강자성 반도체 산화물의 스핀정렬과 전하수송에 관한 연구

Abstract

Recently, magnetic-transition-metal doped n-type ZnO has attracted much attention because of its Curie temperature higher than room temperature (RT), unlike the theoretical prediction. So far, many researchers have tried to figure out the nature of ferromagnetism and to establish the appropriate ferromagnetic model. Despite many efforts, the magnetism has not been definitely understood until now. Thus, the minimum requirements for elucidating the origin are the fabrication of high-quality samples by careful control of the growth parameters and the detailed characterizations. Thus, this thesis mainly describes the fabrication of high-quality Mn-doped ZnO and hydrogenated Mn-doped ZnO film, exhibiting ferromagnetic behavior at RT, and the investigations on their magnetic and charge-transport properties. The main results are: - Mn-doped ZnO films with a Curie temperature above RT were fabricated successfully on a Si wafer by co-sputtering at a relatively low process temperature. Magnetic ordering in the polycrystalline Mn-doped ZnO film is very sensitive to the oxygen deficiency (δ). The ferromagnetic ordering at RT occurs in the polycrystalline Mn-doped ZnO film only with δ ~ 0.03. - The magnetic ordering in Mn-doped ZnO is observed when the carrier concentration is lower (~1018 cm-3) with respect to the theoretical RKKY model and the carrier mobility is very low, which indicates that the RKKY-type mechanism of ferromagnetic ordering is unlikely for Mn-doped ZnO. - The nature of ferromagnetism cannot also be described by the double-exchange model, since Mn dopant is not mixed-valence but exists only as Mn2+. - RT ferromagnetic Zn0.96Mn0.04O:H film with a substantially improved carrier mobility (~ 400% larger than that of the as-grown sample) was successfully fabricated through the hydrogen-implantation and a subsequent heat treatment. - The hydrogen and the oxygen deficiencies in Mn-doped ZnO play significant roles for improving the carrier mobility as well as the carrier concentration, so that these enhancements make carriers mediate the ferromagnetic interaction among Mn ion. This result could be explained well by magnetic bound-polaron model. - The H-implanted Mn-doped ZnO is a promising candidate for the spintronic-device application, where a high mobility is required for the high-speed devices.