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Virtually Defect-Free Materials Set to Advance Electronics
Engineers at Ohio State University’s Electronic Materials & Devices Laboratory have surmounted a key obstacle in the manufacture of high-quality light emitting devices and solar cells. Steven Ringel, professor of electrical engineering, and his colleagues have developed hybrid materials that are virtually defect-free—an important first step in producing ultra-efficient electronics. The same technology could also lead to faster, less expensive computer chips.
Ringel and his staff grew thin films of “III-V” semiconductors—materials made from elements, such as gallium and arsenic, which reside in groups III and V of the chemical periodic table. Because III-V materials absorb and emit light with much greater efficiency than silicon, these materials could integrate traditional silicon computer chips and light-related technologies, such as lasers, displays and fiber optics.
For years, researchers have tried to combine III-V materials with silicon, but have only been partially successful. In contrast, Ringel has produced the combination with record quality, and now has his eye on a more ambitious goal. “Ultimately, we’d like to develop materials that will let us integrate many different technologies on a single platform,” Ringel says.
To accomplish this, Ringel is working on a “virtual substrate”—a generic chip-like surface that would be compatible with many different kinds of technologies and could easily be adapted to accommodate different applications.
Ringel’s current materials design consists of a substrate of silicon (0.7 millimeters thick) topped with III-V materials such as gallium and arsenide, with hybrid silicon-germanium layers sandwiched in-between. The gallium arsenide layer is only 3 micrometers—millionths of a meter—thick.
While other labs have experimented with III-V materials grown on silicon, none have been able to bring defect levels down to below a critical level that would enable devices like light emitting diodes and solar cells to be produced, Ringel says. Defects are the result of the misalignment of the thin layers of atoms in a film. Slight mismatches between layers prevent the material from transmitting electrical charge efficiently.
Ringel and his colleagues grew films of III-V semiconductors using a technique known as molecular beam epitaxy, in which evaporated molecules of a substance settle in thin layers on the surface of the silicon-germanium alloy. They then employed techniques such as transmission electron microscopy to look for defects, which are missing or misplaced atoms that trap electrons within the material, Ringel explains. Because defects disrupt the movement of electrons, engineers usually measure the quality of a solar cell material in terms of carrier lifetime—how long an electron can travel freely through a material without falling into a defect.
Other experimental III-V materials grown on silicon have attained carrier lifetimes of approximately two nanoseconds, or two billionths of a second. In comparison, Ringel’s materials have carrier lifetimes of over 10 nanoseconds.
The engineers have fashioned the III-V material into one-square-inch versions of solar cells in the laboratory and achieved 17% efficiency at converting light to electricity. They have also assembled bright light-emitting diodes (LEDs) on silicon substrates that have a display quality comparable to that of traditional LEDs.
The next research phase will bring Ringel´s materials into space, as part of NASA´s Materials International Space Station Experiment (MISSE). An international partner spacecraft will transport samples of the materials to the space station so they can be analyzed and possibly developed for use in future spacecraft.
The Army Research Office and the National Science Foundation funded this work.
Source:
Materials Could Make for Super LEDs, Solar Cells, Computer Chips
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