Scientists are figuring out the physics behind a seemingly magical way to produce high-quality crystals.

by Patrick L. Barry

"Any sufficiently advanced technology is indistinguishable from magic."
Arthur C. Clarke

At first, it might remind you of a magician making his assistant levitate above a table, apparently suspended only by the power of the magician's mind.

But this is a laboratory - an unlikely setting for such a show. Nevertheless, it's where Frank Szofran and colleagues are growing high-quality crystals using a method as amazing as any conjuring trick.

By carefully cooling a molten germanium-silicon mixture inside a cylindrical container, they coax it into forming a single large and extraordinarily well-ordered crystal. Such crystals have very few defects because, remarkably, they never touched the walls of the very container in which they grew.

Amazing. But like the spectacle of the lovely levitating lady, the sense of "magic" elicited by this way of making crystals is only a product of not fully knowing how the trick is done.

When this crystal-growing procedure was serendipitously discovered in experiments flown on Skylab missions in the 1970s, scientists were as baffled as an audience at a magic show. Since then, crystal growers have begun to piece together an explanation, but there are still unanswered questions about the process - called "detached Bridgman growth."

Image courtesy WebElements.com A close-up view of atoms in a germanium crystal. (The spaces between the atoms are exaggerated for clarity.) The goal of Szofran's crystal-growing experiments is to minimize the defects in the orderly arrangement of the atoms.

Growing well-ordered crystals is important because they are used in a mind-boggling variety of devices here on Earth: microchips, video cameras, radiation detectors, digital watches, high-power semiconductors, and record players, to name a few. Crystals formed by detached Bridgman growth, in particular, could lead to improved windows and substrates for infrared sensors, more accurate cosmic-ray detectors, and tiny solid-state lasers for next-generation flat-panel displays. And hard-to-predict spin-offs could create whole new categories of electronic products.

"In general, when people grow crystals [for electronics applications], they would like them to be the highest quality possible - the lowest number of impurities, the lowest number of dislocations," Szofran says. Detached Bridgman growth is one way to make that happen.

Szofran explains: "When crystal growth takes place in contact with the container wall, the container pushes on the crystal, and that causes the atoms to be nudged out of alignment. Such 'defects,' as they're called, can cause the crystal not to perform as well [for certain applications]."

During detached Bridgman growth, the crystal doesn't touch the container walls, so a higher quality crystal with fewer defects can be produced.

An ideal crystal is a paragon of order and structure. The atoms that comprise it are arranged in a geometrical pattern that repeats over and over - like the tiles in a tile floor, but in three dimensions. The precise ordering of atoms can lend a crystalline substance special properties of use in, e.g., electronic devices.

Although detached Bridgman growth was discovered during experiments done in space, scientists have learned how to grow crystals using this method on the ground, too. That's important because, otherwise, it wouldn't be of much use to industry.

Image courtesy MSFC The surface of this crystal grown by the Bridgman method is visibly smoother where it was detached from the container wall. The "hands off" growth of detached Bridgman crystals creates fewer defects in the crystals' internal structure.

It's too expensive to grow bulk quantities of inorganic crystals in space, Szofran says. The purpose of space-based experiments "is to learn about the process of crystal growth, which hopefully can then be applied to improve the way crystals are grown on Earth."

Growing crystals in space is revealing "because, in a sense, gravity is a variable," he explains. "Any time a scientist can gain control over a variable, they can learn more by changing its value and doing experiments under different conditions."

Here on the ground where gravity is "normal," Szofran and colleagues are studying crystals grown from an alloy of germanium and silicon - materials with well-understood properties thanks to decades of intense research by the semiconductor industry. He notes that other materials are candidates for detached Bridgman growth, too, including alkali halides used in cosmic ray detectors; indium antimonide and gallium antimonide, which are used for infrared detectors and lasers; and calcium fluoride for short-wavelength lenses used in ultra-high definition lithography.

more Many inorganic solids have a crystalline structure. This diagram shows the arrangement of atoms in a "unit cell" of germanium. The spaces between the atoms are exaggerated for clarity.

Their ongoing experiments aim to answer some important questions. For example: What causes the crystal to grow detached from the walls of its container? and What is the exact range of conditions in which detached growth can happen? Szofran says that, "a breakthrough in our understanding of detached terrestrial growth could lead to commercial technologies for preparing advanced semiconductor materials with reduced defect densities, while a better understanding of detached growth in microgravity could allow crystal growers to better control this phenomenon during future flight experiments."

Szofran is looking forward to 2004 or 2005, when he and his colleagues will be able to conduct crystal-growing tests aboard the orbiting International Space Station. The results could provide the understanding they need to master this "magical" technique and to produce crystals of exceptional quality right here on Earth.