Ge/SiO₂纳米镶嵌薄膜光电性质的模拟计算分析

【作者】 王丙杰；

【导师】 柴跃生；

【作者基本信息】 太原科技大学 ， 材料物理与化学， 2010， 硕士

【摘要】 本论文采用第一性原理分子动力学方法,使用Materals Studio程序中基于密度泛函理论（DFT）的CASTEP模块,对Ge、含Ge杂质缺陷SiO₂及Ge/SiO₂纳米镶嵌材料的几何结构、电子能带和光学性质进行了系统的研究。对单晶Ge和基质材料SiO₂晶体进行了计算,分析了得到的晶胞结构、能带、能量态密度和光吸收谱。结果显示,室温下单晶Ge光吸收峰在5.0eV处,位于紫外区域,对可见区光吸收较弱;Ge为间接带隙半导体,带隙约为0.677eV。SiO₂的带隙约为6.06eV,其导带态密度较小,有两个光吸收峰值,分别位于12.5eV和16.5eV处,对0eV～6.5eV能量范围内的光几乎没有吸收。基质材料SiO₂晶体与单晶Ge的计算结果,特别是优化后的SiO₂与Ge的晶格参数与实验数据基本吻合,这说明本论文所采用的计算方法的可靠性较高。建立了SiO₂中Ge杂质缺陷的结构模型,Ge原子以替位式杂质的形式存在于SiO₂晶体中,掺杂摩尔比约为2.08%。研究模拟计算所得到的几何结构、能带结构特征和光学吸收谱。分析表明,Ge杂质缺陷结构吸收谱向长波方向移动,光吸收强度相对于SiO₂晶体有所减小,但吸收谱整体上变化不明显。含杂质缺陷材料光折射率低于SiO₂晶体;Ge原子在SiO₂带隙中引入了很浅的杂质能级,并使SiO₂导带向禁带内延伸,这会提高SiO₂的导电性。Ge-O键中的O原子的S轨道对这个杂质能级有重要影响。在方石英SiO₂晶体中镶嵌不同尺寸的纳米Ge晶粒,计算得到镶嵌Ge3结构和镶嵌的Ge5结构的禁带宽度分别为4.1eV、2.7eV。结果显示,对可见光区的吸收镶嵌Ge3结构比镶嵌Ge5结构强,Ge5结构相对于Ge3结构光吸收边发生了蓝移,这与量子限制模型中随纳米晶粒减小,光吸收边蓝移不同。Ge3和Ge5结构在SiO₂带隙深处费米能级以上约3.2eV处引入了中间能级,此能级与导带底能级差约为1.9eV。结果说明,超精细纳米Ge结构光学性质主要决定于晶粒界面的Ge-O键。实验中观察到的Ge/SiO₂材料中位于2.1eV的发光峰可能来自这种小尺寸Ge晶粒界面的Ge-O键。计算了SiO₂晶体中镶嵌不同尺寸纳米晶Si结构,与纳米Ge晶结构进行了比较讨论。结果表明,Si晶结构中,量子限制效应是决定其光学性质的主要因素,而Ge晶结构中,量子限制效应不再是决定镶嵌结构光学性质的主要因素。综合对Ge、含Ge杂质缺陷SiO₂及Ge/SiO₂镶嵌材料的计算结果分析,结合对已有Ge/SiO₂镶嵌材料的实验分析,研究表明,Ge/SiO₂材料中分散的Ge原子杂质缺陷对材料光学性质影响不明显,微小尺寸Ge纳米晶镶嵌SiO₂材料的光学性质主要决定于纳米晶体Ge的界面成键,量子限域效应不再起主要作用。更多还原

【Abstract】 The research in this dissertation is based on the density functional theory in first-principle calculation. The software we used is CASTEP module in Materals Studio. The electronic structure, geometric structure and optic properties of Ge, SiO₂ with Ge impurity defect and Ge nanocrystals embedded in SiO₂ were discussed.The electron property,geometric property and optic property of Ge crystal and matrix SiO₂ were calculated and analyzed. The results show that the absorption peak of porous germanium is at about 5.0eV, uitraviolet area. It’s visible light absorption is very weak at room temperature. The Ge crystal is indirect gap semiconductor and it’s band gap is about 0.677eV. The band gap of SiO₂ is 6.06eV , there is almost no absorption in 0eV⁶.5eV region. The matrix SiO₂ has two absorption peak, which are at about 12.5eV and 16.5eV respectively. The results of Ge and SiO₂ , especially the parameters of opetimized lattice are accord with experiment result. This indicate that the calculation method and parameter of software set is credible to research the photoelectric properties of Ge, Ge-doped SiO₂ and Ge/SiO₂ nano embedded materials.The model of SiO₂ with Ge impurity defect in which one silicon atom is replaced by germanium atom is fabricated and calculated. The proportion of impurity is about 2.08%. The result show that the absorption spectrum of Ge-doped SiO₂ move to red side and the absorption desity is lower than matrix SiO₂ ,but the change is not very remarkable.The refractive index is lower than pure SiO₂ . In this structure, the impurity levels are brought in the band gap of SiO₂ ,but these levels are very near to the condution level. The S orbit of O atoms in Ge-O bond mostly contribute to these impurity level.The properties of germanium nanocrystals Ge3 and Ge5 in different dimensions embedded in SiO₂ were calculated. The band gap of Ge3 and Ge5 embedded structure are 4.1eV and 2.7eV. The results show, the absorption of visible light of Ge3 structure is better than that of Ge5 structure. The intermediate level of Ge3 and Ge5 embedded structure are all at about 3.2eV above Fermi level, which are at about 1.9eV below conduction level. The results indicate that the optic properties of mini Ge nano structure are mostly contingent on the Ge-O bond in the interface of nanocrystals. This may also caused the 2.1eV PL peak observed in experiment.The Si nanocrystals in different dimension embedded in SiO₂ were calculated, the difference between Ge and Si nano structure were discussed. The results show, quantum confinement mostly decide the optic properties of Si nano structure, but in Ge nano structure, the optic properties are not only determined by the effect of quantum confinement, but also by other factors.In conclusion, the optic properties of Ge/SiO₂ mostly decided by the bond of nanocrystals interface than by quantum confinement or disperse impurity Ge atoms.更多还原