Born inside distant, pulsating stars, germanium is a rare element on Earth most closely related to silicon and tin. Researchers at USD are working on ways to turn this shiny, grayish white metalloid into extremely sensitive sensors that may someday reveal the secrets of dark matter.
USD is one of only a handful of places in the world where scientists are growing extremely pure germanium crystals. Detectors made of the crystals will be used in deep underground experiments, like those at the Sanford Underground Research Facility in the old Homestake Gold Mine in the Black Hills. There scientists hope to study dark matter and double-beta decay.
Germanium has been used since the 1950s to manufacture electronics, but because it is so rare and so costly the semi-conductor industry has switched to silicon, which comes from ordinary sand. Germanium is found in the ore used to produce zinc, although it also can be recovered from the ores of silver, lead and copper. Germanium can be found concentrated in some coal deposits in Russia and China.
Scientists at USD are working to develop a process that will grow very large, very pure germanium crystals in a deep underground environment. When operational, the deep underground lab at Homestake will be the only site in the world where such crystal production is possible.
Germanium is one of the few substances that expands as it solidifies. In the labs on the USD campus, workers are growing germanium crystals inside specially designed crystal pullers. The workers dip a seed crystal into molten germanium, and then the crystal rotates as it slowly withdrawn from the liquid. The molten germanium “freezes” around the seed crystal, resulting in a long, tapered ingot of solid, ultra-high purity germanium.
Ultimately the USD scientists want to develop an economic way to process large amounts of germanium, both to satisfy the deep underground experiments and to sell commercially. Typically, one-third of each grown germanium crystal produced in the lab is of sufficient purity to allow it to be manufactured into a detector. The remaining fraction of each crystal may be fabricated into wafers that can be used in electro and optical devices and solar panels, with the potential to be sold commercially.