Amorphous silicon has been the “king of the hill” when it comes to thin, fast, and flexible semiconductors, but researchers at the University of Pennsylvania believe they have knocked the king off his throne and maybe right into the past.
The U Penn research team, led by doctoral students David Kim and Yuming Lai along with Professor Cherie Kagan, have used cadmium selenide nanocrystals (which are proving themselves useful in a number of areas) to deliver devices that can move electrons 22 times faster than in amorphous silicon.
Cadmium selenide nanocrystals are within a class of colloidal semiconductor nanocrystals that have been found effective for making thin-film field-effect transistors. Essentially taking the form of ink, these colloidal nanocrystals have tantalized researchers looking to create inexpensive thin-film electronics. But until this most recent research they had not been demonstrated for use in the high-performance field-effect transistors needed in large-area integrated circuits.
The Penn research, which was published in the journal Nature Communications (“Flexible and low-voltage integrated circuits constructed from high-performance nanocrystal transistors”), may have found a way to achieve these high-performance large-area integrated circuits.
The researchers started with a flexible polymer on which they used a masking technique to stencil one level of electrodes for the circuit. Another area on the polymer was stenciled off for a conducting gold that would later serve as the electrical connection to the upper levels of the circuit. After putting down an insulating aluminum oxide layer, a spincoating deposition technique was used to deposit a 30-nanometer layer of nanocrystals on top.
What might be the main distinguishing factor between this technique and previous methods using colloidal semiconductor nanocrystals was the use of a new ligand. These ligands extend out from the surface of the nanocrystals and aid conductivity of the nanocrystals as they are packed tightly together.
“There have been a lot of electron transport studies on cadmium selenide, but until recently we haven’t been able to get good performance out of them,” says Kim in a press release. “The new aspect of our research was that we used ligands that we can translate very easily onto the flexible plastic; other ligands are so caustic that the plastic actually melts.”
While the nanocrystal-based devices that the researchers developed are giving amorphous silicon a run for the money in terms of electron mobility, it doesn’t seem that the researchers are targeting amorphous silicon’s main application of flat-panel displays. Instead they envision these flexible and easy-to-produce circuits in pervasive sensors used in either security or biomedical applications.
Dexter Johnson is a contributing editor at IEEE Spectrum, with a focus on nanotechnology.