Nov. 22, 2001 Sliced into almost paper-thin discs called wafers, semiconductors hold the circuitry that receives, transmits and processes information.
Traditionally, scientists "grow" quantities of single-crystalline semiconducting materials by immersing the tip of a pencil-shaped starter crystal, or "seed," in a melt of the same composition. They then slowly withdraw and rotate the seed to form a thick rod shape. To make the crystal develop certain desired properties, they add special impurities to the melt before crystal growth.
However, as the crystal grows, it rejects those impurities into the melt or takes them in. As a result, the melt composition can change during growth - and since the crystal grows from the melt, the crystal composition can continue to change.
When the process is finished, the resulting crystal's composition and properties can vary along its length, so many parts built upon wafers from one crystal can be inconsistent in performance.
UW-Madison Materials Science and Engineering Professor Sindo Kou and graduate student Jia-Jie He have devised a method to ensure the melt composition stays constant.
They first lengthened the crucible in which the materials melt. Then they added a low-temperature heater around the crucible's lower half and moved the existing high-temperature heater to the upper half. The bottom of the crucible holds a solid material identical in composition to the desired crystal; the upper part holds the melt.
As the crystal grows and the melt level decreases, an existing mechanism pushes the crucible upward so that the solid material gradually enters the high-temperature heat zone, melts and keeps the melt composition constant.
Scientists can apply this method to crystals that are a mixture (an alloy) of two different semiconductors and grow them with a uniform composition. With a few modifications, users can adapt this technology easily to their existing equipment. Kou and He are patenting their discovery through the Wisconsin Alumni Research Foundation.
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