Semiconductor Factory In Space

Another facility that had flown on board USML-1 was the Crystal Growth Furnace (CGF), capable of achieving very high operating temperatures in excess of 1,000 Celsius and processing large crystals of semiconducting materials, together with metals and alloys. During USML-2, it was employed to grow samples of cadmium-zinc telluride, mercury-cadmium telluride, gallium arsenide and mercury-zinc telluride. Cadmium-zinc telluride crystals grown on the June 1992 mission had already proved themselves to be the most defect-free specimens ever produced and achieved particular perfection when they did not come into contact with the container walls.

In response to this discovery, the primary CGF sample chamber for USML-2 was outfitted with a spring-loaded piston, which moved to reduce the volume of the cylinder as the material contracted while cooling. This helped to eliminate air voids in the crystal and ensured that it maintained an 'even' contact with the container walls along its entire surface.

Cadmium-zinc telluride is routinely used as a base on which infrared-detecting mercury-cadmium telluride crystals can be grown; the alloyed material - zinc - helps to reduce strain and crystal defects, but makes the resultant alloy relatively soft and easily deformable when produced on Earth. In space, on the other hand, it is possible for investigators to pinpoint the constituents within a crystal that could be altered to improve growth technologies. Already, USML-1 had produced cadmium-zinc telluride crystals one thousand times better than any grown on Earth. During USML-2, Principal Investigator David Larson hoped to reproduce this success.

Mercury-cadmium telluride is also used as a construction material for infrared detectors, while gallium arsenide crystals form an integral part of high-speed, low-power digital circuits, optoelectronic-integrated circuits and solid-state lasers. However, during USML-2 scientists were keen to learn more about the processes by which impurities were distributed throughout compounds during their growth. Finally, mercury-zinc telluride crystals were known to have properties which, theoretically at least, make them superior infrared detectors for use in defence, space exploration, medicine and industry.

Moreover, by carefully modifying the proportions of mercury, zinc and tellurium within the alloy, it was hoped to adjust a resultant material's electronic and optical properties for a wide range of applications. Each of the materials was processed successfully by the CGF, in addition to a fifth sample - of germanium, with a trace of gallium - to demonstrate the influence of changes in Columbia's attitude on the crystal growth process. In total, eight semiconducting crystals were grown, as well as a very thin crystal and two crystals that could form the basis for faster and less power-hungry computer chips.

Early on 26 October, the furnace finished melting David Matthiesen's gallium arsenide crystal and slowly resolidified it at just 1.8 mm per hour, operating at its highest-ever temperature, close to 1,250 Celsius! Throughout the resolidification, an electric pulse 'marked' its exact growth rate, as well as its liquid-to-solid 'boundary' for Matthiesen's team. The experiment also involved the addition of a small amount

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- known as a 'dopant' - of selenium. ''There is less than one part per million of selenium in this sample,'' said CGF researcher Frank Szofran. ''Yet it greatly alters the electrical conductivity of the semiconductors.''

Two days later, Thornton and Sacco completed the furnace's shortest-ever processing run: depositing a thin layer of mercury-cadmium telluride on a base material in less than 90 minutes. ''We're examining ways to reduce crystal 'birth defects', which are transferred from defects in the substrate material that can't be eliminated,'' said Principal Investigator Heribert Weidemeier of the Rensselaer Polytechnic Institute. ''In the crystal we grew on USML-1, the interface between the substrate and the first layer was much smoother than in crystals produced on the ground. This was totally new, something we had never seen before and had not expected.''

On USML-2, Weidemeier's team grew thinner layers to determine the propagation limits of substrate defects in crystals. During similar experiments on Earth, the crystal material firstly forms separate 'islands' on the base, which join to form a complete layer after about two hours of growth. ''We deliberately stopped growth in less time than it takes for a layer to form on Earth,'' Weidemeier said on 29 October. ''However, there is a good chance that under microgravity we may get a complete layer in the shorter time period.'' He hoped this might eventually lead to faster and cheaper terrestrial growth methods.

Other investigations on USML-2 included a record-breaking 1,500 protein samples in both Columbia's middeck and the Spacelab module. In the latter, much

Kathy Thornton works with the Glovebox in the Spacelab module.

of the work was performed in ESA's Glovebox, which Sacco used on 22 October to run an experiment for the Center for Macromolecular Crystallography in Birmingham, Alabama. It included feline calcivirus, akin to a virus responsible for causing digestive problems in humans, and proved so well-formed that it elicited applause from team members. Sacco also set up initial conditions for the growth of a collagen-binding domain protein, which is important in the study of arthritis and joint disease.

Later, Coleman activated a protein known as duck delta crystallin, which is similar to the protein responsible for causing a rare, but deadly, disease in humans. ''The reason we do protein crystal growth is to be able to find out what the protein looks like in order to find a cure for a disease,'' she told journalists during a space-to-ground news conference; the shape of a protein molecule controls its function. The USML-1 mission had proved tremendously successful: of the 30 or so proteins flown, nearly half yielded crystals that were large enough for X-ray analyses of their structures.

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