Experiment 13: The Study of Dopant Segregation Behavior During the Growth of GaAs in Microgravity on USML-2

An investigation into the segregation behavior of selenium doped gallium arsenide (Se/GaAs) during directional solidification in the microgravity environment was conducted using the Crystal Growth Furnace (CGF) aboard the second United States Microgravity Laboratory (USML-2). Two crystals were successfully processed on USML-2, which lasted from October 20 to November 7, 1995. The first sample was processed for 67 hours, 45 minutes (MET 5/04:53:45-8/00:23:50) and included 19 hours of growth at 0.5 microns/sec which yielded 3.42 cm of sample length, and 5 hours of growth at 1.5 microns/sec which yielded 2.7 cm of sample. During the second experiment, the furnace temperature was adjusted to move the melt-solid interface position towards the hot end of the furnace. The second sample was processed for 50 hours, 10 minutes (MET 8/18:48:49-10/21:58:54) and included 11 hours of growth at 0.5 microns/sec which yielded 1.98 cm of sample, and 1 hour, 25 minutes of growth at 5.0 microns/sec which yielded 2.6 cm of sample. This sample provides an order of magnitude change in growth rate and reproduces one of the growth rates used during USML-1. In contrast to the results from USML-1, no voids were present in either crystal grown on USML-2. The absence of voids in either sample indicates that growth rate changes alone were not responsible for the formation of voids found in the crystals grown on USML-1. Sections of the ground-based and flight crystals grown on USML-2 were cut and polished. All of the interface demarcation lines expected from the current pulse interface demarcation (CPID) system have been identified. These measurements have been analyzed for interface positions, interface shapes, and growth rates. Using a newly developed technique, based on experimental and numerical results, the seeding interface reproducibility from run to run was <= 2.5 mm. The seeding interface position could be controllably moved, with respect to the furnace zones, by adjusting the control set points of the heating zones. The interface shapes flattened slightly as the interface position moved closer to the hot zone but was always an unfavorable concave into the solid shape. The growth rate was found to equal the furnace translation rate, after a 2 -hour transient, for growth rates <= 1.0 microns/sec. Segregation measurements for the ground-based crystals are indicative of complete mixing behavior, as expected. Segregation measurements of the flight crystals are still in progress.