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Researchers Find New Uses for an Old Semiconductor

Photonics Spectra
Jun 2000
Daniel S. Burgess

Scientists at Sandia National Laboratories are putting an old alloy to work in new applications. Indium gallium arsenide nitride was developed almost 10 years ago, but it had remained a curiosity. Today, a "crunch program" is under way to prepare the semiconductor material as a laser source for telecommunications and as a solar cell for satellites.

The key is that nitrogen greatly changes the bandgap structure of gallium arsenide, so the researchers should be able to tailor the material's electrical and optical properties to a desired application by controlling the concentration of nitrogen in the alloy. "We're hoping for 1- to 1.3-µm lasers or for it to be used as a sandwich layer in a multilayer solar cell," said Eric Jones, a senior scientist at the facility.

Theoretically, both are within reach, and the researchers should be able to tune the bandgap to anywhere between about 870 nm and 1.4 µm. Four-layer photovoltaic cells using InGaAsN could have an efficiency of 40 percent, nearly twice that of silicon cells, although their higher production cost likely would limit them to space-based applications, where a smaller footprint and lower weight are essential. Vertical-cavity surface-emitting lasers (VCSELs) based on the material should be suitable for short-distance optical networks.

Jones said that the needs of the photovoltaics and telecommunications industries were more responsible for the researchers' reconsideration of the material than any easing in the difficulty to produce it. Currently, they use molecular beam epitaxy and metallorganic chemical vapor deposition, but neither accurately controls the highly reactive nitrogen-bearing compounds they must use. "It isn't like mixing oatmeal," he said.

Researchers at Sandia National Laboratories are developing a tunable semiconductor for use in telecommunications and as a solar cell for satellites. From left, Normand Modine, Andrew Allerman and Eric Jones display InGaAsN wafers.

But the desire for a 1.3-µm bandgap in photovoltaics and changing requirements in telecommunications also brought the scientists back to InGaAsN. When the material was developed, the telecommunications industry had little interest in 1.3-µm lasers, Jones said. "Now there's different fiber, and it's, 'You dumb engineers, go do it.' "

The project is still in the research phase, but the team has built an edge-emitting laser and hopes to produce a VCSEL by the end of the year. "If you talk to the commercial companies," Jones said, "they want it yesterday." Meanwhile, theorists at the facility are working to better understand the material so the team can optimize the growth process.

"For the solar cells, it would have been more interesting if Iridium [LLC] hadn't fallen out," Jones said of the Washington-based satellite phone and paging service company that declared bankruptcy in August 1999. "Now the military is interested, but they'll use it to put twice as much electronics on a satellite."

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