纳米氮化硅的光学性能及其线、薄膜力学行为的模拟研究

发布时间：2018-06-17 10:59

本文选题：纳米氮化硅 + 第一性原理　；参考：《西安交通大学》2017年博士论文

【摘要】：作为一种先进的结构和功能陶瓷,Si_3N_4已经在各个领域展示出了广泛的应用,特别是随着材料向小尺度化的发展,纳米Si_3N_4陶瓷以其独特的力、电和光学性能将会在纳米器件上展现巨大的应用潜力,但目前由于各种原因,还很难通过实验对这类材料性能进行测试与评价,基于此,本文利用第一性原理对掺杂和吸附的Si_3N_4模型进行电子结构和光学性能模拟研究,利用分子动力学对Si_3N_4纳米线和薄膜进行力学行为模拟研究,为其进一步的应用提供理论基础。(1)利用第一性原理模拟了Al、Ga、As和P掺杂Si_3N_4体系的电子结构和光学性质,结果表明:Al和Ga掺杂后体系带隙分别为4.21和4.12e V,而As和P掺杂之后带隙减小,甚至消失,说明后两种元素掺杂对体系的带隙影响较大;掺杂后四种体系的形成能按照Al、Ga、P和As的顺序逐渐增加,说明Al掺杂体系比其他体系的结构更稳定;差分电荷密度图表明,P掺杂后周围的电子缺失增加,说明P与N成键的共价性有所提高,而As、Ga和Al掺杂后周围的电子缺失降低,表明其与N成键的共价性有所降低,并且Al掺杂时,原子附近的电子缺失几乎消失,电子云逐渐向N原子附近靠近,说明共价性向离子性转化的程度最高,掺杂元素与N原子成键的共价性强弱顺序为:PAsGaAl,与计算的布居值相一致;掺杂体系静态介电常数随共价半径的增加先降低,后升高,其中As掺杂体系最低,且在低能区的介电损耗较小,有利于在光电材料上的应用。(2)利用第一性原理模拟了稀土元素(Yb、Gd、Sc、Sm、La和Lu)吸附氮化硅体系的电子结构和光学性能,稀土元素吸附成键后周围的电子缺失变弱,与N成键的离子性的增强,其中Yb,Gd和Sc原子附近还出现了不同程度的电子富集,说明电子云分布的不均匀性增加,与N成键共价性强弱顺序为YbGdScSm(La,Lu),和计算的布居值相一致;静态介电常数和介电损耗都随共价半径的增加先增大,后降低,最后趋于平缓,其中Sc吸附体系最低;[010]极化方向对总介电函数的贡献大于其他两个方向,说明吸附后体系都显示了一定的各向异性;780~2500nm(0.496~1.59eV)近红外光区,La,Yb,Gd和Sc吸附体系的反射率较低,都小于6%,而Lu和Sm吸附体系可达20%,反射程度相对较高,对应折射率较低,说明光在前四种吸附体系中更容易传播;390~780nm(1.59~3.18eV)可见光区,六种吸附体系都具有较低的吸收系数和反射率,说明吸附体系具有“透明型”性质;80~390nm(3.18~15.5eV)紫外光区,吸附体系对光的吸收较强,呈现出“阻隔型”性质。(3)将第一性原理和分子动力学相结合,建立了[001]方向氮化硅纳米线模型,利用分子动力学模拟了氮化硅纳米线的压缩和拉伸力学行为,结果表明:在拉伸应力下,材料弹性极限独立于纳米线长径比,并大都发生在应变为0.05处,纳米线的断裂应力随长径比的升高而减少,同时在变形中产生大量的硅-硅键和团簇状氮原子缺陷;在压缩应力下,当应力达到最大值时,在纳米线下表面的中心部位和上表面的两个对称位置产生了新的四重硅原子缺陷,使应力开始下降。(4)利用分子动力学模拟了氮化硅纳米线在弯曲应力作用下的力学行为,结果表明:应力位移曲线由准弹性阶段和非线性阶段组成,随着长径比的增加(3:1、5:1和7:1),材料断裂应力下降;在压头接触纳米线之后,弯曲应力随压头位移的增加而增大,当初始断裂出现时,弯曲应力急速下降;弯曲应力下,在上表面中心位置,可以观察到硅-硅键和配位数为2的氮原子缺陷,随压头位移的增加,氮的配位数逐渐转变为0和1,硅的配位数转变为6和7。(5)建立了基面氮化硅纳米薄膜,原子的数量分别为5600、7406和9464个,利用最快下降法进行弛豫,得到了稳定结构。利用分子动力学模拟了薄膜的拉伸行为,结果表明:在拉伸加载过程中,当应变小于0.06时,薄膜展示了非线性应力应变关系;在0.06-0.09应变范围内,薄膜展示了线性应力应变关系;超过0.09时,又变成非线性应力应变关系,直到最后的断裂。断裂应力、应变随薄膜边长的增加而增加,杨氏模量变化不大,y方向的断裂应力大于x方向的断裂应力,拉伸过程中的断裂源来自N6h-Si键。随着应变速率的增加,初始N6h-Si断键缺陷出现的位置和应变点是相同的,是不依赖于应变速率的,当应变速率增加到5.3×109s-1时薄膜两部分完全分离的应变数值降到0.113,其原因是由于在N6h-Si键的断键缺陷扩展过程中出现了N2c-Si键的断键缺陷,加速了薄膜裂纹的扩展过程。随着拉伸温度的升高,最大拉伸应力先升高后降低,对应的拉伸应变逐渐降低。
[Abstract]:As a kind of advanced structural and functional ceramics, Si_3N_4 has been widely used in various fields, especially with the development of material to small scale. Nano Si_3N_4 ceramics, with its unique force, electrical and optical properties, will show great potential in the application of nano devices. The performance of this kind of material is tested and evaluated. Based on this, this paper uses the first principle to simulate the electronic structure and optical properties of the Si_3N_4 model of doping and adsorption. The mechanical behavior of Si_3N_4 nanowires and films is simulated by molecular dynamics, which provides a theoretical basis for its further application. (1) use the first theory. The first principle simulates the electronic structure and optical properties of the Al, Ga, As and P doped Si_3N_4 systems. The results show that the band gaps of Al and Ga doped systems are 4.21 and 4.12e V respectively, while the band gap decreases and even disappeared after As and P doping, indicating that the doping of the two elements has great influence on the band gap of the system, and the formation of the four systems after doping can be based on Al. The order of As is increasing gradually, indicating that the Al doping system is more stable than the structure of other systems. The differential charge density diagram shows that the electron deletion around P increases after doping, indicating that the covalence of P and N is improved, while the electron deletion around As, Ga and Al decreases, which indicates that the covalence with N is reduced, and Al doping is reduced. The electron loss near the atom almost disappeared, and the electron cloud gradually approached the N atom, indicating that the covalently to the ionic conversion was the highest. The covalence order of the doped elements and N atoms was PAsGaAl, which was the same as the calculated distribution value; the static dielectric constant of the doping system decreased first and then increased with the increase of covalent radius. The As doping system is the lowest, and the dielectric loss in the low energy region is small, which is beneficial to the application on the photoelectric materials. (2) the electronic structure and optical properties of the adsorption of rare earth elements (Yb, Gd, Sc, Sm, La and Lu) are simulated by the first principle. The electron loss around the rare earth element is weaker and the ionic property of the bond with N There are different degrees of electron enrichment in the vicinity of Yb, Gd and Sc atoms, indicating that the distribution of the electron cloud increases. The order of the bond covalence with N is YbGdScSm (La, Lu), which is the same as that of the calculated values. The static dielectric constant and dielectric loss increase first, then decrease, and finally tend to be flat. Slowly, the Sc adsorption system is the lowest; the contribution of the [010] polarization direction to the total dielectric function is greater than the other two directions, indicating that the adsorption system shows a certain anisotropy; the reflectance of the 780~2500nm (0.496~1.59eV) near infrared light region, La, Yb, Gd and Sc adsorption system is lower than 6%, and Lu and Sm adsorption system can reach 20%, reflection degree phase 390~780nm (1.59~3.18eV) visible light region, the six adsorption systems have low absorption coefficient and reflectivity, indicating that the adsorption system has a "transparent" property, and 80~390nm (3.18~15.5eV) ultraviolet light region, the absorption system has a stronger absorption of light. "Barrier type" is presented. (3) a [001] orientation silicon nitride nanowire model is established by combining the first principle with molecular dynamics, and the compressive and tensile mechanical behavior of the nanowires of silicon nitride are simulated by molecular dynamics. The results show that the elastic limit of the material is independent of the nanoscale length to diameter ratio under tensile stress and mostly occurs in the nanowire ratio of nanoscale. When the strain is 0.05, the fracture stress of nanowires decreases with the increase of the ratio of length to diameter. At the same time, a large number of silicon silicon bonds and cluster like nitrogen atoms are produced in the deformation. Under the compressive stress, when the stress reaches the maximum, a new four silicon atom defect is produced at the center of the surface of the nanowire and the two symmetrical positions of the upper surface. The stress begins to decline. (4) the mechanical behavior of silicon nitride nanowires under the action of bending stress is simulated by molecular dynamics. The results show that the stress displacement curve is composed of quasi elastic phase and nonlinear stage. With the increase of the ratio of length to diameter (3:1,5:1 and 7:1), the fracture stress of the material decreases, and the bending stress follows the pressure head contact nanowires. The increase of the head displacement increases, when the initial fracture occurs, the bending stress rapidly decreases. Under the bending stress, the silicon silicon bond and the coordination number of 2 nitrogen atom defects can be observed. With the increase of the pressure head displacement, the coordination number of nitrogen is gradually changed to 0 and 1, the coordination number of silicon to 6 and 7. (5) is to establish the base silicon nitride. The number of nanometers, the number of atoms is 56007406 and 9464 respectively, is relaxed by the fastest descent method and the stable structure is obtained. The tensile behavior of the film is simulated by molecular dynamics. The results show that the film shows the nonlinear stress-strain relationship when the strain is less than 0.06 during the tensile loading process, and it is within the strain range of 0.06-0.09. The film shows the linear stress-strain relationship; more than 0.09, it becomes nonlinear stress strain relationship until the final fracture. The fracture stress and strain increase with the increase of the length of the film, the young's modulus changes little, the fracture stress in the direction of Y is larger than the X direction, and the fracture source in the tensile process comes from the N6h-Si bond. Along with the strain. As the rate increases, the position of the initial N6h-Si breaking defect is the same and the strain point is the same. It is not dependent on the strain rate. When the strain rate increases to 5.3 * 109s-1, the total separation strain value of the two part of the film is reduced to 0.113. The reason is that the broken bond defect of the N2c-Si bond appears during the break down extension of the N6h-Si key. With the increase of tensile temperature, the maximum tensile stress increases first and then decreases, and the corresponding tensile strain decreases.
【学位授予单位】：西安交通大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TQ174.1

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