Nanocrystalline Germanium Could Drive Next Speed Revolution in Computer Chips
Comparable to "wonder material" graphene, scientists successfully created for the first time a one-atom-thick sheet of germanium that has incredible properties, promising a new pathway to take electronics to the next level. Interestingly, germanium also was the material that allowed the development of the first solid-state transistors ever in 1947, awarded with a nobel prize, and followed 7 years later by the first silicon transistors -- we all know the incredible success story of the microchip that ensued!
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But after 60 years it is now time for the next revolution in material, since the physical limits of silicon are now approaching fast. Nanocrystalline materials look like a good option: The chemists at The Ohio State University who developed the technology for making the atom-thin germanium sheets found that it conducts electrons more than ten times faster than silicon and five times faster than conventional germanium.
The material's structure is closely related to that of graphene -- a much-touted two-dimensional material comprised of single layers of carbon atoms. As such, graphene shows unique properties compared to its more common multilayered counterpart, graphite. Graphene has yet to be used commercially, but experts have suggested that it could one day form much faster computer chips, and maybe even function as a superconductor, so many labs are working to develop it.
Joshua Goldberger, assistant professor of chemistry at Ohio State, decided to take a different direction and focus on more traditional materials, which could allow for similar innovation but at a much lower price-point since the same or similar production methods can be used.
"Most people think of graphene as the electronic material of the future," Goldberger said. "But silicon and germanium are still the materials of the present. Sixty years' worth of brainpower has gone into developing techniques to make chips out of them. So we've been searching for unique forms of silicon and germanium with advantageous properties, to get the benefits of a new material but with less cost and using existing technology."
Researchers have tried to create germanane before but this is the first time anyone has succeeded at growing sufficient quantities of it to measure the material's properties in detail.
In nature, germanium tends to form multilayered crystals in which each atomic layer is bonded together; the single-atom layer is normally unstable. To get around this problem, Goldberger's team created multi-layered germanium crystals with calcium atoms wedged between the layers. Then they dissolved away the calcium with water, and plugged the empty chemical bonds that were left behind with hydrogen. The result: they were able to peel off individual layers of germanane.
The primary thing that makes germanane desirable for optoelectronics is that it has what scientists call a "direct band gap," meaning that light is easily absorbed or emitted. Materials such as conventional silicon and germanium have indirect band gaps, meaning that it is much more difficult for the material to absorb or emit light.
"When you try to use a material with an indirect band gap on a solar cell, you have to make it pretty thick if you want enough energy to pass through it to be useful. A material with a direct band gap can do the same job with a piece of material 100 times thinner," Goldberger said.
With its high electron mobility (electrons move through germanane ten times faster through silicon), germanane could carry the increased load in future high-powered computer chips.
"Mobility is important, because faster computer chips can only be made with faster mobility materials," Goldberger said. "When you shrink transistors down to small scales, you need to use higher mobility materials or the transistors will just not work," Goldberger explained.
Next, the team is going to explore how to tune the properties of germanane by changing the configuration of the atoms in the single layer.
Lead author of the paper was Ohio State undergraduate chemistry student Elizabeth Bianco, who recently won the first place award for this research at the nationwide nanotechnology competition NDConnect, hosted by the University of Notre Dame. Other co-authors included Sheneve Butler and Shishi Jiang of the Department of Chemistry and Biochemistry, and Oscar Restrepo and Wolfgang Windl of the Department of Materials Science and Engineering.
The research was supported in part by an allocation of computing time from the Ohio Supercomputing Center, with instrumentation provided by the Analytical Surface Facility in the Department of Chemistry and Biochemistry and the Ohio State University Undergraduate Instrumental Analysis Program.