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 the future.   Growth of high quality semiconductor crystals is one of several research themes that were selected for this purpose.

Since the successful experiments on InSb by Walter and Witt et al. in the Skylab project in 1975, many investigators attempted to grow homogeneous and low defect density compound semiconductor crystals in space from a melt.   However, satisfactory results have rarely been obtained since then.   The reason may be due to insufficient considerations to residual acceleration and effects of g-jitter on the melt behavior, which could have prevented the purely diffusion-controlled mass and heat transport growth in the past experiments.   An example is Pb_1-xSn_xTe crystal growth experiment in the FMPT mission.   The compositional profile was not uniform and suggested partial mixing of the melt during crystal growth.   This result agreed well with the computer simulation of the convection due to residual acceleration.

Objectives of this research are therefore to make clear the effects of residual acceleration and g-jitter on crystal growth from a melt and to grow high quality crystals in microgravity which have never been grown on the ground.   We selected In_0.3Ga_0.7As, which is promising as substrates of laser diodes for 1.3 mm wavelength but its large homogeneous single crystals have never been grown on the ground.

The graded solute concentration method was proposed for reducing the effect of residual acceleration and g-jitter in microgravity.   The method has been improved into a concentration gradient partial melting method for further improving compositional homogeneity of grown crystals.   We simulated microgravity environments by growing crystals in capillary tubes (1.6 - 2.0 mm bores) because driving force for convection is suppressed in such capillary tubes even on the ground.   We have succeeded in growing homogeneous In_xGa_1-xAs (with x = 0.2 - 0.33) crystals having single crystal region of about 20 mm in length.