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Hao-Chih Yuan

Electrical and Computer Engineering Graduate Student Hao-Chih Yuan holds a sample of a semiconductor film on plastic. (Large image)

Materials Science and Engineering

Fast, flexible, low-power 3-D computer chips on plastic

New thin-film semiconductor techniques invented by UW-Madison engineers promise to add sensing, computing and imaging capability to an amazing array of materials.

A team, led by Professor Max Lagally and Electrical and Computer Engineering Assistant Professor Zhenqiang (Jack) Ma, developed a process to remove a single-crystal film of semiconductor from the substrate on which it is built. Only a couple of hundred nanometers thick, this thin layer can be transferred to glass, plastic or other flexible materials, opening a wide range of possibilities for flexible electronics. In addition, the semiconductor film can be flipped as it is transferred to its new substrate, making its other side available for more components. This doubles the possible number of devices that can be placed on the film. By repeating the process, layers of double-sided, thin-film semiconductors can be stacked together, creating powerful, low-power, three-dimensional electronic devices.

“This is potentially a paradigm shift,” says Lagally. “The ability to create fast, low-power, multilayer electronics has many exciting applications. Silicon germanium membranes are particularly interesting. Germanium has a much higher adsorption for light than silicon. By including the germanium without destroying the quality of the material, we can achieve devices with two to three orders of magnitude more sensitivity.”

That increased sensitivity could be applied to create superior low-light cameras, or smaller cameras with greater resolution. Ma, Lagally, Assistant Professor Paul Evans, Physics Associate Professor Mark Eriksson, and graduate students Hao-Chih Yuan and Guogong Wang are patenting the new techniques through the Wisconsin Alumni Research Foundation.

The team’s work was supported in part by grants from the National Science Foundation Materials Research Science and Engineering Center, the Department of Energy and the Air Force Office of Scientific Research.

Just one nanosecond: Clocking events at the nanoscale

As scientists and engineers build devices at smaller and smaller scales, grasping the dynamics of how materials behave when they are subjected to electrical signals, sound and other manipulations has proven to be beyond the reach of standard scientific techniques.

Postdoctoral fellow Alexei Grigoriev, Assistant Professor Paul Evans and Professor Chang-Beom Eom led a team in developing a way to time such effects at the nanometer scale, in essence clocking the movements of atoms as they are manipulated using electric fields. The accomplishment gives scientists a way to probe another dimension of a material at the scale of nanometers. Adding time to their view of the nanoworld promises to enhance the ability to develop materials for improved memory applications in microelectronics of all kinds, among other things.

The traditional tools of nanotechnology—the atomic force microscope and the scanning tunneling microscope—enable scientists to see atoms, but not their response to events, which at that scale occur on the order of a billionth of a second or less.

The ability to time events that occur in materials used in nanofabrication means that scientists now can view dynamic events at the atomic scale in key materials as they unfold. That ability, in turn, promises a more detailed understanding—and potential manipulation—of the properties of those materials.

In addition to Evans, Eom and Grigoriev, the team included Dal-Hyun Do and Dong Min Kim of UW-Madison; and Bernhard Adams and Eric M. Dufresne of Argonne National Laboratory.

Why nanoscale silicon is an unexpected conductor

When graduate student Pengpeng Zhang successfully imaged a piece of silicon just 10 nanometers—or a millionth of a centimeter—in thickness, she and her co-researchers were puzzled. According to established thinking, the feat should be impossible. The microscopy method used required samples that conduct electricity. At 10 nanometers, the sample should have been very hard to run a current through. So why did it work?

A team led by Assistant Professor Paul Evans, Professor Max Lagally, Electrical and Computer Engineering Assistant Professor Irena Knezevic and Physics Associate Professor Mark Eriksson found that when the surface of nanoscale silicon is specially cleaned, the surface facilitates current flow in thin layers that ordinarily won’t conduct. In fact, conductivity at the nanoscale is completely independent of the added impurities, or dopants, that usually controls the electrical properties of silicon. The team reported its findings in the journal Nature.

“What this tells us is that if you’re building nanostructures, the surface is really important,” says Evans. “And the next logical step would be to try to manipulate the surface chemistry in order to control conductivity. What we have now is a knob for controlling the properties of materials that people haven’t exploited before.”

The results also mean that the powerful concepts, methods and instruments of silicon electronics honed by scientists and the semiconductor industry over decades (many of which require conductive samples, like the scanning tunneling microscopy method employed by Zhang) can also be used to explore the nanoworld.

“We’re working at the crossover between silicon electronics and nanoelectronics,” says Evans. “This material is the same size as nano-devices like silicon nanowires and quantum dots. But now we can use the tools from silicon electronics we already have to probe it.”

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