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Nano-clusters structure and magnetic properties of high fluence Mn+ ion-implanted GaN
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NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 2797–2800 www.elsevier.com/locate/nimb
Nano-clusters structure and magnetic properties of high fluence Mn+ ion-implanted GaN Di Chen, Zhibo Ding, Shude Yao *, Wei Hua, Kun Wang, Tianxiang Chen School of Physics, Peking University, Beijing 100871, PR China Available online 22 March 2008
Abstract The n-type GaN film was grown on sapphire substrate by metal organic chemical vapour deposition (MOCVD). Mn+ ions of 75 keV with the fluence of 8 1017/cm2 were implanted in the GaN at 350 °C. The implanted sample was annealed at 850 °C to recrystallize the sample and to remove implantation damage. We investigated the structural and magnetic properties of Mn+ ion-implanted GaN by using Rutherford back-scattering (RBS), high resolution transmission electron microscopy (HRTEM) and superconducting quantum interference device (SQUID). RBS results showed the Mn+ ions were concentrated near the surface. HRTEM results showed nano-clusters structure in the sample. The temperature dependence of magnetization taken in zero-field-cooling and field-cooling conditions with the block temperature was 267 K and hysteresis loops which exhibited a transformation from ferromagnetism to superparamagnetism showed the features of magnetic nano-clusters system. And it could be explained the magnetic property of this film originated from Mn-rich clusters. Ó 2008 Elsevier B.V. All rights reserved. PACS: 41.75.Ak; 07.55.Je; 81.07.b; 68.37.Lp Keywords: Nano-structure; Magnetic properties; TEM; Ion-implantation
1. Introduction The dilute magnetic semiconductors based on III–V semiconductors have attracted great attention. Among them, GaN is a very important material because of the theoretical prediction of possible ferromagnetism with high Curie temperature TC above room temperature [1] and may be the candidate of spintronic devices [2]. However, the origin of the ferromagnetism observed is far from being understood. Some reports have attributed it to the MnxNy phase [3] or GaxMn1xN since the second phase was not detected by the X-ray diffraction [4,5]. Other reports discussed that the ferromagnetism comes from some ferromagnetic impurities [6,7]. Also some reports related it to the formation of Mn-rich clusters [8]. So the origin of the
*
Corresponding author. Tel.: +86 10 62757534; fax: +86 10 62751875. E-mail address: [email protected] (S. Yao).
0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2008.03.120
ferromagnetism of Mn+ implanted or doping GaN is complicated. Among them, the magnetic cluster is an important origin. In order to research magnetic nano-scale clusters structure in ion-implanted GaN, high fluence Mn+ ions were implanted in n-type GaN. Novel outstanding properties of such nano-structures can be directly correlated to their nano-scale in one or more dimensions. For example, temperature related micro-magnetism lead to transitions of their magnetic behavior from multi-domain to single domain structure, or even to superparamagnetic behavior if the size of clusters is small enough for the thermal energy can overcome magnetic anisotropy energy barrier. In this paper, we report on the investigation of structure and magnetic properties of n-type GaN film in which Mn+ ions were implanted at fluence of 8 1017/cm2. After annealing in N2 atmosphere at 850 °C, we observed nano-scale clusters structure by HRTEM. Theory predicts this Mn-rich clusters structure may have giant magnetic moments [9]. The magnetic properties also show the
2798
D. Chen et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2797–2800
existence of this nano-scale clusters properties in our experiment. 2. Experiments
3. Results and discussion
The 2-lm thick n-type GaN film was grown by metal organic chemical vapour deposition (MOCVD) on c-plane sapphire (0 0 0 1) substrate. 75 keV Mn+ ions with the fluence of 8 1017/cm2 were implanted in it at 350 °C. The implanted samples were annealed at 850 °C performed for 30 min under flowing N2 atmosphere with the sample facing down on the wafers to recrystallize the samples and remove implantation damage. The structural properties of the Mn+ implanted GaN were investigated by HRTEM and Rutherford back-scattering (RBS). RBS is a standard technique in analyzing structure of materials, and used to determine the composition, the thickness and the interface of films. The magnetic properties were measured in a quantum design magnetic property measurement system (MPMS) superconducting quantum interference device (SQUID). In all of the magnetization measurements,
Fig. 1. RBS/C measurements of the Mn which the fluence is 8 1017/cm2.
the magnetic field was applied parallel to the sample plane.
+
ion-implanted GaN film of
Fig. 1 shows the random and h0 0 0 1i aligned RBS spectra of the high fluence Mn+ implanted GaN (a) and a GaN film without implantation (b). The inset shows the geometry used in the back-scattering measurements. The arrows indicate the energy for back-scattering from Ga and Mn atoms at the surface, respectively. The spectrum (b) indicates that GaN film without implantation has minimum yield vmin = 3%, while the spectrum (a) indicates that the Mn+ implanted GaN film has minimum yield vmin = 53% and high dechannelling rate, showing that the damage is serious. These results mean that the density of defects in
Fig. 2. Bright-field TEM micrographs of the Mn+ ion-implanted GaN film. The scale on the left micrograph is 40 nm (a) while in the right micrograph the scale is 20 nm (b).
D. Chen et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2797–2800
2799
the film is extremely high. A simulation of the random spectrum is given by the RUMP program [10], which is used to simulate the random RBS spectrum, revealing that the thickness of the GaN (Mn) film is about 140 nm. It means that Mn ions were concentrated near the surface of the film, so it is difficult to resolve so many Mn ions for GaN. Fig. 2 shows the TEM micrographs with scales of 40 nm (a) and 20 nm (b) for the GaN without implantation is a homogeneous layer without any evidence of a second phase [8]. Yet, in the TEM micrographs of high fluence Mn+ ionimplanted GaN nm-size clusters are obvious. The sizes of these nm-size clusters are not identical and distribution of these clusters is also not homogeneous. The range of the diameter of these clusters is from 3–5 nm to 10–15 nm (the arrows indicate some big ones). Fig. 3 shows the temperature dependence of the magnetism for zero-filed cooling (ZFC) and filed-cooling (FC) conditions. The zero-filed cooling (ZFC) curve was achieved by cooling the sample initially in a zero field to 20 K, and magnetization was recorded in a parallel
magnetic field where H = 100 Oe as the temperature increased. The filed-cooling (FC) magnetization was measured by gradually cooling the sample from 300 K to 20 K, and the magnetization was recorded in the presence of a 100 Oe field. The magnetization increases at first and then decreases in ZFC curve, there is a peak at about
Fig. 3. ZFC and FC magnetization as function of temperature for the 8 1017/cm2 Mn+ implanted GaN film. The left graph is measurement of ZFC only (a). The right graph is measurement both ZFC and FC (b).
Fig. 4. Hysteresis loop for 8 1017/cm2 Mn+ ion-implanted GaN at 100 K (a) and 300 K (b). And GaN without implantation only exhibits diamagnetism (c).
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D. Chen et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2797–2800
267 K while the magnetization decreases in FC. The FCZFC curves are irreversible at temperature below 267 K. Similar traits of temperature dependence of the magnetism have also been achieved by Theodoropoulou [11]. We also measured hysteresis loops at 300 K and 100 K. Fig. 4 shows the superparamagnetism properties were detected at 300 K (a) and ferromagnetism was found at 100 K (b), which indicated there is a transition from ferromagnetism to superparamagnetism, while GaN without implantation only exhibit diamagnetism (c). It shows the features of magnetic nano-clusters system with the block temperature is 267 K. The magnetic properties of Mn is governed by the 3d electrons, according to Hunter’s rule an atomic moment is 5 lB because of its half-filled 3d shell. However because the Mn–Mn coupling is antiferromagnetism, the moment of a single Mn atom in pure Mn clusters is below 1.5 lB [12]. Based on the calculations by Rao [9], N mediated ferromagnetic coupling in MnxN can give rise to giant magnetic moments of 4 lB, 9 lB, 12 lB, 17 lB, 22 lB for x = 1–5. So It could be deduced that Mn-rich clusters were formed in this high fluence Mn+ ion-implanted GaN. 4. Conclusion In conclusion, magnetic and structural properties of high fluence Mn+ ion-implanted GaN were investigated. HRTEM results and the temperature dependence of magnetization taken in zero-field-cooling and field-cooling conditions with the block temperature was 267 K and hys-
teresis loops which exhibited a transformation from ferromagnetism to superparamagnetism showed the features of magnetic nano-clusters system under above implantation and annealing condition. It is an effective method to form magnetic nm-clusters. It could be explained that the magnetic property in this film originated from Mn-rich clusters. Considerable works need to be done to understand the composition and structure of these clusters and establish the relationship between the size and magnetic properties. References [1] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) 019. [2] H. Ohno, Science 281 (1998) 951. [3] Jeong Min Baik, Ho Won Jang, Jong Kyu Kim, Jong-Lam Lee, Appl. Phys. Lett. 82 (2003) 583. [4] M.L. Reed, N.A. El-Masry, H.H. Stadelmaier, M.K. Ritums, M.J. Reed, C.A. Parker, J.C. Roberts, S.M. Bedair, Appl. Phys. Lett. 79 (2001) 3473. [5] K. Ando, Appl. Phys. Lett. 82 (2003) 100. [6] Y. Shon, Y.H. Kwon, D.Y. Kim, X. Fan, D. Fu, T.W. Wang, Jpn. J. Appl. Phys. Part 1 40 (2001) 5304. [7] S. Kuwabara, T. Kondo, T. Chikyow, P. Ahmet, H. Munekata, Jpn. J. Appl. Phys. Part 2 40 (2001) L724. [8] S. Dhar, O. Brandt, A. Trampert, L. Da¨weritz, K.J. Friedland, K.H. Ploog, Appl. Phys. Lett. 82 (2003) 2077. [9] B.K. Rao, P. Jena, Phys. Rev. Lett. 89 (2002) 185504. [10] L.R. Doolittle, Nucl. Instr. and Meth. B 9 (1985) 344. [11] N. Theodoropoulou, A.F. Hebard, Appl. Phys. Lett. 78 (2001) 3475. [12] Mark B. Knickelbein, Phy. Rev. Lett. 86 (2001) 5255.
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 266 (2008) 2797–2800 www.elsevier.com/locate/nimb
Nano-clusters structure and magnetic properties of high fluence Mn+ ion-implanted GaN Di Chen, Zhibo Ding, Shude Yao *, Wei Hua, Kun Wang, Tianxiang Chen School of Physics, Peking University, Beijing 100871, PR China Available online 22 March 2008
Abstract The n-type GaN film was grown on sapphire substrate by metal organic chemical vapour deposition (MOCVD). Mn+ ions of 75 keV with the fluence of 8 1017/cm2 were implanted in the GaN at 350 °C. The implanted sample was annealed at 850 °C to recrystallize the sample and to remove implantation damage. We investigated the structural and magnetic properties of Mn+ ion-implanted GaN by using Rutherford back-scattering (RBS), high resolution transmission electron microscopy (HRTEM) and superconducting quantum interference device (SQUID). RBS results showed the Mn+ ions were concentrated near the surface. HRTEM results showed nano-clusters structure in the sample. The temperature dependence of magnetization taken in zero-field-cooling and field-cooling conditions with the block temperature was 267 K and hysteresis loops which exhibited a transformation from ferromagnetism to superparamagnetism showed the features of magnetic nano-clusters system. And it could be explained the magnetic property of this film originated from Mn-rich clusters. Ó 2008 Elsevier B.V. All rights reserved. PACS: 41.75.Ak; 07.55.Je; 81.07.b; 68.37.Lp Keywords: Nano-structure; Magnetic properties; TEM; Ion-implantation
1. Introduction The dilute magnetic semiconductors based on III–V semiconductors have attracted great attention. Among them, GaN is a very important material because of the theoretical prediction of possible ferromagnetism with high Curie temperature TC above room temperature [1] and may be the candidate of spintronic devices [2]. However, the origin of the ferromagnetism observed is far from being understood. Some reports have attributed it to the MnxNy phase [3] or GaxMn1xN since the second phase was not detected by the X-ray diffraction [4,5]. Other reports discussed that the ferromagnetism comes from some ferromagnetic impurities [6,7]. Also some reports related it to the formation of Mn-rich clusters [8]. So the origin of the
*
Corresponding author. Tel.: +86 10 62757534; fax: +86 10 62751875. E-mail address: [email protected] (S. Yao).
0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2008.03.120
ferromagnetism of Mn+ implanted or doping GaN is complicated. Among them, the magnetic cluster is an important origin. In order to research magnetic nano-scale clusters structure in ion-implanted GaN, high fluence Mn+ ions were implanted in n-type GaN. Novel outstanding properties of such nano-structures can be directly correlated to their nano-scale in one or more dimensions. For example, temperature related micro-magnetism lead to transitions of their magnetic behavior from multi-domain to single domain structure, or even to superparamagnetic behavior if the size of clusters is small enough for the thermal energy can overcome magnetic anisotropy energy barrier. In this paper, we report on the investigation of structure and magnetic properties of n-type GaN film in which Mn+ ions were implanted at fluence of 8 1017/cm2. After annealing in N2 atmosphere at 850 °C, we observed nano-scale clusters structure by HRTEM. Theory predicts this Mn-rich clusters structure may have giant magnetic moments [9]. The magnetic properties also show the
2798
D. Chen et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2797–2800
existence of this nano-scale clusters properties in our experiment. 2. Experiments
3. Results and discussion
The 2-lm thick n-type GaN film was grown by metal organic chemical vapour deposition (MOCVD) on c-plane sapphire (0 0 0 1) substrate. 75 keV Mn+ ions with the fluence of 8 1017/cm2 were implanted in it at 350 °C. The implanted samples were annealed at 850 °C performed for 30 min under flowing N2 atmosphere with the sample facing down on the wafers to recrystallize the samples and remove implantation damage. The structural properties of the Mn+ implanted GaN were investigated by HRTEM and Rutherford back-scattering (RBS). RBS is a standard technique in analyzing structure of materials, and used to determine the composition, the thickness and the interface of films. The magnetic properties were measured in a quantum design magnetic property measurement system (MPMS) superconducting quantum interference device (SQUID). In all of the magnetization measurements,
Fig. 1. RBS/C measurements of the Mn which the fluence is 8 1017/cm2.
the magnetic field was applied parallel to the sample plane.
+
ion-implanted GaN film of
Fig. 1 shows the random and h0 0 0 1i aligned RBS spectra of the high fluence Mn+ implanted GaN (a) and a GaN film without implantation (b). The inset shows the geometry used in the back-scattering measurements. The arrows indicate the energy for back-scattering from Ga and Mn atoms at the surface, respectively. The spectrum (b) indicates that GaN film without implantation has minimum yield vmin = 3%, while the spectrum (a) indicates that the Mn+ implanted GaN film has minimum yield vmin = 53% and high dechannelling rate, showing that the damage is serious. These results mean that the density of defects in
Fig. 2. Bright-field TEM micrographs of the Mn+ ion-implanted GaN film. The scale on the left micrograph is 40 nm (a) while in the right micrograph the scale is 20 nm (b).
D. Chen et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2797–2800
2799
the film is extremely high. A simulation of the random spectrum is given by the RUMP program [10], which is used to simulate the random RBS spectrum, revealing that the thickness of the GaN (Mn) film is about 140 nm. It means that Mn ions were concentrated near the surface of the film, so it is difficult to resolve so many Mn ions for GaN. Fig. 2 shows the TEM micrographs with scales of 40 nm (a) and 20 nm (b) for the GaN without implantation is a homogeneous layer without any evidence of a second phase [8]. Yet, in the TEM micrographs of high fluence Mn+ ionimplanted GaN nm-size clusters are obvious. The sizes of these nm-size clusters are not identical and distribution of these clusters is also not homogeneous. The range of the diameter of these clusters is from 3–5 nm to 10–15 nm (the arrows indicate some big ones). Fig. 3 shows the temperature dependence of the magnetism for zero-filed cooling (ZFC) and filed-cooling (FC) conditions. The zero-filed cooling (ZFC) curve was achieved by cooling the sample initially in a zero field to 20 K, and magnetization was recorded in a parallel
magnetic field where H = 100 Oe as the temperature increased. The filed-cooling (FC) magnetization was measured by gradually cooling the sample from 300 K to 20 K, and the magnetization was recorded in the presence of a 100 Oe field. The magnetization increases at first and then decreases in ZFC curve, there is a peak at about
Fig. 3. ZFC and FC magnetization as function of temperature for the 8 1017/cm2 Mn+ implanted GaN film. The left graph is measurement of ZFC only (a). The right graph is measurement both ZFC and FC (b).
Fig. 4. Hysteresis loop for 8 1017/cm2 Mn+ ion-implanted GaN at 100 K (a) and 300 K (b). And GaN without implantation only exhibits diamagnetism (c).
2800
D. Chen et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 2797–2800
267 K while the magnetization decreases in FC. The FCZFC curves are irreversible at temperature below 267 K. Similar traits of temperature dependence of the magnetism have also been achieved by Theodoropoulou [11]. We also measured hysteresis loops at 300 K and 100 K. Fig. 4 shows the superparamagnetism properties were detected at 300 K (a) and ferromagnetism was found at 100 K (b), which indicated there is a transition from ferromagnetism to superparamagnetism, while GaN without implantation only exhibit diamagnetism (c). It shows the features of magnetic nano-clusters system with the block temperature is 267 K. The magnetic properties of Mn is governed by the 3d electrons, according to Hunter’s rule an atomic moment is 5 lB because of its half-filled 3d shell. However because the Mn–Mn coupling is antiferromagnetism, the moment of a single Mn atom in pure Mn clusters is below 1.5 lB [12]. Based on the calculations by Rao [9], N mediated ferromagnetic coupling in MnxN can give rise to giant magnetic moments of 4 lB, 9 lB, 12 lB, 17 lB, 22 lB for x = 1–5. So It could be deduced that Mn-rich clusters were formed in this high fluence Mn+ ion-implanted GaN. 4. Conclusion In conclusion, magnetic and structural properties of high fluence Mn+ ion-implanted GaN were investigated. HRTEM results and the temperature dependence of magnetization taken in zero-field-cooling and field-cooling conditions with the block temperature was 267 K and hys-
teresis loops which exhibited a transformation from ferromagnetism to superparamagnetism showed the features of magnetic nano-clusters system under above implantation and annealing condition. It is an effective method to form magnetic nm-clusters. It could be explained that the magnetic property in this film originated from Mn-rich clusters. Considerable works need to be done to understand the composition and structure of these clusters and establish the relationship between the size and magnetic properties. References [1] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) 019. [2] H. Ohno, Science 281 (1998) 951. [3] Jeong Min Baik, Ho Won Jang, Jong Kyu Kim, Jong-Lam Lee, Appl. Phys. Lett. 82 (2003) 583. [4] M.L. Reed, N.A. El-Masry, H.H. Stadelmaier, M.K. Ritums, M.J. Reed, C.A. Parker, J.C. Roberts, S.M. Bedair, Appl. Phys. Lett. 79 (2001) 3473. [5] K. Ando, Appl. Phys. Lett. 82 (2003) 100. [6] Y. Shon, Y.H. Kwon, D.Y. Kim, X. Fan, D. Fu, T.W. Wang, Jpn. J. Appl. Phys. Part 1 40 (2001) 5304. [7] S. Kuwabara, T. Kondo, T. Chikyow, P. Ahmet, H. Munekata, Jpn. J. Appl. Phys. Part 2 40 (2001) L724. [8] S. Dhar, O. Brandt, A. Trampert, L. Da¨weritz, K.J. Friedland, K.H. Ploog, Appl. Phys. Lett. 82 (2003) 2077. [9] B.K. Rao, P. Jena, Phys. Rev. Lett. 89 (2002) 185504. [10] L.R. Doolittle, Nucl. Instr. and Meth. B 9 (1985) 344. [11] N. Theodoropoulou, A.F. Hebard, Appl. Phys. Lett. 78 (2001) 3475. [12] Mark B. Knickelbein, Phy. Rev. Lett. 86 (2001) 5255.