Will space processing yield better semiconductor crystals?

JOM, Oct 2000

A professor at the University of Illinois and scientists at Marshall Space Flight Center are working together to study the growth of alloy crystals on the International Space Station. Alloy crystals, special blends of germanium and silicon, act as semiconductor materials for use in nextgeneration communication systems and advanced computers. The problem with developing these alloy crystals is that the Earth's gravity makes them difficult to grow without defects.

"Germanium is about three times heavier than silicon, so it generally sinks to the bottom of the melt in the crucible, destroying the desired homogenous concentration in the crystal," said John Walker, a professor of mechanical and industrial engineering at the University of Illinois. "On Earth, gravity also presses the liquid against the walls of the container, resulting in the formation of numerous faults,dislocations, and contact stresses in the growing crystal."

In the absence of gravity, however, the ingredients don't separate as readily and the molten material tends to pull away from the container shortly before solidifying. According to Walker, the crystals could be grown in special ampules within magnetic damping furnaces on the space staHon. The magnetic fields would act as a brake, suppressing all movement in the molten material and thereby preventing the mixture from separating.

While growing crystals in space will likely never be commercially viable, Walker and his colleagues hope to show that space-- grown crystals consistently create better semiconductor materials. "Then, once we understand the fundamental materials science, we can search for a way to reliably reproduce these crystals on Earth in the presence of gravity," Walker said.