Study of crystal growth mechanism Utilizing microgravity

在微重力条件下，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。本项目利用这种理想的生长条件，研究了溶液中晶体的生长机理。特别研究了宏步，以发现基本的生长过程。首先，利用光学显微镜对空间生长InP的横截面进行了观察，并对宏观台阶的行为进行了详细的研究。发现在生长初期会形成宏台阶，但当生长接近稳态时，宏台阶消失。在空间实验室D-2和EURECA使命中，在德国一侧生长了InP晶体。所采用的生长技术是THM（移动加热器法）。在该技术中，在生长前沿施加相当强的温度梯度。结果表明，温度梯度和生长速度是控制宏台阶产生和消失的主要因素。通过对三维扩散方程的数值求解，在宏观台阶形状边界条件下，发现随着温度梯度的增大和生长速度的降低，宏观台阶与冒口的夹角减小。本文还研究了在室温下ADP和KDP水溶液生长过程中的宏观台阶稳定性。结果表明，宏观台阶的出现或消失取决于流动方向对邻面取向的影响。这被解释在与宏观台阶相关的溶质浓度的不均匀性方面。这个想法是一致的理论开发的InP宏台阶。分析认为，宏观台阶的产生是由于三维扩散和界面不平整引起的生长表面的不稳定性。

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.