Semiconductors in flexible devices
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.”