Magnetic and structural properties of Co ion-implanted GaN

A GaN epilayer was grown on Al₂O₃ substrate by metal-organic chemical vapor deposition, and Co^- ions with a dose of 3×10¹⁶ cm^-2 were implanted into GaN at 350°C. The implanted samples were postannealed at 700°C-900°C to recrystallize the samples and to remove implantation damage. We have investigated the magnetic and structural properties of Co ion-implanted GaN by using X-ray diffraction (XRD), superconducting quantum interference device (SQUID) magnetometer, and X-ray photoelectron spectroscopy (XPS). XRD results did not show any peaks associated with the second phase formation, and only the diffraction from the GaN layer and substrate structure were observed. The temperature dependence of magnetization taken in zero-field-cooling and field-cooling conditions showed the features of superparamagnetic system in films annealed at 700°C-900°C. The magnetization curves at 5 K for samples annealed at 700°C-900°C exhibits ferromagnetic hysteresis loops, and the highest residual magnetization (M_R) and coercivity (H_c) of M_R=1.5×10^-4 emu/g and H_c=107 Oe were found in the 800°C annealed sample. XPS measurement showed the metallic Co 2p core levels and the metallic valence band spectra for as-implanted and 700°C-900°C annealed samples. From these, it could be explained that the magnetic property of our films originated from Co and CoGa magnetic clusters.