Background and Objectives of the Research
NASDA has recently established the Space Utilization Research Program (SURP) for performing team research in collaboration with researchers outside NASDA. The role of SURP in microgravity science is to perform systematic research for demonstrating the potential of microgravity utilization by producing benchmark results and to promote microgravity utilization in future. Growth of high quality semiconductor crystals is one of several research themes that were selected for this purpose.
Control of solute transport rate at the growth interface is the most important issue to grow compositionally homogeneous crystals. We aim at inventing a new growth method to control solute concentration and concentration gradient at the growth interface for growing homogeneous mixed crystals. The benefit of microgravity in crystal growth is improvement of controllability of solute transport by the suppression of thermal and solutal convections in fluids. Study on growth mechanism can also become much easier in microgravity by the suppression of convection. Therefore, we utilize this advantage of microgravity in crystal growth. We selected In0.3Ga0.7As as a target material because In0.3Ga0.7As is promising as a substrate of laser diodes for 1.3 mm wavelength but large homogeneous single crystals have never been grown on the ground.
When we review past crystal growth experiments in space, very little satisfactory results have been obtained in spite of many attempts to grow homogeneous crystals. The reason may be due to insufficient considerations to the effects of residual acceleration and g-jitter on the melt behavior which could have prevented the purely diffusion-controlled mass and heat transport during crystal growth. An example is a Pb1-xSnxTe crystal growth experiment in the FMPT mission. The compositional profile was not uniform and suggested partial mixing of a melt during crystal growth. This result agreed well with the result of computer simulation on fluid flow in a melt; residual gravity on the order of 10-4 g causes convective flow whose velocity exceeds PbTe-SnTe interdiffusion rate.
We, therefore, proposed the graded solute concentration method as a new crystal growth method in which solute concentration decrease at the growth interface due to residual acceleration and g-jitter is compensated by preinstalled solute concentration gradient in a feed. The method has been developed by introducing a technique of partial melting method for further improving compositional homogeneity of grown crystals. We have succeeded in growing homogeneous InxGa1-xAs crystals (with x = 0.2 - 0.33 and fluctuation less than 竺0.02) having single crystal region of about 20 mm in length and we named the newly invented method ﾅﾕhe traveling liquidus-zone method・after the principle of the growth method.
In 2000 fiscal year, we developed the traveling liquidus-zone method experimentally and theoretically for obtaining larger size crystals and for elucidating mechanism of the homogeneous crystal growth and we applied to the 1st International Announcement of Opportunity for microgravity experiments aboard the International Space Station. In this booklet, we report results of our activities in the last year.