UB Researchers Develop First Flexible Semiconductors

November 05, 1996

BUFFALO, N.Y. -- New, flexible semiconductors -- some of which were made with ordinary weatherstripping silicone and can be peeled right off their substrates -- have been developed by a group of University at Buffalo physicists.

Described recently in Applied Physics Letters, they are the first single-crystal, semiconducting nanomaterials that bend, but don't break.

"When we think of semiconductors, we think of a crystal, something that is very hard and very fragile," said Hong Luo, Ph.D., assistant professor of physics at UB who headed the research group with Athos Petrou, Ph.D., professor of physics at UB. "But these semiconductors can bend like rubber."

They can be peeled right off their supports almost as though one were peeling an address label from a sheet of labels.

Because they retain both their structural integrity and optical properties, the semiconductors are seen as particularly significant for future advances in optical computing, where information will be carried by light instead of by electrons.

"These semiconductors could help expedite the transition from electronics to optical computers by allowing us to exploit optics in semiconductors much more efficiently than has been possible," said Luo.

The new semiconductors are ideal for optical computing because they will allow for optical waveguides -- the optical equivalent of wires -- and semiconductors to be contained inside the same component.

Their flexibility also makes it possible to transmit optical signals in three-dimensional optical circuits, making their applications far more efficient and allowing for far more versatile design than now is possible with two-dimensional transmission o f light.

Other applications lay in optical waveguides for telecommunications and in high-efficiency solar cells for the military, which should be lightweight but sturdy and flexible enough to withstand rocket blastoff and battlefield conditions.

"We have developed a general technology to be used with all semiconductors," Luo explained.

He said the new semiconductors are flexible because they are deposited on substrates in such thin layers by molecular-beam epitaxy (MBE), a technique that involves the deposition of thin films on substrates in an ultra-high-vacuum chamber.

"Theoretically, if you could make it thin enough, even a diamond could be flexible," he said. "But such thin materials are, of course, extremely fragile. They need to be supported by something, which makes it a physicist's problem. We have figured ou t a way to give mechanical support to this type of semiconductor structure."

Using MBE, the researchers grew quantum wells -- structures that are so thin that they follow the rules of quantum physics, not classical physics. They grew them out of zinc selenide and zinc cadmium selenide on gallium arsenide, a typical substrate f or semiconductors.

The MBE-grown sample was then bonded to the silicone and the gallium arsenide was etched away, leaving just the one-micron-thick quantum well structure on top of the silicone.

The discovery by Luo's and Petrou's graduate students that the semiconductors the UB researchers had developed were flexible was actually an accident.

"They were trying to glue the semiconductor to another piece of semiconductor, but it didn't glue very well and it just came off," Luo recalled. "They thought it was ruined."

But the next day, the researchers decided to do some optical testing on that material. To their surprise, they found all of its properties intact.

Luo explained that while some semiconductors that have been constructed out of polymeric materials are flexible and inexpensive, they have not always performed as well as inorganic semiconductors in their ability to emit light or to maintain structural integrity.

"The flexible semiconductors we developed are man-made structures that are fabricated using conventional semiconducting elements," he said. "Such materials possess superior optical properties and can be combined with polymeric materials because both ar e flexible."

So far, the UB researchers have fabricated semiconductors that are about one centimeter in diameter, but Luo said that in facilities used to make industrial products, samples of up to five inches in diameter and even larger could be fabricated.

Co-authors on the paper, which appeared in the September issue of Applied Physics Letters, are Jens J. Haetty, Myunghee Na and Huicheng Chang, all doctoral candidates in physics at UB.

University at Buffalo

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