Room-temperature Ferromagnetic Zn 0.95 Co 0.05 O Diluted Magnetic Semiconducting Thin Films by Pulsed Laser Deposition
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0928-GG05-04
Room-temperature Ferromagnetic Zn0.95Co0.05O Diluted Magnetic Semiconducting Thin Films by Pulsed Laser Deposition Yuebin Zhang1, Qing Liu1, Thirumany Sritharan1, and Sean Li1,2 1 School of Materials Science and Engineering, Nanyang Technological University, Singapore, Singapore, 639798, Singapore 2 School of Materials Science and Engineering, The University of New South Wales, Sydney, NSW, 2052, Australia
ABSTRACT Co-doped ZnO thin films with room-temperature ferromagnetism have been successfully synthesized on (001) Si substrates at 450 °C by pulsed-laser deposition using a Zn0.95Co0.05O ceramic target. Their microstructural properties are carefully studied using atomic force microscopy, x-ray diffraction and high-resolution transmission electron microscopy. The oxidation state of Co and the ratio of Co/Zn are examined by x-ray photoelectron spectroscopy, and magnetic measurements are performed using SQUID. The results show that a single-phase crystalline Co-doped ZnO film was grown with (002) preferential orientation and some edge dislocations formed during the film growth. The origin of room-temperature ferromagnetism is explored. The presence of nanoclusters of any magnetic phase can be ruled out. The dislocations, coupled with oxygen vacancy, may contribute to the ferromagnetic properties in the much diluted magnetic semiconductor.
INTRODUCTION Recently oxide-diluted magnetic semiconductors (O-DMS) have initiated enormous scientific interests because of their potential spintronics applications utilizing semiconductor physics and ferromagnetism [1,2]. As a wide band gap semiconductor ZnO has electron (n-type) conductivity naturally but p-type conductivity can also be induced by using a co-doping technique [3]. Therefore, the ZnO:Co system is a highly promising candidate for O-DMS in practical applications utilizing stable room-temperature ferromagnetism since an ideal ferromagnetic semiconductor would have a Curie temperature (TC) above room temperature and would be able to incorporate both p-type and n-type dopants. Ab initio calculations based on the local spin density approximation predict that n-type ZnO:Co should be a thermally robust ferromagnet [4,5]. Subsequently many studies were done on ZnO:Co films deposited by pulsed laser ablation using a KrF laser [6-11] and it was found that the sample properties delicately rely on the growth process [7,11]. Ueda et al. demonstrated a ferromagnetic Zn0.95Co0.05O film grown on (110) sapphire with a TC of 280 K [6]. Later, Prellier et al. reported high-quality 5%Co-doped ZnO thin films grown on (001) sapphire by alternative deposition with a TC close to the room temperature [8]. Recently anisotropic room-temperature ferromagnetism has been observed in Zn0.95Co0.05O films grown on (110) sapphire [11]. Due to the wurtzite (hexagonal) structure of ZnO, which is conserved upon doping, sapphire has been a common substrate in the
past because of its low lattice mismatch of 2% with the film. However, for easy integration into current semiconductor device
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