Tuning The Spin States Of Vacancy Defects In Silicon Carbide Crystals

Microelectronics based on electronic charges and transportation has greatly promoted the advancement of society and brought people into information era. However, in conventional microelectronics, we only take advantage of the charge property of the electron, while the spin degree of freedom of the electron is neglected. Until 1980s the discovery of Giant magnetoresistance had revolutionized applications in magnetic recording and memory. Thus, spin-dependent electrical transport was inspired because of its huge potential application. And this launched the new field of spin electronics-'spintronics', which is centered on the spin of electrons including their generation, transport and detection.Magnetic semiconductors (MS) have attracted considerable attention because of their potential applications in spintronics devices. Ferromagnetic semiconductor is one of the key materials of spintronics. Magnetic semiconductors are usually synthesized by doping transitional metal elements such as V, Cr, Mn, Fe, Co, and Ni, into III-V, II-VI, and IV group compounds such as InAs, GaAs, ZnO, TiO2, SnO2, ZnTe, etc. In the early research, the Curie temperature of ferromagnetic semiconductors is mostly lower than 10 K, which limits its practical applications. Much effort has been devoted to search for high Curie temperature ferromagnetic semiconductors. In 1990s, the Curie temperature of Ga1-xMnxAs materials reaches up to 170 K. Subsequently, a lot of results on II-VI compounds (concentrated on oxide) and group IV magnetic semiconductors were reported rapidly. The group-IV SiC semiconductor has similar electronic structure with silicon, and is compatible with the current silicon technology. However, the concentration of transition metals doped in SiC is always very low due to phase separation.In this thesis, we preformed first-principles calculations combined with group theory to study the spin-polarization of silicon vacancy defects in silicon carbide at different charge states and magnetic coupling between them. Our results reveal the origination of local magnetic moments in undoped SiC materials and open a promising route to achieve room temperature ferromagnetism in SiC semiconductor.The thesis includes the following aspects.(1) We predicted that the silicon vacancy defect (Vsi) in cubic silicon carbide crystal (3C-SiC) possess high-spin states with spin S depending on the charge states of this defect. The Vsi defects at-1 charge states (Vsi)-1 have S=3/2 and prefer to interact in an antiferromagnetic way. For the VSi defects at-2 charge states (Vsi)-2, however, the net spin of each defect is S=1, and long-rang ferromagnetic ordering is energetically favorable. The vacancy charge states, as well as the spin states, can be modulated by adjusting the concentration of n-type doping, e.g. nitrogen impurities.(2) We revealed for the first time the role of Al impurities in the magnetism of 4H-SiC crystal. Our calculations show that for the 4H-SiC free from vacancy defects, the substitution of Si atoms with Al atoms cannot induce spin-polarization of the defect states, and thus has no contribution to magnetism. For the 4H-SiC containing Vsi defects, however, the doped Al atoms modify the charge states of Vsi defects and trigger spin-polarization of defect states. The local magnetic moments of the Vsi defects are sensitive to the doping site. This is consistent with the experimental findings of magnetism in Al-doped 4H-SiC.(3) We investigated the electronic structure of the nitrogen-silicon-vacancy complex (N-Vsi center) in 3C-SiC crystal. We show that the negatively-charged N-Vsi center at 3C-SiC possess electronic structures and spin states are very similar to those of N-V center in diamond. Considering the great achievement of N-V center in solid-state qubit operation, our results imply a promising way to achieve solid-state qubit in the 3C-SiC based on N-Vsi centers.