改进热交换法生长蓝宝石晶体的气泡研究

采用改进的热交换法生长的蓝宝石晶体,气泡是其主要缺陷之一.本文采用数值模拟研究了晶体生长过程中氦气流量对坩埚内温场、固液界面形状的影响.并结合晶体生长实验结果,分析了在实际的晶体生长过程中,氦气流量的线性增加对晶体内气泡的尺寸、形态和分布的影响.     

泡生法生长蓝宝石晶体中的气泡分布

气泡是蓝宝石晶体中的主要缺陷之一.本文中研究了采用泡生法生长蓝宝石晶体中气泡的俘获机理以及在晶体中的分布情况.研究结果表明气泡的分布与温场的结构相关,气泡产生的直接原因是结晶前沿较快的晶体生长速度.晶体中的气泡可以通过对生长速率的适当调整予以消除.     

热交换器中氦气流量对生长蓝宝石温场影响的研究

采用热交换法生长蓝宝石晶体,通过数值模拟与实验研究了晶体生长过程中热交换器中氦气流量对温场的影响.结果表明:晶体生长过程中固液界面为近弧面;随氦气流量增大,晶体与熔体的温度下降,温度梯度增大;加热器功率缓慢上升.     

蓝宝石晶体热性能的各向异性对SAPMAC法晶体生长的影响

采用有限元法对冷心放肩微量提拉法蓝宝石晶体生长过程中晶体内的温度、应力分布进行了模拟计算,结合实验结果讨论了蓝宝石晶体热性能的各向异性对晶体生长的影响.研究结果表明,对于冷心放肩微量提拉蓝宝石晶体生长系统,较大的轴向热导率有利于提高晶体的生长速率和界面稳定性,而稍大的径向热导率则有利于保持微凸的生长界面.晶体内的热应力受径向热膨胀系数的影响显著,随着径向热膨胀系数的增大而增大,最大热应力总是出现在籽晶与新生晶体的界面区域.在实验中选α轴为结晶取向,成功生长出了直径达230mm、高质量蓝宝石晶体.     

Bridgman法晶体生长的瞬态数值模拟

采用变形有限元法对坩埚下降法晶体生长过程进行了瞬态数值模拟.对随下降距离生长位置、生长速度、界面位置处的法线方向温度梯度和凹度的变化进行了分析说明,将生长过程划分为三个阶段.结果表明在整个生长过程中,晶体生长条件一直处于变化之中,因而晶体的质量也在整块晶体中呈现一定的波动性.     

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。     

大尺寸晶体快速生长理论与技术的研究进展

“如何突破大尺寸晶体材料的制备理论和技术”是中国科协发布的2021年度的十大前沿科学问题之一,揭示晶体生长机制和突破生长关键技术是大尺寸功能晶体发展的两个趋势。在原子分子尺度上,晶体生长可以是有势垒的热激活过程,也可以是无势垒的超快结晶过程,这与具体的体系以及晶面有关。从界面属性角度来看,光滑界面是以台阶拓展的方式生长;粗糙界面没有明显的固-液分层,通过局部原子固化进行生长。本文从晶体生长理论模型、生长技术及其应用实例,以及分子动力学方法在晶体生长中的应用等方面探讨了近些年大尺寸晶体快速生长理论和技术的研究进展。目前有多种方法制备大尺寸晶体,但普遍存在制备的晶体质量差和性能不稳定等问题。需要突破对晶体生长微观机制上的认识,建立机制与温度、流速等外界因素的内在联系。而利用机器学习力场以及分子动力学模拟方法,建立固-液界面,模拟晶体生长,将是探究晶体生长微观机制的一种有效方式。     

晶体生长时的固液相结构变化分析

晶体的溶解、熔化以及结晶成核生长时的固液相原子结构是怎样变化的,晶体生长时的生长基元是原子还是原子团.本文根据各种材料液相结构的最新研究结果,提出不饱和配位结构转换模式,并以此模式对各种常见晶体材料从溶解、熔化到结晶生长时的液态母相结构变化以及晶体成核过程进行了描述和分析,认为晶体生长时的界面结构和液相结构十分接近,溶解、熔化主要是晶体表面的不饱和配位原子(离子)转换到液相结构的过程,晶体生长主要是液相中的不饱和配位原子(离子)转换到固液生长界面的位错位置,使配位结构更饱和的过程.随着液相过饱和度的增大,液相结构单元的原子数越来越多,吸附到晶体生长界面若来不及转换回液相,就形成新的位错生长中心,形成晶体生长缺陷.     

改进布里奇曼法生长HgCdTe晶体的生长速度对固液界面形态的影响

为选择合理的晶体生长速度,在用改进Bridgman法生长直径为φ19mm的HgCdTe(x=0.21)晶体过程中,对正在生长的单晶体及熔体进行淬火,以观察其固液界面形态.初步的实验结果表明:在2mm/d及9mm/d的两种生长速度条件下,石英安瓶中的固液界面形态均为凹形抛物面,但其凹陷深度分别为10mm和14mm.较低的晶体生长速度条件下,凹陷深度较小,固液界面形态较平.由实验和讨论得知,宜选择较低的晶体生长速度用改进Bridgman法生长HgCdTe晶体.     

液面位置对φ300mm硅单晶固液界面形状影响的数值计算

数值模拟技术已经成为分析和优化工业化晶体生长工艺必不可少的工具.本文利用有限元分析软件计算了φ300 mm直拉硅单晶生长过程中,不同液面位置时的晶体固液界面形状,模拟计算考虑了热传导、辐射、气流等物理现象,分析了晶体长度和液面位置对晶体生长界面形状的影响,得出了随着晶体长度的增加固液界面的凸度会增大的规律.     

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。     

Crystallisation Mechanism in Gradient Temperature Zone Melting

Theory of zone recrystallization in the framwork of already known models of normal crystal growth, growth by screw dislocations and by two-dimensional nucleation is discussed. By mathematical treatment, different from Tillers approach, analytical expressions for supersaturation at the crystallisation interface, for superheating at the dissolution interface, growth rate and some other parameters have been obtained for both cases of normal and screw dislocations growth. It is possible to determine the growth mechanism.     

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 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.     

Numerical investigation of crystal growth process of bulk Si and nitrides – a review

Heat and mass transfer during crystal growth of bulk Si and nitrides by using numerical analysis was studied. A three‐dimensional analysis was carried out to investigate temperature distribution and solid‐liquid interface shape of silicon for large‐scale integrated circuits and photovoltaic silicon. The analysis enables prediction of the solid‐liquid interface shape of silicon crystals. The result shows that the interface shape became bevel like structure in the case without crystal rotation. We also carried out analysis of nitrogen transfer in gallium melt during crystal growth of gallium nitride using liquid‐phase epitaxy. The result shows that the growth rate at the center was smaller than that at the center. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A variational approach to cellular crystal growth

The variational method is applied to the study of crystal growth from melt under the condition that the boundary between the liquid and solid phases has a cellular structure. The surface energy of the interface is taken into account.     

On the growth mechanism of polycrystalline snow crystals with a specific grain boundary

Observed frequently in a natural snowfall at temperatures lower than -20°C, the crossed-plate crystal is an assemblage of two to four thin basal plates, each plate extending along a CSL grain boundary with a relation of 70.3°/[11$\text{2}$0]. A possible growth mechanism is proposed to account for the characteristics growth in the direction of the axis [11$\text{2}$0]. If two or more grain boundary dislocation (GBD) structures are contained at a CSL grain boundary, growth layers will overflow alternately on the (10$\text{1}$0) planes of each component crystal at the intersection of these planes. Thus, the distinctive structure of the CSL grain boundary, like the feature of a screw dislocation step, will induce the preferential growth in this direction.     

Experimental Study of Interface Inversion of Tb3ScxAl5‐xO12 Single Crystals Grown by the Czochralski Method

Thermal conditions and rotation rate were examined experimentally for obtaining a flat interface growth of high melting‐point oxide (Tb₃Sc_xAl_5‐xO₁₂ ‐ TSAG) by the Czochralski method. The critical crystal rotation rate can be significantly reduced, of about twice at low and very low temperature gradients comparing to medium temperature gradients in the melt and surroundings of the crystal. The interface shape of TSAG crystals is not very sensitive on crystal rotation rate at small rotations and becomes very sensitive at higher rotations, when the interface transition takes place. The range of crystal rotation rates during the interface transition from convex to concave decreases with a decrease of temperature gradients. At low temperature gradients interface inversion crystals takes place in very narrow range of rotation rates, which does not allow one to growth such crystals with the flat interface. Even changing crystal rotation rate during the growth process in a suitable manner did not prevent the interface inversion from convex to concave and thus did not allow to obtain and maintain the flat interface.     

Control of crystal growth in bridgman furnace

Control of crystal quality during crystal growth requires accurate implementation of thermal boundary conditions. We identify this problem as the furnace temperature control problem. The thermal boundary conditions, in turn, dictate the interface shape between the solid and the liquid region of the material. Determination of the boundary conditions for a given desired interface shape is considered as the material temperature control problem in this paper. We outline the current efforts for the solution of the furnace temperature control and the material temperature control problems. We restrict our review to Bridgman growth control techniques.     

Orientation changes during recovery of compressed Mo crystals

The type of the recovery variety which is accompanied by changes in texture is studied. Reorientation of the surface of compressed Mo crystal was observed during its annealing before recrystallization. As a result, new macroscopic {100} orientation areas appear on the contact crystalline surface of {111} orientation. Turned areas have a more perfect inner structure and correspondingly lesser density of deformation defects. As a result of rotation a high-angle boundary interface occurs as a “broken boundary”. The transition layer considered is highly stressed and contains microcracks. The boundary interface is shown to be the intensive recrystallization source, where the recrystallization grains are nucleated at the expence of interface dislocation bulgings. The driving forces of the rotation observed and a scheme of a residual stress distribution in the compressed crystal are analysed. According to the scheme a deformed crystalline surface is affected by the compressive stresses and the inner layers are affected by the tensile ones. A relaxation of the internal stresses results from plastic rotations. A rotation moment arising between the surfaces and internal layers of the crystal leads to reorientation.