- Email: [email protected]
Ferromagnetic modification of ZnO film by Fe+ ions implantation
Nuclear Instruments and Methods in Physics Research B 266 (2008) 4891–4895
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Ferromagnetic modification of ZnO film by Fe+ ions implantation B. Zhang a,*, Q.H. Li b, L.Q. Shi a, H.S. Cheng a, J.Z. Wang a a b
Applied Ion Beam Physics Laboratory, Institute of Modern Physics, Fudan University, No. 220 Handan Road, Shanghai 200433, P.R. China Research Center for Green Photo-Science and Technology, Department of Applied Chemistry, Tokyo University of Science, Tokyo 162-8601, Japan
a r t i c l e
i n f o
Article history: Received 2 July 2008 Received in revised form 20 August 2008 Available online 11 September 2008 PACS: 32.30.Rj 82.80.Ej 75.50.Pp Keywords: PIXE Fe content Ion implantation ZnO-based DMS
a b s t r a c t The ZnO-based diluted magnetic semiconductors (DMSs) were achieved by ion implantation. Eighty kiloelectron-volt Fe+ ions were implanted into n-type ZnO films at room temperature with doses ranging from 1 1016 cm 2 to 8 1016 cm 2 and subsequently annealed at 700 °C for 1 h in air ambient. PIXE was employed to determine the Fe-implanted content. The magnetic property was measured by the Quantum Design MPMS SQUID magnetometer. No secondary phases or clusters were detected within the sensitivity of XRD. Raman spectrum measurement showed that the Fe ions incorporated into the crystal lattice positions of ZnO through substitution of Zn atoms. Apparent ferromagnetic hysteresis loops measured at 10 K were presented. The relationships between the Fe-implanted content and the ferromagnetic property are discussed. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Zinc oxide (ZnO) is one of the II–VI semiconductor materials with wide band gap (3.37 eV at room temperature) and large exciton binding energy (60 meV). So it has great potential applications in various devices such as varistors, UV light-emitters, piezoelectric transducers and ferroelectric-memory devices [1–4]. Especially, the ZnO-based dilute magnetic semiconductors (DMSs) have recently attracted much attention because charge and spin degrees of freedom could be accommodated into a single material system with magneto-optical or magneto-electronic properties. Molecular-beam epitaxy (MBE) can be employed to manufacture the ZnO-based DMS, but there is a major obstacle during production, which is the low solubility of magnetic elements (e.g. Mn, Fe, Co and Ni etc.) in ZnO [5–7]. Ion implantation has obvious advantages in solving this problem. It can introduce impurities without any limitation of solubility and achieving reproducible high level doping. However, it could bring the crystal lattice damage, which could be recovered by the annealing process. Since the ion implantation is very simple effective method for introducing iron magnetic elements into ZnO film, it is desirable to manufacture the ZnO-based DMS film by this method.
* Corresponding author. Tel.: +86 2155665192; fax: +86 2165642787. E-mail address: [email protected] (B. Zhang). 0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2008.08.015
In this paper, we implanted Fe+ ions into n-type ZnO film to make ZnO:Fe DMS. Ferromagnetic property and the structure of these ZnO:Fe films were investigated. At the same time, the relationship between the Fe-implanted content and its magnetic property was discussed. 2. Experimental The ZnO films used in this work were grown by metal organic chemical deposition (MOCVD) on a (0 0 0 1) sapphire substrate, using (C2H5)2Zn (DEZn) as Zn source and H2O as O source. The 50-nm-thick ZnO buffer was grown first, followed by a growth of 3-lm-thick n-type ZnO. After the growth of the films, Fe+ ions were implanted into the n-type ZnO films at 80 keV with doses ranging from 1 1016 cm 2 to 8 1016 cm 2. Subsequently the samples were annealed at 700 °C for 1 h in air ambient. PIXE [8–10] was taken in order to confirm the existence and the concentration of Feimplanted non-destructively. The magnetic property was measured by the Quantum Design MPMS SQUID magnetometer. The PIXE experiment was performed at the NEC 9SDH-2 3MV pelletron tandem accelerator of Fudan University. In this study the external-beam PIXE was used. Samples were placed at 10 mm outside the beam exit window (7.5 lm Kapton film). A ORTEC Si(Li) detector (165 eV FWHM at 5.9 KeV) placed at 90° relative to the beam direction, was used to detect the X-ray emitted from the sample. The X-rays traveled through 10–15 mm of air before
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B. Zhang et al. / Nuclear Instruments and Methods in Physics Research B 266 (2008) 4891–4895
reaching the detector. A 3.0 MeV collimated proton beam with diameter of 1 mm was used and beam current was 0.1–1.0 nA keeping the dead time less than 5%. The obtained PIXE spectrum was recorded and analyzed with conventional electronic system followed by a multi-channel analyzer. The yields of the characteristic X-ray peaks were obtained after background fitting and subtraction using the program GUPIX-96 [11]. The X-ray diffraction measurements were performed on a Bruker D4 Endeavor X-ray diffractometer with a Cu Ka1 radiation source (k = 0.154176 nm). Raman spectrum measurements were carried out at room temperature on the LabRam-1B with the resolution of 1 cm 1, using a He–Ne laser device with the wavelength of 632.8 nm as the excited light source. The incident laser beam is normal to the surface of the samples and the beam spot diameter on the sample after focusing was approximately 1 lm. 3. Results and discussion According to the SRIM 2008 program [12], the mean projected range of 80 keV Fe+ ions in ZnO film is 359 Å and the longitudinal straggling r is 164 Å. Due to the straggling, the projected range with Gaussian distribution lies nearby the mean projected range 359 Å. Thus about 99.7% Fe+ ions exist in the region from surface to 851 Å depth of ZnO. Fig. 1 shows the action of 80 keV Fe+ ions in ZnO film using the SRIM 2008. Fig. 2 is typical PIXE spectra from the virgin ZnO and the Fe+-implanted sample with the dose of 5 1016 cm 2. To the implanted sample, the X-ray peaks from Fe and Zn can be seen clearly, while to the virgin ZnO, the X-ray peak of Zn only exists, which means that the ZnO film used in this work is very pure. By using of the program GUPIX-96, the mean Fe/Zn mass ratios in the top region of 851 Å, which well agree with the theoretic value calculated from the SRIM 2008, are listed in Table 1. Fig. 3 is the magnetization curves at 10 K measured by SQUID magnetometer. Their magnetic parameters are listed in Table 1. These magnetization curves were obtained with the applied field parallel to the plane of the sample. The diamagnetic background of ZnO substrate was subtracted. The hysteresis loops showed clear ferromagnetic behavior of these Fe-implanted ZnO films. From the Table 1, we find that the residual magnetization (Mr), the saturation magnetization (Ms) and the magnetic hysteresis loss (Q) are
Fig. 1. (a) Project range profile along target depth. (b) Project range showing Feimplanted dose to target depth.
Fig. 2. Typical PIXE spectra of the virgin ZnO and Fe-implanted ZnO film.
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B. Zhang et al. / Nuclear Instruments and Methods in Physics Research B 266 (2008) 4891–4895 Table 1 The relationship between the Fe content and the magnetic parameter in Fe-implanted ZnO films in 10 K Fe+-implanted dose (cm 16
1 10 3 1016 5 1016 8 1016
2
)
Fe/Zn (%)
Hc (Oe)
Mr (emu/cm3)
Ms (emu/cm3)
Mr/Ms
Q (Oe emu/cm3)
4.23 5.29 8.46 16.92
199 250 198 199
1.12 1.42 2.61 1.63
2.95 5.72 12.01 6.39
0.38 0.25 0.22 0.26
19,714 41,969 86,445 46,569
the strongest when Fe content is 8.46%. However, the maximum of the coercive field (Hc) exists when Fe-implanted content is 5.29%. The magnetic hysteresis loss (Q) is an indicator of the ferromagnetic magnitude. Generally, the stronger the ferromagnetic signal, the larger the magnetic hysteresis loss. So the ferromagnetic signal firstly increased and then decreased with Fe-implanted content from 4.23% to 16.92% and it was the strongest when Fe content was 8.46%, which can obviously be concluded from the magnetization curves in Fig. 3. The reduced ferromagnetism possibly originates from anti-ferromagnetic interaction in the 16.92% Feimplanted ZnO film. In ZnO, the Fe has the deep acceptor behavior. With more Fe atoms substituting into the ZnO lattice, the Fermi le-
vel will shift towards the valence band. Under such condition, the donor-like defects such as oxygen vacancies may form. These donor-like negative defects can compensate for the Fe atoms and induce the anti-ferromagnetic coupling and further decrease the ferromagnetism. The anti-ferromagnetic interaction induced by oxygen vacancies is supported by the phenomenological band structure model [13]. Fig. 4 shows XRD spectra of the virgin and Fe-implanted ZnO films. The bottom is the spectrum of the virgin ZnO film. From the bottom, the virgin ZnO film clearly shows a sapphire (0 0 0 6) peak at about 41.8° from sapphire substrate and (0 0 0 1) oriented wurtzite ZnO characteristic peaks at about 34.6° and 72.9° due to
Fig. 3. Hysteresis loops of the Fe-implanted ZnO film. (a) Implanted dose of 1 1016 cm implanted dose of 8 1016 cm 2.
2
, (b) implanted dose of 3 1016 cm
2
, (c) implanted dose of 5 1016 cm
2
and (d)
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B. Zhang et al. / Nuclear Instruments and Methods in Physics Research B 266 (2008) 4891–4895
Fig. 4. XRD spectra of the virgin and Fe-implanted ZnO film.
Fig. 5. Raman spectra of the Al2O3 substrate, the virgin and Fe-implanted ZnO film.
the (0 0 0 2) and (0 0 0 4) diffractions of the wurtzite ZnO, respectively. The existence of the strong (0 0 0 2) diffraction peak with a very narrow full width at half maximum (FWHM) value of 0.1 and the high order ZnO (0 0 0 4) diffraction peak confirm a very good quality of the ZnO films grown on sapphire substrate. To the Fe-implanted ZnO film with the dose of 8 1016 cm 2, the 2h angle of the ZnO (0 0 0 2) and (0 0 0 4) diffraction peaks also lie at 34.6° and 72.9°, respectively. Obviously, no secondary phases or clusters are detected within the sensitivity of XRD. The Raman spectra ranging from 200 to 1200 cm 1 at room temperature are showed in Fig. 5. In the Raman spectrum of the unimplanted sample, the sharpest and strongest peak at about 435 cm 1 can be assigned to the E2 high frequency branch [E2 (high)] of ZnO, which is the strongest mode in the wurtzite crystal
structure according to the Raman selection rules. While the other characteristic peak of a hexagonal wurtzite ZnO phase at about 578 cm 1, due to A1 longitudinal optical (LO) mode [A1 (LO)], is overlapped in the peak at about 578 cm 1 from the Al2O3 substrate. The peak at about 335 cm 1, originated from two-phonon process, is the second-order phonon structure of ZnO, which is interpreted as 2E2 (M) by Calleja and Cardora [14]. To the implanted-Fe+ sample, we find that the Raman spectrum still shows the E2 (high), A1 (LO) and 2E2 (M) modes, which means that these ZnO:Fe films keep the crystal structure of ZnO and the Fe ions incorporate into the crystal lattice positions of ZnO through substitution of Zn atoms. The non-covalent substitutions of Fe in the Zn site act as acceptors and at the same time provide localized spins, which play a key role in the ferromagnetism.
B. Zhang et al. / Nuclear Instruments and Methods in Physics Research B 266 (2008) 4891–4895
4. Conclusions The ZnO-based diluted magnetic semiconductors (DMSs) were achieved by implanting Fe+ ions into n-type ZnO films and subsequent annealing at 700 °C for 1 h in air ambient. Apparent ferromagnetic hysteresis loops measured at 10 K were presented. The relationships between the Fe-implanted content and the ferromagnetic property are discussed. No secondary phases or clusters are detected within the sensitivity of XRD. Raman spectrum measurement showed that the Fe ions incorporated into the crystal lattice positions of ZnO through substitution of Zn atoms.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant No. 10775033) and partly supported by the Fund of Fudan University. We also gratefully acknowledge support from Shanghai Leading Academic Discipline Project (B107).
4895
[11] [12] [13] [14]
D.M. Bagnall et al., Appl. Phys. Lett. 70 (1997) 2230. M.H. Huang et al., Science 292 (2001) 1897. M.T. Björk et al., Appl. Phys. Lett. 80 (2002) 1058. H. Hayashi et al., Appl. Phys. Lett. 82 (2003) 1365. H. Ohno, Science 281 (1998) 951. Y. Shi, L. Lin, C.Z. Jiang, X.J. Fan, D.J. Fu, T.W. Kang, J. Korean Phys. Soc. 46 (2005) S39. B. Zhang, L.Q. Shi, C.C. Chen, D.G. Zhao, Nucl. Instr. and Meth. B 252 (2006) 225. B. Zhang, H.S. Cheng, B. Ma, Q.H. Li, P. Zhang, F.X. Gan, F.J. Yang, Nucl. Instr. and Meth. B 240 (2005) 559. B. Zhang, B.H. Pan, Z.Q. Zhang, H.S. Cheng, M.H. Gao, F.J. Yang, X.B. Peng, Nucl. Instr. and Meth. B 219–220 (2004) 26. B. Zhang, H.S. Cheng, W.J. Zhao, Z.Y. Gao, G.X. Li, J.Z. Xie, M. Gao, F.J. Yang, XRay Spectrom. 35 (2006) 27. J.L. Campbell, T.L. Hopman, J.A. Maxwell, Z. Nejedly, Nucl. Instr. and Meth. B 170 (2000) 193. J.F. Ziegler, Nucl. Instr. and Meth. B 219–220 (2004) 1027. G.M. Dalpian, S.H. Wei, X.G. Gong, J.R. Silva, A. Fazzio, Solid State Commun. 138 (2006) 353. J.M. Calleja, M. Cardona, Phys. Rev. B 16 (1977) 3753.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Ferromagnetic modification of ZnO film by Fe+ ions implantation B. Zhang a,*, Q.H. Li b, L.Q. Shi a, H.S. Cheng a, J.Z. Wang a a b
Applied Ion Beam Physics Laboratory, Institute of Modern Physics, Fudan University, No. 220 Handan Road, Shanghai 200433, P.R. China Research Center for Green Photo-Science and Technology, Department of Applied Chemistry, Tokyo University of Science, Tokyo 162-8601, Japan
a r t i c l e
i n f o
Article history: Received 2 July 2008 Received in revised form 20 August 2008 Available online 11 September 2008 PACS: 32.30.Rj 82.80.Ej 75.50.Pp Keywords: PIXE Fe content Ion implantation ZnO-based DMS
a b s t r a c t The ZnO-based diluted magnetic semiconductors (DMSs) were achieved by ion implantation. Eighty kiloelectron-volt Fe+ ions were implanted into n-type ZnO films at room temperature with doses ranging from 1 1016 cm 2 to 8 1016 cm 2 and subsequently annealed at 700 °C for 1 h in air ambient. PIXE was employed to determine the Fe-implanted content. The magnetic property was measured by the Quantum Design MPMS SQUID magnetometer. No secondary phases or clusters were detected within the sensitivity of XRD. Raman spectrum measurement showed that the Fe ions incorporated into the crystal lattice positions of ZnO through substitution of Zn atoms. Apparent ferromagnetic hysteresis loops measured at 10 K were presented. The relationships between the Fe-implanted content and the ferromagnetic property are discussed. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Zinc oxide (ZnO) is one of the II–VI semiconductor materials with wide band gap (3.37 eV at room temperature) and large exciton binding energy (60 meV). So it has great potential applications in various devices such as varistors, UV light-emitters, piezoelectric transducers and ferroelectric-memory devices [1–4]. Especially, the ZnO-based dilute magnetic semiconductors (DMSs) have recently attracted much attention because charge and spin degrees of freedom could be accommodated into a single material system with magneto-optical or magneto-electronic properties. Molecular-beam epitaxy (MBE) can be employed to manufacture the ZnO-based DMS, but there is a major obstacle during production, which is the low solubility of magnetic elements (e.g. Mn, Fe, Co and Ni etc.) in ZnO [5–7]. Ion implantation has obvious advantages in solving this problem. It can introduce impurities without any limitation of solubility and achieving reproducible high level doping. However, it could bring the crystal lattice damage, which could be recovered by the annealing process. Since the ion implantation is very simple effective method for introducing iron magnetic elements into ZnO film, it is desirable to manufacture the ZnO-based DMS film by this method.
* Corresponding author. Tel.: +86 2155665192; fax: +86 2165642787. E-mail address: [email protected] (B. Zhang). 0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2008.08.015
In this paper, we implanted Fe+ ions into n-type ZnO film to make ZnO:Fe DMS. Ferromagnetic property and the structure of these ZnO:Fe films were investigated. At the same time, the relationship between the Fe-implanted content and its magnetic property was discussed. 2. Experimental The ZnO films used in this work were grown by metal organic chemical deposition (MOCVD) on a (0 0 0 1) sapphire substrate, using (C2H5)2Zn (DEZn) as Zn source and H2O as O source. The 50-nm-thick ZnO buffer was grown first, followed by a growth of 3-lm-thick n-type ZnO. After the growth of the films, Fe+ ions were implanted into the n-type ZnO films at 80 keV with doses ranging from 1 1016 cm 2 to 8 1016 cm 2. Subsequently the samples were annealed at 700 °C for 1 h in air ambient. PIXE [8–10] was taken in order to confirm the existence and the concentration of Feimplanted non-destructively. The magnetic property was measured by the Quantum Design MPMS SQUID magnetometer. The PIXE experiment was performed at the NEC 9SDH-2 3MV pelletron tandem accelerator of Fudan University. In this study the external-beam PIXE was used. Samples were placed at 10 mm outside the beam exit window (7.5 lm Kapton film). A ORTEC Si(Li) detector (165 eV FWHM at 5.9 KeV) placed at 90° relative to the beam direction, was used to detect the X-ray emitted from the sample. The X-rays traveled through 10–15 mm of air before
4892
B. Zhang et al. / Nuclear Instruments and Methods in Physics Research B 266 (2008) 4891–4895
reaching the detector. A 3.0 MeV collimated proton beam with diameter of 1 mm was used and beam current was 0.1–1.0 nA keeping the dead time less than 5%. The obtained PIXE spectrum was recorded and analyzed with conventional electronic system followed by a multi-channel analyzer. The yields of the characteristic X-ray peaks were obtained after background fitting and subtraction using the program GUPIX-96 [11]. The X-ray diffraction measurements were performed on a Bruker D4 Endeavor X-ray diffractometer with a Cu Ka1 radiation source (k = 0.154176 nm). Raman spectrum measurements were carried out at room temperature on the LabRam-1B with the resolution of 1 cm 1, using a He–Ne laser device with the wavelength of 632.8 nm as the excited light source. The incident laser beam is normal to the surface of the samples and the beam spot diameter on the sample after focusing was approximately 1 lm. 3. Results and discussion According to the SRIM 2008 program [12], the mean projected range of 80 keV Fe+ ions in ZnO film is 359 Å and the longitudinal straggling r is 164 Å. Due to the straggling, the projected range with Gaussian distribution lies nearby the mean projected range 359 Å. Thus about 99.7% Fe+ ions exist in the region from surface to 851 Å depth of ZnO. Fig. 1 shows the action of 80 keV Fe+ ions in ZnO film using the SRIM 2008. Fig. 2 is typical PIXE spectra from the virgin ZnO and the Fe+-implanted sample with the dose of 5 1016 cm 2. To the implanted sample, the X-ray peaks from Fe and Zn can be seen clearly, while to the virgin ZnO, the X-ray peak of Zn only exists, which means that the ZnO film used in this work is very pure. By using of the program GUPIX-96, the mean Fe/Zn mass ratios in the top region of 851 Å, which well agree with the theoretic value calculated from the SRIM 2008, are listed in Table 1. Fig. 3 is the magnetization curves at 10 K measured by SQUID magnetometer. Their magnetic parameters are listed in Table 1. These magnetization curves were obtained with the applied field parallel to the plane of the sample. The diamagnetic background of ZnO substrate was subtracted. The hysteresis loops showed clear ferromagnetic behavior of these Fe-implanted ZnO films. From the Table 1, we find that the residual magnetization (Mr), the saturation magnetization (Ms) and the magnetic hysteresis loss (Q) are
Fig. 1. (a) Project range profile along target depth. (b) Project range showing Feimplanted dose to target depth.
Fig. 2. Typical PIXE spectra of the virgin ZnO and Fe-implanted ZnO film.
4893
B. Zhang et al. / Nuclear Instruments and Methods in Physics Research B 266 (2008) 4891–4895 Table 1 The relationship between the Fe content and the magnetic parameter in Fe-implanted ZnO films in 10 K Fe+-implanted dose (cm 16
1 10 3 1016 5 1016 8 1016
2
)
Fe/Zn (%)
Hc (Oe)
Mr (emu/cm3)
Ms (emu/cm3)
Mr/Ms
Q (Oe emu/cm3)
4.23 5.29 8.46 16.92
199 250 198 199
1.12 1.42 2.61 1.63
2.95 5.72 12.01 6.39
0.38 0.25 0.22 0.26
19,714 41,969 86,445 46,569
the strongest when Fe content is 8.46%. However, the maximum of the coercive field (Hc) exists when Fe-implanted content is 5.29%. The magnetic hysteresis loss (Q) is an indicator of the ferromagnetic magnitude. Generally, the stronger the ferromagnetic signal, the larger the magnetic hysteresis loss. So the ferromagnetic signal firstly increased and then decreased with Fe-implanted content from 4.23% to 16.92% and it was the strongest when Fe content was 8.46%, which can obviously be concluded from the magnetization curves in Fig. 3. The reduced ferromagnetism possibly originates from anti-ferromagnetic interaction in the 16.92% Feimplanted ZnO film. In ZnO, the Fe has the deep acceptor behavior. With more Fe atoms substituting into the ZnO lattice, the Fermi le-
vel will shift towards the valence band. Under such condition, the donor-like defects such as oxygen vacancies may form. These donor-like negative defects can compensate for the Fe atoms and induce the anti-ferromagnetic coupling and further decrease the ferromagnetism. The anti-ferromagnetic interaction induced by oxygen vacancies is supported by the phenomenological band structure model [13]. Fig. 4 shows XRD spectra of the virgin and Fe-implanted ZnO films. The bottom is the spectrum of the virgin ZnO film. From the bottom, the virgin ZnO film clearly shows a sapphire (0 0 0 6) peak at about 41.8° from sapphire substrate and (0 0 0 1) oriented wurtzite ZnO characteristic peaks at about 34.6° and 72.9° due to
Fig. 3. Hysteresis loops of the Fe-implanted ZnO film. (a) Implanted dose of 1 1016 cm implanted dose of 8 1016 cm 2.
2
, (b) implanted dose of 3 1016 cm
2
, (c) implanted dose of 5 1016 cm
2
and (d)
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B. Zhang et al. / Nuclear Instruments and Methods in Physics Research B 266 (2008) 4891–4895
Fig. 4. XRD spectra of the virgin and Fe-implanted ZnO film.
Fig. 5. Raman spectra of the Al2O3 substrate, the virgin and Fe-implanted ZnO film.
the (0 0 0 2) and (0 0 0 4) diffractions of the wurtzite ZnO, respectively. The existence of the strong (0 0 0 2) diffraction peak with a very narrow full width at half maximum (FWHM) value of 0.1 and the high order ZnO (0 0 0 4) diffraction peak confirm a very good quality of the ZnO films grown on sapphire substrate. To the Fe-implanted ZnO film with the dose of 8 1016 cm 2, the 2h angle of the ZnO (0 0 0 2) and (0 0 0 4) diffraction peaks also lie at 34.6° and 72.9°, respectively. Obviously, no secondary phases or clusters are detected within the sensitivity of XRD. The Raman spectra ranging from 200 to 1200 cm 1 at room temperature are showed in Fig. 5. In the Raman spectrum of the unimplanted sample, the sharpest and strongest peak at about 435 cm 1 can be assigned to the E2 high frequency branch [E2 (high)] of ZnO, which is the strongest mode in the wurtzite crystal
structure according to the Raman selection rules. While the other characteristic peak of a hexagonal wurtzite ZnO phase at about 578 cm 1, due to A1 longitudinal optical (LO) mode [A1 (LO)], is overlapped in the peak at about 578 cm 1 from the Al2O3 substrate. The peak at about 335 cm 1, originated from two-phonon process, is the second-order phonon structure of ZnO, which is interpreted as 2E2 (M) by Calleja and Cardora [14]. To the implanted-Fe+ sample, we find that the Raman spectrum still shows the E2 (high), A1 (LO) and 2E2 (M) modes, which means that these ZnO:Fe films keep the crystal structure of ZnO and the Fe ions incorporate into the crystal lattice positions of ZnO through substitution of Zn atoms. The non-covalent substitutions of Fe in the Zn site act as acceptors and at the same time provide localized spins, which play a key role in the ferromagnetism.
B. Zhang et al. / Nuclear Instruments and Methods in Physics Research B 266 (2008) 4891–4895
4. Conclusions The ZnO-based diluted magnetic semiconductors (DMSs) were achieved by implanting Fe+ ions into n-type ZnO films and subsequent annealing at 700 °C for 1 h in air ambient. Apparent ferromagnetic hysteresis loops measured at 10 K were presented. The relationships between the Fe-implanted content and the ferromagnetic property are discussed. No secondary phases or clusters are detected within the sensitivity of XRD. Raman spectrum measurement showed that the Fe ions incorporated into the crystal lattice positions of ZnO through substitution of Zn atoms.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
Acknowledgements This work is supported by the National Natural Science Foundation of China (Grant No. 10775033) and partly supported by the Fund of Fudan University. We also gratefully acknowledge support from Shanghai Leading Academic Discipline Project (B107).
4895
[11] [12] [13] [14]
D.M. Bagnall et al., Appl. Phys. Lett. 70 (1997) 2230. M.H. Huang et al., Science 292 (2001) 1897. M.T. Björk et al., Appl. Phys. Lett. 80 (2002) 1058. H. Hayashi et al., Appl. Phys. Lett. 82 (2003) 1365. H. Ohno, Science 281 (1998) 951. Y. Shi, L. Lin, C.Z. Jiang, X.J. Fan, D.J. Fu, T.W. Kang, J. Korean Phys. Soc. 46 (2005) S39. B. Zhang, L.Q. Shi, C.C. Chen, D.G. Zhao, Nucl. Instr. and Meth. B 252 (2006) 225. B. Zhang, H.S. Cheng, B. Ma, Q.H. Li, P. Zhang, F.X. Gan, F.J. Yang, Nucl. Instr. and Meth. B 240 (2005) 559. B. Zhang, B.H. Pan, Z.Q. Zhang, H.S. Cheng, M.H. Gao, F.J. Yang, X.B. Peng, Nucl. Instr. and Meth. B 219–220 (2004) 26. B. Zhang, H.S. Cheng, W.J. Zhao, Z.Y. Gao, G.X. Li, J.Z. Xie, M. Gao, F.J. Yang, XRay Spectrom. 35 (2006) 27. J.L. Campbell, T.L. Hopman, J.A. Maxwell, Z. Nejedly, Nucl. Instr. and Meth. B 170 (2000) 193. J.F. Ziegler, Nucl. Instr. and Meth. B 219–220 (2004) 1027. G.M. Dalpian, S.H. Wei, X.G. Gong, J.R. Silva, A. Fazzio, Solid State Commun. 138 (2006) 353. J.M. Calleja, M. Cardona, Phys. Rev. B 16 (1977) 3753.