【作者】 宋久旭；

【导师】 杨银堂；

【作者基本信息】 西安电子科技大学 ， 微电子学与固体电子学， 2008， 博士

【摘要】 本文首先对碳纳米管和碳化硅纳米管的研究进展进行了较全面的论述,包括其制备、纯化以及应用等,其中较深入的论述碳纳米管的布里渊区及其电学性质相关的应用。接下来对在纳米材料电子结构以及纳米器件输运特性研究中取得较满意结果的密度泛函理论和非平衡格林函数法进行了详细的描述。采用该方法对碳纳米管和碳化硅纳米管的电子结构和输运特性进行了较深入的研究。采用基于密度泛函理论的第一性原理计算方法,对本征和掺杂(8, 0)碳纳米管的结构和电子结构进行了计算。本征(8, 0)碳纳米管的计算结果表明它是典型的直接带隙半导体,其能带间隙为0.46 eV。掺杂碳纳米管拥有与本征纳米管不同的性质,也有更广泛的应用前景,为此分别计算了掺氮和掺硼碳纳米管的电子结构。掺杂的氮原子与其相邻的碳原子形成的氮碳键的键长与本征纳米管对应的碳碳键相比有所增加,这种趋势会随着掺杂浓度的增大而变的更加明显,这与实验制备的掺氮碳纳米管呈现竹节状是一致的。掺杂的氮原子与碳原子相比多出了一个电子,从分子的最高占据轨道可以看出,多出的电子主要分布在氮原子及其相邻的碳原子上。这增加了电子在不同原子间转移的可能性,使得碳纳米管能带间隙有所减小。掺硼碳纳米管结构的变化与掺氮呈现了相同的趋势,掺杂原子所在的碳环的半径有所增大。掺杂的硼原子少出了一个电子,在掺杂原子附近形成了明显的空穴,碳纳米管的能带间隙也有所增大。采用结合密度泛函理论的非平衡格林函数法,对孤立(8, 0)碳纳米管和耦合于金电极的(8, 0)碳纳米管的输运特性进行了计算。从孤立碳纳米管的平衡态(未施加偏压)透射谱可以看出它是半导体型的,这与前面的计算是一致的。其透射谱呈现了明显的台阶状;其伏安特性可以看出当偏压小于1.2 V时几乎没有电流流过纳米管,而大于该偏压时是接近指数关系的。在碳纳米管应用中,通常要与金属电极相连接,为此计算了耦合于金电极的(8, 0)碳纳米管的输运特性,在计算的过程中考虑了长度对输运特性的影响。其结果表明随着长度的增加碳纳米管由金属型转化为半导体型。在金电极与碳纳米管形成接触时,由于功函数的不同,电子会在碳纳米管和电极之间转移,其结果是形成了较明显的带隙态。在长度较短时,带隙态在输运特性中起了重要的作用,但是其影响会随着长度的增长而削弱。这解释了为什么当前实验未观测到带隙态的原因。采用与研究碳纳米管电子结构相同的方法,计算了本征和掺杂(8, 0)碳化硅纳米管的电子结构。本征碳化硅纳米管的结构优化的结果显示碳环的半径要略大于硅环的半径,其电子结构显示,(8, 0)碳化硅纳米管的能带间隙为0.94 eV要大于(8, 0)碳纳米管的,这是由于碳化硅纳米管中碳硅键为含有离子键成分共价键的结果。氮原子与硼原子是碳化硅体材料的常用掺杂原子,在计算掺杂碳化硅纳米管电子结构的过程中仍然选择它们为杂质。在替位掺杂的过程中,参考体碳化硅材料掺杂的结果,氮原子取代碳原子所在的晶格。掺氮碳化硅纳米管结构优化的结果显示一氮掺杂碳化硅纳米管中氮原子与相邻的硅原子形成氮硅键的键长有较明显的减小,而二氮掺杂结构优化的结果表明氮原子倾向占据相邻碳环中位置最接近的碳晶格所在的位置并且在纳米管的表面形成明显的突起。氮原子与碳原子相比多出的电子主要分布在相邻的硅原子上。掺氮碳化硅纳米管的能带间隙会减小。掺硼碳化硅纳米管的几何结构优化的结果显示其所在硅环的半径会减小,从掺硼碳化硅纳米管的最高占据轨道可以看出,在硼原子附近电子出现的概率明显降低,形成明显的空穴态,限制了电子在不同原子间的转移,导致了能带间隙的增大。本文还计算了孤立和耦合于金电极的(7, 0)碳化硅纳米管的输运特性。孤立(7, 0)碳化硅纳米管的透射谱显示碳化硅纳米管为半导体型的,从其伏安特性可以看出电流发生明显变化的偏压为2.2V,要大于碳纳米管的。对耦合于金电极(7, 0)碳化硅纳米管的研究发现,电荷转移导致了带隙态的形成,它们使得透射谱在费米能级附近不为0,其伏安特性在较小偏压下为线性;当偏压在+1.4 V到+1.6V之间,电流随偏压的增大呈现了下降的趋势,这就是微分负阻效应。从不同偏压下系统的透射谱可以看出,偏压导致的传输特性的变化是微分负阻效应产生的原因。更多还原

【Abstract】 The research progresses on the carbon nanotubes (CNTs) and silicon carbide nanotubes (SiCNTs) are reviewed entirely at first, such as preparation, purification and application, in which the Brillouin zone of the CNTs and the applications related to their electronic structures are described. The density functional theoty and nonequilibrium Green’s function are discussed in detail. With this method, achievements have been obtained in the study of the electronic structures and transport properties of the nano-materials and nano-electronics. Using the above method, the electronic structures and transport properties of the CNTs and SiCNTs are calculated in this paper.The structures and electronic properties of the intrinsic and doped (8, 0) CNT are calculated by first-principles calculation based on the density functional theory (DFT). The intrinsic (8, 0) CNT is a direct band-gap semiconductor with a value of 0.46 eV. The electronic properties of the doped CNTs are quite different from the intrinsic CNTs’, which broadens the range of their application. From the optimized structure, we can see that the lengths of C-N bonds are longer than that of the C-C bonds. This tendency becomes more obvious with the increase of the doping concentration. It is consistent with the structure of the synthesized nitrogen-doped CNTs, which is bamboo-shaped. As the nitrogen atoms supply excess electrons, these electrons centralize on the doped nitrogen atoms and adjacent carbon atoms. This leads to the increase of the possibility of the charge transfer between different atoms in the CNT. The band-gap of the nitrogen-doped CNT is narrowed. The influence of the boron atoms on the structure of the CNT is similar to the nitrogen and the radius of the boron atom located is increased. Holes are formed by the doped boron atoms, in which the boron atoms localize and the band-gap of the boron-doped CNT is broadened.The transport properties of the isolated (8, 0) CNT and coupled to Au electrodes are investigated with the method combined non-equilibrium Green’s function (NEGF) with DFT. The step–shaped equilibrium transmission spectrum of the isolated (8, 0) CNT shows that the CNT is a semiconductor, which is coincident with the result achieved by first-principles calculations. The current voltage curve of the isolated (8, 0) CNT can be divided into two parts, when the bias voltage smaller than 1.2V, the current is near zero, while the voltage greater than 1.2V, the relationship between the current and the voltage is nearly exponential. In practical applications, CNTs are usually connected to metal electrodes. We calculated the transport properties of the (8, 0) CNT coupled to Au electrodes, in which the influence of length on the transport properties of CNT is considered. With the increase of the CNT’s length, the two probe system transforms from metallic to semiconductoring. In the formation of the contact between Au electrodes and the CNT, charge transfer between electrodes and the SiCNT will occur due to the difference in their work functions, which results in the metal-induced gap states (MIGS). In short CNTs, the MIGS plays an important role in its transport properties. Its influence weakens with the increase of the CNT’s length. This is the reason why no MIGS are observed in experiments.Using the method in the study of CNT’s electronic structures, the electronic structures of the intrinsic and doped (8, 0) SiCNT are obtained. The radius of the carbon rings are greater than that of the silicon rings in the optimized intrinsic (8, 0) SiCNT. The intrinsic (8, 0) SiCNT is a direct band-gap semiconductor with a value of 0.94 eV, which is broader than the CNT’s. This is owing to the ionicity of the Si-C bonds in SiCNTs. As boron and nitrogen are the common doping material in bulk SiC, they are selected as impurities in the study of the doped SiCNT. In substitution doping of the SiCNT, carbon atoms are replaced by nitrogen atoms, which is the same in bulk SiC doping. When one nitrogen atom is doped into the (8, 0) SiCNT, the lengths of Si-N bonds are shorten. In two nitrogen atoms doped SiCNT, the nitrogen atoms are like to occupy the nearest crystal lattice in the adjacent carbon rings and a salient is formed on the SiCNT’s surface. The excess electrons provided by nitrogen atoms locate mainly on the silicon atoms adjacent to the impurity atoms. The band-gap of nitrogen-doped SiCNT is narrowed. The radiuses of the boron atoms located silicon rings are decreased. The appearance probability of the electrons near the doped boron atoms is low and holes are formed, which results in the broadening of the band-gap.The transport properties of isolated (7, 0) SiCNT and coupled to Au electrodes are studied with the same method in the investigation of CNTs’transport properties. The transmission spectrum of the isolated SiCNT shows that is a semiconductor and its turning on voltage is about 2.2 V. In the (7, 0) SiCNT coupled to Au electrodes, the MIGS are found, which leads to the transmission coefficient near the Fermi energy is no zero and under small bias, the relationship between the current and bias voltage is linear. In the bias voltage range from +1.4 V to +1.6 V, the current decreases with the increase of the bias voltage. This means the appearance of the negative differnetial resistance (NDR). The origin of the NDR is the variation of the transmission spectrum caused by the applied voltage.更多还原