스핀트로닉스 응용을 위한 스핀분극 전류 생성 및 ZnO 기반 자성반도체 연구

Abstract

For spintronic applications, ferromagnetic-semiconductor spin-source material and direct spin-polarized current generation in paramagnetic materials were studied. Firstly, the growth of ZnO-based diluted magnetic semiconductor and its characterization were performed. ZnO is a promising candidate for the base material of diluted magnetic semiconductor with a high Curie temperature above room temperature and accumulated spin density of states occurs in the ferromagnetic semiconductor. In this study, the ZnO films were grown on sapphire by pulsed laser deposition. High-quaility single crystallized ZnO films were fabricated and the magnetic impurities, Mn and Co, were also doped into ZnO. The properties of Mn-, Co-doped ZnO were characterized by x-ray diffraction, capacitance-voltage, deep level transient spectroscopy, photoluminescence, and M-H measurement. Especially, through an analysis with a hydrogen-plasma annealing, it was verified that wide-bandgap semiconductor-based ZnO diluted magnetic semiconductor, in an wurzite structure, has ferromagnetism which is due to the spin-spin double-exchange interaction related to hydrogen atom in the magnetic-impurity dimer. Secondly, a direct generation of spin-polarized current in a paramagnet was also studied. An new way of spin pumping by using a radio-frequency electric field was suggested and its experimental realization was also performed in this thesis. The spin-polarized state can be measured by spin Hall effect. So, the Hall devices that can apply a radio-frequency electric field were designed. The measurement system was also designed, which is for Hall-voltage shifts by the radio-frequency field in a range of 10 MHz ~ 6 GHz. The system can scan a resonance frequency with resolution of 1 Hz, 1 fA and 10 nV. Al-metal Hall device and InAs/InGaAs 2-dimensional electron gas semiconductor Hall device were fabricated. The spin Hall-voltage shift, which is due to the radio-frequency spin pumping was observed in the devices at room temperature. In Al device, the spin pumping occurs at 1.4 ~ 1.8 GHz and the spin polarization ratio is 0.1 (10%). The spin-polarized current generation by radio-frequency fields was tried in this study. Generation of a spin-polarized state is very important for spintronics and quantum mechanical logic devices.