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The Theoretical Studies on Magnetism and Optical Properties of ZnO-based and TiO2-based Diluted Magnetic Semiconductors

Author: DanXu
Tutor: YaoKaiLun
School: Huazhong University of Science and Technology
Course: Physical Electronics
Keywords: Diluted magnetic semiconductors Half-metalic ferromagnet Densityfunctional theory Formation energy Absorption spectrum
CLC: TN304.2
Type: PhD thesis
Year: 2013
Downloads: 1
Quote: 0
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Abstract


In conventional semiconductor devices, only the charge property of the electron has been used. However, according to quantum mechanics, we know that the electron has two fundamental degrees of freedom, e.g. charge and spin. In the past twenty years, electronic spin has attracted more and more attention, and then launched a new field of spintronics, which combines both charge and spin of electron as the carriers of information. Compared to the conventional semiconductor devices, the spintronics devices have the advantages of smaller size and lower energy consumption. Diluted magnetic semiconductors and half-metallic ferromagnets are of potential applications in spintronics devices, and it is a hotspot field of the condensed matter physics.With the rapid development of the technique of computers, predicting new functional materials via materials simulation softwares becomes more and more powerful. Compared to artificial experiments, computational simulations have the advantage of few time and lower cost. In this dissertation, we performed the studies of magnetism and electronic structure on the zinc-blend ZnO-based as well as rutile TiO2-based diluted magnetic semiconductors using the materials simulation softwares of WIEN2k and CASTERFor the C-doped zinc-blende ZnO system, our computational results indicate that this system exhibits half-metallic ferromagnetism with a stable ferromagnetic ground state. The calculated low formation energy suggests C-doped zinc-blende ZnO is energetically stable and the estimated Curie temperature is476K, meaning that C-doped zinc-blende ZnO is an ideal candidate for spintronics applications. Since the Zn, C, O elements and their compounds are all nonmagnetic, we can conclude that the ferromagnetism in C-doped ZB ZnO is intrinsic, and the ferromagnetism in C-doped zinc-blende ZnO can be explained by the hole-mediated double exchange mechanism. Holes induced by the carbon dopants are the main carriers and the spin-polarized holes in C2p states couple with the O2p localized spins via a p-d type p-p interaction, which results in the long-range ferromagnetic interaction between the two C dopants.The calculations of the electronic structure, the magnetism and the optical properties of N-doped, Co-doped as well as N,Co-codoped rutile TiO2indicates that N-doped rutile TiO2exhibits a semiconducting characteristic with a stable antiferromagnetic ground state. Co-doped rutile TiO2and N,Co-codoped rutile TiO2both have a stable ferromagnetic ground state. When the GGA for the electronic exchange-correlation functional is adoped, Co-doped rutile TiO2shows half-metallic ferromagnetism and N,Co-codoped rutile TiO2has a nearly half-metallic nature, while, both the two doped system are a semiconductor within GGA+U. N-doped, Co-doped and N,Co-codoped TiO2all can greatly increase the optical absorption in the visible-light region. N-doped rutile TiO2enhances the absorption intensity in short wavelength region dramatically, while, in visible-light region, the absorption intensity and absorption region of Co-doped rutile TiO2are both the biggest among the three doped syestems. N,Co-codoped rutile TiO2has the similar absorption spectra shape with Co-doped rutile TiO2in the visible-light region, but its absorption intensity is less than that of the Co-doped rutile TiO2, which is due to the lower Co concentration in N,Co-codoped system than that in Co-doped system.In the case of V-doped rutile TiO2and V,N-codoped rutile TiO2, our computational results show that the two doped systems both exhibit half-metallic characteristic when the GGA for the electronic exchange-correlation functional is adoped. V-doped rutile TiO2can enhance the absorption intensity in short wavelength region dramatically, while, V,N-codoped rutile TiO2can not only enhance the absorption intensity but also extend the absorption region to more than700nm.

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