Experts at Purdue University say the United States could cut its total energy consumption and related carbon emissions by approximately 10 percent through the broad adoption of light-emitting diode technology. Known to be about four times more efficient than incandescent lights, one LED “negative” has prevented the technology’s widespread domestic use: prohibitive cost. LEDs are “at least 20 times” more expensive than incandescent and fluorescent bulbs, a Purdue press release says. This situation may be about to change, however, based on the same release’s announcement that Purdue researchers have developed a new technique that promises to lower the cost of producing LEDs.
As described in the journal Applied Physics Letters, the technique enables sapphire, a costly LED component, to be replaced with cheaper silicon. The technique owes its development to a research team headed by Timothy Sands, director of the Birck Nanotechnology Center at Purdue, the release says. In the release, Sands – a professor of materials, electrical and computer engineering – explains that sapphire more expensive than silicon and doesn’t offer silicon’s scalability. Another factor that adds to a sapphire LED’s costliness, Sands says, is that they “require a separate mirror-like collector to reflect light that ordinarily would be lost.” Costly collectors aren’t required in silicon-based LEDs, Sands says, because the Purdue team “metallized” the silicon substrate with a “built-in reflective layer of zirconium nitride.”
The team overcame zirconium nitride’s natural tendency to become unstable in the presence of silicon “by placing an insulating layer of alumininum nitride between the silicon substrate and the zirconium nitride,” he says. Sands stresses that this step is crucial to the success of his technique. In fact, he calls “placing a barrier on the silicon substrate to keep the zirconium nitride from reacting,” one of his work’s “main achievements.” The achievement was accomplished by utilizing reactive sputter deposition. This enabled Sands’ team to shower “the metals zirconium and aluminum with positively charged ions of argon gas in a vacuum chamber. The argon ions caused metal atoms to be ejected, and a reaction with nitrogen in the chamber resulted in the deposition of aluminum nitride and zirconium nitride onto the silicon surface.” The release then describes how Sands’ team deposited gallium nitride, the light-emitting ingredient in LEDs, onto the silicon via organometallic vapor phase epitaxy, performed in a reactor, at temperatures of about 1000°C. As the zirconium nitride, aluminum nitride and gallium nitride are deposited on the silicon, they self assemble into a crystalline structure that matches that of silicon. “We call this epitaxial growth, or the ordered arrangement of atoms on top of the substrate,” Sands says. “The atoms travel to the substrate, and they move around on the silicon until they find the right spot.” This crystalline formation, he says, is “critical” to the proper performance of silicon LEDs. “It all starts with silicon, which is a crystal, and you end up with gallium nitride that’s oriented with respect to the silicon through these intermediate layers of zirconium nitride and aluminum nitride,” he explains. “If you just deposited gallium nitride on a glass slide, for example, you wouldn’t get the ordered crystalline structure and the LED would not operate efficiently.” Sands says, because many LEDs can be manufacturered from a large silicon wafer, silicon-based LED technology also will enable manufacturers to “scale up” processing and, thereby, further reduce production costs. He notes similar economies of scale are “not possible using sapphire.”
One major obstacle remains before silicon-based LED technology will be ready for market, Sands admits. He says his team must find a way to prevent the gallium nitride layer from cracking while the silicon wafer cools after manufacturing.”The silicon wafer expands and contracts less than the gallium nitride,” Sands explains. “When you cool it down, the silicon does not contract as fast as the gallium nitride, and the gallium nitride tends to crack.” The Purdue professor is optimistic, however, and calls this challenge an “engineering issue” and “not a major show stopper.” For this reason, he believes affordable silicon LEDs will be “on the market within two years.”