新型二维半导体材料及自旋相关器件量子输运的第一性原理研究

发布时间：2018-01-12 13:32

本文关键词：新型二维半导体材料及自旋相关器件量子输运的第一性原理研究　出处：《北京科技大学》2016年博士论文　论文类型：学位论文

【摘要】：自旋电子学已经成为一个热门的研究领域,大量的研究工作致力于探究其相关器件在介观尺度下的物理机制以及提高其性能。与此同时,二维层状半导体材料也在蓬勃的发展之中,优异的机械、电学以及光学性能使得其受到广泛的关注。而将自旋电子学与二维半导体材料相结合,也已经成为了新的研究趋势,它亟需人们大量相关的理论工作来预测及解释其新异的量子输运效应。本论文的研究工作是利用密度泛函理论结合非平衡格林函数方法,通过第一性原理计算从理论上研究自旋相关器件以及新型二维半导体材料的量子输运特性。主要研究内容及成果如下：1)提出并理论验证了一种全新非常有潜力的两端口磁性隧道结(MTJ)结构,Fe(001)/O/NaCl(001)/O/Fe(001),隧穿磁电阻率(TMR)值最大可超过3600%。巨大的TMR来源于MTJ结构的自旋过滤效应,它能过滤掉几乎所有向下自旋通道的隧穿作用。另外向上自旋通道的输运过程主要由对称性为△1与△s态所贡献,与此同时通过计算散射态密度发现在MTJ结构中的FeO界面处存在很强的谐振态,使得△l和△5态的输运过程得到非常大的增强。2)利用非平衡格林函数方法的第一性原理量子输运理论研究了Fe和Ni四端口纳米结构的反常霍尔效应(AHE)。在Landauer-Buttiker公式的框架下,从量子输运的角度来定义非零反常霍尔电阻的内禀项。通过分析自旋轨道耦合作用,发现纳米尺度下的Fe和Ni表现出相反的反常霍尔系数,与它们各自的块体材料保持一致。另外通过分析局域散射态态密度(LDOSS),发现纳米尺度下Ni的反常霍尔系数的负值主要由四端口电极的散射作用所贡献。最后,我们通过散射矩阵的方法在四电极AHE结构中发现了相似的Onsager关系。3)研究了包含自旋轨道耦合作用的单层WSe2薄膜的场效应晶体管(FET)的量子输运性质。在强大的自旋轨道耦合作用以及特殊的二维的原子结构,使得单层WSe2的自旋劈裂类似于Zeeman型,通过从外部施加栅极电压向薄膜中引入一个Rashba型的自旋轨道耦合作用,实现利用外电场改变WSe2薄膜中载流子自旋取向,当与FET漏极处自旋取向完全相匹配时系统的电导率最大；当与漏极处自旋取向完全不匹配时,系统的电导率最小。最后我们把FET电流Iτ,s同时由valley指数τ和spin指数s表示,在同一器件中实现valleytronics和spintronics。4)研究出了一套能定性以及定量预测非平衡态下的极化valley流的第一性原理量子输运计算方法。由于单层WSe：薄膜具有非常强的自旋轨道耦合作用,它的量子态能够用倒空间valley指数和spin空间指数来标定。构建出的WSe2-FET在外偏压下,通过照射圆偏振光,能选择性的将净极化的valley和spin流Iτ,s从源极输送至漏极。同时由于WSe2-FET在输运方向缺少平移对称性,由此发现了其中的去valley极化效应,该效应随着晶体管的长度减小而增大。5)从理论上研究了能够与单层黑磷(BP)有良好接触的金属材料。通过分析接触面几何结构、成键状态、电子结构、及电荷转移等,找到了Cu(111),Zn(0001),In(110),Ta(110)-BP,和Nb(110)-BP这5种合适的金属表面。由接触面势垒的分析,发现Cu(111)是能与单层BP形成完美欧姆接触的最佳金属面,其他的四种金属或组合的表面与单层BP接触面有较大肖特基势垒。同时研究了单层半导体BP与电极形成的面内电流输运模型(current-in-plane, CIP)的能带弯曲特性,通过功函数及非平衡格林函数法分析CIP结构的电子结构,发现单层独立BP薄膜与以上几种电极相连接处均为n型能带弯曲。
[Abstract]:Spintronics has become a hot research field, a lot of research work is devoted to exploring the related devices in the meso scale mechanism and to improve its performance. At the same time, a two-dimensional layered semiconductor material in the vigorous development, excellent mechanical, electrical and optical properties of the attention and the phase. Combination of spintronics and two-dimensional semiconductor materials, has become a new trend of research, theoretical work it needs a large number of relevant people to predict and explain the new quantum transport effects. The study is combined with non-equilibrium Green function method using density functional theory, the first principle calculation of spin and related devices quantum new two-dimensional semiconductor material from the theory of transport properties. The main research contents and results are as follows: 1) proposed and verified the theory A new two port magnetic tunnel is a potential node (MTJ), Fe (001) /O/NaCl (001) /O/Fe (001), tunneling magnetoresistancet rate (TMR) value of the spin filter effect can exceed 3600%. huge TMR derived from the MTJ structure, it can filter out almost all spin down channel the tunneling effect. In addition to the transport process on the spin channel mainly by symmetry for delta 1 and delta s states to contribute, at the same time by calculating the scattering density of States found resonant strong FeO interface in the MTJ structure, the transport process of delta L and delta 5 states to enhance.2 very large anomalous Holzer effect) of first principle quantum using the non-equilibrium Green function method, the transport theory of Fe and Ni four port nano structure (AHE). In the framework of Landauer-Buttiker formula, the intrinsic quantum transport from the perspective of the definition of non zero anomalous Holzer resistance. Analysis of effect of spin orbit coupling, found that Fe and Ni nano scale showed abnormal Holzer coefficient on the contrary, consistent with their respective bulk materials. In addition, through the analysis of local scattering states of the density of states (LDOSS), found that negative value anomalous Holzer coefficient of nano scale Ni is mainly composed of four port scattering electrode the contribution. Finally, we through the scattering matrix method in four electrode AHE structure found in a similar Onsager.3) field effect transistor of the single-layer WSe2 film contains the effect of spin orbit coupling (FET) quantum transport properties. The effect of spin orbit coupling and strong special two-dimensional atomic structure so, the spin splitting of single WSe2 is similar to Zeeman, the effect of spin orbit coupling applied gate voltage from the outside into a Rashba type to the film, the realization of the use of external electric field change WSe2 The spin orientation in the film, and when the drain of the FET conductivity spin orientation completely matched system; and when the drain at the spin orientation do not match, the system. Finally we got the minimum conductivity FET current I s by Valley and tau, tau and spin index index s, valleytronics and spintronics.4 in the same device) developed a qualitative and quantitative prediction of first principle quantum polarization Valley nonequilibrium flow transport method. The single WSe thin film with spin orbit coupling effect is very strong, its quantum state can use reciprocal space Valley index and spin index of spatial calibration. Build a WSe2-FET in the external bias, by irradiating the circularly polarized light, can selectively net polarized Valley and spin flow I, s from the source to the drain. At the same time as the delivery of WSe2-FET in the transport direction of Que Shaoping Shift symmetry, the discovery of valley to the polarization effect, the effect with the length of the transistor decreases with the increase of.5) is studied theoretically with single black phosphorus (BP) metal materials have good contact. Through the analysis of contact surface geometry structure, electronic structure, bonding, and charge transfer. Find the Cu (111), Zn (0001), In (110), Ta (110) -BP and Nb (110) -BP these 5 kinds of suitable metal surface. By the analysis of the contact surface of the barrier, Cu (111) is the best metal surface can form perfect ohmic contact with monolayer surface BP. Four kinds of metal or a combination of the BP and other single contact surface is also studied. The current Schottky barrier formed monolayer semiconductor BP and electrode surface transport model (current-in-plane, CIP) of the band bending characteristics, the work function and the non-equilibrium Green function method analysis of the electronic structure of the CIP, found that single B The junction of the P film and the above several electrodes is n type band bending.

【学位授予单位】：北京科技大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TN304

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