【作者】 潘凤春；

【导师】 梅良模； 赵明文；

【作者基本信息】 山东大学 ， 凝聚态物理， 2010， 硕士

【摘要】 以研究和控制电子的荷电特性及其输运特性为主要内容的微电子学,使人类进入了信息时代。在传统微电子学之中,电子只是被当作电荷的载体,它的自旋特性一直未被引起重视。二十世纪八十年代末,巨磁电阻效应(GMR, Giant magnetoresistance)的发现引发了磁存储和磁记录领域的革命,其重要的应用前景极大地激发了人们对磁性材料输运的兴趣,并在此基础上逐渐形成了一门以研究、利用和控制自旋极化的电子输运过程为核心的新兴学科：自旋电子学(Spintronics)。铁磁性半导体是自旋电子学领域的关键材料,作为一种具有丰富物理内涵和重要应用前景的新型电子材料,磁性半导体已成为自旋电子学这个新领域的研究热点。磁性半导体通常是通过掺杂过渡族金属元素TM如V, Cr, Mn, Fe, Co, Ni等进入III-V族、Ⅱ-Ⅵ族和Ⅳ族化合物或单质,如InAs,GaAs,ZnO,TiO2, ZnTe, Ge, Si等得到的。早期研究中铁磁性半导体材料的居里温度非常低,一般在10 K以下,这极大的限制了铁磁性半导体材料的实际应用。于是寻找具有高居里温度的磁性半导体材料成为自旋电子学的研究重点之一。上世纪90年代,首先在Ⅲ-Ⅴ族半导体材料(如GaAs)的研究中取得了巨大的进展,但其居里温度仍低于室温。之后,对于以氧化物为代表的Ⅱ-Ⅵ族材料和以碳化硅(SiC)为代表的Ⅳ-Ⅳ族材料的研究也相继展开。碳化硅是一种Ⅳ-Ⅳ族的半导体材料,与当前主流的半导体材料硅材料具有相似的电子结构,在制作工艺也相互兼容。但是,碳化硅中过渡金属的掺杂浓度低,容易出现相分离。磁化强度和居里温度的提高受到了限制。本论文从量子力学第一性原理出发结合群论的分析方法,系统研究了碳化硅中空位缺陷的自旋极化、缺陷之间的磁耦合机制及缺陷的电荷态对自旋极化的调控作用。为揭示非掺杂(undoped)碳化硅的磁矩起源提供了理论依据,同时也为碳化硅铁磁半导体材料提供新的思路。本论文主要的研究结果如下：(1).立方碳化硅(3C-SiC)中的阳离子空位(Vsi具有高,低自旋极化的电子态,不同价态具有不同的自旋极化和自旋耦合方式。-1价的硅空位(Vsi)-1具有S=3／2的自旋态,且空位之间倾向形成自旋反平行的排列(反铁磁)；而-2价的硅空位(VSi)-2则具有S=1的自旋态,倾向形成自旋平行的排列(铁磁)。可以通过控制碳化硅中n型掺杂的浓度来调控其中(VSi)的电荷态,进而调控其自旋极化和自旋耦合。以氮原子掺杂为例,当N:VSi= 1:1时,VSi处于-1价态,并具有S=3／2的自旋和反平行的自旋排列；当N:VSi=2:1时,VSi处于-2价态,并具有S=1的自旋和平行的自旋排列。这为在3C-SiC中实现铁磁性提供了新的思路。(2)理想的4H-SiC(不含空位缺陷)中Al原子替代Si原子的掺杂(p型掺杂)并不会引起缺陷态电子的自旋极化。但是,当其中存在Vsi缺陷时,Al原子掺杂改变了VSi电荷态,诱发了VSi自旋极化,VSi磁矩的大小与Vsi和掺杂Al原子的相对位置有关。这解释了实验上发现的Al掺杂4H-SiC的自旋玻璃铁磁态的起源。(3)首次研究了3C-SiC中氮原子和硅空位的组合(N-Vsi center)的电子结构和自旋极化和自旋耦合。从理论上预言：3C-SiC中的(N-Vsi)-1缺陷具有与金刚石中(N-V)-1缺陷相似的电子结构,因此,有希望利用(N-Vsi)-1在3C-SiC实现固体量子比特。更多还原

【Abstract】 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.更多还原