Conjuring Crystals

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

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

December 11, 2001: 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 NASA 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.

Above: 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. Image courtesy WebElements.com.

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."

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.

Above: 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. Image courtesy MSFC.

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.

Right: 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. [more]

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.

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."

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Detached Bridgman Growth
How Does it Work?

Scientists have considered several theories to explain why the crystal forms a small distance away from the container wall. All agree that the gas pressure in the gap is important, but they differ on other points.

Some scientists think gases produced from volatile substances within the material can provide enough pressure to cause the separation. But others believe this source of gas is inconsequential and the needed pressure difference between p1 and p2 (see illustration) has to be provided by other means.

Szofran and his colleagues hope to answer this question. They will do experiments where the necessary gas is sealed into the container and others where no gas is added to see if enough comes out of the material to cause detachment.

Szofran explains the value of doing such experiments in space: "You'd like to have the pressure in the gap between the solid and the container [p2 in the illustration] to be higher than the pressure above the liquid [p1]. And you're limited on the ground as to how high that difference can be, because if it gets to be very large, a bubble will form and release the pressure in the gap. In space that won't happen because there's no buoyancy to move the bubble through the liquid."

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.

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."

Left: The First Materials Science Research Rack, or MSRR-1. Future flight experiments with germanium-silicon crystals will be conducted inside the MSRR-1. Says Szofran: "During processing, the sample will be behind the circular door in the middle-right of the rack." [more]

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.

Editor's Note: Szofran's collaborators include materials science researchers from the University of Illinois in Urbana-Champaign, Cape Simulations, Inc., the Albert-Ludwigs-Universität Freiburg (Germany), and the Institut für NE-Metallurgie und Reinststoffe (Germany). Work in the United States is sponsored by NASA and the work in Germany is sponsored by the German space agency DLR (Deutsches Zentrum für Luft- und Raumfahrt).

Web Links

Office of Biological and Physical Research -- sponsors Szofran's work and more like it.

Microgravity Research Program Office -- Gravity is an important variable in many physical processes. The Microgravity Research Program helps scientists learn what happens in weightless environments.

Reduction of Defects in Germanium-Silicon -- (MSFC) one application of detached Bridgman growth, includes a movie.

Right: a comparison of three different crystal growth techniques. [more]

First Materials Science Research Rack -- (MSFC) the "MSRR-1" is a mini-laboratory for materials science experiments on the ISS. The MSRR-1 is part of the Materials Science Research Facility being developed at the NASA Marshall Space Flight Center.

Structures of Simple Solids -- plenty of information about solid crystals, from a University of Oxford 1st year inorganic chemistry course.

Germanium -- detailed information about the element, from WebElements.com

Crystals -- basic information from Encyclopedia.com

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