Semi­con­duc­tors in flex­i­ble de­vices

DEMM Engineering & Manufacturing - - ELECTRICAL TECHNOLOGY -­quiry #141233a

Re­searchers from North Carolina State Univer­sity have de­vel­oped a new way to trans­fer thin semi­con­duc­tor films, which are only one atom thick, onto ar­bi­trary sub­strates, paving the way for f lex­i­ble com­put­ing or pho­tonic de­vices. The tech­nique is much faster than ex­ist­ing meth­ods and can per­fectly trans­fer the atomic scale thin films from one sub­strate to oth­ers, with­out caus­ing any cracks.

At is­sue are molyb­de­num sul­fide (MoS2) thin films that are only one atom thick, first de­vel­oped by Dr Linyou Cao, an as­sis­tant pro­fes­sor of ma­te­ri­als sci­ence and en­gi­neer­ing at NC State. MoS2 is an in­ex­pen­sive semi­con­duc­tor ma­te­rial with elec­tronic and op­ti­cal prop­er­ties sim­i­lar to ma­te­ri­als al­ready used in the semi­con­duc­tor in­dus­try.

“The ul­ti­mate goal is to use th­ese atomic-layer semi­con­duct­ing thin films to cre­ate de­vices that are ex­tremely f lex­i­ble, but to do that we need to trans­fer the thin films from the sub­strate we used to make it to a f lex­i­ble sub­strate,” says Cao, who is se­nior au­thor of a pa­per on the new trans­fer tech­nique.

“You can’t make the thin film on a flex­i­ble sub­strate be­cause flex­i­ble sub­strates can’t with­stand the high tem­per­a­tures you need to make the thin film.”

Cao’s team makes MoS2 films that are an atom thick and up to 5cm in di­am­e­ter. The re­searchers needed to find a way to move that thin film with­out wrin­kling or cracking it, which is chal­leng­ing due to the film’s ex­treme del­i­cacy.

“To put that chal­lenge in per­spec­tive, an atom-thick thin film that is 5cm wide is equiv­a­lent to a piece of pa­per that is as wide as a large city,” Cao said. “Our goal is to trans­fer that big, thin pa­per from one city to another with­out caus­ing any dam­age or wrin­kles.”

Cao’s team has de­vel­oped a tech­nique that takes ad­van­tage of the MoS2’s phys­i­cal prop­er­ties to trans­fer the thin film us­ing only room-tem­per­a­ture wa­ter, a tis­sue and a pair of tweez­ers.

MoS2 is hy­dropho­bic – it re­pels wa­ter. But the sap­phire sub­strate the thin film is grown on is hy­drophilic – it at­tracts wa­ter. Cao’s new trans­fer tech­nique works by ap­ply­ing a drop of wa­ter to the thin film and then pok­ing the edge of the film with tweez­ers or a scalpel so that the wa­ter can be­gin to pen­e­trate be­tween the MoS2 and the sap­phire. Once it has be­gun to pen­e­trate, the wa­ter pushes into the gap, f loat­ing the thin film on top. The re­searchers use a tis­sue to soak up the wa­ter and then lift the thin film with tweez­ers and place it on a f lex­i­ble sub­strate.

The whole process takes a cou­ple of min­utes. Chem­i­cal etch­ing takes hours.

“This new trans­fer tech­nique gets us one step closer to us­ing MoS2 to cre­ate flex­i­ble com­put­ers,” Cao adds. “We are cur­rently in the process of de­vel­op­ing de­vices that use this tech­nol­ogy.”

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