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have pushed the concept further by developing an optical ‘nanocavity’ that boosts the amount of light that ultrathin semiconductors absorb. The advancement could lead to, among other things, more powerful photovoltaic cells and faster video cameras; it also could be useful for splitting water using energy from light, which could aid in the development of hydrogen fuel. The team, comprised of faculty and students from the University at Buffalo and two Chinese universities, presented its findings February 24th in the journalAdvanced Materials.
far less expensive, can be used to increase the amount of light that semiconducting materials absorb.”
The nanocavity consists of, from bottom to top: aluminium, aluminium oxide and germanium. In the experiment, light passed through the germanium, which is 1.5 to 3 nanometres thick, and circulated in a closed path through the aluminium oxide and aluminium. The absorption rate peaked at 90 percent, with germanium absorbing roughly 80 percent of the blue-green light and aluminium absorbing the rest. This is ideal, says Haomin Song, PhD candidate in electrical engineering at UB and the paper’s first author, because the bulk of the light stays within the semiconducting material. “The nanocavity has many potential applications. For example, it could help boost the amount of light that solar cells are able to harvest; it could be implanted on camera sensors, such as those used for security purposes that require a high-speed response. It also has properties that could be useful for photocatalytic water splitting, which could help make hydrogen fuel a reality,” Song says.
A rendering shows a beam of light interacting with an optical nanocavity. The nanocavity boosts light absorption in ultrathin semiconductors. (Credit: Advanced Materials)
“We’re just scratching the surface, but the preliminary work that we’ve done is very promising,” says Qiaoqiang Gan, PhD, lead author and UB assistant professor of electrical engineering. “This advancement could lead to major breakthroughs in energy-harvesting and conversion, security and other areas that will benefit humankind.” The most common semiconductor material, silicon, is used to make microchips for cellular phones, computers and other electronic devices. Industry has kept pace with the demand for smaller, thinner and more powerful optoelectronic devices, in part, by shrinking the size of the semiconductors used in these devices. The problem, however, is that these ultrathin semiconductors do not absorb light as well as conventional bulk semiconductors.
Therefore, there is an intrinsic trade off between the ultrathin semiconductors’ optical absorption capacity and their ability to generate electricity. As a result, researchers worldwide are trying to find ways to boost the amount of light that ultrathin semiconductors can absorb. Harvard University researchers recently had varying degrees of success by combining thin films of germanium, another common semiconductor, on a gold surface. “While the results are impressive, gold is among the most expensive metals,” says Suhua Jiang, associate professor of materials science at Fudan University in China. “We illustrated a nanocavity, made with aluminium or other whitish metals and alloys that are
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Before any of that happens, however, more research must be done, especially as it relates to how the semiconductor would turn the light into power as opposed to heat. Gan’s research group is collaborating with Alexander Cartwright, PhD, UB professor of electrical engineering and vice president for research and economic development, and Mark Swihart, PhD, UB professor of chemical and biological engineering, to develop ultrathin energy-harvesting devices. Gan is also working with Hao Zeng, PhD, UB associate professor of physics, to study its effect on photocatalysis. The National Science Foundation supported the research. The paper, called “Nanocavity enhancement for ultra-thin film optical absorber,” by Haomin Song et al is available via the following link: DOI: 10.1002/adma.201305793
SiGe chip sets new speed record
The silicon-germanium technology could be used in cold- temperature applications such as in outer space, where temperatures can be extremely low A research collaboration consisting of IHP-Innovations for High Performance Microelectronics in Germany and the Georgia Institute of Technology has demonstrated what it claims is the world’s fastest silicon-based device to date. The investigators operated a silicon-germanium (SiGe) transistor at 798 gigahertz (GHz) fMAX, exceeding the previous speed record for SiGe chips by about 200 GHz.
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