Concept of Experiment

We developed the graded solute concentration method for avoiding effects of residual acceleration and g-jitter in the space craft and for improving compositional homogeneity of grown crystal.   The principle of the graded solute concentration method is shown in Fig. 1.   In this method, a feed with graded solute concentration is used instead of a uniform feed.   If the residual acceleration on the order of 10^-4 g exists, it will cause weak convection and the solute at the solid-liquid (S/L) interface will be transported away from the interface by the convection.   Such transportation of the solute results in the concentration profile of the solute similar to that of partial mixing of the melt during crystal growth (Fig. 1a), when a uniform feed is used.   On the other hand, if the solute concentration in the feed decreases from the top to the end as is shown in Fig. 1b, the excess solute will be compensated by the solute loss due to weak convection in the melt and will result in a uniform crystal.   In this method, choosing an appropriate soaking period is important because the preinstalled concentration profile in the feed will smear out by diffusion in the melt.   Solidification rate should also be determined so as to maintain constant solute concentration at the S/L interface.   If weak convection exists, apparent diffusion coefficient of the solute increases.   Therefore, the growth rate should be higher than the case of diffusion limited growth.

In order to determine appropriate growth rate, InAs-GaAs interdiffusion coefficients were measured by the sounding rocket TR-IA#7 launched in September in 1998.   The coefficients range form 1.2 to 4.1ﾗ10^-8 m²/s.   According to the numerical analysis, growth rates of higher than 3 mm/h are deduced so as to obtain uniform concentration of the solute in the grown crystal.   However, these growth rates are too high in the crystal growth of the InAs-GaAs system because constitutional supercooling easily occurs at this growth rate.

Therefore, we have improved the graded solute concentration method by adopting an idea of partial melting and named the method as the concentration gradient partial melting method.   In this method, a narrow melt zone is formed by heating the feed with graded solute concentration at a low temperature gradient: the high InAs concentration part has low melting temperature and is melted, while the low InAs concentration part has high melting temperature and remains as a solid.   As is shown in Fig. 2, it is easier in this method to maintain the solute concentration at C₀/k at the S/L interface (C₀: solute concentration in the original melt, k: segregation coefficient) than the case of entire melting of the feed.   Diffusion rate of the solute is smaller in this method because diffusion rate decreases at the interface of the melt and the feed.   Then, homogeneous crystals can be grown at slower growth rates and constitutional supercooling can be avoided.