Compositional uniformity of a Si0.5Ge0.5 crystal grown on board the International Space Station

• Homogeneous Si0.5Ge0.5 crystal growth in microgravity.
• Axial compositional fluctuation less than 2 at% for 14.5 mm length.
• Radial compositional fluctuation less than 1 at% for 10 mm diameter.
• Marking of growth interface by the step temperature change.
• Precise growth rate determination based on striations.

A Si0.5Ge0.5 crystal was grown on board the International Space Station (ISS) using the traveling liquidus-zone method. Average Ge concentration was 49±2 at% for the growth length of 14.5 mm. Radial compositional uniformity was excellent especially between the growth length of 3 and 9 mm; concentration fluctuation was less than 1 at%. In this experiment, cartridge surface temperatures were monitored and heater temperatures were adjusted based on the monitored temperatures for improving compositional uniformity of a grown crystal. A step temperature change by 1 °C was imposed for adjusting heater temperatures. This procedure made it possible to observe growth interface shape; striations due to heater temperature change were observed by a backscattered electron image. Growth rates were precisely determined by the relation between interval of heater temperature change and the distance between striations. Based on the measured growth rates, two-dimensional growth model for the traveling liquidus-zone method was discussed.