二氧化铪体系电子结构及硅纳米线力学行为的第一性原理研究
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摘要
本文采用基于密度泛函理论的第一性原理计算方法,对二氧化铪体系的电子结构和硅纳米线的力学行为进行了理论研究。主要研究内容包括：掺杂钇对立方相二氧化铪的结构稳定性和缺陷性质的影响；不同晶相二氧化铪的低密勒指数表面体系的表面弛豫、表面磁性、表面能和表面电子结构的研究分析；非晶相二氧化铪模型的构造以及非晶相二氧化铪-硅界面的原子构型和能带偏移值的计算；一维硅纳米线在拉伸应力下的结构演化和断裂机制的研究。
本论文分为六章。第一章介绍二氧化铪体系和硅纳米线体系在实验研究、工业应用中的重要性以及采用理论模拟的必要性。第二章具体讨论文中计算模拟方法的理论基础以及实现方法。
第三章、第四章和第五章都是有关二氧化铪体系的模拟、分析和结果讨论。具体地,第三章讨论了掺杂钇对立方相二氧化铪晶体结构和电子结构的影响。通过对掺杂前后体系能量的比较,发现掺杂钇能够使得常温下并不稳定的立方相二氧化铪得到稳定；对体系中氧空位缺陷的研究表明：未掺杂之前,中性的氧空位结构更容易形成,但是掺杂钇之后,缺陷体系更倾向于失去一个电子,并且对掺杂前后缺陷体系的电子结构分析发现,掺杂会使得原来存在的杂质能级消失,结果和实验上光电子能谱的观察相吻合,从而从原子层次解释了掺杂钇对立方相二氧化铪结构稳定性和缺陷行为的影响。
第四章讨论了单斜相、四方相和立方相二氧化铪的低密勒指数表面的结构弛豫、表面形成能、表面磁性和电子结构性质。结果表明：表面体系的弛豫能够不同程度地降低表面能,从而使表面结构得到更好的稳定；铪氧键的离子性特征对于解释表面正负离子的不同弛豫方式起着关键的作用。一般来说,化学配比的表面体系具有更低的表面形成能。但是某些非化学配比的亚稳表面结构的形成能与最稳定表面的形成能之间的差异不是很大,即在实验中的某些化学环境下,可以观察到这些体系的存在。通过自旋极化的计算,发现多氧的表面体系,即体系中氧铪的比例大于二比一的情形,存在着d0铁磁性,分析表明这是由氧原子2p轨道很强的自旋交换作用和价带顶附近很高的氧2p电子的态密度共同作用的结果。这在一定程度上解释了实验和理论上关于二氧化铪体系中d0铁磁性的争论。
第五章是关于非晶相二氧化铪-硅界面的原子构型和电子结构性质的研究。首先,我们利用第一性原理分子动力学的方法,在“熔化-淬火”方案下,构造得到了非晶相的二氧化铪结构,分析表明：得到的二氧化铪非晶相的结构特征能够很好地与实验的观察相符合。进而,我们研究了非晶二氧化铪和硅之间界面的原子构型和电子结构。结果发现,体系中的界面态主要是来源于界面附近的硅铪键；对能带偏移值的计算显示,硅和二氧化铪之间的价带偏移值和导带偏移值都能很好地与实验比较,并且能够满足器件应用中的性能要求。
第六章对沿[001]方向生长的硅纳米线的拉伸过程进行了准静态的模拟,研究了在拉伸应变下硅纳米线的结构演化和断裂机制。
First principles calculations based on density functional theory have been performed to investigate the electronic structures of HfO2 systems and the tensile properties of silicon nanowires. For HfO2, the Y defect in cubic structure, the low Miller index surfaces and the interface between silicon and amorphous HfO2 are studied. For silicon nanowire, the tensile behavior of Si[001] nanowire is simulated.
This thesis consists of six chapters. ChapterⅠintroduces the experimental progress, the industrial usage of the HfO2 and Si nanowires. It is emphasized that theoretical simulations should be conducted to help understand and predict the physical phenomena. ChapterⅡgoes through the basic theories and the explicit methods adopted.
ChapterⅢ, chapter IV and chapterⅤare all concerned with HfO2 systems. In chapterⅢ, we discuss the effect of Y doping on the structural stability and defect properties of cubic HfO2·It is found that Y doping would increase the stability of cubic phase relative to the monoclinic phase. The energetics indicates that the complex defect, formed by oxygen vacancy and Y substitute, prefers the single positively charged states. The corresponding defect level is located within the valence band. This explains the experimental observation that gap states related to oxygen vacancy defects become non-detectable in Y-doped HfO2 films.
ChapterⅣis related to the structural, electronic and magnetic properties of low Miller index surfaces in monoclinic, tetragonal and cubic HfO2·We find the atomic relaxation lowers the surface energies of the systems; The ionic feature of Hf-O bond is important to understand the relaxation of the anions and cations; The most energetically favorable surfaces are stoichiometric, but the less stable non-stoichiometric ones could be observed in experiments under some chemical environments if the energy difference is not large; And d0 magnetism has been found in some oxygen rich non-stoichiometric surfaces, which results from the large spin exchange energy of the surface oxygen 2p orbital and the high density of states of the oxygen 2p electrons near the Fermi level. With the energet-ics, we propose a reasonable explanation for recent controversial observations of ferromagnetism in HfO2·
ChapterⅤdeals with the atomistic and electronic properties of amorphous HfO2/Si(001) interface. The amorphous HfO2 model structure is generated by ab initio molecular dynamics simulations within the "melt-and-quench" scheme. Calculations indicate that the simulated amorphous HfO2 essentially shows the characteristics of the experimental amorphous HfO2 structure. The results sug-gest that atomic coordination of interface Si atoms would significantly affect the interface electronic properties. With the band lineup of the core level, the va-lence band offset is determined to be 2.62±0.35 eV, in good agreement with the experimental data.
In chapterⅥ, quasi-static simulations are carried out to study the structural evolution and the breaking mechanism of the Si[001] nanowire with small radius.
本论文分为六章。第一章介绍二氧化铪体系和硅纳米线体系在实验研究、工业应用中的重要性以及采用理论模拟的必要性。第二章具体讨论文中计算模拟方法的理论基础以及实现方法。
第三章、第四章和第五章都是有关二氧化铪体系的模拟、分析和结果讨论。具体地,第三章讨论了掺杂钇对立方相二氧化铪晶体结构和电子结构的影响。通过对掺杂前后体系能量的比较,发现掺杂钇能够使得常温下并不稳定的立方相二氧化铪得到稳定；对体系中氧空位缺陷的研究表明：未掺杂之前,中性的氧空位结构更容易形成,但是掺杂钇之后,缺陷体系更倾向于失去一个电子,并且对掺杂前后缺陷体系的电子结构分析发现,掺杂会使得原来存在的杂质能级消失,结果和实验上光电子能谱的观察相吻合,从而从原子层次解释了掺杂钇对立方相二氧化铪结构稳定性和缺陷行为的影响。
第四章讨论了单斜相、四方相和立方相二氧化铪的低密勒指数表面的结构弛豫、表面形成能、表面磁性和电子结构性质。结果表明：表面体系的弛豫能够不同程度地降低表面能,从而使表面结构得到更好的稳定；铪氧键的离子性特征对于解释表面正负离子的不同弛豫方式起着关键的作用。一般来说,化学配比的表面体系具有更低的表面形成能。但是某些非化学配比的亚稳表面结构的形成能与最稳定表面的形成能之间的差异不是很大,即在实验中的某些化学环境下,可以观察到这些体系的存在。通过自旋极化的计算,发现多氧的表面体系,即体系中氧铪的比例大于二比一的情形,存在着d0铁磁性,分析表明这是由氧原子2p轨道很强的自旋交换作用和价带顶附近很高的氧2p电子的态密度共同作用的结果。这在一定程度上解释了实验和理论上关于二氧化铪体系中d0铁磁性的争论。
第五章是关于非晶相二氧化铪-硅界面的原子构型和电子结构性质的研究。首先,我们利用第一性原理分子动力学的方法,在“熔化-淬火”方案下,构造得到了非晶相的二氧化铪结构,分析表明：得到的二氧化铪非晶相的结构特征能够很好地与实验的观察相符合。进而,我们研究了非晶二氧化铪和硅之间界面的原子构型和电子结构。结果发现,体系中的界面态主要是来源于界面附近的硅铪键；对能带偏移值的计算显示,硅和二氧化铪之间的价带偏移值和导带偏移值都能很好地与实验比较,并且能够满足器件应用中的性能要求。
第六章对沿[001]方向生长的硅纳米线的拉伸过程进行了准静态的模拟,研究了在拉伸应变下硅纳米线的结构演化和断裂机制。
First principles calculations based on density functional theory have been performed to investigate the electronic structures of HfO2 systems and the tensile properties of silicon nanowires. For HfO2, the Y defect in cubic structure, the low Miller index surfaces and the interface between silicon and amorphous HfO2 are studied. For silicon nanowire, the tensile behavior of Si[001] nanowire is simulated.
This thesis consists of six chapters. ChapterⅠintroduces the experimental progress, the industrial usage of the HfO2 and Si nanowires. It is emphasized that theoretical simulations should be conducted to help understand and predict the physical phenomena. ChapterⅡgoes through the basic theories and the explicit methods adopted.
ChapterⅢ, chapter IV and chapterⅤare all concerned with HfO2 systems. In chapterⅢ, we discuss the effect of Y doping on the structural stability and defect properties of cubic HfO2·It is found that Y doping would increase the stability of cubic phase relative to the monoclinic phase. The energetics indicates that the complex defect, formed by oxygen vacancy and Y substitute, prefers the single positively charged states. The corresponding defect level is located within the valence band. This explains the experimental observation that gap states related to oxygen vacancy defects become non-detectable in Y-doped HfO2 films.
ChapterⅣis related to the structural, electronic and magnetic properties of low Miller index surfaces in monoclinic, tetragonal and cubic HfO2·We find the atomic relaxation lowers the surface energies of the systems; The ionic feature of Hf-O bond is important to understand the relaxation of the anions and cations; The most energetically favorable surfaces are stoichiometric, but the less stable non-stoichiometric ones could be observed in experiments under some chemical environments if the energy difference is not large; And d0 magnetism has been found in some oxygen rich non-stoichiometric surfaces, which results from the large spin exchange energy of the surface oxygen 2p orbital and the high density of states of the oxygen 2p electrons near the Fermi level. With the energet-ics, we propose a reasonable explanation for recent controversial observations of ferromagnetism in HfO2·
ChapterⅤdeals with the atomistic and electronic properties of amorphous HfO2/Si(001) interface. The amorphous HfO2 model structure is generated by ab initio molecular dynamics simulations within the "melt-and-quench" scheme. Calculations indicate that the simulated amorphous HfO2 essentially shows the characteristics of the experimental amorphous HfO2 structure. The results sug-gest that atomic coordination of interface Si atoms would significantly affect the interface electronic properties. With the band lineup of the core level, the va-lence band offset is determined to be 2.62±0.35 eV, in good agreement with the experimental data.
In chapterⅥ, quasi-static simulations are carried out to study the structural evolution and the breaking mechanism of the Si[001] nanowire with small radius.
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