外加氩气流下脉冲激光烧蚀制备纳米硅晶粒成核生长动力学研究

为研究纳米硅晶粒成核生长动力学过程,采用脉冲激光烧蚀(PLA)技术,在室温,50～200 Pa的氩气氛围中,通过引入垂直于烧蚀羽辉轴线的外加气流,在水平放置的衬底上沉积了一系列纳米Si晶薄膜.扫描电子显微镜( SEM)、拉曼(Raman)散射和X射线衍射(XRD)检测结果表明,未引入气流时,衬底上相同位置处晶粒尺寸随气体压强的增大逐渐减小;在距靶1 ～2cm范围内引入气流后,尺寸变化规律与未引入气流时相反.通过分析晶粒尺寸及其在衬底上的位置分布特点,结合流体力学模型和热动力学方程,分析得出在激光能量密度一定的条件下,环境气体压强、烧蚀粒子温度和密度共同影响着纳米晶粒的成核生长.     

不同能量密度下脉冲激光烧蚀制备纳米Si晶粒成核生长动力学研究

在室温、10 Pa氩气环境下,采用脉冲激光烧蚀(PLA)技术,通过改变激光能量密度,在烧蚀点正下方、与烧蚀羽辉轴线平行放置的衬底上沉积制备了一系列纳米Si晶薄膜.采用SEM、Raman散射谱和XRD对纳米Si晶薄膜进行了表征.结果表明:沉积在衬底上的纳米Si晶粒分布在距靶一定的范围内,晶粒尺寸随与靶面距离的增加先增大后减小;随着激光能量密度的增加,晶粒在衬底上的沉积范围双向展宽,但沉积所得最大晶粒尺寸基本保持不变,只是沉积位置随激光能量密度的增加相应后移.结合流体力学模型、成核分区模型和热动力学方程,通过模拟激光烧蚀靶材的动力学过程,对纳米Si晶粒的成核生长动力学过程进行了研究.     

真空中脉冲激光烧蚀制备银纳米晶粒在水平衬底上的分布特性

在室温、真空环境中,通过XeCl准分子脉冲激光烧蚀银靶,烧蚀产物沉积在水平放置的Si(111)衬底上.利用扫描电子显微镜(SEM)、X射线衍射(XRD)仪、探针式表面轮廓(PTSP)仪和选区电子衍射(SAED)对沉积所得的样品进行表征.结果表明,样品主要由具有不同尺寸的银纳米晶粒组成;随着衬底与靶面水平距离的增加,样品厚度和晶粒尺寸逐渐减小;样品的(111)和(200)晶面的XRD特征谱线强度,随着衬底位置的改变而变化.在增加激光能量密度、衬底与烧蚀焦点垂直距离的情况下,晶粒尺寸及样品XRD特征谱线变化规律并未发生明显变化.结合银纳米晶粒传输特性和沉积样品沿不同晶面生长所需表面能的差异,对实验结果进行了分析.     

激光烧蚀硅靶下荷电粒子和纳米晶粒的电流响应研究

采用波长为308 nm的XeCl脉冲准分子激光器,在5～50 Pa的Ar气压下烧蚀高阻抗单晶硅(Si)靶.在烧蚀点正前方、距靶1.5 cm处放置一个中心开孔直径为2 mm的挡板,挡板后面上下对称放置两个极板,然后串联一个标准电阻接地,电阻大小为10 Ω,利用高分辨数字存储示波器记录并测量回路中产生的瞬间电流来研究荷电粒子和纳米晶粒的电流响应.实验结果表明烧蚀产物中的正离子在气压为5 Pa时到达极板上数量最多.纳米Si晶粒带正电并且随着气压的增加纳米Si晶粒下落到极板的数量呈现先增大后减小趋势,在气压为8 Pa时达到最大值,这与扫描电子显微镜的测量结果基本一致.所得结果为进一步研究烧蚀粒子在环境气体中的输运动力学过程提供了依据.     

基于朗缪尔探针的硅离子空间分布特性及纳米晶粒生长过程研究

在室温、10 Pa氩气环境气体中,采用脉冲激光烧蚀技术,在以烧蚀点为圆心、半径为2 cm的玻璃弧形支架的不同角度处放置衬底,沉积了纳米Si晶薄膜.通过扫描电子显微镜、拉曼散射仪对制备样品的形貌和特性进行分析.结果表明:纳米Si晶粒以羽辉轴线为轴呈对称分布,在轴线处平均尺寸最大,随着衬底同轴线夹角的增加,晶粒尺寸逐渐减小.结合朗缪尔探针对空间不同位置羽辉中Si离子密度和热运动温度分布的诊断情况,从晶粒生长过程的角度对其尺寸随空间位置变化的结果进行了研究,得到了晶粒尺寸正比于烧蚀粒子密度和热运动温度的结论.     

混合环境气体配比对纳米硅晶粒角度分布的影响

为了研究混合环境气体配比对脉冲激光烧蚀制备纳米硅(Si)晶粒角度分布的影响,采用XeCl准分子激光器,烧蚀高阻抗单晶Si靶,改变混合环境气体配比(He/Ar =41、1∶10、1∶1、1∶4、1∶2),在半圆环衬底上成功制备了一系列纳米Si晶薄膜.使用扫描电子显微镜(SEM)图像、X射线衍射谱(XRD)和拉曼光谱(Raman)对其进行表征分析.结果表明,在五种配比下,纳米Si晶粒的平均尺寸均随着偏离羽辉轴向夹角的增大而减小;各个角度处,纳米Si晶粒的平均尺寸均随着混合环境气体平均原子质量的增加而呈现先减小后增大的趋势.从传输动力学角度,对结果进行了定性分析.     

掺硼纳米晶粒多晶Si薄膜的结构特征与电学特性

采用低压化学气相沉积(LPCVD)工艺,以SiH4作为反应气体源和B2H6作为硼(B)掺杂剂,在单晶Si或石英表面上,通过原位掺杂制备了掺B的纳米晶粒多晶Si(nc-poly-Si(B))薄膜.利用扫描电子显微镜(SEM)、原子力显微镜(AFM)和拉曼光谱等手段,检测和分析了沉积膜层的表面形貌、晶粒尺寸和密度分布等结构特征.结果表明,在典型工艺条件下,获得了晶粒尺寸为大约15 nm和密度分布为大约9 × 1010cm-2的nc-poly-Si(B)薄膜.样品经退火处理后,Si晶粒尺寸变大,排列更加有序,而且电导特性明显改善.利用常规四探针法测量了样品的薄层电阻.并讨论了B掺杂浓度和退火温度对薄膜电学性质的影响.     

不同衬底表面上大晶粒多晶Si薄膜的制备

利用高频感应加热化学气相沉积(HFCVD)工艺,以H2稀释的SiH4作为反应气体源,分别在n-(111)Si衬底上常规热生长的SiO2层、织构的SiO2层和纳米晶粒多晶Si薄膜表面上,制备了具有均匀分布的大晶粒多晶Si膜.采用扫描电子显微镜(SEM)和X射线衍射(XRD)等检测手段,测量和分析了沉积膜层的表面形貌、晶粒尺寸、密度分布与择优取向等结构特征.结果表明,多晶Si膜中Si晶粒的尺寸大小和密度分布不仅与衬底温度、SiH4浓度与反应气压等工艺参数有关,而且强烈依赖于衬底的表面状态.本实验获得的最好的薄膜中,Si晶粒平均尺寸约为2.3 μm,密度分布约为3.8×107/cm2.对薄膜的沉积机理分析表明,衬底表面上Si原子基团的吸附、迁移、成核与融合等热力学过程支配着大晶粒多晶Si膜的生长.     

温度对SiO2表面上磁控溅射制备Ge纳米点的生长模式和尺寸的影响

采用磁控溅射技术在SiO2/Si(100)表面上制备了一系列不同生长温度的Ge纳米点样品.原子力显微镜(AFM)的实验结果表明:不同衬底温度下Ge纳米点在SiO2薄膜上的生长模式和尺寸分布有所不同.当衬底生长温度达到500 ℃时,SiO2开始与Ge原子发生化学反应,并形成"Ge纳米点的Si窗口".在此温度条件下,外延生长实验可获得尺寸均匀且密度高达3.2×1010 cm-2的Ge纳米点.     

激光退火实现非晶Si晶化的成核势垒研究

采用脉冲激光烧蚀技术,在真空条件下沉积了一系列非晶Si薄膜,并对薄膜样品进行不同能量密度的激光退火处理.通过扫描电子显微镜(SEM)、X射线衍射仪(XRD)、Raman散射仪(Raman)等手段对退火后的薄膜进行形貌、晶态成分表征,确定了非晶Si薄膜晶化的激光能量密度阈值(85 mJ/cm2).结合激光晶化机理进行定量计算.结果显示:形成一个18 nm的Si晶粒所需要的能量,即成核势垒大小约为1.4×10-9 mJ.     

Modelling of mechanism of agglomeration of KCl crystallization

The article presents an analysis into agglomeration during KCl vacuum crystallization. The theoretical and experimental investigations into the mechanism of agglomeration during mass crystallization result in an extension of the growth phenomena within the known model equations. The basis for this is essentially constituted by the collision model concepts of the theory of floculation in disperse systems. The parameters derived from the microprocess analysis (energy dissipation, content of solids, growth rate of individual grains) lead to model equations which are confirmed by laboratory and test trials.     

Relationship of Morphological Types of Dynamic Forms of Crystals

Crystallography Reports - The relationship of morphological spectra (sets of data on the morphological types of real polyhedral crystals and their probabilities under current physicochemical...     

Development of Pauling's concepts of geometric characteristics of atoms

The evolution of the geometric characteristics introduced by Pauling and their dependence on the specific features of the structure and chemical bonds have been considered. The values of the covalent and van der Waals radii are given as well as their relationships and mutual transitions.     

Velocities of Disappearance and Lifetime of Faces of Growing Crystals

The formulae for absolute R_disap and relative R velocities of disappearance and lifetime τ of faces of growing crystals have been derived for stationary growth. It was shown that the quantities are determined by the relative growth velocity R_A/R_critA of the vanishing face A with respect to the critical growth velocity R_critA and by the geometry of a crystal expressed by the trigonometric functions of interfacial angles β and γ formed between face A and the adjacent faces. R increases and τ decreases with the increase in R_A/R_critA to certain limiting values. The calculations have been verified and illustrated by the experimental results for triclinic potassium bichromate (KBC) crystals. Results enable ones to predict values of velocities of disappearance and lifetimes of undesirable, supplementary faces of any real crystal.     

Comparative analysis of two methods of calculation of the orientation of domain walls in ferroelastics

Two types of domain-wall equations are analyzed: the equations derived by the Sapriel method and the equations obtained by interface matching of the thermal-expansion tensor. It is shown that, for W-type domain walls, these methods yield the same equations. For W′-type domain walls, the equations obtained by different methods coincide for proper ferroelastics and differ for improper ferroelastics.     

Dependence of the parameters of equations of viscous flow on chemical composition of silicate melts

The temperature dependence of viscosity of silicate melts is discussed in the framework of the Avramov–Milchev (AM) equation. The composition is described by means of two parameters: the molar fraction, x, and the “lubricant fraction”, l. The molar fraction is the sum of the molar parts x_i of all oxides dissolved in SiO₂, the molar fraction of the latter being 1 ? x. It is shown that, with sufficient precision, two of the parameters of the AM equation can be presented as unique functions of the molar fraction. On the other hand, x is not sufficient to determine properly the reference temperature T_r , at which viscosity is η_r = 10¹³ [dPa.s]. Therefore, additional parameter, “lubricant fraction” l, is introduced. For each of the components, l_i is a product of molar part x_i and a specific dimensionless coefficient 0 ≤ k_i ≤ 1 accounting for the specific contribution of this component to the increased mobility of the system. It is demonstrated that, for l > 0, the reference temperature is related to the “lubricant fraction” l through the reference temperature T_r,SiO2 of pure SiO₂.     

A review of measurement of thermophysical properties of silicon melt

Measurements of thermophysical properties of Si melt and supplementary study of X-ray scattering/diffraction by the authors' group were reviewed. The values obtained differed variously from those of literature. Density was 2–3% larger, surface tension 20–30% smaller, viscosity up to 40% larger, electrical conductivity 8% smaller, spectral emissivity more or less in good agreement with literature values, and thermal diffusivity a few percent larger. An anomalous density jump was found near the melting point. Surface tension and viscosity also showed anomaly. A strange time-dependent change of density was observed over 3 h after melting. X-ray analyses suggested a slight change in local atom ordering, but showed no sign of cluster formation. An addition of 0.1 at% gallium caused the density jump to disappear, while that of boron caused no change. An EXAFS study of the former melt indicated a strong interaction between Ga and Si atoms as if molecules of GaSi₃ existed. The implications of the measured properties are a possibility of soft-turbulence in an Si melt in a relatively large crucible, a more complicated manner of intake of oxygen depleted molten Si from the free surface region to underneath the growing crystal, and a relaxation of the melt after melting arising from trapped gas species.     

Generation of crystal structures of heteromolecular compounds by the method of discrete modeling of packings

Within the method of discrete modeling of packings, an algorithm of generation of possible crystal structures of heteromolecular compounds containing two or three molecules in the primitive unit cell, one of which has an arbitrary shape and the other (two others) has a shape close to spherical, is proposed. On the basis of this algorithm, a software package for personal computers is developed. This package has been approved for a number of compounds, investigated previously by X-ray diffraction analysis. The results of generation of structures of five compounds—four organic salts (with one or two spherical anions) and one solvate—are represented.     

A SAXS study of kinetics of aggregation of silica sols

SAXS in situ experiments on the evolution of TMOS solutions during hydrolysis and polycondensation lead to power laws with scaling exponents ≈ 2. It is suggested that this could be the result of the polydispersity of the samples and that only an apparent fractal dimension can be obtained in this way. Kinetic studies tend to indicate that agglomeration in the sol is the result of a diffusion-controlled process.