- Email: [email protected]
Magnetic clusters in Co ion-implanted GaN
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 310 (2007) e729–e731 www.elsevier.com/locate/jmmm
Magnetic clusters in Co ion-implanted GaN Woochul Kima, Hee Jae Kangb, Sam Kyu Nohc, Jonghan Songd, Chul Sung Kima, a
Department of Physics, Kookmin University, 861-1, Cheongnung-dong, Songbuk-gu, Seoul 136-702, South Korea b Department of Physics, Chungbuk National University, Cheongju 361-763, South Korea c Korea Research Institute of Standards and Science, Daejeon 305-600, South Korea d Korea Institute of Science and Technology, Seoul 130-650, South Korea Available online 27 November 2006
Abstract The structural and optical properties of Co ion-implanted GaN were investigated by various measurements. HRXRD, TEM, and SADP results did not show any peaks associated with second phase formation. XPS depth measurements showed the metallic Co 2p core levels for 700–900 C annealed-samples. From XPS results, it could be explained that magnetic property of our films originated from Co or CoGa magnetic clusters. The PL spectra obtained at 10 K display a clear intensity modulation, indicating that the optical interference effect occurs in the layers. r 2006 Elsevier B.V. All rights reserved. PACS: 75.50.Pp; 75.70.Ak Keywords: Ion implantation; Magnetic semiconductor
The wide bandgap semiconductor GaN have very recently received great interest as promising host material for making DMS since its Curie temperature (T c Þ higher than room temperature is predicted by theoretical studies [1,2]. Although Mn-doped GaN is the most intensively investigated depending on the theoretical studies, the GaN system doped with other transition elements such as Co, Fe, and Ni may also offer suitable properties. Very recently, we have reported the magnetic properties of Co ion-implanted GaN as a function of annealing temperature [3]. In this study, we report on additional experimental results about structural and optical properties of the samples. We have conducted the experiments about structural and optical characterizations with the samples used for magnetic measurements. High-resolution X-ray diffraction (HRXRD), high-resolution transmission electron microscopy (TEM), selective area diffraction pattern (SADP), and X-ray photoelectron spectroscopy (XPS) using Mg K a radiation source ðhn ¼ 1253:6 eVÞ were employed for Corresponding author. Tel.: +82 2 910 4752; fax: +82 2 910 5170.
E-mail address: [email protected] (C.S. Kim). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.11.034
structural characterization. Magnetization measurements were carried out using a superconducting quantum interference device (SQUID) magnetometer system. In all the magnetization measurements, the magnetic field was applied parallel to the sample plane. Optical properties were analyzed by photoluminescence (PL). PL measurements were carried out with a 325-nm continuous-wave He–Cd laser as the optical excitation source. For the lowtemperature PL measurements, the films were cooled in a cryostat with a He recycled displex cooling system. We had investigated the magnetic characterization of Co ion-implanted GaN samples as a function of annealing temperature [3]. The magnetization curves at 5 K of samples showed clear ferromagnetic behavior. But at room temperature, the ferromagnetic hysteresis of samples seems to have disappeared within the resolution of the measurements. Also in zero-field-cooled (ZFC) and field-cooled (FC) magnetization measurements, we have observed a cusp in the ZFC magnetization curve and a clear irreversibility of the two different conditions for the samples at 700–900 C. This ZFC and FC curve as well as magnetization curve of sample are characteristic of a superparamagnetic system.
ARTICLE IN PRESS W. Kim et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e729–e731
2p3/2
2p1/2
Co 2p
Intensity (Arb.Units)
metallic Co
69 min 57 min 45 min 33 min 21 min 9 min 3 min
ΔE=14.8 eV 810
800
790
780
770
Binding energy (eV) Fig. 2. Co 2p spectra measured by XPS with Ar sputtering time for sample annealed at 800 C.
GaN implanted Co
-
:3x1016 cm-2 10 K
3.49 eV
GaN:
PL Intensity (Arb.Unit)
To obtain a better understanding on the origin of magnetic properties in samples, we have conducted the HRXRD, TEM, SADP, and XPS depth profile experiments. HRXRD measurements were not showed the existence of Co clusters and the reaction between Co and GaN formed during the implantation and post-annealing process. Fig. 1 shows a cross-sectional TEM micrograph (left) and selected-area-diffraction pattern (right) for 800 C-annealed sample. The TEM micrograph in Fig. 1 shows a significant degree of distortion. However, there is no evidence for second phases with sizes greater than the 2 nm resolution of the microscope. Also the SADP in Fig. 1 confirms the absence of precipitates. It shows that there are no obvious extra spots from the secondary phase formation. Despite the absence of precipitates within the detectable limits of the characterization techniques, we do not exclude the existence of undetected clusters. In order to observe the chemical states of implanted Co existing on the GaN inside, XPS depth measurements was carried out with sputtering time for samples annealed at 700, 800, and 900 C. Fig. 2 show the evolution of the Co 2p spectra with sputtering time for 800 C-annealed sample. XPS measurements of Co 2p3=2 core levels show only one peak at 778 and 14.8 eV energy difference between Co 2p3=2 and Co 2p1=2 core level. This corresponds to the peak for metallic Co bonding [4]. Fig. 3 shows PL spectra of Co ion-implanted GaN samples with post-annealing. The PL spectra of unimplanted GaN epilayer exhibited sharp and intense donorbound exciton peaks at 3.49 eV. With intrinsic He–Cd excitation, noticeable changes are observed in the spectra of implanted GaN films with post-annealing. After implantation with annealing, the PL spectrum of Co ion-implanted GaN taken 10 K displays a clear intensity modulation, indicating that the optical interference effect [5] occurs in the layers with a sufficiently small surface roughness. It also shows only one broad band appearing at around 2.7 eV. In conclusion, the structural and optical properties of Co ion-implanted GaN were investigated. HRXRD, TEM, and SADP results did not show any second phases formation. XPS depth measurement showed the only metallic Co formation at the depth where Co was implanted. The PL spectra obtained at 10 K display a clear intensity modulation.
PL Intensity (Arb. Units)
e730
10 K
as-grown
2.0
2.5
3.0
3.5
Photon energy (eV)
900 °C
800 °C
700 °C
as-implanted
2.0
Fig. 1. TEM cross-sectional micrograph (left) and selected-area-diffraction pattern (right) of 800 C-annealed sample.
2.2
2.4
2.6 2.8 3.0 Photon energy (eV)
3.2
3.4
3.6
Fig. 3. PL spectra at 10 K for as-grown, as-implanted, and annealed samples at 700, 800, and 900 C.
ARTICLE IN PRESS W. Kim et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e729–e731
This work was supported by the research program 2006 of Kookmin University in Korea. References [1] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 87 (2000) 1019. [2] K. Sato, H. Katayama-Yoshida, Jpn. J. Appl. Phys. 40 (Part 2) (2001) L485.
e731
[3] W. Kim, H.J. Kang, S.K. Oh, S. Shin, J.-H. Lee, J. Song, S.K. Noh, S.J. Oh, C.S. Kim, IEEE Trans. Nanotechnol. 5 (2006) 149. [4] J.H. Moulder, W.F. Strickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer, Eden Prairie, MN, 1992. [5] R. Smirani, F. Martin, G. Abel, Y.Q. Wang, M. Chicoine, G.G. Ross, J. Luminescence 115 (2005) 62.
Journal of Magnetism and Magnetic Materials 310 (2007) e729–e731 www.elsevier.com/locate/jmmm
Magnetic clusters in Co ion-implanted GaN Woochul Kima, Hee Jae Kangb, Sam Kyu Nohc, Jonghan Songd, Chul Sung Kima, a
Department of Physics, Kookmin University, 861-1, Cheongnung-dong, Songbuk-gu, Seoul 136-702, South Korea b Department of Physics, Chungbuk National University, Cheongju 361-763, South Korea c Korea Research Institute of Standards and Science, Daejeon 305-600, South Korea d Korea Institute of Science and Technology, Seoul 130-650, South Korea Available online 27 November 2006
Abstract The structural and optical properties of Co ion-implanted GaN were investigated by various measurements. HRXRD, TEM, and SADP results did not show any peaks associated with second phase formation. XPS depth measurements showed the metallic Co 2p core levels for 700–900 C annealed-samples. From XPS results, it could be explained that magnetic property of our films originated from Co or CoGa magnetic clusters. The PL spectra obtained at 10 K display a clear intensity modulation, indicating that the optical interference effect occurs in the layers. r 2006 Elsevier B.V. All rights reserved. PACS: 75.50.Pp; 75.70.Ak Keywords: Ion implantation; Magnetic semiconductor
The wide bandgap semiconductor GaN have very recently received great interest as promising host material for making DMS since its Curie temperature (T c Þ higher than room temperature is predicted by theoretical studies [1,2]. Although Mn-doped GaN is the most intensively investigated depending on the theoretical studies, the GaN system doped with other transition elements such as Co, Fe, and Ni may also offer suitable properties. Very recently, we have reported the magnetic properties of Co ion-implanted GaN as a function of annealing temperature [3]. In this study, we report on additional experimental results about structural and optical properties of the samples. We have conducted the experiments about structural and optical characterizations with the samples used for magnetic measurements. High-resolution X-ray diffraction (HRXRD), high-resolution transmission electron microscopy (TEM), selective area diffraction pattern (SADP), and X-ray photoelectron spectroscopy (XPS) using Mg K a radiation source ðhn ¼ 1253:6 eVÞ were employed for Corresponding author. Tel.: +82 2 910 4752; fax: +82 2 910 5170.
E-mail address: [email protected] (C.S. Kim). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.11.034
structural characterization. Magnetization measurements were carried out using a superconducting quantum interference device (SQUID) magnetometer system. In all the magnetization measurements, the magnetic field was applied parallel to the sample plane. Optical properties were analyzed by photoluminescence (PL). PL measurements were carried out with a 325-nm continuous-wave He–Cd laser as the optical excitation source. For the lowtemperature PL measurements, the films were cooled in a cryostat with a He recycled displex cooling system. We had investigated the magnetic characterization of Co ion-implanted GaN samples as a function of annealing temperature [3]. The magnetization curves at 5 K of samples showed clear ferromagnetic behavior. But at room temperature, the ferromagnetic hysteresis of samples seems to have disappeared within the resolution of the measurements. Also in zero-field-cooled (ZFC) and field-cooled (FC) magnetization measurements, we have observed a cusp in the ZFC magnetization curve and a clear irreversibility of the two different conditions for the samples at 700–900 C. This ZFC and FC curve as well as magnetization curve of sample are characteristic of a superparamagnetic system.
ARTICLE IN PRESS W. Kim et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e729–e731
2p3/2
2p1/2
Co 2p
Intensity (Arb.Units)
metallic Co
69 min 57 min 45 min 33 min 21 min 9 min 3 min
ΔE=14.8 eV 810
800
790
780
770
Binding energy (eV) Fig. 2. Co 2p spectra measured by XPS with Ar sputtering time for sample annealed at 800 C.
GaN implanted Co
-
:3x1016 cm-2 10 K
3.49 eV
GaN:
PL Intensity (Arb.Unit)
To obtain a better understanding on the origin of magnetic properties in samples, we have conducted the HRXRD, TEM, SADP, and XPS depth profile experiments. HRXRD measurements were not showed the existence of Co clusters and the reaction between Co and GaN formed during the implantation and post-annealing process. Fig. 1 shows a cross-sectional TEM micrograph (left) and selected-area-diffraction pattern (right) for 800 C-annealed sample. The TEM micrograph in Fig. 1 shows a significant degree of distortion. However, there is no evidence for second phases with sizes greater than the 2 nm resolution of the microscope. Also the SADP in Fig. 1 confirms the absence of precipitates. It shows that there are no obvious extra spots from the secondary phase formation. Despite the absence of precipitates within the detectable limits of the characterization techniques, we do not exclude the existence of undetected clusters. In order to observe the chemical states of implanted Co existing on the GaN inside, XPS depth measurements was carried out with sputtering time for samples annealed at 700, 800, and 900 C. Fig. 2 show the evolution of the Co 2p spectra with sputtering time for 800 C-annealed sample. XPS measurements of Co 2p3=2 core levels show only one peak at 778 and 14.8 eV energy difference between Co 2p3=2 and Co 2p1=2 core level. This corresponds to the peak for metallic Co bonding [4]. Fig. 3 shows PL spectra of Co ion-implanted GaN samples with post-annealing. The PL spectra of unimplanted GaN epilayer exhibited sharp and intense donorbound exciton peaks at 3.49 eV. With intrinsic He–Cd excitation, noticeable changes are observed in the spectra of implanted GaN films with post-annealing. After implantation with annealing, the PL spectrum of Co ion-implanted GaN taken 10 K displays a clear intensity modulation, indicating that the optical interference effect [5] occurs in the layers with a sufficiently small surface roughness. It also shows only one broad band appearing at around 2.7 eV. In conclusion, the structural and optical properties of Co ion-implanted GaN were investigated. HRXRD, TEM, and SADP results did not show any second phases formation. XPS depth measurement showed the only metallic Co formation at the depth where Co was implanted. The PL spectra obtained at 10 K display a clear intensity modulation.
PL Intensity (Arb. Units)
e730
10 K
as-grown
2.0
2.5
3.0
3.5
Photon energy (eV)
900 °C
800 °C
700 °C
as-implanted
2.0
Fig. 1. TEM cross-sectional micrograph (left) and selected-area-diffraction pattern (right) of 800 C-annealed sample.
2.2
2.4
2.6 2.8 3.0 Photon energy (eV)
3.2
3.4
3.6
Fig. 3. PL spectra at 10 K for as-grown, as-implanted, and annealed samples at 700, 800, and 900 C.
ARTICLE IN PRESS W. Kim et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e729–e731
This work was supported by the research program 2006 of Kookmin University in Korea. References [1] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 87 (2000) 1019. [2] K. Sato, H. Katayama-Yoshida, Jpn. J. Appl. Phys. 40 (Part 2) (2001) L485.
e731
[3] W. Kim, H.J. Kang, S.K. Oh, S. Shin, J.-H. Lee, J. Song, S.K. Noh, S.J. Oh, C.S. Kim, IEEE Trans. Nanotechnol. 5 (2006) 149. [4] J.H. Moulder, W.F. Strickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer, Eden Prairie, MN, 1992. [5] R. Smirani, F. Martin, G. Abel, Y.Q. Wang, M. Chicoine, G.G. Ross, J. Luminescence 115 (2005) 62.