金刚石及相关材料纳米结构电子性质第一性原理研究
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摘要
第Ⅳ主族材料(金刚石、硅和锗),一直是半导体领域研究的热点材料。随着纳米材料在上世纪末的兴起,这一族半导体纳米材料受到了广泛关注,特别是纳米尺度的零维量子点。这些材料在设计纳米电子,光电子器件以及作为纳米器件构造模块等方面具有很大潜力。这些体系的量子点在纳米尺度下体现出量子限制效应,会显著改变其电子及光电子学等相关性质。本论文借助第一性原理密度泛函计算,研究了表面和空位缺陷对金刚石、硅和锗量子点的量子限制效应的影响,涉及电子性质,电子亲和势等。
2003年,Dahl等人在《科学》杂志上报导了他们在实验上成功离析出纳米量级尺寸的具有三维金刚石笼状结构和高度稳定性的碳氢化合物金刚烷,使得对sp3杂化电子结构的实验研究推进到了分子水平。在纳米尺度下,这些量子点会体现出显著的量子限制效应,即随着尺寸减小,通常来说能隙的价带边会下移而导带边会上移,导致能隙不断增大。2005和2006年,Willey等人报导了这类金刚石量子点的尺寸效应实验研究结果,他们的结果表明：随着尺寸减小,金刚石量子点的价带边会发生蓝移,而导带边没有变化。同时他们还发现导带边是由表面处的碳氢键主导,较大尺寸颗粒的带隙要小于块体的带隙。然而,对于硅和锗量子点,价带边和导带边同时具有量子限制效应。密度泛函理论计算也表明这两类量子点的能隙大于相应的块体带隙。实验研究还表明,氢化金刚石颗粒具有负的电子亲和势,对开发金刚石基电子发射器具有重要意义。而氢化的硅和锗量子点则具有正的电子亲和势,且电子亲和势的数值随量子点尺寸增大而增大。对于由碳和硅这两种元素组成的化合物半导体碳化硅量子点,实验结果也表明其电子结构和性质及其电子亲和势在一定的条件下也具有量子限制效应,但是相关的理论研究还比较滞后。
对这一族材料量子点的研究还涉及到磁学性质,如经过氮和碳元素离子注入的金刚石纳米颗粒具有室温铁磁性。2007年,Liou等人报导了纳米锗颗粒具有室温铁磁性,同时还受量子限制效应的影响。
众所周知,纳米体系具有较大的比表面,因而表面对其性质有着极其重要的影响。在本论文中,我们借助密度泛函理论计算研究了表面环境和重构对金刚石材料电子结构和性质的影响,以及C、Si和Ge纳米颗粒中空位缺陷引起的自旋极化及其量子限制效应,并研究了由C和Si元素组成的碳化硅半导体量子点的电子结构尺寸效应。
在本论文中,第一章简要回顾了第Ⅳ主族量子点材料的研究背景,以及需要解决的问题,阐明了本论文研究的意义。并对量子限制效应的产生根源做了简要说明。
第二章简要介绍了密度泛函理论的基本框架和近年来的理论发展。密度泛函理论的发展以寻找合适的交换相关能量泛函为主线。从最初的局域密度近似(LDA)、广义梯度近似(GGA)到现在的杂化泛函,使计算结果的精确度越来越高。最后对本论文工作所采用的主要程序ADF做了简要介绍。
第三章主要讨论了表面重构对氢化金刚石纳米颗粒的几何结构、稳定性、电子结构和电子亲和势的影响和作用。计算结果表明几何结构的变化可以在一定程度上调控禁带宽带。最低非占据分子轨道空间分布表明电荷分布主要依赖于表面碳氢键的键长,而不是表面的碳氢基团。对此我们对这一现象的起因做了详细地分析。对电子亲和势的研究结果表明：在表面碳原子被氢原子饱和的前提下,随着氢覆盖度的降低,负电子亲和势的数值呈现下降趋势,分析表明这是由于碳氢偶极矩的加强导致的。这些研究结果可以为纳米尺度金刚石基光电和电子发射器件的设计提供有益的帮助和借鉴。
第四章研究了表面不同碳氢基团与金刚石纳米颗粒的几何结构,电子结构和电子亲和势及稳定性的相关性。研究结果显示,由于甲基CH3构型的存在,当尺寸大于1nm时,这类颗粒的能带隙大于不含甲基颗粒的能带隙,也大于块体金刚石的禁带宽度。电子结构研究表明,最低非占据分子能级是由表面碳氢键提供的,这与实验上观测到的X光吸收光谱特征相一致。此外,我们还发现对于氢饱和的金刚石纳米颗粒,其负电子亲和势性质依赖于表面上碳原子与氢原子数的比值C／H。
第五章研究了金刚石,硅和锗纳米颗粒中空位缺陷态引发的自旋极化以及缺陷态之间的磁耦合作用。研究结果表明,在金刚石纳米颗粒中,缺陷态具有自旋极化的基态而且这一性质不随颗粒尺寸的增大而改变,即不具有量子尺寸效应。但对于硅和锗纳米颗粒,只有在尺寸很小时空位才会引发自旋极化的基态,即具有量子尺寸效应,这与实验上观察到的磁现象结果一致。另外,我们还发现在金刚石纳米颗粒中空位引起的缺陷态在邻近的位置形成铁磁耦合,而在硅和锗纳米颗中相应的位置却易于形成反铁磁耦合。
第六章通过密度泛函理论计算和化学键分析,研究了氢原子饱和的第Ⅳ主族及其组合量子点的量子限制效应和电子亲和势。结果表明碳化硅(SiC)和碳锗(GeC)量子点的最高占据分子轨道(HOMO)展现出量子限制效应,而这些量子点的最低非占据分子轨道(LUMO)不显示量子限制效应。当表面被碳氢键终结时,碳化硅和碳锗量子点表现出具有负的电子亲和势,而被硅氢键或锗氢键终结时,体系的电子亲和势则为正值。化学键分析表明第Ⅳ主族量子点在量子限制效应和电子亲和势方面的上述差异起源于最近邻和次近邻原子的价层p轨道之间的相互作用。
第七章对本论文进行了总结,并对以后工作做了展望。本论文在密度泛函理论框架下从理论上研究了不同的表面碳氢基团和表面重构对金刚石纳米颗粒性质的影响,研究了空位缺陷在金刚石,硅和锗量子点中自旋极化的量子限制效应,对相关实验给出了很好的解释；并对碳化硅量子点电子结构的量子限制效应做了预测,并结合已有的结果做了深入的理论分析。
Group-IV semiconductors have been the focused central materials in semiconductor fields. Following the advent of nanoscience and nanotechnology at the end of last century, the nanoscale counterpart of the group-IV semiconductors have been paid much attention, particularly for the zero-dimensional quantum dots, which have shown great potential for fabricating nanoelectronic, optoelectronic devices and the candidate of building blocks for nanodevices. Quantum dots (QD) can exhibit quantum confinement effects, which can substantially affect the electronic, optical properties of nanostructure systems. In this dissertationthesis, based on first principles density functional calculations, we investigated the influence of surface conditions and vacancy defects on the quantum confinement of group-IV quantum dots, including the electronic property, electron affinities and so on.
In 2003, Dahl et al. reported their successful isolation of diamondoids, a new class of nanosized hydrocarbons with high thermodynamic stability in, which have three-dimensional diamond-cage structures. This experimental finding makes the study of electronic structure of sp3 hybridizational system reach molecular level. At nanoscale, quantum dots exhibit quantum confinement effect, generally, with decreasing the size, their valence band edge (conduction band edge) will move downwards (upwards) resulting in the increasing of the band gap. In 2005 and 2006, Willey et al. reported their experimentally studies of quantum confinement effect on the band edge states of hydrogenated diamond quantum dots. Their resutls showed that the valence band edge have blueshift with decreasing dot size, while the conduction band edge exhibits non-shift feature, which is mainly composed of the C-H bonds at surface. Their results confirmed that the gap of larger diamond QD may be smaller than that of the bulk diamond. For Si and Ge QDs, both the valence band edge and the conduction band edge exhibit quantum confinement effect. Moreover, density functional calculations showed that the gap of Si and Ge QDs are always larger than that of the corresponding bulks. Experiments displayed that hydrogenated diamond QDs have negative electron affinities, which has potential applications for diamond based electron-emitter devices. In contrast, hydrogenated Si and Ge QDs have positive electron affinities, and their values increase with the dot size. For SiC QDs composed of Si and C elements, experimental observations of quantum confinement have been reported, but there is rare related theoretical studies.
Investigations on group-IV QDs also concern the magnetic property, such as the room temperature ferromagnetism observed in nanosized diamond QDs by C-/N- ion implantation. In 2007, Liou et al. reported the room temperature ferromagnetism in Ge nanostructures, which is also affected by quantum confinement effect.
It is known that nanoscale systems have relatively larger surface-to-volume ratio, thus the surface has substantial influence on the properties of such systems. In this dissertation, we investigated the effect of surface enviroment and reconstruction on the electronic structure by means of density functional theory (DFT) calculations, and the quantum confinement effect on the spin polarization of vacancy defects in group-IV QDs, and such effect on the HOMO/LUMO levels of SiC QDs are also examined and explored.
In chapter 1, we present a brief introduction to the study background of group-IV QDs and the origin of quantum confinement effect. We outlined the questions to be resolved and clarified the significance of the dissertation.
In chapter 2, we briefly introduce the basic concept of density functional theory and reviewed its recent progress. Developing good approximation for exchange-correlation functional is one of the main targets of DFT research. Following the development of exchange-correlation functionals, DFT method can give more and more accurate description from the initial LDA, GGA to hybridization functional. At the end, we briefly introduce the ADF code.
In chapter 3, we investigated the influence of the surface reconstructions on the geometries, stability based on DFT calculations. Our results show that the changes of the geometries can modulate the energy gaps. The spatial variation of the LUMOs depends rather on the C-H bond length than on the respective surface sites and the causes are analyzed. For the hydrogenated surface, the values of the negative electron affinity show lowering trend with the hydrogen coverage decreasing due to the increase of the surface C-H dipoles. These results are heplfull for the design of nanoscale diamond-based optoelectronic and electron-emitter devices.
In chapter 4, we investigate the effect of different surface terminations on the geometry, electronic structure, electron affinity and the stability of hydrogenated diamond QDs. Our results indicated that with the existence of CH3 species, the QDs with size larger than 1 nm have larger gaps than that without CH3 species, and even larger than that of bulk diamond. The studies of electroinc structure indicate that the compositions of HOMO and LUMO are responsible for the individual behavior associated with the quantum confinement, which agrees with the experimentally observed spectral feature in the x-ray absorption measurement. In addition, our results show that the negative electron affinity is strongly dependent on the C/H ratio for the hydrogenated diamond nanoparticles.
In chapter 5, we examine the vacancy-induced spin polarization in diamond, silicon and germanium nanoparticles and the magnetic coupling between the vacancy-induced defect states in those nanoparticles. Our results show that the vacancy-induced defect states are spin-polarized in diamond nanoparticles regardless of their size, but this happens in silicon and germanium nanoparticles only when their size is small, which is in reasonable agreement with the experimentally observed magnetic behaviors. It is found that the vacancy-induced defect states on adjacent vacancies prefer to couple ferromagnetically in C nanoparticles, but antiferromagnetcally in Si and Ge nanoparticles.
In chapter 6, by density functional calculations and chemical bonding analysis, we examine the quantum confinement effect and electron affinities of hydrogen-passivated group-IV quantum dots (C, Si, Ge, SiC and GeC). The results show that the HOMOs of SiC and GeC quantum dots show quantum confinement effect, but their LUMOs do not. SiC and GeC quantum dots have negative and positive electron affinities when their surfaces are terminated by C-H and A-H (A = Si, Ge) bonds, respectively. The chemical bonding analysis reveals that the differences in the quantum confinement effect and electron affinities of the group-IV quantum dots originated from the nearest-neighbor and next-nearest-neighbor interactions between the valence atomic p-orbitals in their HOMO/LUMO levels.
In chapter 7, we summarized the contents of the dissertation and draw a preview for future works.
2003年,Dahl等人在《科学》杂志上报导了他们在实验上成功离析出纳米量级尺寸的具有三维金刚石笼状结构和高度稳定性的碳氢化合物金刚烷,使得对sp3杂化电子结构的实验研究推进到了分子水平。在纳米尺度下,这些量子点会体现出显著的量子限制效应,即随着尺寸减小,通常来说能隙的价带边会下移而导带边会上移,导致能隙不断增大。2005和2006年,Willey等人报导了这类金刚石量子点的尺寸效应实验研究结果,他们的结果表明：随着尺寸减小,金刚石量子点的价带边会发生蓝移,而导带边没有变化。同时他们还发现导带边是由表面处的碳氢键主导,较大尺寸颗粒的带隙要小于块体的带隙。然而,对于硅和锗量子点,价带边和导带边同时具有量子限制效应。密度泛函理论计算也表明这两类量子点的能隙大于相应的块体带隙。实验研究还表明,氢化金刚石颗粒具有负的电子亲和势,对开发金刚石基电子发射器具有重要意义。而氢化的硅和锗量子点则具有正的电子亲和势,且电子亲和势的数值随量子点尺寸增大而增大。对于由碳和硅这两种元素组成的化合物半导体碳化硅量子点,实验结果也表明其电子结构和性质及其电子亲和势在一定的条件下也具有量子限制效应,但是相关的理论研究还比较滞后。
对这一族材料量子点的研究还涉及到磁学性质,如经过氮和碳元素离子注入的金刚石纳米颗粒具有室温铁磁性。2007年,Liou等人报导了纳米锗颗粒具有室温铁磁性,同时还受量子限制效应的影响。
众所周知,纳米体系具有较大的比表面,因而表面对其性质有着极其重要的影响。在本论文中,我们借助密度泛函理论计算研究了表面环境和重构对金刚石材料电子结构和性质的影响,以及C、Si和Ge纳米颗粒中空位缺陷引起的自旋极化及其量子限制效应,并研究了由C和Si元素组成的碳化硅半导体量子点的电子结构尺寸效应。
在本论文中,第一章简要回顾了第Ⅳ主族量子点材料的研究背景,以及需要解决的问题,阐明了本论文研究的意义。并对量子限制效应的产生根源做了简要说明。
第二章简要介绍了密度泛函理论的基本框架和近年来的理论发展。密度泛函理论的发展以寻找合适的交换相关能量泛函为主线。从最初的局域密度近似(LDA)、广义梯度近似(GGA)到现在的杂化泛函,使计算结果的精确度越来越高。最后对本论文工作所采用的主要程序ADF做了简要介绍。
第三章主要讨论了表面重构对氢化金刚石纳米颗粒的几何结构、稳定性、电子结构和电子亲和势的影响和作用。计算结果表明几何结构的变化可以在一定程度上调控禁带宽带。最低非占据分子轨道空间分布表明电荷分布主要依赖于表面碳氢键的键长,而不是表面的碳氢基团。对此我们对这一现象的起因做了详细地分析。对电子亲和势的研究结果表明：在表面碳原子被氢原子饱和的前提下,随着氢覆盖度的降低,负电子亲和势的数值呈现下降趋势,分析表明这是由于碳氢偶极矩的加强导致的。这些研究结果可以为纳米尺度金刚石基光电和电子发射器件的设计提供有益的帮助和借鉴。
第四章研究了表面不同碳氢基团与金刚石纳米颗粒的几何结构,电子结构和电子亲和势及稳定性的相关性。研究结果显示,由于甲基CH3构型的存在,当尺寸大于1nm时,这类颗粒的能带隙大于不含甲基颗粒的能带隙,也大于块体金刚石的禁带宽度。电子结构研究表明,最低非占据分子能级是由表面碳氢键提供的,这与实验上观测到的X光吸收光谱特征相一致。此外,我们还发现对于氢饱和的金刚石纳米颗粒,其负电子亲和势性质依赖于表面上碳原子与氢原子数的比值C／H。
第五章研究了金刚石,硅和锗纳米颗粒中空位缺陷态引发的自旋极化以及缺陷态之间的磁耦合作用。研究结果表明,在金刚石纳米颗粒中,缺陷态具有自旋极化的基态而且这一性质不随颗粒尺寸的增大而改变,即不具有量子尺寸效应。但对于硅和锗纳米颗粒,只有在尺寸很小时空位才会引发自旋极化的基态,即具有量子尺寸效应,这与实验上观察到的磁现象结果一致。另外,我们还发现在金刚石纳米颗粒中空位引起的缺陷态在邻近的位置形成铁磁耦合,而在硅和锗纳米颗中相应的位置却易于形成反铁磁耦合。
第六章通过密度泛函理论计算和化学键分析,研究了氢原子饱和的第Ⅳ主族及其组合量子点的量子限制效应和电子亲和势。结果表明碳化硅(SiC)和碳锗(GeC)量子点的最高占据分子轨道(HOMO)展现出量子限制效应,而这些量子点的最低非占据分子轨道(LUMO)不显示量子限制效应。当表面被碳氢键终结时,碳化硅和碳锗量子点表现出具有负的电子亲和势,而被硅氢键或锗氢键终结时,体系的电子亲和势则为正值。化学键分析表明第Ⅳ主族量子点在量子限制效应和电子亲和势方面的上述差异起源于最近邻和次近邻原子的价层p轨道之间的相互作用。
第七章对本论文进行了总结,并对以后工作做了展望。本论文在密度泛函理论框架下从理论上研究了不同的表面碳氢基团和表面重构对金刚石纳米颗粒性质的影响,研究了空位缺陷在金刚石,硅和锗量子点中自旋极化的量子限制效应,对相关实验给出了很好的解释；并对碳化硅量子点电子结构的量子限制效应做了预测,并结合已有的结果做了深入的理论分析。
Group-IV semiconductors have been the focused central materials in semiconductor fields. Following the advent of nanoscience and nanotechnology at the end of last century, the nanoscale counterpart of the group-IV semiconductors have been paid much attention, particularly for the zero-dimensional quantum dots, which have shown great potential for fabricating nanoelectronic, optoelectronic devices and the candidate of building blocks for nanodevices. Quantum dots (QD) can exhibit quantum confinement effects, which can substantially affect the electronic, optical properties of nanostructure systems. In this dissertationthesis, based on first principles density functional calculations, we investigated the influence of surface conditions and vacancy defects on the quantum confinement of group-IV quantum dots, including the electronic property, electron affinities and so on.
In 2003, Dahl et al. reported their successful isolation of diamondoids, a new class of nanosized hydrocarbons with high thermodynamic stability in
Investigations on group-IV QDs also concern the magnetic property, such as the room temperature ferromagnetism observed in nanosized diamond QDs by C-/N- ion implantation. In 2007, Liou et al. reported the room temperature ferromagnetism in Ge nanostructures, which is also affected by quantum confinement effect.
It is known that nanoscale systems have relatively larger surface-to-volume ratio, thus the surface has substantial influence on the properties of such systems. In this dissertation, we investigated the effect of surface enviroment and reconstruction on the electronic structure by means of density functional theory (DFT) calculations, and the quantum confinement effect on the spin polarization of vacancy defects in group-IV QDs, and such effect on the HOMO/LUMO levels of SiC QDs are also examined and explored.
In chapter 1, we present a brief introduction to the study background of group-IV QDs and the origin of quantum confinement effect. We outlined the questions to be resolved and clarified the significance of the dissertation.
In chapter 2, we briefly introduce the basic concept of density functional theory and reviewed its recent progress. Developing good approximation for exchange-correlation functional is one of the main targets of DFT research. Following the development of exchange-correlation functionals, DFT method can give more and more accurate description from the initial LDA, GGA to hybridization functional. At the end, we briefly introduce the ADF code.
In chapter 3, we investigated the influence of the surface reconstructions on the geometries, stability based on DFT calculations. Our results show that the changes of the geometries can modulate the energy gaps. The spatial variation of the LUMOs depends rather on the C-H bond length than on the respective surface sites and the causes are analyzed. For the hydrogenated surface, the values of the negative electron affinity show lowering trend with the hydrogen coverage decreasing due to the increase of the surface C-H dipoles. These results are heplfull for the design of nanoscale diamond-based optoelectronic and electron-emitter devices.
In chapter 4, we investigate the effect of different surface terminations on the geometry, electronic structure, electron affinity and the stability of hydrogenated diamond QDs. Our results indicated that with the existence of CH3 species, the QDs with size larger than 1 nm have larger gaps than that without CH3 species, and even larger than that of bulk diamond. The studies of electroinc structure indicate that the compositions of HOMO and LUMO are responsible for the individual behavior associated with the quantum confinement, which agrees with the experimentally observed spectral feature in the x-ray absorption measurement. In addition, our results show that the negative electron affinity is strongly dependent on the C/H ratio for the hydrogenated diamond nanoparticles.
In chapter 5, we examine the vacancy-induced spin polarization in diamond, silicon and germanium nanoparticles and the magnetic coupling between the vacancy-induced defect states in those nanoparticles. Our results show that the vacancy-induced defect states are spin-polarized in diamond nanoparticles regardless of their size, but this happens in silicon and germanium nanoparticles only when their size is small, which is in reasonable agreement with the experimentally observed magnetic behaviors. It is found that the vacancy-induced defect states on adjacent vacancies prefer to couple ferromagnetically in C nanoparticles, but antiferromagnetcally in Si and Ge nanoparticles.
In chapter 6, by density functional calculations and chemical bonding analysis, we examine the quantum confinement effect and electron affinities of hydrogen-passivated group-IV quantum dots (C, Si, Ge, SiC and GeC). The results show that the HOMOs of SiC and GeC quantum dots show quantum confinement effect, but their LUMOs do not. SiC and GeC quantum dots have negative and positive electron affinities when their surfaces are terminated by C-H and A-H (A = Si, Ge) bonds, respectively. The chemical bonding analysis reveals that the differences in the quantum confinement effect and electron affinities of the group-IV quantum dots originated from the nearest-neighbor and next-nearest-neighbor interactions between the valence atomic p-orbitals in their HOMO/LUMO levels.
In chapter 7, we summarized the contents of the dissertation and draw a preview for future works.
引文
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