A computational investigation of the thermodynamics of the Stillinger-Weber family of models at supercooled conditions

The existence of metastable liquid-liquid phase transitions (LLPTs) in tetrahedral liquids such as water, silicon, and silica has been the subject of vigorous scientific debate. Because high crystallization rates hinder experimental investigation at deeply supercooled conditions, computer simulation has been widely employed to investigate the existence of LLPTs in molecular models of tetrahedral liquids. The Stillinger-Weber (SW) model of silicon (and more generally, the SW family of models) has been actively studied along these lines. Whereas some studies observe evidence of an LLPT in this model, others report that only a single metastable liquid exists under deeply supercooled conditions. Here, we perform extensive state-of-the-art free energy calculations to investigate the possibility of an LLPT in the SW model of silicon. A similar analysis is also presented for the generalized SW family of models constructed by varying the strength of the three-body energetic term. Our analysis does not show any evidence of an LLPT in SW silicon nor in the generalized family of SW models over the parameter ranges studied. Explanations for the aforementioned discrepancies between previous studies are provided, along with explicit demonstrations of how these discrepancies may have occurred. Outstanding ambiguities and directions for future work are also discussed.     

Genesis of hole states of small germanium clusters in silicon

The results of calculations of electronic states of germanium clusters in a silicon matrix are discussed. The influence of intersubband interaction and smooth interface potential on the position of hole size-quantization levels of ultrasmall Ge clusters is studied.     

空位的第一性原理及经验势函数的对比研究

利用第一性原理及Stillinger-Weber（SW）,EDIP和Tersoff经验势函数对比研究了硅中单空位(V₁)、双空位(V₂)和六边形空位环(V₆)的结构特性及形成能.讨论了经验势函数描述空位时的优点和缺点.结果发现,第一性原理方法可以精确描述空位的原子结构及能量特性,而短程有效的经验势函数无法描述空位所固有的量子效应,如Jahn-Teller变形等.另外,由于经验势函数自身的缺陷,EDIP和T3无法应用于空位结构特性的计算.虽然 关键词： 空位 第一性原理 经验势函数     

Rotational spectrum and structure of Si3

The rotational spectrum of a pure silicon cluster, the Si3 trimer, has been observed for the first time. From the rotational constants of the normal and the 29Si and 30Si isotopic species, a precise geometrical structure has been derived: the trimer is an isosceles triangle with a bond to the apex Si of length 2.177(1) A and an apex angle of 78.10(3) degrees. The substantial inertial defect and fairly large centrifugal distortion suggest that the molecule possesses a shallow bending potential. Si3 is a good candidate for astronomical detection because radio lines of comparably massive silicon molecules (e.g., SiC2, SiC4, and SiS) are readily observed in at least one astronomical source. The rotational spectra of Si6, Si9, and even larger polar silicon clusters may be detectable with the present technique, as well as similar germanium clusters.     

Agn-1Si（n=5-10）团簇结构与性质研究

采用密度泛函理论研究了Agn-1Si团簇（n = 5-10）的几何结构和物理性质。首先得到体系最低能量结构，其中Ag5Si 和 Ag7Si团簇的结构比之前研究中结构能量更低。通过分析相应结构的能隙，平均结合能，二阶能量差可以发现Si原子的加入可以加强团簇结构稳定性，使团簇更加紧凑。在团簇尺寸n = 5-10的范围里，拥有八个价电子的Ag4Si团簇在以上三个方面都显示出非常稳定的特点。通过分析Agn-1Si团簇 (n = 5-10)的差分电荷发现，电荷的转移主要发生在Si原子与其相邻的Ag原子之间， Si原子和附近的Ag原子之间产生了强烈的共价相互作用，是团簇稳定性增强的重要原因。     

Novel metal-encapsulated caged clusters of silicon and germanium

We report the recent findings of metal (M) encapsulated clusters of silicon from computer experiments based on ab initio total energy calculations and a cage shrinkage and atom removal approach. Our results show that using a guest atom, it is possible to wrap silicon in fullerenelike (f) structures, as sp² bonding is not favorable to produce empty cages unlike for carbon. Transition M atoms have a strong bonding with the silicon cage that are responsible for the compact structures. The size and structure of the cage change from 14 to 20 Si atoms depending upon the size and valence of the M atom. Fewer Si atoms lead to relatively open structures. We find cubic, f, Frank-Kasper (FK) polyheral type, decahedral, icosahedral and hexagonal structures for M@Si_n with n = 12-16 and several different M atoms. The magic behavior of 15 and 16 atom Si cages is in agreement with experiments. The FK polyhedral cluster, M@Si₁₆ has an exceptionally large density functional gap of about 2.35 eV calculated within the generalized gradient approximation. It is likely to give rise to visible luminescence in these clusters. The cluster-cluster interaction is weak that makes such clusters attractive for cluster assembled materials. Further studies to stabilize Si₂₀ cage with M = Zr, Ba, Sr, and Pb show that in all cases there is a distortion of the f cage. Similar studies on M encapsulated germanium clusters show FK polyhedral and decahedral isomers to be more favorable. Also perfect icosahedral M@Ge₁₂ and M@Sn₁₂ clusters have been obtained with large gaps by doping with divalent M atoms. Recent results of the H interaction with these clusters, hydrogenated silicon fullerenes as well as assemblies of clusters such as nanowires and nanotubes are briefly presented.     

氧化硅层中的锗纳米晶体团簇量子点

采用氧化和析出的方法在氧化硅中凝聚生成锗纳米晶体量子点结构. 其形成的锗晶体团簇没有突出的棱角和支晶结构，锗晶体团簇的轮廓较圆混，故可以用球形量子点模型来模拟实际的锗晶体团簇. 对比了在长时间退火氧化条件下和在短时间退火用激光照射氧化条件下所生成的锗纳米晶体结构的PL光谱和对应的锗纳米晶体团簇的尺寸分布. 短时间退火氧化条件下生成的锗纳米晶体较小(3.28—3.96nm)，长时间退火用激光照射氧化条件下所生成的锗纳米晶体较大(3.72—4.98nm)；其分布结构显示某些尺寸的锗纳米晶体团簇较稳定，适当的氧化条件可以得到尺寸分布范围较窄的锗纳米晶体团簇. 用量子点受限模型计算了锗纳米晶体团簇的能隙结构，用Monte Carlo方法模拟了PL光谱和对应的锗纳米晶体团簇的尺寸分布，分别与实验结果符合较好. 关键词： 锗晶体团簇 纳米晶体 量子点 激光照射     

HmTiSin(m=1～2;n=2～8)团簇结构、电子性质和红外光谱的密度泛函理论研究

近些年,由于硅半导体材料在微电子工业中的潜在应用,其理论和实验研究备受人们广泛关注。尤其是过渡金属掺杂的硅团簇材料在物理化学性质方面表现了极好的稳定性。这些主要归因于过渡金属含有未填满的d轨道电子,可以填充硅团簇表面的空轨道,减少团簇表面的悬挂键,进而提高整个掺杂硅团簇的结构稳定性,同时产生各种特殊光学、磁性和超导等性质。采用密度泛函理论DFT-B3LYP方法对H_mTiSi_n(m=1～2; n=2～8)团簇的几何结构和电子性质进行了理论计算,讨论了Ti掺杂硅团簇TiSi_n(n=2～8)及其氢化团簇基态结构的变化规律、解离通道和HOMO-LUMO能隙等特征。结果表明,随着Si原子数目的增加,在TiSi_n(n=2～8)团簇中其掺杂Ti原子依次吸附在团簇的棱、面及结构内部。当在掺杂团簇表面吸附氢原子时,都优于吸附在团簇的硅原子上,而且绝大多数的氢化结构采纳了TiSi_n团簇的骨架构型。解离能和HOMO-LUMO能隙的分析结果表明在团簇表面吸附两个H原子时能够明显提高整个团簇的结构稳定性。二阶能量差分的研究发现TiSi₂和TiSi₆团簇相对其他团簇具有较高的稳定性,同时两个H₁TiSi₇和H₂TiSi₇氢化团簇的稳定性更高。此外,模拟了这些氢化团簇的红外振动特征峰,对主要特征峰进行了归属。这些研究将为过渡金属掺杂硅基团簇材料的实验制备和表征提供重要的理论参考。     

State equations and properties of various polymorphous modifications of silicon and germanium

The state equations and the pressure dependences of the lattice properties have been obtained for various polymorphous modifications of silicon and germanium using the Mie–Lennard-Jones pair interatomic potential and the Einstein crystal model. It is shown that the elastic-type interatomic potential gives the best results for the semiconductor phase and the plastic-type interatomic potential for the metalized phases whose potential well depth is significantly smaller. The pressure dependences of the lattice properties are calculated along isotherm 300 K and the jumps of the properties during the phase transition from the diamond structure to the β-Sn phase are evaluated for both silicon and germanium. The calculated results agree well with the experimental data.     

分子动力学模拟研究熔态硅的局部结构

采用Tersoff势,修正试用不同的粒子间相互作用距离R,S的取值,进行了液态硅的分子动力学模拟.模拟的结果表明,修正Tersoff势下得到的径向分布函数能与X射线衍射、中子散射实验相一致.模拟得到在液态硅中,Si的配位数为6.9,键长为0.254nm.分子动力学模拟表明,液态硅中Si原子间联接成一种网络状结构,但大多数Si原子与其近邻Si原子仍保持近似于正四面体的局部构型.键角概率分布出现两个峰值~57°和~102°.通过键序参量分析,得到在液态硅近邻结构中,正四面体构型约占82%,键取向波动方差为5. 关键词：     

Phonon localization in Ge nanoislands and its manifestation in Raman spectra

Multilayer structures with germanium nanoislands that are formed on the silicon (111) surface upon submonolayer deposition by molecular-beam epitaxy have been investigated using Raman spectroscopy. To interpret the experimental Raman spectra, numerical calculations of the spectra have been performed for nanoislands containing from several to several hundred germanium atoms. The calculations demonstrate that the in-plane sizes of nanoislands (with sizes less than 2–3 nm) substantially affect the frequencies of phonons localized in these nanoislands. The experimental Raman spectra confirm the occurrence of the quantum size effect.     

硅和锗纳米线的原子排布和电荷分布的密度泛函紧束缚方法研究

低维硅锗材料是制备纳米电子器件的重要候选材料，是研发高效率、低能耗和超高速新一代纳米电子器件的基础材料之一，有着潜在的应用价值。采用密度泛函紧束缚方法分别对厚度相同、宽度在0.272 nm~0.554 nm之间的硅纳米线和宽度在0.283 nm~0.567 nm之间的锗纳米线的原子排布和电荷分布进行了计算研究。硅、锗纳米线宽度的改变使原子排布，纳米线的原子间键长和键角发生明显改变。纳米线表层结构的改变对各层内的电荷分布产生重要影响。纳米线中各原子的电荷转移量与该原子在表层内的位置相关。纳米线的尺寸和表层内原子排列结构对体系的稳定性产生重要影响。     

Electronic properties of silicon with ultrasmall germanium clusters

The electronic states of silicon with a periodic array of spherical germanium clusters are studied within the pseudopotential approach. The effects of quantum confinement in the energies and wave functions of the localized cluster states are analyzed. It is demonstrated that clusters up to 2.4 nm in size produce one localized s state whose energy monotonically shifts deep into the silicon band gap as the cluster size increases. The wave function of the cluster level corresponds to the single-valley approximation of the effective-mass method. In the approximation of an abruptly discontinuous potential at the heterointerface, the quantities calculated using the effective-mass method for clusters containing more than 200 Ge atoms are close to those obtained by the pseudopotential method. For smaller clusters, it is necessary to take into account the smooth potential at the interface.     

锗团簇Ge65, Ge70, Ge75的稳定结构及其电子结构性质

将两种全局结构搜索方法(压缩液态法、遗传算法)与锗的紧束缚势模 型相互结合, 对Ge₆₅, Ge₇₀, Ge₇₅的稳定结构进行了大规模的搜寻,提 出能量较低的可能结构, 然后进一步利用第一性原理方法对这些低能结构进行精确 的优化计算, 确定出了这三种尺寸团簇的基态结构. 发现这三种团簇各具有两种稳定的并且能量相近的异构体: 类球形和类椭球形, 这与实验上报道的大尺寸团簇Ge_n (65 ≤ n ≤ 80) 的结构特征相符合. 简要地分析了这三种团簇基态结构的电子性质. 关键词： 锗团簇 紧束缚势 遗传算法 压缩液态法     

Structure and mechanical properties of Al-Si(Ge) alloys upon melt centrifugation and quenching

The structures of eutectic binary Al-12.7 at. % Si and Al-29.7 at. % Ge alloys and a ternary Al-10 at. % Si-10 at. % Ge alloy produced by quenching levitated melts or through solidification either in the presence or in the absence of a centrifugal acceleration of 7g are studied. Centrifugation is found to cause an increase in the silicon content in the Al-Si alloy in the direction opposite to the direction of centrifugal acceleration and an increase in the germanium content in the Al-Ge alloy in the direction of centrifugal acceleration. These differences are explained by the fact that the densities of silicon and germanium clusters and solidification centers differ from the liquid-phase density at temperatures of solidification. The related changes in the values of the Young’s modulus and in the stress-strain curves can be due to sedimentation-induced changes in the composition of samples cut from the middle part of an ingot. The processes of decomposition and recovery are shown to have a substantial effect on the elastic moduli of these alloys.     

Fe掺杂ZnSe团簇结构和磁性质

采用密度泛函理论研究了Fe原子单掺杂和双掺杂(ZnSe)12团簇的结构、电子性质和磁性质.我们考虑了三种掺杂方式:替代掺杂,外掺杂和内掺杂.单掺杂时,外掺杂团簇是最稳定结构;而双掺杂时,内掺杂团簇是最稳定结构.团簇磁矩主要来自Fe-3d态的贡献,4s和4p态也贡献了一小部分磁矩.由于轨道杂化,相邻的Zn和Se原子上也产生少量自旋.不同掺杂团簇的总磁矩不同,在可调磁矩的磁性材料领域有潜在应用价值.     

锗硅表面结构和动态过程的STM研究

尽管作为微电子工业的基础,硅和锗的表面和界面几十年来一直是研究的热点,但和纳米技术等不断提出的问题相比,对它们的了解仍很不够。为此,最近我们用扫描隧道显微镜和低能电子衍射方法,对锗硅表面的稳定性、宏观小面化、纳米小面化、小面化的规律、稳定表面的比自由能、表面原子结构以及表面和亚表面原子的动态过程进行了大量的系统的研究。文章综述已取得的研究结果。这些结果除具有重要的基础意义外,对半导体异质外延生长衬底选择,以及量子线和量子点自组织生长模板的选择都会有帮助。     

Phenomenological interatomic potentials for silicon, germanium and their binary alloy

A phenomenological potential is constructed for silicon, germanium, and their binary alloy. As in the modified embedded atom model (MEAM), the interatomic interactions are assumed to arise from the overlap of the electron wave functions. However, the calculation of energy is quite different in the two models. The proposed potential has seven adjustable parameters in contrast to ten or more in MEAM but gives perfect fit with the measured values of seven most important quantities for characterizing strained silicon: cohesive energy, equilibrium lattice constant, vacancy formation energy, Raman frequency, and the three elastic constants. The potential should be suitable for lattice statics calculations on strained silicon.     

注入离子漂移管技术测量原子团簇结构

团簇结构测量是团簇科学研究中的一个重要课题。文章对近几年发展的注入离子漂移管技术进行了详细介绍与分析，该技术是上前测量团簇结构近似程度最高的技术，已在Ｃ，Ｓｉ，Ｇｅ及Ａｌ等原子团簇的结构研究中获得广泛应用，并取得了许多重要结果。