溶液、熔体中负离子配位多面体生长基元的分布与缔合

根据对晶体生长溶液、熔体拉曼光谱的测试结果,剖析了溶液和熔体中负离子配位多面体的分布及其缔合过程,总结出了不同过饱和溶液和不同过冷度熔体中负离子配位多面体生长基元的缔合形式和维度的规律.在靠近晶体的边界层处已出现与晶体结构相同或相似的大维度生长基元.实验表明,生长基元的分布和缔合与溶液过饱和度和熔体过冷度密切相关,从而提出用拉曼光谱进行实时监控,寻找最佳生长物化条件,优化晶体生长边界层的厚度和大维度生长基元的数量,为选择最佳工艺条件提出理论依据.     

Effect of the Temperature Fluctuation of the Melt on β‐BaB2O4 (BBO) Crystal Growth

In this paper the effect of the growth temperature fluctuation, for instance, the transient furnace temperature variation due to a short‐term electric power supply interruption on BBO crystal growth was investigated based on the theory of temperature wave transmitting in melt and the boundary layer theory of melt. It was found that the critical width of the temperature pulse to avoid the temperature wave penetrating through the boundary layer and reaching to the growth interface at a constant rotation speed (9∼4 r/min) is 69∼150 s and the corresponding amplitude of the temperature pulse is high more than 60 °C due to the large thickness of the velocity boundary layer of the melt. This result indicates that a small transient temperature fluctuation has no significant effect on the crystal quality, and therefore implies that not only transport processes but interface growth kinetics, a two‐dimensional nucleation growth mode at the interface may also dominate the crystal growth.     

Mass transport effect within the boundary layers in solution crystal growth: Holographic study of KTP and KDP crystal growth

Using holographic phase-contrast interferometric microphotography, we have carried out real-time investigations of the mass transport processes taking place during the high temperature solution growth of KTiOPO₄ (KTP) crystal and low-temperature solution growth of KH₂PO₄ (KDP) crystal. Our experiments demonstrate that a mere diffusion boundary layer is not existing. The mass transport process within the boundary layers is a result of the coupled effect of diffusion and convection actions, no matter whether it is high-temperature or low-temperature solution growth. Under free convection state, the influence of bulk supersaturation on the thickness of solute boundary layer exists in the two different regions. The solute concentration distribution within the layer is an exponential function of the position.     

Three dimensional numerical study of the effect of large Grashof number on HEM crystal growth

Heat transfer and fluid flow in HEM crystal growth of silicon in cylindrical cavity is studied numerically. The walls of the crucible are heated to a fixed temperature. The exchanger that causes and induces natural convection is seated at the middle‐bottom of the crucible. The finite‐volume method is employed to solve the governing equations with proper boundary conditions. The effects of transport mechanism on the temperature distribution, melt flow, pressure and stream function are presented. We focus our work on the pressure field which has not yet been studied in HEM crucible. Also, we extend our work on a wide range Grashof number and for large numbers until 10¹² not yet studied in HEM furnace. It is found that the onset of flow fluctuations appears at Gr = 10¹⁰. Uniform temperature is observed in the entire melt at high Grashof number with development of a thermal boundary layer close to the exchanger. The thermal boundary layer thickness is calculated for strong buoyancy regime. Besides, for very high Gr number, buoyancy has less effect on temperature and then on melt‐crystal interface shape. During enlarging Gr, pressure evolution is related to temperature variation more than flow pattern.     

移动加热器法生长碲锌镉晶体的组分输运与界面形貌研究

移动加热器法(THM)生长碲锌镉晶体时,界面稳定性对晶体生长的质量有很大影响。本文基于多物理场有限元仿真软件Comsol建立了THM生长碲锌镉晶体的数值模拟模型,讨论了Te边界层与组分过冷区之间的关系,对不同生长阶段的物理场、Te边界层与组分过冷区进行仿真研究,最后讨论了微重力对物理场分布的影响,并对比了微重力与正常重力下的生长界面形貌。模拟结果表明,Te边界层与组分过冷区的分布趋势是一致的,在不同生长阶段,流场中次生涡旋的位置会发生移动,从而导致生长界面的形貌随着生长的进行发生变化,同时微重力条件下形成的生长界面形貌最有利于单晶生长。因此,在晶体生长的中前期,对次生涡旋位置的控制和对组分过冷的削弱,是THM生长高质量晶体的有效方案。     

晶体生长界面相研究

在分析前人的晶体生长理论时,作者认为晶体生长过程中可能存在界面相;在分析各种晶体生长现象后认为,晶体生长过程中界面相是存在的,并起着十分重要的作用;通过分析研究,将晶体生长过程中的界面相划分为3个有机的组成部分：界面层、吸附层和过渡层;并进一步论述了界面层、吸附层和过渡层在晶体生长过程中的地位与作用;在此基础上提出了界面相模型。     

Study of the dissolution and growth of salt single crystals in inhomogeneous acoustic fields: II. A focused acoustic field

The growth and dissolution of the isolated (100) face of a KDP crystal at exposure of the phase boundary to focused acoustic fields have been investigated. Visualization tools have been developed and the scale and dynamics of the arising acoustic flows have been established. A quadratic dependence of the dissolution rate on the sound pressure has been obtained; such a dependence is in agreement with the theoretical concepts about the dependence of the mass exchange rate in the acoustic boundary layer on the field parameters. A significant growth response of the face under study, normally exposed to ultrasound along the acoustic axis in the prefocal and postfocal planes of a spherical concentrator (f = 1.4 MHz), has been revealed. It is shown that the mechanisms of mass exchange enhancement in these acoustic modes are radically different. The results obtained show a possibility of controlling growth and dissolution of crystals by varying the parameters of an inhomogeneous acoustic field.     

Influence of magnetic field on the director profile in nematic layer with curved surface

A nematic liquid crystal layer with a curved upper boundary exposed to a magnetic field directed along the layer plane has been studied. Strong nematic anchoring at the lower boundary surface is assumed. A phenomenological expression for the director profile is proposed to solve the problem on the director profile distribution in this system and its stability with respect to the rotational deformations in relation to the degree of the upper-surface roughness and the magnetic field value. The distribution parameters are found by optimizing the expression for the free energy of the liquid crystal system.     

Theoretical model of crystal growth shaping process

A theoretical investigation of the crystal growth shaping process is carried out on the basis of the dynamic stability concept. The capillary dynamic stability of shaped crystal growth processes for various forms of the liquid menisci is analyzed using the mathematical model of the phenomena in the axisymmetric case. The catching boundary condition of the capillary boundary problem is considered and the limits of its application for shaped crystal growth modeling are discussed.

The static stability of a liquid free surface is taken into account by means of the Jacobi equation analysis. The result is that a large number of menisci having drop-like shapes are statically unstable. A few new non-traditional liquid meniscus shapes (e.g., bubbles and related shapes) are proposed for the case of a catching boundary condition.     

Numerical design of Metal‐Organic Vapour Phase Epitaxy process for gallium nitride epitaxial growth

The paper presents the results of numerical simulations and experimental measurements of the epitaxial growth of gallium nitride in Metal Organic Vapor Phase Epitaxy within a AIX‐200/4RF‐S reactor. The aim was to develop optimal process conditions for obtaining the most homogeneous crystal layer. Since there are many factors influencing the chemical reactions on the crystal growth area such as: temperature, pressure, gas composition or reactor geometry, it is difficult to design an optimal process. In this study various process pressures and hydrogen volumetric flow rates have been considered. Due to the fact that it is not economically viable to test every combination of possible process conditions experimentally, detailed 3D modeling has been used to get an overview of the influence of process parameters. Numerical simulations increased the understanding of the epitaxial process by calculating the heat and mass transfer distribution during the growth of gallium nitride. Appropriate chemical reactions were included in the numerical model which allowed for the calculation of the growth rate of the substrate. The results obtained have been applied to optimize homogeneity of GaN film thickness and its growth rate.     

A review of radial segregation in crystal growth during microgravity

This review describes radial segregation results from crystal growth experiments in microgravity, together with their corresponding theoretical treatments. The paper is structured in terms of the different factors influencing radial segregation during crystal growth, such as: curved growth interfaces, variations in boundary layer thickness, weak convection, facets and magnetic fields.In a number of experiments considerably stronger radial segregation occurs in space than is normally observed on earth. The theoretical treatments lead to a sound understanding of all of the results. Possible ways to avoid the problem, such as the application of magnetic fields, are outlined.     

相场方法及其在晶体生长中的应用

相场方法已被发掘出用于直接求解时的自由边界问题－著名的斯特藩方程。该方法作为晶体生长过程中模拟复杂图形成图的计算工具，已呈现出强有力的生命力。目前的研究在于努力发展精巧的计算技术，以便于晶体生长和金属凝固过程进行了理论模拟，而这些技术将有可能方法地应用于工业流程。相场方法之所以具有吸引力，基于如下事实：在计算机模拟过程中，即可避免对于边界实时追踪，又不需要反复判别是否满足显式边界条件。在过去的10年中，它已逐步被用于研究晶体生长的基础课题。诸如；热质输运，晶体生长动力学，二维和三维枝晶生长，图形选择，生长形态和显微结构等。本文对相场方法进行评述，同时给出其最新应用结果。     

Effect of the growth rate on thermal conditions during the growth of single crystals with melt feeding

An attempt is made to establish a correlation between the radial and axial growth rates and the change in the conditions of heat transfer from a growing crystal to the atmosphere of the water-cooled vacuum furnace for the growth of large alkali halide single crystals. It is found experimentally that an increase in the growth rate leads to an increase in the automatic compensation of the melt temperature by the main heater. In this case, the thickness of the layer of melt condensate on the end face and the lateral surface of the crystal decreases. It is revealed that the possibility of growing infinitely long ingots in the presence of intense melt evaporation is restricted by the possibilities of the heat transport through the boundary between the furnace atmosphere and the cooled furnace walls, onto which melt condensate deposits.     

Optical properties of a nematic liquid crystal under the binary effect of periodic shear and compression

The optical response (clearing) of a liquid crystal cell, containing a layer of a nematic crystal with homeotropic alignment, under the binary effect of periodic shear and compression has been theoretically described. The effect was calculated taking into account the presence of orientational waves arising at displacement of one of the boundary plates in its plane, and the mobility of this plate along the normal upon statement of boundary conditions. The influence on the effect of the alignment energy of molecules at the layer boundaries is analyzed. The calculation results are compared with the experimental data.     

Flux growth of La‐doped lead zirconate stannate titanate antiferroelectric crystals

Relaxor antiferroelectric single crystals lead lanthanum zirconate stannate titanate (PLZST) with the composition around the morphotropic phase boundary (MPB) have been grown by flux method using 50 wt% PbO‐PbF₂‐B₂O₃ as a flux. The obtained crystals are light yellow in color. The XRD patterns revealed that the habitual faces of the obtained crystal are (001). The crystal morphology was studied and related to a layer growth mechanism controlled by two‐dimensional growth. The chemical composition of as‐grown crystal was analyzed by inductively coupled plasma atomic emission spectrometry (ICP), indicating a slight decrease of the amount of Ti compared to the starting materials. The result was verified by the XRD patterns with the phase transformation from the co‐existence of tetragonal and rhombohedra phases to the single tetragonal phase. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Investigation of the variations in the crystallization front shape during growth of gadolinium gallium and terbium gallium crystals by the Czochralski method

Numerical investigation of the variations in the crystallization front shape during growth of gadolinium gallium and terbium gallium garnet crystals in the same thermal zone and comparison of the obtained results with the experimental data have been performed. It is shown that the difference in the behavior of the crystallization front during growth of the crystals is related to their different transparency in the IR region. In gadolinium gallium garnet crystals, which are transparent to thermal radiation, a crystallization front, strongly convex toward the melt, is formed in the growth stage, which extremely rapidly melts under forced convection. Numerical analysis of this process has been performed within the quasistationary and nonstationary models. At the same time, in terbium gallium garnet crystals, which are characterized by strong absorption of thermal radiation, the phase boundary shape changes fairly smoothly and with a small amplitude. In this case, as the crystal is pulled, the crystallization front tends to become convex toward the crystal bulk.     

Computational analysis of three-dimensional flow and mass transfer in a non-standard configuration for growth of a KDP crystal

Computational analysis of three-dimensional flow and mass transfer in a non-standard configuration for growth of a KDP crystal was conducted. The results show that the surface shear stress is mainly affected by the inlet velocity, and the distribution of the surface supersaturation is determined by the bulk supersaturation and the inlet velocity. By adjusting the inlet velocity, the homogeneity of surface supersaturation can be improved, which is helpful for reducing the occurrence of inclusions and enhancing the crystal quality. The thickness of solute boundary layer is closely related to the flow intensity, but it is almost free from the impact of the bulk supersaturation.     

衬底的Al化处理对AlN性能的影响

研究了衬底的Al化处理对采用MOCVD法在c面蓝宝石衬底上高温生长AlN外延层的影响机制.通过原位监测监控整个外延生长过程,同时对AlN外延层的表面形貌和晶体质量以及应变状态进行表征研究.结果表明衬底的Al化处理导致AlN外延层的表面更加平整但是晶体质量下降,同时对外延层的应变也有很明显的影响.     

Behaviour of Nematic Liquid Crystals in Inhomogeneous Electric Fields

The behaviour of homogeneously aligned nematic liquid crystal layers in inhomogeneous electric fields has been theoretically and experimentally investigated. The variation of the phase retardation of monochromatic light along nematic liquid crystal layer at the edges of a cell electrode has been obtained. Anisotropic character of the resolution of liquid crystal devices is shown.     

Acousto-optic effect in a nematic liquid-crystal layer under the binary effect of sound and viscous waves

The optical effect in a liquid crystal cell containing a homeotropic layer of nematic liquid crystal (NLC) is analyzed. An NLC layer, located between crossed polaroids and opaque in the absence of external effect, is cleared after irradiation by an ultrasonic beam with a sharp spatial boundary. This enlightenment is suggested to be caused by the reorientation of crystal molecules in the acoustic flows that arise under the binary effect of the layer compression in the irradiated region and the viscous waves propagating from the layer boundaries. The flows were calculated taking into account the stress caused by the velocity convection and crystal structure relaxation. An expression is derived for the cell transparency, and the relative role of the convection and relaxation processes in the effect is determined.