本文關鍵詞：新型二維半導體材料及自旋相關器件量子輸運的第一性原理研究　出處：《北京科技大學》2016年博士論文　論文類型：學位論文

  更多相關文章： 量子輸運 第一性原理計算 密度泛函 非平衡格林函數(shù) 二維層狀半導體材料

【摘要】：自旋電子學已經(jīng)成為一個熱門的研究領域,大量的研究工作致力于探究其相關器件在介觀尺度下的物理機制以及提高其性能。與此同時,二維層狀半導體材料也在蓬勃的發(fā)展之中,優(yōu)異的機械、電學以及光學性能使得其受到廣泛的關注。而將自旋電子學與二維半導體材料相結合,也已經(jīng)成為了新的研究趨勢,它亟需人們大量相關的理論工作來預測及解釋其新異的量子輸運效應。本論文的研究工作是利用密度泛函理論結合非平衡格林函數(shù)方法,通過第一性原理計算從理論上研究自旋相關器件以及新型二維半導體材料的量子輸運特性。主要研究內(nèi)容及成果如下：1)提出并理論驗證了一種全新非常有潛力的兩端口磁性隧道結(MTJ)結構,Fe(001)/O/NaCl(001)/O/Fe(001),隧穿磁電阻率(TMR)值最大可超過3600%。巨大的TMR來源于MTJ結構的自旋過濾效應,它能過濾掉幾乎所有向下自旋通道的隧穿作用。另外向上自旋通道的輸運過程主要由對稱性為△1與△s態(tài)所貢獻,與此同時通過計算散射態(tài)密度發(fā)現(xiàn)在MTJ結構中的FeO界面處存在很強的諧振態(tài),使得△l和△5態(tài)的輸運過程得到非常大的增強。2)利用非平衡格林函數(shù)方法的第一性原理量子輸運理論研究了Fe和Ni四端口納米結構的反�；魻栃�(AHE)。在Landauer-Buttiker公式的框架下,從量子輸運的角度來定義非零反�；魻栯娮璧膬�(nèi)稟項。通過分析自旋軌道耦合作用,發(fā)現(xiàn)納米尺度下的Fe和Ni表現(xiàn)出相反的反�；魻栂禂�(shù),與它們各自的塊體材料保持一致。另外通過分析局域散射態(tài)態(tài)密度(LDOSS),發(fā)現(xiàn)納米尺度下Ni的反�；魻栂禂�(shù)的負值主要由四端口電極的散射作用所貢獻。最后,我們通過散射矩陣的方法在四電極AHE結構中發(fā)現(xiàn)了相似的Onsager關系。3)研究了包含自旋軌道耦合作用的單層WSe2薄膜的場效應晶體管(FET)的量子輸運性質。在強大的自旋軌道耦合作用以及特殊的二維的原子結構,使得單層WSe2的自旋劈裂類似于Zeeman型,通過從外部施加柵極電壓向薄膜中引入一個Rashba型的自旋軌道耦合作用,實現(xiàn)利用外電場改變WSe2薄膜中載流子自旋取向,當與FET漏極處自旋取向完全相匹配時系統(tǒng)的電導率最大；當與漏極處自旋取向完全不匹配時,系統(tǒng)的電導率最小。最后我們把FET電流Iτ,s同時由valley指數(shù)τ和spin指數(shù)s表示,在同一器件中實現(xiàn)valleytronics和spintronics。4)研究出了一套能定性以及定量預測非平衡態(tài)下的極化valley流的第一性原理量子輸運計算方法。由于單層WSe：薄膜具有非常強的自旋軌道耦合作用,它的量子態(tài)能夠用倒空間valley指數(shù)和spin空間指數(shù)來標定。構建出的WSe2-FET在外偏壓下,通過照射圓偏振光,能選擇性的將凈極化的valley和spin流Iτ,s從源極輸送至漏極。同時由于WSe2-FET在輸運方向缺少平移對稱性,由此發(fā)現(xiàn)了其中的去valley極化效應,該效應隨著晶體管的長度減小而增大。5)從理論上研究了能夠與單層黑磷(BP)有良好接觸的金屬材料。通過分析接觸面幾何結構、成鍵狀態(tài)、電子結構、及電荷轉移等,找到了Cu(111),Zn(0001),In(110),Ta(110)-BP,和Nb(110)-BP這5種合適的金屬表面。由接觸面勢壘的分析,發(fā)現(xiàn)Cu(111)是能與單層BP形成完美歐姆接觸的最佳金屬面,其他的四種金屬或組合的表面與單層BP接觸面有較大肖特基勢壘。同時研究了單層半導體BP與電極形成的面內(nèi)電流輸運模型(current-in-plane, CIP)的能帶彎曲特性,通過功函數(shù)及非平衡格林函數(shù)法分析CIP結構的電子結構,發(fā)現(xiàn)單層獨立BP薄膜與以上幾種電極相連接處均為n型能帶彎曲。
[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.

【學位授予單位】：北京科技大學
【學位級別】：博士
【學位授予年份】：2016
【分類號】：TN304

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