Study of crystal growth mechanism Utilizing microgravity

在微重力下，由于热对流停止，如果避开自由表面，就可以消除溶液中的各种对流。在本项目中，利用这种理想的生长条件，研究了THR晶体在溶液中的生长机理。重点研究了MacroStep的基本生长过程。首先，用光学显微镜观察了空间生长的InP的横截面，并详细研究了宏观台阶的行为。研究发现，宏观台阶在生长初期形成，但在生长趋于稳定时消失。InP晶体是在D-2空间实验室和欧洲原子能机构任务中在德国一侧生长的。所采用的生长技术为THM(移动加热法)。在这种技术中，在生长前沿施加了相当强的温度梯度。结果表明，温度梯度和生长速度是控制宏观相产生和湮灭的主要因素。日本和德国的研究小组也联合进行了理论研究。采用宏观台阶形状边界条件数值求解三维扩散方程，结果表明，随着温度梯度的增大和生长速度的降低，宏观台阶与立管的夹角减小。俄罗斯研究小组还研究了ADP和KDP溶液在室温下从水溶液中生长的宏观步骤稳定性。结果表明，宏观台阶的出现或消失取决于流动方向与邻近表面取向的关系。这是根据与宏观步骤相关的溶质浓度的不均匀来解释的。这一观点与InP宏步进理论是一致的。结果表明，宏观台阶的产生是由三维扩散和非平坦界面引起的生长表面的不稳定性造成的。

Under microgravity since thermal convection is stopped, one can eliminate all kinds of convection in the solution if one avoids free surfaces. In this project, thr crystal growth mechanism in a solution was studied by making use of this ideal condition for the growth. Macrostep is especially studied to find elementary growth process. Firstly, the cross section of space grown InP was investigated by optical microscope and the behavior of macrostep was studied in detail. It was found that macrostep is formed in the beginning of the growth but it disappears when the growth approaches to a steady state. The Inp crystals were grown in Germany side in spacelab D-2 and EURECA mission. The employed growth technique was THM (traveling heater method). In this technique, rather strong temperature gradient was applied at the growth front. It turned out that the temperature gradient and the growth velocity are major factors which govern the creation and annihilation of the macrostep.Theoretical studies were also made jointly by Japanese and German groups. By solving 3D diffusion equation numerically with a shape boundary condition of macrostep, it was shown that the angle between macrostep terrace and riser decreases with increasing temperature gradient and with decreasing the growth velocity. This means that the macrostep disappears in such conditions.The macrostep stability was also studied for the solution growth of ADP and KDP at room temperature from water solution by Russian group. It was shown that macrostep appears or disappears depending on the flow direction against the orientation of vicinal surface. This was explained in terms of the non-uniformity of the solute concentration associated with the macrostep. This idea is consistent with the theory developed for the InP macrosteps. It was concluded that the origin of macrostep is the instability of the growing surface caused by the 3D diffusion and non-flat interface.