锗烯在金属衬底上的结构及电子性质的第一性原理研究

基于密度泛函理论的第一性原理计算,我们系统的研究了锗烯在Pt(111)、Au(111)和Al(111)表面的几何和电子结构.在这三种金属衬底上寻找到了9种结构,其中Al(111)-a、Au(111)-b和Pt(111)-c结构就是目前实验上已经成功制备的结构.我们还发现了其余6种目前理论和实验均没有提出,此外,Al(111)-b、Au(111)-a和Pt(111)-b结构的Dirac态仍保留,这些结构的形成能均大于范德瓦尔斯作用,因此非常有希望在实验上制备出来,应用于量子自旋霍尔效应的研究.本文的研究为锗烯在半导体衬底上的制备及应用奠定了理论基础.     

类石墨烯锗烯研究进展

近年来,伴随石墨烯研究的深入开展,考虑到兼容半导体工业,构筑类石墨烯锗烯并探究其奇特电学性质已成为凝聚态物理领域的研究前沿.本文首先简要介绍了锗烯这一全新二维体系的理论研究进展,包括锗烯的几何结构、电子结构及其调控以及它们之间的关系.理论研究表明,因最近邻原子间距大,锗烯比硅烯更难构筑,实验上构筑锗烯颇具挑战性.针对这一问题,介绍了实验上制备锗烯的一些进展,重点介绍了金属表面外延制备锗烯,并对本征锗烯的制备及其在未来纳电子学器件的潜在应用做出了展望.     

GeH/π层间弱相互作用调控锗烯电子结构的机制

锗烯是继石墨烯、硅烯发现以来最重要的二维纳米材料之一, 以其优异的物理化学性质迅速得到人们的广泛关注. 然而, 锗烯具有的零带隙能带特点(狄拉克点)极大程度地限制了其在微电子纳米材料方面的应用. 本文采用范德华力修正的密度泛函计算方法, 研究了锗烯、锗烷、锗烯/锗烷的几何和电学性质. 研究发现, 锗烯和锗烷可以通过弱相互作用形成稳定的双层结构, 并在锗烯中打开一个85 meV的带隙. 电子结构分析表明, Ge-H/π 的存在破坏了锗烯子晶格的对称性, 从而在狄拉克点上打开一个带隙. 差分电荷密度图分析表明有部分电荷从H原子的s轨道转移至Ge的p_z轨道. 该电荷转移机制增强了锗烯与锗烷之间的相互作用力, 是形成锗烯/锗烷双层二维纳米结构的主要原因. 进一步研究还发现, 锗烷/锗烯/锗烷的三明治结构无法在锗烯中打开带隙. 这是由于两侧的锗烷对夹层的锗烯作用力等价, 无法破坏锗烯的子晶格对称性, 所以无法打开锗烯带隙. 最后, 所有计算结果都在高精度杂化密度泛函HSE06计算精度下得到进一步验证. 因此, 本文从理论上提出了一种切实可行的打开锗烯狄拉克点的方法, 为锗烯在场效应管和其他纳米材料中的应用提供了理论指导.     

S钝化GaAs(100)表面的电子特性

用TB-LMTO方法研究单层的S原子在理想的GaAs (100) 表面的化学吸附, 对GaAs(100)表面是Ga-和As-中断两种情况分别进行考虑. 计算了S原子在不同位置的吸附能、吸附体系与清洁的GaAs(100)表面的层投影态密度, 以及电子转移情况. 结果表明, 两种情况下S原子都是桥位吸附最稳定, S-Ga相互作用比S-As稍强, S钝化GaAs(100)表面可以取得明显的钝化效果.     

掺杂对硅烯的性能影响

硅烯是单原子层的硅薄膜,具有类石墨烯结构.因此拥有与石墨烯相似的各种奇特的热学、化学、光学和电学性质.近年来,硅烯引起了研究者的广泛关注,作为一种新型的二维狄拉克电子材料,硅烯在理论计算和实验上都取得了不少新的进展.本文主要在前人对硅烯实施边缘钝化、掺杂、外加电场、加应力或者表面官能团修饰和吸附等研究的情况下,结合当前硅烯的研究发展趋势,重点研究了不同掺杂对硅烯性质的影响,并探讨硅烯在未来硅基电子器件的应用前景.     

单层硅烯表面的CoPc分子吸附研究

由于低维材料表面上的单原子和分子具有丰富的物理化学性质,现已经成为量子器件及催化科学等领域的研究热点.单层硅烯在不同的衬底制备温度下,表现出丰富的超结构,这些超结构为实现有序的单原子或分子吸附提供了可靠的模板.利用原位硅烯薄膜制备,分子沉积,超高真空扫描隧道显微镜以及扫描隧道谱,本文研究了Ag(111)衬底上3种硅烯超结构((4×4),(■×■),(2■×2■))的电子态结构,表面功函数随超结构的变化,以及CoPc分子在这3种超结构硅烯上的吸附行为.研究结果表明,这3种超结构的硅烯具有类似的电子能带结构,且存在电子从Ag(111)衬底转移到硅烯上的可能性,从而导致硅烯的表面功函数增大,表面功函数在原子级尺度上的变化对分子的选择性吸附起着重要作用.此外,还观察到分子与硅烯的相互作用导致CoPc分子的电子结构发生对称性破缺.     

硅烯饱和吸附碱金属原子的第一性原理研究

基于密度泛函理论的第一性原理计算, 研究了硅烯饱和吸附碱金属元素原子的稳定性、微观几何结构和电子性质, 并与纯硅烯及其饱和氢化结构进行了对比分析. 研究发现复合物SiX(X=Li, Na, K, Rb)的形成能都是负的, 相对于纯硅烯来说可以稳定存在. Bader电荷分析表明, 电荷从碱金属原子转移至硅原子. 从成键方式来看, 硅烯与氢原子形成共价键, 而与碱金属原子之间形成的键主要是离子性成键, 但还存在部分共价关联成分. 能带计算表明, 锂原子饱和吸附在硅烯形成的复合物SiLi是直接带隙的半导体, 带隙大小为0.34 eV. 其他碱金属饱和吸附在硅烯上形成的复合物都表现为金属性.     

门电压控制的硅烯量子线中电子输运性质

硅烯是由单层硅原子形成的二维蜂窝状晶格结构,具有石墨烯类似的电学性质,由于硅烯中存在比较强的自旋轨道耦合而备受关注.本文利用非平衡格林函数方法研究了门电压控制的硅烯量子线中电子输运性质和能带结构.研究发现,只有在较强的门电压下,而且硅烯量子线具有较好的锯齿形或扶手椅形边界而不存在额外硅原子时,硅烯量子线中才存在无能隙的自旋极化边缘态.另外,计算结果表明这种门电压控制的硅烯量子线中边缘态在每个能谷处自旋是极化的.这些计算结果将为实验上利用电场制作硅烯纳米结构提供理论支持.     

有机分子吸附和衬底调控锗烯的电子结构

锗基集成电子学的发展潜力源于其极高的载流子迁移率以及与现有的硅基和锗基半导体工业的兼容性,而锗烯微小带隙能带特点极大程度地阻碍其应用.因此,在不降低载流子迁移率的情况下,打开一个相当大的带隙是其应用于逻辑电路中首先要解决的问题.本文采用范德瓦耳斯力修正的密度泛函理论计算方法,研究了电场作用下有机分子吸附和衬底对锗烯原子结构和电学性质的影响.研究结果表明,有机分子吸附和衬底通过弱相互作用破坏了锗烯亚晶格的对称性,从而在狄拉克点上打开了相当大的带隙.苯/锗烯和六氟苯/锗烯体系均在K点打开了带隙.当使用表面完全氢化的锗烯(锗烷HGeH)衬底时,苯/锗烯/HGeH和六氟苯/锗烯/HGeH体系的带隙可进一步变宽,带隙值分别为0.152和0.105 eV.在外电场作用下,上述锗烯体系可实现大范围的近似线性可调谐带隙.更重要的是,载流子迁移率在很大程度上得以保留.本文提出了一种有效的可调控锗烯带隙的设计方法,为锗烯在场效应管和其他纳米电子学器件中的应用提供了重要的理论指导.     

卤化硅烯结构、电子和光学性质的第一性原理研究

二维硅烯的商业用途通常受到其零带隙的抑制,限制了其在纳米电子和光电器件中的应用.利用基于密度泛函理论的第一性原理计算,单层硅烯的带隙通过卤原子的化学官能化被成功打开了,并综合分析了卤化对单层硅烯的结构,电子和光学性质的影响.研究结果表明卤化使结构变得扭曲,但保持了良好的稳定性.通过HSE06泛函,全功能化赋予硅烯1.390至2.123 eV的直接带隙.键合机理分析表明,卤原子与主体硅原子之间的键合主要是离子键.最后,光学性质计算表明,I-Si-I单层在光子频率为10.9 eV时达到最大光吸收,吸收值为122000 cm^-1,使其成为设计新型纳米电子和光电器件的有希望的候选材料.     

Self-organized in-plane incorporation of Si atoms in GaAs by molecular beam epitaxy

The preferential attachment of Si atoms at misorientation steps on vicinal GaAs(001) surfaces has been studied by RHEED. By analysing the time evolution of the specular beam intensity and the change in surface reconstruction during Si deposition we show that a self-organized Si incorporation along the step edges takes place. The observed (3×2) structure is due to an ordered array of dimerized Si atoms with missing dimer rows. Taking into account the structure of the (3×2) unit mesh and its orientation with respect to the As-terminated or Ga-terminated steps, a characteristic minimum in the RHEED intensity recording corresponds to the number of Ga step-edge sites. Since the preferential path for Ga as well as for Si adatom diffusion is along the [110] direction, the critical terrace width for wirelike Si attachment is much larger for a misorientation toward (111)As than for a misorientation toward (111)Ga. Despite the high local impurity concentration, the Si-modified surface can be overgrown with GaAs without adverse effects on the growth front. This is promising for the fabrication of doping wires.     

Tight—binding calculation of the electronic states of bulk—terminated GaAs（311）A and B surfaces

We have carried out theoretical investigations on the electronic structure of GaAs(311)A and GaAs(311)B surfaces. The bulk electronic structure of GaAs has been described by the second-neighbour tight-binding formalism and the surface electronic structure was evaluated via an analytic Green function method. First, we present the surface band structure together with the projected bulk band of both Ga-terminated and As-terminated for GaAs(311)A and GaAs(311)B surfaces, respectively. In each case, the number of surface states is determined, and the localized surface features and orbital properties of these surface states along Γ-Y-S-X-Γ high symmetry lines of the surface Brillouin zone are discussed. For the Ga-terminated GaAs(311)A (1×1) surface, we have tested two possible structure models, i.e. "the bridge site" and "the hollow site" models. In comparison with the angle-resolved photoelectron spectroscopy studied recently, the results have shown that the surface electronic states of the hollow site model are in good agreement with the experiments, whereas those of the bridge site model are not. So we have concluded that the hollow site model is favourable for the Ga-terminated GaAs(311) (1×1) surface and the bridge site model should be excluded.     

Cluster model approach for electronic structure of Si and Ge(111) and GaAs(110) surfaces

For the purpose of exploring how realistic a cluster model can be for semiconductor surfaces, extended Huckel theory calculations are performed on clusters modeling Si and Ge(111) and GaAs(110) surfaces as prototypes. Boundary conditions of the clusters are devised to be reduced. The ideal, relaxed, and reconstructed Si and Ge(111) surfaces are dealt with. Hydrogen chemisorbed (111) clusters of Si and Ge are also investigated as prototypes of chemisorption systems. Some comparison of the results with finite slab calculations and experiments is presented. The cluster-size dependence of the calculated energy levels, local densities of states, and charge distributions is examined for Si and Ge(111) clusters. It is found that a 45-atom cluster which has seven layers along the [111] direction is large enough to identify basic surface states and study the hydrogen chemisorption on Si and Ge(111) surfaces. Also, it is presented that surface states on the clean Si and Ge(111) clusters exist independent of relaxation. Further, the calculation for the relaxed GaAs(110) cluster gives the empty and filled dangling-orbital surface states comparable to experimental data and results of finite slab calculations. The cluster approach is concluded to be a highly useful and economical one for semiconductor surface problems.     

Features of atomic processes at the formation of a wetting layer and nucleation of three-dimensional Ge islands on Si(111) and Si(100) surfaces

The intermediate stages of the formation of a Ge wetting layer on Si(111) and Si(100) surfaces under quasiequilibrium grow conditions have been studied by means of scanning tunneling microscopy. The redistribution of Ge atoms and relaxation of mismatch stresses through the formation of surface structures of decreased density and faces different from the substrate orientation have been revealed. The sites of the nucleation of new three-dimensional Ge islands after the formation of the wetting layer have been analyzed. Both fundamental differences and common tendencies of atomic processes at the formation of wetting layers on Si(111) and Si(100) surfaces have been demonstrated. The density of three-dimensional nuclei on the Si(111) surface is determined by changed conditions for the surface diffusion of Ge adatoms after change in the surface structure. Transition to three-dimensional growth on the Si(100) surface is determined by the nucleation of single {105} faces on the rough Ge(100) surface.     

The surface structures growth’s features caused by Ge adsorption on the Au(111) surface

The initial stage of the adsorption of Ge on an Au(111) surface was investigated. The growth and stability of the structures formed at the surface were studied by ultrahigh-vacuum low-temperature scanning tunneling microscopy and analyzed using density functional theory. It was established that the adsorption of single Ge atoms at the Au(111) surface at room temperature leads to the substitution of Au atoms by Ge atoms in the first surface layer. An increasing of surface coverage up to 0.2–0.4 monolayers results in the growth of an amorphous binary layer composed of intermixed Au and Ge atoms. It was shown that the annealing of the binary layer at a temperature of T _s ? 500 K, as well as the adsorption of Ge on the Au(111) surface heated to T _s ? 500 K for coverages up to 1 monolayer lead to a structural transition and the formation of an Au–Ge alloy at least in the first two surface layers. Based on experimental and theoretical data, it was shown that the formation of single-layer germanene on the Au(111) surface for coverages ≤1 monolayer in the temperature range of T _s = 297–500 K is impossible.     

Hydrogen chemisorption on the 100 (2 × 1) surfaces of Si and Ge

This paper combines a theoretical study of the Si(100) surface having a monolayer of atomic hydrogen chemisorbed to it with an experimental study of the analogous Ge(100) and Ge(110) surfaces. In the theoretical work the underlying (100) silicon surface is taken to be reconstructed according to the Schlier-Farnsworth-Levine pairing model with the hydrogen located on the unfilled tetrahedral bonds of this structure. Self-consistent calculations of the electronic potential, charge density, spectrum, and occupied surface density of states are carried out. The force on the hydrogen atoms is then calculated using the Hellman-Feynman theorem. This force is found to be close to zero, confirming that the hydrogen atoms are indeed at the equilibrium position for the chosen silicon geometry. Features in the calculated photoemission spectrum for the Si(100) 2 × 1 : H surface are discussed in terms of related features in the photoemission spectrum of Si(111) : H, but are found not to agree with the previously measured photoemission spectrum of Si(100) 2 × 1 : H. Measured photoemission and ion-neutralization spectra for Ge(100) 2 × 1 : H agree in their major features with what is calculated for Si(100) 2 × 1 : H, however, suggesting that the Ge(100) 2 × 1 : H surface is reconstricted according to the pairing model. Similarly, measured spectra for clean Ge(100) 2 × 1 agree with calculations for the row dimerized Si(100) surface.     

用中能电子向前散射图像研究表面原子结构

从中能电子向前散射产生的实空间图像研究了Cu（111）1×1，Si（111）（3^1/2×3^1/2）R30°-In及Ge（111）（3^1/2×3^1/2）R30°-Ag的表面结构．Cu（111）1×1的图像不仅说明表面有三重旋转轴对称性，而且还表明fcc结构一直保持到最表面一层原子．Si（111）（3^1/2×3^1/2）R30°-In表面的图像说明In原子占据T4位，而不是H3位．Ge（111）（3^1/2×3^1/2）R30°-Ag的图像说明HCT模型是正确的．这些成功的应用说明从中能电子向前散射图像可以直观而且快捷地获得表面结构类型的可靠信息，而无需类似于低能电子衍射（LEED）表面结构分析那样的复杂计算 关键词：     

Surface structure of GaAs(211)

The atomic structures of the two inequivalent (211) surfaces of GaAs have been investigated by LEED. Both surfaces, prepared by etching and heat-cleaning or ion-sputtering and annealing, are unstable and develop large (110) facets which exhibit the atomic geometry of the (110) GaAs surface. These facets entirely cover the surface. Three sets of facets, making 30°, 30° and 54° angles with the (211) plane, are detected on one surface. Only two sets, making 30° angles with the (211) plane, are detected on the other. The reasons for this difference are not understood at this time. The LEED study of Si(211) and Ge(211) shows that the Si surface is flat whereas the Ge surface exhibits reconstructed (311) facets. The structural difference between the (211) surfaces of GaAs and Ge and the facetting of the compound are invoked to explain the problems encountered in the MBE growth of GaAs on Ge(211).     

On a rudimentary model of instability and (2 x 1) reconstruction of Si (001) and (111) surfaces

A recently proposed simple qualitative theory of instability and reconstruction of surfaces of solids exhibiting a certain covalent component of bonding is applied to (2 x 1) Si (001) and (111) surfaces with a possible extension to analogous surfaces of germanium and GaAs (001)., Shockley surface states from the vicinity of the Fermi energy are supposed to play a crucial role in the electron-phonon coupling. For the Si (001) surface the interaction between bridge and dangling bonds causes the dimerization (primary effect) and the buckling distortions to couple. If the latter effect is pronounced, the asymmetric dimer might exist as a stable or metastable surface phase. For the Si (111) surface the Pandey -bonded chain model seems to be a natural candidate for reconstruction from the present simple point of view.