Þ 100% was varied from 0% to 8% to modify the films’ properties. The planar magnetron cathode was operated with an rf power of 140 W. Pre-sputtering of the target was carried out prior to each deposition. Then BaM films were deposited onto polished 10 10 0.5 mm Si (1 0 0) substrates at a substrate temperature of 300 °C. After deposition, it was found that the as-deposited BaM films were amorphous and nonmagnetic. Therefore, post-deposition thermal annealing was performed at 800 °C for 2 h to obtain the magnetic properties desired. 2.2. Sample analysis and measurement The crystallographic properties of the films were characterized by DX 1000 Xray diffractometry (XRD) (Cu target, Ka radiation, 35 kV, 25 mA) at room temperature. The surface morphologies, thicknesses and stoichiometric compositions of the films were observed by field emission scanning electron microscopy (FESEM) with an energy disperse spectroscopy (EDS) probe. The magnetic properties were measured by Model IBHV-525 vibrating sample magnetometer (VSM) at room temperature. The molar ratios of elements and oxidation states of Fe ions in the films were analyzed by X-ray photoelectron spectroscopy (XPS) (XSAM800). The linear source of X-rays was Al Ka (which has an energy level of 1486.6 eV). The spectrometer was calibrated by the reference binding energy of C1s (284.8 eV), then the peaks were fitted by an XPSPEAK 41 program and the straight-line method was used for background correction.
3. Results and discussion 3.1. Composition and crystallographic characteristics Fig. 1 shows the deposition rates of BaM films deposited with different oxygen partial pressure ratios. It can be seen that the deposition rate declines drastically when the oxygen partial pressure ratio increases from R = 0% to R = 1%, and then decreases slowly with the oxygen partial pressure ratio further increasing to R = 8%. As is known, oxygen is a reactive gas. By introducing oxygen during deposition, high energetic atomic ions ejected from the target may react with the oxygen ions due to collisions and transform into oxides on the target surface [11]. With the oxygen partial pressure increasing, the collision probability between the reactive gas and the atomic ions increases. Hence the sputtering and deposition rates decline, as is shown in Fig. 1. In this experiment, all of the BaM films were with the same thickness of 1 lm, which was realized by adjusting the deposition time. The compositions of the as-deposited films have been analyzed by EDS. It is found that all the films are in lack of Ba content despite of different oxygen partial pressure ratios, and a lower oxygen partial pressure ratio is more beneficial to obtain a stoichiometric BaM
film. The change in the composition of the as-deposited films in comparison to the target composition has been reported in the case of radio frequency sputtered BaM films [12–14]. And Ba-rich nonstoichiometric targets were always adopted to compensate for the lack of Ba content in the films and to deposit the films with stoichiometric composition [8,10,15]. This change has been attributed to the preferential re-sputtering of Ba atoms due to high mass ratio of Ba to Fe [6], thus the numbers of Ba atoms and Fe atoms arriving at the substrate are not in stoichiometry. The typical XRD patterns of BaM films deposited with different oxygen partial pressure ratios are presented in Fig. 2, clearly showing the drastic change of diffraction peak intensities with the oxygen partial pressure ratios. All the diffraction peaks are well indexed on the hexagonal P63/mmc symmetry and labeled by the Miller indices for BaFe12O19 [16], but their relative intensities exhibit strong dependency on the oxygen partial pressure ratios. For the sample deposited without oxygen gas (R = 0%), there are not only (0 0 l) diffraction peaks but also other diffraction peaks, such as (1 0 7), (1 1 4), and (2 0 3). A comparison of the peak relative intensity of the film and that of the BaM powders indicates that the film does not show any preferential orientation. When the oxygen partial pressure ratio increases to R = 1%, the relative intensities of (0 0 l) diffraction peaks become stronger and dominant, whereas relative intensities of other diffraction peaks are diminished. This implicates that preferential perpendicular c-axis orientation growth occurs when the sample is deposited with R = 1%. As the oxygen partial pressure further increases, the (0 0 l) diffraction peaks become weaker. At the same time, the (1 0 7), (1 1 4) and (2 0 3) diffraction peaks gradually become stronger with respect to (0 0 l) peaks. Especially when the oxygen partial pressure increases to R = 6% and R = 8%, the diffraction peaks of (0 0 6) and (0 0 8) almost disappear, whereas the (1 0 7), (1 1 4) and (2 0 3) diffraction peaks become dominant. Meanwhile, it is seen from Fig. 2 that with the increase of oxygen partial pressure ratios, the diffraction peaks of films slightly shift towards larger angles. Corresponding lattice parameters (a and c) for the films calculated by using Bragg’s law are listed in Table 1, which demonstrates that the lattice parameters decrease with the increase of oxygen partial pressure ratios. In particular, the film deposited with 1% oxygen partial pressure has the values of a and c comparable to those of bulk BaM, namely, a = 5.892 Å and c = 23.183 Å, respectively [16]. It is well known that in stoichiometric BaM, there is no possibility in its closed-packed O2 structure for the interstitial sites to accommodate excesses of Ba, Fe, or O without drastic lattice
4
(0014) (2011)
(d) R=4% (c) R=2%
3
0
2 4 6 Oxygen partial pressure ratio R=PO2/PAr+O2 (%)
8
Fig. 1. Deposition rates of BaM films deposited with different oxygen partial pressure ratios.
20
30
40
(a) R=0% 50
(0014) (2011)
(205) (206)
(114)
(203)
(008) (107)
(b) R=1% (006)
2
(f) R=8% (e) R=6%
Intensity (a.u.)
5
(205) (206)
(101) (102)
6 Deposition rate (nm/min)
(110) (107) (114) (108) (203)
7
60
2 Theta (deg) Fig. 2. XRD patterns of BaM films deposited with different oxygen partial pressure ratios.
13
Z. Xu et al. / Journal of Alloys and Compounds 538 (2012) 11–15 Table 1 Lattice parameters for the films deposited with different oxygen partial pressure ratios. Oxygen partial pressure ratio R (%)
0
1
2
4
6
8
Lattice parameter a (Å) Lattice parameter c (Å)
5.9088 23.2130
5.8969 23.1818
5.8879 23.1773
5.8797 23.1648
5.8707 23.1594
5.8604 23.1289
(b) R=1%
(a) R=0%
Raw spectrum
Raw spectrum
Fe3+2P3/2
Fe3+2P1/2 Fe2+2P1/2 2+ Fitting spectrum Fe 2P3/2
700
710
720
730
Intensity (a. u.)
Intensity (a. u.)
Fe3+2P3/2
Fe3+2P1/2
Fe2+2P1/2
Fitting spectrum Fe2+2P3/2
700
710
Bingding Energy (eV)
720 Bingding Energy (eV)
730
Fig. 3. XPS spectra of the Fe 2P in the BaM films deposited with different oxygen partial pressure ratios: (a) R = 0%; (b) R = 1%.
0.65
360
S⊥
0.60
S//
320
Squareness
Ms (kA/m)
0.55 280
0.50 0.45
240
0.40 200
0.35 0
2
4
6
8
Oxygen partial pressure ratio R=PO2/PAr+O2 (%)
0
2
4
6
8
Oxygen partial pressure ratio R=PO2 /PAr+O2 (%)
Fig. 4. Saturation magnetization (Ms) of the BaM films as a function of the oxygen partial pressure ratio (R).
Fig. 5. The dependence of out-of-plane squareness (S\) and in-plane squareness (Sk) on the oxygen partial pressure ratios.
distortions. Previous studies reported that when deposited with low oxygen partial pressure, the films become oxygen deficient [8,17–19] and have oxygen vacancies in them, which then induce randomly localized perturbations to the oxygen lattice. Considering the global charge distribution, oxygen vacancies can act as donors which then change, in the ionic model, the Fe valence to 2+ state as the Ba is stable in the 2+ state. Formation of single ionized oxygen in BaM films appears less probable since the ground state of Fe3+ in iron oxides lies generally above the oxygen valence band, thus favoring a localization of the hole near the Fe3+ ion. Therefore random Fe2+ ions are formed in the case of low deposition oxygen partial pressure. While with the increase of oxygen partial pressure, the oxygen vacancies gradually getting compensated and the ratio of Fe2+ ions reduces. In addition, EDS analysis indicates that with the increase of oxygen partial pressure, the molar ration of Ba ions in the films decreases. The decrease in numbers of Fe2+
ions and Ba2+ ions produces localized deformations of the crystal lattice in the films, whose overall effect then results in the shrinkage of lattice parameters as seen in Table 1, since the ionic radius of Fe2+ ion (0.74 Å) is larger than that of Fe3+ ion (0.67 Å) and the ionic radius of Ba2+ ion (1.43 Å) is larger than that of O2 ion (1.32 Å). In order to confirm the existence of Fe2+ ions, the oxidation states of Fe ions in the films were analyzed by XPS. Fig. 3 presents the typical XPS spectra of the Fe element in the BaM films deposited with oxygen partial pressure ratios R = 0% and R = 1%. Fitting was carried out on the Fe2p peaks using an XPSPEAK 41 program. By calculating the ratio of photoelectron counting areas of Fe2+ and Fe3+ ions, the molar ratios of Fe2+/(Fe2++Fe3+) were obtained, which are 12.86% and 5.37% for R = 0% and R = 1% respectively. Thus it can be concluded that when deposited with low oxygen partial pressure, some of the Fe3+ ions change to Fe2+ ions, otherwise the ratio of Fe2+ ions reduces with the increase of oxygen partial pressure.
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Z. Xu et al. / Journal of Alloys and Compounds 538 (2012) 11–15
3.2. Magnetic properties Magnetic hysteresis loops (M vs H) for the films were measured in both the out-of-plane (field applied normal to the film plane) and the in-plane (field applied along the film plane) geometry by VSM at room temperature with a maximum available applied magnetic field of 20 kOe. The saturation magnetization (Ms) was deduced by dividing the EMU as measured by the VSM in the out-of-plane and multiplying it by 1/v, where v is the volume of the sample. In Fig. 4 the saturation magnetizations of the BaM films as a function of the oxygen partial pressure ratio are illustrated. Here, seen from Fig. 4, it is found that with the oxygen partial pressure ratio increasing from 0% to 8%, the saturation magnetization initially increases to a maximum (Ms = 358 kA/m) at R = 1% and then for higher oxygen partial pressure ratios, the saturation magnetization decreases and seems to saturate at a value of around 200 kA/m for R > 4%. The initial increase of saturation magnetization can be related to the decrease of oxygen vacancies. As discussed above, when deposited 4000
Hc⊥ Hc//
Coercivity (Oe)
3500
3000
2500
2000
1500 0
2
4
6
8
Oxygen partial pressure ratio R=PO2 /PAr+O2 (%) Fig. 6. The dependence of out-of-plane coercivity (HC\) and in-plane coercivity (HCk) on the oxygen partial pressure ratios.
with low oxygen partial pressure, the films become oxygen deficient and have oxygen vacancies in them, which then modify globally the charge balance in per unit cell. Considering the number of cations remains constant, in particular all iron sub-lattices remain unaltered for their occupation numbers, on average certain Fe ions will assume a valence state of Fe2+. The Fe2+ ion has a lower magnetic moment (4 lB) compared to Fe3+ ion (5 lB) and prefers to occupy the 2a sub-lattices in the S-blocks of Ba ferrite crystallite [20,21]. Thus the saturation magnetization is lower when the film deposited without oxygen partial pressure. While with the oxygen partial pressure increasing to 1%, the oxygen vacancies gradually get compensated and the ratio of Fe2+ ions reduces, leading to the initial increase of saturation magnetization. However, for higher oxygen pressure ratios (R P 2%), the films might have iron and barium deficiencies, like the situation in the Y3Fe5O12 system [22,23]. In the ionic model, since the barium valence is stable, this implies the formation of Fe4+ in the vicinity of Fe3+ or Ba2+ vacancy. The magnetic moments of both iron vacancy (0 lB) and Fe4+ ion (4 lB) are smaller than that of Fe3+ ion (5 lB), resulting in the decrease of saturation magnetization. Fig. 5 demonstrates variation of the loop squareness (SQ = Mr/ Ms), where Mr is the remanent magnetization and Ms is the saturation magnetization, obtained from the hysteresis loops taken in both the out-of-plane (S\) and in-plane (Sk) of the films as a function of the oxygen partial pressure ratio. Also, the dependence of out-of-plane coercivity (HC\) and in-plane coercivity (HCk) on the oxygen partial pressure ratio is illustrated in Fig. 6. It is found that when the film deposited with 1% oxygen partial pressure, the values of S\ and HC\ are a little bigger than those of Sk and HCk in the film. While for the film deposited without oxygen partial pressure (R = 0%) or higher oxygen partial pressure ratio (R > 1%), the values of squareness and coercivity for the out-of-plane are almost the same with those for the in-plane. This implies that the film deposited at the oxygen partial pressure ratio R = 1% shows better perpendicular c-axis orientation in comparison with those deposited at R = 0% or R > 1%, which show random c-axis orientation. To understand the cause of the different c-axis orientations, microstructures of the films were studied by FESEM. Fig. 7 presents the surface morphologies of BaM films deposited with different
Fig. 7. Surface morphologies of BaM films deposited with different oxygen partial pressure ratios: (a) R = 0%; (b) R = 1%; (c) R = 4%; (d) R = 8%.
Z. Xu et al. / Journal of Alloys and Compounds 538 (2012) 11–15
oxygen partial pressure ratios. It shows that the grains can be classified into two major categories: those with acicular shape and those which are platelets. Previous studies have concluded that the acicular grains have their c-axis in the film plane and perpendicular to their long axis, while platelets have their c-axis perpendicular to the film plane [1,10]. And the large difference between the crystal growth rate in the basal plane and along the c-axis causes a large volume fraction difference between the two kinds of grains. In the film deposited without oxygen partial pressure, the acicular grains occupy a lager area than the platelet grains (see Fig. 7(a)), resulting in the in-plane c-axis orientation of the film. While for the film deposited with 1% oxygen partial pressure ratio, the platelet grains are dominated (see Fig. 7(b)), so the film shows perpendicular c-axis orientation as confirmed in the XRD data (see Fig. 2(b)). Further increase of oxygen partial pressure ratio (R > 1%) seems to gradually enhance the growth of the in-plane and randomly oriented acicular grains, which then fuse each other and limit the platelet grains growth (see Fig. 7(c) and (d)), thus gradually worsening the perpendicular c-axis orientation. 4. Conclusions With the oxygen partial pressure ratio increasing from 0% to 8% during deposition, the films change from a state of oxygen deficiency to oxygen excess, which then results in the different oxidation states of Fe ions in the films. In addition, oxygen partial pressure induces difference between the crystal growth rate in the basal plane and along the c-axis and causes a large volume fraction difference between the acicular grains and platelet grains. Thus it significantly affects the material characteristics and magnetic properties of BaM thin films. The X-ray diffraction peaks shift slightly towards the larger angles and the corresponding lattice parameters (a and c) decrease. The saturation magnetization (Ms) ranges from 200 to 358 kA/m as the oxygen partial pressure ratio is varied from 0% to 8%. When deposited with 1% oxygen partial pressure, the film possesses the highest saturation magnetization (358 kA/m) and exhibits slight perpendicular c-axis orientation in comparison with those deposited without oxygen partial pressure or with higher oxygen partial pressure.
15
Acknowledgements This work is financially supported by the National Natural Science Foundation of China under Grant No. 51101028, the Fundamental Research Funds for the Central Universities under Grant No. E022050205, and the State Key Laboratory of Electronic Thin Films and Integrated Devices of China under Grant No. ZZKT200901 & No. KFJJ2010006. The authors are grateful for the discussion with Prof. Xin Chuan about the XPS analysis. References [1] X.Y. Sui, M.H. Kryder, B.Y. Wong, et al., IEEE Trans. Magn. 29 (6) (1993) 3751– 3753. [2] X.Y. Sui, M. Schergea, M.H. Kryder, et al., J. Magn. Magn. Mater. 155 (1–3) (1996) 132–139. [3] V.G. Harris, Z.H. Chen, Y.J. Chen, et al., J. Appl. Phys. 99 (08M911) (2006). 08M911-1-08M911-5. [4] V.G. Harris, A. Geiler, Y.J. Chen, et al., J. Magn. Magn. Mater. 321 (2009) 2035– 2047. [5] V.G. Harris, IEEE Trans. Magn. 48 (3) (2012) 1075–1104. [6] B. Bayard, J.P. Chatelon, M.L. Berre, et al., Sens. Actuators, A 99 (2002) 207–212. [7] S. Capraro, J.P. Chatelon, H. Joisten, et al., J. Appl. Phys. 93 (12) (2003) 9898– 9901. [8] A. Morisako, M. Matsuioto, M. Naoe, IEEE Trans. Magn. MAG33 (1) (1987) 56– 58. [9] A. Morisako, M. Matsuioto, M. Naoe, IEEE Trans. Magn. 24 (6) (1988) 3024– 3026. [10] Z.L. Zhuang, M. Rao, R.M. White, et al., J. Appl. Phys. 87 (9) (2000) 6370–6372. [11] A. Lisfia, J.C. Lodder, E.G. Keim, et al., Appl. Phys. Lett. 82 (1) (2003) 76–78. [12] Y.J. Chen, D.E. Laughlin, X.D. Ma, et al., J. Appl. Phys. 81 (8) (1997) 4380–4382. [13] K. Noma, N. Matsushita, S. Nakagawa, et al., J. Appl. Phys. 81 (8) (1997) 4377– 4379. [14] M. Naoe, S. Hasunuma, Y. Hoshi, et al., IEEE Trans. Magn. 17 (6) (1981) 3184– 3186. [15] N. Matsushita, K. Noma, S. Nakagawa, et al., J. Magn. Magn. Mater. 176 (1) (1997) 41–45. [16] Joint Committee Powder Diffraction Fiels (JCPDF) Card No. 840757. [17] S.R. Shinde, S.E. Lofland, C.S. Ganpule, et al., J. Appl. Phys. 85 (10) (1999) 7459– 7466. [18] S.D. Yoon, S.A. Oliver, C. Vittoria, J. Appl. Phys. 91 (10) (2002) 7379–7381. [19] S.D. Yoon, C. Vittoria, S.A. Oliver, J. Appl. Phys. 92 (11) (2002) 6733–6738. [20] F.K. Lotgering, J. Phys. Chem. Solids 35 (12) (1974) 1633–1639. [21] A.M.V. Diepen, F.K. Lotgering, J. Phys. Chem. Solids 35 (12) (1974) 1641–1643. [22] Y. Dumont, N. Keller, E. Popova, et al., J. Magn. Magn. Mater. 272–276 (Supplement) (2004) E869–E871. [23] Y. Dumont, N. Keller, E. Popova, et al., Phys. Rev. B 76 (2007). 104413-1104413-6.