【作者】 刘红霞；

【导师】 张鹤鸣；

【作者基本信息】 西安电子科技大学 ， 微电子学与固体电子学， 2009， 博士

【摘要】 纳米管异质结在纳米电子器件中具有较好的应用前景,而纳米管异质结的电子输运特性是其应用的基础,对其进行研究,不仅具有重要的理论意义,而且具有重要的实用价值,也是当前国内外重视的研究领域。论文采用结合密度泛函理论的非平衡格林函数法,分别对碳/掺氮碳纳米管异质结、碳/碳化硅纳米管异质结和碳/氮化硼纳米管异质结的电子输运特性进行了研究,主要的结论如下:1.碳/掺氮碳纳米管异质结电子输运特性研究碳/掺氮碳纳米管异质结的结构和电子输运特性是该异质结的相关研究的理论基础。本文研究中,首先,建立了(8, 0)碳/掺氮碳纳米管异质结的模型,对该模型进行结构优化的结果显示,在掺氮碳纳米管侧,掺杂氮原子所在碳环的直径有所增大,这与已制备的掺氮碳纳米管呈现“竹节状”是一致的。然后,计算分析了异质结的电子输运特性,结果表明,孤立纳米管异质结的伏安特性有较高的对称性,正负偏压下的开启电压均为1.0 V;耦合于金电极异质结的伏安特性与孤立的不同,在负偏压下伏安特性曲线呈现了较明显的线性关系,正偏压下的伏安特性曲线可分成0.0 V到+0.6 V、+0.6 V到+1.2 V和+1.2 V到+2.0 V三部分,在这三个偏压区间内伏安特性都接近为线性,但是第二个偏压区间内电流的增长速度要小于其它两部分。2.碳/碳化硅纳米管异质结电子输运特性研究碳纳米管和碳化硅纳米管是研究较深入的两种纳米管材料,而碳/碳化硅纳米管异质结的结构和电子输运特性是值得研究的领域。本文在这种异质结的研究中,建立了(8, 0)碳/碳化硅纳米管异质结的模型,并对其进行了结构优化,结果可见,异质结的重构主要分布在异质结的界面,碳/碳化硅纳米管界面两侧的第四层原子之外的结构变化可以忽略。孤立和耦合于金电极的碳/碳化硅纳米管异质结的电子输运特性研究表明:孤立异质结导带底和价带顶位于异质结的碳纳米管侧,异质结在正负偏压下的开启电压分别为+1.8 V和-2.2 V;耦合于金电极异质结的伏安特性出现了微分负阻效应,微分负阻效应是偏压引起轨道局域性变化的结果。3.碳/氮化硼纳米管异质结电子输运特性研究碳/氮化硼纳米管异质结是有望制备成功的纳米管异质结,在对其进行的研究中,本文建立了(8, 0)碳/氮化硼纳米管异质结的模型,采用第一性原理计算对该异质结进行了结构优化,优化的结果显示,异质结结构的变化较小,这是由于这两种纳米管的结构具有较高相似性的结果。在结构优化的基础上,对该异质结的电子输运特性进行了研究。研究结果表明,孤立异质结的导带底和价带顶位于碳纳米管侧,异质结具有整流特性;当异质结耦合于金电极后,其电子输运特性发生了显著变化,在正负偏压下都出现了微分负阻效应,该效应是偏压导致异质结分子轨道局域性变化的结果。本文对纳米管异质结结构的研究对于其制备工作的开展具有一定的意义,而对纳米管异质结电子输运特性的研究对其相关电子器件的建模与设计工作具有较高的参考价值。更多还原

【Abstract】 Nanotube heterojunctions have good application prospects in nano-electronic devices, whose electronic transport properties are foundation of relevant investigations. Studies on electronic transport properties of nanotube heterojunctions are not only of high theoretical value, but also of great practical significance. The electronic transport properties of the carbon/nitrogen-doped carbon nanotube heterojunction, the carbon/silicon carbide nanotube heterojunction and the carbon/boron nitride nanotube heterojunction are studied with the method combined density functional theory (DFT) with nonequilibrium Green’s function (NEGF) and the main conclusions are as follows:1. Electronic transport properties of carbon/nitrogen-doped carbon nanotube heterojunctionThe structure and electronic transport properties of the heterojunction may improve related studies. First, the model of the carbon/nitrogen-doped carbon nanotube heterojunction is established and optimized. From the optimized structure, it is can be seen that the diameter of the nitrogen atoms located is enlarged, which is consistent with the structure of the synthesized nitrogen-doped CNT. Then, the electronic transport properties of the heterojunction are studied with the method combined DFT with NEGF. The symmetry of current voltage characteristic for the isolate heterojunction is high. The threshold voltages under positive and negative are±1.0 V respectively. The current voltage curve of the heterojunction coupled to Au electrodes can be divided into two parts: under negative bias and positive bias. Under negative bias, the relationship between the current and the bias voltage is nearly linear. Under positive bias, the current voltage curve consists of three ranges: from 0.0 V to +0.6 V, from +0.6 V to +1.2V and from +1.2 V to +2.0V. In these three ranges, the current voltage curve is nearly linear. While the current in the second bias range increase slower than in the other two bias ranges.2. Electronic transport properties of carbon/silicon carbide nanotube heterojunctionCNT and silicon caride nanotube (SiCNT) are two types of nanotube materials deeply studied. The structure and electronic transport properties of the carbon/SiCNT heterojunction is worth of being studied. Researches on this nanotube heterojunction are focused on two aspects: First, the model of nanotube heterojunction formed by a finite (8, 0) CNT and SiCNT is built and optimized with first principle calculations based on DFT. Results show that the rearrangement of the heterojunction mainly concentrates on its interface and the variation in structure decreses rapidly with the distance increase from the interface. The changes of diameters for the fourth layer in CNT section and SiCNT section can be ignored. Second, the transport properties of the isolate heterojunction and the heterojunction coupled to Au electrodes are studied. From the molecular orbitals of the isolate heterojunction, it can be seen that the conduction band bottom and the valence band top of the heterojunction lie in the CNT. The threshold voltages under negative bias and positive bias are -2.2 V and +1.8 V. In the transport properties of the heterojunction coupled to Au electrodes, negative differential resistance (NDR) effect is discovered. It originates from the variations of the locality for the molecular orbitals caused by the applied bias voltage.3. Electronic transport properties of carbon/boron nitride nanotube heterojunctionThe CNT/boron nitride nanotube (BNNT) heterojunction is the possible fabricated one. The model for an (8, 0) CNT/BNNT heterojunction is established. The geometry optimization of the heterojunction is implemented with the first principle calculations to obtaion the structure of the heterojunction, which is most similar to the fabricated. Results show that the structure variation of the heterojunction is weak in its formation, which is due to the high similarity in structure for two nanotubes. The electronic transport properties of the isolate heterojunction and of the heterojunction coupled to Au electrodes are studed. From the transport properties of the isolate heterojunction, it is can be found that both the conduction band bottom and the valence band top locate on the carbon nanotube section and rectifying property is found in its current voltage characteristic. While the heterojunction is coupled to Au electrodes, its transport properties are influenced by the coupling effect. The most important property is the existence of the NDR under positive bias and negative bias. It derives from the viariation of the locality for molecualr orbitals of the heterojunction caused by the applied bias voltage.The studies on the structure of the heterojunctions are meaningful to their synthesis. Investigations on the transport properties are helpful to modeling and developing novel devices.更多还原