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Stretching silicon: A new method to measure how strain affects semiconductors


A thin-film transistor (TFT) made with silicon nanomembranes

A thin-film transistor (TFT) made with silicon nanomembranes. TFTs are the basis for flexible electronics. (large image)

University of Wisconsin-Madison engineers and physicists have developed a method of measuring the influence strain in thin films of silicon that could lay the foundation for faster flexible electronics.

Silicon is the industry standard semiconductor for electronic devices. Silicon thin films could be the basis for fast, flexible electronics. Researchers have long known that inducing strain into the silicon increases device speed—yet did not fully understand why.

Developed by a team of researchers led by Erwin W. Mueller and Bascom Professor of Materials Science and Engineering Max Lagally, the new method enables the researchers to directly measure the effects of strain on the electronic structure of silicon. The group published their findings in the October 10 online edition of Physical Review Letters, and the paper will soon appear in the print edition of the journal as well.

Standard strained silicon has so many dislocations and defects that strain measurements aren’t accurate, so the research team starts with its own specially fabricated silicon nanomembranes. The team can induce uniform strain in these extremely thin, flexible silicon sheets. “Imagine if you were to attach a ring and a hook on all four corners and pull equally on all four corners like a trampoline, it stretches out like that,” says Lagally.

As a result, the researchers avoid the defects and variations that make it difficult to study standard strained silicon. Uniform strain allows accurate measurement of its effect on electronic properties.

The researchers drew on the powerful X-ray source at the UW-Madison Synchrotron Radiation Center (SRC), which allowed them to measure conduction bands in strained silicon. To study the energy levels, the researchers needed a wavelength-tunable X-ray source. The SRC also houses a monochromator, a device that enabled the team to choose a precise wavelength, giving their readings the required high energy resolution.

By measuring nanomembranes with different percentages of strain, the researchers have determined the direction and magnitude of shifts in the conduction bands. Their findings have shed light on divergent theories and uncovered some surprising properties. Understanding these properties, and the energy shifts in strained materials, could lead to improved fast, flexible electronic devices.

A silicon nanomembrane coming free from its carrier substrate

A silicon nanomembrane coming free from its carrier substrate. The single crystal sheet can then be stretched like a trampoline to study the effects of strain. (large image)

Capitalizing on its techniques for fabricating silicon nanomembranes, the group hopes to use SRC resources to study strain in other semiconductor materials, as well as to make measurements over smaller areas to study the effects of localized strain.

“The ability to make membranes of various materials, to strain them, and make these measurements will enable us to determine strain-dependent band structure of all kinds of semiconductor materials,” says Lagally.