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Structure and giant magnetoresistance of carbon-based amorphous films prepared by magnetron sputtering
Thin Solid Films 556 (2014) 460–463
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Structure and giant magnetoresistance of carbon-based amorphous films prepared by magnetron sputtering L. Ma a,b,⁎, M.F. He a,b, Z.W. Liu c, D.C. Zeng c, Z.F. Gu a,b, G. Cheng a,b a b c
School of Materials Science and Engineering, Guilin University of Electronic Technology, Guilin 541004, China Guangxi Key Laboratory of Information Materials, Guilin University of Electronic Technology, Guilin 541004, China School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China
a r t i c l e
i n f o
Article history: Received 15 June 2013 Received in revised form 16 January 2014 Accepted 17 January 2014 Available online 24 January 2014 Keywords: Granular films Magnetoresistance Magnetron sputtering
a b s t r a c t Pure amorphous carbon (а-C) and Co-doped CoxC1 − x films were prepared on n-Si(100) substrates by dc magnetron sputtering. In Co–C films, the nano-sized amorphous Co particles were homogeneously dispersed in the amorphous cross-linked carbon matrix. The structures of a-C and CoxC1 − x films were investigated by X-ray photoelectron spectroscopy and Raman spectroscopy. The results showed that the a-C films were diamond-like carbon (DLC) films. After doping cobalt into DLC film, the sp3-hybridized carbon content in DLC composite films almost had no change. The as-deposited CoxC1 − x granular films had larger value of magnetoresistance (MR) than the amorphous carbon film. A very high positive MR, up to 15.5% at magnetic field B = 0.8 T and x = 2.5 at.% was observed in a CoxC1 − x granular film with thickness of 80 nm at room temperature when the external magnetic field was perpendicular to the electric current and the film surface. With increase of the film thickness and Co-doped content, the MR decreased gradually. It remains a challenge to well explain the observed MR effect in the CoxC1 − x granular films. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Amorphous carbon (a-C) thin films have attracted much attention recently due to their potential applications in hard coating, field emission and microelectronic devices [1–3]. More than one decade, metal containing diamond-like carbon (Me-DLC) films have been extensively studied because they cannot only maintain high hardness, low friction coefficient and high thermal stability, but also have adjustable electrical conductivity. This is obviously advantageous for its applications in the fields of electronics, optics, magnetics and catalysis [4–6]. Various deposition techniques have been used to fabricate Me-DLC composite films, such as ion beam assisted deposition [5], co-sputtering deposition [7], pulsed laser deposition (PLD) [8–10], electrolysis deposition [11], and electron cyclotron resonance chemical vapor deposition [6]. Currently, the researches on these carbon films mainly focused on their fabrication, microstructure, magnetoelectronic properties, and potential applications. For example, it was reported that the a-C films prepared by pulsed laser deposition had shown an apparent voltage-induced switch effect and a magnetoresistance (MR) of about 1% in 1 T magnetic field [10]. As we know, Co and C are immiscible and the metastable Co carbides (Co2C and Co3C) decompose easily into Co and C [12]. Co–C composite films therefore form a good metal/insulator system to be investigated. Using PLD method, Zhu et al. [9] have prepared Co–C
⁎ Corresponding author. Tel.: +86 7732291680; fax: +86 7732290129. E-mail address: [email protected] (L. Ma). 0040-6090/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2014.01.048
films and observed a large positive MR of 22% in the Co0.02–C0.98 granular film on Si(100) at room temperature with B = 1 T. In this article, we report the structure and MR properties of the pure а-C films and Co-doped amorphous CoxC1 − x granular films deposited on Si(100) substrates by magnetron sputtering at room temperature. 2. Experimental details The CoxC1 − x thin films with x = 0–25 at.% were deposited on nSi(100) substrates by magnetron sputtering at room temperature. The purity of the Co and C was better than 99.95% and the targets were pieced together with Co sectors putting on the well-proportioned graphite target. The Si(100) substrates were lightly doped with P resulting in a ntype material. Before deposition, the Si substrates were ultrasonically cleaned firstly in acetone and then in ethanol, etched by diluted HF solution and rinsed in de-ionized water. The working gas was Ar (99.99% purity). The deposition took place inside a chamber where the pressure was kept at 0.7 Pa, and the substrate was kept at room temperature. For comparison, pure carbon films were also deposited by magnetron sputtering. The structure and magnetic properties of the Co–C films were investigated by conventional X-ray diffraction (XRD, Philips X'Pert MPO Pro, U = 40 kV/I = 40 mA, X'celerator, Cu-Kα1: λ = 1.54056 Å, 0.0167°/step), scanning electron microscopy (SEM, FEI Nova NanoSEM 430) and physical properties measurement system (PPMS-9, Quantum Design Co.). The atomic concentration and binding energy for each element in the films were determined by X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD, AlKα, energy of photoelectron:
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1486.8 eV, U = 15 kV/I = 10 mA), and the intrinsic bonding structure of carbon-based film is also provided by Raman scattering (Ar+ laser of 532 nm) spectroscopy. 3. Results and discussion Co–C films deposited by magnetron sputtering were characterized by XRD, as shown in Fig. 1 for an example. The results show that the film is in amorphous state in the as-deposited films. Scanning electron microscopy observation shows that the film thickness is in the range of 80–240 nm. Deposition rate, determined from the film thickness and deposition time, increases linearly with power about 0.35 Å/s at 120 W and a total pressure of 0.7 Pa. SEM characterization shows that the microstructure of the carbon-based films are homogeneous distributed with mean particle size of about 30–40 nm, and Co particles are well dispersed in amorphous carbon (Fig. 2). The result is in consistent with some previous reports [9]. Fig. 3a is a typical XPS result for the as-deposited Co–C films. The whole spectrum consists of three main peaks at about 284.5, 531.0, and 778.4 eV, corresponding to the binding energy of C-1s, O-1s, and Co-2p, respectively. The relative atomic Co/C ratio calculated from XPS is about 2.5%. The fine spectrogram of Co-2p is shown in Fig. 3b, which can be assigned to Co atoms bonded to oxygen owing to the surface oxidation when exposed to air. It was reported that cobalt component existed in atomic state, and dispersed randomly in the amorphous carbon matrix indicated by TEM [9]. In our work, the binding energies of the Co-2p peak (Fig. 3b) in pure cobalt and cobalt carbides almost coincide with each other. Moreover, the structural change of the asdeposited samples can be determined by the curve-fitted C-1s spectra according to the Gussian fitting: C_C (284.5–284.7 eV), C\C (285.5– 285.8 eV) and C\O (288.1–288.4 eV) [13]. As shown in Fig. 3c, the C-1s peak was fitted by three Gaussian peaks centered at 284.52, 285.38, and 288.29 eV with area percentages of 78.7%, 16.8%, and 4.5%, respectively. The peaks at 284.52 eV and 285.38 eV correspond to the binding energy of carbon–carbon bonds, and the other peak corresponds to those of C\O bond. No peak corresponding to the binding energy of C-1s in cobalt carbide was observed. It indicated that the C-1s peak in Co doping films did not vary evidently from amorphous carbon film to graphite film, and the doping of Co did not enhance the graphitization of carbon film. This is different from the Fe doping which can increase sp3-hybridized carbon in the films [11]. Fig. 3d shows the fitting result of the O-1s peak in the film. Two major peaks centered at 529.84 and 531.68 eV with area percentages of 12.6% and 82.9%, respectively, correspond to the binding energy of O-1s in cobalt oxide (CoO) and oxygen. It is believed that the formation of CoO is due to surface oxidation of the Co clusters in the film. The contribution of this small amount of CoO to the magnetic properties of the film could be ignored.
Fig. 1. XRD pattern of as-deposited CoxC1 − x (x = 2.5 at.%) composite film deposited at room temperature.
Fig. 2. The SEM pattern of as-deposited Co-DLC films (2.5 at.% Co) deposited at room temperature.
Furthermore, the intrinsic bonding structure of carbon-based film is also provided by Raman scattering (Ar+ laser of 532 nm) spectroscopy. And the main characteristics are the D and G bands appeared in the range of 1200–1700 cm− 1 [14]. As shown in Fig. 4, both D and G bands centered at 1388 and 1573 cm−1 appear in the as-deposited samples, which are typical DLC Raman spectrum. Therein, the G band is attributed to E2g vibrational mode arising from the bond stretching of all pairs of sp2 atoms in both rings and chains, whereas the D band is assigned to the breathing modes of sp2 ring structure in disorder graphite [14,15]. To further identify the internal structure of the as-deposited samples, all the Raman spectra are fitted with a Gaussian profile after background correction. From Fig. 4a and b, the positions of D and G bands do not shift obviously, and the ID/IG ratio almost has no variation compared with that of pure DLC films. Ferrari and Robertson found that there is a relationship between the visible Raman spectra and the sp3 fraction in amorphous carbon films and the sp3 content increased
Fig. 3. XPS analysis of as-deposited Co-DLC films (2.5 at.% Co) deposited at room temperature: (a) the full-spectrum of Co-DLC; (b) Co2p; (c) C1s in Co-DLC film; and (d) O1s in Co-DLC film (curve fitted).
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Fig. 4. Raman spectra of as-deposited DLC (a) and Co-DLC films (b).
simultaneous with the lowering of G-peak position and the decrease of and ID/IG ratio [14]. On the contrary, our results indicate that the sp3/sp2 ratio in the DLC films remains the same with the incorporation of Co in the magnetron sputtering process, which agrees well with the XPS analysis. The magnetic properties for the Co-DLC nanocomposite films were determined by the PPMS in this work, and the hysteresis for a selected film is shown in Fig. 5. The coercivity of only around 30 Oe was obtained, which indicates that the incorporation of Co in the carbon matrix leads to good soft magnetic properties. Fig. 6 shows MR measurement results taken at T = 300 K by fourprobe method on the electromagnetic field device with an in-plane or
Fig. 5. In-plane hysteresis loop for the as-deposited CoxC1 − x (x = 2.5 at.%) granular film measured at 300 K (the inset is the enlarged view of coercivity).
Fig. 6. The MR dependence on magnetic field for as-deposited pure C (a) and Co–C (b) films at room temperature.
out-of-plane external magnetic field. The MR is defined as △ R/R = [R(B,T) − R(0,T) / R(0,T)], where R(B,T) and R(0,T) are resistances measured at temperature T with or without a magnetic field B applied, respectively. As shown in Fig. 6a, the pure carbon film shows different MR properties when the magnetic field direction is parallel or perpendicular to the film plane. At 300 K with B = 0.8 T, a large MR value of about 1% is obtained in the pure C film when it is perpendicular to the field direction. Similarly, for Co-doped amorphous CoxC1 − x granular films, a larger value of MR was obtained when the electric current is perpendicular to the external magnetic field instead of parallel it, which indicates that the CoxC1 − x granular films have strong in-plane anisotropy. As shown in Fig. 6b, the as-deposited Co–C granular films have much larger MR values than amorphous carbon film. It is also found that the MR of the CoxC1 − x film decreases with increase of thickness. For a 240-nm thick film, a MR value of ~5% was obtained at room temperature when the external magnetic field is perpendicular to the film surface. A very large positive MR, up to 15.5% at magnetic field B = 0.8 T and x = 2.5 at.%, was observed in the film with thickness of 80 nm. Interestingly, our results are different from other reports. In reference [9], Co–C granular films show fine MR property when the external magnetic field direction is in the film plane. It is possible that the difference is caused by two different deposition methods: magnetron sputtering and PLD. The recently reported morphological peculiarities of the nanoparticle films obtained by PLD are the oblate ellipsoidal shape of the constituent nanograins, with an aspect ratio which depends on the laser intensity, and their major cross section parallel to the deposition substrate, as a consequence of the nanodrop impact with the substrate [16,17]. We believe that the difference arises due to shape anisotropy or magnetic anisotropy. As for this remarkable positive MR effect, Xue and Zhang [10] proposed that a p–n heterojunction is formed when the C film and n–Si are in close contact, because the a-C films are p-type semiconductor. When the Fermi energy of the C film is
L. Ma et al. / Thin Solid Films 556 (2014) 460–463
lower than that of the n–Si, the electrons will flow from the n–Si to the C film. However, there is a conduction band that offsets between the C film and the n–Si substrate [18]. As a result, a two-dimensional electron gas (2DEG) is formed on the Si surface. When the applied electric field (or voltage) reaches a threshold, a large number of electrons in the localized states are excited into the extended states. Under the external magnetic field applied on the 2DEG, the Lorentz force will curve the orbits of the electrons which will result in a positive MR. Moreover, Jiang and Gao [8] proposed that the Co aggregation in the Co–C/Si interface may be a possible origin of the positive colossal MR effect. However, to evaluate the validity of this proposal, more experimental and theoretical works are in need. 4. Conclusions We investigated the structure and room temperature positive MR in Co-doped amorphous carbon film and pure amorphous carbon film, which have simple structure and can be easily prepared on Si(100) substrate by magnetron sputtering method. It was found that the a-C film and CoxC1 − x films had fairly smooth and flat surface, and the Co particles were homogeneously dispersed into the amorphous cross-linked carbon matrix. The results of Raman spectra show that the amorphous carbon films are diamond-like carbon films. After doping cobalt into DLC film, the sp3-hybridized carbon content in DLC composite films has no significant change. Larger values of MR were obtained in these films when the electric current is perpendicular to the external magnetic field instead of parallel to it. The CoxC1 − x films had better MR properties than C film, in particular, the as-deposited CoxC1 − x (x = 2.5 at.%) film with a thickness of 80 nm has shown a positive MR of 15.5% under B = 0.8 T at room temperature. The positive MR effect can be understood by energy band model proposed by previous researchers, but the underlying mechanism is still in need of study.
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Acknowledgments This work is supported by the Natural Science Foundation of Guangxi (2013GXNSFBA019242, 2012GXNSFGA060002), the National Natural Science Foundation of China (U0734001, 51161005, 51261004), the Guangxi Key Laboratory of Information Materials (1110908-04-Z) and the Fundamental Research Funds for the Central Universities (SCUT, 2012ZZ0015). References [1] N.A. Hastas, C.A. Dimitriadis, D.H. Tassis, S. Logothetidis, Appl. Phys. Lett. 79 (5) (2001) 638. [2] D.B. Mahadik, S.S. Shinde, C.H. Bhosale, K.Y. Rajpure, J. Alloy. Compd. 509 (5) (2011) 1418. [3] M.W. Huang, J.Y. Jao, C.C. Lin, W.J. Hsieh, Y.H. Yang, L.S. Cheng, F.S. Shieu, H.C. Shih, Appl. Surf. Sci. 261 (2012) 21. [4] Y.C. Jiang, Z.P. Wu, W. Bao, S.J. Xu, J. Gao, J. Appl. Phys. 111/7 (2012) 07C510. [5] T. Hioki, Y. Itoh, A. Itoh, S. Hibi, J. Kawamoto, Surf. Coat. Technol. 46 (1991) 233. [6] Rusl, S.F. Yoon, Q.F. Huang, H. Yang, M.B. Yu, J. Ahn, Q. Zhang, E.J. Teo, T. Osipowicz, F. Watt, J. Appl. Phys. 88/6 (2000) 3699. [7] K. Bewilogua, C.V. Cooper, C. Specht, J. Schroder, R. Wittorf, M. Grischke, Surf. Coat. Technol. 127 (2000) 224. [8] Y.C. Jiang, J. Gao, Appl. Phys. Lett. 101 (18) (2012) 182401. [9] D.D. Zhu, X. Zhang, Q.Z. Xue, J. Appl. Phys. 95 (4) (2004) 1906. [10] Q.Z. Xue, X. Zhang, Carbon 43 (4) (2005) 760. [11] S.H. Wan, L.P. Wang, Q.J. Xue, Electrochem. Commun. 11 (1) (2009) 99. [12] T.J. Konno, R. Sinclair, Acta Metall. Mater. 42 (4) (1994) 1231. [13] E. Riedo, F. Comin, J. Chevrier, F. Schmithusen, S. Decossas, M. Sancrotti, Surf. Coat. Technol. 125 (2000) 124. [14] A.C. Ferrari, J. Robertson, Phys. Rev. B 61 (20) (2000) 14095. [15] A.C. Ferrari, Solid State Commun. 143 (1–2) (2007) 47. [16] S. Amoruso, R. Bruzzese, N. Spinelli, R. Velotta, M. Vitiello, X. Wang, G. Ausanio, V. Iannotti, L. Lanotte, Appl. Phys. Lett. 84 (22) (2004) 4502. [17] G. Ausanio, A.C. Barone, V. Iannotti, L. Lanotte, S. Amoruso, R. Bruzzese, M. Vitiello, Appl. Phys. Lett. 85 (18) (2004) 4103. [18] N.L. Rupesinghe, R.J. Cole, M. Chhowalla, G.A.J. Amaratunga, P. Weightman, Diam. Relat. Mater. 9 (2000) 1148.
Contents lists available at ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Structure and giant magnetoresistance of carbon-based amorphous films prepared by magnetron sputtering L. Ma a,b,⁎, M.F. He a,b, Z.W. Liu c, D.C. Zeng c, Z.F. Gu a,b, G. Cheng a,b a b c
School of Materials Science and Engineering, Guilin University of Electronic Technology, Guilin 541004, China Guangxi Key Laboratory of Information Materials, Guilin University of Electronic Technology, Guilin 541004, China School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China
a r t i c l e
i n f o
Article history: Received 15 June 2013 Received in revised form 16 January 2014 Accepted 17 January 2014 Available online 24 January 2014 Keywords: Granular films Magnetoresistance Magnetron sputtering
a b s t r a c t Pure amorphous carbon (а-C) and Co-doped CoxC1 − x films were prepared on n-Si(100) substrates by dc magnetron sputtering. In Co–C films, the nano-sized amorphous Co particles were homogeneously dispersed in the amorphous cross-linked carbon matrix. The structures of a-C and CoxC1 − x films were investigated by X-ray photoelectron spectroscopy and Raman spectroscopy. The results showed that the a-C films were diamond-like carbon (DLC) films. After doping cobalt into DLC film, the sp3-hybridized carbon content in DLC composite films almost had no change. The as-deposited CoxC1 − x granular films had larger value of magnetoresistance (MR) than the amorphous carbon film. A very high positive MR, up to 15.5% at magnetic field B = 0.8 T and x = 2.5 at.% was observed in a CoxC1 − x granular film with thickness of 80 nm at room temperature when the external magnetic field was perpendicular to the electric current and the film surface. With increase of the film thickness and Co-doped content, the MR decreased gradually. It remains a challenge to well explain the observed MR effect in the CoxC1 − x granular films. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Amorphous carbon (a-C) thin films have attracted much attention recently due to their potential applications in hard coating, field emission and microelectronic devices [1–3]. More than one decade, metal containing diamond-like carbon (Me-DLC) films have been extensively studied because they cannot only maintain high hardness, low friction coefficient and high thermal stability, but also have adjustable electrical conductivity. This is obviously advantageous for its applications in the fields of electronics, optics, magnetics and catalysis [4–6]. Various deposition techniques have been used to fabricate Me-DLC composite films, such as ion beam assisted deposition [5], co-sputtering deposition [7], pulsed laser deposition (PLD) [8–10], electrolysis deposition [11], and electron cyclotron resonance chemical vapor deposition [6]. Currently, the researches on these carbon films mainly focused on their fabrication, microstructure, magnetoelectronic properties, and potential applications. For example, it was reported that the a-C films prepared by pulsed laser deposition had shown an apparent voltage-induced switch effect and a magnetoresistance (MR) of about 1% in 1 T magnetic field [10]. As we know, Co and C are immiscible and the metastable Co carbides (Co2C and Co3C) decompose easily into Co and C [12]. Co–C composite films therefore form a good metal/insulator system to be investigated. Using PLD method, Zhu et al. [9] have prepared Co–C
⁎ Corresponding author. Tel.: +86 7732291680; fax: +86 7732290129. E-mail address: [email protected] (L. Ma). 0040-6090/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2014.01.048
films and observed a large positive MR of 22% in the Co0.02–C0.98 granular film on Si(100) at room temperature with B = 1 T. In this article, we report the structure and MR properties of the pure а-C films and Co-doped amorphous CoxC1 − x granular films deposited on Si(100) substrates by magnetron sputtering at room temperature. 2. Experimental details The CoxC1 − x thin films with x = 0–25 at.% were deposited on nSi(100) substrates by magnetron sputtering at room temperature. The purity of the Co and C was better than 99.95% and the targets were pieced together with Co sectors putting on the well-proportioned graphite target. The Si(100) substrates were lightly doped with P resulting in a ntype material. Before deposition, the Si substrates were ultrasonically cleaned firstly in acetone and then in ethanol, etched by diluted HF solution and rinsed in de-ionized water. The working gas was Ar (99.99% purity). The deposition took place inside a chamber where the pressure was kept at 0.7 Pa, and the substrate was kept at room temperature. For comparison, pure carbon films were also deposited by magnetron sputtering. The structure and magnetic properties of the Co–C films were investigated by conventional X-ray diffraction (XRD, Philips X'Pert MPO Pro, U = 40 kV/I = 40 mA, X'celerator, Cu-Kα1: λ = 1.54056 Å, 0.0167°/step), scanning electron microscopy (SEM, FEI Nova NanoSEM 430) and physical properties measurement system (PPMS-9, Quantum Design Co.). The atomic concentration and binding energy for each element in the films were determined by X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD, AlKα, energy of photoelectron:
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1486.8 eV, U = 15 kV/I = 10 mA), and the intrinsic bonding structure of carbon-based film is also provided by Raman scattering (Ar+ laser of 532 nm) spectroscopy. 3. Results and discussion Co–C films deposited by magnetron sputtering were characterized by XRD, as shown in Fig. 1 for an example. The results show that the film is in amorphous state in the as-deposited films. Scanning electron microscopy observation shows that the film thickness is in the range of 80–240 nm. Deposition rate, determined from the film thickness and deposition time, increases linearly with power about 0.35 Å/s at 120 W and a total pressure of 0.7 Pa. SEM characterization shows that the microstructure of the carbon-based films are homogeneous distributed with mean particle size of about 30–40 nm, and Co particles are well dispersed in amorphous carbon (Fig. 2). The result is in consistent with some previous reports [9]. Fig. 3a is a typical XPS result for the as-deposited Co–C films. The whole spectrum consists of three main peaks at about 284.5, 531.0, and 778.4 eV, corresponding to the binding energy of C-1s, O-1s, and Co-2p, respectively. The relative atomic Co/C ratio calculated from XPS is about 2.5%. The fine spectrogram of Co-2p is shown in Fig. 3b, which can be assigned to Co atoms bonded to oxygen owing to the surface oxidation when exposed to air. It was reported that cobalt component existed in atomic state, and dispersed randomly in the amorphous carbon matrix indicated by TEM [9]. In our work, the binding energies of the Co-2p peak (Fig. 3b) in pure cobalt and cobalt carbides almost coincide with each other. Moreover, the structural change of the asdeposited samples can be determined by the curve-fitted C-1s spectra according to the Gussian fitting: C_C (284.5–284.7 eV), C\C (285.5– 285.8 eV) and C\O (288.1–288.4 eV) [13]. As shown in Fig. 3c, the C-1s peak was fitted by three Gaussian peaks centered at 284.52, 285.38, and 288.29 eV with area percentages of 78.7%, 16.8%, and 4.5%, respectively. The peaks at 284.52 eV and 285.38 eV correspond to the binding energy of carbon–carbon bonds, and the other peak corresponds to those of C\O bond. No peak corresponding to the binding energy of C-1s in cobalt carbide was observed. It indicated that the C-1s peak in Co doping films did not vary evidently from amorphous carbon film to graphite film, and the doping of Co did not enhance the graphitization of carbon film. This is different from the Fe doping which can increase sp3-hybridized carbon in the films [11]. Fig. 3d shows the fitting result of the O-1s peak in the film. Two major peaks centered at 529.84 and 531.68 eV with area percentages of 12.6% and 82.9%, respectively, correspond to the binding energy of O-1s in cobalt oxide (CoO) and oxygen. It is believed that the formation of CoO is due to surface oxidation of the Co clusters in the film. The contribution of this small amount of CoO to the magnetic properties of the film could be ignored.
Fig. 1. XRD pattern of as-deposited CoxC1 − x (x = 2.5 at.%) composite film deposited at room temperature.
Fig. 2. The SEM pattern of as-deposited Co-DLC films (2.5 at.% Co) deposited at room temperature.
Furthermore, the intrinsic bonding structure of carbon-based film is also provided by Raman scattering (Ar+ laser of 532 nm) spectroscopy. And the main characteristics are the D and G bands appeared in the range of 1200–1700 cm− 1 [14]. As shown in Fig. 4, both D and G bands centered at 1388 and 1573 cm−1 appear in the as-deposited samples, which are typical DLC Raman spectrum. Therein, the G band is attributed to E2g vibrational mode arising from the bond stretching of all pairs of sp2 atoms in both rings and chains, whereas the D band is assigned to the breathing modes of sp2 ring structure in disorder graphite [14,15]. To further identify the internal structure of the as-deposited samples, all the Raman spectra are fitted with a Gaussian profile after background correction. From Fig. 4a and b, the positions of D and G bands do not shift obviously, and the ID/IG ratio almost has no variation compared with that of pure DLC films. Ferrari and Robertson found that there is a relationship between the visible Raman spectra and the sp3 fraction in amorphous carbon films and the sp3 content increased
Fig. 3. XPS analysis of as-deposited Co-DLC films (2.5 at.% Co) deposited at room temperature: (a) the full-spectrum of Co-DLC; (b) Co2p; (c) C1s in Co-DLC film; and (d) O1s in Co-DLC film (curve fitted).
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Fig. 4. Raman spectra of as-deposited DLC (a) and Co-DLC films (b).
simultaneous with the lowering of G-peak position and the decrease of and ID/IG ratio [14]. On the contrary, our results indicate that the sp3/sp2 ratio in the DLC films remains the same with the incorporation of Co in the magnetron sputtering process, which agrees well with the XPS analysis. The magnetic properties for the Co-DLC nanocomposite films were determined by the PPMS in this work, and the hysteresis for a selected film is shown in Fig. 5. The coercivity of only around 30 Oe was obtained, which indicates that the incorporation of Co in the carbon matrix leads to good soft magnetic properties. Fig. 6 shows MR measurement results taken at T = 300 K by fourprobe method on the electromagnetic field device with an in-plane or
Fig. 5. In-plane hysteresis loop for the as-deposited CoxC1 − x (x = 2.5 at.%) granular film measured at 300 K (the inset is the enlarged view of coercivity).
Fig. 6. The MR dependence on magnetic field for as-deposited pure C (a) and Co–C (b) films at room temperature.
out-of-plane external magnetic field. The MR is defined as △ R/R = [R(B,T) − R(0,T) / R(0,T)], where R(B,T) and R(0,T) are resistances measured at temperature T with or without a magnetic field B applied, respectively. As shown in Fig. 6a, the pure carbon film shows different MR properties when the magnetic field direction is parallel or perpendicular to the film plane. At 300 K with B = 0.8 T, a large MR value of about 1% is obtained in the pure C film when it is perpendicular to the field direction. Similarly, for Co-doped amorphous CoxC1 − x granular films, a larger value of MR was obtained when the electric current is perpendicular to the external magnetic field instead of parallel it, which indicates that the CoxC1 − x granular films have strong in-plane anisotropy. As shown in Fig. 6b, the as-deposited Co–C granular films have much larger MR values than amorphous carbon film. It is also found that the MR of the CoxC1 − x film decreases with increase of thickness. For a 240-nm thick film, a MR value of ~5% was obtained at room temperature when the external magnetic field is perpendicular to the film surface. A very large positive MR, up to 15.5% at magnetic field B = 0.8 T and x = 2.5 at.%, was observed in the film with thickness of 80 nm. Interestingly, our results are different from other reports. In reference [9], Co–C granular films show fine MR property when the external magnetic field direction is in the film plane. It is possible that the difference is caused by two different deposition methods: magnetron sputtering and PLD. The recently reported morphological peculiarities of the nanoparticle films obtained by PLD are the oblate ellipsoidal shape of the constituent nanograins, with an aspect ratio which depends on the laser intensity, and their major cross section parallel to the deposition substrate, as a consequence of the nanodrop impact with the substrate [16,17]. We believe that the difference arises due to shape anisotropy or magnetic anisotropy. As for this remarkable positive MR effect, Xue and Zhang [10] proposed that a p–n heterojunction is formed when the C film and n–Si are in close contact, because the a-C films are p-type semiconductor. When the Fermi energy of the C film is
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lower than that of the n–Si, the electrons will flow from the n–Si to the C film. However, there is a conduction band that offsets between the C film and the n–Si substrate [18]. As a result, a two-dimensional electron gas (2DEG) is formed on the Si surface. When the applied electric field (or voltage) reaches a threshold, a large number of electrons in the localized states are excited into the extended states. Under the external magnetic field applied on the 2DEG, the Lorentz force will curve the orbits of the electrons which will result in a positive MR. Moreover, Jiang and Gao [8] proposed that the Co aggregation in the Co–C/Si interface may be a possible origin of the positive colossal MR effect. However, to evaluate the validity of this proposal, more experimental and theoretical works are in need. 4. Conclusions We investigated the structure and room temperature positive MR in Co-doped amorphous carbon film and pure amorphous carbon film, which have simple structure and can be easily prepared on Si(100) substrate by magnetron sputtering method. It was found that the a-C film and CoxC1 − x films had fairly smooth and flat surface, and the Co particles were homogeneously dispersed into the amorphous cross-linked carbon matrix. The results of Raman spectra show that the amorphous carbon films are diamond-like carbon films. After doping cobalt into DLC film, the sp3-hybridized carbon content in DLC composite films has no significant change. Larger values of MR were obtained in these films when the electric current is perpendicular to the external magnetic field instead of parallel to it. The CoxC1 − x films had better MR properties than C film, in particular, the as-deposited CoxC1 − x (x = 2.5 at.%) film with a thickness of 80 nm has shown a positive MR of 15.5% under B = 0.8 T at room temperature. The positive MR effect can be understood by energy band model proposed by previous researchers, but the underlying mechanism is still in need of study.
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Acknowledgments This work is supported by the Natural Science Foundation of Guangxi (2013GXNSFBA019242, 2012GXNSFGA060002), the National Natural Science Foundation of China (U0734001, 51161005, 51261004), the Guangxi Key Laboratory of Information Materials (1110908-04-Z) and the Fundamental Research Funds for the Central Universities (SCUT, 2012ZZ0015). References [1] N.A. Hastas, C.A. Dimitriadis, D.H. Tassis, S. Logothetidis, Appl. Phys. Lett. 79 (5) (2001) 638. [2] D.B. Mahadik, S.S. Shinde, C.H. Bhosale, K.Y. Rajpure, J. Alloy. Compd. 509 (5) (2011) 1418. [3] M.W. Huang, J.Y. Jao, C.C. Lin, W.J. Hsieh, Y.H. Yang, L.S. Cheng, F.S. Shieu, H.C. Shih, Appl. Surf. Sci. 261 (2012) 21. [4] Y.C. Jiang, Z.P. Wu, W. Bao, S.J. Xu, J. Gao, J. Appl. Phys. 111/7 (2012) 07C510. [5] T. Hioki, Y. Itoh, A. Itoh, S. Hibi, J. Kawamoto, Surf. Coat. Technol. 46 (1991) 233. [6] Rusl, S.F. Yoon, Q.F. Huang, H. Yang, M.B. Yu, J. Ahn, Q. Zhang, E.J. Teo, T. Osipowicz, F. Watt, J. Appl. Phys. 88/6 (2000) 3699. [7] K. Bewilogua, C.V. Cooper, C. Specht, J. Schroder, R. Wittorf, M. Grischke, Surf. Coat. Technol. 127 (2000) 224. [8] Y.C. Jiang, J. Gao, Appl. Phys. Lett. 101 (18) (2012) 182401. [9] D.D. Zhu, X. Zhang, Q.Z. Xue, J. Appl. Phys. 95 (4) (2004) 1906. [10] Q.Z. Xue, X. Zhang, Carbon 43 (4) (2005) 760. [11] S.H. Wan, L.P. Wang, Q.J. Xue, Electrochem. Commun. 11 (1) (2009) 99. [12] T.J. Konno, R. Sinclair, Acta Metall. Mater. 42 (4) (1994) 1231. [13] E. Riedo, F. Comin, J. Chevrier, F. Schmithusen, S. Decossas, M. Sancrotti, Surf. Coat. Technol. 125 (2000) 124. [14] A.C. Ferrari, J. Robertson, Phys. Rev. B 61 (20) (2000) 14095. [15] A.C. Ferrari, Solid State Commun. 143 (1–2) (2007) 47. [16] S. Amoruso, R. Bruzzese, N. Spinelli, R. Velotta, M. Vitiello, X. Wang, G. Ausanio, V. Iannotti, L. Lanotte, Appl. Phys. Lett. 84 (22) (2004) 4502. [17] G. Ausanio, A.C. Barone, V. Iannotti, L. Lanotte, S. Amoruso, R. Bruzzese, M. Vitiello, Appl. Phys. Lett. 85 (18) (2004) 4103. [18] N.L. Rupesinghe, R.J. Cole, M. Chhowalla, G.A.J. Amaratunga, P. Weightman, Diam. Relat. Mater. 9 (2000) 1148.