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Effects of Ni doping and structural defects on magnetic properties of annealed SiC films
Superlattices and Microstructures 96 (2016) 267e272
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Effects of Ni doping and structural defects on magnetic properties of annealed SiC films Yuting Fu, Xin Jin, Ning Sun, Chunjing Li, Yukai An**, Jiwen Liu* School of Material Science and Engineering, Tianjin University of Technology, Tianjin Key Laboratory for Photoelectric Materials and Devices, Tianjin 300384, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 March 2016 Received in revised form 20 May 2016 Accepted 23 May 2016 Available online 1 June 2016
Ni-doped SiC films deposited on Si (100) substrates prepared by RF-magnetron sputtering were discussed in this paper. The results show that with reference to the as-deposited as well as annealing at 800 C. C atoms were substituted by Ni atoms in the 3CeSiC lattice and Ni-related secondary phase cannot be detected. After annealing at 1200 C, the crystal quality improved obviously while the majority of Ni atoms form the Ni2Si secondary phase. Temperature dependent on resistivity reveals that the conduction mechanism is dominated by Mott variable range hopping behavior for the Ni-doped SiC films, confirming that the carriers are localized. All the films are ferromagnetic at 300 K and annealing can evidently improve the room-temperature (RT) ferromagnetism. The bound magnetic polarons should be responsible for the RT ferromagnetism of the Ni-doped SiC films. © 2016 Elsevier Ltd. All rights reserved.
Keywords: SiC film Ni doping Annealing Structural defect Magnetic properties
1. Introduction Dilute magnetic semiconductors (DMSs) have been put on extensive interests due to its multifunctional properties in spintronic devices where both charge and spin degrees of freedom can be manipulated [1]. It is essential to achieve room temperature ferromagnetism (RTFM) in DMS for applications [2]. Among the potential host materials for DMS, SiC is a pioneered and unending promising material because of its excellent properties: high breakdown field, high thermal conductivity, and chemical stability, which makes us more to believe it promising to fabricate spintronic materials [3,4]. Recently, considerable attention has been paid to multifunctional DMS behavior of SiC. Theoretical calculations by Padmaja et al. found that Sc or Ti -doped SiC favoring the magnetic interaction when it replaces C atoms rather than Si atoms [5]. So magnetic properties introduced in cubic SiC not only depend on the types of dopant but also the sites of substitution. Although Mn, Cu, Al and V-doped SiC films have been found RT ferromagnetism [6e9], the origin of ferromagnetic order is still under debate. Many results show that the ferromagnetism may be related to transition metal (TM) clusters, secondary phases, intrinsic defects or other factors [10e12]. Despite of the signatures of ferromagnetic behavior, the origin of the ferromagnetism in doped SiC is not understood and further studies has to be reinvestigated. In this paper, we present our studies on Ni local environment, magnetism and transport properties of Ni-doped SiC thin films prepared by radio-frequency magnetron sputtering technique. The XAFS technique is determined to investigate the local atomic environment of Ni in the SiC films. We purpose of acquiring the dependence of magnetism and transport properties on
* Corresponding author. ** Corresponding author. E-mail address: [email protected] (J. Liu). http://dx.doi.org/10.1016/j.spmi.2016.05.033 0749-6036/© 2016 Elsevier Ltd. All rights reserved.
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the local structure around the Ni atoms and the formation of the Ni2Si secondary phase. In addition, what attracted us more is the origin of the observed RT ferromagnetism. 2. Experimental details Ni-doped SiC films with different annealing temperature were deposited on Si (100) substrates using radio-frequency magnetron sputtering system. A high-purity SiC target with some Ni chips symmetrically attached was used to deposit the Ni-doped SiC films. The base pressure of the sputtering system was 8 105Pa and the working pressure during the deposition process was 1.0 Pa. Ar (purity 99.99%) was used as sputtering gas and the sputtering time was 60 min. To improve the crystalline quality of the films, the films were annealed at 800 C and 1200 C for 120 min in condition of vacuum at lower than 8 105Pa. The concentrations of Ni in the films were determined as 4 at% basing on the energy dispersive spectroscopy (EDS). The crystal structure and phase were analyzed by X-ray diffraction (XRD) using Cu Ka radiation (l ¼ 0.15406 nm). XPS spectra were measured with a PHI-1600 photoelectric spectrometer (Mg Ka, X-ray) to determine the chemical states of the film. The Ni K-edge X-ray absorption near edge structure (XANES) data was obtained at the 4B9A beam line of Beijing Synchrotron Radiation Facility. The dependence of resistance on temperature was measured by the standard four probe method on a Physical Property Measurement System (PPMS). Before testing, four ribbon-shaped indium electrodes with same width were welded on the sample surface, then the samples with indium electrodes were put between the pole pieces of an electromagnet in the condition of a continuous flow of helium. When the samples were tested, the cooling rate of helium was kept at 5 K/min over a temperature range of 13e300 K. The electrical properties of the samples were measured by the Hall effect measurement system with the van der Pauw method. The magnetic properties were measured with a superconducting quantum interference device magnetometer (SQUID) at 300 K temperature. 3. Results and discussion The XRD curves of the as-deposited, 800 C and 1200 C annealed Ni-doped SiC films are shown in Fig. 1(a). The diffraction peak corresponding to the crystalline SiC was not observed for the as-deposited film, which shows that the as-deposited film was amorphous. After 800 C and 1200 C annealing, it is obvious that the 3CeSiC (111) diffraction peak appears. And the intensity of (111) diffraction peak increases with increasing the annealing temperature, indicating that the crystal quality of SiC is improved. The Ni metal or Ni-related secondary phases were not detected for the as-deposited and the films annealing at 800 C. However it is clear after annealing at 1200 C, (112), (103), (210), (013) and (020) diffraction peaks of Ni2Si compound obviously exist, which confirms that the Ni2Si emerges as secondary phase. In order to ascertain the valences of Ni in the films, the films are measured by XPS. Fig. 1(b) shows the Ni 2p XPS spectra of the as-deposited and 1200 C annealed Nidoped SiC films. It is well known that for the Ni metal and NiO, the 2p3/2 peak of Ni are located at 852.3 and 853.3 eV, with the energy separation between the 2p3/2 and 2p1/2 peaks D ¼ 17.4 and 18.4 eV, respectively. For the as-deposited and 1200 C annealed films, the Ni 2p3/2 peaks can be fitted into two different peaks situated at 853.58 eV and 853.06 eV with D ¼ 18.03 and 18.2 eV respectively. Compared with the energy separation of Ni metal and NiO, it is most possible that the doped Ni atoms are mainly in Ni2þ states and the existence of Ni metal (Ni0) can be excluded. In order to accurately detect the existing forms of the doped Ni atoms in the films, we performed the extend X-ray absorption fine structure (EXAFS) measurements as a sensitive local-structure probe to further investigate the structural
(b)
Ni 2p o
Intensity (a.u.)
Ni2Si(020)
Ni2Si(112) Ni2Si(103) Ni2Si(210) Ni2Si(013)
1200 C annealed 3C-SiC(111)
Intensity (a.u.)
Si(100)
(a)
18.2 eV 2p3/2 2p1/2
1200 C annealed
as-deposited
800 C annealed as-deposited
20
25
30
35
40
45
2 (degree)
50
55
60
18.03 eV 845
850
855
860
865
870
875
880
Binding Energy (eV)
Fig. 1. (a) XRD curves of the as-deposited, 800 C and 1200 C annealed Ni-doped SiC films. (b) Ni 2p XPS spectra of the as-deposited and 1200 C annealed Nidoped SiC films.
Y. Fu et al. / Superlattices and Microstructures 96 (2016) 267e272
269
(b)
Intensity (a.u.)
Ni Ni 2O3 Ni 2Si o
1200 C annealed o
800 C annealed
Intensity (a.u.)
(a)
o
800 C annealed
as-deposited 0
1
2
3
4
5
6
o
7
as-deposited 8
0
2
R (A)
4
o
R (A)
6
8
10
Fig. 2. (a) Experimental Fourier transform curves of Ni K-edge EXAFS oscillation functions k3c (k) for the as-deposited, 800 C and 1200 C annealed Ni-doped SiC films as well as Ni, Ni2O3 and Ni2Si foils. (b) Fitting Fourier transform curves of Ni K-edge EXAFS oscillation functions k3c (k) for the as-deposited and 800 C annealed Ni-doped SiC films. Solid lines: experimental, Dashed lines: fitting.
Table 1 The best fitted parameters of the Ni K-edge Fourier transform curves of the as-deposited and 800 C annealed films. Samples
N
R (Å)
R-factor:s2
As-deposited 800 annealed
1.12 2.56
1.95 1.99
0.006 0.004
N, R and s2 are the coordination number, bond length and Debye-Waller factor, respectively. The uncertainties for N, R, and s2 are 5%, 0.01 Å, and 5%, respectively.
characteristics. Fig. 2(a) shows the experimental Fourier transform curves of Ni K-edge EXAFS oscillation functions k3c (k) for the as-deposited, 800 C and 1200 C annealed films as well as Ni metal, Ni2O3 and Ni2Si foils. It has been seen clearly that the films annealing at 1200 C exists one strong peak located at 1.88 Å, corresponding to the first shell of Ni atoms in the film. The peak position is very similar to that of Ni2Si (1.89 Å), but different from Ni metal (2.17 Å) and Ni2O3 (1.56 and 2.48 Å). On the other hand, for the as-deposited film, there is one peak located at 1.81 Å and for 800 C annealed films, there is one peak located at 1.76 Å. The positions of the two peaks are obviously different from those of Ni metal, Ni2O3 and Ni2Si. So it is very possible that the doped Ni atoms are incorporated into the SiC lattice and occupy the C or Si sites for the as-deposited and 800 C annealed films. As the ionic radius of Ni2þ(0.69 Å) is very close to the covalent radius of C (0.77 Å) but far less than the covalent radius of Si(1.12 Å), Ni ions are more easier to substitute C sites in the 3CeSiC lattice. Absence of NieC bonds in the XPS spectra further confirms the substitution of Ni for C site. In order to determine the accurate location of Ni atoms in the as-deposited and 800 C annealed films, we fitted the main Fourier transform peaks located at
Fig. 3. Variation of resistivity r and carrier concentration pc of the Ni-doped SiC films with the annealing temperature.
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Fig. 4. (a)The r-T curves of the as-deposited Ni-doped SiC films. (b)The r-T curves of the 800 C annealed Ni-doped SiC films. (c)The r-T curves of the 1200 C annealed Ni-doped SiC films. The insets show the plot of ln(r) versus T1/4 for the as-deposited, 800 C and 1200 C annealed Ni-doped SiC films. The solid lines show the fitting curves.
1.81 Å and 1.76 Å. The best fit can be obtained by the structural model of assuming Ni atom substitution for C site of the 3CeSiC lattice, as shown in Fig. 2(b). The fitted bond length, coordination number and Debye-Waller factors (s) are shown in Table 1. The obtained fit values for the NieSi bond length (RNieSi) in the as-deposited and 800 C annealed films are about 1.95 and 1.99 Å, respectively. The obtained NieSi bond length is slightly larger than the SieC bond length (RCeSi ¼ 1.89 Å) in SiC [13], which is reasonable since RNi2þ ¼ 0.069 nm is larger than RC4þ ¼ 0.016 nm. These results manifest that the doped Ni ions are incorporated into the C sites of 3CeSiC lattice for the as-deposited and 800 C annealed Ni-doped SiC films. The carrier concentration pc and resistivity r are measured to investigate the possible magnetic mechanism responsible for the observed ferromagnetism in the Ni-doped SiC films. Results from the Hall measurements indicate that the Ni-doped SiC is p-type. The dependence of carrier concentration pc and resistivity r on the annealing temperature is shown in Fig. 3. It can be seen that the pc and r are not almost changed for the as-deposited and 800 C annealed films. However, the r remarkably increases and pc decreases after annealing at 1200 C. The Ni doping (the substitution of Ni2þ for C4þ) can be considered as the possible sources contributing to the hole carriers for the as-deposited and 800 C annealed films. On the other hand, the doped Ni atoms mainly exist in the form of Ni2Si phase for 1200 C annealed films, resulting in the number of Ni2þ ions substitute for the C sites of SiC lattice decreases and also lead to decrease of the hole carriers concentration pc. So it can be considered that the remarkable change in r and pc for the 1200 C annealed film may be due to the formation of Ni2Si phase. The resistivity r as a function of temperature T was measured for the as-deposited, 800 C and 1200 C annealed Ni-doped SiC films are shown in Fig. 4(a)e(c). It can be seen that r and temperature emerge inversely proportional in the whole temperature range, indicating semiconductor characteristic for the Ni-doped SiC films. It is obvious that the linear relationship between lnr and T1/4 for the as-deposited and 800 C annealed films in the insets of Fig. 4(a)e(b) shows that the
Y. Fu et al. / Superlattices and Microstructures 96 (2016) 267e272
4
2
2
800 C
0
as-deposited
o
800 C annealed
-2 -4
-1000
-500
0
500
as-deposited
1000
H (Oe) 3.0 3
0
1200 C annealed
o
M (emu/cm )
3
M(emu/cm )
3
M (emu/cm )
4
o
o
1200 C
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271
-2 -4
2.8
FC
2.6 2.4
Tc1
ZFC
2.2 2.0
-6
0
50 100 150 200 250 300 350
T (K)
-20000-15000-10000 -5000
0
5000 10000 15000 20000
H (Oe) Fig. 5. MeH curves of the as-deposited, 800 and 1200 C annealed Ni-doped SiC films at 300 K. The top inset shows the enlarged MeH curves at low magnetic fields. The bottom inset shows the temperature dependence of ZFC and FC curves under a magnetic field of 500 Oe for the 1200 C annealed Ni-doped SiC film.
transport mechanism is governed by the Mott variable range hopping (VRH) behavior, namely r ~ exp[(T0/T)1/4], indicating the carriers are strongly localized [14]. For the 1200 C annealed film, the best understanding of transport mechanisms can be achieved by the combination of the Mott VRH model in the low temperature range and the hard band gap hopping, r ~ exp(Ed/ KBT), in the high temperature range. Fig. 5 shows the magnetization as functions of magnetic field at 300 K for the as-deposited, 800 and 1200 C annealed Ni-doped SiC films. The magnetization saturates at high magnetic fields and exhibits a clear hysteresis at lower fields, indicating that all the films are RT ferromagnetism (top inset of Fig. 5). With the annealing temperature increasing, the saturation magnetization of the films increases from 2.1emu/cm3 to 5.4emu/cm3. It suggests that annealing can enhance the saturation magnetization of the films. Until 800 C annealing, the possible TM clusters or secondary phase can be excluded by XRD, XPS and EXAFS measurements. It can be concluded that the observed RT ferromagnetism in the as-deposited and 800 C annealed films is intrinsic property. After annealing at 1200 C, majority of Ni atoms form the Ni2Si secondary phase compounds. Recently, some researches have indicated that the Ni2Si secondary phase is ferromagnetic with the Curie temperature of about 200 K [15]. This can also be verified by the measurements of zero-filed-cooling (ZFC) and field-cooling (FC) curves. From the inset of Fig. 5, it is clear that there exists an obvious FM transitions temperatures located at TC1 in the FC curve, corresponding a FM phases for the film annealing at 1200 C. By the trend of linear fitting the FC curves of the FM phase, the Curie temperature TC1 about 210 K can be obtained, as shown by the red dash lines in the bottom inset of Fig. 5. So it is reasonably concluded that the secondary phase Ni2Si cannot be responsible for the origin of room-temperature FM order in the 1200 C annealed film. In addition, many types of defects could induce ferromagnetism by the formation of bound magnetic polarons [16]. The process of co-sputtering and high temperature annealing can make many defects emerge in the films. The defects can play a crucial role on the RT ferromagnetism. Because the bound magnetic polarons take effect on the condition of ordered crystal structure, the saturated magnetic moment enhanced with the increase of annealing temperature suggests that the crystal quality of SiC has a great influence on ferromagnetism. Especially after 1200 C annealing, the two reasons that the crystal quality of SiC obviously improves and the number of defects increase because of phase boundaries between the Ni-doped SiC and Ni2Si compounds make the saturated magnetic moment obviously enhance. Therefore, it can be concluded that the origin of RT ferromagnetic order in Ni-doped SiC films does not come from the contribution of Ni2Si phase and the defects are crucial for introducing ferromagnetic order into the Ni-doped SiC system. 4. Conclusions The Ni-doped SiC films were prepared by RF-magnetron sputtering technique. The doped Ni atoms exist in forms of Ni2þ in as-deposited and annealed films. For the as-deposited and 800 C annealed films, the doped Ni atoms substitute for the C sites in the 3CeSiC lattice and Ni-related secondary phase can not be detected. For the 1200 C annealed films, the crystal quality of
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SiC is improved and the majority of Ni atoms form the Ni2Si secondary phase. For the as-deposited and 800 C annealed films, the transport properties are governed by Mott variable range hopping (VRH) behavior, indicating that the carriers are strongly localized. However the transport mechanisms can be achieved by the combination of the Mott VRH model in the low temperature range and the hard band gap hopping in the high temperature range for the 1200 C annealed film. All the films are ferromagnetic at 300 K and annealing temperature can obviously improve their ferromagnetism. The bound magnetic polarons induced by the defects of oxygen vacancies should be responsible for the room temperature ferromagnetism of the films. Acknowledgement This work was supported by National Natural Science Foundation of China (Grant No 10904110, 11174217). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) 1019. G. Shanmuganathan, I.B. Shameem Banu, Superlattices Microstruct. 75 (2014). K. Bouziane, M. Mamor, M. Elzain, Phys. Rev. B 78 (2008) 195305. J. Adell, I. Ulfat, J. Sadowski, L. Ilver, J. Kanski, Solid State Commun. 151 (850) (2011). P. Patnaik, G. Mukhopadhyay, P.P. Singh, AIP Conf. Proc. 1349 (2011) 1087. S.Q. Wang, Y.K. An, X. Li, Z.H. Wu, J.W. Liu, J. Alloys Compd. 584 (2014) 339. H.W. Zheng, Y.L. Yan, Z.C. Lv, S.W. Yang, X.G. Li, J.D. Liu, B.J. Ye, C.X. Peng, C.L. Diao, W.F. Zhang, Appl. Phys. Lett. 102 (2013) 142409. C.H. Zhang, J.B. Lian, H.L. Dickon, J. Am. Ceram. Soc. 96 (2013) 3494. H. Wang, C.F. Yan, E.W. Shi, Appl. Phys. Lett. 101 (2012) 142404. S.Y. Zhuo, X.C. Liu, E.W. Shi, Chin. Phys. B 21 (2012) 067503. F. Stromberg, W. Keune, X. Chen, S. Bedanta, H. Reuther, A. Mucklich, J. Phys. 18 (2006) 9881. B. Song, H.Q. Bao, H. Li, M. Lei, T. H.Peng, J. K.Jian, J. Liu, W.Y. Wang, W.J. Wang, X.L. Chen, J. Am. Chem.Soc. 131 (2009) 1376. H.W. Zheng, Z.Q. Wang, X.Y. Liu, C.L. Diao, H.R. Zhang, Y.Z. Gu, Appl. Phys. Lett. 99 (2011) 222512. N.F. Mott, E.A. Davies, in: Electronic Processes in Noncrystalline Materials, second ed., Oxford University, New York, 1979. C.Y. Chen, Y.K. Lin, C.W. Hsu, C.Y. Wang, Y.L. Chueh, L.J. Chen, S.C. Lo, L.J. Chou, Nano. Lett. 12 (2012) 2254. J.M.D. Coey, M. Venkatesan, C.B. Fitzgerald, Nat. Mater. 4 (2005) 173.
Contents lists available at ScienceDirect
Superlattices and Microstructures journal homepage: www.elsevier.com/locate/superlattices
Effects of Ni doping and structural defects on magnetic properties of annealed SiC films Yuting Fu, Xin Jin, Ning Sun, Chunjing Li, Yukai An**, Jiwen Liu* School of Material Science and Engineering, Tianjin University of Technology, Tianjin Key Laboratory for Photoelectric Materials and Devices, Tianjin 300384, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 March 2016 Received in revised form 20 May 2016 Accepted 23 May 2016 Available online 1 June 2016
Ni-doped SiC films deposited on Si (100) substrates prepared by RF-magnetron sputtering were discussed in this paper. The results show that with reference to the as-deposited as well as annealing at 800 C. C atoms were substituted by Ni atoms in the 3CeSiC lattice and Ni-related secondary phase cannot be detected. After annealing at 1200 C, the crystal quality improved obviously while the majority of Ni atoms form the Ni2Si secondary phase. Temperature dependent on resistivity reveals that the conduction mechanism is dominated by Mott variable range hopping behavior for the Ni-doped SiC films, confirming that the carriers are localized. All the films are ferromagnetic at 300 K and annealing can evidently improve the room-temperature (RT) ferromagnetism. The bound magnetic polarons should be responsible for the RT ferromagnetism of the Ni-doped SiC films. © 2016 Elsevier Ltd. All rights reserved.
Keywords: SiC film Ni doping Annealing Structural defect Magnetic properties
1. Introduction Dilute magnetic semiconductors (DMSs) have been put on extensive interests due to its multifunctional properties in spintronic devices where both charge and spin degrees of freedom can be manipulated [1]. It is essential to achieve room temperature ferromagnetism (RTFM) in DMS for applications [2]. Among the potential host materials for DMS, SiC is a pioneered and unending promising material because of its excellent properties: high breakdown field, high thermal conductivity, and chemical stability, which makes us more to believe it promising to fabricate spintronic materials [3,4]. Recently, considerable attention has been paid to multifunctional DMS behavior of SiC. Theoretical calculations by Padmaja et al. found that Sc or Ti -doped SiC favoring the magnetic interaction when it replaces C atoms rather than Si atoms [5]. So magnetic properties introduced in cubic SiC not only depend on the types of dopant but also the sites of substitution. Although Mn, Cu, Al and V-doped SiC films have been found RT ferromagnetism [6e9], the origin of ferromagnetic order is still under debate. Many results show that the ferromagnetism may be related to transition metal (TM) clusters, secondary phases, intrinsic defects or other factors [10e12]. Despite of the signatures of ferromagnetic behavior, the origin of the ferromagnetism in doped SiC is not understood and further studies has to be reinvestigated. In this paper, we present our studies on Ni local environment, magnetism and transport properties of Ni-doped SiC thin films prepared by radio-frequency magnetron sputtering technique. The XAFS technique is determined to investigate the local atomic environment of Ni in the SiC films. We purpose of acquiring the dependence of magnetism and transport properties on
* Corresponding author. ** Corresponding author. E-mail address: [email protected] (J. Liu). http://dx.doi.org/10.1016/j.spmi.2016.05.033 0749-6036/© 2016 Elsevier Ltd. All rights reserved.
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the local structure around the Ni atoms and the formation of the Ni2Si secondary phase. In addition, what attracted us more is the origin of the observed RT ferromagnetism. 2. Experimental details Ni-doped SiC films with different annealing temperature were deposited on Si (100) substrates using radio-frequency magnetron sputtering system. A high-purity SiC target with some Ni chips symmetrically attached was used to deposit the Ni-doped SiC films. The base pressure of the sputtering system was 8 105Pa and the working pressure during the deposition process was 1.0 Pa. Ar (purity 99.99%) was used as sputtering gas and the sputtering time was 60 min. To improve the crystalline quality of the films, the films were annealed at 800 C and 1200 C for 120 min in condition of vacuum at lower than 8 105Pa. The concentrations of Ni in the films were determined as 4 at% basing on the energy dispersive spectroscopy (EDS). The crystal structure and phase were analyzed by X-ray diffraction (XRD) using Cu Ka radiation (l ¼ 0.15406 nm). XPS spectra were measured with a PHI-1600 photoelectric spectrometer (Mg Ka, X-ray) to determine the chemical states of the film. The Ni K-edge X-ray absorption near edge structure (XANES) data was obtained at the 4B9A beam line of Beijing Synchrotron Radiation Facility. The dependence of resistance on temperature was measured by the standard four probe method on a Physical Property Measurement System (PPMS). Before testing, four ribbon-shaped indium electrodes with same width were welded on the sample surface, then the samples with indium electrodes were put between the pole pieces of an electromagnet in the condition of a continuous flow of helium. When the samples were tested, the cooling rate of helium was kept at 5 K/min over a temperature range of 13e300 K. The electrical properties of the samples were measured by the Hall effect measurement system with the van der Pauw method. The magnetic properties were measured with a superconducting quantum interference device magnetometer (SQUID) at 300 K temperature. 3. Results and discussion The XRD curves of the as-deposited, 800 C and 1200 C annealed Ni-doped SiC films are shown in Fig. 1(a). The diffraction peak corresponding to the crystalline SiC was not observed for the as-deposited film, which shows that the as-deposited film was amorphous. After 800 C and 1200 C annealing, it is obvious that the 3CeSiC (111) diffraction peak appears. And the intensity of (111) diffraction peak increases with increasing the annealing temperature, indicating that the crystal quality of SiC is improved. The Ni metal or Ni-related secondary phases were not detected for the as-deposited and the films annealing at 800 C. However it is clear after annealing at 1200 C, (112), (103), (210), (013) and (020) diffraction peaks of Ni2Si compound obviously exist, which confirms that the Ni2Si emerges as secondary phase. In order to ascertain the valences of Ni in the films, the films are measured by XPS. Fig. 1(b) shows the Ni 2p XPS spectra of the as-deposited and 1200 C annealed Nidoped SiC films. It is well known that for the Ni metal and NiO, the 2p3/2 peak of Ni are located at 852.3 and 853.3 eV, with the energy separation between the 2p3/2 and 2p1/2 peaks D ¼ 17.4 and 18.4 eV, respectively. For the as-deposited and 1200 C annealed films, the Ni 2p3/2 peaks can be fitted into two different peaks situated at 853.58 eV and 853.06 eV with D ¼ 18.03 and 18.2 eV respectively. Compared with the energy separation of Ni metal and NiO, it is most possible that the doped Ni atoms are mainly in Ni2þ states and the existence of Ni metal (Ni0) can be excluded. In order to accurately detect the existing forms of the doped Ni atoms in the films, we performed the extend X-ray absorption fine structure (EXAFS) measurements as a sensitive local-structure probe to further investigate the structural
(b)
Ni 2p o
Intensity (a.u.)
Ni2Si(020)
Ni2Si(112) Ni2Si(103) Ni2Si(210) Ni2Si(013)
1200 C annealed 3C-SiC(111)
Intensity (a.u.)
Si(100)
(a)
18.2 eV 2p3/2 2p1/2
1200 C annealed
as-deposited
800 C annealed as-deposited
20
25
30
35
40
45
2 (degree)
50
55
60
18.03 eV 845
850
855
860
865
870
875
880
Binding Energy (eV)
Fig. 1. (a) XRD curves of the as-deposited, 800 C and 1200 C annealed Ni-doped SiC films. (b) Ni 2p XPS spectra of the as-deposited and 1200 C annealed Nidoped SiC films.
Y. Fu et al. / Superlattices and Microstructures 96 (2016) 267e272
269
(b)
Intensity (a.u.)
Ni Ni 2O3 Ni 2Si o
1200 C annealed o
800 C annealed
Intensity (a.u.)
(a)
o
800 C annealed
as-deposited 0
1
2
3
4
5
6
o
7
as-deposited 8
0
2
R (A)
4
o
R (A)
6
8
10
Fig. 2. (a) Experimental Fourier transform curves of Ni K-edge EXAFS oscillation functions k3c (k) for the as-deposited, 800 C and 1200 C annealed Ni-doped SiC films as well as Ni, Ni2O3 and Ni2Si foils. (b) Fitting Fourier transform curves of Ni K-edge EXAFS oscillation functions k3c (k) for the as-deposited and 800 C annealed Ni-doped SiC films. Solid lines: experimental, Dashed lines: fitting.
Table 1 The best fitted parameters of the Ni K-edge Fourier transform curves of the as-deposited and 800 C annealed films. Samples
N
R (Å)
R-factor:s2
As-deposited 800 annealed
1.12 2.56
1.95 1.99
0.006 0.004
N, R and s2 are the coordination number, bond length and Debye-Waller factor, respectively. The uncertainties for N, R, and s2 are 5%, 0.01 Å, and 5%, respectively.
characteristics. Fig. 2(a) shows the experimental Fourier transform curves of Ni K-edge EXAFS oscillation functions k3c (k) for the as-deposited, 800 C and 1200 C annealed films as well as Ni metal, Ni2O3 and Ni2Si foils. It has been seen clearly that the films annealing at 1200 C exists one strong peak located at 1.88 Å, corresponding to the first shell of Ni atoms in the film. The peak position is very similar to that of Ni2Si (1.89 Å), but different from Ni metal (2.17 Å) and Ni2O3 (1.56 and 2.48 Å). On the other hand, for the as-deposited film, there is one peak located at 1.81 Å and for 800 C annealed films, there is one peak located at 1.76 Å. The positions of the two peaks are obviously different from those of Ni metal, Ni2O3 and Ni2Si. So it is very possible that the doped Ni atoms are incorporated into the SiC lattice and occupy the C or Si sites for the as-deposited and 800 C annealed films. As the ionic radius of Ni2þ(0.69 Å) is very close to the covalent radius of C (0.77 Å) but far less than the covalent radius of Si(1.12 Å), Ni ions are more easier to substitute C sites in the 3CeSiC lattice. Absence of NieC bonds in the XPS spectra further confirms the substitution of Ni for C site. In order to determine the accurate location of Ni atoms in the as-deposited and 800 C annealed films, we fitted the main Fourier transform peaks located at
Fig. 3. Variation of resistivity r and carrier concentration pc of the Ni-doped SiC films with the annealing temperature.
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Fig. 4. (a)The r-T curves of the as-deposited Ni-doped SiC films. (b)The r-T curves of the 800 C annealed Ni-doped SiC films. (c)The r-T curves of the 1200 C annealed Ni-doped SiC films. The insets show the plot of ln(r) versus T1/4 for the as-deposited, 800 C and 1200 C annealed Ni-doped SiC films. The solid lines show the fitting curves.
1.81 Å and 1.76 Å. The best fit can be obtained by the structural model of assuming Ni atom substitution for C site of the 3CeSiC lattice, as shown in Fig. 2(b). The fitted bond length, coordination number and Debye-Waller factors (s) are shown in Table 1. The obtained fit values for the NieSi bond length (RNieSi) in the as-deposited and 800 C annealed films are about 1.95 and 1.99 Å, respectively. The obtained NieSi bond length is slightly larger than the SieC bond length (RCeSi ¼ 1.89 Å) in SiC [13], which is reasonable since RNi2þ ¼ 0.069 nm is larger than RC4þ ¼ 0.016 nm. These results manifest that the doped Ni ions are incorporated into the C sites of 3CeSiC lattice for the as-deposited and 800 C annealed Ni-doped SiC films. The carrier concentration pc and resistivity r are measured to investigate the possible magnetic mechanism responsible for the observed ferromagnetism in the Ni-doped SiC films. Results from the Hall measurements indicate that the Ni-doped SiC is p-type. The dependence of carrier concentration pc and resistivity r on the annealing temperature is shown in Fig. 3. It can be seen that the pc and r are not almost changed for the as-deposited and 800 C annealed films. However, the r remarkably increases and pc decreases after annealing at 1200 C. The Ni doping (the substitution of Ni2þ for C4þ) can be considered as the possible sources contributing to the hole carriers for the as-deposited and 800 C annealed films. On the other hand, the doped Ni atoms mainly exist in the form of Ni2Si phase for 1200 C annealed films, resulting in the number of Ni2þ ions substitute for the C sites of SiC lattice decreases and also lead to decrease of the hole carriers concentration pc. So it can be considered that the remarkable change in r and pc for the 1200 C annealed film may be due to the formation of Ni2Si phase. The resistivity r as a function of temperature T was measured for the as-deposited, 800 C and 1200 C annealed Ni-doped SiC films are shown in Fig. 4(a)e(c). It can be seen that r and temperature emerge inversely proportional in the whole temperature range, indicating semiconductor characteristic for the Ni-doped SiC films. It is obvious that the linear relationship between lnr and T1/4 for the as-deposited and 800 C annealed films in the insets of Fig. 4(a)e(b) shows that the
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H (Oe) Fig. 5. MeH curves of the as-deposited, 800 and 1200 C annealed Ni-doped SiC films at 300 K. The top inset shows the enlarged MeH curves at low magnetic fields. The bottom inset shows the temperature dependence of ZFC and FC curves under a magnetic field of 500 Oe for the 1200 C annealed Ni-doped SiC film.
transport mechanism is governed by the Mott variable range hopping (VRH) behavior, namely r ~ exp[(T0/T)1/4], indicating the carriers are strongly localized [14]. For the 1200 C annealed film, the best understanding of transport mechanisms can be achieved by the combination of the Mott VRH model in the low temperature range and the hard band gap hopping, r ~ exp(Ed/ KBT), in the high temperature range. Fig. 5 shows the magnetization as functions of magnetic field at 300 K for the as-deposited, 800 and 1200 C annealed Ni-doped SiC films. The magnetization saturates at high magnetic fields and exhibits a clear hysteresis at lower fields, indicating that all the films are RT ferromagnetism (top inset of Fig. 5). With the annealing temperature increasing, the saturation magnetization of the films increases from 2.1emu/cm3 to 5.4emu/cm3. It suggests that annealing can enhance the saturation magnetization of the films. Until 800 C annealing, the possible TM clusters or secondary phase can be excluded by XRD, XPS and EXAFS measurements. It can be concluded that the observed RT ferromagnetism in the as-deposited and 800 C annealed films is intrinsic property. After annealing at 1200 C, majority of Ni atoms form the Ni2Si secondary phase compounds. Recently, some researches have indicated that the Ni2Si secondary phase is ferromagnetic with the Curie temperature of about 200 K [15]. This can also be verified by the measurements of zero-filed-cooling (ZFC) and field-cooling (FC) curves. From the inset of Fig. 5, it is clear that there exists an obvious FM transitions temperatures located at TC1 in the FC curve, corresponding a FM phases for the film annealing at 1200 C. By the trend of linear fitting the FC curves of the FM phase, the Curie temperature TC1 about 210 K can be obtained, as shown by the red dash lines in the bottom inset of Fig. 5. So it is reasonably concluded that the secondary phase Ni2Si cannot be responsible for the origin of room-temperature FM order in the 1200 C annealed film. In addition, many types of defects could induce ferromagnetism by the formation of bound magnetic polarons [16]. The process of co-sputtering and high temperature annealing can make many defects emerge in the films. The defects can play a crucial role on the RT ferromagnetism. Because the bound magnetic polarons take effect on the condition of ordered crystal structure, the saturated magnetic moment enhanced with the increase of annealing temperature suggests that the crystal quality of SiC has a great influence on ferromagnetism. Especially after 1200 C annealing, the two reasons that the crystal quality of SiC obviously improves and the number of defects increase because of phase boundaries between the Ni-doped SiC and Ni2Si compounds make the saturated magnetic moment obviously enhance. Therefore, it can be concluded that the origin of RT ferromagnetic order in Ni-doped SiC films does not come from the contribution of Ni2Si phase and the defects are crucial for introducing ferromagnetic order into the Ni-doped SiC system. 4. Conclusions The Ni-doped SiC films were prepared by RF-magnetron sputtering technique. The doped Ni atoms exist in forms of Ni2þ in as-deposited and annealed films. For the as-deposited and 800 C annealed films, the doped Ni atoms substitute for the C sites in the 3CeSiC lattice and Ni-related secondary phase can not be detected. For the 1200 C annealed films, the crystal quality of
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SiC is improved and the majority of Ni atoms form the Ni2Si secondary phase. For the as-deposited and 800 C annealed films, the transport properties are governed by Mott variable range hopping (VRH) behavior, indicating that the carriers are strongly localized. However the transport mechanisms can be achieved by the combination of the Mott VRH model in the low temperature range and the hard band gap hopping in the high temperature range for the 1200 C annealed film. All the films are ferromagnetic at 300 K and annealing temperature can obviously improve their ferromagnetism. The bound magnetic polarons induced by the defects of oxygen vacancies should be responsible for the room temperature ferromagnetism of the films. Acknowledgement This work was supported by National Natural Science Foundation of China (Grant No 10904110, 11174217). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
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