X-ray absorption spectroscopy study the origin of ferromagnetism in the Co doped ZnO diluted magnetic semiconductor

• Main Authors: Liao, Yen-Fa, 廖彥發
• Other Authors: Lee, Chih-Hao
• Format: Others
• Language: en_US
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/28951166143426319850

博士 === 國立清華大學 === 工程與系統科學系 === 98 === The purpose of this thesis is to study the origin of ferromagnetism of the Co doped ZnO diluted magnetic semiconductor (DMS) by synchrotron x-ray absorption spectroscopy. The origin of ferromagnetism in oxide-based magnetic semiconductor is closely related to the BMP model, which can be understood clearly by the formation of the electron structure in this system. Firstly, we prepared the Co doped ZnO nanoparticles by thermal hydrolysis. The room temperature ferromagnetism was observed in these nanoparticles. The magnetization of the Co doped ZnO nanoparticles was decreased as the Co concentration increased. An impurity band structure caused by hybridization between the Co 3d states and Zn 4s states was observed by the soft x-ray absorption spectroscopy. The mechanism of the origin ferromagnetism that caused by the cobalt cluster effect was excluded in our system. The annealing process has been used to change in the number of oxygen vacancies in the nanoparticles. The effect of oxygen vacancies leads to the magnetization in the Co doped ZnO nanoparticles. Decrease in the number of oxygen vacancies which are as follows the annealed temperature increased was determined by the x-ray absorption spectroscopy and photoluminescence spectroscopy. The magnetization of the sample was absence after annealing treatment. We consider that the magnetization of the samples are attributable to the effect of the oxygen vacancies in the Co doped ZnO nanoparticles. Furthermore, we had studied the influence of the magnetization related to the carrier effect in DMS. High quality ZnO epitaxial thin film was deposited by ion beam sputtering. The concentration of the carrier and the cobalt dopants in ZnO host matrix are controlled by ion implantation method. With the observation of various cobalt doses, the origin of the room temperature ferromagnetism was caused by the metallic cobalt cluster in the Co implanted ZnO thin film. The effect of the cobalt cluster was observed positively as the implanted cobalt dose increased. The effect of the cobalt clusters leads to magnetic properties in high dose samples. The annealing treatment has been used to study the origin of ferromagnetism in the Co implanted ZnO thin film. Intrinsic DMSs phase in the Co implanted ZnO thin film was observed after annealing process. Intrinsic DMSs is defined as the characteristic that the structure of the Co substituted on Zn sites results in the ferromagnetic coupling. We study the structural and magnetic properties of thermally annealed Co implanted ZnO thin film at 600℃, 700℃ and 800℃, respectively. Two mechanisms of ferromagnetism in DMSs are present in the following examples: the cobalt cluster effect and the intrinsic DMSs. We observed the formation of the cobalt cluster before annealing treatment. The magnetic properties of the 600℃ annealing sample show two ferromagnetic phases in these systems. The effect on the Co substituted in ZnO host matrix results in the magnetization in the 700℃ annealing sample. An impurity band observed at Zn L edge was measured by soft x-ray absorption spectroscopy. The origin of ferromagnetism in DMS might be due to the charge transfer that the Co substituted on Zn site leads to the interaction between the Co electron and Zn electron through oxygen vacancies in DMS. Hence, we believed that the oxygen vacancy model could explain to the origin of the ferromagnetism in oxide-based DMS.